Sub-Terahertz Complex Permittivity Measurement Method Using Cavity Switches
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Sub-Terahertz Complex Permittivity Measurement Method Using Cavity Switches Maxim L. Kulygin 1
& Ilya A. Litovsky
1
Received: 24 April 2020 / Accepted: 4 September 2020/ # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
New method of measurement of dielectric constant and loss tangent for small samples of lossy dielectrics, including plain semiconductors, using non-oversized resonators being parts of the recently developed cavity switches for sub-terahertz bands is proposed. Strong influence of the tangent loss value to the Q-factor of the resonator is calculated and observed in experiments. A simple monosemantic algorithm to retrieve explicit mathematical expressions for the complex permittivity from the experimental data is demonstrated. The advantage of the new method over the known ones is much lower (up to million times) volume of the sample needed to analyze. Keywords Permittivity measurement . Method resonator . Cavity switch . Terahertz . Dielectric . Semiconductor . Loss tangent . Q-factor
1 Introduction The recently developed sub-terahertz cavity switches [1, 2], with the fastest switching duration of nanoseconds [3, 4], microseconds [5, 6], and even up to 10 s [7, 8], based on rapid changing of the resonant cavity’s Q-factor [9] due to laser-induced photoconductivity [10] in active elements made of plain semiconductors, have many different applications from radars to spectroscopy [11–13]. The switches are intended to be driven by a standard 532-nm laser with pulse duration of several nanoseconds and energy value of about 10 nJ per pulse [1, 4, 5]. The most of the prospective applications intend us to solve the direct problem, i.e., we have a semiconductor with known properties, and so we need to analyze frequency characteristics of the switch [14]. The present research is devoted to a reversed problem where the resonator’s properties are known and calibrated, and we have to measure the complex permittivity ε = ε ' + iε ' ' of the active element’s semiconductor (or just a lossy dielectric in case of no driving laser input signal) being the part of the sub-terahertz resonator (see Fig. 1). * Maxim L. Kulygin [email protected]–nnov.ru
1
Institute of Applied Physics (IAP) RAS, Nizhny Novgorod, Russia
Journal of Infrared, Millimeter, and Terahertz Waves
Fig. 1 Cross section (a) and 3D scheme (b) of the resonator cavity for the sub-terahertz cavity switch
The resonator cavity is formed by the main single-mode TE10 waveguide containing the microwave input and output, the transversal subcritical waveguide with the laser input, and the plate of active element—the rectangular wafer of an arbitrary dielectric, with known thickness d and unknown complex permittivity ε, placed nearby the junction of the two waveguides. The field distribution inside the switch is calculated from the Maxwell’s set of equations with a modification of the direct time integration method of finite difference time domain (FDTD) [15, 16]. The field distribution in the time domain lets us to obtain the po
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