Abstract-Ultrafast nonlinear travel of hot carriers driven by high-field terahertz pulse

Hee Jun Shin, Van Luan Nguyen, Seong Chu Lim, and Joo-Hiuk Son

http://iopscience.iop.org/article/10.1088/1361-6455/aac59a

We aim to generate high-intensity terahertz (THz) electric fields and study nonlinear phenomena in GaAs and graphene to investigate their applications. To obtain a high-efficiency intense THz field, we employ the tilted pump-pulse front technique using a LiNbO3 crystal. With this technique, we obtain a THz field strength of over 300 kV cm−1. We investigate the hot-carrier dynamics in n- and p-type GaAs driven by high-field THz pulses. Although both samples show similar carrier concentrations, the nonlinear THz responses show different trends. Owing to hot-carrier generation, intervalley scattering is dominant in n-type GaAs, and intervalence band scattering is the main cause in p-type GaAs. In addition, we study the hot-carrier dynamics in graphene with the grain-size dependency. Although graphene has the same Fermi level regardless of the grain size, the THz responses are different for large- and small-grained graphene: charged impurity scattering in large-grained graphene and defect scattering in small-grained graphene. From these results, our study provides insights into high-speed electronics applications.

Abstract-Ultrafast Spectral Photoresponse of Bilayer Graphene: Optical Pump-Terahertz Probe Spectroscopy

Srabani Kar, Van Luan Nguyen, Dipti R. Mohapatra, Young Hee Lee,  A. K. Sood

http://pubs.acs.org/doi/abs/10.1021/acsnano.7b08555?journalCode=ancac3

Photoinduced terahertz conductivity Δσ(ω) of Bernal stacked bilayer graphene (BLG) with different dopings is measured by time resolved optical pump terahertz probe spectroscopy. The real part of photoconductivity Δσ(ω) (ΔσRe(ω)) is positive throughout the spectral range 0.5-2.5 THz in low doped BLG. This is in sharp contrast to Δσ(ω) for high doped bilayer graphene where ΔσRe(ω) is negative on low frequency and positive on the high frequency side. We use Boltzmann transport theory to understand quantitatively the frequency dependence of Δσ(ω), demanding the energy dependence of different scattering rates such as short-range impurity scattering, Coulomb scattering, carrier-acoustic phonon scattering, and substrate surface optical phonon scattering. We find that the short-range disorder scattering dominates over other processes. The calculated photoconductivity captures very well the experimental conductivity spectra as a function of lattice temperature varying from 300 K to 4K, without any empirical fitting procedures adopted so far in the literature. This helps to understand intraband conductivity of photo-excited hot carriers in 2D materials.