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1. Terahertz Signal Generation Measurements in Photoconductive Antennas using Time Domain Spectroscopy System

   Santos, J; El-Shenawee, M. (December 2021, Antennas and Propagation Society International Symposium)

   This paper presents the signal generation measurements of LT-GaAs photoconductive antenna (PCA) emitters. These measurements were developed in an open-bench time-domain spectroscopy (TDS) system. The main challenges presented here are associated with the alignment of the PCA devices and the location of the terahertz (THz) pulse with respect to the optical delay in the system. The position of the slow delay line was crucial for the location of the THz pulse, which helped in the correct alignment process of the emitter and detector devices. 

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2. Experimental and Computational Analysis of Broadband THz Photoconductive Antennas

   https://doi.org/10.1155/2023/6682627  

   Uttley, Zachary; Santos Batista, Jose; Pirzada, Bilal; El-Shenawee, Magda (February 2023, International Journal of Antennas and Propagation)

   Gennarelli, Claudio (Ed.)

   The work presents the fabrication and measurements of four LT-GaAs photoconductive terahertz (THz) antennas with different geometries of metallic electrodes. The goal is to analyze the overall bandwidth of the antennas through a comparison between the spectra of the generated photocurrent in the antenna gap, the radiated electric field THz pulse, and the S11 parameter of the metallic electrodes. The photocurrent density and the S11 parameters are computed using COMSOL multiphysics, while the generated THz pulse was experimentally measured using a time-domain spectroscopy system. The polarizations of the photoconductive antennas are experimentally measured, using x-cut quartz crystal halfwave plates, showing polarization in the direction of the electrode’s long axis. Pinholes are used to verify system alignment and quality of the radiated signal spectra. The results show that the spectra of the radiated THz pulses in all four antennas considered in this work are dominated by the behavior of the S11 parameter at the lower part of the frequency band, but with the decreasing photocurrent dominating the spectra at higher frequencies. 

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3. Non-invasive low charge electron beam time-of-arrival diagnostic using a plasmonics-enhanced photoconductive antenna

   https://doi.org/10.1063/1.5052631  

   Snively, E_C; Yardimci, N_T; Jacobson, B_T; Jarrahi, M.; Musumeci, P.; Murokh, A. (November 2018, Applied Physics Letters)

   The use of a plasmonics-enhanced photoconductive antenna (PCA) optically gated by a near infrared (NIR) pulse enables non-invasive time-of-arrival measurements of a low charge electron beam with respect to the NIR reference, achieving picosecond resolution. The measured signal values show the expected scaling with the beam charge and distance from PCA to the beam axis, as the PCA samples the electric field of the passing electron beam. We operate the device with an NIR spot size much larger than the PCA active-area, resulting in a very simple optical setup and alignment procedure, making the plasmonics-enhanced PCA a preferred alternative to more complex timing diagnostics for applications requiring non-invasive picosecond or sub-picosecond timestamping. 

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4. Black phosphorus photoconductive terahertz antenna: 3D modeling and experimental reference comparison

   https://doi.org/10.1364/JOSAB.419996  

   Batista, Jose_Santos; Churchill, Hugh_O_H; El-Shenawee, Magda (March 2021, Journal of the Optical Society of America B)

   This paper presents a 3D model of a terahertz photoconductive antenna (PCA) using black phosphorus, an emerging 2D anisotropic material, as the semiconductor layer. This work aims at understanding the potential of black phosphorus (BP) to advance the signal generation and bandwidth of conventional terahertz (THz) PCAs. The COMSOL Multiphysics package, based on the finite element method, is utilized to model the 3D BP PCA emitter using four modules: the frequency domain RF module to solve Maxwell’s equations, the semiconductor module to calculate the photocurrent, the heat transfer in solids module to calculate the temperature variations, and the transient RF module to calculate the THz radiated electric field pulse. The proposed 3D model is computationally intensive where the PCA device includes thin layers of thicknesses ranging from nano- to microscale. The symmetry of the configuration was exploited by applying the perfect electric and magnetic boundary conditions to reduce the computational domain to only one quarter of the device in the RF module. The results showed that the temperature variation due to the conduction of current induced by the bias voltage increased by only 0.162 K. In addition, the electromagnetic power dissipation in the semiconductor due to the femtosecond laser source showed an increase in temperature by 0.441 K. The results show that the temperature variations caused the peak of the photocurrent to increase by $\sim <\#comment/>3.4\mathrm{\%<\#comment/>}$and $\sim <\#comment/>10\mathrm{\%<\#comment/>}$, respectively, under a maximum bias voltage of 1 V and average laser power of 1 mW. While simulating the active area of the antenna provided accurate results for the optical and semiconductor responses, simulating the thermal effect on the photocurrent requires a larger computational domain to avoid false rise in temperature. Finally, the simulated THz signal generation electric field pulse exhibits a trend in increasing the bandwidth of the proposed BP PCA compared with the measured pulse of a reference commercial LT-GaAs PCA. Enhancing signal generation and bandwidth will improve THz imaging and spectroscopy for biomedical and material characterization applications. 

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5. Challenges in Measurement of Broadband THz Photoconductive Antennas

   Zach Uttley; Jose Santos Batista; Mahmudul Doha; Katie Welch; Hugh Churchill; and Magda El-Shenawee (January 2023, ARFTG Microwave Measurement Conference)

   ARFTG (Ed.)

   This work utilizes an open bench time-domain spectroscopy system to measure the THz pulse and spectrum of photoconductive antennas (PCAs) with two different active layer materials: low temperature gallium arsenide (LT-GaAs) and 2D black phosphorous (BP). COMSOL Multiphysics modeling of the PCAs has shown that using BP as an active layer greatly increases the optical to THz conversion efficiency compared to LT-GaAs. However, the fabrication and measurement of both devices has demonstrated the opposite. 

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