near-ﬁeld nanoimprinting using colloidal monolayers · near-ﬁeld nanoimprinting using colloidal...

Near-field nanoimprinting usingcolloidal monolayers

Christin David,1,2,6 Paul Kuhler,1,3,6 F. Javier Garcıa de Abajo,4,5,⇤ andJan Siegel1,7

1Instituto de Optica, Consejo Superior de Investigaciones Cientıfcas, Madrid, Spain2Instituto de Quımica-Fısica “‘Rocasolano”’, Consejo Superior de Investigaciones

Cientıfcas, Madrid, Spain3Photonics and Optoelectronics Group, Ludwig-Maximilians-University Munich, Germany

4ICFO - Institut de Ciencies Fotoniques, Mediterranean Technology Park, Castelldefels, Spain5ICREA - Institucio Catalana de Recerca i Estudis Avancats, Barcelona, Spain

6These authors contributed equally to the [email protected]

⇤[email protected],

Abstract: We experimentally and theoretically explore near-field nanopat-terning obtained by irradiation of hexagonal monolayers of micron-sizedpolystyrene spheres on photosensitive Ge2Sb5Te5 (GST) films. The im-printed patterns are strongly sensitive to the illumination conditions, as wellas the size of the spheres and the orientation of the monolayer, which wechange to demonstrate control over the resulting structures. We show thatthe presence of multiple scattering effects cannot be neglected to describethe resulting pattern. The experimental patterns imprinted are shown tobe robust to small displacements and structural defects of the monolayer.Our method enables the design and experimental verification of patternswith multiple focii per particle and complex shapes, which can be directlyimplemented for large scale fabrication on different substrates.

© 2014 Optical Society of America

OCIS codes: (350.4600) Optical engineering; (160.2900) Optical storage materials; (050.5298)Photonic crystals; (000.2190) Experimental physics; (000.6800) Theoretical physics.

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6. Z. B. Wang, M. H. Hong, B S. Luk’yanchuk, Y. Lin, Q. F. Wang, and T. C. Chong, “Angle effect in lasernanopatterning with particle-mask,” J Appl. Phys. 96, 6845 (2004).

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#203796 - $15.00 USD Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014(C) 2014 OSA 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8226

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14. P. Kuhler, F. J. Garcıa de Abajo, J. Solis, M. Mosbacher. P. Leiderer, C. Afonso, and J. Siegel, “Imprinting theOptical Near Field of Microstructures with Nanometer Resolution,” Small 5, 1825 (2009).

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16. J. Siegel, A. Schropp, J. Solis, C. N. Afonso, and M. Wuttig, “Rewritable phase-change optical recording inGe2Sb2Te5 films induced by picosecond laser pulses,” Appl. Phys. Lett. 84, 2250 (2004).

17. J. Siegel, W. Gawelda, D. Puerto, C. Dorronsoro, J. Solis, C. N. Afonso, J. C. G. de Sande, R. Bez, A. Pirovano,and C. Wiemer, “Amorphization dynamics of Ge2Sb2Te5 films upon nano- and femtosecond laser pulse irradia-tion,” J. Appl. Phys. 103, 023516 (2008).

18. P. Kuhler, F. J. Garcıa de Abajo, P. Leiprecht, A. Kolloch, J. Solis, P. Leiderer, and J. Siegel, “Quantitativeimaging of the optical near field,” Opt. Express 20, 22063–22078 (2012).

19. B. Lee, J. Abelson, S. Bishop, D. Kang, B. Cheong, and K. Kim, “Investigation of the optical and electronicproperties of Ge2Sb5Te5,” J. Appl. Phys. 97, 18 (2005).

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band-structure calculations of photonic crystals,” Nat. Nano 3, 413 (2008).23. R. Sainidou, N. Stefanou, I. Psarobas, and A. Medinos, “A layer-multiple-scattering method for phononic crystals

and heterostructures of such,” Nat. Nano 3, 413 (2008).

1. Introduction

Optical near-fields in the vicinity of metal or dielectric microstructures upon external illu-mination display complex patterns that can be used for nanoscale imprinting on substratesurfaces.[1, 2, 3, 4] This is a promising concept for nanoprocessing large regular patterns, forexample by arranging colloidal particles to form closed-packed layers that act as a mask. Thisapproach has been used with different types of surfaces and for various particle materials andshapes.[5, 6, 7, 8, 9, 10] Engineered structures exploiting the focal points of the colloids be-come realizable in practice by angular beam scanning[11]. It is generally known that the result-ing patterns not only correspond to a linear superposition of the optical near-fields of the singleparticles, but also that particle interaction and mutliple scattering effects become important fortouching particles in a colloidal monolayer.[12] Recent FDTD simulations for periodic bound-ary conditions in a Si particle monolayer have been used to identify optimum particle array andirradiation parameters for maximum field enhancement at the substrate surface near the contactpoint with the particle. [13] Moreover, collective effects in the resulting 2D photonic crystalproduce a wealth of features, beyond a single region of field enhancement as typically reportedand exploited, which enlarge the suite of achievable imprinting structures. In this combinedexperiment-theory study, we address the optical near-fields imprinted by colloidal particle ar-rays to gain insight into the underlying mechanisms of pattern formation.

2. Experiments

We use chalcogenide phase-change substrates to record and subsequently image the near-fieldlight intensity with subwavelength resolution.[14] Upon irradiation with short laser pulses, a


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Fig. 1. (a) Hexagonal lattice of polystyrene spheres with an underlying calculated near-fieldpattern, which is imprinted into a GST substrate. The calculation details are as in (d) (seebelow). The incident light is p-polarized and has wave vector along a nearest-neighborsbond direction (f = 0�). (b) Dispersion diagram showing the reflection coefficient |r| ofa closed-packed monolayer of polystyrene spheres on an fcc GST planar substrate. (c)-(e)Near-field associated with specific points in the dispersion diagram for a = 817 nm (seecorresponding symbols in (b)). Wavelengths and angles of incidence used: (c) l = 409 nm,q = 40.8�; (d) l = 799 nm, q = 52.2�; (e) l = 925 nm, q = 30.0�.

change from the crystalline to the amorphous phase is induced in regions where the local in-tensity is large enough, effectively mapping complex intensity patterns for varying illuminationconditions with great detail.[15, 16, 17] The phase transition induced is also accompanied by achange in material density, topography, and electric conductivity, which makes scanning elec-tron microscopy (SEM) a suitable high-resolution read-out technique.[18] We have applied thismethod to study the near-field distributions of 2D hexagonally arranged arrays of polystyrene(PS) spheres (nPS = 1.58, k = 0.003 at l = 800nm) as a function of several experimental pa-rameters, as illustrated in Fig. 1(a). The substrates consist of 40-nm-thick, face-centered-cubic(fcc) polycrystalline Ge2Sb5Te5 (GST) films[15, 16, 17] sputter-deposited on Si [001] waferscovered by a 10-nm-thick amorphous SiO2 buffer layer (Numonyx, Italy). The complex in-dex of refraction (n+ ik) of these materials at the experimental laser wavelength (800nm) is5.72+4.09i for fcc-GST[19], 4.74+1.45i for amorphous GST[19], 1.453+0i for amorphousSiO2, and 3.69+0.006i for Si. Closed-packed monolayers of spherical PS particles (diameters817nm and 1704nm, Microparticles GmbH, polydispersity 2.6%) were self-assembled on a wa-ter surface and then deposited onto the substrate. Given their large Mie parameter 2p

lnPSd

2 � 1,each particle produces collimating lensing, involving the participation of many multipoles upto a high order.

Laser irradiation of the particle covered films was performed in air using a regenerativelyamplified Ti:sapphire laser system operating at 800nm central wavelength with a pulse durationof 350ps. The laser beam was focused onto the sample at an angle of incidence q = 52.2� toa measured elliptical spot size of 270⇥150 µm2 (1/e2 diameter). A single pulse was selectedfrom a 100Hz pulse train by means of an electromechanical shutter to irradiate the targeted area.The sample was mounted on a motorized 3D translation stage and observed with a home-builtmicroscope for in-situ control. Subsequent to laser irradiaton, the particles were removed witha scotch tape in order to access the patterns because the laser irradiation at the fluences requiredfor imprinting the near-field patterns did not lead to removal of the particles. Additionally,


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Fig. 2. Imprinted near-field intensity calculated for increasing angles of incidence withfixed azimuthal sample lattice rotation f = 21�, particle size a = 1730 nm, and p-polarizedlight of wavelength l = 900 nm. Gray circles signal particle positions. The maximum ofintensity beneath the GST-air interface is given in each figure, while the color scale isnormalized as indicated in the lower color bar. The imprinted intensity drops for increasinginclination.

images were taken using a Gemini Ultra Plus field emission scanning electron microscope(SEM) operated at 5kV and yielding 5nm spatial resolution.

3. Results and interpretation

We performed multiple scattering calculations using a layer-KKR approach[20, 21, 22, 23] forperiodic particle arrays, which are converged with respect to both the number of multipolesused for each sphere and the number or reciprocal lattice vectors in the layer-substrate cou-pling. Fig. 1(b) shows the calculated band diagram for a hexagonal PS sphere monolayer oncrystalline GST. The upper triangular region above the light line (k = w

c nPS ⌘ kk) allows us toidentify configurations of particle diameter a (equal to the lattice spacing in the closed-packedstructures), incident wavelength l = 2p

k , and incidence angle (related to the parallel wave vectorthrough kk = k sinq ) associated with resonant optical modes, where high local field enhance-ment is expected. Incidentally, the kinematical small-particle bands given by |kk �Gnm| = k,where Gnm runs over reciprocal lattice vectors (superimposed curves in Fig. 1(b)), differ fromthe numerical bands due to inter-particle interaction (see Supporting Information (SI) for moredetails).

The remarkable diversity in the near-field distribution patterns at the GST-air interface for


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V

III

V

III

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0.18

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Fig. 3. Influence of incident light polarization on the imprinted near-field for l = 799 nm,a = 1704 nm, and q = 52.2�. Gray circles in the calculated graphs indicate the sphere-substrate contact points. Measured SEM images (a,b) are compared with theory (c,d) forthe following parameters: (a),(c) s-polarization, and lattice rotation f = 22.3� (see inset to(c)); (b),(d) p-polarization, f = 20.6�.

selected points of the band diagram (see Fig. 1(c)-(e)) clearly illustrates the large sensitivity ofthe imprinted structures to geometrical and illumination parameters. This allows a rich varietyof patterns to be created that display the periodicity of the colloidal monolayer mask. Noticethat the change in the particle size-to-wavelength ratio of the patterns shown in Fig. 1 variesthe complexity of the near-fields. The near-field intensity is given as the ratio of the intensityI directly beneath the GST surface normalized by the intensity of the incident field I0. Thisindicates the absorbed field intensity that produces phase-change nanoimprinting.

In Fig. 2 we compare simulations for fixed azimuthal sample orientation and increasing an-gles of p-polarized light incidence. The imprinted intensity drops with increasing angle of inci-dence, but it acquires a more complex structure, presumably due to the involvement of a richerstructure of modes, as observed in the dispersion diagram of Fig. 1(b) with increasing kk.

Additionally, pattern complexity is influenced by the incident-light polarization, see Fig. 3.From here on, light is considered to be coming from the left in all figures. The upper panelsshow measured SEM images of structures created with a single laser pulse at an angle of inci-dence of 52.2�. The dark spots at position II in Fig. 3(a) can also be found in regions that werenot irradiated, and thus, they are not associated with light irradiation effects (see SI, Fig. 2).They can only be detected with the inlens detector, indicating they are small modifications ofthe GST surface that result from adsorbants deposited at the PS spheres contact region. They aretherefore suitable to determine the position during illumination of the spheres even after theirremoval. In contrast, the dark regions with a brighter circle inside (position I) are a direct resultof irradiation. By comparison with optical micrographs, the observed modifications of the GSTfilm can be attributed to amorphization of the otherwise crystalline film (outer ring) and ablation


500 nm

(a)

(c) (d)

(b)

I/I0

0.18

0.06

I/I0

0.20

0.06

Fig. 4. Influence of lattice rotation f on the imprinted near-field for a = 817 nm and p-polarized light incident with l = 799 nm and q = 52.2�. Gray circles signal contact pointsin the calculated images. Measured SEM images (a,b) (acquired with a sideways detectorthat is insensitive to the contact-point surface modifications) are compared with theory (c,d)for f = 41� in (a,c) and f = 23� in (b,d).

(enclosed by the bright rim). [18] A relevant parameter is the lattice orientation angle f relativeto the light incidence direction (see inset of Fig. 3(c)). Regions of similar orientation f wereselected in Fig. 3, so the main differences between panels (a) and (b) is the light polarization(s and p, respectively). While displaying only a simple elliptically shaped maximum at I for spolarization, the imprint is more complex for p polarization. Beside the main maximum at IV,which is still well pronounced, several less intense features are revealed (see III), including anauxiliary maximum at the contact point V, which can not be observed for s polarization. The re-markable modifications observed in the spatial patterns when the light polarization is changedcan be traced back to the involvement of several array modes for any given light frequencyand direction of incidence. Their excitation strongly depends on the orientation and amplitudeof the in-plane electric field vector of the incoming light, see Fig. 1 (b). Essentially, differentpolarizations couple with different strengths to these modes, thus producing different spatialpatterns. The same arguments apply to the variations on the produced patterns with the angleof incidence (Fig. 2) and the lattice rotation that we discuss in the following.

The lower panels in Fig. 3 depict our corresponding calculations with grey circles superim-posed to indicate the positions of contact points. The agreement between theory and experimentis good. In particular, the occurence of multiple local maxima as well as their detailed shapeand position depending on the laser polarization is well reproduced in the calculations. As in-dicated in Fig. 1(c), the number of near-field maxima is not limited by the number of particlesand can exceed it.

The dependence on the lattice orientation relative to the projected light incidence direction(angle f ) is discussed in Fig. 4. The left (right) side corresponds to f = 41� (f = 23�). Sym-metry is reduced with respect to the horizontal axis and significant differences between both of


(d) I/I0

1.33

0.63

(c) I/I0

0.23

0.12

(a)

2µm

(b)

10µm

I II

Fig. 5. (a) Observation of an imprinted pattern imaged with a SEM inlens mode detector.The sample and illumination parameters are l = 799 nm, a = 1730 nm, q = 52.2�, p-polarization, and f = 26.4�. The single lattice defect is marked by an arrow. No pertur-bation in the imprinted near-field pattern is observed. (b) Optical microscopy image of adifferent, larger region of the same monolayer before irradiation, showing the presenceof single defects (vacancies). (c) Multiple- scattering calculation of the corresponding fulllattice (no defect). (d) Simple superposition model.

them are observable in the detailed shape of the position of the near-field maxima relative tothe spheres.

We want to emphasize that the intensity pattern produced by a colloidal monolayer cannot bedescribed by a simple superposition of patterns produced by single spheres, without consideringparticle interactions. To demonstrate this, we have performed an experiment using conditionsfor imprinting a relatively simple pattern (Fig. 5(a)), which we compare with calculations bothfor the multiple scattering method noted above (Fig. 5(c)), as well as with a simple superpo-sition of the scattered field of each individual particle[14] (single scattering, Fig. 5(d)). Thelatter was carried out for a hexagonal lattice of 11⇥ 11 particles, neglecting collective effectssuch as multiple scattering from neighboring particles. This superposition model does not re-produce the position of the dominant intensity maxima, which are predicted to lie much closerto the contact points than found experimentally. At the same time, the multiple scattering modelpredicts a distance between maxima and contact points that is in agreement with experiment.Notice that the contact points are more pronounced here than in Fig. 3, presumably as a resultof slightly varying conditions during sample preparation. Furthermore, the intensity maximacalculated with the superposition model display considerably different shapes and amplitudeswith respect to those measured in Fig. 5(a) or calculated with the full model. This demon-strates that the patterns produced cannot be explained by simple interference of incoming andscattered light because each particle reacts to significant field contributions originating in scat-tering from its neighbors. The experimental conditions in Figs. 3 and 5 are very similar, with


only a small variation in both lattice rotation and particle size. However, the imprinted patternslook different, thus emphasizing the sensitivity small parameter changes.

A remarkable feature can be observed in Fig. 5(a): the absence of a single contact point (atthe position marked by a dashed circle), indicating that the particle at this position might havebeen missing in the monolayer during illumination. Such lattice defects in form of vacanciesare frequently observed in colloidal monolayers, as shown in Fig. 5(b). Despite this defect, theimprinted pattern in Fig. 5(a) features the corresponding main maximum next to the defect, justas if there was no defect. We speculate that the strong collective interaction between the parti-cles might render the optical response of the monolayer robust against the presence of latticedefects, featuring collective photonic crystal-like properties. Yet, we cannot exclude the possi-bility that the defect was not a vacancy but a particle slightly elevated above the substrate, thusnot forming a contact point but contributing in a slightly different way to the optical response.Nonetheless, both explanations imply that the imprinted patterns are not too sensitive to smalldisplacements and structural defects of the colloids. Further analysis of the degree of toleranceagainst monolayer defects is still needed.

4. Conclusion

In conclusion, we have experimentally imaged the complex optical near-fields of colloidalmonolayers by imprinting them on thin GST films. The measured intensity distributions arein excellent agreement with electromagnetic simulations. This simple yet effective concept iswell suited for nanostructuring and mapping arbitrary intensity distributions with high spatialresolution, which can be implemented directly for large scale fabrication on other substrates.The imprinted near-fields inherit the full translational invariance from the colloidal monolayer.At the same time, by studying the influence of various setup parameters, we conclude thatthe detailed near-field distribution is not only determined by the particle arrangement but alsostrongly depends on light polarization and angle of incidence. In particular, the number of near-field maxima is not limited by the number of particles and can be increased by the proper choiceof illumination conditions. The orientation of the lattice with respect to the light incidence di-rection is found to have a significant impact on the near-field distribution, thus providing anadditional degree of freedom to tailor patterned imprinted structures.

Acknowledgments

The authors thank E. Varesi, A. Pirovano, and R. Bez for supplying the GST films andMatthias Hagner for help with the SEM measurements. We also thank P. Leiderer and J.Solis for stimulating discussion. This work was performed within a Joint Project betweenCSIC and Konstanz University funded by the Spanish Government and the DAAD. We wouldlike to acknowledge national support from Spanish National Research Projects (ConsoliderNanoLight.es, MAT2010-14885, and TEC2011-22422). C.D. acknowledges a FPU fellowshipfrom the Spanish Ministerio de Educacion.


near-ﬁeld nanoimprinting using colloidal monolayers · near-ﬁeld nanoimprinting using colloidal...

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