Inspired by nanoscience and nanoengineering, numerous nanostructured materials developed by multidisciplinary approaches exhibit excellent photoelectronic properties ranging from ultraviolet to terahertz frequencies. As a new class of building block, nanoscale elements in terms of quantum dots, nanowires, and nanolayers can be used for fabricating photodetectors with high performance. Moreover, in conjunction with traditional photodetectors, they exhibit appealing performance for practical applications including high density of integration, high sensitivity, fast response, and multifunction. Therefore, with the perspective of photodetectors constructed by diverse low-dimensional nanostructured materials, recent advances in nanoscale photodetectors are discussed here; meanwhile, challenges and promising future directions in this research field are proposed.

Nanostructured Photodetectors: From Ultraviolet to Terahertz.

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

https://iopscience.iop.org/article/10.1088/1361-6463/ab31d5/pdf

A review of terahertz detectors

Terahertz frequency (THz) radiation lies in between the microwave and infrared ranges. While it is strongly absorbed by water, it is nonionizing and has low possibility of causing tissue damage as it involves low energy photons. Recent technological progress in developing THz instrumentation, means that commercial THz systems are being produced with improving performance which are easier to operate and more reliable. THz phase imaging, an advanced imaging technology which combines the benefits of THz and commonly used phase imaging techniques, has recently received significant attention. In this paper, the current state of such imaging systems is reviewed. This review deals with both pulsed and continuous-wave (CW) imaging systems. Pulsed THz phase imaging is a coherent measurement, which includes terahertz pulsed imaging (TPI) based on femtosecond laser and holographic imaging in the time domain, both allow phase and amplitude information of the electric field to be recorded. CW THz phase imaging is mainly based on digital holography, interferometry and ptychography. These systems can obtain the complex amplitude by capturing diffraction patterns and applying numerical reconstruction techniques. Biomedical applications of such THz systems are highlighted.

Terahertz phase imaging and biomedical applications

In the last several decades, glow discharge detectors (GDD) have been utilized to detect microwave radiation. Recently, these detectors were used to study higher frequencies within the terahertz (THz) band of the electromagnetic spectrum. Commercial neon lamps can act as GDDs, and their performance at 0.2 THz was investigated. A THz continuous-wave imaging system with the GDD was constructed and the images obtained by the GDD and a Schottky diode were compared. The experimental results indicate that an optimally designed GDD will be an efficient, fast, and inexpensive detector for THz imaging systems.

Terahertz Wave Imaging System Based on Glow Discharge Detector

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

Continuous-wave (cw) terahertz (THz) phase imaging can accurately and noninvasively present the depth information of an object’s surface and interior. However, a 2π ambiguity limits the measurement of a sample with a thickness larger than the detection wavelength of THz waves. A multiwavelength phase unwrapping method is introduced to the cw THz phase imaging to reconstruct the exact phase map of the object. By using this method, three different types of high-density polyethylene samples were measured, and their phase profiles were well extracted. The result shows that this method is effective in cw THz phase imaging and has the potential to improve the applications of cw THz imaging.

Continuous-wave terahertz interferometry with multiwavelength phase unwrapping.

The high-gain operating mode of gallium arsenide (GaAs) photoconductive semiconductor switch (PCSS) brings the hope that a higher power PCSS will be triggered by a lower energy laser. However, the lock-on effect of high-gain PCSS restricts its lifetime and some other applications. In this paper, it is experimentally demonstrated that dynamically controlling the electrical field across a GaAs:EL2 PCSS can terminate the carrier avalanche and that there exists a lower bound of electrical field (about 3.64 kV/cm) for maintaining the high-gain operating mode of the PCSS under different conditions. This lower bound of electrical field is approximately equal to that of the transferred-electron effect; hence, it supports the hypothesis of a photon-activated charge domain to explain the physical phenomena of the high-gain PCSS.

Lower Bound of Electrical Field for Maintaining a GaAs Photoconductive Semiconductor Switch in High-Gain Operating Mode

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