Material characterization using micro-, mm-, and THz radiation

1. Surface Loss Bulk Loss Lab/ Frequency tan  3.8 x 10-4 2.0 x 10-5 2.7 x 10-4 NCCU/162 GHz Karlsruhe/140 GHz 2.0 x 10-5 2.3 x 10-4 3.5 x 10-4 Material characterization using micro-, mm-, and THz radiation Current BWO setup at NCCU for materials research Biswadev Roy, Charles R. Jones, Marvin Wu, and , Branislav Vlahovic Department of Mathematics and Physics, North Carolina Central University, Durham, NC 27707 Stabilizer Proposed Work Using BWO facility Millimeter Wave loss tangent setup and results Past Work Focus on time resolved mm-wave conductivity (TRMMWC) and TRTS for photovoltaic materials • Past measurement of dielectric properties (loss-tangents) for ultra-low loss materials in D-Band ( CVD diamond, and SiC) conducted by C.R. Jones, and J.M. Dutta at NCCU using backward wave oscillator (BWO) source and an Open Resonator System (1980’s - 2008). Focus:To characterize materials for Gyrotron and MW fusion vacuum windows • The challenge: characterizing high power windows for nuclear fusion • Bottle-Neck : Electron-Cyclotron-Wave Resonance (ECR)-Current Drive (CD) heating-Systems • ECR current drive systems contribute a major power source for heating (H) and for power supply (CD) in next generation large scale plasma experiments:Tokamaks: ITER ( 20 MW at 170 GHz) • Stellarators: W7-X (8-10 MW at 140 GHz) • Key components in the development of ECH&CD systems: • MM-wave sources: High power gyrotrons in continuous wave operation (1MW) • - Output window at the gyrotron: Risk of failure by thermal crack formation and temperature escalation • Barrier window at torus: proven limits of radiation tolerance. • NCCU created BWO-based Open-Resonator laboratory for characterizing gyrotron windows SiC, Diamond Lorentzian fit: Tuning Frequency vs. Power (1000 point high resolution scan of resonance curve) • dBm-> nW-> non-linear least squares fit (Lorentzian line shape) 1. fR and Δf(3B linewidth) calculated, Q = fR / Δf 2. Q computed for “antinodes” where samples absorption max 3. Tan δ then computed at various Temperature. photoresponse cycle of the microwave absorption probe. ( From: NREL, FP-TRMC training slides) From fig. 5 of Ying Diao et al. (Nature materials, Vol.12, 2013). a) shows the histogram of hole mobility between single-crystalline At millimeter wavelengths, photons can interact only with transverse phonons with wavenumber near zero however, direct creation of a single TO phonon is forbidden. In the two-phonon difference process, the absorption of an photon results in excitation of an optical phonon and annihilation of an acoustic phonon, satisfying the equality: 1 - 2 =  , where 1 , 2 , and  are frequencies of the optic and acoustic phonons, and the electromagnetic quanta respectively. The millimeter-wave photon energy is less than the gap between the transverse optical (TO) and transverse acoustic (TA) phonon branches, so this process is not energetically allowed in the limit of infinite phonon lifetimes. Lifetime broadening of the phonons makes the process possible. When the process described here is dominant, the dependence of loss tangent on frequency, f, and temperature, T, has been predicted* to take the form domain measured along the solution shearing direction from 50-60 FETs – it also gives the mobility average, COV, b) transfer characteristics, and c) hysteresis characteristics. Two-phonon Difference Process Experimentally (Jones et al., 2011) • TRMMWC should enable measurement of intrinsic properties of these near-ideal semi conducting material • TRMMWC Study for photoconductance decay in P3HT using 75-300 GHz BWO as probe • TRMMWC for measuring mobility as function of crystalline polymorphs (possible collaboration with N.C. State University, and Stanford University) Temperature at millimeter wavelengths is well-described in crystalline silicon carbide by a two phonon difference process in which an optical phonon is created and an acoustic phonon is annihilated. The data is well fit by Garin’s theory and yields for the fitting parameters: C = (16.71 ± 0.94 )x 103 TD = 1071± 24 Setup for TRMMWC Comparison with Institut für Materialforschung,Germany 3.8 x 10-4 Possible to perform THz pump-probe using CW UV laser, pulsed THz electron accel., and BWO in series Tan [ Angle(Resistive (E-lossy)/Reactive (E-lossless) ] Acknowledgement Reference: Jones, C.R., J.M. Dutta, Guofen Yu and Yuanci Gao, Journal of Infrared, Millimtere and THz Waves, Vol. 32, Issue 6, pp 838-847, (2011) Supported by: NASA: NNX09AV07A BWO “ISTOK” 130-180 GHz, model OTK433