Office of Naval Research

MURI Annual Progress Report (June 5, 2001-May 31, 2002)

For Dr. Kathryn Wahl and Dr. V. Browning

Under Grant Number: N00014-01-1-0803

DOD/ONR MURI

Scalable and Reconfigurable Metamaterials

Principle Investigator: Xiang Zhang

Co-PIs: G. Chen, T. Itoh, E. Yablonovitch, J. D. Joannopoulos, J. Pendry, D. Smith, and S. Schultz

University of California in Los Angeles

Engineering IV, 420 Westwood Plaza

Los Angeles, CA 90095-1597

1. Goals and Planned Milestones Ultimate Goal: The goals of this project are to develop new synthesis technologies for fabrication of 3D scalable and reconfigurable meta-materials, to explore the new physics and simulation methods of meta-materials, to develop an integrated design tool – Meta-CAD for the design and optimization of meta-materials and devices, to experimentally characterize the physical properties of meta-materials, to demonstrate prototype meta-material-based devices for novel electromagnetic wave applications and to transfer the developed technology to defense industries and to develop an interdisciplinary educational program that trains a new generation of graduate students and post-docs in the science and technology of meta-materials. Planned Milestones for This Year:

1. Study the fundamental physics of metamaterials; with focus on identification of key parameters of superlens resolution, extraordinary reflectivities of left-handed metamaterials, time-domain study of negative refraction, perfect lens effects in photonic crystals, exploration of new design of metamaterials, such as using arrays of engineered defects in photonic crystals.

2. Develop 3D microfabrication methods for metamaterials in GHz to THz range; optimize process stability and yield; demonstrate 1st prototype of metamaterials including artificial plasma wires, micro bubble arrays and test their electromagnetic properties.

3. Establish transmission/reflection methods for characterizing the performance of 1st prototypes of metamaterials in GHz and THz frequencies.

4. Understand metamaterials, in particular LH structures from a practical/engineer point of view, and consequently develop a different approach based on well-established theories and techniques, demonstrate the fundamental effects of metamaterials in simplified lumped-element circuits and infer a rigorous characterization of them, investigate novel meta-structures using active devices.

5. Simulation study and design of 1st prototype devices such as artificial plasmonic reflector, plasma wire filters, LH cross-coupled filter.

2. Major Accomplishments during This Period Considerable progresses have been made during this first year on metamaterial synthesis, novel physics study, and metamaterial devices and metamaterial characterizations. (1) Metamaterial Synthesis: A new high resolution projection stereolithography system (PµSL) has been set up with stepping function implemented which allows for fabricating complex three dimensional structures with large sample size while in high spatial resolution. Chemical compositions of the UV-curing resins and their exposure conditions have been investigated and optimized. Upon the successes of these instrumentation efforts, several sets of plasmon wire samples with THz optical activities have been fabricated and characterized. Designed more complex structures are under the

way of process optimization and testing. 2D bubble arrays have been fabricated and will be tested. (2) Metamaterial Physics: Defects in photonic crystals have been investigated for building blocks of metamaterials. A novel perfect lens by employing an effect called All-Angle-Negative-refraction in a photonic crystal has been proposed and theoretically studied, and theorists and experimentalists have been working together to design and demonstrate this new meta-material structure. Substantial efforts have also been made on understanding the physics of perfect lenses. Especially, the performances of a perfect lens made of practical materials with parameters which deviates from the ideal case has been investigated and shown to be still very attractive for sub-wavelength applications. Alternative structures such as multilayers to alleviate these limitations of real material properties have been studied. Novel periodic metallic structures have been proposed and investigated to reduce the stop-band edge by making use of large capacitive loading. The amplification of evanescent wave in metal silver film, an essential effect for perfect lens, has been studied and demonstrated experimentally. (3) Metamaterial Characterization and Devices: A stable relationship has been established with our collaborators in UCSD for characterizing metamaterials, and fabricated samples have been tested. New metamaterial microwave devices such as left-handed transmission line, retrodirective array, left-handed forward coupler have been proposed and demonstrated. 3. Detailed Description of Accomplishments and Results Metameterials synthesis: Micro stereo Lithography System: Progresses on the development of the high resolution

micro stereo lithography process have been made on several facets in Zhang’s group.

1) As for the system, a high-resolution projection optic system has been set up for the projection micro stereolithography system (PµSL) to improve the fabrication resolution. Meanwhile, the larger aperture of the new projection lens provides with broader field view. So that the new PµSL system allows for fabrication of samples of larger sizes than our old system while with high spatial resolution.

2) As for the chemicals, study has been performed to control optical curing depth by using various chemical absorbers. The chemical absorber with desired absorption and solubility has been identified and its concentration is optimized through experimental measurement and numerical simulations.

3) As for the system control, a stepping function has been developed and implemented to the system to synthesize larger samples for FTIR

Fig. 2 Metal coated wires recently fabricated in Zhang’s group

measurement. The motorized translation stage and motion controller have been purchased and implemented into current system. The control algorithm has been coded, and being implemented in the current system. The design for the second generation of µSL has been finalized, the purchased optical components have arrived, and the custom-designed mechanical fixtures have been machined and assembled. Now Zhang’s group is constructing this new µSL system. In concurrent to the system construction, they are also working on developing the process control program, which delivers many attractive advantages over the current µSL.

4) Design and process optimization of THz artificial plasma wires and split ring

resonator arrays: 1st prototype of THz plasmon wire samples are fabricated with wire diameter 30-40µm, lattice constant 80-160µm, and overall sample size 1x2x1mm. They are designed with a plasma frequency at 0.5-1.2THz, with isolation ratio higher than 40dB. The 1st set of THz 2D split-ring resonator arrays are also in design and process development phase.

Metallization and Molding: sputter coating and electrodeposition turned out to be

successful for the metallization of 3D metamaterials with feature size down to 10µm. This is proven by the FTIR characterization of 1st prototype of artificial plasma wires. Elastomer thermal plastic molding has also been optimized toward the development of tunable devices. For mold transferring into other candidate dielectric materials, various methods of infiltration and molding processes have been compared. Zhang’s group determined to focus on two methods, i.e., sol-gel and powder-infiltration. Purchase orders for chemicals and powders have been filed, and accessible ovens for the thermal annealing have been found at UCLA.

Plasmonic Filter Synthesis: Using the advanced projection µSL system, Zhang’s group

synthesized the first several sets of THz plasmon wire samples with wire diameter 30-40µm, lattice constant 80-160µm, and overall sample size 1x2x1mm (Fig. 1). These samples are being tested at UCSD for transmission and reflection measurements in an FTIR system. Tunable plasma wires embedded in elastomeric substrates (PDMS) are being fabricated and will be tested on Q1 of 2nd financial year.

Figure 3 1D microdevice with bubbles turned on at desired sites

Figure 4 Experimental and numerical values of transmissivity for the microstrip (red –experimental, black – numerical)

Bubble and Droplet based reconfigurable Meta-materials: Gang’s group had begun to work in two different directions – one in the area of microwaves and the other in the infrared and optical wavelength range. The intention was to further the research in both directions by conducting further experiments on the fabricated microstrips and complete the fabrication of micro-devices for the IR range and conduct preliminary experiments.

1) They have succeeded in

the fabrication of microdevices for the generation and control of bubbles along fixed positions in a straight line to form 1D lattice as well as in a square to form 2D lattices. The devices have been fabricated such that the lattice constant as well as the shape of the lattice can be adjusted. Initial experiments have been conducted to observe the generation and control of bubbles in the 1D lattice. The bubbles can be turned on or off to increase or decrease the lattice constant. Fig. 3 shows the 1D microdevice with alternate bubbles turned on. The photonic properties of 2D bubble arrays thus formed have been calculated using Prof. Pendry’s program.

2) They have also compared the

experimentally determined values of transmissivity of the microstrip with the results from the numerical simulation from Prof. Pendry’s program and the results match well as shown in Fig. 4.

3) They fabricated a test device for

making 2 dimensional bubble arrays, which is shown in Figure 5. The device in the left figure is for making simple lattice bubble arrays and the one in the right is the figure for bubble arrays of both a simple lattice and a face centered cubic lattice. In the figure the bright area is a platinum film deposited by e-beam sputtering on the glass substrate. In order to generate a micro bubble a metal with low thermal conductivity, in this case platinum is necessary. The developed wires of their devices are discontinuous at a point in order to pass the electric current through water. Electric resistivity of water is much higher than that of platinum. Hence, water in a tiny region is heated, thereby creating a vapor bubble. To enhance the

Fig. 5 Test devices for 2D bubble arrays

generation of bubbles silicon dioxide is deposited by PECVD on the surface and cavities are fabricated at the wire gap by reactive ion enhanced (RIE) etching RIE. 2x2 bubble arrays are tested with full reconfigurability. Their next step is to make

4x4 bubble arrays using these devices. 4) They are also working towards a new optical approach for testing the bubble

arrays as sell as seeking the possibility of application of the devices to a new research field, i.e., configurable bubble optics. They have theoretically simulated the potential use of the configurable arrays as controllable micro light rejecters and diffusers.

New Physics in Metamaterials Negative Refraction and Superlens Effect The Imperial College (IC), UCSD and MIT

have made considerable collaborative efforts investigating the physics of negative refractive media and the perfect lens effect. The understanding of the focusing properties of the perfect lens is crucial to constructing alternative configurations and using alternative processes for the optimization of any practical near-field imaging system.

1) Clarifying the concepts of the perfect lens (Pendry):

After publication of the original proposal for the perfect lens using negative refractive index, some doubts had been expressed on the perfect lens effect in Comments on the original publication. Prof. Pendry has clarified that the perfect lens effect does not violate any fundamental laws of physics and have shown that the objections were based on flawed arguments [1].

2) Effects of deviations in the lens parameters from the ideal conditions (IC and UCSD):

A slab of negative refractive index acts as a perfect lens under the conditions

εs = −ε0, µs = − µ0, (1)

0.01

0.1

1

10

100

1000

0 2 4 6 8 10 12 14 16

µ = µ' + 0.001 iε = -1 + 0.001 i

d/λ= 0.1

kx/k

0

|τeik

zd |

µ' = -2

-1.5

-1.2

-1.1

-1.05

-1.02

-1.01

-1.005

-1.002-1.001

-1

Figure 6 Left: the transfer function for a left-handed slab. Right: the results of applying a finite aperture to the ‘perfect lens’ (with small losses)

where εs and µs are the dielectric permittivity and the magnetic permeability of the slab material respectively, and ε0 and µ0 are the dielectric permittivity and the magnetic permeability of the surrounding medium respectively. Under these conditions the lens brings to a focus both the propagating and the evanescent waves associated with a source. It is important to note that Eq. (1) is the exact condition for the exciting surface states on an interface between semi-infinite positive and negative media. For a slab, however, the presence of the other interface detunes the resonant frequency of the surface plasmon states. The near-field lens works by the off-resonance excitation of the surface states on these surfaces. The detuning ensures that they are excited to the correct degree by the incident field for a focused image.

For the design and optimization of a practical near-field lens, it is important to investigate the consequences of departing from the perfect and ideal conditions in Eq. (1). Any deviations from these always results in the on-resonance excitation of slab plasmon polariton modes, which degrades the image resolution as the transfer of waves with large transverse wave-vectors is hindered. Absorption in the lens also results in a similiar limitation. They have carried out a thorough exploration of the parameter space of the material parameters of the negative refractive slab for which sub-wavelength imaging is still possible. Their conclusion is that imperfections put severe limitations on lens materials and even the best available materials will restrict the performance of a practical lens. But reasonable sub-wavelength resolution is possible using the currently available materials.

They have also investigated the effects of a finite transverse size of the negative refractive slab, which is an important restriction in conventional imaging. They have analysed the situation when the lens has a finite transverse size by phenomenologically modelling the finite size effects by an aperture that is completely absorbing outside the aperture region. Their results indicate that the finite aperture effects do not affect the imaging process appreciably so long as the

Figure 7 Left: an asymmetric lens bounded on one side by air and on the other by dielectric. Right: enhancement of resolution for the asymmetric lens when ε2 = −ε3 + 0.4i, i.e. the favorable configuration.

aperture size does not become comparable to the feature sizes of the source that is being imaged.

The asymmetric lens configuration (IC and UCSD):

Another line of investigation has been to ask how perfectly the geometry of the lens needs to be realised. The ideal lens should be symmetrical with the same medium surrounding the negative refractive on both the sides. However we have shown that the geometry of the lens can be asymmetric with different media on either side, and yet evanescent waves can be amplified within the slab provided that surface plasmon conditions in Eq. (1) is satisfied at one of the interfaces. This asymmetric configuration has a few disadvantages, such as non-zero reflectivity and small amounts of aberration, but it has the advantage of being mechanically more rugged for nanoscale applications as the lens material can now be coated onto a dielectric substrate and integrated detection becomes possible. They have investigated in detail the focussing properties for an asymmetric configuration of silver coated on a dielectric or semiconductor substrate. They have shown that it is possible to compensate for the deviation of µ from the perfect lens conditions in this system (µ = +1 here) by corresponding deviations in ε. We conclude that a sub-wavelength image resolution of 50-60 nm (corresponding to ~λ/10) should be possible.

3) Layered configuration for the perfect lens (IC): Another line of investigation of Prof. Pendry’s group has been to look at alternative structures of the negative media that can be used for imaging with sub-wavelength resolution but are less susceptible to imperfections in the materials. In this connection we have been examining the effects of making the slab of negative refractive materials into very thin slices and distributing the layers in between the object and image planes. Such a multi-layer stack of alternating positive and negative refracting media can also behave as a perfect lens, but it is

less sensitive to imperfections in the lens material. It now has the novel property of behaving as a fibre-optic bundle that can transfer the perfect image of an object across its thickness. The point is that by distributing the amplification for the evanescent waves, the fields do not grow to the extreme values as in a single slab, and therefore the dissipation is much less. The degree of image resolution is much improved in this case. Also the process of slicing the lens renders the layered stack into a highly anisotropic medium with ε⊥ → ∞ and ε|| → 0 under the perfect lens conditions (ε+ = ε− and µ+ = µ−). Radiation in an anisotropic medium propagates in an anisotropic medium with the dispersion (kx

2 +ky2)/εz + kz

2/εx = ω2/c2, and hence for the perfect lens conditions it is always true that kz = 0. Each Fourier component of the image passes through this unusual effective medium without change of phase or attenuation.

Although it is difficult to have negative magnetic permeability at optical frequencies, metals with negative dielectric permittivity (ε) can mimic the effects of negative refraction to some extent. It has been numerically verified the above effects for a multilayer stack of alternating thin layers of silver and a positive dielectric medium. Making the layers very thin reduces the effects of retardation and the effects of absorption are also less deleterious. The effects of absorption can be minimized further by making the magnitude of the real part of the dielectric constant much larger. The dissipation in the system appears to set the eventual limit on the highest possible image resolution.

Figure 8 Left: Schematic of the field distribution for an incident evanescent wave on a layered perfect lens, when the original lens is cut into three pieces placed symmetrically between object and image. Right: field intensity at the image plane (i) using a single slab of silver of thickness 40 nm and, (ii) a layered stack of 8 alternating positive and negative dielectric layers of 5 nm layer thicknesses

4) Use of optical amplification to compensate for absorption in the layered perfect lens (IC)

Prof. Pendry’s group has examined the possibility of using optically (laser) amplifying positive dielectric media to compensate for dissipation in the negative medium. The use of optical gain in the positive layers is implied in Eq. (1) when the negative medium is absorbing. Their results indicate that compensation for dissipation is possible and that one can literally reach the limits set by retardation.

For a layered stack consisting of silver/gain medium, calculations demonstrate a dramatic improvement in lens performance. Further, the transfer of evanescent waves becomes almost independent of the total stack thickness and can result in transfer of sub-wavelength image information over several wavelengths. However, such a medium with distributed absorption and amplification could be susceptible to laser instabilities.

Figure 9 Left: Schematic of the layered structure. The positive amplifying and negative dissipative dielectric layers are assumed to be of equal. Right: The transmission coefficient for a layered stack of positive and negative dielectric layers when the positive media are optically amplifying.

5) Negative refraction effect in photonic crystals with positive refractive index (MIT, IC and UCSD)

The possibility of negative refraction at all angles of incidence by using photonic bandgap materials has been investigated. This negative refraction effect occurs for a photonic crystal with effective positive refractive index but negative effective photon mass in the band. It does not involve negative group velocity. The concept involves the convex shape of the equi-frequency surface at the M-point for a 2D square lattice of cylinders. An incident wave then couples to a single Bloch mode

that propagates into the crystal on the negative side of the boundary normal. This enables the design of a novel microlens in the form of a slab of such a material.

ob ject

image

ne gative

refrac tionmedium

Figure 10. A superlens can focus a point object into a point image without needing to be convex.

During this past year, Prof. Joannopoulos’ group at MIT, collaborated with Prof. Pendry at IC have also collaborated and begun investigating a new direction of research. Negative refraction of electromagnetic waves has become of great interest recently because it is the foundation for a variety of novel phenomena, including a super-lensing effect (as shown in the figure below) that can potentially overcome the diffraction limit inherent in conventional lenses. These phenomena

have been described in the context of an effective medium theory with negative permittivity and negative permeability. To explore the possibility of negative refraction at optical frequencies, they have turned to photonic crystal structures as interesting alternatives. Recent work by Notomi indicates that negative refraction phenomena in photonic crystals are possible in a limited sense in regimes of negative group velocity and negative effect index. In a collaborative effort between MIT, IC, they have recently shown that single-beam negative refraction in photonic crystals is possible for all incoming angles in a regime of positive effective index of refraction. In particular, they focus on the lowest photonic band near a Brillouin zone corner furthest from Γ.

Interestingly, this band has a positive group velocity but a negative photonic “effective mass”. The frequency range is chosen so that for all incident angles, one obtains a single negative-refracted beam. They call this effect All-Angle Negative Refraction (AANR) and have determined the set of sufficient criteria for its observation. To illustrate this phenomenon, we succeeded in designing and numerically simulating a photonic crystal micro-superlens. The results are shown in Figure 6. They are currently working with the Chen and Schultz/Smith groups

Figure 11. An oscillating point diploe on the left can focussed to an oscillating point image on the right using a parallel-sided photonic crystal superlens.

at UCSD to fabricate this photonic crystal superlens and experimentally demonstrate its performance. It is believed that this work could help pave the way towards realization of all-angle negative refraction at optical frequencies and they are working with our experimental colleagues Chen, Smith and Schultz in order to realize this.

6) Theoretical demonstration of the negative refraction effect at an interface for a beam of light with finite width (IC and UCSD)

A modulated Gaussian beam has been shown to undergo negative refraction at an interface between positive and negative refractive media. While the negative refraction of the beam is clearly negative, the interference fronts due to the modulation appear to suggest positive refraction. This is because the velocity of the interference fronts is not coincident with the group velocity in the negative medium due to the dispersive nature of such media. This apparent positive refraction for the interference fronts masks the underlying negative refraction for a plane wave and becomes obvious only when a beam with finite transverse width is considered.

Publications during this year: 1. J.B. Pendry, “Replies to comments on ‘Negative refraction makes a perfect

lens’”, Phys. Rev. Lett. 87, 249702 (2001); 249704 (2001).

2. D.R. Smith, D. Schurig, M. Rosenbluth, S. Schultz, S.A. Ramakrishna and J.B. Pendry, “Limitations on Sub-Diffraction Imaging with a Negative Refractive Index Slab”, J. Phys. Condens. Matter (Submitted).

3. D. Schurig, D.R. Smith, S. Schultz, S.A. Ramakrishna and J.B. Pendry, “Focussing properties of a negative refractive slab”, (To be submitted).

4. S.A. Ramakrishna, J.B. Pendry, D. Schurig, D. R. Smith and S. Schultz, “The asymmetric lossy near-perfect lens”, J. Mod. Optics, In Press.

5. S.A. Ramakrishna, J.B. Pendry, M.C.K. Wiltshire and W.J. Stewart, “Imaging the near field”, J. Mod. Optics (Submitted).

6. S. Anantha Ramakrishna and J.B. Pendry, “Optical amplification removes absorption and improves resolution in a near-field lens”, (To be submitted).

7. Luo, S.G. Johnson, J.D. Joannopoulos and J.B. Pendry, “All angle negative refraction without negative effective index”, Phys. Rev. B 65, 201104(R) (2002).

8. D. R. Smith, J.B. Pendry and D. Schurig, “Refraction of a modulated Gaussian beam into a medium with negative refractive index”, (Submitted).

Defect Engineering During the past year, Prof. Joannopoulos’ group at MIT has also

been investigating the possibility of exploiting the ability of photonic crystals to control the flow of light in truly unique ways, in order to design point-defects in photonic crystals as basic building blocks for engineering metamaterials with

novel dielectric and magnetic responses. In fact, point-defects in photonic crystals consisting of dielectrics can be designed to have single mode resonance properties that can be either primarily electric field or magnetic field in nature. In order to optimize a defect state’s electric field or magnetic field nature, it is preferable to work in a photonic crystal environment that can distinguish, as much as possible, between TM-like modes (where the electric field will be of primary interest) and TE-like modes (where the magnetic field is of primary importance). A natural way to distinguish between TM-like modes and TE-like modes is to employ 2D photonic crystal slab systems, two examples of which are illustrated in Fig. 12.

Figure 12. Examples of 2D photonic crystal slab geometries. In particular, it is now known that 2D slab configurations involving dielectric rods in air are useful for isolating TM-like modes, while 2D slab configurations involving air holes in dielectric are useful for isolating TE-like modes. Unfortunately, point-defects in such 2D slab configurations are intrinsically lossy because of their coupling to photon modes.in the radiation manifold. Recently, however, the MIT group succeeded in

Figure 13. The new 3D photonic crystal and associated bandstructure

designing a new 3D photonic crystal that possesses a large omnidirectional photonic band gap (up to 27% using Si at 1.5 microns) and consists of alternating stacks of the two basic 2D slab configurations discussed above. The new 3D photonic crystal and its associated bandstructure are shown in Fig. 13. The key advantage of this structure is that point-defects can now be introduced systematically in the layers consisting of dielectric-rods-in-air (to create electric

moments) or in the layers consisting of air-holes-in-dielectric (to create magnetic moments), without incurring intrinsic radiation losses due to the lack of a complete photonic band gap. They have recently performed calculations of prototype point-defects for both types of slab layers. The results are shown in Figs. 14 and 15. Note, that Fig. 14 describes an electric dipole, while Fig. 15 describes what is in effect a magnetic dipole.

Figure 14. A point defect-state behaving like an electric dipole moment.

Figure 15. A point defect-state behaving like a magnetic dipole moment.

Proceeding in this fashion one can envision incorporating 1D, 2D, or 3D arrays of such defects in this photonic crystal host, leading to the design of new metamaterials with potentially novel permittivities or permeabilities. Of course the frequency of operation of the metamaterial will be dictated by the photonic band gap, which can be easily designed to operate anywhere between the visible and the far infrared. This approach is closely linked with and complements the work of Pendry using metallic systems. Indeed, the incorporation of metallic components to a photonic crystal system would enable the design of electromagnetic moments that could operate at wavelengths much longer than the scale of the periodicity.

-1 1Hz

-1 1Ez

In the coming year we plan to continue these efforts and design defect structures that emulate magnetic dipoles that can be fabricated and measured by our experimental colleagues Chen, Schultz and Smith.

Artificial Plasmonic Media The ultimate goal of this research direction of Prof. Yablonovitch’s group is the application of plasmon-assisted phenomena in the microwave frequency range. Near-field imaging with a super lens is but one phenomenon achievable with plasmonic media. Demonstration and application of plasmonic behaviour at microwave frequencies requires the development of a metamaterial with a suitable dispersion resembling that of a conventional plasma. They implement such an artificial medium with a periodic metallic mesh loaded with dielectric. The primary challenge however is to produce a plasmonic metamaterial that is both compact and lightweight, and thus suitable for potential use outside of a research laboratory. Two research milestones towards their ultimate goal were thus set for this year. The first milestone for the year was the conception and design of a practical plasmonic structure. The required criteria were the following: i) the design must be truly three-dimensional, behaving as a plasma for all angles of incidence and polarization ii) the period of the structure a must be a fraction of the resonant wavelength λ iii) the filling fraction of both metal and dielectric must be minimized to reduce weight (a) (b) Fig.16 – Plasmonic metamaterial design. In (a), two layers of the structure are illustrated with color representation: green, vertical rods; red, transverse strips; and grey scale, capacitive ribbons. In (b), a top view of one layer illustrates the transverse strips and etch pattern of the capacitive ribbons, forming the resonant loops in the metamaterial. A lattice constant of 12.7 mm and a 4 pF capacitance is expected to yield a resonant frequency of order ~1GHz.

Prof. Yablonovitch’s group has designed a metallic mesh formed of interlocking rods through which capacitive ribbons are wound. The capacitive loading reduces the resonant frequency and dielectric filling ratio as required. Fabrication proved impractical due to the severe deflection from stress relief in the milled metallic rods. The difficulty in fabrication of the metallic mesh due to non-ideal material behaviour was not fully appreciated. An alternative design strategy, satisfying all the stated criteria was finally formulated as illustrated in Fig 16. A series of vertical, copper plated rods are bound together by perforated, transverse, copper strips, thus establishing a metallic mesh. The capacitive ribbons are fabricated with conventional circuit lithography techniques to define capacitor plates upon a flexible, 25 µm polyimide substrate. The second milestone for the year was the experimental demonstration of an omni-directional plasmonic response from the metamaterial. At the time of this writing, components for the metamaterial design are in the process of fabrication, thus precluding experimental demonstration. The work reported here is a logical continuation of previous work in the field by MURI team members. Research groups led by Prof. E. Yablonovitch and Prof. J.B. Pendry performed initial investigations of plasmonic metamaterials. Furthermore, Prof. J.B. Pendry’s theoretical investigation of plasmon-assisted imaging is a strong motivating factor for our development of metamaterials with plasmonic behaviour.

Thin Silver Films: With improved uniformity of grating fabrication, Zhang’s group repeated the measurement of evanescent field amplification through a thin silver film using grating couplers. Counter-intuitively, the coupling efficiency of the grating coupler to s-polarization is found to be higher than p-polarization. Furthermore, the thickness dependence of enhancement factor as extracted from the grating coupling measurement cannot confirm the exponential growth of evanescent field in silver film. The result is explained by strong modulation of the deep grating to transmitted field. Alternative coupling mechanism is being devised and tested. Following the suggestions from Prof. Pendry, Zhang’s group deposited multilayer Ag/SiO2 film. In quasi-static limit, these multilayer film samples are expected to exhibit significant anisotropy in εx and εz. The anisotropy of the effective permittivity is being examined by ellipsometric measurements, and the values will be applied to testify the enhancement factor of evanescent field in such a stratified lens.

Electromagnetic Characterization and Devices

Metamaterial Characterization With the help of Basov's group in UCSD, Zhang’s group has measured the normal reflectivity of two sets of artificial plasma wires with 0.5-5THz activities. The reflection curve is in good accordance with a plasma edge from 0.5 to 0.7 THz. Above the plasma edge, additional resonance are found with their higher order harmonics, which is likely related to the shunt resonance of wire length of 1mm. Polarized reflection reveals significant anisotropy of permittivity on the directions along and normal to the wires. Further simulation designs are in progress to improve the extinction ratio below and above the plasma edge.

Metamaterial Devices

1) Introduction of a TL approach of LH materials

A TL approach of LH materials has been developed. This approach is based on the dual of the conventional right-handed (RH) line, the infinitesimal model of which is shown in Fig. 18 for the lossless case. In this line, left-handedness is straightforwardly understood from the propagation constant β(ω) =-j√(Z’Y’) = -1/ (ω√LC), which yields the antiparallel velocities vp = -ω2√LC < 0 and vg = +ω2√LC > 0. This line, in which the characteristic impedance is still the same as in the conventional line Zc = √(L/C), exhibits equivalent negative parameters ε(ω) = -1/ (ω2L) and µ(ω) = -1/ (ω2C), which can be shown to satisfy the entropy condition and to be associated with a negative index of refraction n = -√(εrµr). This LH-TL has been fully characterized and systematically compared to the RH-TL. The LH-TL is fundamentally of high-pass type. The zero-reference of transmission phase is located at ω=∞, instead of at ω=0 (RH case), and increases then parabolically to infinity as frequency decreases to DC. From the expression of β(ω), it is seen that the guided wavelength is proportional to frequency in a LH-TL, as a consequence of the low-pass to high-pass transformation, which means that the frequency axis is reversed with respect to the RH case.

( )CjZ ′=′ ω1

( )LjY ′=′ ω1pass-high

length dz

( )CjZ ′=′ ω1

( )LjY ′=′ ω1pass-high

length dz

Fig. 18: Infinitesimal model of the (lossless) LH-TL.

Fig. 21: Prototype of the artificial microstrip LH-TL.

2) Realization and characterization of a lumped-element (LE) LH-TL

An artificial LE-LH-TL has been investigated. This line can be realized by cascading N high-pass sections corresponding to the model of Fig. 18. The resulting circuit is a high-pass filter, with cutoff ωc =1/(2√(LuCu)) where Lu/Cu are the inductance/capacitance of a unit-cell, which perfectly mimics an ideal (non-existing) LH-TL above cutoff. Figs. 19 and 20 represent the typical behavior of such a line (N=20). Fig. 19 shows that lossless transmission can be achieved over an unlimited bandwidth along the LE-LH-TL. Fig. 20, obtained by unwrapping the phase of S21, demonstrate an excellent agreement above cutoff between the dispersion curves of theory and of the LE realization for a sufficient N (N>10p/(2π)2f√(L’C’),where p is the length of the corresponding artificial line in a particular technology). Left-handedness was also demonstrated by probing time-responses at different points of the circuit using a transient analysis.

ep1(GHz)frequency ∝

1−= NN peaks

ionapproximat)(cutoff cf

ωλωλ

1!!!

∝=⇒∝

ppe

> lossless> unlimited BW

parameters-S of Magnitude

ep1(GHz)frequency ∝

1−= NN peaks

ionapproximat)(cutoff cf ionapproximat)(cutoff cf

ωλωλ

1!!!

∝=⇒∝

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> lossless> unlimited BW

parameters-S of Magnitude

( ) ( )CL ′′−= ββω 1

)unwrapping phase by (obtained

Nω-β vs.diagram

)m1( β

( ) ( )CL ′′−= ββω 1

)unwrapping phase by (obtained

Nω-β vs.diagram

)m1( β Fig. 19: S-parameters for the LE-LH-TL Fig. 20: Dispersion diagram for the LE-LH-TL

3) Microstrip Implementation of the LH-TL

The LE-LH-TL was then implemented in a quasi-lumpded microstrip form. A prototype of this structure, including 7 unit cells, is shown in Fig. 21. The series capacitances are provided by interdigital capacitors, while the shunt inductances are provided by shorted stub inductors. These reactive components are naturally strongly dispersive (hence “quasi”-lumped) and will therefore impose a high-frequency limit, fmax, in addition to the low-frequency limit, fmin, which

input through

coupledisolated

1

3

2

4

input through

coupledisolated

1

3

2

4

Fig. 24: LH forward coupler prototype

corresponds to the frequency below which the lumped-elements approximation is not valid (slightly higher than cutoff). In the present design, Lu=4.62 nH and Cu =2.06 pF at f=1.5 GHz.

The measured S-parameters of the resulting high-pass filter are shown in Fig. 22, where the estimated LH range is of the order of 100 %. Although the structure is a poor highpass filter, it exhibits reasonable insertion losses, smaller than 5 dB. Fig. 23 shows the measured dispersion diagram, obtained by unwrapping the phase of S21, along with the theoretical curve of the ideal LH-TL, ( ) ( )''1 CLωωβ −= , for comparison. Although some discrepancies, expected from the dispersive nature of the interdigital capacitors and stub inductors, are noticeable, a fairly good agreement with theory is obtained in the LH range.

range LH

parameters-S of Magnitude

(GHz)frequency

range LH

parameters-S of Magnitude

(GHz)frequency

( ) ( )CL ′′−= ββω 1

cutoffminf

maxf

range LH

(1/m) β

diagram - βω

( ) ( )CL ′′−= ββω 1

cutoffminf

maxf

range LH

(1/m) β

diagram - βω

Fig. 22: S-parameters for the microstrip LH-TL Fig. 23: Dispersion diagram for the

microstrip LH-T

4) A Microstrip Forward Coupler

A microstrip forward coupler, consisting of two parallel LH-TLs, as shown in Fig. 24, was then investigated. The coupler was analyzed using a LE 4-ports equivalent circuit, reduced to the problem of 2 complementary 2-ports networks through the even/odd modes decomposition technique. Fig. 8 demonstrates that the reversal of the frequency axis with respect to the RH case is also true for this

coupler structure, although the coupling capacitance between the lines is still of the same nature that in the conventional case (capacitive edge-coupling through transverse evanescent fields). Theoretically, coupled/through power transfer can be achieved down to DC, and therefore complete coupling can be obtained at very low frequencies (i.e. small coupling length); however, this necessitates a large number of cells N, to push cutoff to correspondingly small frequencies. Fig. 25 verifies that the parabolic behavior characteristic the left-handedness is present in both the through and coupled signals. Despite important losses, due to mutual coupling between adjacent cells, the microstrip structure showed a similar behavior, as well as an importantt reduction of the coupling length (around 4 times) in comparison to the conventional forward coupler.

Magnitude, LE-ckt model

0 1000 2000 3000 4000 5000 6000 7000-40

-35

-30

-25

-20

-15

-10

-5

0

Frequency (MHz)

S-P

aram

eter

s (d

B)

S11S21S31S41

approx.fmin

dBf321S

31S

0fcf

Magnitude, LE-ckt model

0 1000 2000 3000 4000 5000 6000 7000-40

-35

-30

-25

-20

-15

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0

Frequency (MHz)

S-P

aram

eter

s (d

B)

S11S21S31S41

approx.fmin

dBf321S

31S

0fcf

2 4 6 8 10Frequency (GHz)

( ) ( )ωλ

ωβπλωωβ

12

1

∝=⇒∝=

′′−=

pp

CL

e

g

0f

21S

31Sapprox.fmin

cf

Phase, LE-ckt model

2 4 6 8 10Frequency (GHz)

( ) ( )ωλ

ωβπλωωβ

12

1

∝=⇒∝=

′′−=

pp

CL

e

g

0f

21S

31Sapprox.fmin

cf

Phase, LE-ckt model

Fig. 25: S-paramaters for the coupler Fig. 26: Dispersion diagram for the coupler

5) An Active Retrodirective Meta-Surface

Finally, we quitted the field of LH materials to develop a novel meta-material, which is in fact an “meta-surface” in the form of an active retrodirective array antenna, using field effect transistors (FETs) to achieve the required non-linear susceptibility necessary for retrodirectivity. This meta-surface is characterized by a surface impedance (inhomogeneous and anisotropic), which perfectly reflects an incoming signal toward the source, as shown in Fig. 27, instead of away from the

Fig. 27: Principle of a retrodirective meta-surface Fig. 28: Phased-array implementation

source on the other side of the normal, whatever the nature/position of the source and type of propagation medium (e.g dispersive). This operation is achieved through phase reversal, which is also called phase conjugation, of the incoming signal. In the phased-array implementation of the surface, each array element acts as a non-linear “dipole” of a material (such as Kerr), and operates phase conjugation through the use of an LO (local oscillator) with frequency twice that of the incoming signal (using the conventional difference modulation term), as shown in Fig. 28.

A balanced-architecture 4-elements prototype, including matching circuits, appropriate delay lines and FET biasing circuitry, is shown in Fig. 29. The interest of the balance architecture proposed is that it automatically absorbs the incoming RF and reradiate the desired IF (intermediate frequency), as well as provides excellent LO-IF leakage isolation, by judicious combinations of phases in the two channels. To obtain a more compact structure, slightly different RF and IF frequencies were used, which allows incoming RF and outgoing IF to share the same polarization. A great advantage of this active implementation is that it provides conversion gain through the FETs and allows the radiation of information through LO modulation. The bistatic measurement of Fig. 30, where a transmitting antenna (source) illuminates the array at a fixed angle while the receiving antenna is rotated, demonstrate the excellent retrodirectivity of the meta-surface designed.

Fig. 29: The balanced retrodirective array

Fig. 30: Bistatic RCS measurements for the balanced retrodirective array

4 Synergy and Interactions

During this year, there have been many formal and informal interactions among members of our MURI team. For instance, we have had two teleconferences to exchange with each other with new results. Prof. J.B. Pendry visited the UCSD and UCLA groups in November 2001, and S.A. Ramakrishna visited the UCSD group and Prof. Zhang’s group at UCLA in February 2002, for discussions, interactions and to finalize publication of results. We have also collaborated with colleagues outside our team such as Prof. Basov at UCSD.

5 Efforts and Milestones Planned for Next Year

Metamaterial Synthesis Prof. Zhang’s group will continue working on the projection micro stereolithography system (PµSL) to raise processing speed so that larger sample size can be made in a reasonable period of time. The target for 2nd year is to reach sample size >6mm (about 50 to 100 wavelength) with feature size down to 1µm. Meanwhile, the main effort of synthesis research will be developing nanofabrication methods to meet the demand of submicron features in THz up to optical frequencies. At these frequencies, the mold transfer process is more challenging. Therefore they will continue working on molding through sol-gel and particle-infiltration methods. On the other hand, they will continue working on the plasmonic wire filter design and fabrications. Elastomer PDMS will be employed as substrate for reconfigurable devices. Prof. Chen’s group plans to continue working on the 2D micro bubble arrays. They will also collaborate closely with Prof. Joannopoulos’ group to fabricate designed photonic crystals with negative refraction and to demonstrate all-angle negative refraction at high frequencies. New Physics of Metamaterials Negative Refraction and Perfect Lens Prof. Pendry’s group plans to continue their studies of how to design new materials with better performance. Mainly this means minimizing the effects of loss in the system. The concept of a layered medium has been very fruitful in this respect, and will be explored further. In addition the introduction of amplifying media has been an important step forward and they plan to explore further the viability of this concept as applied to meta materials operating in the optical region of the spectrum. Secondly, Prof. Pendry’s group shall develop further the application of negative refractive index materials to controlling sub wavelength radiation. The mapping that they have developed between layered structures and a ‘near field optical fibre’ (see my report

at the June 2002 MURI meeting) is particularly promising in this respect. Another avenue of exploration which has opened is the use of cylindrically shaped meta materials as magnifying lenses. We are only just beginning to understand these new structures and more work needs to be done on their performance. We believe that the work on Negative Refraction in Photonic Crystals that Prof. Joannopoulos has suggested could help pave the way towards realization of all-angle negative refraction at optical frequencies. Prof. Joannopoulos’ group is working with our experimental colleagues Chen, Smith and Schultz in order to realize this. In the coming year we plan to extend these ideas to 3D photonic crystal systems Artificial Plasmonic Media The immediate goal is to complete an experimental demonstration of omni-directional plasmonic behaviour of the metamaterial design. Prof. Eli Yabolonovitch’s group anticipates the tuning of capacitance to bring the plasma frequency to a convenient frequency for further experimental work. The primary information we hope to gleam from the experimental demonstration is the quality of the resonance at the plasma frequency of the structure. A steep roll off in transmission is desired on the long wavelength side of the plasma frequency, but the slope is likely to be limited by the uniformity in capacitance and metallic mesh from cell to cell. The next stage of research will focus on the application of the omni-directional plasmonic medium to investigations of near field imaging. Near field imaging experiments with a slab of plasmonic metamaterial will provide insight into practical issues affecting image resolution and insertion loss. The reflection, and thus insertion loss, due to the mismatch of the wavefront outside and inside the plasmonic metamaterial is but one effect that warrants experimental investigation. Microwave frequency experiments will provide information applicable to plasma-assisted phenomena at higher frequencies as well, where experiments are more difficult to perform. Defect Engineering In the coming year, Prof. Joannopoulos’ group plans to continue their efforts on defect engineering and design defect structures that emulate magnetic dipoles that can be fabricated and measured by our experimental colleagues Chen, Schultz and Smith. Metamaterial Characterization and Metamaterial Devices Prof. Zhang’s group plans to continue characterizing more designed and fabricated metamaterials, such as metal plasmonic wires, split-ring resonators. In collaborating, they are also going to develop a near-field probing characterization method to determining ε and µ of the metamaterials. Optimization of scattering parameters/phase advancement should be attempted on the design of split-rings, wires and other new candidates of metamaterials. Special efforts will be focused on study of chirality and high frequency magnetism. Finally, they will attempt the first set of experiments on examining the near-field imaging using slab superlens.

Prof. Itoh’s group plans to extend the transmission line approach of LH materials to non-uniform LH-TLs, and study the leakage radiation in the LH-TL so that they can develop

novel types of a scanning leaky antenna with backward radiation characteristics. They are also planning to study dynamically-controllable PBG meta-structures for millimeter-wave applications