Samuel Brem, Florian Wendler, and Ermin Malic
https://journals.aps.org/prb/accepted/77077O7aQ331c431f4bb34537ee0565d99b84ec19
In the presence of strong magnetic fields the electronic bandstructure of graphene drastically changes. The Dirac cone collapses into discrete non-equidistant Landau levels, which can be externally tuned by changing the magnetic field. In contrast to conventional materials, specific Landau levels are selectively addressable using circularly polarized light. Exploiting these unique properties, we propose the design of a tunable laser operating in the technologically promising terahertz spectral range. To uncover the many-particle physics behind the emission of light, we perform a fully quantum mechanical investigation of the non-equilibrium dynamics of electrons, phonons, and photons in optically pumped Landau-quantized graphene embedded into a high quality optical cavity. The gained microscopic insights allow us to predict optimal experimental conditions to realize a technologically promising terahertz laser.
- 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.
Florian Wendler; Ermin Malic
http://spie.org/Publications/Proceedings/Paper/10.1117/12.2186880
We investigate the carrier dynamics in Landau-quantized graphene within the density matrix formalism. In particular, we focus on the carrier-light interaction addressing the impact of higher-order polarizations beyond the optical selection rules. We find that these terms are in general negligible, however, there are regimes, where they even become crucial for the carrier dynamics. Our calculations show that for short excitation pulses, very small Landau level broadenings, and certain configurations of magnetic field strength, Fermi energy, and excitation energy, higher-order polarizations need to be taken into account.
http://www.nature.com/srep/2015/150729/srep12646/full/srep12646.html
Terahertz (THz) technology has attracted enormous interest with conceivable applications ranging from basic science to advanced technology. One of the main challenges remains the realization of a well controlled and easily tunable THz source. Here, we predict the occurrence of a long-lived population inversion in Landau-quantized graphene (i.e. graphene in an external magnetic field) suggesting the design of tunable THz Landau level lasers. The unconventional non-equidistant quantization in graphene offers optimal conditions to overcome the counteracting Coulomb- and phonon-assisted scattering channels. In addition to the tunability of the laser frequency, we show that also the polarization of the emitted light can be controlled. Based on our microscopic insights into the underlying many-particle mechanisms, we propose two different experimentally realizable schemes to design tunable graphene-based THz Landau level lasers.
