Method of manufacture of a metasurface

Abstract

The present invention relates to a new method for making metasurfaces comprising liquid gating.

Claims

1. Method for making metasurfaces comprising the steps of: I. providing a substrate which exhibits at least two distinct measurable states of an optical property and which can stably but reversibly be transitioned from i. a first state of the optical property into at least ii. one second state of the optical property which is measurably distinct from the first state, by a change in the chemical composition of the substrate II. creating on a surface of the substrate at least one area i. outside of which the substrate is in the first sate of the optical property, and ii. inside of which the substrate is in one of the at least one second state of the optical property, or iii. wherein the states of the optical property of the inside area and the outside area are inverted, and III. wherein step II further comprises defining and delineating a desired-to-be inside area on the surface of the substrate which is in the first state of the optical property and subsequently contacting the surface in the defined and delineated inside area with a liquid which reacts with the substrate material to yield in or to bring about the transition into the at least one second state of the optical property within the inside area only, or wherein in step III the states of the optical property of the surface and the inside area are inverted.

2. The method of claim 1, wherein the optical property is selected from one or more of refraction, diffraction, extinction, scattering, absorption, reflection, polarization or transmittance.

3. The method of claim 1, wherein the substrate material is VO.sub.2, WO.sub.3, MoO.sub.3, SrCoO.sub.3 or SrTiO.sub.3.

4. The method of claim 1, wherein the substrate has a thickness in the range of 1-100 nm.

5. The method of claim 1, wherein the liquid brings about a change in the chemical composition of the substrate material.

6. The method of claim 1, wherein the transition is instigated by applying an electric potential to the liquid.

7. The method of claim 6, wherein the electric potential is 0.1-10 Volts.

8. The method of claim 1, wherein the liquid is selected from one or more of water, an alcohol, a hydrocarbon, or a halogen containing hydrocarbon.

9. The method of claim 1, wherein the liquid is an ionic liquid.

10. The method of claim 9, wherein the ionic liquid is selected from one or more of 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI), 1-Propyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide, 1-Butyl-3-methylimidazolium bis(tri-fluoromethylsulfonyl)imide and 1-Hexyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide.

11. A metasurface manufactured according to the method of claim 1, wherein said metasurface controls a wavefront of electromagnetic waves.

12. A planar lens, vortex generator, beam deflector, axicon, electromagnetic absorber, polarization converter or spectrum filters, for wireless communications, energy harvesting, imaging, or cloaking comprising the metasurface of claim 11.

13. The method of claim 3, wherein the substrate material is VO.sub.2.

14. The method of claim 4, wherein the thickness is in the range of 5-50 nm.

15. The method of claim 4, wherein the thickness is in the range of 7-15 nm.

16. The method of claim 7, wherein the electric potential is 1-5 Volts.

17. The method of claim 7, wherein the electric potential is 2-4 Volts.

18. The method of one of claim 8, wherein the alcohol is ethanol or methanol; the hydrocarbon is pentane or hexane, and the halogen containing hydrocarbon is chloroform.

Description

BRIEF DESCRIPTION OF FIGURES

[0049] FIG. 1

[0050] a Electron beam lithography patterning of boomerang-shaped antennas array on VO.sub.2/TiO.sub.2.

[0051] b Ionic liquid gating on VO.sub.2/TiO.sub.2 with resist mask on the top.

[0052] c Metasurface of boomerang-shaped gated VO.sub.2 array in initial VO.sub.2 matrix after removal of ionic liquid and resist.

[0053] d Resistivity-temperature curves for VO.sub.2 films of initial and gated states.

[0054] e Refractive index and

[0055] f extinction coefficient of initial (300 and 330 K), and gated 20 nm VO.sub.2 thin film (+3V 0.5 hour, 300K).

[0056] FIG. 2

[0057] a Representative conductive atomic force microscope image,

[0058] b Amplitude and

[0059] c phase of near-field optical microscopy images,

[0060] d Amplitude and

[0061] e phase of simulated near-field maps for one period of the gated VO.sub.2 boomerang-shaped antennas. The periodic length of the array is 16 μm.

[0062] f The extracted near-field phase at the right-most tip of the boomerang-shaped antennas (blue circles from experimental results c while black square from simulations results e) across one period. The black dashed line is the theoretical phase change at different positions.

[0063] FIG. 3

[0064] a Schematic experimental setup for reflection measurements. The incidence angle (θ.sub.i) and reflection angle (θ.sub.r) are marked by black and red arrows. The intensity mapping (θ.sub.r versus θ.sub.i) of far-field reflection for VO.sub.2 thin films

[0065] b without and

[0066] c with ionic liquid gating through resist mask at a λ.sub.0=8.05 μm. The dot lines indicate the theoretical prediction of the peak position using the generalized Snell's law of different orders (N=0, ±1, ±2, and ±3).

[0067] d Typical simulated phase distribution of x-polarized electric field (ϕ.sub.Ex) under the illumination of a normally incident y-polarized EM wave. The corresponding positions of simulated results are marked by red circles in c.

[0068] FIG. 4

[0069] Conductive atomic force microscope images of VO.sub.2 after ionic liquid gating through boomerang-shaped antennas array resist mask with different periodic lengths:

[0070] a 40 μm and

[0071] b 8 μm.

[0072] The intensity mapping (θ.sub.r versus θ.sub.i) of far-field reflection for VO.sub.2 after ionic liquid gating through boomerang-shaped antennas array resist mask with different periodic lengths:

[0073] c 40 μm and

[0074] d 8 μm.

[0075] The dot lines indicate the theoretical prediction of the peak position using the generalized Snell's law of different orders.

[0076] e The anomalous reflections (N=−1) for VO.sub.2 metasurface with different periodic lengths. The incidence angle is fixed at 45°. The dashed lines indicate the position of theoretical anomalous reflection positions.

[0077] FIG. 5

[0078] Sketch for the resist mask

[0079] FIG. 6

[0080] CAFM images of VO.sub.2 after patterned ionic liquid gating at varied temperatures

[0081] FIG. 7

[0082] Sketch for the near-field simulation. The TiO.sub.2 substrate is only partly shown.

[0083] FIG. 8

[0084] a Amplitude and

[0085] b phase of near-field optical microscopy images,

[0086] c Amplitude and

[0087] d phase of simulated near-field maps for one period of the gated VO.sub.2 boomerang-shaped antennas. The periodic length of the array is 16 μm. The angle between the polarization of incident light and x (or y) axis is 45°.

[0088] FIG. 9

[0089] a Sketch for the far-field simulation.

[0090] b-n Simulated Ex field patterns on the x-z plane for a model consisting of metallic V-shape VO.sub.2 array in insulating VO.sub.2 matrix under the illumination of a y-polarized EM wave at different incidence angles from 90° to −30°. The scale bar is 8 μm.

[0091] The invention will now be described by way of an example.

Example

[0092] Based on ILC technology according to the present invention, electron beam lithography was used to fabricate an array of boomerang-shaped holes on the resist coated on the surface of 20 nm VO.sub.2 thin films grown on pure as well as a Nb-doped TiO.sub.2 (001) substrate (see FIG. 1a). VO.sub.2 films on Nb-doped TiO.sub.2 are used for the conductive atomic force microscopy (CAFM) measurements, while VO.sub.2 films on pure TiO.sub.2 are used in all the other measurements. On different samples, the feature sizes and periodic lengths of these boomerang-shaped holes change from 100 to 500 nm and 8 to 40 μm, respectively. Such a boomerang-shaped antenna array is chosen to achieve the phase coverage of 2π while maintaining large scattering amplitudes. Secondly, a drop of ionic liquid [EMIM-TFSI, (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide] is applied to the surface of the sample with a resist mask and an isolated gold electrode. Then a gate voltage of +3 V is applied for 0.5 hour between the gold electrode and the VO.sub.2 layer under vacuum conditions (pressure better than 6×10−5 mbar) (see FIG. 1b). Finally, the ionic liquid gated boomerang-shaped array is achieved by removing the ionic liquid and resist (see FIG. 1c).

[0093] The change of transport properties caused by ILG is measured in a Hall bar device with a lateral gate electrode and shown in FIG. 1d. The initial (as grown) VO.sub.2 thin film of 20 nm grown on TiO.sub.2 (001) substrate shows the MIT transition (metal-insulator transition) with a resistance change of around four orders of magnitude at ˜310 K, which is lower than that of the bulk state (˜340 K) because of the tensile strain (“a” lattice parameter TiO2=4.59 Å, “a” lattice parameter VO.sub.2=4.55 Å). Differently, a progressive suppression of the MIT in gated VO.sub.2 is observed and the metallic state is kept in the temperature range of 200-340 K. The conductivities of the initial (warming branch) and gated VO.sub.2 are 1.83×10.sup.−4 Ωcm and 9.92×10.sup.−1 Ωcm at 300 K, respectively. The metallization is a consequence of the electric-field-induced extraction of oxygen ions during the gating.

[0094] The optical constants of the VO.sub.2 layer were determined before and after the gating process using an ellipsometer. For this measurement, a 10×10 mm.sup.2 thin film sample was used and after the measurement of the initial state, the sample was gated without any resist mask. FIGS. 1e and f show refractive indices and extinction coefficients of VO.sub.2 thin films as a function of wavelength in the initial and gated (gating voltage (V.sub.G)=+3 V) states, respectively. The refractive indices of initial and gated VO.sub.2 are very close to each other between 200 and 450 nm. For larger wavelengths, the refractive index of gated VO.sub.2 first decreases from 2.92 to 1.51 and then dramatically increases to 7.53 at 10 μm. Whilst the refractive index of initial (ungated) VO.sub.2 only shows a gradual decline from 2.64 to 1.98 as the wavelength is changed from 2 to 10 μm. The extinction coefficient of gated VO.sub.2 monotonously rises between 600 nm and 10 μm to a value of 10.22, while the ungated VO.sub.2 shows an extinction coefficient close to zero for the whole spectral range. For VO.sub.2 thin film of initial state at 330 K, which is higher than the MIT transition temperature, a very similar refractive index and extinction coefficient spectra was found compared with those of the gated one, confirming the changes of optical constants in the gated sample are related to ILG induced insulator-to-metal transition.

[0095] Subsequently, the ILG was done on the sample through the resist mask to make an array of boomerang-shaped antennas inside the VO.sub.2 layer.

CAFM Measurement on a Sample Grown on Nb-Doped TiO.SUB.2 .(001) Substrate

[0096] After the gating and cleaning process, the sample was affixed to a metal holder by conductive silver paint on the bottom. In this measurement, a 1 MΩ resistor was connected in series with the VO.sub.2/Nb-doped TiO.sub.2 heterostructure and the CAFM was used to measure the current flowing across the sample (perpendicular to the surface) at a constant voltage of 1 V. The leakage current mapping shows one period of boomerang-shaped metallic VO.sub.2 regions in the insulating VO.sub.2 matrix with a periodic length of 16 μm. There is clear evidence that only within the boomerang-shaped holes there are changes in the film conductivity, with a substantial increase in leakage current while the regions covered by resist are still insulating after ILG (see FIG. 2a). The highly face-dependent gating effect only allows the out-of-plane oxygen extraction without notable in-plane diffusion to create sharp metallic VO.sub.2 regions in the boomerang-shape. These boomerang-shaped conductive regions act as antennas just like those in conventional metasurfaces that are formed from, for example, Au regions on or within a Si host. What is necessary is that the antennas are formed from materials with distinct dielectric coefficients for the relevant wavelength. The temperature dependent CAFM results show that the conductive antennas become inactive at 330 K but recovered by cooling the sample down (see FIG. 6). The corresponding topography images (not shown) indicate that the conductive regions after gating are slightly higher than the matrix due to the lattice expansion after introducing oxygen vacancies via ILG. For clarity, the metasurface in x-y plane is defined and the unit cell of boomerang-shaped antennas repeat with the periodicity of Γ in the x direction, while the direction vertical to the metasurface is set to be z. These definitions are applicable to the following geometries of all the measurements.

s-SNOM Measurement on a Sample Grown on Pure TiO.sub.2 (001) Substrate

[0097] To investigate the phase change in the boomerang-shaped antennas, phase-resolved scattering-type scanning near-field optical microscopy (s-SNOM) is used. The experimental results are compared with simulations. A period of boomerang-shaped metallic VO.sub.2 regions in the insulating VO.sub.2 matrix with the same shape and size as those in CAFM measurement but grown on pure TiO.sub.2 substrate is used for the s-SNOM measurements. A y-polarized incident light beam [wavelength (λ.sub.0) of 6.2 μm] vertically illuminates the sample from the back side of a double side polished substrate (transmission mode) to get a fully in-plane polarization and resultant lossless phase gradient. Meanwhile, the near-field behavior of the structures is simulated using the completely same condition as that in the experiment (see FIG. 7). In this way, the amplitude and phase maps of near-field intensity (I.sub.z-exp.) and phase ((φ.sub.z-exp.) of the optical electric-field component along z direction for VO.sub.2 metallic structures are recorded for one period and shown in FIGS. 2b and c, respectively, along with corresponding simulations (I.sub.z-sim. and φ.sub.z-sim.) in FIGS. 2d and e. Both the experimental and simulated I.sub.z maps (FIGS. 2b and d) indicate that the metallic boomerang-shaped VO.sub.2 successfully support both symmetric and antisymmetric charge-oscillation eigenmodes under the y-polarized incident light (λ.sub.0=6.2 μm), which is similar to the previous works where gold boomerang-shaped antennas were employed. The results for the structures under purely symmetric and antisymmetric excitation modes are shown in FIG. 8.

[0098] The individual near-field phase (φ.sub.z-exp. and φ.sub.z-sim.) images in FIGS. 2c and e show gradients along the antenna arms, which are the characteristics of simultaneous excitation of both symmetric and antisymmetric antenna modes. The extracted near-field phases at the right-most tip of the boomerang-antennas from both experimental and simulated results (respectively marked by blue hole circles and black hole squares) exhibit a phase change from about −π to about π across the unit cell where the eight antenna data points are summarized in FIG. 2f. Such a realization of the phase shifts covering 2π range in one unit cell length provides a full control of the wavefront. Similar intensity and phase change can be also observed in the samples with a periodic length of 8 μm (not shown but similar to FIG. 2).

[0099] A −π to π phase change in one period of boomerang-shaped metallic VO.sub.2 antennas is expected to achieve anomalous reflections at far-field based on the generalized Snell's law:

[00001]$\begin{array}{cc}\sin {\theta }_r-\sin {\theta }_i=N\frac{{\lambda }_0}{2\pi n_i}\frac{d\phi }{\mathrm{dx}}& \left(1\right)\end{array}$

[0100] Where, the θ.sub.r is the reflection angle, θ.sub.i is the incident angle, λ.sub.0 is the vacuum wavelength, n.sub.i is the refractive index, and

[00002]$\frac{d\phi }{\mathrm{dx}}$

is the phase gradient. Here, N is an integer (0, ±1, ±2, ±3 . . . ) and stands for the order of reflection, in which N=0 stands for the ordinary reflection, while others for anomalous reflections. To firmly demonstrate the manipulated light propagation by a metasurface created through the patterned ILG according to the present invention, a far-field reflection measurement was carried out using a quantum cascade laser with λ.sub.0=8.05 μm. The geometry of the far-field measurement setup is schematically illustrated in FIG. 3a. The incident light is y-polarized while a polarizer is used to select the anomalous reflection beam that is supposed to be cross-polarized with respect to the excitation. The y-polarized incident light is chosen to get a 45° angle with respect to the symmetric axis of each antenna so that both symmetric and antisymmetric modes can be excited and the scattered light has a substantial component polarized orthogonal to that of the incident light with a large range of phases and amplitudes for a given wavelength. The

[00003]$\frac{d\phi }{\mathrm{dx}}$

is 2π/16 μm.sup.−1 and n.sub.i is set to be 1 for air.

[0101] In the θ.sub.r versus θ.sub.i for the reflection of initial VO.sub.2/TiO.sub.2 sample, only weak ordinary (specular) reflection is observed [suppressed by the orthogonal polarizer (relative to the incident light polarization) in front of the detector], suggesting the absence of metasurface before ILG (see FIG. 3b). However, the situation changes dramatically once the sample is gated through a resist mask as shown in FIG. 3c. Despites the weak, two strong anomalous reflection curves are observed, which agree well with the theoretical prediction of generalized Snell's law (Eq. 1) using different orders of −1 and 1 (black dotted lines). In addition, very weak reflection curves, which originate from higher order diffraction of the periodic antenna array, are also observed (indicated by ±2 and ±3 in FIG. 3c). Due to restrictions of the measurement setup, the angles marked with the gray area cannot be measured, because at these angles the incident light path is blocked by the detector. Note that the anomalous reflections observed here are caused by the formation of a VO.sub.2 metasurface rather than the diffraction effect as the latter one is suppressed in the cross-polarizer measurement.

[0102] To further confirm the origin of the anormalous reflection behavior from the metasurface, the first-order phase patterns (ϕ.sub.Ex) of x-polarized scattered field (E.sub.x) are simulated with representative incident angles from 90° to −30° scattered by the metasurface under the illumination of a y-polarized incident light (λ.sub.0=8 μm). The patterns in FIG. 3d clearly show that the incident light is anomalously reflected by the metasurface in three typical cases of θ.sub.i=0°, 30° and 60.sup.0 (marked by the red circles in FIG. 3c). Here the simulated reflection light is filtered by a polarizer orthogonal to the polarization of incident light to remove the ordinary reflection. Apparently, the incident lights with θ.sub.i=0°, 30° and 60° are all reflected nonspecularly (θ.sub.r≠θ.sub.i) with θ.sub.r=30.0°, 0° and 21.2°, respectively, which are in agreement with the experimental measurements as shown in FIG. 3c.

Leakage Current Mappings of VO.sub.2/Nb-Doped TiO.sub.2

[0103] In the following, a series of gated VO.sub.2 metasurface are studied with different Γ of 40 μm and 8 μm. Firstly, the leakage current mappings of VO.sub.2/Nb-doped TiO.sub.2 clearly show that boomerang-shaped metallic VO.sub.2 antenna arrays are successfully created in the insulating VO.sub.2 matrix with Γ of 40 μm and 8 μm (see FIGS. 4a and b), which are consistent with what is observed in the case of Γ=16 μm.

Far-Field Response of VO.sub.2 Metasurface Grown on Pure TiO.sub.2 Substrate

[0104] FIGS. 4c and d show the θ.sub.r versus θ.sub.i for the reflection of VO.sub.2 metasurface with periodicity of Γ=40 μm and 8 μm, respectively. Remarkably, both samples show anomalous reflections, which agree well with theoretical prediction of Eq. 1. Compared with the complex reflection mapping for the case of Γ=40 μm with diffractions, only anomalous reflections with orders of −1 and 1 could be observed in the case of Γ=8 μm. FIG. 4e summarizes the results of the ordinary and the anomalous reflections for samples with different Γ with a fixed incident angle of 45°. With the shrinkage of F from 40 μm to 8 μm, the anomalous reflections (N=−1) shift from 30.0° to −18.0° gradually. In theory, the sample with smaller F corresponds to the larger phase gradient and the resultant larger divergence between ordinary and anomalous reflections. The positions of anomalous reflections calculated by Eq. 1 are marked by black dotted lines in FIG. 4e, which agree with the experimental data.

[0105] It has thus been shown that ILG can induce an insulator-to-metal transition followed by dramatic changes in optical properties of VO.sub.2 thin film. By introducing a resist mask, such a metallic phase can be locally created in the insulating parent phase by ILG. The metallic VO.sub.2 antenna array with the insulating VO.sub.2 host material realizes a full 2π phase manipulation of the optical phase in the near-field and leads to anomalous reflections at far-field consistent with the generalized Snell's law. Such a spatially selective gating engineering represents a new paradigm for active photonic structures and devices.

Methods

Sample Preparation

[0106] The VO.sub.2 films of 20 nm thickness were deposited on (001) oriented pure or 0.5 wt % Nb-doped TiO.sub.2 substrate by pulsed laser deposition (PLD) in an oxygen pressure of 1.9×10-2 mbar at 380° C.

[0107] A boomerang-shaped antenna array was designed to excite both symmetric and the antisymmetric modes by y-polarized incident light for all the antennas. The boomerang-shaped antennas consist of two rectangular arms with width a, which are connected at the center of one end at an angle Δ. The symmetry axis ŝ of the first four antennas of the unit cell is oriented along the 45° diagonal between the x- and y-axes, and their opening angles are Δ=60°, 90°, 120°, and 180°, respectively. The second four antennas are copies of the first four with rotating clockwise by 90° (FIG. 5). Thus, the metasurface unit cell containing eight boomerang-shaped antennas will be able to introduce constant phase gradient 2π/Γ along the x-axis to the light scattered in cross-polarization, where Γ is the length of the unit cell in x-axis direction (periodic length). W is the width of the unit cell in y-axis direction. The a of arms in boomerang-shaped antennas is changed from 50 to 500 nm, with corresponding Γ and W change from 4 to 40 μm and 0.5 to 5 μm, respectively.

[0108] Such boomerang-shaped antennas hole arrays are made on the positive resist (ARP 6200.09, Allresist) using electron beam lithography (EBL) (Raith Nanofabrication system). After spin coating at 400 rpm, ˜200 nm resist was achieved on the surface of sample and then baked at 150° C. for 1 minute. After exposure at a dose of 150 μC/cm.sup.2, the sample was developed in AR 600-60 for 30 s. Before ionic liquid gating, the resin was etched using reactive ion etching (Plasmalab100, Oxford) to slightly remove ˜20 nm residual resist after development. Such a pattern could be made in a larger area of 4.8×3.2 mm.sup.2 for the far-field reflection measurements. One unit cell of boomerang-shaped antennas with resist pattern is schematically shown in FIG. 5.

[0109] The ionic liquid EMIM-TFSI [1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide], was used for all gating experiments. The IL gating was performed in a probe station (pressure<10.sup.−6 mbar) and VG=+3 V was applied for 30 minutes. After gating the IL and resist were removed using acetone and isopropanol. Transistor devices for transport measurements were prepared by photo-lithography and wet etching in the form of Hall-bars with lateral gate electrodes located in the vicinity of the channel. The channel is 400 μm long and 100 μm wide. Electrical contacts to the edge of the channel were formed from Au (60 nm)/Cr (10 nm) that was deposited by thermal evaporation.

Sample Characterization

[0110] The transport properties were carried out in a Quantum Design DynaCool. The CAFM function in a Cypher atomic force microscopy (Asylum Research) was used to measure the current flowing across the sample (perpendicular to the surface). A 1 MΩ resistor was connected in series with the VO.sub.2/Nb-doped TiO.sub.2 at a constant voltage of 2 V. A silicon tip with a Ti/Ir coating (Asyelec-01) was used (tip radius˜28±10 nm). The optical constants before and after gating were measured by ellipsometry (M-2000 and IR-VASE Ellipsometer from J. A. Wollam).

[0111] The Neaspec scattering-type scanning optical near field microscope (s-SNOM) utilizes a metal-coated AFM cantilever operated in tapping mode (f=250 kHz). During the measurement the sample surface is scanned underneath the cantilever. The area of the sample in the vicinity of the cantilever is illuminated with 6.20 μm mid-infrared light focused via a parabolic mirror from the backside in a transmission mode. The incident light excites plasmonic modes of the fabricated nanostructures, which in response generate local electrical fields. Due to the operation in tapping mode, the distance between cantilever and sample surface is modulated sinusoidally. Because of the strongly nonlinear interaction of the tip with the surrounding electrical field with respect to the tip-sample distance, a sinusoidal distance modulation results in the generation of high harmonic signal components in the scattered light intensity. The scattered light is collected and focused on an MCT-detector. A lock-in demodulation at higher harmonics of the tapping frequency ensures an almost background-free measurement. Amplitude and phase information in the detected signal are separated with the aid of an interferometric technique. This so called pseudoheterodyne detection involves a Michelson interferometer where the light from the s-SNOM tip interferes with a reference.

[0112] FIGS. 8a and b shows the intensity and phase maps of the electric-field component along z direction at near field for the VO.sub.2 metasurface (Γ=16 μm) when the angle between the polarization of incident light and x (or y) axis is 45°. In this configuration, the left four metallic VO.sub.2 antennas are excited in an antisymmetric mode while the right four ones are excited in a symmetric mode. In the amplitude map (FIG. 8a), both the tips and the vertex of each antenna are bright in symmetric mode, while in the antisymmetric mode the vertex stays dark indicating no field buildup at the center. On the other hand, the phases of the two arms of antenna look similar in symmetric mode (symmetric along the incident polarization direction), but those in antisymmetric mode show a difference of ˜π. The simulated amplitude (FIG. 8c) and phase (FIG. 10d) are well in line with the experimental results. All of these observations are in line with previous work in Au/Si system.

[0113] In the far-field reflection measurement setup, a quantum cascade laser (QC) with a wavelength 8.05 μm is used. A polarizer allows to select the polarization. The incident light is slightly focused using a Kepler telescope (f=50 cm). The sample is mounted on a motorized 360° rotation stage and located at the focus of the telescope. This enables the setting of any incident angle of the laser with respect to the sample surface normal. The slight focusing of the incident beam ensures a small spot size on the sample surface. The reflected light intensity is measured using a detector mounted on a second 360° rotation stage. Thus, all possible incident and reflected angles can be addressed, except for those where the detector blocks the path of the incident light (reflected angle close to incident angle). An aperture in front of the detector is used to increase the angular resolution to the sub-degree range. Additionally, an analyzer (2nd polarizer) in front of the detector can be used to selectively only detect the parallel or the cross-polarized component of the scattered light. For the mid-infrared light a mercury cadmium telluride (MCT) detector is used. Lock-in amplification is used to increase the dynamic range of the detection. For this an optical chopper located at the inner focal point of the Kepler telescope is used. In this way a dynamic range of about six orders of magnitude is obtained.

Simulation

[0114] The near-field and far-field simulations for one unit cell of antenna array were performed using the RF module from COMSOL Multiphysics® on a server workstation.