GaAs Mid-IR Electrically Tunable Metasurfaces.

In this work, we explore III-V based metal-semiconductor-metal structures for tunable metasurfaces. We use an epitaxial transfer technique to transfer a III-V thin film directly on metallic surfaces, realizing III-V metal-semiconductor-metal (MSM) structures without heavily doped semiconductors as substitutes for metal layers. The device platform consists of gold metal layers with a p-i-n GaAs junction. The target resonance wavelength can be tuned by modifying the geometry of the top metal grating on the GaAs, while systematic resonance tunability has been shown through the modulation of various carrier concentration injections in the mid-IR range. Electrically tunable metasurfaces with multilevel biasing can serve as a fundamental building block for electrically tunable metasurfaces. We believe that our demonstration can contribute to understanding the optical tuning of III-V under various biased conditions, inducing changes in metasurfaces.

Black Phosphorus Molybdenum Disulfide Midwave Infrared Photodiodes with Broadband Absorption-Increasing Metasurfaces.

Black phosphorus (BP) has been established as a promising material for room temperature midwave infrared (MWIR) photodetectors. However, many of its attractive optoelectronic properties are often observable only at smaller film thicknesses, which inhibits photodetector absorption and performance. In this work, we show that metasurface gratings increase the absorption of BP-MoS2 heterojunction photodiodes over a broad range of wavelengths in the MWIR. We designed, fabricated, and characterized metasurface gratings that increase absorption at selected wavelengths or broad spectral ranges. We evaluated the broadband metasurfaces by measuring the room temperature responsivity and specific detectivity of BP-MoS2 photodiodes at multiple MWIR wavelengths. Our results show that broadband metasurface gratings are a scalable approach for boosting the performance of BP photodiodes over large spectral ranges.

Symmetry breaking of dark-mode metamaterials for voltage-switchable infrared absorption.

We propose electrically reconfigurable absorbers with switchable narrowband resonances in the infrared. Our absorbers consist of two coupled, identical resonators and support a dark supermode. We show that by dynamically breaking the symmetry of the system, the dark supermode can be made to couple to an incoming plane wave, producing a narrowband absorption peak in the spectrum. We use this effect to design and optimize absorbers consisting of coupled metal-insulator-metal resonators based on gallium arsenide. We show that the switching functionality of the designed device is robust to fabrication imperfections, and that it additionally serves as a spectrally tunable absorber. Our results suggest exciting possibilities for designing next-generation reconfigurable absorbers that could benefit several applications, such as energy harvesting and sensing.

Dynamically switchable self-focused thermal emission.

The ability to manipulate thermal emission is paramount to the advancement of a wide variety of fields such as thermal management, sensing and thermophotovoltaics. In this work, we propose a microphotonic lens for achieving temperature-switchable self-focused thermal emission. By utilizing the coupling between isotropic localized resonators and the phase change properties of VO2, we design a lens that selectively emits focused radiation at a wavelength of 4 µm when operated above the phase transition temperature of VO2. Through direct calculation of thermal emission, we show that our lens produces a clear focal spot at the designed focal length above the phase transition of VO2 while emitting a maximum relative focal plane intensity that is 330 times lower below it. Such microphotonic devices capable of producing temperature-dependent focused thermal emission could benefit several applications such as thermal management and thermophotovoltaics while paving the way for next-generation contact-free sensing and on-chip infrared communication.

Monolithic III-V on Metal for Thermal Metasurfaces.

It has been proposed that metal-semiconductor-metal (MSM) structures can be used to tune the absorptivity of a metasurface at infrared wavelengths. Indium arsenide (InAs) is a low-band-gap, high-electron-mobility semiconductor that may enable rapid index tuning for dynamic control over the infrared spectrum. However, direct growth of III-V thin films on top of metals has typically resulted in small-grain, polycrystalline materials that are not amenable to high-quality devices. Previously, epitaxial wafers were used for this purpose. However, the epitaxial constraints required that InAs be used for both the tuning layer and the bottom "metallic" layer, limiting the range of accessible designs. In this work, we show a demonstration of direct growth of single-crystalline InAs on metal to build tunable absorbers/emitters in the infrared regime. The growth was carried out at a temperature of 300 °C by the low temperature templated liquid phase (LT-TLP) method. The size of InAs single-crystalline mesas is ∼2500 µm2, enabling the desired device sizes. The proposed growth and device enable scalable and tunable infrared devices for various thermal-photonic applications.

Resonant Grating-Enhanced Black Phosphorus Mid-Wave Infrared Photodetector.

Black phosphorus (BP) has emerged as a promising materials system for mid-wave infrared photodetection because of its moderate bandgap, high carrier mobility, substrate compatibility, and bandgap tunability. However, its uniquely tunable bandgap can only be taken advantage of with thin layer thicknesses, which ultimately limits the optical absorption of a BP photodetector. This work demonstrates an absorption-boosting resonant metal-insulator-metal (MIM) metasurface grating integrated with a thin-film BP photodetector. We designed and fabricated different MIM gratings and characterized their spectral properties. Then, we show that an MIM structure increased room temperature responsivity from 12 to 77 mA W-1 at 3.37 µm when integrated with a thin-film BP photodetector. Our results show that MIM structures simultaneously increase mid-wave infrared absorption and responsivity in a thin-film BP photodetector.

Spectral emissivity modeling in multi-resonant systems using coupled-mode theory.

The ability to design multi-resonant thermal emitters is essential to the advancement of a wide variety of applications, including thermal management and sensing. These fields would greatly benefit from the development of more efficient tools for predicting the spectral response of coupled, multi-resonator systems. In this work, we propose a semi-analytical prediction tool based on coupled-mode theory. In our approach, a complex thermal emitter is fully described by a set of coupled-mode parameters, which can be straightforwardly calculated from simulations of unit cells containing single and double resonators. We demonstrate the accuracy of our method by predicting and optimizing spectral response in a coupled, multi-resonant system based on hBN ribbons. The approach described here can greatly reduce the computational overhead associated with spectral design tasks in coupled, multi-resonant systems.

Photonic crystal integrated logic gates and circuits.

This paper presents and demonstrates the three logic processing levels based on complementary photonic crystal logic devices through photonic integrated circuit modeling. We accomplished a set of logic circuits including AND, OR, NAND, NOR, XOR, FAN-OUT, HALF ADDER, and FULL ADDER based on photonic crystal slab platforms. Furthermore, we achieved efficient all-optical logic circuits with contrast ratios as high as 5.5 dB, demonstrated in our simulation results, guaranteeing well-defined output power values for logic representations; a clock-rate up to 2 GHz; and an operating wavelength at λ ≈ 1550 nm. Thus, we can now switch up for high computing abstraction levels to build photonic integrated circuits rather than isolated gates or devices.

Generalized multi-channel scheme for secure image encryption.

The ability of metamaterials to manipulate optical waves in both the spatial and spectral domains has provided new opportunities for image encoding. Combined with the recent advances in hyperspectral imaging, this suggests exciting new possibilities for the development of secure communication systems. While traditional image encryption approaches perform a 1-to-1 transformation on a plain image to form a cipher image, we propose a 1-to-n transformation scheme. Plain image data is dispersed across n seemingly random cipher images, each transmitted on a separate spectral channel. We show that the size of our key space increases as a double exponential with the number of channels used, ensuring security against both brute-force attacks and more sophisticated attacks based on statistical sampling. Moreover, our multichannel scheme can be cascaded with a traditional 1-to-1 transformation scheme, effectively squaring the size of the key space. Our results suggest exciting new possibilities for secure transmission in multi-wavelength imaging channels.

Vanadium-dioxide microstructures with designable temperature-dependent thermal emission.

We propose gold-vanadium dioxide microstructures for which the difference in thermally radiated power between the low and high temperature states can be tuned via structural design. We start by incorporating VO2 in a gold-dielectric-gold waveguide to achieve a temperature-dependent mode effective index. We show that a cavity formed in this waveguide structure has a fundamental resonance wavelength that shifts with temperature. We calculate the thermal radiated power from the cavity at temperatures above and below the phase transition of VO2 for wavelengths between 8 and 14 µm. We show that the difference in radiated power can be made positive, negative, or zero simply by adjusting the cavity length. Finally, we use our cavity to design thermally emissive metasurfaces with spatial emission patterns that can be inverted with temperature. Our emitters could serve as building blocks in the realization of metasurfaces enabling complex thermal radiation control.

Coupled metamaterial optical resonators for infrared emissivity spectrum modulation.

We study the absorptivity of coupled metamaterial resonators in the mid-infrared range. We consider resonators supporting either a bright mode or a dark mode, introducing an additional degree of freedom for spectral modulation relative to bright modes alone. In a dark-bright coupled resonator system, we demonstrate tunable spectral splitting by changing the separation between resonators. We show via coupled mode theory that resonator separation can be mapped to coupling constant. We further introduce a dark-dark coupled resonator system, which gives rise to an emissive bright mode only in the presence of inter-resonator coupling. The dark-dark system yields a broadband emissivity that decays to zero exponentially with resonator separation, providing a design method for strong thermal emissivity control.

Complementary photonic crystal integrated logic devices.

We theoretically propose and demonstrate through numerous simulations complementary photonic crystal integrated logic (CPCL) devices. Simulation results provide demonstration of a highly efficient clock rate, higher than 20 GHz, guaranteeing operation at both input and output with the same wavelength (around λ=1550nm). The proposed devices show well-defined output power values representing the two logic states 1 and 0, with a contrast ratio as high as 6 dB. The results presented here provide countless possibilities for future research, targeting the development of photonic crystal logic and communications systems with CPCLs acting as the core hardware devices.

Experimental demonstration of dynamic thermal regulation using vanadium dioxide thin films.

We present an experimental demonstration of passive, dynamic thermal regulation in a solid-state system with temperature-dependent thermal emissivity switching. We achieve this effect using a multilayered device, comprised of a vanadium dioxide (VO2) thin film on a silicon substrate with a gold back reflector. We experimentally characterize the optical properties of the VO2 film and use the results to optimize device design. Using a calibrated, transient calorimetry experiment we directly measure the temperature fluctuations arising from a time-varying heat load. Under laboratory conditions, we find that the device regulates temperature better than a constant emissivity sample. We use the experimental results to validate our thermal model, which can be used to predict device performance under the conditions of outer space. In this limit, thermal fluctuations are halved with reference to a constant-emissivity sample.

Gold-black phosphorus nanostructured absorbers for efficient light trapping in the mid-infrared.

We propose a gold nanostructured design for absorption enhancement in thin black phosphorus films in the 3-5 µm wavelength range. By suitably tuning the design parameters of a metal-insulator-metal (MIM) structure, lateral resonance modes can be excited in the black phosphorus layer. We compare the absorption enhancement due to the resonant light trapping effect to the conventional 4n2 limit. For a layer thickness of 5 nm, we achieve an enhancement factor of 561 at a wavelength of 4 µm. This is significantly greater than the conventional limit of 34. The ability to achieve strong absorption enhancement in ultrathin dielectric layers, coupled with the unique optoelectronic properties of black phosphorus, makes our absorber design a promising candidate for mid-IR photodetector applications.

Spectral emissivity design using aluminum-based hybrid gratings.

We propose a strategy to design infrared emitters with predefined spectral response using aluminum gratings as building blocks. We begin by identifying 3 target spectra with resonances in the 7-15 µm wavelength range. Next, we use FDTD simulations and interpolation to create a reference library of gratings relating their structural parameters to attributes of their infrared spectra. By using a search algorithm based on minimization of errors in spectral attributes, we identify gratings from this library corresponding to peaks in the target spectra. Finally, we discuss an approach for designing hybrid structures from these gratings to generate each of the 3 target spectra. This strategy can be extended to design structures with complex spectral responses.

Photonic crystal lightsail with nonlinear reflectivity for increased stability.

Recent research has studied the feasibility of using laser radiation pressure to propel lightweight spacecraft, such as sails, at relativistic speeds. One major challenge is the effect of laser beam distortion on sail stability. We propose and investigate the use of lightsails based on Kerr nonlinear photonic crystals as a passive method for increasing sail stability. The key concept is to flatten the dependence of reflected intensity on incident intensity at the laser wavelength, using a specially designed, guided-resonance mode of the nonlinear photonic crystal. We use coupled-mode theory to analyze the resonance characteristics that yield the flattest curve. We then design a silicon nitride photonic crystal that supports a resonance with the desired properties. We show that our design simultaneously provides both high stability and high thrust on the sail, unlike designs based on linear materials.

Design of VO2-coated silicon microspheres for thermally-regulating paint.

Recent work has proposed an approach to design materials that regulate their own temperature. The concept is based on a temperature-dependent thermal emitter that has minimal emissivity below a target temperature, and maximal emissivity above it. Here we propose a microparticle approach suitable for scaling to large areas. We use electromagnetic and thermal simulations to show that the designed particles provide a 13x reduction in temperature variation relative to an uncoated aluminum substrate.

Tunable, polarization-sensitive, dual guided-resonance modes in photonic crystals.

We present a photonic-crystal design which supports multiple guided-resonance modes in a narrow spectral range. Introduction of mutually-orthogonal slots within a conventional lattice allows us to create polarization-sensitive guided modes with distinct near-field periodicities and tunable resonance wavelengths. The device can potentially be used as a reconfigurable optical trap, multiband tunable filter, or differential sensor.

Highly selective ultraviolet aluminum plasmonic filters on silicon.

We report the use of aluminum patterning to make highly selective UV bandpass filters. We design and fabricate a periodic array of nanoholes in Al thin film on a bare silicon substrate as an analog for potential integration with a Si photodetector. Arrays were designed to operate in the wavelength range of 200-400 nm. Our results show that we can obtain a single dominant peak filter with a linewidth of 30 nm at normal incidence, in contrast to similar structures on glass substrates, where multiple modes influence the UV spectrum. Varying the angle of incidence is shown to split the plasmonic mode and further decrease the linewidth of the maximum wavelength mode down to 10 nm. Our results therefore show high potential for applications in solid-state image sensors for astronomy and planetary studies.

Sequential and selective localized optical heating in water via on-chip dielectric nanopatterning.

We study the use of nanopatterned silicon membranes to obtain optically-induced heating in water. We show that by varying the detuning between an absorptive optical resonance of the patterned membrane and an illumination laser, both the magnitude and response time of the temperature rise can be controlled. This allows for either sequential or selective heating of different patterned areas on chip. We obtain a steady-state temperature of approximately 100 °C for a 805.5nm CW laser power density of 66 µW/µm2 and observe microbubble formation. The ability to spatially and temporally control temperature on the microscale should enable the study of heat-induced effects in a variety of chemical and biological lab-on-chip applications.