Abstract-Ultrafast terahertz photoresponse of single and double-walled carbon nanotubes: Optical pump-terahertz probe spectroscopy

Srabani Kar, A.K.Sood,



https://www.sciencedirect.com/science/article/pii/S0008622318312156?via%3Dihub

Photocarrier excitation in conventional semiconductors enhances the conductivity due to increased intraband absorption of terahertz (THz) radiation. We report frequency dependent photoconductivity in terahertz range after 800 nm optical pump excitation in (6,5) semiconducting single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) containing both metallic and semiconducting tubes. The real and imaginary parts of photoconductivity (Δσ(ω)) show non-Drude behavior. In SWCNT, the real part ΔσRe(ω) is positive for low frequency and negative on the high-frequency side of the terahertz spectra. In contrast, DWCNTs show negative ΔσRe(ω) on low frequency and positive on the high-frequency side. This contrasting behavior is explained using Boltzmann transport theory, where the carrier scattering rate is energy dependent. Taking the scattering rate to be dominated by short-range disorder scattering, we show that the Boltzmann transport model captures the unique experimental features of Δσ(ω), for SWCNT as well as DWCNT. Both the semiconducting and metallic nanotubes in DWCNT are shown to contribute to the observed photoconductivity.

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.

Abstract-Tuning photoinduced terahertz conductivity in monolayer graphene: Optical-pump terahertz-probe spectroscopy

Srabani Kar, Dipti R. Mohapatra, Eric Freysz, and A. K. Sood

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.165420

Optical-pump terahertz-probe differential transmission measurements of as-prepared single layer graphene (AG) (unintentionally hole doped with Fermi energy EF at ∼−180meV), nitrogen doping compensated graphene (NDG) with EF∼−10 meV, and thermally annealed doped graphene (TAG) are examined quantitatively to understand the opposite signs of photoinduced dynamic terahertz conductivity Δσ. It is negative for AG and TAG but positive for NDG. We show that the recently proposed mechanism of multiple generations of secondary hot carriers due to Coulomb interaction of photoexcited carriers with the existing carriers together with the intraband scattering can explain the change of photoinduced conductivity sign and its magnitude. We give a quantitative estimate of Δσ in terms of controlling parameters—the Fermi energy EF and momentum relaxation time τ. Furthermore, the cooling of photoexcited carriers is analyzed using a supercollision model which involves a defect mediated collision of the hot carriers with the acoustic phonons, thus giving an estimate of the deformation potential.

DOI: http://dx.doi.org/10.1103/PhysRevB.90.165420

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• Published 16 October 2014
• Received 11 June 2014
• Revised 16 August 2014

©2014 American Physical Society