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

• Published in: ACS nano Vol. 12; no. 2; pp. 1785 - 1792
• Main Authors: Kar, Srabani, Nguyen, Van Luan, Mohapatra, Dipti R, Lee, Young Hee, Sood, A. K
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 27-02-2018
• Subjects: optical pump−terahertz probe; doping; short-range scattering; bilayer graphene; photoinduced terahertz conductivity
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/acsnano.7b08555

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 at 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 to 4 K, without any empirical fitting procedures adopted so far in the literature. This helps us to understand the intraband conductivity of photoexcited hot carriers in 2D materials.