Photoconductive antennas (PCAs) have been extensively utilized for the generation and detection of both pulsed broadband and single frequency continuous wave terahertz (THz) band radiation. These devices form the basis of many THz imaging and spectroscopy systems, which have demonstrated promising applications in various industries and research fields. The development of THz PCA technology through the last 30 years is reviewed. The key modalities of improving device performance are identified, and literature is reviewed to summarize the progress made in these areas. The goal of this review is to provide a collection of all relevant literature to bring researchers up to date on the current state and remaining challenges of THz PCA technology.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-56/issue-1/010901/Review-of-terahertz-photoconductive-antenna-technology/10.1117/1.OE.56.1.010901.pdf

Review of terahertz photoconductive antenna technology

We describe the saturation properties and power scaling of large-aperture planar photoconducting antennas. These antennas have been used to emit and detect electromagnetic pulses with terahertz bandwidth. At high optical fluences, the amplitude of the radiated electric field saturates, at a value comparable to the bias field in agreement with a simple model of the radiation. We also discuss the saturation properties of the large-aperture photoconducting antennas with different semiconductor materials.

Saturation properties of large-aperture photoconducting antennas

We present the first experimental demonstration of a new fiber-chip grating coupler concept that exploits the blazing effect by interleaving the standard full (220 nm) and shallow etch (70 nm) trenches in a 220 nm thick silicon layer. The high directionality is obtained by controlling the separation between the deep and shallow trenches to achieve constructive interference in the upward direction and destructive interference toward the silicon substrate. Utilizing this concept, the grating directionality can be maximized independent of the bottom oxide thickness. The coupler also includes a subwavelength-engineered index-matching region, designed to reduce the reflectivity at the interface between the injection waveguide and the grating. We report a measured fiber-chip coupling efficiency of -1.3  dB, the highest coupling efficiency achieved to date for a surface grating coupler in a 220 nm silicon-on-insulator platform fabricated in a conventional dual-etch process without high-index overlays or bottom mirrors.

High-directionality fiber-chip grating coupler with interleaved trenches and subwavelength index-matching structure.

Photoconductive switches were the key components that allowed the generation and detection of coherent broadband electromagnetic pulses at terahertz frequencies, opening the possibility for performing spectroscopy and, therefore, measuring complex dielectric properties of materials in this band, which was mostly unexplored. In this paper, we present a brief introduction to the operation principles of these devices. Subsequently, we present a review of the current state-of-the-art in this field and discuss the challenges to be faced in future development of these devices.

Photoconductive devices for terahertz pulsed spectroscopy: a review [Invited]

Generation of ultra-wideband microwave utilizing a low-energy-triggered bulk gallium arsenide (GaAs) avalanche semiconductor switch with ultrafast switching is presented. The switch biased at 5.2 kV is able to reach the conducting state with a time as low as ~200 ps, and a 5.6-nJ laser is required. The switching transient features such as delay, ultrafast switching, and voltage locking are described and explained by multiple powerful avalanche domains, and a comparison with previous discussions and findings available in the literatures is provided. The application of the bulk GaAs avalanche semiconductor switches for generating ultra-wideband microwave is demonstrated. We produce a bipolar pulse with peak-to-peak voltage amplitude of ~5 kV and a unipolar pulse with peak amplitude of ~10 kV.

Ultra-Wideband Microwave Generation Using a Low-Energy-Triggered Bulk Gallium Arsenide Avalanche Semiconductor Switch With Ultrafast Switching

Semiconductor material is important to the performance of the terahertz (THz) photoconductive antenna. In this paper, we investigate the relationship between THz wave emission efficiency, noise, and stability to the material properties of fabricated low-temperature (LT) GaAs and semi-insulating GaAs antennas with the same structure, and compare their emission power, signal-to-noise ratio, and stability under the same experimental conditions. Both theoretical analysis and experimental results reveal that the LT-GaAs antenna has high THz wave emission efficiency, low noise, and high stability due to its short carrier lifetime and high resistivity, which provides us with guidance for the fabrication of high-performance photoconductive antennas.

An LT-GaAs Terahertz Photoconductive Antenna With High Emission Power, Low Noise, and Good Stability

A fast terahertz (THz) continuous-wave detector based on weakly ionized plasma is developed. Its response time can be less than a microsecond according to calculation. By comparing with a Schottky diode in a THz interferometer, we illustrated that our detector has faster response time, linear responsivity, and higher signal-to-noise ratio.

Fast Terahertz Continuous-Wave Detector Based on Weakly Ionized Plasma

A 3-mm-gapped GaAs photoconductive semiconductor switch is triggered by a 1.6 μJ laser diode. Effects of the rectangular spot's layout and the triggering position on the electric field threshold of nonlinear mode are investigated. As the illuminated position moves from the cathode to the anode, the effective illumination optical energy varies. In that case, when the rectangular spot is perpendicular to two electrodes the electric field threshold increases linearly. And when the rectangular spot is parallel to the electrodes, the electric field threshold exhibits an asymmetric parabola-like curve.

Investigation of electric field threshold of GaAs photoconductive semiconductor switch triggered by 1.6 μJ laser diode

The photoactivated charge domain (PACD) plays an important role in the nonlinear modes of semi-insulating GaAs and LnP photoconductive switches. The formation and transporting process of photoactivated charge domain are discussed in this paper, which indicate that it is the shielded electric field that induced the unique distribution and evolution law of the PACD. The PACD restricts space-charge current in the photoconductor and the output current of the photoconductive switch by its shielded effect.

Current limiting effects of photoactivated charge domain in semi-insulating GaAs photoconductive switch

This paper presents an experimental study on the failure mechanism of high-power Gallium Arsenide (GaAs)-based photoconductive semiconductor switches (PCSS). Two of the typical failure scenarios of high-power GaAs PCSS are discussed: 1) a failure of a 3-mm-gap GaAs PCSS at output current of 45 A and 2) the failure of two 2-mm-gap GaAs PCSSs at output currents of 1.45 and 1.8 kA, respectively. The failure mechanisms of these two cases are analyzed and summarized as follows: The failure of high-power GaAs PCSS is mainly dominated by the development of current filaments under low output currents. A large number of energetic carriers in the bulk region of the current filament can cause the dot damage. When the dot damage is dense enough to form a continuous chain connecting the PCSS electrodes, a damage path is created and a catastrophic breakdown occurs. The thermal stress is the main cause of the failure of high-power GaAs PCSS under high current scenarios. The stress can cause local microscopic fracture on the surface of PCSS. At the time when these local microscopic fractures reach a critical level, a macroscopic fracture occurs resulting in a disintegration of the high-power PCSS.

Research on the Failure Mechanism of High-Power GaAs PCSS

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