Mona Jarrahi

Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes

Even though the terahertz spectrum is well suited for chemical identification, material characterization, biological sensing and medical imaging, practical development of these applications has been hindered by attributes of existing terahertz optoelectronics. Here we demonstrate that the use of plasmonic contact electrodes can significantly mitigate the low-quantum efficiency performance of photoconductive terahertz optoelectronics. The use of plasmonic contact electrodes offers nanoscale carrier transport path lengths for the majority of photocarriers, increasing the number of collected photocarriers in a subpicosecond timescale and, thus, enhancing the optical-to-terahertz conversion efficiency of photoconductive terahertz emitters and the detection sensitivity of photoconductive terahertz detectors. We experimentally demonstrate 50 times higher terahertz radiation powers from a plasmonic photoconductive emitter in comparison with a similar photoconductive emitter with non-plasmonic contact electrodes, as well as 30 times higher terahertz detection sensitivities from a plasmonic photoconductive detector in comparison with a similar photoconductive detector with non-plasmonic contact electrodes.

Terahertz generation using plasmonic photoconductive gratings

A photoconductive terahertz emitter based on plasmonic contact electrode gratings is presented and experimentally demonstrated. The nanoscale grating enables ultrafast and high quantum efficiency operation simultaneously, by reducing the photo-generated carrier transport path to the photoconductor contact electrodes. The presented photoconductor eliminates the need for a short-carrier lifetime semiconductor, which limits the efficiency of conventional photoconductive terahertz emitters. Additionally, the photo-absorbing active area of the plasmonic photoconductive terahertz emitter can be increased without a significant increase in the capacitive loading to the terahertz radiating antenna, enabling high quantum-efficiency operation at high pump power levels by preventing the carrier screening effect and thermal breakdown. A plasmonic photoconductive terahertz emitter prototype based on the presented scheme is implemented and integrated with dipole antenna arrays on a semi-insulating In0.53Ga0.47As substrate. Emitted terahertz radiation is characterized in a terahertz time-domain spectroscopy setup, measuring a terahertz pulse width of 590?fs full-width at half maximum in response to 150?fs pump pulses at 925?nm.

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%.

Analysis of periodic metallic nano-slits for efficient interaction of terahertz and optical waves at nano-scale dimensions

We analyze the unique property of periodic arrays of subwavelength metallic slits to allow extraordinary electromagnetic transmission at multiple frequency bands. The diffraction limit in periodic arrays of subwavelength metallic slits is mitigated by excitation of surface waves which assist efficient coupling of a transverse magnetic–polarized incident electromagnetic wave into the TEM waveguide modes of the subwavelength slab waveguides formed by metallic slits. By investigating the geometry dependence of the electromagnetic guided modes supported by periodic arrays of subwavelength metallic slits, we present the design of a periodic array of metallic nanoslits which enables efficient interaction of terahertz and optical waves at nanoscale dimensions.

High power terahertz generation using 1550 nm plasmonic photomixers

We present a 1550 nm plasmonic photomixer operating under pumping duty cycles below 10%, which offers significantly higher terahertz radiation power levels compared to previously demonstrated photomixers. The record-high terahertz radiation powers are enabled by enhancing the device quantum efficiency through use of plasmonic contact electrodes, and by mitigating thermal breakdown at high optical pump power levels through use of a low duty cycle optical pump. The repetition rate of the optical pump can be specifically selected at a given pump duty cycle to control the spectral linewidth of the generated terahertz radiation. At an average optical pump power of 150 mW with a pump modulation frequency of 1 MHz and pump duty cycle of 2%, we demonstrate up to 0.8 mW radiation power at 1 THz, within each continuous wave radiation cycle.

Generation of high power pulsed terahertz radiation using a plasmonic photoconductive emitter array with logarithmic spiral antennas

An array of 3 × 3 plasmonic photoconductive terahertz emitters with logarithmic spiral antennas is fabricated on a low temperature (LT) grown GaAs substrate and characterized in response to a 200 fs optical pump from a Ti:sapphire mode-locked laser at 800 nm wavelength. A microlens array is used to split and focus the optical pump beam onto the active area of each plasmonic photoconductive emitter element. Pulsed terahertz radiation with record high power levels up to 1.9 mW in the 0.1–2 THz frequency range is measured at an optical pump power of 320 mW. The record high power pulsed terahertz radiation is enabled by the use of plasmonic contact electrodes, enhancing the photoconductor quantum efficiencies, and by increasing the overall device active area, mitigating the carrier screening effect and thermal breakdown at high optical pump power levels.

Advanced Photoconductive Terahertz Optoelectronics Based on Nano-Antennas and Nano-Plasmonic Light Concentrators

High power sources and high sensitivity detectors are highly in demand for terahertz imaging and sensing systems. Use of nano-antennas and nano-plasmonic light concentrators in photoconductive terahertz sources and detectors has proven to offer significantly higher terahertz radiation powers and detection sensitivities by enhancing photoconductor quantum efficiency while maintaining its ultrafast operation. This is because of the unique capability of nano-antennas and nano-plasmonic structures in manipulating the concentration of photo-generated carriers within the device active area, allowing a larger number of photocarriers to efficiently contribute to terahertz radiation and detection. An overview of some of the recent advancements in terahertz optoelectronic devices through use of various types of nano-antennas and nano-plasmonic light concentrators is presented in this article.

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.

Design of reconfigurable metallic slits for terahertz beam modulation

We analyze the interaction of electromagnetic waves with double-layered subwavelength metallic slits on a dielectric substrate. This structure allows efficient transmission of an incident TM-polarized electromagnetic wave into the dielectric substrate, due to the presence of surface modes which couple the incident wave to the TEM waveguide modes supported by the subwavelength metallic slits. Our study shows that electromagnetic transmission through double-layered subwavelength metallic slits is strongly geometry dependent. Based on this observation, a terahertz modulation scheme is presented which, compared to existing terahertz modulator solutions, has the promise of significant enhancement in modulation index over a broad range of terahertz frequencies.