Mohammad R. Hashemi

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%.

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.

Plasmonic photoconductive detectors for enhanced terahertz detection sensitivity

A photoconductive terahertz detector based on plasmonic contact electrodes is presented. The use of plasmonic electrodes mitigates the inherent tradeoff between high quantum efficiency and ultrafast operation of the employed photoconductor, enabling significantly higher detection sensitivities compared to conventional photoconductive terahertz detectors. Prototypes of comparable photoconductive detectors with and without plasmonic contact electrode gratings were fabricated and characterized in a time-domain terahertz spectroscopy setup under the same operation conditions. The experimental results show that the plasmonic photoconductive detector offers more than 30 times higher terahertz detection sensitivities compared to the comparable conventional photoconductive detector without plasmonic contact electrodes over 0.1-1.5 THz frequency band.

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation (1-8). Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors (9). Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 (10).

Nanoscale contact electrodes for significant radiation power enhancement in photoconductive terahertz emitters

We present a photoconductive terahertz emitter that incorporates nanoscale plasmonic contact electrodes to offer significantly higher radiation powers compared to conventional photoconductive terahertz emitters. Radiation enhancement is the result of reducing the average transport path of photogenerated carriers to the plasmonic contact electrodes and, thus, increasing the number of the photocarriers that can be routed to the terahertz antenna in a sub-picosecond time-scale to efficiently contribute to terahertz radiation. We experimentally demonstrate up to 50 times higher terahertz radiation powers from the presented plasmonic photoconductive terahertz emitter compared to a similar conventional photoconductive terahertz emitter without plasmonic contact electrodes.

Terahertz detection sensitivity enhancement by using plasmonic contact electrode gratings

A novel photoconductive terahertz detector based on plasmonic contact electrode gratings is presented, which provides significantly higher detection sensitivities compared to conventional photoconductive terahertz detectors. Terahertz detection sensitivity enhancement is the result of reducing the average transport path of photo-generated carriers to the plasmonic contact electrodes, making it possible to generate higher output photocurrents under the same terahertz and pump power levels. We experimentally demonstrate a plasmonic photoconductive detector, which offers more than 30 times higher terahertz detection sensitivities compared to a comparable conventional photoconductive detector without plasmonic contact electrodes over 0.1-1.5 THz frequency range and under the same operation conditions.

Terahertz radiation enhancement through use of plasmonic photomixers

We demonstrate significant enhancement in terahertz radiation power from a photomixer that incorporates plasmonic contact electrodes. The plasmonic contact electrodes reduce the average transport path of photocarriers to the contact electrodes, increasing the ultrafast photocurrent that drives the terahertz antenna. We experimentally demonstrate an order of magnitude higher radiated power from a photomixer with plasmonic contact electrodes in comparison with an analogous photomixer without plasmonic contact electrodes in the 0.25-2.5 THz frequency range.

Fully integrated broadband terahertz modulators

Recent advances in terahertz sources and detectors have made terahertz radiation much more accessible for many practical imaging and sensing applications, such as identifying chemicals, characterizing materials, biological sensing, and biomedical imaging, increasing demand for high-performance terahertz passive components. Among these, terahertz modulators play a key role in single-pixel compressive sensing and imaging systems,1, 2 especially when high-sensitivity detector arrays with a large number of pixels are not available. It is challenging to construct high-performance terahertz modulators based on conventional optical and IR modulation schemes3–8 due to a lack of materials with the desired properties at terahertz frequencies and practical difficulties in scaling device dimensions to operate efficiently in this regime.9 In contrast, the spectral response of reconfigurable metamaterials (constructed by assembling so-called meta-molecule building blocks in a specific arrangement) can be engineered by their geometry, rather than being limited by the characteristics of natural materials at terahertz frequencies. As a result, they offer a very promising platform for manipulating terahertz waves.10, 11 However, until now the operational bandwidth of terahertz modulators based on metamaterials has been limited by the resonant nature of the device configurations employed. There is a good reason for using resonant structures in previously demonstrated reconfigurable metamaterials: the response is highly sensitive to small changes in the metamaterial’s constituent parts as a result of external stimuli at resonance frequency. To enable broadband modulation, we have developed a reconfigurable metasurface that, unlike previous attempts, offers extreme switching in the scattering parameters without use of any resonant enhancement within the device operation frequency range.12 Therefore, it enables the broadband modulation required in time-domain and frequency-tunable terahertz Figure 1. Scanning electron microscopy images (above) and schematic diagram (below) of a microelectromechanical systems (MEMS)reconfigurable metasurface that offers broadband terahertz modulation. E, H, KTHz: Electric field, magnetic field, and wave vector of the incident terahertz wave. Au: Gold. Si: Silicon. SiO2: Silicon dioxide.

Plasmonics-enhanced photoconductive terahertz emitters

We present a photoconductive terahertz emitter that incorporates nanoscale plasmonic contact electrodes to offer significantly higher radiation powers compared to conventional photoconductive terahertz emitters. Radiation enhancement is the result of reducing the average transport path of photo-generated carriers to the plasmonic contact electrodes and, thus, increasing the number of the photocarriers that can be routed to the terahertz antenna in a sub-picosecond time-scale to efficiently contribute to terahertz radiation. We experimentally demonstrate up to 50 times higher terahertz radiation powers from the presented plasmonic photoconductive terahertz emitter compared to a similar conventional photoconductive terahertz emitter without plasmonic contact electrodes.