Mid-infrared and terahertz free-electron laser spectroscopy of zero- and two-dimensional semiconductors

The free-electron laser (FEL) facility FELBE in Dresden provides unique opportunities to advance our knowledge on the interaction of intense mid-infrared and terahertz fields with materials and devices. Its nearly transform-limited ps pulses, which can also be combined with synchronous ps or fs pulses from near-infared tabletop lasers, represent a versatile radiation source for novel experiments, which is also available to external users. This talk reviews some of our recent experimental studies, which are also representative for advanced measurement techniques at FELBE.
In high-quality semiconductor quantum wells, time-resolved photoluminescence allows us to investigate the dynamics of excitons, i.e. two-dimensional, hydrogen-like electron-hole quasiatoms. In particular, applying time-delayed FEL pulses tuned to the intra-excitonic 1s-2p transition (at 9 meV for an 8 nm wide GaAs/AlGaAs quantum well), we have studied the population transfer between the 2p and 2s exciton states. Moreover, strong terahertz pumping in resonance with the 1s-2p transition results in a characteristic splitting (Rabi splitting) of the 1s exciton state, which is a manifestation of the intra-excitonic Autler-Townes effect [1].
In semiconductor quantum dots, resonant excitation between s, p and d sublevels in these zero-dimensional atom-like nanostructures is shown to produce an absorption contrast in aperture-less scattering scanning near-field optical microscopy (s-SNOM). This effect allows us to obtain functional s-SNOM images with deep sub-wavelength resolution, where far-infrared absorption by single electrons produces sufficient contrast to map individual quantum dots [2].
In graphene, FEL-based pump-probe spectroscopy has shown evidence of different relaxation times for excitation energies above and below those of the optical phonons, as well as a transition from induced transmission (bleaching) to induced absorption if the photon energy becomes smaller than twice the Fermi energy [3]. In particular, the latter observation is indicative for interesting potential applications of this two-dimensional semiconductor material, e.g. as an electro-optical modulator.
[1] J. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010)
[2] R. Jacob et al., Nano Lett. 12, 4336–4340 (2012))
[3] S. Winnerl et al., Phys. Rev. Lett. 107, 237401 (2011)