Electrical and Electronic Engineering - Research Publications
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ItemElectrical tuning of reflectance of graphene metasurface for unpolarized long wavelength infrared light(OSA, 2018-01-01)We demonstrate a graphene-metal metasurface for unpolarized long wavelength infrared light with electrically-tunable reflectance. By applying a gate voltage, we shift the wavelength of a resonant reflectance dip centered at ~9.4 micron by~156 nm.
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ItemMid-infrared Magnetic Mirror Based on a Hybrid Metal/Dielectric Metasurface(IEEE, 2018)We propose a hybrid metal/dielectric metasurface that functions as a mid-infrared magnetic mirror. It consists amorphous silicon cuboids on gold. The physical mechanism is explained by image theory. Measured reflection spectra agree with simulations.
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ItemSolution-Synthesized High-Mobility Tellurium Nanoflakes for Short-Wave Infrared Photodetectors(AMER CHEMICAL SOC, 2018-07)Two-dimensional (2D) materials, particularly black phosphorus (bP), have demonstrated themselves to be excellent candidates for high-performance infrared photodetectors and transistors. However, high-quality bP can be obtained only via mechanical exfoliation from high-temperature- and high-pressure-grown bulk crystals and degrades rapidly when exposed to ambient conditions. Here, we report solution-synthesized and air-stable quasi-2D tellurium (Te) nanoflakes for short-wave infrared (SWIR) photodetectors. We perform comprehensive optical characterization via polarization-resolved transmission and reflection measurements and report the absorbance and complex refractive index of Te crystals. It is found that this material is an indirect semiconductor with a band gap of 0.31 eV. From temperature-dependent electrical measurements, we confirm this band-gap value and find that 12 nm thick Te nanoflakes show high hole mobilities of 450 and 1430 cm2 V-1 s-1 at 300 and 77 K, respectively. Finally, we demonstrate that despite its indirect band gap, Te can be utilized for high-performance SWIR photodetectors by employing optical cavity substrates consisting of Au/Al2O3 to dramatically increase the absorption in the semiconductor. By changing the thickness of the Al2O3 cavity, the peak responsivity of Te photoconductors can be tuned from 1.4 μm (13 A/W) to 2.4 μm (8 A/W) with a cutoff wavelength of 3.4 μm, fully capturing the SWIR band. An optimized room-temperature specific detectivity ( D*) of 2 × 109 cm Hz1/2 W-1 is obtained at a wavelength of 1.7 μm.
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ItemHigh-resolution mid-infrared spectral reconstruction using a subwavelength coaxial aperture array(OSA & IEEE, 2019-01-01)We demonstrate mid-infrared computational spectroscopy using an array of coaxial aperture filters. We experimentally determine material transmission spectra using an algorithm whose inputs are the transmission spectra and the power transmitted through each filter.
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ItemMid-infrared computational spectroscopy with an electrically-tunable graphene metasurface(OSA - Optical Society of America, 2019-01-01)We demonstrate graphene-plasmonic metasurfaces whose mid-infrared reflection spectra are electrically-tunable. Using measurements of the power reflected by the metasurfaces at different drive voltages, the source spectrum is computationally reconstructed by the recursive least squares method.
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ItemPolarization-resolved black phosphorus/molybdenum disulfide mid-wave infrared photodiodes with high detectivity at room temperature(NATURE PUBLISHING GROUP, 2018-10-01)Infrared photodetectors are currently subject to a rapidly expanding application space, with an increasing demand for compact, sensitive and inexpensive detectors. Despite continued advancement, technological factors limit the widespread usage of such detectors, specifically, the need for cooling and the high costs associated with processing of iii–v/ii–vi semiconductors. Here, black phosphorous (bP)/MoS2 heterojunction photodiodes are explored as mid-wave infrared (MWIR) detectors. Although previous studies have demonstrated photodiodes using bP, here we significantly improve the performance, showing that such devices can be competitive with conventional MWIR photodetectors. By optimizing the device structure and light management, we demonstrate a two-terminal device that achieves room-temperature external quantum efficiencies (ηe) of 35% and specific detectivities (D*) as high as 1.1 × 1010 cm Hz1/2 W−1 in the MWIR region. Furthermore, by leveraging the anisotropic optical properties of bP we demonstrate the first bias-selectable polarization-resolved photodetector that operates without the need for external optics.
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ItemExperimental demonstration of infrared spectral reconstruction using plasmonic metasurfaces(OPTICAL SOC AMER, 2018-09-15)We computationally reconstruct short- to long-wave infrared spectra using an array of plasmonic metasurface filters. We illuminate the filter array with an unknown spectrum and measure the optical power transmitted through each filter with an infrared microscope to emulate a filter-detector array system. We then use the recursive least squares method to determine the unknown spectrum. We demonstrate our method with light from a blackbody. We also demonstrate it with spectra generated by passing the light from the blackbody through various materials. Our approach is a step towards miniaturized spectrometers spanning the short- to long-wave infrared based on filter-detector arrays.
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ItemMid- to long-wave infrared computational spectroscopy using a subwavelength coaxial aperture array(Nature Publishing Group, 2019-09-19)Miniaturized spectrometers are advantageous for many applications and can be achieved by what we term the filter-array detector-array (FADA) approach. In this method, each element of an optical filter array filters the light that is transmitted to the matching element of a photodetector array. By providing the outputs of the photodetector array and the filter transmission functions to a reconstruction algorithm, the spectrum of the light illuminating the FADA device can be estimated. Here, we experimentally demonstrate an array of 101 band-pass transmission filters that span the mid- to long-wave infrared (6.2 to 14.2 μm). Each filter comprises a sub-wavelength array of coaxial apertures in a gold film. As a proof-of-principle demonstration of the FADA approach, we use a Fourier transform infrared (FTIR) microscope to record the optical power transmitted through each filter. We provide this information, along with the transmission spectra of the filters, to a recursive least squares (RLS) algorithm that estimates the incident spectrum. We reconstruct the spectrum of the infrared light source of our FTIR and the transmission spectra of three polymer-type materials: polyethylene, cellophane and polyvinyl chloride. Reconstructed spectra are in very good agreement with those obtained via direct measurement by our FTIR system.