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International School of Liquid Crystals 22nd Course

1st International school of the IEEE Photonics Society - Italy Chapter

“Photonic Integration: advanced materials, new technologies and applications”

E. Majorana Centre for Scientific Culture, Erice (Italy)

25th September - 1st October 2016

Directors of the Course: Director of the School: A. d’Alessandro, P. Pasini, C. Zannoni S. Selleri, C. Zannoni

COST Action IC1208 “Integrating devices and materials: a challenge for new instrumentation in ICT”

Scientific Program

Please find below the updated program. Click on the title of each lecture to download the corresponding abstract.

25th September

Afternoon: Arrival

21.15: Welcome Reception at the Marsala Lecture Hall (S. Rocco)

26th September

9.00-9.20 Welcome by C. Zannoni – Director of the ISLC and S. Selleri (IEEE PS President)

9.20 – 10.20 Lecture 1 “Photonic Integration: what, where, why, when, how?” (Lecturer: Meint Smit)

10.20 – 11.20 Lecture 2 The food chain for photonics integration (Lecturer: Marco Romagnoli)

11.20 – 11.50 Coffee Break

11.50 – 12.50 Lecture 3 “Fundamentals on integrated optics” (Lecturer: Gabriella Cincotti)

12.50 – 15.00 Lunch break

15.00 – 16.00 Lecture 4 “Advances on integrated optics” (Lecturer: Andrea Melloni)

16.00 – 17.00 Lecture 5 “Materials and technologies: Si” (Lecturer: Bertrand Szelag)


17.30 – 18.30 Lecture 6 “Materials, technologies and devices: InP and III-V” (Lecturer: Meint Smit)

27th September

9.00 – 10.00 Lecture 7 “Si3N4 waveguide technology for broadband and ultra-low-loss photonic integrated circuits” (Lecturer: Martijn Heck)

10.00 – 11.00 Lecture 8 “Glass and Glass-Ceramic Photonic Systems: advances and perspectives” (Lecturer: Maurizio Ferrari)


11.30 – 12.30 Lecture 9 “InP-on-Silicon Integration Through Waferbonding and Epitaxy” (Lecturer: Dries Van Thourhout)

Antonio_MBP

Rettangolo

12.30 – 13.30 Lecture 10 “Integration of Si and SiN PICs with New Active Materials” (Lecturer: Dries Van Thourhout)


15.00 – 16.00 Lecture 11 “Materials and technologies: electro-optic dielectrics” (Lecturer: Alessandro Busacca)

16.00 – 17.00 Lecture 12 “Liquid crystals and photonic integration: materials, properties, technologies, devices” (Lecturer: Antonio d’Alessandro)


17.30- 19.00 Poster Session

28th September

9.00 – 10.00 Lecture 13 “Control layer: Electronics at service of photonics” (Lecturer: Francesco Morichetti)

10.00 – 11.00 Lecture 14 “Optical Interconnections: photonics at service of electronics” (Lecturer: Antonio La Porta)


11.30 – 12.30 Lecture 15 “Photonic packaging at glance” (Lecturer: Antonello Vannucci)

12.30 – 13.30 Lecture 16 “Design tools and design flows for integrated photonics” (Lecturer: Twan Korthorst)


15.30 – 16.30 Lecture 17 “Optical spatial solitons and nonlinear guided waves: the legacy of Prof. George Stegeman and some recent developments” (Lecturer: Gaetano Assanto)

16:30 -17:30 Lecture 18 “From nonlinear integrated optics to microresonator frequency combs” (Lecturer: Stefan Wabnitz)


18.00 – 19.00 POSTER SESSION and e-COST ACTION IC 1208 Meeting

29th September

8.00 – 20.00 Social Program: Visit to the archeological sites of Selinunte and Segesta.

30th September

9.00 – 10.00 Lecture 19 “Applications: biosensing in integrated optics” (Lecturer: Ivo Rendina)

10.00 – 11.00 Lecture 20 “Photonics technologies for future 5G mobile networks” (Lecturer: Roberto Sabella)


11.30 – 12.30 Lecture 21 “Applications: Imaging microscopy for biomicrofluidic platform” (Lecturer: Pietro Ferraro)

12.30- 13.30 Lecture 22 “Integrated Microwave Photonics” (Lecturer: José Capmany)


15.00 – 16.00 Lecture 23 “Quantum integrated photonics” (Lecturer: Fabio Sciarrino)

16.00 – 17.00 Lecture 24 “The generic photonic foundry perspective: existing foundries, manufacturing, access, expectations, philosophy” (Lecturer: Meint Smit)

17.00 – Concluding remarks

20.30 Social dinner

1st October

Departures

LECTURES

Photonic Integration: what, where, why, when, how?

Meint K. Smit

Institute for Photonic Integration, TU Eindhoven, The Netherlands e-mail: [email protected]

The twentieth century is sometimes referred to as the century of electronics. The twenty-first century could well be the century of photonics. In other words the century of light but then light used in technical applications such as screens, solar cells, LED lighting, optical communication, 3D printers, medical diagnostics, metrology and sensors. The market for photonic technology is currently about one fifth of the global market for electronics but it is growing faster and Europe has a strong position in this market. Electronics and photonics can be found almost everywhere but electronics is currently further developed and relatively cheaper, especially due to the emergence of microelectronic and nanoelectronic integration technology. In photonics, integration technology is still in its infancy but it is developing rapidly. Integrated optical chips are the optical counterparts of microelectronic integrated circuits. Electronic equipment used to consist of a box or a cabinet full of electronic components such as vacuum tubes, resistors, capacitors and coils. After the invention of the transistor, electronics became more compact and circuits that were not too big could be integrated on a single print plate. However the real breakthrough came in the 1960s and 1970s when a growing volume of electronics could be integrated in a piece of silicon with dimensions of just a few millimetres. In photonics we are now in the same situation as the initial years of microelectronics. Most optical systems still consist of separate components such as lasers, modulators, detectors and filters, which are connected to each other with lenses or glass fibres or plastic fibres. However the technology to integrate tens to hundreds of optical components on a small piece of semiconductor material, an optical chip, has made considerable progress in the past twenty years. In the lecture an overview will be given of where we are presently.

The food chain for photonics integration

Marco Romagnoli CNIT (National Interuniversity Consortium for Telecommunications)

TeCIP (Institute of Communication, Information and Perception Technologies) - Scuola Superiore Sant'Anna via Moruzzi, 1 - 56124 Pisa, Italy

e-mail: [email protected]

As digital applications, services, communications are dramatically increasing with super-linear growth rate the need of bandwidth is increasing as well and the exploitation of photonics is becoming always more pervasive. The lecture will illustrate the present and perspective situation starting from the vision of the applications, in general, and the communication infrastructure for ubiquitous connections. Given the present situation and the perspectives, an analysis of the requirements enabling scaling will be presented. Technologies enabling scalable bandwidth will be indicated and discussed in general with a focus on the key/critical building blocks that still require improvements..

Fundamentals on Integrated Optics

Gabriella Cincotti Engineering Department, University Roma Tre, via Vito Volterra 62, 00143 Rome, Italy


The lecture provides students with the basic knowledge of photonic passive devices for optical communications. The topics include fundamentals of optical waveguides, fibers, filters, multiplexers, couplers splitters, evidencing for each device the design guidelines and performance. The optical modes and dispersion equations of planar waveguides and fibers are described, as well as the effects of intermodal and chromatic dispersion. The main features of power splitters, directional couplers, MMI couplers, optical filters, Mach Zehnder interferometers, Bragg gratings and arrayed waveguide gratings (AWG) are analyzed in details.

Advances on Integrated Optics

Andrea Melloni DEIB – Politecnico di Milano, Piazza Leonardo da Vinci, 32 - 20131 Milano, Italy

e-mail: [email protected] http://photonics.deib.polimi.ithttp://www.polifab.polimi.it

The analysis and design of advance and complex photonic integrated circuit is an art that have to take into account subtle aspects, technological details, tricks and skills that expert in the filed accumulate in years of activity. Well aware that in a short time it is impossible to cover exhaustively the various aspects related to technologies, passive and active devices, linear and non linear, and so on, the aim of this lecture is to consider with some detail aspects apparently trivial or negligible that can have a large impact on the overall performance of the entire circuits. The topics include aspects related to the index contrast and effective and group index with a comparison between technologies, the surface of the waveguide, backscatter, crosstalk and attenuation. Rings resonators will be treated in detail, starting from an historical survey and going through the theory and applications with potentials and limits. The combination of basic building blocks towards the design of complex circuit will be considered with an introduction towards circuit analysis and synthesis. The effect of tolerances will be introduced and simple consideration on robust design, statistical analysis and yield estimation mentioned. Several arguments have the scope and are essential to introduce the other lectures. Some References D. Melati, A. Melloni, and F. Morichetti, “Real photonic waveguides: guiding light through imperfections,” Adv. Opt. Photon. 6, 156-224 (2014) S. Grillanda, F. Morichetti, “Light-induced metal-like surface of silicon photonic waveguides”, Nature Communications 6, Article number: 8182, 2015. D. Melati, F. Morichetti, G.G. Gentili, and A. Melloni, “Optical radiative crosstalk in integrated photonic waveguides”, Optics Letters Vol. 39, Iss. 13, pp. 3982–3985 (2014). D. Melati, F. Morichetti, and A. Melloni, “A unified approach for radiative losses and backscattering in optical waveguides”, J. Opt. 16 055502 (2014), (Featured Article) A. Melloni, P. Monguzzi, R. Costa, and M. Martinelli, “Design of curved waveguides: the matched bend,” J. Opt. Soc. Am. A 20, 130-137 (2003) D. Jalas, A. Petrov, M. Eich, W. Freude, S.H. Fan, Z.F. Yu, R. Baets, M. Popovic, A. Melloni, J.D. Joannopoulos, M. Vanwolleghem, C.R. Doerr and H. Renner, “What is: and what is not an optical isolator” Nature Photonics 7(8), 2013

Lecture 5: Materials and technologies Silicon photonic process integration

Bertrand SZELAG

LETI, MINATEC campus, CEA-Grenoble, Grenoble, France, F38054 e-mail: [email protected]

After more than a decade of exploratory research, silicon photonics has now become a business segment coveted by various industrials. The rationale behind the emergence of silicon photonics is to take advantage of the production capacities of CMOS foundries, that is, big volume and low cost manufacturability. The semiconductor industry benefits from a long experience in silicon processing take advantage of process maturity of this non-zero impulse to launch silicon photonics and ensure a quick qualification of technology nodes with respect to targeted photonic device performances. We learn from recent publications that most of the processes associated to the current silicon photonic production can be imported with minimum implementation from CMOS fab processes. Nevertheless, a few exceptions still exist to fulfill completely the needs of a totally integrated photonic circuit. In this lecture, we will review the motivations for photonic integration using silicon technology and more precisely cmos fabrication capabilities. Typical silicon photonic fabrication flow will be detailed, highlighting the specific needs versus cmos process. The major particularities of silicon photonics are mainly related to the silicon patterning. For example, in silicon photonics the integrated devices are optically interconnected at silicon level. Thus, silicon patterning must be done at one step to guarantee the auto-alignment of structures. Another issue is the sidewall waveguide roughness which directly impacts the optical performances of the devices. A specificity of silicon photonic technology is the use of pure germanium epitaxy as an absorbing material for photo detection function. Figure 2 presents a tilted SEM pictures of an integrated Butt-coupled lateral Germanium photodiode. Doped silicon junctions required also some specific controls and integration strategies to address the needs of active photonic device, mainly modulator. Process robustness and impact on variability on silicon photonics device will be also discussed in this lecture.

Fig.1: Array Wave Guide Fig.2: Integrated Butt-coupled lateral Germanium photodiode

Materials, technologies and devices: InP and III-V

Meint K. Smit

Institute for Photonic Integration, TU Eindhoven, The Netherlands e-mail: [email protected]

III-V materials have the advantage over silicon of a direct bandgap which allows for optical amplification and short detectors. Further they have a higher electron-mobility as silicon, which gives them better rf-properties. The most widely used III-V semiconductors are based on InP, GaAs and GaN. InP is particularly suited for telecommunication purposes because it can generate and detect light in a wide wavelength range from 1.2 to 1.6 mm. Also for non-telecom application it is an interesting platform, however, because of the unprecedented functionality that it offers. In the lecture we will discuss the most important components available in InP-based integration platforms. The most important one is the optical amplifier. It is at the heart of a number of different lasers: tunable lasers, pulsed lasers and multi-wavelength lasers. Further InP supports a number of modulators: low-loss phase modulators, which are usually applied in a Mach-Zehnder structure to provide high-speed amplitude modulation, up to 50 Gb/s. Mach-Zehnder modulators are fairly long, in the order of 1 mm or longer. They have excellent chirp and loss properties. Much shorter high-speed modulators can be created by using the electro-absorption effect. Electro-absorption modulators have higher losses and more chirp, however. InP detectors are very compact and can have bandwidths well beyond 50 GHz. Propagation losses in passive waveguides are typically in the order of 2 dB/cm. In undoped waveguides they can be lower than 1 dB/cm. The most important passive components used in InP-based Photonic ICs are MMI-couplers and AWG demultiplexers. In the lecture the most important properties of the InP-based integration platform and its most frequently used components will be discussed.

Si3N4 waveguide technology for broadband and ultra-low-loss

photonic integrated circuits

Martijn J. R. Heck

Department of Engineering, Aarhus University, Finlandsgade 22, 8200 Aarhus, Denmark


Traditionally, routing, switching and filtering functions for communications are performed by

photonic integrated circuits (PICs) fabricated on a silica or silicon nitride based platform [1]. In

such a platform, doped silica, silicon oxynitride (SiON), or stoichiometric silicon nitride (Si3N4) is

used for the light guiding core, embedded in a silicon oxide upper and lower cladding. Only passive

components and thermo-optic tuners are available.

The reason for the widespread use is the low intrinsic material loss over a wide wavelength range,

from the ultra-violet to the mid-infrared. Traditionally, doped silica and SiON were the preferred

solutions, as the waveguide core index can be tuned by material composition. This allows for low-

index contrast waveguides, e.g., 0.3% - 3% [2], thereby minimizing scatter losses.

The interest in silicon nitride waveguide technology has significantly increased recently, due to its

potential compatibility with CMOS fabrication technology. Si3N4 has a relatively large index

contrast with the oxide cladding. This allows for small waveguide cores and tight bending radii,

with a potential for large-scale photonic integration. Another benefit of this tight confinement is the

optimization of nonlinearities. Silicon nitride resonators have been used for comb generation [3]

and frequency doubling [4]. At first sight, silicon nitride might not seem like a good approach for

low-loss waveguide design. This is due to the high index contrast with silicon dioxide and, hence,

the large scattering. However, it has been shown that the lowest loss waveguides can be made using

very thin silicon nitride waveguides of only 30 nm – 40 nm thin [5] (Figure 1). The CMOS-

compatibility of the process also allows for further integration with silicon photonics, leading to

highly functional and high-end PICs [6].

Figure 1. (left) Silicon nitride waveguide in a spiral layout, showing red light propagation. (right) Overview of waveguide

propagation loss as a function of bend radius. Data for silica and silicon nitride before 2008 (red, open), recent silicon nitride (red,

solid), silicon (blue), indium phosphide (green squares), and gallium arsenide (green triangles) are given [7].

References

[1] C. R. Doerr, and K. Okamoto, Journ. of Lightw. Technol. vol. 24, pp. 4763-4789, 2006.

[2] M. Kawachi, IEE Proceedings-Optoelectronics vol. 143, pp. 257-62, 1996.

[3] Y. Okawachi et al., Optics Letters vol. 36, pp. 3398-400, 2011.

[4] J. S. Levy et al., Optics Express vol. 19, pp. 11415-21, 2011.

[5] J. F. Bauters et al., Optics Express vol. 19, pp. 24090-24101, 2011.

[6] J. F. Bauters et al., Optics Express vol. 21, pp. 544-555, 2013.

[7] M. J. R. Heck et al., Laser & Photonics Reviews vol. 8, pp. 667-686, 2014.

Acknowledgment: supported by the Aarhus University Research Foundation AUFF with a Lektor Starting Grant

Glass and Glass-Ceramic Photonic Systems:

advances and perspectives

Lidia Zur1,2, Thi Ngoc Lam Tran3,2,4, Marcello Meneghetti5,2, Stefano Varas2, Cristina

Armellini2, Alessandro Chiasera2, Francesco Scotognella6,7, Daniele Zonta3,2,8, Dominik Dorosz9,

Anna Lukowiak10, Giancarlo C. Righini1,11, Roberta Ramponi15, Maurizio Ferrari2,1,*

1. Centro di Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 1, 00184 Roma, Italy

2. IFN-CNR CSMFO Lab. and FBK Photonics Unit via alla Cascata 56/C Povo, 38123 Trento, Italy

3. Department of Civil, Environmental and Mechanical Engineering, Trento Univ. Via Mesiano, 77, 38123 Trento, Italy

4. Ho Chi Minh City University of Technical Education, Thu Duc District, Ho Chi Minh City, Viet Nam

5. Dipartimento di Fisica, Università di Trento, via Sommarive 14, Povo, 38123 Trento, Italy

6. Center for Nano Science and Technology@PoliMi, IIT, via Giovanni Pascoli, 70/3, 20133, Milano, Italy 7. Politecnico di Milano, Dip. Fisica and IFN-CNR,Piazza Leonardo da Vinci 32, 20133 Milano, Italy

8. Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow, G11XJ, UK

9. Bialystok University of Technology, Dep. of Power Eng., Photonics and Lighting Technology, Bialystok, Poland.

10. Institute of Low Temperature and Structure Research PAS, Okolna St. 2, 50-422 Wroclaw, Poland

11. MDF Lab. IFAC - CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy

12. IFN-CNR and Department of Physics, Politecnico di Milano, p.zza Leonardo da Vinci 32, 20133 Milano, Italy.

*e-mail: [email protected]

The development of optically confined structure is a major topic in both basic and applied physics

not solely ICT oriented but also concerning lighting, laser, sensing, energy, environment, biological

and medical sciences, and quantum optics. Glasses and glass-ceramics activated by rare earth ions

are the bricks of such structures. Glass-ceramics are nanocomposite systems that exhibit specific

morphologic, structural and spectroscopic properties allowing to develop new physical concepts, for

instance the mechanism related to the transparency, as well as novel photonic devices based on the

enhancement of the luminescence. The dependence of the final product on the specific parent glass

and on the fabrication protocol still remain an important task of the research in material science.

Looking to application, the enhanced spectroscopic properties typical of glass ceramic in respect to

those of the amorphous structures constitute an important point for the development of integrated

optics devices, including optical amplifiers, monolithic waveguide laser, novel sensors, coating of

spherical microresonators, and up and down converters. This lecture presents some results obtained

by our consortium regarding glass-based photonics systems. After a short history of research in

glass ceramics we will comment the energy transfer mechanism in transparent glass ceramics taking

as examples the up and down conversion systems and the role of SnO2 nanocrystals as sensitizers.

Coating of spherical resonators by glass ceramics, 1D-Photonic Crystals for luminescence

enhancement, laser action and disordered 1-D photonic structures, polymeric-based structures for

integrated optics, will be discussed. Finally, RF-Sputtered rare earth doped P2O5-SiO2-Al2O3-Na2O-

Er2O3 planar waveguides, will be presented.

Acknowledgment: Effort sponsored by the projects COST MP1401 “Advanced Fibre Laser and Coherent Source as

tools for Society, Manufacturing and Lifescience” (2014 - 2018), PAS-CNR (2014-2016), PLANS - Centro Fermi,

“Plasmonics for a better efficiency of solar cells”bilateral project between South Africa and Italy (contributo del

Ministero degli Affari Esteri e della Cooperazione Internazionale, Direzione Generale per la Promozione del Sistema

Paese), “Grandi progetti 2012” PAT: “Developing and Studying novel intelligent nano Materials and Devices towards

Adaptive Electronics and Neuroscience Applications” - MaDEleNA Project.

InP-on-Silicon Integration Through Waferbonding and Epitaxy

Dries Van Thourhout

1. Photonics Research Group, Ghent University – imec, Technologiepark 15, 9052 Gent e-mail: [email protected]

Silicon Photonics is rapidly evolving to a mature platform for realizing complex Photonic ICs and several companies are currently introducing first commercial products based on this platform. However the platform is missing a natural light source. In this talk I will discuss two approaches for overcoming this issue. Using wafer bonding high quality epitaxial layers can be directly integrated on the silicon circuits. Following a decade of research this technology now allows to realize device that can compete and in some cases even outperform standard InP-based telecom devices. I will introduce the basics of this III-V on silicon platform and present some new developments. A second and much more exploratory approach is the direct epitaxial growth of III-V materials on silicon substrates. We recently demonstrated InP and InP/InGaAs DFB lasers directly grown on silicon. I will present these results together with other recent results from literature. References: [1] M. Paladugu, C. Merckling, R. Loo, O. Richard, H. Bender, J. Dekoster, W. Vandervorst, M. Caymax, and M. Heyns, “Site selective integration of III-V materials on Si for nanoscale logic and photonic devices,” Cryst. Growth Des., vol. 12, no. 10, pp. 4696–4702, 2012. [2] C. Merckling, N. Waldron, S. Jiang, W. Guo, N. Collaert, M. Caymax, E. Vancoille, K. Barla, A. Thean, M. Heyns, and W. Vandervorst, “Heteroepitaxy of InP on Si (001) by selective-area metal organic vapor-phase epitaxy in sub-50 nm width trenches : The role of the nucleation layer and the recess engineering,” J. Appl. Phys., vol. 115, p. 023710, 2014. [3] Z. Wang, B. Tian, M. Paladugu, M. Pantouvaki, N. Le Thomas, C. Merckling, W. Guo, J. Dekoster, J. Van Campenhout, P. Absil, and D. Van Thourhout, “Polytypic InP nanolaser monolithically integrated on (001) silicon.,” Nano Lett., vol. 13, no. 11, pp. 5063–9, Nov. 2013. [4] B. Tian, Z. Wang, M. Pantouvaki, W. Guo, M. Clement, and J. Van Campenhout, “InP Nanowire lasers Epitaxially Grown on (001) Silicon ‘V-groove’ templates,” in IPRM 2014, paper Thu–B1–4. [5] Z. Wang, B. Tian, M. Pantouvaki, W. Guo, P. Absil, J. Van Campenhout, C. Merckling, and D. Van Thourhout, “Room Temperature InP DFB Laser Array Directly Grown on (001) Silicon,” http://arxiv.org/abs/1501.03025, Jan. 2015.

Integration of Si and SiN PICs with New Active Materials Dries Van Thourhout

1. Photonics Research Group, Ghent University – imec, Technologiepark 15, 9052 Gent

e-mail: [email protected] In this presentation we will report on our recent work on new materials that can be monolithically integrated on high-index contrast silicon or silicon nitride photonic ICs to enhance their functionality. This includes graphene and other 2D-materials for realizing compact electro-absorption modulators and non-linear devices, ferroelectric materials for realizing phase modulators and colloidal quantumdots for lasers and integrated quantum optics devices. [1] Y. Hu, M. Pantouvaki, J. Van Campenhout, S. Brems, I. Asselberghs, C. Huyghebaert, P. Absil, D. Van Thourhout, Broadband 10 Gb/s operation of graphene electro-absorption modulator on silicon, Laser & Photonics Reviews, 10(2), p.307-316 (2016). [2] K. Alexander, Y. Hu, M. Pantouvaki, S. Brems, I. Asselberghs, S.-P. Gorza, C. Huyghebaert, J. Van Campenhout, B. Kuyken, D. Van Thourhout, “Electrically Controllable Saturable Absorption in Hybrid Graphene-Silicon Waveguides”, accepted for publication in Conference on Lasers and Electro-Optics (CLEO), United States, 2015 [3] J. George, J. Beeckman, W. Woestenborghs, P.F. Smet, W. Bogaerts, “Preferentially oriented BaTiO3 thin films deposited on silicon with thin intermediate buffer layers”, Nanoscale Research Letters, 8, p.1-7 (2013)

Materials and technologies: electro-optic dielectrics

Prof. Alessandro Busacca

When an electric field is applied across an optical medium, the distribution of electrons within is

distorted, so that the polarizability and hence the refractive index of the medium changes

anisotropically. The result of this electro-optic effect may be to introduce new optic axes into

naturally doubly refracting crystals.

In solids, the linear variation in the refractive index associated with the applied field is known as

the Pockels effect while the variation arising from the quadratic term is called the Kerr effect.

During the lecture, we will investigate the two phenomena also giving some examples of practical

interest. In particular, we will discuss with regard to light-wave modulation using electro-optic

materials.

Liquid crystals and photonic integration: materials, properties, technologies, devices

Antonio d’Alessandro

Department of Information Engineering, Electronics and Telecommunications, Sapienza University, Rome, Italy


Liquid crystals (LC) are materials with good electro-optic and nonlinear optical properties suitable to make switchable and reconfigurable devices using low driving power. These materials have been mainly developed for flat panel displays but their mature technology can be effectively used to make integrated optic devices for other applications such telecom, sensors, datacom and so on. In the first part of this lecture molecular orientation of LC mesophases and the corresponding dielectric, elastic, thermal and optical properties will be presented. In particular the relationship between molecular orientation and optical anisotropy of LC will be shown. Linear and nonlinear propagation of light will be discussed. The basic fabrication technologies of LC electrooptic devices and the most used design techniques will be illustrated. In the second part the state of art of the most recent techniques will be presented in which liquid crystals are combined with other materials to make active integrated optic devices such as tuneable lasers, photonic switches, optical tunable filters based on Bragg gratings and so on.

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Design tools and design flows for integrated photonics

Twan Korthorst

PhoeniX Software, Hengelosestraat 705, 7521PA Enschede, the Netherlands


The lecture will discuss the available design tools and some example design flows for integrated

photonic circuits (PICs). It will show the special requirements for photonics and will compare this,

so called, Photonic Design Automation (PDA) with Electronic Design Automation (EDA). The

different steps of an ideal design flow will be shown in detail. The current situation will be

discussed showing the various available tools and improvement points will be addressed. One of the

main improvements for PDA is the use of a mature and complete Process Design Kit (PDK) as

widely being used in the electronics semiconductor industry and has been introduced for photonics

by European collaborations over the last few years. The PDK will be discussed in more detail.

Another improvement that can be controlled from the PDK are automated Design Rule Checks

(DRC). A couple of examples of Design Rule Checks will be given, showing the huge benefits

when this can be performed automatically.

References

[1] Korthorst, Twan, Stoffer, Remco, Bakker, Arjen, Photonic IC design software and process design kits , Advanced

Optical Technologies, Volume 4, Issue 2, pp.147-155, 04/2015.

[2] Heins, Mitchell, et. al., Design Flow Automation for Silicon Photonics Challenges, Collaboration and

Standardization , Silicon Photonics III, Chapter 4, 2016.

Optical spatial solitons and nonlinear guided waves: the legacy of Prof.

George Stegeman and some recent developments

Gaetano Assanto

NooEL, Nonlinear Optics and Optoelectronics Lab

CNISM, INFN, CNR-ISC, Department of Engineering

University of Rome "Roma Tre" - Italy

Optics Lab, Department of Physics

Tampere University of Technology - Finland

Abstract

After a brief and incomplete summary of the professional career and achievements of Prof. George

I. Stegeman, passed away last year, I will pinpoint a few of his scientific results on optical

bistability and spatial optical solitons, linking them to some recent achievements in the area of

liquid crystal photonics.

In particular, the extension of optical bistability and spontaneous symmetry breaking to nonlinear

propagating beams in reorientational anisotropic media will be discussed with reference to

nematicons, spatial optical solitons in nematic liquid crystals.

From Nonlinear Integrated Optics to Microresonator Frequency Combs

S. Wabnitz1, T. Hansson2, F. Leo3, I. Ricciardi4, M. De Rosa4, J. Anthony5, S. Coen5,

and M. Erkintalo5 1. Dipartimento di Ingegneria dell’Informazione, Università di Brescia, via Branze 38, 25123 Brescia, Italy

2. INRS-EMT, 1650 Blvd. Lionel-Boulet, Varennes, Quebec J3X 1S2, Canada 3. OPERA-photonics, ULB, 50 Avenue F. D. Roosevelt, CP 194/5, B-1050 Bruxelles, Belgium

4. CNR-INO, Istituto Nazionale di Ottica, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy 5. Dodd-Walls Centre, Department of Physics, The University of Auckland, Auckland 1142, New Zealande-

mail: [email protected] Perhaps one of the most spectacular current applications of nonlinear integrated optics, a field which was pioneered by George Stegeman more than thirty years ago [1], is that of nonlinear microresonator based optical frequency comb light sources. Optical frequency comb sources are characterized by a spectrum comprising many equally spaced components [2], and have a wide range of scientific and technological applications. Although commercial comb generators are based on mode-locked lasers and fiber supercontinuum generation, nonlinear integrated optics provides a low-cost and chip-scale alternative, based on a low-power cw laser coupled into a high-Q microresonator [3]. So far microresonator frequency combs have exploited the third order “Kerr” nonlinearity, which permits to generate successive comb lines with a spacing equal to the resonator free-spectral range via cascaded four-wave mixing [4-5]. Modeling of microresonator frequency combs can be greatly simplified by a single partial differential equation approach [4-6], analogous to the case of other coherently driven Kerr spatially diffractive [7] or temporally dispersive [8-9] nonlinear cavities. In order to lower the threshold power and extend the spectral range of frequency comb generation, for example into the visible or mid-infrared, while still using near-infrared cw laser pumps, quadratic nonlinear cavities can be exploited [10]. These quadratic microresonator frequency comb sources operate close to the phase-matching condition for the underlying quadratic processes, and not in the cascading regime that reduces the dynamics to the Kerr case [11]. Quite remarkably, a single time domain partial differential equation with an effective delayed third-order nonlinearity was derived to describe with excellent accuracy the dynamics of quadratic frequency comb generation [12-13]. In situations where multiple processes are present, and the frequency combs generated around the interacting waves over multiple octaves overlap, we carried out numerical modeling based on a single envelope equation approach [14]. References [1] G.I. Stegeman, E.M. Wright, N. Finlayson, R. Zanoni, and C.T. Seaton, J. Lightwave Technology 6, 953 (1988). [2] T. Udem, R. Holzwarth, and T. W. Hänsch, Nature 416, 233 (2002). [3] P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, Nature 450, 1214 (2007). [4] S. Coen, H. G. Randle, T. Sylvestre, and M. Erkintalo, Opt. Lett. 38, 37 (2013). [5] T. Hansson, D. Modotto, and S. Wabnitz, Phys. Rev. A 88, 023819 (2013). [6] T. Hansson, D. Modotto, and S.Wabnitz, Opt. Comm. 312, 134 (2014). [7] L. A. Lugiato and R. Lefever, Phys. Rev. Lett. 58, 2209 (1987). [8] M. Haelterman, S. Trillo, and S. Wabnitz, Opt. Commun. 91, 401 (1992). [9] F. Leo, S. Coen, P. Kockaert, S.-P. Gorza, P. Emplit, and M. Haelterman, Nature Photon. 4, 471 (2010). [10] I. Ricciardi, S. Mosca, M. Parisi, P. Maddaloni, L. Santamaria, P. De Natale, and M. De Rosa, Phys. Rev. A 91, 063839 (2015). [11] G. I. Stegeman, D. J. Hagan, and L. Torner, Optical and Quantum Electronics 28, 1691 (1996). [12] F. Leo, T. Hansson, I. Ricciardi, M. De Rosa, S. Coen, S. Wabnitz, and M. Erkintalo, Phys. Rev. Lett. 116, 033901 (2016). [13] F. Leo, T. Hansson, I. Ricciardi, M. De Rosa, S. Coen, S. Wabnitz, and M. Erkintalo, Phys. Rev. A 93 (2016). [14] T. Hansson, F. Leo, M. Erkintalo, J. Anthony, S. Coen, I. Ricciardi, M. De Rosa, and S. Wabnitz, J. Opt. Soc. Am. B 33, 1207 (2016).

Applications: Biosensing in integrated optics

Ivo Rendina

National Research Council, Institute for Microelectronics and Microsystems

Area della Ricerca CNR, Via P. Castellino 111, 80131 Napoli, Italy


In the last years, an increasing interest has been devoted to the exploitation of integrated optics to

biosensing for biomedicine, food analysis, environmental monitoring and security applications. The

lecture provides students with a survey starting from the basic concepts applied to biosensing up to

the most advanced lab-on-chip technologies. A particular emphasis will be given to new ideas

emerging in label-free biosensing, exploiting strong light confinement and resonance in micro and

nanostructures.

Photonics technologies for future 5G mobile networks

Roberto Sabella ERICSSSON, Research Corporate Center, Pisa , Italy, Italy


The evolution of mobile networks towards 5G will really enable the networked society and create significant opportunities for Industry and Society. Huge traffic growth as well as a significant reduction of latency dictated by time-sensitive MTC services will require a substantial transformation of the radio access networks (RAN) and, consequently, imposes a rethinking of the underlying transport network. The conventional point-to-point fronthaul concept is evolving towards a geographical network connecting a pool of DUs with a plurality of RRUs using the CPRI protocol. Centralization of radio baseband processing functions is gaining great interest for its potential to allow a consolidation of nodes and network elements, so as to lower CapEx and OpEx (e.g. fewer nodes to install, to maintain, to upgrade, and to power supply), while at the same time increase radio coordination functions. Optical technologies with their conventional benefits of high bandwidth, protocol transparency, scalability, low latency, high resiliency and network re-configurability, are today perceived as a promising key piece of the radio access network puzzle, in both front haul and back haul transport areas. But previous generation of optical networking technologies (e.g. SDH/SONET, WDM, OTN etc.), based on discrete components and modules, that played a relevant role to realize an affordable transport medium in metro and core networks are not adequate for the needs of the emerging RAN transport segments requiring low cost, lower power consumption and a level of miniaturization. Re-configurability features, provided by WDM technologies, can further increase CPRI transport efficiency. Photonic Integration and in particular silicon photonics with its recent advances in integrating many optical circuits and functions (for instance multiplexer, attenuator, switches, couplers) in a single chip using the well-developed CMOS production infrastructure, is the ideal technology to fit the RAN needs. In addition to this, the exponential growth of traffic is driving important evolutions in the development of HW platforms of next generation telecom and data-com equipments. Future hardware platforms will have to be much more efficient with regard to energy consumption, footprint and cost. Hence, the critical challenges for telecom/data-com vendors will be to continuously increase bandwidth density at every point in the communication infrastructure. This leads to put more and more features onto the same hardware unit (e.g. a chip, a module, or even a board); and integrate as much as possible multiple functions in a single chip and also integration of many chips in the same module. This is crucial. Photonics has become the key technology for board to board and chip to chip interconnects for its characteristics of large bandwidth, nearly reach and data rate agnostic characteristics and reduced energy consumption. Similarly to RAN transport application, also in the new HW platform evolution, the traditional optical devices based on discrete components cannot be used for cost and footprint reasons. Whole new optical devices based on photonic integration have to be envisaged. In particular, rapidly maturing CMOS compatible photonics which is mass-producible at low cost and high level of integration is a proven candidate to ensure effective integration with the control and host electronics.

Applications: Imaging microscopy for biomicrofluidic platform

Pietro Ferraro National Research Council, Institute of Applied Sciences & Intelligent Systems

www.isasi.cnr.it

Via Campi Flegrei 34, 800078 Pozzuoli (Napoli) Italy e-mail: [email protected]

Lab-on-a-chip(LoC)devicesareextremelypromisingtobringclinicaldiagnosticfunctionsatthepoint-of-care. At this scope, an important goal is to design imaging schemes integrated withmicrofludic platforms. In fact, imaging inmicroscopymodality is one ofmost powerful tool forclinic diagnostic. An ideal microscopy system for LoC systems should satisfy three mainrequirements, i.e. high-throughput data collection, label-free imaging, and quantitativemeasurements.Recent advancements in the fieldwill be reviewed and illustrated. In particularlatest evolutions of imaging systems based on Quantitative Phase Imaging (QPI) developed byleadingresearchgroupsworldwideforin–flowcytometryandwillbeconsidered.

Integrated Microwave Photonics

José Capmany Universidad Politecnica de Valencia, Spain


Integrated microwave photonics (IMWP) deals with the application of integrated photonics technologies to microwave photonics (MWP) systems. The lecture will cover this topic from a multi-facet point of view. After briefly introducing the basic concept behind MWP and IMWP and their application to emerging ICT system, it will briefly outline the salient characteristics of available material platforms that can be employed for the implementation of IMWP chips. I will cover basic features of mature material platforms such as InP, SOI and Si3N4. The second part of the lecture will be devoted to describe the two salient approaches that are available for the implementation of IMWP chips from a functional point of view. On one hand, we will review the recent progress in Application-Specific Photonic Integrated Circuits (ASPICs), where a particular circuit and chip configuration is designed to optimally perform a particular MWP functionality. On another hand, recent progress will be reported on a radically different approach, the universal MWP signal processor architecture that can be integrated on a chip and is capable of performing all the main functionalities by suitable software programming of its control signals.

Quantum integrated photonics

Fabio Sciarrino

Dipartimento di Fisica, Sapienza Università di Roma P.le Aldo Moro 2, 00185 Roma, Italy e-mail: [email protected]

Integrated photonic circuits have a strong potential to perform quantum information processing [1, 2]. Indeed, the ability to manipulate quantum states of light by integrated devices may open new perspectives both for fundamental tests of quantum mechanics and for novel technological applications. By exploiting waveguides fabricated by femtosecond laser waveguide, integrated circuits with three dimensional geometry can be designed to carry out several quantum information processing tasks. Our aim has been to develop and implement quantum simulation by exploiting 3-dimensional integrated photonic circuits. As first we implemented an integrated beam splitter able to support polarization-encoded qubits. As following step we addressed the implementation of discrete quantum walk: we investigated how the particle statistics, either bosonic or fermionic, influences a two-particle discrete quantum walk both in ordered and disordered systems [3]. We will discuss the perspectives of optical quantum simulation: the implementation of the boson sampling to demonstrate the computational capability of quantum systems [4,5,6] and the development of integrated architecture with three-dimensional geometries [7]. References [1] T. D. Ladd, et al, “Quantum computers”, Nature 464, 45 (2010). [2] J. L. O’Brien, A. Furusawa, and J. Vuckovic, “Photonic quantum technologies”, Nature Photonics 3, 687 (2009). [3] L. Sansoni, F. Sciarrino, G. Vallone, P. Mataloni, A. Crespi, R. Ramponi, R. Osellame, “Two-particle bosonic-fermionic quantum walk via 3D integrated photonics”, Phys. Rev. Lett. 108, 010502 (2012); A. Crespi, R. Osellame, R. Ramponi, V. Giovannetti, R. Fazio, L. Sansoni, F. De Nicola, F. Sciarrino, and P. Mataloni, “Anderson localization of entangled photons in an integrated quantum walk”, Nature Photonics 7, 322 (2013). [4] A. Crespi, R. Osellame, R. Ramponi, D. J. Brod, E. F. Galvao, N. Spagnolo, C. Vitelli, E. Maiorino, P. Mataloni, and F. Sciarrino, “Experimental boson sampling in arbitrary integrated photonic circuits”, Nature Photonics 7, 545 (2013). [5] N. Spagnolo, C. Vitelli, M. Bentivegna, D. J. Brod, A. Crespi, F. Flamini, S. Giacomini, G. Milani, R. Ramponi, P. Mataloni, R. Osellame, E. F. Galvao, F. Sciarrino, “Efficient experimental validation of photonic boson sampling”, Nature Photonics 8, 615 (2014). [6] M. Bentivegna, N. Spagnolo, C. Vitelli, F. Flamini, N. Viggianiello, L. Latmiral, P. Mataloni, D. J. Brod, E. F. Galvao, A. Crespi, R. Ramponi, R. Osellame, F. Sciarrino, Experimental scattershot boson sampling, Science Advances 1, e1400255 (2015). [7] N. Spagnolo, C. Vitelli, L. Aparo, P. Mataloni, F. Sciarrino, A. Crespi, R. Ramponi, and R. Osellame, “Three- photon bosonic coalescence in an integrated tritter”, Nature Communications 4, 1606 (2013).

The generic photonic foundry perspective: existing foundries, manufacturing, access, expectations, philosophy

Meint K. Smit

Institute for Photonic Integration, TU Eindhoven, The Netherlands


The development of the manufacturing technology for complex optical chips is very expensive. The costs of a well-equipped chip factory add up to several hundred million euros and then you only have the equipment and the building. Also the development of the integration processes that involve a large number of lithography, deposition and etching steps costs seceral millions of euros. There are few markets large enough to justify such huge investments. In microelectronics this problem has been solved by the development of standardised technology in which a number of building blocks, such as transistors, resistors and capacitors, can be integrated in large numbers – billions of transistors per chip – in a single standardized manufacturing process. As a result of this the high investment costs can be earned back across a combined market that is much larger than the markets for the individual applications. To drastically reduce the high costs for the use of optical chips a similar development is underway in photonics. If you have a technology with which components for manipulating the amplitude, phase and polarisation of light can be integrated as basic building blocks, then you can realise chips with different functionalities in a single integration process. Such a technology is called a generic integration technology. By providing open access to such a technology through generic foundries, the entry costs for developing PICs for a variety of applications are dramatically reduced: You do not need to build an expensive cleanroom, and you also do not have to develop an expensive integration process: both the cleanroom and the standardized process are available and their costs can be shared by many users. In the lecture the philosophy of the generic foundry approach will be explained and the capabilities of the existing InP-foundry platforms will be discussed.

POSTERS

Optimal conditions for amplified spontaneous emission collection with

the aid of a nematicon

Serena Bolis 1,2

, Jeroen Beeckman2 and Pascal Kockaert

1

1. OPERA-Photonics Group, Université libre de Bruxelles, CP 194/5, 50 Av. F.D. Roosevelt, 1050 Bruxelles, Belgium

2. ELIS Department, Ghent University, Technologiepark-Zwijnaarde 15, 9052 Gent,


The huge Kerr-like nonlinearity of liquid crystals (LC) allows the formation of solitons (nematicons

in nematic LC) with a low power threshold (~mW) [1]. Recently we have shown that a nematicon

generated in a dye-doped LC cell can efficiently collect and inject amplified spontaneous emission

(ASE) into an optical fiber [2].

A 75 µm thick planar antiparallel cell (rubbing direction at 45° with respect to the edges) is filled

with 1 wt.% PM597 in E7 LC. A CW infrared (1064 nm) laser beam is injected into the cell through

a non-standard optical fiber (core diameter 2.9 µm) slid inside the cell. When a pump beam (532

nm, 400 ps, 2.0 µJ/pulse at 10 Hz) is focused onto the sample with cylindrical lenses, ASE is

emitted along the excited stripe axis. If the stripe is oriented towards the fiber, a small part of the

ASE is coupled into the fiber even in the absence of the soliton (Fig.1a-b). However, when the

nematicon is present, the soliton-induced waveguide increases the collected ASE by one order of

magnitude (Fig.1c-d). The guiding efficiency of the soliton increases with the soliton power (i.e. the

molecular reorientation). However at high IR powers the local LC director fluctuations cause the

deviation of the nematicon from its path (Fig.1e), with the spatial oscillations of the soliton that

increase with the soliton power (Fig.1f). An optimum power of 2 mW for the soliton is found. A

numerical modeling of the thermal noise impact on the nematicon propagation is still ongoing.

Figure 1. (a)-(d) Intensity profile of the ASE at the fiber output without (a)-(b), and with (c)-(d) the nematicon, for

polarizations parallel (a)-(c) or orthogonal (b)-(d) to the substrate. (e) Example of nematicon profiles for high (3.7mW) IR

powers (the pump is not present). The dotted black line is a straight line. (f) Soliton position after 3.6 mm of propagation for

three different powers and 100 acquisitions.

References

[1] A. Piccardi, A. Alberucci, U. Bortolozzo, S. Residori and G. Assanto, “Soliton gating and switching in liquid crystal

light valve”, Appl. Phys. Lett., vol. 96, 071104, 2010.

[2] S. Bolis, T. Virgili, S.K. Rajendran, J. Beeckman and P. Kockaert, “Nematicon-driven injection of amplified

spontaneous emission into an optical fiber”, Opt. Lett., vol. 41, 2245–2248, 2016.

Acknowledgment: Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA); Belgian

Science Policy Office (BELSPO) (IAP7-35)

(b) (d)

(e)

(f)

(a) (c)

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Ultrafast Optical Kerr Effect method indicates negative nonlinear refractive index for graphene

Evdokia Dremetsika*,1, Bruno Dlubak2, Simon-Pierre Gorza1, Charles Ciret1,

Marie-Blandine Martin3, Stephan Hofmann3, Pierre Seneor2, Daniel Dolfi2, Serge Massar1, Philippe Emplit1, and Pascal Kockaert1

1.OPÉRA-photonique, Université libre de Bruxelles, Brussels, Belgium

2.Unité Mixte de Physique CNRS/Thales (UMR137), Palaiseau Cedex, France 3.Department of Engineering, University of Cambridge, Cambridge, United Kingdom


Recent advances in integrated photonics lead to a growing demand for ultrathin materials compatible with CMOS technology. Graphene appears to be a promising candidate for all-optical signal processing in photonic integrated applications, as it presents broadband optical properties and a high and broadband optical nonlinearity [1-4]. However, researchers do not agree on the value of its nonlinear refractive index [2-4]. In this work, we report on the use of the ultrafast optical Kerr effect method with optical heterodyne detection (OHD-OKE) [5] for the characterization of the nonlinear refractive index of monolayer CVD graphene on quartz [6] at telecom wavelength. This method is advantageous over the previously used Z-scan, as it is neither sensitive to the inhomogeneities of the sample, nor to thermal nonlinearities. Our measurements in Fig. 1(a) and (b) indicate a negative nonlinear refractive index for monolayer CVD graphene. We confirmed this negative sign by Z-scan measurements on the same samples [Fig. 1(c)]. We discuss the different parameters that could affect the sign of the nonlinearity.

Figure 1. (a), (b) OHD-OKE signals for silicon and graphene respectively. Graphene presents a response with opposite sign and

slower relaxation time than silicon. (c) Z-scan trace resulting from division of closed-aperture by open-aperture data. Savitzky-Golay filtering has been used for data smoothing before fitting. The peak-valley trace is typical of a self-defocusing material [M.Sheik-

Bahae et al, IEEE J. Quant. Elect. 1990, 26 760]. References [1] S. A. Mikhailov and K. Ziegler, J. Phys.-Condens. Mat. 2008, 20, 384204. [2] H. Zhang, S. Virally, Q. Bao, L. Kian Ping, S. Massar, N. Godbout, and P. Kockaert, Opt. Let. 2012, 37, 1856. [3] W. Chen, G. Wang, S. Qin, C. Wang, J. Fang, J. Qi, X. Zhang, L. Wang, H. Jia, and S. Chang, AIP Adv. 2013, 3, 042123. [4] D. Chatzidimitriou, A. Pitilakis, and E. E. Kriezis, J. App. Phys. 2015, 118, 023105. [5] N. A. Smith and S. R. Meech, Int. Rev. Phys. Chem. 2002, 21, 75. [6] P. R. Kidambi, C. Ducati, B. Dlubak, D. Gardiner, R. S. Weatherup, M.-B.Martin, P. Seneor, H. Coles, and S. Hofmann, J. Phys. Chem. C 2012, 116, 22492. Acknowledgment: Effort sponsored by the Belgian Science Policy Office (BELSPO) (IAP7- 35), the EU FP7 Work Program (Graphene Flagship) (604391). E. Dremetsika is funded by the Fund for Research Training in Industry and Agriculture (FRIA), Belgium.

Infrared detection in multifunctional graphene-based transistors

M. A. Giambra1,2, A. Benfante1, R. Pernice1, S. Stivala1, E. Calandra1, A. C. Busacca1 W. H. P. Pernice2, R. Danneau2

1. DEIM – University of Palermo, Viale delle Scienze, Bldg. 9, 90128, Palermo, Italy

2. Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), 76344 Egg.-Leopoldshafen, Germany e-mail: [email protected]

In the last years great attention has been paid to graphene-based devices for optoelectronic applications such as photodetection [1, 2]. In this work, we report on Graphene Field Effect Transistors (GFETs) photoelectrical response due to the photo-transistor effect [3]. The devices fabrication steps and their electro-optic characterization are herein presented. The transistors, whose cross section is shown in Fig. 1 (a), were built on a sapphire substrate. First, the dual-finger back-gate was patterned on a sapphire substrate by e-beam lithography followed by the evaporation of a thin Ti/Au bilayer (∼ 5/40 nm). A ~ 10 nm thick Al2O3 film was directly grown via atomic layer deposition at 90° as dielectric layer. A CVD-grown graphene film was directly transferred onto the oxide layer and etched in a meandered pattern by Reactive Ion Etching (RIE) to minimize the contact resistance. Subsequently, source/drain electrodes were patterned onto the graphene sheet using E-beam lithography followed by a Ti / Au (~ 5 / 100 nm) deposition. Finally, a ~ 300 nm Au was directly grown by PVD as contact pads. Photoelectrical measurements were performed using a 1.55 μm erbium fiber laser with the output output beam, chopped at 667 Hz and coupled into a single mode optical fiber through a microscope objective. The output of the fiber was placed above the sample at a distance of 1.55 mm from the photoactive area (i.e., the 20 μm × 1 μm graphene area above the gates) that was illuminated by an IR beam with a spot radius (1/e2) of 144 μm. An auxiliary visible laser was employed for alignment purpose. The GFETs electrical drain-source voltages (VDS) were measured using a lock-in amplifier synchronized to the chopper frequency. A sketch of the set-up is depicted in Fig. 1 (b). Optical measurements as a function of both the incident laser power and the DC bias of the devices have been carried out. As it can be easily noticed, the photocurrent (Iph, in Fig. 1 (c)) increases with the power of the IR beam illuminating the sample and a maximum responsivity of 0.34 A/W has been obtained.

References [1] Z. Sun et al., “Graphene and Graphene-like Two-Dimensional Materials in Photodetection: Mechanisms and

Methodology”, ACS Nano, vol. 8, n. 5, 2014, pp. 4133–56. [2] A. C. Ferrari et al., “Science and technology roadmap for graphene, related two-dimensional crystals, and

hybrid systems,” Nanoscale, vol. 7, 2014, pp. 4598–4810. [3] B. K. Sarker et al., “Gate-tunable and high responsivity graphene phototransistors on undoped semiconductor

substrates”, arXiv preprint arXiv:1409.5725, 2014.

Acknowledgment: This activity was supported by PON03PE_00214_1 "Nanotecnologie e nanomateriali per i beni culturali" (TECLA) Research Program.

Figure 1. (a) GFET cross-section, (b) measurement setup, (c) GFET photo-response at different values of IR power (VDS = 0.4 V).

(a) (b) (c)

Light coupling in microscale polymer photonic circuits

Manuel Gil-Valverde 1, Manuel Caño-García1, David Poudereux1, Morten A. Geday1,José M. Otón1, Xabier Quintana1

1. CEMDATIC, Universidad Politécnica de Madrid, Av. Complutense, 30, 28040 Madrid, Spaine-mail: [email protected]

Photonic integrated circuits (PICs) are devices that integrate multiple photonic functions. Thevisible light is guided between components by light waveguides of the same order as lightwavelength, i.e. 1-5μm or even less [1]. Because of the size of connections, coupling light into PICsis a challenging task previous to characterizing them. The aim of our group is to turn a passive PICinto an active one by adding layers of electro-optics materials, either as a component within thelight path, or deposited onto the waveguide affecting the evanescent field of the guided light.Testing the light behavior inside the PICs may be hampered by the arduous light coupling fromexternal sources. This problem may be faced up from several points of view.

Figure 1. Different views of the X-Y-Z nanopositioner setup for coupling light to PICs.

We are developing improved versions of classical coupling setups. It is possible to appreciate in thefigure 1 one of our latest realizations to couple light into the PICs. It consists of an X-Y-Zelectrically-manual controlled nanopositioned microscope objective, two microscopes and green/redHe-Ne lasers. The nanopositioner let us focus the laser beam into the small cross section of thewaveguide, where the first microscope is placed. The second microscope is used to analyze theoutput signal in the opposite end of the waveguide, which can be eventually brought to anotherdetection system. Moreover, others alternative mechanisms are being developed now in the groupfor light coupling, being the most promising solutions those in which light is coupled by difractiveelements [2] or generating it internally.

References[1] D. Geuzebroek, R. Dekker, E. Klein, J. van Kerkhof , “Photonic Integrated Circuits for visible light and near infrared: Controlling transport and properties of light”, Sensors and Actuators B: Chemical, vol. 223,p.952956, 2015.[2] J. Zhang, J. Yang, H. Lu, W. Wu, J. Huang, S. Chang, “Subwavelength TE/TM grating coupler based on silicon-on-insulator”, Infrared Physics & Technology, vol. 71, p. 542546, 2015.

Acknowledgment: This work has been supported by Spanish Government RETOS Program grant no. TEC2013-47342-C2-R, the R&D Program SINFOTON S2013/MIT-2790 of the Comunidad de Madrid, and the Euopean COST Action IC1208.

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Fabrication of high quality alignment layer for nematic liquid crystal cell with variable orientations

B. W. Klus, M. Kwaśny and U. A. Laudyn

Faculty of Physics, Warsaw University of Technology

Koszykowa 75, 00-662 Warsaw, Poland e-mail:[email protected]

Concerning the control of the liquid crystal alignment by means of substrate surface treatment there

are different treatment methods for achieving such a control, among others directional rubbing,

photoalignment, ion-beam exposure on polymer substrates, irradiation by plasma beam, etc. Any

one of these methods has its advantages and disadvantages and what is suitable for some materials

or applications may not be suitable for another. In this work we present an alignment method for the

near-zero pretilt angle of NLCs using electron beam lithography. Much attention will be paid to the

uniformity of the obtained orientation and the anchoring strength. We highlight that proposed

method allows to obtain high quality alignment layer that is not worse than that achieved by other

commonly used methods, and in many ways much better, allowing to obtain molecules orientation

difficult or impossible to achieve by other methods.

Our work relates generally to nematic liquid crystal cell, particularly, it relates to fabrication of

micro structure waveguides in NLC cell for light beam guiding and switching. The huge advantage

of the proposed method is the ability to obtain variable direction of orientation of the long axes of

NLC molecules along light beam propagation with the nanoscale resolution. This allows for

designing precise paths for light beam, in general with curved trajectory. Such fabricated liquid

crystal waveguide supports a full control on the direction of propagation of the light beam both in

linear as well as in nonlinear regime. It operates due to the high quality alignment layer with

adequate anchoring energy and variable alignment conditions along propagation distances.

References [1] I. Ostromęcka, M. Kwasny, P. Jung, B. W. Klus, U. A. Laudyn, “Measurements of the quality of nematic liquid crystal alignment”, Phot. Lett. Poland, vol. 8, No 1 (2016). [2] K. Takatoh, M. Hasegawa, M. Koden, N. Itoh, R. Hasegawa, and M. Sakamoto, “Alignment Technologies and Applications of Liquid Crystal Devices”, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (2006). Acknowledgment: Effort sponsored by the National Centre for Research and Development by the grant agreement LIDER/018/309/L-5/13/NCBR/2014.

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Longitudinal writing of vertical waveguides in fused silica

D. P. Lopes*1,2, R. Martinez Vazquez2, R. Osellame1,2

1. Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133, Milano

2. IFN-CNR, Politecnico di Milano, Piazza L. da Vinci, 32, 20133, Milano.

*e-mail: [email protected]

The past decade has seen femtosecond direct writing fully developing into a solid tool that provides

fabrication of optofluidic devices with great flexibility of geometry with its truly 3D direct writing

[1]. Longitudinal writing is less commonly used due to its limitations: writing depth limited by the

working distance of the objective used and the modification profile being highly dependent on the

writing depth due to spherical aberrations [2]. Still, it provides naturally a symmetrical

modification, and for writing vertical waveguides it enables writing in any position on the sample.

The main objective of this research is to fabricate 1 mm length vertical waveguides in fused silica

that must be compatible with telecom photonic circuits.

We first explored to fabricate waveguides with a dry low numerical aperture objective (20x, 0.35

NA), in order to reduce aberrations during longitudinal writing. The modification profile was too

small (~1 um) for single mode waveguiding (SMW) at 1550 nm. In order to enlarge the index

modification region, two approaches were followed: 1-multiscanning across the waveguiding area,

enlarging the waveguide by building up the positive modification; 2- write a helix shape by direct

writing pilled circles in order to have waveguiding in the highly stressed central area.

In figure 1a we report the microscope images from the edge of three multiscan waveguides, varying

the scan distances. It is possible to build up the modification by separating the scans by less than 1

µm. Each scan was made with 10 µm/s scan speed, repetition rate of 1 MHz and pulse energy of

0.15 µJ. After enlarging the modified region with three layers, with 0.75 µm separation, SMW was

observed on these waveguides but with very high insertion losses (>10 dB).

In the cladding writing, SMW at 1550 nm was achieved by writing with 1 MHz repetition rate, 0.16

µJ pulse energy, ring diameter of 16 µm and 1 µm pitch, obtaining an insertion loss of 1.7 dB.

However, the waveguide was not uniform since the guided modes observed coupling from the two

edges were much different (12 µm vs 11 µm).

In alternative to the above mentioned objective we tried a water immersion objective (20x, 0.5 NA).

With these updates we should avoid the presence of ablation at both surfaces of the glass, and

reduce the spherical aberrations by making a better phase matching with water instead of air. We

obtained homogeneous modification with the single scan modification too small for SMW at 1550

nm. Using the cladding geometry, we observed again SMW at 1550 nm and the modes on both

sides are roughly symmetric and close to the fiber (figure 1b), which shows improvements in

uniformity with depth.

Figure 1a: Merging of the modification observed by making 6 equidistant scans with different distances (r[µm]) from the

central line. 1b: cladding waveguides characterization: i- microscope image, ii-imaged waveguiding mode, iii- fiber mode.

References

[1] R. Osellame, et al. Femtosecond Laser Micromachining: Photonic and Microfluidic Devices in Transparent

Materials. Vol. 123. Springer Science & Business Media, 2012.

[2] C. Mauclair, et al., Optics express 16.8: 5481-5492, (2008).

Acknowledgment: This research was supported by the European project TERABOARD

a b i ii

iii

mailto:[email protected]

�� !"#$%&�'�(�(�")#�*++!�%&�,��(*%&�(�-��'!./�0%&�'�(�1�"23�456��27�%�89:;<=>?@AB>=CD>CEFAGHCD<CAID=JCKE=<LF�MK=CDCK@??DANAONNPAQRFASDE>TCUCFA<TCAQC<TCK@?DUEAAACVW?=@XA>9=9J?DCWWCK=YZ[<HCD<C9D@A��\]_�abcdef�cag]�]hhii]jef�ekjdlm]�aj�noic�mdp]p�eode�ac]de]�ij�qihh]]je�gdr]m]jseo�djs]p�d]�ibcaedje�ha�dccmideiajp�ij�aceidm�p]jpijsf�pc]eapac_f�b]eamas_�djq�e]m]abbkjideiajpt�uj�eo]�vwxyz�ca{]e�|vd]�wdeo�xar]m�yj}oic�zak ]p~�eo]�q]r]macb]je�ah�oiso}ajedpe�d]}]deo�iaj�qac]q�caedppikb�qaklm]�ekjspede]�|vw��|�y�~��a��~�il�gdr]skiq]p�ip�cacap]qt��oap]�gdr]skiq]p�d]�j]]ppd_�ea�q]r]macf�ij�d�mde]�peds]f�abcde�ijs�]pajdeapt�uj�eoip�ga�f�d�hdlideiaj�po]b]�ea�caqk]�ij�d�ajeammdlm]�djq�]caqkilm]�gd_�pko�oisom_�ajhij]q�gdr]skiq]p�ip�cacap]qt��>YD�H@CU�WCD<XAGT=EAH�KYA=EA�[DUCUA�LA<TCAS[K�;C?DA�CEC?K>TA��[D>=@9A�

Gain-ratio Brillouin Optical Correlation Domain Analysis for

distributed strain and temperature monitoring

Jacopo Morosi Politecnico di Milano - Dipartimento di Elettronica, Informazione e Bioingegneria (DEIB)

Fiber-optic Brillouin sensors for temperature and strain distributed monitoring offer unique

performance in terms of distance, high spatial resolution and accuracy. Most of the available

systems rely on Brillouin Optical Time-Domain Analysis (BOTDA) techniques, which employ

pulsed pump signals guaranteeing a spatial resolution fundamentally limited to 1 meter [1]. An

attractive alternative approach is the so-called Brillouin Optical Correlation-Domain Analysis

(BOCDA), based on the simultaneous phase modulation of both pump and probe waves [2]. This

way, efficient amplification is confined only in a narrow fiber section, called correlation peak,

where the Brillouin phase-matching condition in satisfied. Correlation peak width can be made

arbitrarily narrow and its position along the sensing fiber can be moved by properly tuning phase

modulation rate. Phase-BOCDA sensors can thus guarantee high flexibility in measurement

configuration, with spatial resolution enhanced to centimetric or even sub-centimetric scale, at the

expense of longer acquisition times and lower number of monitored points. In this work, a method

similar to that described in [3] has been applied for the first time to a traditional phase-BOCDA

scheme, without any modification of the experimental setup shown in Fig. 1(a), leading to a ten-

times reduction in measurement time while retaining high resolution and measurement accuracy.

Instead of performing a reconstruction of the full Brillouin Gain Spectrum (BGS) thickly scanning

pump-probe frequency difference (ΔυB) in a ~200 MHz span for each monitored point, it is

sufficient to measure gain in two fixed positions corresponding to positive and negative slopes of

the unstrained BGS at room temperature. The ratio of these two values gives information on the

direction and amount of shift induced by temperature or strain variations. A calibration procedure is

required to convert measured RB values into correspondent Brillouin Frequency Shift (BFS) values,

from which the actual strain/temperature distribution over the entire FUT can be estimated. Fig.

1(b) shows preliminary results for an 8cm strained fiber section near the end of a 10m SSMF FUT,

confirming 2 cm spatial resolution and dynamic range of more than 600 µε.

References [1] A. Motil, A. Bergman, M. Tur, “State of the art of Brillouin fiber-optic distributed sensing”, in Optics & Laser

Technology, vol. 78, pp. 81-103, 2016..

[2] A. Zadok, Y. Antman, N. Primerov, A. Denisov, J. Sancho, and L. Thevenaz, “Random-access distributed fiber sensing”,

in Laser & Photon. Rev., 6: L1-L5, 2012.

[3] A. Motil, O. Danon, Y. Peled and M. Tur, "Pump-Power-Independent Double Slope-Assisted Distributed and Fast

Brillouin Fiber-Optic Sensor," in IEEE Photonics Technology Letters, vol. 26, no. 8, pp. 797-800, 2014.

Figure 1 – (a) Experimental phase-BOCDA setup and (b) Measured strain distributions on a 8 cm section near the end of the FUT

Spatial and temporal imaging of polymeric integrated circuits

Dmitry Nuzhdin1,2, Sara Nocentini1, Lorenzo Pattelli1, Simone Zanotto1,3 and Diederik S. Wiersma1

1. European Laboratory for Nonlinear Spectroscopy (LENS), University of Florence, Via Nello Carrara 1, 50019

Sesto Fiorentino (FI), Italy 2. Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany

3. CNR-INO, U.O.S. Sesto Fiorentino, University of Florence, Via Nello Carrara 1, 50019 Sesto Fiorentino (FI), Italy

[email protected] Integrated photonic circuits, realized with many different materials and advanced technologies, are nowadays applied to many fields, including telecommunication, optical sensors and healthcare diagnostics. The functional characterization of such structures is extremely important to highlight differences between the intended design and the fabricated device, allowing to identify the possible presence of defects. Quite often, though current state-of-the-art techniques in CW can be applied only to simple optical devices, time or frequency resolved domains provide only limited information. We propose a wide-field, single-shot characterization technique combining ultrafast temporal and spatial imaging, and use it to investigate the propagation of light inside waveguides coupled to whispering gallery mode resonators. The sample under analysis is a three-dimensional polymeric circuit made of a commercial polymer (Nanoscribe GmbH) using the Direct Laser Writing lithographic technique (DLW). The light is coupled into the waveguide through a grating coupler designed for telecom wavelengths. A whispering gallery mode resonator is therefore vertically coupled to the waveguide and images of the transmitted light from the output coupler are recorded with sub-ps resolution with a CCD and a PMT for comparison, exploiting sum-frequency optical gating on a non-linear crystal [1]. From these data, we can reconstruct the evolution in time of the pulse through the photonic structure and highlight the different characteristics of the circuits and their defects using both a spatial and temporal imaging. The final aim of this experiment is to visualize the light inside the photonic circuits in time domain and perform a direct imaging of the propagating modes.

Figure 1. a) Scanning Electron Microscope image of a integrated polymeric circuit. b) Time resolved setup scheme.

References [1] L. Pattelli, R. Savo, M. Burresi, and D. S. Wiersma, “Spatio-temporal visualization of light transport in complex photonic structures.” Light: Science & Applications, 5(5), e16090, (2016). Acknowledgment: Effort sponsored by the funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n° [291349] on photonic micro-robotics and by the Erasmus Mundus Doctorate Program Europhotonics (Grant No. 159224-1-2009-1-FR-ERA MUNDUS-EMJD).

1

Fabrication and Characterization of PDMS Optical Waveguides structured by means of Two-Photon Polymerization Effect

Giulia Panusa1,2, Antonio d'Alessandro1,2, Ye Pu3, Demetri Psaltis3.

1. Informatics, Electronics and Telecommunication Engineering Department, Sapienza Unversity of Rome, Via Eudossiana 18, 00184, Roma.

2. Nanotechnology Enginnering, Sapienza Unversity of Rome, Via Eudossiana 18, 00184, Roma.

3. Optics Laboratory, School of Engingeering, École Polytechnique Fédérale de Lausanne, BM 4101, Station 17, CH-1015, Lausanne.

email: [email protected]

Sensorineural hearing loss (SNHL) is the most common type of hearing loss that affects more than 10% of the population worldwide [1]. Cochlear implants (CIs) are devices that generate hearing sensation through electrical stimulation of the auditory sensory neurons using an array of electrodes in patiens with partial or complete hearing loss. Significant damage to the inner ear often occurs due to the misplacement of the CI electrode array, resulting in the loss of residual hearing ability. Therefore, visualization within the cochlea would help diagnosing the status of the important intracochlear hearing structures.

In this work, polidimethylsiloxane (PDMS) optical waveguides made by using Two-Photon Polymerization (2PP) for cochlear implants applications are presented. The waveguides will be used to make a CI guiding endoscope: an "auxiliary" optical microendoscope to be incorporated into cochlear implants. 2PP is based on a nonlinear absorption effect that occurs when very high intense laser pulses are focused inside a material [2],[3]. By direct femtosecond laser writing, it is possible to polymerize a PDMS matrix material, that contains a suitable photoinitiator and an appropriate monomer mixture.

In this work, many commercially available photoinitiators and monomers have been investigated: depending on the molecular structure of the used photoinitiators, the source wavelength has to change in order to trigger the photo-polymerization. Photo-polymerization occurs only where the laser beam is focused. The unreacted monomers can be removed by heating the sample. We obtained optical waveguides with a diameter of 1-1.5 µm and we characterized the optical waveguides in terms of refractive index change through holographic phase change recording (Digital Holographic Interferometry). We measured a refractive index change of about 0.06, which allows light confinement in the waveguides.

References

[1] Ye Pu, Demetri Psaltis, Christophe Moser, Integration of Optical Guidance Mechanism in Cochlear Implants, CTI Proposal, EPFL.

[2] R. Inführ, N. Pucher, C. Heller, H.Lichtenegger, R. Liska, V. Schmidt , L. Kuna, A. Haase, J. Stampfl, Functional Polymers by Two Photon 3D Lithography, Applied Surface Science 254 (2007) 836-840.

[3] J. Stampfl, R. Inführ, K. Standlmann, N. Pucher, V. Schmidt, R. Liska, Materials for the Fabrication of Optical Waveguides with Two Photon Photopolymerization, Proceedings of the Fifth International WLT-Conference on Laser Manufacturing 2009.

Acknowledgment: Effort sponsored by the Optics Laboratory of École Polytechnique Fédérale de Lausanne.

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Reconfigurable Fabry-Perot Cavity Leaky-Wave Antennas Based on Nematic Liquid Crystals for THz Applications

Silvia Tofani 1,2, Walter Fuscaldo1,3, Romeo Beccherelli2, Alessandro Galli1

1. Dipartimento di Ingegneria dell’Informazione, Elettronica e delle Telecomunicazioni (DIET), “Sapienza” Università di Roma, Via Eudossiana 18, 00184, Roma, Italia.

2. Istituto per la Microelettronica e Microsistemi, Consiglio Nazionale delle Ricerche (CNR-IMM), via del Fosso del Cavaliere 100, 00133, Roma, Italia.

3. Institut d’Électronique et de Télécommunications de Rennes, UMR CNRS 6164, Université de Rennes 1, 35700, Rennes, France.

Reconfigurable antennas are usually designed by means of metamaterials in both the microwave and optical range. In the THz range, graphene has been recently as a tuning element, but preliminary results revealed the low efficiency of graphene-based antennas [1]. Here, we propose the use of nematic liquid crystals (NLCs) as tuning elements for the design of a THz Fabry-Perot cavity leaky-wave antenna (FPC-LWA). Such an antenna allows for achieving the beamscanning property at fixed-frequency through the simple application of a bias to the NLC layers. A theoretical model is first investigated as a proof concept . Then, two more practical designs are considered to estimate the technological implementation of such device. The structure consists of a multilayered stack of alternating dielectric layers, placed above a grounded dielectric slab. The alternation of tunable low-permittivity layers (NLC layers) and high-permittivity layers (Al2O3), with thicknesses fixed at odd multiples of a quarter wavelength in their respective media, produces a resonance condition, which makes possible to obtain a narrow radiated beam at broadside [2]. The application of a common bias voltage to the NLC layer allows for changing the resonance condition and, thus, the propagation constant of the fundamental leaky modes with consequent beam steering capability. A suitable circuit model for the dispersive analysis of planar structures in the presence of anisotropic layers is developed [3]. Dispersion curves of the fundamental TM leaky mode of three different layouts [4] are shown in Figure 1: a theoretical design (a) is compared with two realistic implementations (b, c) with the aim to design an antenna suitable for fabrication. In the next months, prototyping and measurements of the structures are expected as well as further theoretical investigations.

Figure 1. Normalized complex propagation constant of the TM leaky mode for applied voltages starting from 0 V (red lines) to

high-voltage limit V∞ (blue lines) for ideal 1 THz (a), realistic 0.59 THz (b), more realistic 0.56 THz (c) FPC-LWAs. References [1] W. Fuscaldo, P. Burghignoli, P. Baccarelli, A. Galli, “Reconfigurable Substrate–Superstrate Graphene-Based Leaky-Wave THz Antenna”, IEEE Antenna Wireless Propag. Lett. 15 (2016). [2] D. R. Jackson, A. A. Oliner, A. Ip, “Leaky-wave propagation and radiation for a narrow-beam multiple-layer structure”, IEEE Trans. Antennas Propagat. 41, pp. 344-348 (1993). [3] G. Valerio, D. R. Jackson, A. Galli, “Fundamental properties of surface waves in lossless stratified structures”, Proc. R. Soc. A, 466, pp. 2447-2469 (2010). [4] W. Fuscaldo, S.Tofani, D. C. Zografopoulos, P. Burghignoli, P. Baccarelli, R. Beccherelli, A. Galli, “Tunable Fabry-Perot cavity THz antenna based on leaky-wave propagation in nematic liquid crystals”, submitted to Opt. Lett.

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Ferroelectric liquid crystals with short helical pitch

Dorota Węgłowska and Michał Czerwiński

Faculty of Advanced Technologies and Chemistry, Military University of Technology

2 Kaliskiego Str., 00-908 Warsaw 49, Poland


The deformed helix ferroelectric liquid crystal effect (DHF) [1] exhibits short switching times

(<100 ms) at a very low applied voltage (1 V/mm) and hysteresis-free V-shape switching curve,

nearly independent of the frequency of the applied voltage in a broad frequency range (10 Hz-4

kHz). The above features make ferroelectric liquid crystals (FLCs) working in the DHF mode very

useful for photonic applications.

For the DHF mode FLCs having a low melting point, a broad temperature range of SmC*

phase, a short helical pitch, a high tilt angle and a high spontaneous polarization are especially

promising. Several chiral compounds with a benzoate rigid core with the general structure:

C3F7CH2O(CH2)mO O

O

X

O

C6H13

CH3 wherein: m=2-7 and X=H or F

have been synthesized, and their mesomorphic properties have been studied [2]. The tilt angle, the

spontaneous polarisation as well as the helical pitch of the compounds have been evaluated in the

full temperature domain. New mesogenic compounds exhibit short helical pitch (< 0.8m) and

unique, so called orthoconic, behavior at the synclinic smectic SmC* phase: the tilt angles measured

in the SmC* phase reveal extremely high values at saturation approaching 45. The values of the

spontaneous polarization for all investigated compounds are between 80 and 100 nC/cm2.

References

[1] L. A. Baresnev, V. Chigrinov, D. I. Dergachev, E. P. Poshidaev, J. Funfshilling,

M. Schadt, Liq. Cryst., 1989, 5, 1171.

[2] D. Węgłowska, P. Perkowski, W. Piecek, M. Mrukiewicz, R. Dąbrowski, RSC Adv., 2015, 5, 81003.

Acknowledgements: This work has been supported by the Polish Ministry of Science and Higher Education, grant RMN

No. 08-796/2016.

mailto:[email protected]

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