Huygens' Metasurfaces Enabled by Magnetic Dipole Resonance

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Huygens’ metasurfaces enabled by magnetic dipole resonance tuning in split dielectric nanoresonators Sheng Liu, Aleksandr Vaskin, Salvatore Campione, Omri Wolf, Michael B. Sinclair, John L. Reno, Gordon A. Keeler, Isabelle Staude, and Igal Brener Nano Lett., Just Accepted Manuscript • Publication Date (Web): 07 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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Huygens’ metasurfaces enabled by magnetic dipole resonance tuning in split dielectric nanoresonators Sheng Liu,1,* Aleksandr Vaskin2, Salvatore Campione,1 Omri Wolf,1,3 Michael B. Sinclair,1 John Reno,1,3 Gordon A. Keeler,1 Isabelle Staude2, Igal Brener1,3 1

Sandia National Laboratories, Albuquerque, NM 87185, USA

2

Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, AlbertEinstein-Str. 15, 07745 Jena, Germany 3

Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, NM 87185, USA

*Corresponding author: [email protected]

Abstract: Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance towards the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator.

For

example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances, and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nano-antenna that provides strong unidirectional emission from an electric dipole source.

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Keywords: Dielectric metasurfaces, GaAs, III-V semiconductors, resonant tuning, Kerker condition, Huygens’ metasurface

Introduction: Recent developments in nano-fabrication have enabled artificial assemblies of micro- and nanostructures, specifically metamaterials, that greatly enrich the types of electromagnetic behavior at optical frequencies. Benefiting from integrated circuit fabrication processes, noble metals such as gold and silver have been the workhorses of metallic-based metamaterials. Metallic nanostructures exhibiting electric dipole, quadrupole, and other high-order multipoles arising from their localized surface plasmon resonances can strongly interact with incoming electric fields, and lead to effective permittivities when assembled in arrays. This approach was initially applied at microwave and radio frequencies1,

2

for applications such as antennas, wireless

communications, etc. At optical frequencies, natural materials usually do not interact strongly with the magnetic component of an electromagnetic wave. However, plasmonic metamaterials with specific designs such as split-ring resonators3 have been demonstrated to generate circulating electric currents that dramatically modify the effective permeabilities, and couple to magnetic fields. These engineering capabilities have the potential to enable a diverse set of applications such as sensing, solar and thermal energy harvesting, and high-density data storage; however, in practice there has been limited success due to the high losses of metals at optical frequencies4-8. More recently, high-index dielectric structures and nanoparticles have been attracting much attention due to the low intrinsic material loss and resonant interaction with both the electric and 2 ACS Paragon Plus Environment

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magnetic fields. In this regard, semiconductors have emerged as attractive materials due to their high permittivities; compatibility with advanced nanofabrication techniques; and the ability to tune their material properties by varying the carrier density through doping or dynamic electron injection.

The resonant enhancements of the electromagnetic fields inside isolated

semiconductor dielectric resonators have been demonstrated to improve nonlinear frequency generation9-11, ultrafast all-optical modulation12, photoluminescence spectral shaping13, and optical magnetic mirror behavior14. Most of the work done within the realm of semiconductor dielectric metamaterials has employed silicon, which is the most obvious choice due to its broad use in microelectronic fabrication. However, silicon is not an efficient light emitter, and is not suitable for second-order nonlinear optical processes. These shortcomings can be overcome using III-V semiconductors such as gallium arsenide (GaAs). We recently developed a monolithic top-down fabrication process15 for realizing III-V semiconductors based dielectric metamaterials that has enabled resonantly enhanced second-harmonic generation9 and ultrafast switching16. Moreover, the epitaxial growth technique of III-V semiconductors enables more complex metamaterial designs that can be challenging to realize in Silicon.

For example, we demonstrated multi-layer dielectric

metamaterials15 showing unity reflectivity across a wide spectral range. Semiconductor dielectric resonators exhibit both electric and magnetic17-19 Mie resonances with comparable strengths20, 21 within a single resonator. The properties of metasurfaces consisting of arrays of dielectric resonators are governed by the interplay of the electric and magnetic dipole resonances (between neighboring particles22 or within a single unit23, 24) which can lead to new strategies for realizing ultrathin optical components with novel functionalities25 such as Huygens’ metasurfaces26, holograms27, lenses28, beam deflectors29,

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, polarization converters31-34, high3

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quality Fano resonant structures22, 24, just to name a few. However, the spectral positions of the Mie dipole resonances are fixed by the resonator geometries and there are limited strategies available for tuning the relative frequencies of the multipoles. One such strategy for independent tuning of the resonances relies on a perturbative approach35-37, where perturbations (such as splits or metallic dipoles) are arranged in such a manner as to selectively interact with the electric field patterns of the magnetic or electric resonances and shift the (magnetic/electric dipole) resonance frequency towards the (electric/magnetic dipole) resonance frequency. Here we introduce a nano-structure consisting of split dielectric resonators (SDRs) as the metasurface building block that enables the tuning of the spectral position of magnetic dipole resonances while leaving the electric dipole resonances almost unaffected. The low-index splits are arranged in such a manner as to selectively interact with the electric field pattern of the magnetic dipole resonance and shift the resonance frequency upwards towards the electric dipole resonance frequency. This demonstration shows the versatility of the combined technology of III-V epitaxy and our fabrication process, thus providing a new avenue for metamaterial device engineering.

Moreover, this resonance tuning approach is distinct from our previously

demonstrated approach where both the magnetic and electric dipole resonances are tuned simultaneously by changing the aspect ratio of the resonators’ height and diameter23. This approach is also very different from the ultrafast all-optical tuning we recently demonstrated16, where the use of high energy femtosecond pulses limits practical applications. Also, both dipole resonances were tuned in Ref. 16 by introducing a high density of free carriers, with a spectral tuning range (~30 nm) much smaller than what shown here. We envision that the spectral tuning offered by SDRs can be used for other applications as well, such as directional nonlinear optical emission11 that relies on the interference between electric and magnetic dipole resonances. More 4 ACS Paragon Plus Environment

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importantly, we experimentally observe low reflectivity over a broad spectral range by suppressing the backward scattering. This is achieved when the magnetic and electric dipole resonances overlap in frequency and exhibit comparable strengths, such that the first Kerker condition is satisfied at resonance35, 38-40. The enhanced forward scattering could be useful for photovoltaic applications, antireflection coatings, and high-transmission phase modulators. Finally, we show that a single, isolated SDR can act as an optical nano-antenna providing strong directional emission from a dipole source. Simulation and design of SDR metasurfaces To avoid absorption loss, we designed GaAs SDRs to support electric and magnetic dipole resonances below the bandgap of GaAs (~1.42 eV). Figure 1(a) shows a schematic of an SDR metasurface consisting of GaAs SDRs arranged in a square lattice. Each SDR is composed of two identical GaAs nanodisks stacked together vertically but separated by a low-index AlGaO gap15. The AlGaO is created by a selective oxidation process that will be discussed in detail below. The upper thin oxide gap is used to split the resonator into two parts. The bottom isolation oxide layer is used to separate the SDRs from the high-index substrate for tight electromagnetic confinement inside the SDRs. Figure 1(b) shows a reflectivity contour image of a series of SDR metasurfaces simulated using a finite-difference time-domain (FDTD) solver. These metasurfaces consist of GaAs SDRs with the same height (both upper and lower GaAs nanodisks) of 160 nm, the same diameter of 300 nm, but varying AlGaO gap thicknesses between 0 and 120 nm. The SDR metasurfaces’ periodicity is kept constant at 750 nm and therefore the duty cycle, which is defined as the diameter divided by the periodicity, is 40%. The SDR metasurfaces are arranged on a 400 nm thick AlGaO isolation layer that lies on top of an undoped GaAs substrate. In the FDTD simulation, we used the permittivity of GaAs from the 5 ACS Paragon Plus Environment

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database of Lumerical Inc. and a refractive index of n = 1.6 for AlGaO15. When the thickness of the low-index AlGaO gap approaches zero, the two reflectivity peaks at ~1.2 and ~1 µm correspond to the magnetic and electric dipole resonances, respectively. As the oxide gap thickness increases, the magnetic dipole resonance blue shifts while the electric dipole resonance remains almost unaffected at around 1 µm. As we further increase the thickness of the low-index gap to above 100 nm, the reflectivity of the SDR metasurfaces in the vicinity of the original electric dipole resonance wavelength decreases dramatically indicating the spectral overlap of the two resonances and the attainment of the first Kerker condition at resonance. To confirm that the electromagnetic behavior of the SDR metasurfaces is not strongly affected by the bottom oxide isolation layer or the GaAs substrate, we also simulated SDR metasurfaces that are suspended in vacuum (see Supporting Information, Section 1). The metasurfaces in the two different environments show similar behavior indicating negligible impact of the substrate and the isolation layer. To compare with dielectric resonators without the low index gap, we also simulate GaAs metasurfaces with the same material of GaAs as the gap (see Supporting Information, Section 2). In this case, both electric and magnetic resonances would move.


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Figure 1. (a) Schematic of an array of GaAs SDRs each consisting of two identical GaAs nanodisks separated by a low refractive index AlGaO gap. The green and grey colors represent GaAs and low-index AlGaO, respectively. (b) Simulated reflectivity contour image of GaAs SDR metasurfaces that consist of SDRs with different oxide gap size varying between 0 and 120 nm. The black dashed lines are for eye guidance, and depict the locations of the electric dipole (ED) and the magnetic dipole (MD) resonances for a given wavelength and gap size. To confirm that the magnetic dipole resonance blue shifts as the gap thickness increases, we performed multipolar decomposition calculations35. Figure 2(a) & 2(b) show the scattering cross-section of two different SDRs with gap thicknesses of 0 and 30 nm. The other dimensions are the same as used in Figure 1(b). Note that the calculation was performed for isolated SDRs in free space. Comparing the two figures, the amplitude and spectral position (~0.97 µm) of the electric dipole scattering cross-section remain the same, while the magnetic dipole resonance blue shifts towards the electric dipole resonance as the gap thickness increases from 0 to 30 nm (from ~1.25 µm to ~1.16 µm).

The multipolar decomposition results agree well with the

simulated reflectivity spectra and unambiguously show the independent tuning of the magnetic dipole resonance. Figure 2(c)-(d) and Fig. 2(e)-(f) show the simulated electric field profiles, where the arrows represent the direction while the color represents the magnitude for the two SDRs. Although the electric field profile is distorted by the low-index gap, we can still clearly identify the circular and linear displacement currents indicating the magnetic and electric dipole resonances, respectively. We found that the rate of change of the blue shift of the magnetic resonance lessens with increasing gap thickness. This agrees with the “saturation” effect we theoretically demonstrated using perturbation theory36,

37

, where we showed that the shifting saturates when the gap 7 ACS Paragon Plus Environment

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thickness is half of the side dimension of the disk. Moreover, the maximum wavelength shift depends on the refractive index contrast between the resonator and the gap material.

We

simulated an SDR with a higher index contrast (refractive indices of resonator and gap are 5.66 and 1) and observed complete overlap between the two dipole resonances (see Supporting Information, Section 3). However, with the lower index contrast between GaAs and AlGaO, for isolated SDRs, blue shift saturation occurs prior to complete spectral overlap. Despite the saturation observed for isolated resonators, dipole-resonance overlap can still be achieved when the SDRs are arranged in a closely spaced array, since a dielectric resonator’s behavior can be strongly affected by other neighboring resonators26, 32, 41. We initially chose a low duty cycle (40%) to focus on the individual SDR’s behavior.

To explore the impact of resonator

interactions on the resonance positions, we simulated the reflectivity of metasurfaces when SDRs are closer—a larger duty cycle of 60% (see Supporting Information, Section 4). We show that, similar to our previous work23, closely placed SDRs are critical for spectral overlap of electric and magnetic dipole resonances, and the concomitant reduction of backscattering.


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Figure 2. Scattering cross-sections associated with the electric and magnetic dipoles computed using multipolar decomposition for isolated SDRs when the thicknesses of the gaps are (a) 0 and (b) 30 nm. Simulated electric field magnitude (|E|) profile at the (c) & (e) magnetic and (d) & (f) electric dipole resonances in the x-z plane located half way through the SDR. The edges of (c), (d), (e), (f) are the boundaries of the GaAs resonators in the x-z plane, axes orientation from fig. 1a. Fabrication of GaAs SDRs Gap thicknesses of 0, 40, and 100 nm were chosen to show both the tuning of magnetic dipole resonances and the spectral overlap of electric and magnetic dipole resonances. We started our fabrication process by epitaxially growing three wafers. The grown layers from top to bottom 9 ACS Paragon Plus Environment

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are a 160 nm thick GaAs layer, an Al0.85Ga0.15As gap layer (0, 40, and 100 nm thickness), another 160 nm thick GaAs layer, a 500 nm thick Al0.85Ga0.15As isolation layer residing on a semi-insulating GaAs substrate at the bottom.

Then we employed standard electron-beam

lithography and chlorine-based inductively coupled-plasma etch to create the SDR arrays with desired dimensions.

The AlGaAs was subsequently oxidized into AlGaO that has a low

refractive index of about 1.615,

42, 43

. The details of sample fabrication can be found in the

Methods section. For each of the grown wafers, we fabricated SDR metasurfaces with various diameters and two different duty cycles of 60% and 40%. Figure 3(a) shows a 75º side-view scanning-electron microscope (SEM) image of a metasurface consisting of an array of SDRs showing clear color contrast between the top 160 nm GaAs layer, the 100 nm low index oxide gap layer, the lower 160 nm GaAs layer, and the AlGaO isolation layer at the bottom. Note that the oxide gap thickness is slightly different from the original AlGaAs layer thickness due to the different lattice constants between the two materials. Figure 3(b) & (c) show top view SEM images of two SDR metasurfaces with different duty cycles of 60% and 40%, respectively.

Figure 3. (a) A side view SEM image of a metasurface consisting of SDRs with a 100 nm thick low refractive index gap that separates the two-part 160 nm thick GaAs nanodisks. Top view SEM images of GaAs SDR metasurfaces with different duty cycles of (b) ~60% and (c) ~40%. 10 ACS Paragon Plus Environment

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Experimental results of GaAs SDR metasurfaces First, we show spectral tuning of the magnetic dipole resonance as the gap in the SDR is varied. Following the previous discussion, it is easier to identify the two dipole resonances from the two reflectivity peaks when the SDR metasurfaces have a duty cycle of 40% (rather than 60%). Thus, we first concentrate on the GaAs SDR metasurfaces with 40% duty cycle, with 0 and 40 nm thick AlGaAs gap layers. For reflectivity measurements, we used a 20× Mitutoyo near-infrared objective (numerical aperture = 0.4) to both focus the incident broadband “white” light onto the samples and collect the reflected light. The reflected light was then directed into a near-IR spectrometer and measured using a liquid nitrogen cooled InGaAs detector. The measured reflectivity spectra were then normalized to the reflectivity spectrum of a gold mirror measured under the same conditions. Figure 4(a) shows the experimental reflectivity spectra of two metasurfaces that consist of SDRs with the same diameter of 260 nm and periodicity of 650 nm (duty cycle 40%), but with differing gap widths. The blue curve corresponds to a metasurface that does not have an oxide gap while the red curve is for a metasurface with a 40 nm oxide gap layer. The blue curve resembles the reflectivity of a typical dielectric metasurface with two well-separated peaks corresponding to the electric and magnetic dipole resonances (at ~0.92 µm and ~1.05 µm, in agreement with simulations). In comparison, for the metasurface with the 40 nm thick gap, the two reflectivity peaks merge into one single peak with a larger spectral width (at ~0.95 µm). This is because the magnetic dipole resonance has been shifted towards the electric dipole resonance so the two resonances are spectrally close but not overlapping.


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Figure 4. Experimentally measured reflectivity spectra of four different GaAs SDR metasurface samples with two different gap thicknesses (blue curve: 0 nm and red curve: 40 nm) and two different diameters of (a) 260 nm and (b) 300 nm. Simulated reflectivity contour images of


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GaAs SDR metasurfaces consisting of SDRs with various diameters and two different oxide gap thicknesses of (c) 0 and (d) 40 nm. The blue square and triangular markers represent the experimentally measured reflectivity peak wavelengths of ten different SDR metasurfaces. The dashed lines in (d) are for eye guidance, and indicate the locations of the two dipole resonances with wavelength and resonator diameter. (e) Experimentally measured reflectivity spectra of three GaAs SDR metasurfaces consisting of SDRs with the same diameter of 340 nm but different AlGaO gaps thicknesses of 0, 40, and 100 nm. (f) Simulated reflectivity contour image of GaAs SDR metasurfaces consisting of SDRs with varying diameters and the same gap thickness of 100 nm. The square markers represent the measured reflectivity minima of five SDR metasurfaces. Figure 4(b) shows the reflectivity spectra of two metasurfaces consisting of SDRs with similar designs to the ones discussed above except for the larger diameter of 300 nm (duty cycle unchanged, 40%). These two metasurfaces better illustrate the tuning of the magnetic dipole resonance since the two reflectivity peaks can still be identified when a 40 nm oxide gap is included (at ~1.0 µm and ~1.1 µm, in agreement with simulations). Moreover, the spectral position of the electric dipole resonance remains unchanged when the oxide gap is incorporated, confirming the independent tuning of the magnetic dipole resonance. The experimental data were then compared with the simulation results. Figure 4(c) & (d) show two reflectivity contour plots of a series of SDR metasurfaces without and with a 40 nm thick oxide gap, respectively.

Experimentally measured reflectivity peaks (of both electric and

magnetic resonances) of ten different samples are shown as square and triangular markers. The diameters of SDRs were measured using SEM (The full experimental spectra of these samples can be found in the Supporting Information, Section 5). For the metasurfaces without an oxide 13 ACS Paragon Plus Environment

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gap, the two dipole resonances are well separated and both red shift as the diameter increases. For the metasurfaces with a 40 nm oxide gap, we see that as the diameter increases, the two reflectivity peaks separate spectrally. Overall, simulations and experimental results agree well. Next, we explore the minimization of reflection by spectrally overlapping the electric and magnetic dipole resonances. For each of the three wafers, we fabricated metasurfaces with a constant 60% duty cycle but various resonator diameters. Figure 4(e) shows reflectivity spectra of three representative metasurfaces consisting of SDRs with the same diameter of 340 nm but different oxide gaps with thickness of 0, 40, and 100 nm. As discussed above, for the metasurface without an oxide gap, the two reflectivity peaks corresponding to the two dipole resonances can be identified, although not as distinct as when the duty cycle is 40%.

In

comparison, as the oxide gap thickness increases to 40 and then 100 nm, the reflectivity of the metasurfaces decreases dramatically.

Moreover, a reflectivity dip instead of the original

reflection peak appears around the electric dipole resonance of ~1.05 µm. Noticeably, when the gap thickness is 100 nm, the metasurface exhibits broadband low reflection, and the reflectivity approaches zero at ~1.05 µm, which matches well with the electric dipole resonance. Figure 4(f) shows the simulated reflectivity spectra of SDR metasurfaces with the same gap thickness of 100 nm. As the SDR’s diameter increases, the wavelength corresponding to the reflectivity minimum increases, as expected.

The five white square markers represent the

wavelengths of the experimentally measured reflectivity minima of the five samples. (The full experimental spectra of these samples can be found in the Supporting Information, Section 5) The experimental results agree well with simulation, further confirming the spectral overlap between the two dipole resonances35, and attainment of the first Kerker condition at resonances. This condition has been used recently to create holographic metasurfaces with high transmission 14 ACS Paragon Plus Environment

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efficiency27; however, fine tuning of the phase response was obtained by simultaneous changes of diameter and resonator spacing. The first Kerker condition wavelength enabled by SDRs can be tuned by varying the diameters while the other dimensions, determined during the epitaxial growth, remain the same. Directional scattering and SDR as an optical nano-antenna Apart from their use as building blocks of dielectric metasurfaces, conventional high-index dielectric nanoresonators were previously suggested as nanoantennas that are able to shape the emission from a nearby source into a directional pattern with high radiation efficiency23. In single-element nanoantennas, the formation of the directional pattern usually relies on the controlled interference of two or several multipolar contributions of the electromagnetic fields emitted by the coupled emitter-nanoantenna system44-46.

In order to provide a broader

perspective and outlook for possible areas of application of SDR geometries beyond Huygens’ metasurfaces, in the following we demonstrate numerically that multipolar SDRs can be used as low-loss optical nanoantennas with greatly enhanced directivity. A sketch of the considered SDR nanoantenna geometry is shown in Fig. 5(a). It consists of two GaAs nanodisks, which are separated by a low-index oxide layer. Each nanodisk has a height of 220 nm and a diameter of 620 nm. A transverse oriented electric dipole source is located on the nanodisk axis at a distance of 80 nm from the surface. The entire coupled system is embedded in a dielectric medium with refractive index of 1.45. The thickness of the oxide layer is varied from 0 to 200nm. Using the commercial software package COMSOL, we performed numerical calculations to determine the maximum directivity of the emission and the corresponding emission pattern. The directivity is ସగ

defined as ‫ܦ‬ሺߠ, ߮ሻ = ௉

‫݌‬ሺߠ, ߮ሻ, where ‫݌‬ሺߠ, ߮ሻ is the radiated power per unit solid angle in the

ೝೌ೏


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desired direction ሺߠ, ߮ሻ and ܲ௥௔ௗ is the total radiated power. The results are shown in Fig. 5(b) and indicate that the oxide layer thickness provides an efficient way to tailor the directivity spectra. The directivity maximum of 13.5 is reached for the oxide layer thickness of 30 nm and the emission pattern for this SDR resonator is shown in Fig. 5(c). In contrast, the maximum directivity of a single GaAs nanodisk antenna of the same dimensions in homogeneous n=1.45 environment is limited to approximately 8 (see Supporting Information, Section 6), which is in accord with previous studies for silicon nanodisk antennas45. Note that the directivity decreases significantly if the oxide layer thickness approaches zero, clearly showing the significance of the layer. Our results demonstrate that SDR nanoantennas may allow for strong enhancement of the light collection efficiency from an emitter even without using a back-reflector or a lens. A route for experimental realization of the suggested structure including the excitation dipole source is provided by the possibility to include an additional layer of quantum wells (InGaAs) or quantum dots (InAs) during the wafer growth process.

Figure 5. (a) Schematic of a single SDR used as an optical nano-antenna. (b) Numerically calculated directivity in the forward direction as a function of wavelength and the oxide layer


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thickness. (c) Emission pattern at a wavelength of 990 nm and oxide thickness of 30 nm, where the maximum directivity is achieved. Conclusions In summary, we experimentally demonstrated that epitaxially grown and lithographic nanofabricated split dielectric resonators are novel building blocks that enable relative tuning of electric and dipole magnetic resonances. The SDR structure consists of two nano-disks separated by a low-index oxide; metasurfaces consisting of SDR arrays were fabricated using our recently developed III-V dielectric metasurface fabrication technique.

By fabricating GaAs SDR

metasurfaces with different diameters and gap thicknesses, we demonstrated independent tuning of the magnetic dipole resonance, while the electric dipole resonance was kept at a fixed wavelength. This allowed us to spectrally overlap electric and magnetic dipole resonances and satisfy the first Kerker condition as manifested by a broadband low reflectivity. The ability to tune the resonance provides direct control over the interference between the two dipole resonances. Therefore, our SDR structure provides a powerful platform for engineering light scattering properties by providing additional design flexibility, which is important for multifunctional metasurfaces that require complex optimization. Our demonstration not only shows the versatility of III-V semiconductor based metamaterials, but also paves the way for realizing new functionalities using more complex dielectric metasurfaces. The simulated results reveal that a multipolar SDR can function as an optical nano-antenna for highly directional emission from a dipole source. Methods (sample fabrication):


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Following the epitaxial growth, the SDR arrays were fabricated by spin-coating diluted negative tone hydrogen silsesquioxane (HSQ Fox-16) resist on the sample which were then baked at 90 ºC for 3 minutes. The circular disks are patterned using standard electron-beam lithography that converted the HSQ to SiOx. The unexposed HSQ is developed using tetramethylammonium hydroxide leaving ∼150 nm tall SiOx nanodisks as etch masks, whose shape is then transferred onto the GaAs and AlGaAs layers using a chlorine-based inductively coupled-plasma etch. Finally, a selective wet oxidization process is performed by placing the samples in a tube furnace at ∼420 °C. By flowing high temperature water vapor, the layers of Al0.85Ga0.15As are converted into its oxide (AlxGa1−x)2O3 which has a low refractive index of n∼1.642. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nano-lett.xxxxx Experimental results, numerical modeling and Figures S1-S6 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes: The authors declare no competing financial interests.

Acknowledgement


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Parts of this work were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. A.V. and I.S. gratefully acknowledge financial support from the German Research Foundation (STA 1426/2-1) and by the Thuringian State Government through its ProExcellence Initiative (ACP2020). TOC Graphic

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Figure 1. (a) Schematic of an array of GaAs SDRs each consisting of two identical GaAs nanodisks separated by a low refractive index AlGaO gap. The green and grey colors represent GaAs and low-index AlGaO, respectively. (b) Simulated reflectivity contour image of GaAs SDR metasurfaces that consist of SDRs with different oxide gap size varying between 0 and 120 nm. The black dashed lines are for eye guidance, and depict the locations of the electric dipole (ED) and the magnetic dipole (MD) resonances for a given wavelength and gap size. 397x142mm (96 x 96 DPI)


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Figure 2. Scattering cross-sections associated with the electric and magnetic dipoles computed using multipolar decomposition for isolated SDRs when the thicknesses of the gaps are (a) 0 and (b) 30 nm. Simulated electric field magnitude (|E|) profile at the (c) & (e) magnetic and (d) & (f) electric dipole resonances in the x-z plane located half way through the SDR. The edges of (c), (d), (e), (f) are the boundaries of the GaAs resonators in the x-z plane, axes orientation from fig. 1a. 319x340mm (96 x 96 DPI)


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Figure 3. (a) A side view SEM image of a metasurface consisting of SDRs with a 100 nm thick low refractive index gap that separates the two-part 160 nm thick GaAs nanodisks. Top view SEM images of GaAs SDR metasurfaces with different duty cycles of (b) ~60% and (c) ~40%. 309x120mm (96 x 96 DPI)


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Figure 4. Experimentally measured reflectivity spectra of four different GaAs SDR metasurface samples with two different gap thicknesses (blue curve: 0 nm and red curve: 40 nm) and two different diameters of (a) 260 nm and (b) 300 nm. Simulated reflectivity contour images of GaAs SDR metasurfaces consisting of SDRs with various diameters and two different oxide gap thicknesses of (c) 0 and (d) 40 nm. The blue square and triangular markers represent the experimentally measured reflectivity peak wavelengths of ten different SDR metasurfaces. The dashed lines in (d) are for eye guidance, and indicate the locations of the two dipole resonances with wavelength and resonator diameter. (e) Experimentally measured reflectivity spectra of three GaAs SDR metasurfaces consisting of SDRs with the same diameter of 340 nm but different AlGaO gaps thicknesses of 0, 40, and 100 nm. (f) Simulated reflectivity contour image of GaAs SDR metasurfaces consisting of SDRs with varying diameters and the same gap thickness of 100 nm. The square markers represent the measured reflectivity minima of five SDR metasurfaces. 337x405mm (96 x 96 DPI)


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Figure 5. (a) Schematic of a single SDR used as an optical nano-antenna. (b) Numerically calculated directivity in the forward direction as a function of wavelength and the oxide layer thickness. (c) Emission pattern at a wavelength of 990 nm and oxide thickness of 30 nm, where the maximum directivity is achieved. 340x160mm (96 x 96 DPI)


Huygens' Metasurfaces Enabled by Magnetic Dipole Resonance

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