Detection of Sub-Terahertz Waves Modulated by Ionization-induced Charge Carriers using a Sub-wavelength Antenna

• Main Authors: Kim, Yushin, Jeong, Diana, Coffee, Ryan, Levin, Craig S.
• Format: Conference Proceeding
• Language: English
• Subjects: Antenna measurements; Charge carriers; HEMTs; Ionizing radiation; Probes; Transmitting antennas; Ultrafast optics
• ISSN: 2577-0829
• DOI: 10.1109/NSS/MIC44867.2021.9875756

We are currently exploring the modulation of optical properties as an alternative mechanism for ionizing radiation detection for time-of-flight positron emission tomography (TOF-PET) rather than scintillation. "Pump-probe spectroscopy" is one of the most promising approaches for ultrafast free charge carrier detection, with a goal to achieve sub-10 ps coincidence time resolution for TOF-PET. Electromagnetic waves with sub-terahertz wavelength can interact strongly with the charge carriers generated from the ionization process due to the well-matched photon energy and the intraband transition energy. However, the mismatch between the millimeter-length wavelength of the sub-terahertz waves and the size of the spatial distribution of charge carriers limit the amount of probe wave modulation. We employed a sub-wavelength antenna array to overcome this spatial mismatch. The probe waves were manipulated to be confined below the diffraction limit to maximize the light-matter interaction using the antenna. The transmission through the antenna decreased 32% in a finite-difference time-domain (FDTD) simulation as a result of free charge carriers produced from 511 keV interactions. We experimentally measured the transmission changes of the continuous terahertz wave while it transmits to the antenna on a silicon substrate. The terahertz waves were focused on the antenna to maximize the field intensity in the 200 μm spacing between each antenna. A high electron mobility transistor (HEMT) attached to the sample measured the intensity of the near-field transmitted wave, with triggering to focus on only the possible ionization events. We observed the probe wave was modulated by charge carriers generated as a result of the ionizing radiation absorbed in the sub-wavelength antenna. The experimentally measured transmission spectrum shows a 6.3 times larger standard deviation than the noise at 1.745 GHz under irradiation with a Tl-204 beta source. This result suggests the further investigation of this novel method of ionizing radiation detector using sub-THz probe waves.