Abstract-Comparative study on calculated terahertz absorption spectra of different heterostructure materials with external magnetic field

Lei Cao

http://iopscience.iop.org/article/10.1088/2399-6528/aadbba

Room temperature operation and frequency tunability are attractive advantages of terahertz (THz) detectors based on the two-dimensional electron gas (2DEG) in the heterostructure material with external magnetic field. This work compared absorption spectra of four typical heterostructure materials (AlGaN/GaN, InAlN/GaN, AlGaAs/GaAs and SiGe/Si) with the nonlocal magnetoconductivity model at ambient and cryogenic temperatures in the frequency range 0-5 THz. The GaN based materials have the highest absorption amplitude, while the AlGaAs/GaAs material owns the largest frequency shift as the magnetic field increases up to 10 tesla, although superconducting magnets at cryogenic temperature are usually employed to provide that high magnetic field. The numerical results showed that the absorption properties (amplitude and frequency) could be further optimized by other parameters, such as the period and filling factor of the grating coupler, and the barrier thickness.

Abstract-Electro-optic sampling of optical pulses and electron bunches for a compact THz-FEL source

Bang Wu, 

Zhe Zhang, 

Lei Cao,  

Qiang Fu, 

Yongqian Xiong

https://www.sciencedirect.com/science/article/pii/S1350449518302895

A systematic one dimensional analytical model describing the full electromagnetic propagation in electro-optic (EO) diagnostic is proposed and designed for a terahertz FEL source at HUST. Our model contains two main aspects: the propagation of the THz pulse (1.5–6 THz) along the transport line and the THz EO process in the detection crystal. The diffraction, Fabry–Perot and focusing effects are included in the THz propagation process. The phase mismatch, the frequency-dependent EO coupling coefficient and geometrical overlap between probe and THz pulses as well as the group velocity dispersion and finite duration of the probe pulse are considered in the THz EO process. The calculation shows that the diffraction in the TPX window and the Fabry–Perot effect in the GaP crystal are crucial for retrieving the details of the THz optical pulse. While for electron bunches, the detected Coulomb electric field will be significantly broadened due to the relatively low relativistic Lorenz factor (γ=10–20$\mathrm{}\mathrm{}$). Therefore, a convenient deconvolution algorithm is utilized to reconstruct THz electric field from the distorted EO signals, where the truncated singular value decomposition method is employed. Our algorithm is verified to work well even with a low signal-to-noise ratio (20 dB) in the future measurement.

Abstract- Terahertz Electro-Optic Sampling in Thick ZnTe Crystals Below the Reststrahlen Band With a Broadband Femtosecond Laser

Bang Wu,  Lei Cao,   Zhe Zhang,   Qiang Fu,   Yongqian Xiong

https://ieeexplore.ieee.org/document/8319525/

The method of electro-optic (EO) sampling of terahertz (THz) pulses in thick EO crystals leads to waveform distortions due to effects of phase mismatch, dispersive propagation, and absorption. In this paper, we demonstrate theoretically and experimentally that EO sampling with a broadband femtosecond (fs) laser could significantly eliminate these distortions below the Reststrahlen band. Our simulation results show that the oscillations and dips in the signals of EO response function of thick crystals are smoothed out by the broadband spectrum of the femtosecond laser. In the experiment, we use a laser with a bandwidth of 100 nm and a low temperature GaAs photoconductive antenna to generate typical THz pulses (0.1-3 THz). The measurement results confirm that a {110}-oriented 3-mm-thick ZnTe crystal is an attractive EO material for sampling THz pulse below 3 THz and agree with simulation results. This technique is particularly useful in areas where a large time window is needed, for example in the THz time-domain spectroscopy system.

Abstract-Terahertz magnetoplasmon-polaritons with nonlocal corrections for lossy two dimensional electron gas in GaN-based heterostructures

Lei Cao,  Qiang Fu, Bang Wu and Yongqian Xiong

http://iopscience.iop.org/article/10.1088/1361-648X/aa80c6

The high electron concentration in GaN-based heterostructures necessitate the consideration of nonlocal corrections in the magnetoconductivity for the study of magnetoplasmon-polaritons (MPPs) in the lossy two dimensional electron gas system with grating couplers. With the complete set of Maxwell equations, the complex dispersion frequency and absorption spectrum are calculated numerically at different magnetic field (B max  =  10 T) and plasmon wavevectors (k max  =  5  ×  107m−1). Taking the AlGaN/GaN structure as a representative case, we observe the cyclotron resonance and its high order harmonics, as well as the MPP modes. The effective aspect ratio of the grating is determined according to the absorption spectrum. Effects of the nonlocal corrections on the dispersion frequency and absorption spectrum are analyzed in detail. The contributions of LO phonons, electron collisions, THz wave polarizations and retardations are also discussed.