Abstract-Microscopic origins of the terahertz carrier relaxation and cooling dynamics in graphene

• Momchil T. Mihnev,
• Faris Kadi,
• Charles J. Divin,
• Torben Winzer,
• Seunghyun Lee,
• Che-Hung Liu,
• Zhaohui Zhong,
• Claire Berger,
• Walt A. de Heer,
• Ermin Malic,
• Andreas Knorr
• & Theodore B. Norris

http://www.nature.com/ncomms/2016/160525/ncomms11617/abs/ncomms11617.html

The ultrafast dynamics of hot carriers in graphene are key to both understanding of fundamental carrier–carrier interactions and carrier–phonon relaxation processes in two-dimensional materials, and understanding of the physics underlying novel high-speed electronic and optoelectronic devices. Many recent experiments on hot carriers using terahertz spectroscopy and related techniques have interpreted the variety of observed signals within phenomenological frameworks, and sometimes invoke extrinsic effects such as disorder. Here, we present an integrated experimental and theoretical programme, using ultrafast time-resolved terahertz spectroscopy combined with microscopic modelling, to systematically investigate the hot-carrier dynamics in a wide array of graphene samples having varying amounts of disorder and with either high or low doping levels. The theory reproduces the observed dynamics quantitatively without the need to invoke any fitting parameters, phenomenological models or extrinsic effects such as disorder. We demonstrate that the dynamics are dominated by the combined effect of efficient carrier–carrier scattering, which maintains a thermalized carrier distribution, and carrier–optical–phonon scattering, which removes energy from the carrier liquid.

Abstract-Terahertz-visible two-photon rotational spectroscopy of cold OD−

Seunghyun Lee, Daniel Hauser, Olga Lakhmanskaya, Steffen Spieler, Eric S. Endres, Katharina Geistlinger, Sunil S. Kumar, and Roland Wester

Phys. Rev. A 93, 032513 – Published 21 March 2016

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.93.032513

We present a method to measure rotational transitions of molecular anions in the terahertz domain by sequential two-photon absorption. Ion excitation by bound-bound terahertz absorption is probed by absorption in the visible on a bound-free transition. The visible frequency is tuned to a state-selective photodetachment transition of the excited anions. This provides a terahertz action spectrum for just a few hundred molecular ions. To demonstrate this we measure the two lowest rotational transitions, J=1←0 and J=2←1 of OD− anions in a cryogenic 22-pole trap. We obtain rotational transition frequencies of 598 596.08(19) MHz for J=1←0 and 1 196 791.57(27) MHz forJ=2←1 of OD−, in good agreement with their only previous measurement. This two-photon scheme opens up terahertz rovibrational spectroscopy for a range of molecular anions, in particular for polyatomic and cluster anions.

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Abstract-Terahertz-visible two-photon rotational spectroscopy of cold OD-

Seunghyun Lee, Daniel Hauser, Olga Lakhmanskaya, Steffen Spieler, Eric S. Endres, Katharina Geistlinger, Sunil S. Kumar, Roland Wester

(Submitted on 26 Feb 2016)

http://arxiv.org/abs/1602.08303

We present a method to measure rotational transitions of molecular anions in the terahertz domain by sequential two-photon absorption. Ion excitation by bound-bound terahertz absorption is probed by absorption in the visible on a bound-free transition. The visible frequency is tuned to a state-selective photodetachment transition of the excited anions. This provides a terahertz action spectrum for just few hundred molecular ions. To demonstrate this we measure the two lowest rotational transitions, J=1<-0 and J =2<-1 of OD- anions in a cryogenic 22-pole trap. We obtain rotational transition frequencies of 598596.08(19) MHz for J=1<-0 and 1196791.57(27) MHz for J=2<-1 of OD-, in good agreement with their only previous measurement. This two-photon scheme opens up terahertz rovibrational spectroscopy for a range of molecular anions, in particular for polyatomic and cluster anions.