## Masterarbeit "ZnO nanowire lasers"

Zinc oxide (ZnO) nanowires provide all requirements for realizing a nanolaser: ZnO is a well studied gain medium, and the cylindrical geometry, together with the end facets, forms a waveguide cavity [1]. In the experiment the population inversion in ZnO is created by multiphoton absorption induced by sub-100 fs laser pulses. Lasing sets in when the gain in the ZnO material overcomes the cavity losses. While the cavity losses are given by the nanowire nature, the gain is a function of the excited electron density and thus depends on the generation rate of electrons during the laser pulse.

The experiments show a very nontrivial dependence of the electron generation rate on the pumping wavelength. By tuning the pumping wavelength from 0.8 µm up to 3.9 µm, which corresponds to absorption of from 3 up to 12 photons and changing the regime of excitation from multiphoton absorption to tunneling [2], the hysteresislike dependence of the lasing threshold on the pumping intensity is observed. With increase in the amount of absorbed photons the lasing threshold grows rapidly increasing by the order of magnitude for the 1.4 µm pumping wavelength (5 photon absorption) but drops to the value close to the 3-photon absorption threshold when mid-IR 3.9 µm pump is used. These results suggest that the generation rate in the tunnel ionization regime is higher than for multiphoton absorption. The master student will work on the theoretical understanding of the origin of this unexpected and important phenomenon.

Simulations can already describe successfully the dynamics of the guided-modes of the electromagnetic field in optically pumped semiconducting nanowires [3][4]. We want to focus now on first-principles calculations of excited electrons in same system, including many-body effects on the carriers. To this end the student will learn timedependent density functional theory [5] and perform ab initio calculations of electronic excitations and electron dynamics in ZnO nanowires.

**Further reading**

- [1] M. A. Zimmler, F. Capasso, S. Müller and C. Ronning, "Optically pumped nanowire lasers: invited review", Semiconductor Science and Technology, 25(2):024001 (2010)
- [2] L. V. Keldysh, "Ionization in the field of a strong electromagnetic wave", Soviet Physics JETP, 20:1307-1314 (1965)
- [3] R. Buschlinger, M. Lorke, and U. Peschel, "Light-matter interaction and lasing in semiconductor nanowires: A combined finite-difference time-domain and semiconductor Bloch equation approach", Phys. Rev. B 91: 045203 (2015)
- [4] R. Buschlinger, M. Lorke, and U. Peschel, "Coupled-Mode Theory for Semiconductor Nanowires", Phys. Rev. Applied, 7:034028 (2017)
- [5] "Fundamentals of Time-Dependent Functional Theory", edited by M.A.L. Marques, N. Maitra, F.M.S. Nogueira, E.K.U. Gross, and A. Rubio, (Springer-Verlag, Berlin Heidelberg, 2012)

**The student's profile**

A sound knowledge of quantum mechanics and solid state physics is requested. Programming capabilities are a plus.