Shang-Hua Yang

High-Power Terahertz Generation Using Large-Area Plasmonic Photoconductive Emitters

In this paper, we present a novel design of large-area photoconductive emitters which incorporates plasmonic contact electrodes to offer significantly higher optical-to-terahertz conversion efficiencies compared with conventional designs. Use of plasmonic contact electrodes enables a more efficient separation and acceleration of photocarriers, enhancing the effective dipole moment induced within the device active area in response to an incident optical pump. At an optical pump power level of 240 mW, we demonstrate broadband, pulsed terahertz radiation with radiation power levels as high as 3.8 mW over the 0.1-5-THz frequency range, exhibiting an order of magnitude higher optical-to-terahertz conversion efficiency compared with conventional designs.

7.5% Optical-to-Terahertz Conversion Efficiency Offered by Photoconductive Emitters With Three-Dimensional Plasmonic Contact Electrodes

We present a photoconductive terahertz emitter that incorporates three-dimensional plasmonic contact electrodes to offer record high optical-to-terahertz power conversion efficiencies. By use of three-dimensional plasmonic contact electrodes the majority of photocarriers are generated within nanoscale distances from the photoconductor contact electrodes and drifted to the terahertz radiating antenna in a sub-picosecond time-scale to efficiently contribute to terahertz radiation. We experimentally demonstrate 105 μW of broadband terahertz radiation in the 0.1-2 THz frequency range in response to a 1.4 mW optical pump, exhibiting a record high optical-to-terahertz power conversion efficiency of 7.5%.

Switchable Scattering Meta-Surfaces for Broadband Terahertz Modulation

Active tuning and switching of electromagnetic properties of materials is of great importance for controlling their interaction with electromagnetic waves. In spite of their great promise, previously demonstrated reconfigurable metamaterials are limited in their operation bandwidth due to their resonant nature. Here, we demonstrate a new class of meta-surfaces that exhibit electrically-induced switching in their scattering parameters at room temperature and over a broad range of frequencies. Structural configuration of the subwavelength meta-molecules determines their electromagnetic response to an incident electromagnetic radiation. By reconfiguration of the meta-molecule structure, the strength of the induced electric field and magnetic field in the opposite direction to the incident fields are varied and the scattering parameters of the meta-surface are altered, consequently. We demonstrate a custom-designed meta-surface with switchable scattering parameters at a broad range of terahertz frequencies, enabling terahertz intensity modulation with record high modulation depths and modulation bandwidths through a fully integrated, voltage-controlled device platform at room temperature.

Enhanced light-matter interaction at nanoscale by utilizing high-aspect-ratio metallic gratings.

We report the design, fabrication, and experimental characterization of high-aspect-ratio metallic gratings integrated with nanoscale semiconductor structures, which enable efficient light-matter interaction at the nanoscale over interaction lengths as long as two times of the effective optical wavelength. The efficient light-matter interaction at the nanoscale is enabled by excitation of the guided modes of subwavelength slab waveguides formed by the high-aspect-ratio metallic gratings. By controlling the height of the high-aspect-ratio gratings, the wavelength of the guided modes through the nanoscale semiconductor structures is determined.

Frequency-tunable continuous-wave terahertz sources based on GaAs plasmonic photomixers

We present frequency-tunable, continuous-wave terahertz sources based on GaAs plasmonic photomixers, which offer high terahertz radiation power levels at 50% radiation duty cycle. The use of plasmonic contact electrodes enhances photomixer quantum efficiency while maintaining its ultrafast operation by concentrating a large number of photocarriers in close proximity to the device contact electrodes. Additionally, the relatively high thermal conductivity and high resistivity of GaAs allow operation under high optical pump power levels and long duty cycles without reaching the thermal breakdown limit of the photomixer. We experimentally demonstrate continuous-wave terahertz radiation with a radiation frequency tuning range of more than 2 THz and a record-high radiation power of 17 μW at 1 THz through plasmonic photomixers fabricated on a low temperature grown GaAs substrate at 50% radiation duty cycle.

Tunable terahertz wave generation through a bimodal laser diode and plasmonic photomixer

We demonstrate a compact, robust, and stable terahertz source based on a novel two section digital distributed feedback laser diode and plasmonic photomixer. Terahertz wave generation is achieved through difference frequency generation by pumping the plasmonic photomixer with two output optical beams of the two section digital distributed feedback laser diode. The laser is designed to offer an adjustable terahertz frequency difference between the emitted wavelengths by varying the applied currents to the laser sections. The plasmonic photomixer is comprised of an ultrafast photoconductor with plasmonic contact electrodes integrated with a logarithmic spiral antenna. We demonstrate terahertz wave generation with 0.15-3 THz frequency tunability, 2 MHz linewidth, and less than 5 MHz frequency stability over 1 minute, at useful power levels for practical imaging and sensing applications.

Quantum-dot laser with a metal-coated waveguide under continuous-wave operation at room temperature

We fabricated and characterized Fabry–Perot quantum-dot lasers with metal-coated waveguides. Lasing action at room temperature under continuous-wave operation was observed, contrary to the belief that metal is too lossy to serve as the waveguide material at optical frequencies. We extracted the optical gain and group index from the amplified spontaneous emission spectra. A high group index of about 4.2 has been observed from these metal-coated devices, which is much larger than a value of 3.2 measured from an uncoated quantum-dot laser. It is believed that the metal coating contributes to the high group index in these devices.

Electronically-Controlled Beam-Steering through Vanadium Dioxide Metasurfaces.

Engineered metamaterials offer unique functionalities for manipulating the spectral and spatial properties of electromagnetic waves in unconventional ways. Here, we report a novel approach for making reconfigurable metasurfaces capable of deflecting electromagnetic waves in an electronically controllable fashion. This is accomplished by tilting the phase front of waves through a two-dimensional array of resonant metasurface unit-cells with electronically-controlled phase-change materials embedded inside. Such metasurfaces can be placed at the output facet of any electromagnetic radiation source to deflect electromagnetic waves at a desired frequency, ranging from millimeter-wave to far-infrared frequencies. Our design does not use any mechanical elements, external light sources, or reflectarrays, creating, for the first time, a highly robust and fully-integrated beam-steering device solution. We demonstrate a proof-of-concept beam-steering metasurface optimized for operation at 100 GHz, offering up to 44° beam deflection in both horizontal and vertical directions. Dynamic control of electromagnetic wave propagation direction through this unique platform could be transformative for various imaging, sensing, and communication applications, among others.

Spectral characteristics of terahertz radiation from plasmonic photomixers.

We present a comprehensive analysis of spectral characteristics of terahertz radiation from plasmonic photomixers. We fabricate plasmonic photomixer prototypes with plasmonic contact electrode gratings on a low temperature grown GaAs substrate and characterize the spectral properties of the generated terahertz radiation by use of a heterodyne detection scheme. Our analysis shows that linewidth, stability, and frequency tuning range of the generated terahertz radiation are directly determined by linewidth, stability, and wavelength tuning range of optical pump beam and not affected by device geometry, substrate properties, optical pump power level and other operational settings. Our study indicates the crucial role of optical sources in realizing high performance terahertz spectroscopy and wireless communication systems based on plasmonic photomixers.

Optical characteristics of a quantum-dot laser with a metallic waveguide

We report the optical characteristics of a quantum-dot laser with a metal-coated waveguide, which shows a large group index of 4.2 compared to 3.2 for an uncoated laser. Temperature-dependent measurements reveal a high characteristic temperature in the temperature range of 7 degrees C-25 degrees C. The optical gain, refractive index change, and linewidth enhancement factor are extracted from the measured Fabry-Perot amplified spontaneous emission spectra. We use the pulse measurements to eliminate the thermal effect and obtain a low linewidth enhancement factor of 0.35.