Publications

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A prominent nonlinear optical phenomenon that is extensively studied using nanostructured materials is second-harmonic generation (SHG) as it has applications in various fields. Achieving efficient SHG from a nanostructure requires a large second-order nonlinear susceptibility of the material system and large electromagnetic fields. For practical applications, the nanostructures should also have low losses, high damage thresholds, large bandwidths, wavelength scalability, dual mode operation in transmission and reflection, monolithic integrability, and ease of fabrication. While various approaches have demonstrated efficient SHG, to the best of our knowledge, none have demonstrated all these desired qualities simultaneously. Here, we present a hybrid approach for realizing efficient SHG in an ultrathin dielectric-semiconductor nonlinear device with all the above-mentioned desired properties. Our approach uses high quality factor leaky mode resonances in dielectric metasurfaces that are coupled to intersubband transitions of semiconductor quantum wells. Using our device, we demonstrate SHG at pump wavelengths ranging from 8.5 to 11 μm, with a maximum second-harmonic nonlinear conversion factor of 1.1 mW/W2 and maximum second-harmonic conversion efficiency of 2.5 × 10-5 at modest pump intensities of 10 kW/cm2. Our results open a new direction for designing low loss, broadband, and efficient ultrathin nonlinear optical devices.

Metamaterials research has developed perfect absorbers from microwave to optical frequencies, mainly featuring planar metamaterials, also referred to as metasurfaces. In this study, we investigated vertically oriented metamaterials, which make use of the entire three-dimensional space, as a new avenue to widen the spectral absorption band in the infrared regime between 20 and 40 THz. Vertically oriented metamaterials, such as those simulated in this work, can be experimentally realized through membrane projection lithography, which allows a single unit cell to be decorated with multiple resonators by exploiting the vertical dimension. In particular, we analyzed the cases of a unit cell containing a single vertical split-ring resonator (VSRR), a single planar split-ring resonator (PSRR), and both a VSRR and PSRR to explore intra-cell coupling between resonators. We show that the additional degrees of freedom enabled by placing multiple resonators in a unit cell lead to novel ways of achieving omnidirectional super absorption. Our results provide an innovative approach for controlling and designing engineered nanostructures.

Terahertz (THz) photoconductive devices are used for generation, detection, and modulation of THz waves, and they rely on the ability to switch electrical conductivity on a subpicosecond time scale using optical pulses. However, fast and efficient conductivity switching with high contrast has been a challenge, because the majority of photoexcited charge carriers in the switch do not contribute to the photocurrent due to fast recombination. Here, we improve efficiency of electrical conductivity switching using a network of electrically connected nanoscale GaAs resonators, which form a perfectly absorbing photoconductive metasurface. We achieve perfect absorption without incorporating metallic elements, by breaking the symmetry of cubic Mie resonators. As a result, the metasurface can be switched between conductive and resistive states with extremely high contrast using an unprecedentedly low level of optical excitation. We integrate this metasurface with a THz antenna to produce an efficient photoconductive THz detector. The perfectly absorbing photoconductive metasurface opens paths for developing a wide range of efficient optoelectronic devices, where required optical and electronic properties are achieved through nanostructuring the resonator network.

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We use GaAs metasurfaces with (111) crystal orientation to channel the second harmonic generation (SHG) into the zero-diffraction order that is suppressed for SHG obtained from GaAs metasurfaces with (100) orientation.

We demonstrate ultrafast tuning of Fano resonances in a broken symmetry III-V metasurface using optical pumping. The resonance is spectrally shifted by 10 nm under low pump fluences of < 100 uJ·cm-2.

Exposure to chemicals in everyday life is now more prevalent than ever. Air and water pollution can be delivery mechanisms for toxins, carcinogens, and other chemicals of interest (COI). A compact, multiplexed, chemical sensor with high responsivity and selectivity is desperately needed. We demonstrate the integration of unique Zr-based metal organic frameworks (MOFs) with a plasmonic transducer to demonstrate a nanoscale optical sensor that is both highly sensitive and selective to the presence of COI. MOFs are a product of coordination chemistry where a central ion is surrounded by a group of ligands resulting in a thin-film with nano-to micro-porosity, ultra-high surface area, and precise structural tunability. These properties make MOFs an ideal candidate for gaseous chemical sensing, however, transduction of a signal which probes changes in MOF films has been difficult. Plasmonic sensors have performed well in many sensing environments, but have had limited success detecting gaseous chemical analytes at low levels. This is due, in part, to the volume of molecules required to interact with the functionalized surface and produce a detectable shift in plasmonic resonance frequency. The fusion of a highly porous thin-film layer with an efficient plasmonic transduction platform is investigated and summarized. We will discuss the integration and characterization of the MOF/plasmonic sensor and summarize our results which show, upon exposure to COI, small changes in optical characteristics of the MOF layer are effectively transduced by observing shifts in plasmonic resonance.

We use dielectric metasurfaces made from direct bandgap semiconductors to generate high-harmonics and nonlinear mixing simultaneously, without the need of phase matching. Inclusion of broken-symmetry designs and quantum heterostructures can lead to even higher efficiencies.

In this work, we demonstrate high-performance electrically injected GaN/InGaN core-shell nanowire-based LEDs grown using selective-area epitaxy and characterize their electro-optical properties. To assess the quality of the quantum wells, we measure the internal quantum efficiency (IQE) using conventional low temperature/room temperature integrated photoluminescence. The quantum wells show a peak IQE of 62%, which is among the highest reported values for nanostructure-based LEDs. Time-resolved photoluminescence (TRPL) is also used to study the carrier dynamics and response times of the LEDs. TRPL measurements yield carrier lifetimes in the range of 1-2 ns at high excitation powers. To examine the electrical performance of the LEDs, current density-voltage (J-V) and light-current density-voltage (L-J-V) characteristics are measured. We also estimate the peak external quantum efficiency (EQE) to be 8.3% from a single side of the chip with no packaging. The LEDs have a turn-on voltage of 2.9 V and low series resistance. Based on FDTD simulations, the LEDs exhibit a relatively directional far-field emission pattern in the range of pm ± 15°. This work demonstrates that it is feasible for electrically injected nanowire-based LEDs to achieve the performance levels needed for a variety of optical device applications.

A frequency mixer is a nonlinear device that combines electromagnetic waves to create waves at new frequencies. Mixers are ubiquitous components in modern radio-frequency technology and microwave signal processing. The development of versatile frequency mixers for optical frequencies remains challenging: such devices generally rely on weak nonlinear optical processes and, thus, must satisfy phase-matching conditions. Here we utilize a GaAs-based dielectric metasurface to demonstrate an optical frequency mixer that concurrently generates eleven new frequencies spanning the ultraviolet to near-infrared. The even and odd order nonlinearities of GaAs enable our observation of second-harmonic, third-harmonic, and fourth-harmonic generation, sum-frequency generation, two-photon absorption-induced photoluminescence, four-wave mixing and six-wave mixing. The simultaneous occurrence of these seven nonlinear processes is assisted by the combined effects of strong intrinsic material nonlinearities, enhanced electromagnetic fields, and relaxed phase-matching requirements. Such ultracompact optical mixers may enable a plethora of applications in biology, chemistry, sensing, communications, and quantum optics.

The ability to control the light-matter interaction with an external stimulus is a very active area of research since it creates exciting new opportunities for designing optoelectronic devices. Recently, plasmonic metasurfaces have proven to be suitable candidates for achieving a strong light-matter interaction with various types of optical transitions, including intersubband transitions (ISTs) in semiconductor quantum wells (QWs). For voltage modulation of the light-matter interaction, plasmonic metasurfaces coupled to ISTs offer unique advantages since the parameters determining the strength of the interaction can be independently engineered. In this work, we report a proof-of-concept demonstration of a new approach to voltage-tune the coupling between ISTs in QWs and a plasmonic metasurface. In contrast to previous approaches, the IST strength is here modified via control of the electron populations in QWs located in the near field of the metasurface. By turning on and off the ISTs in the semiconductor QWs, we observe a modulation of the optical response of the IST coupled metasurface due to modulation of the coupled light-matter states. Because of the electrostatic design, our device exhibits an extremely low leakage current of ∼6 pA at a maximum operating bias of +1 V and therefore very low power dissipation. Our approach provides a new direction for designing voltage-tunable metasurface-based optical modulators.

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Considering the power constrained scaling of silicon complementary metal-oxide-semiconductor technology, the use of high mobility III-V compound semiconductors such as In0.53Ga0.47As in conjunction with high-κ dielectrics is becoming a promising option for future n-type metal-oxide-semiconductor field-effect-transistors. Development of low dissipation field-effect tunable III-V based photonic devices integrated with high-κ dielectrics is therefore very appealing from a technological perspective. In this work, we present an experimental realization of a monolithically integrable, field-effect-tunable, III-V hybrid metasurface operating at long-wave-infrared spectral bands. Our device relies on strong light-matter coupling between epsilon-near-zero (ENZ) modes of an ultra-thin In0.53Ga0.47As layer and the dipole resonances of a complementary plasmonic metasurface. The tuning mechanism of our device is based on field-effect modulation, where we modulate the coupling between the ENZ mode and the metasurface by modifying the carrier density in the ENZ layer using an external bias voltage. Modulating the bias voltage between ±2 V, we deplete and accumulate carriers in the ENZ layer, which result in spectrally tuning the eigenfrequency of the upper polariton branch at 13 μm by 480 nm and modulating the reflectance by 15%, all with leakage current densities less than 1 μA/cm2. Our wavelength scalable approach demonstrates the possibility of designing on-chip voltage-tunable filters compatible with III-V based focal plane arrays at mid- and long-wave-infrared wavelengths.

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Resonant semiconductor metasurfaces are an emerging versatile platform for nonlinear photonics. In this work, we investigate second-harmonic generation from metasurfaces consisting of two-dimensional square arrays of gallium arsenide nanocylinders as a function of the polarization of the fundamental wave. To this end, we perform nonlinear second harmonic microscopy, where the pump wavelength is tuned to the resonances of the metasurfaces. Furthermore, imaging the generated nonlinear signal in Fourier space allows us to analyze the spatial properties of the generated second harmonic. Our experiments reveal that the second harmonic is predominantly emitted into the first diffraction orders of the periodic arrangements, and that its intensity varies with the polarization angle of the fundamental wave. While this can be expected from the structure of the GaAs nonlinear tensor, the characteristics of this variation itself are found to depend on the pump wavelength. Interestingly, we show that the metasurface can reverse the polarization dependence of the second harmonic with respect to an unstructured GaAs wafer. These general observations are confirmed by numerical simulations using a simplified model for the metasurface. Our results provide valuable input for the development of metasurface-based classical and quantum light sources based on parametric processes.

All-dielectric metasurfaces, two-dimensional arrays of subwavelength low loss dielectric inclusions, can be used not only to control the amplitude and phase of optical beams, but also to generate new wavelengths through enhanced nonlinear optical processes that are free from some of the constraints dictated by the use of bulk materials. Recently, high quality factor (Q) resonances in these metasurfaces have been revealed and utilized for applications such as sensing and lasing. The origin of these resonances stems from the interference of two nanoresonator modes with vastly different Q. Here we show that nonlinear optical processes can be further enhanced by utilizing these high-Q resonances in broken symmetry all-dielectric metasurfaces. We study second harmonic generation from broken symmetry metasurfaces made from III-V semiconductors and observe nontrivial spectral shaping of second-harmonic and multifold efficiency enhancement induced by high field localization and enhancement inside the nanoresonators.

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Metamaterials provide a means to tailor the spectral response of a surface. Given the periodic nature of the metamaterial, proper design of the unit cell requires intimate knowledge of the parameter space for each design variable. We present a detailed study of the parameter space surrounding vertical split-ring resonators and planar split-ring resonators, and demonstrate widening of the perfect absorption bandwidth based on the understanding of its parameter space.

We use broken symmetry III-V semiconductor Fano metasurfaces to substantially improve the efficiency of second-harmonic generation (SHG) in the near infrared, compared to SHG obtained from metasurfaces created using symmetrical Mie resonators.

In this work, we experimentally demonstrate simultaneous occurrence of second-,third-, fourth-harmonic generation, sum-frequency generation, four-wave mixing and six-wave mixing processes in III-V semiconductor metasurfaces with spectra spanning from the UV to the near-IR.

We demonstrate active tuning of high-Q dielectric metasurfaces by embedding asymmetric silicon meta-atoms in liquid crystals, thus controlling the relative refractive index by heating. Spectral tuning of more than three resonance widths is achieved.

Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. However, metasurfaces are no longer adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. We experimentally realise an ultrafast tunable metasurface consisting of subwavelength gallium arsenide nanoparticles supporting Mie-type resonances in the near infrared. Using transient reflectance spectroscopy, we demonstrate a picosecond-scale absolute reflectance modulation of up to 0.35 at the magnetic dipole resonance of the metasurfaces and a spectral shift of the resonance by 30 nm, both achieved at unprecedentedly low pump fluences of less than 400 μJ cm-2. Our findings thereby enable a versatile tool for ultrafast and efficient control of light using light.

Optical nonlinearities are intimately related to the spatial symmetry of the nonlinear media. For example, the second order susceptibility vanishes for centrosymmetric materials under the dipole approximation. The latter concept has been naturally extended to the metamaterials' realm, sometimes leading to the (erroneous) hypothesis that second harmonic (SH) generation is negligible in highly symmetric meta-atoms. In this work we aim to show that such symmetric meta-atoms can radiate SH light efficiently. In particular, we investigate in-plane centrosymmetric meta-atom designs where the approximation for meta-atoms breaks down. In a periodic array this building block allows us to control the directionality of the SH radiation. We conclude by showing that the use of symmetry considerations alone allows for the manipulation of the nonlinear multipolar response of a meta-atom, resulting in e.g. dipolar, quadrupolar, or multipolar emission on demand. This is because the size of the meta-atom is comparable with the free-space wavelength, thus invalidating the dipolar approximation for meta-atoms.

Switchable metasurfaces fabricated on a doped epi-layer have become an important platform for developing techniques to control terahertz (THz) radiation, as a DC bias can modulate the transmission characteristics of the metasurface. To model and understand this performance in new device configurations accurately, a quantitative understanding of the bias-dependent surface characteristics is required. We perform THz variable angle spectroscopic ellipsometry on a switchable metasurface as a function of DC bias. By comparing these data with numerical simulations, we extract a model for the response of the metasurface at any bias value. Using this model, we predict a giant bias-induced phase modulation in a guided wave configuration. These predictions are in qualitative agreement with our measurements, offering a route to efficient modulation of THz signals.

We experimentally demonstrate resonantly enhanced nonlinear optical processes such as 2nd-, 3rd-, and 4th-harmonic generations, sum-frequency generation, four-wave mixing processes, etc., in the visible and near-IR using GaAs dielectric metasurfaces.

We present a study of an electrically modulated nonlinear metamaterial consisting of an array of split-ring resonators fabricated on n-type gallium arsenide. The resonant metamaterial nonlinearity appears as an intensity-dependent transmission minimum at terahertz frequencies and arises from the interaction between local electric fields in the split-ring resonator (SRR) capacitive gaps and charge carriers in the n-type substrate. We investigate the active tuning range of the metamaterial device as the incident terahertz field intensity is increased and conversely the effect of an applied DC bias on the terahertz field-induced nonlinear modulation of the metamaterial response. Applying a DC bias to the metamaterial sample alters the nonlinear response and reduces the net nonlinear modulation. Similarly, increasing the incident terahertz field intensity decreases the net modulation induced by an applied DC bias. We interpret these results in terms of DC and terahertz-field-assisted carrier acceleration, scattering, and multiplication processes, highlighting the unique nature of this DC-field modulated terahertz nonlinearity.

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We demonstrate the active tuning of all-dielectric metasurfaces exhibiting high-quality factor (high-Q) resonances. The active control is provided by embedding the asymmetric silicon meta-atoms with liquid crystals, which allows the relative index of refraction to be controlled through heating. It is found that high quality factor resonances (Q = 270 ± 30) can be tuned over more than three resonance widths. Our results demonstrate the feasibility of using all-dielectric metasurfaces to construct tunable narrow-band filters.