Role of free carriers in the absorption of intense long-wavelength light in solids<i> </i>

Since chip technology based on transistors has reached its economically feasible limit, new approaches are needed to meet the increasing demand for higher computing power.
A novel promising approach uses the nonlinear absorption of intense ultrashort laser pulses in semiconductors and dielectrics. Thus, modulations of the optical properties of the material with frequencies up to the PHz range have been demonstrated. For future applications, a detailed understanding of the nonlinear light absorption processes is needed.
In this contribution, we present our studies on absorption of intense laser light far off a band resonance in ZnO in the wide wavelength range from 1.2 to 10µm. Here, the band gap to the photon energy ratio varies between 4 and 26. In order to monitor the number of excited electrons, we use the onset of lasing, upon population inversion in ZnO thin films. Comparing our experimental results with simulations based on Keldysh and Drude models that include both interband and intraband carrier dynamics, we demonstrate the key role of free-carrier absorption at sub-optical cycle time scale in semiconductors interacting with long wavelength intense ultrashort laser pulses.