Ultrafast high-field transport in semiconductors
Zusammenfassung der Projektergebnisse
The project aims to use intense THz or midinfrared (MIR) pulses to measure electronic properties of solids. These ultrashort THz/MIR pulses are generated by nonlinear optical processes from the output of a femtosecond Ti:sapphire oscillator-amplifier system working in the near-infrared (wavelength around 800 nm) spectral range. Since the optimal pulse shape in the near-infrared strongly depends on the required THz/MIR pulses, in the first part of the project we extended the present setup to two amplifiers. The pulses from the two amplifier are perfectly synchronized since both amplify pulses from the same femtosecond oscillator. Having two amplifiers instead of one amplifier with double the pulse energy has the advantage of being able to separately optimize the spectral intensity and spectral phases of the two output pulses. As an example, one of the two amplifier pulses can be optimal for THz generation, the other for MIR generation. In most of our measurements we use electrooptic sampling to determine the electric fields of the THz/MIR pulses as a function of time. This yields the complete information about the pulses, including the spectrum, the absolute phase and the electric field amplitude. The full information about nonlinear processes using two pulses incident on the sample is obtained from two-dimensional scans along two time axes, the real time (this yields the electric fields as a function of time) and the delay between the two pulses. The nonlinear signal as a function of these two times is obtained as the difference between the electric field when both pulses are present minus the electric fields when only a single pulse is incident on the sample. Choppers in the two beams synchronized to the amplifier repetition rate allow the measurement of these different electric fields. A twodimensional Fourier transform of the nonlinear signal allows the separation of the various nonlinear processes, e.g., pump-probe, four- and six-wave-mixing signals. The setup was used for the investigation of several materials, among them graphene, LiNbO3 , and GaAs/AlGaAs quantum wells. In epitaxial multi-layer graphene, THz-pump–THz-probe measurements show induced absorption. This induced absorption decays with time constants of a few ps. The time constants decrease with temperature and increase with pump intensity. Measurements with pump pulses in the MIR pump and in the near-infrared pump show very similar results. These results can be explained by the pump generating electron-hole pairs by interband transitions and subsequent acceleration of the carriers within their bands (intraband transitions). Thus, the induced absorption is caused by having more carriers available for intraband transitions. In a good approximation it does not matter how the carriers were generated, whether by THz or by near-infrared radiation. This explanation requires that most of the layers in the multi-layer graphene are undoped, which is in contrast to most other types of graphene. LiNbO3 is a ferroelectric insulator. It was one of the first solids to show the photogalvanic effect, i.e., the generation of a electric current when light is shone on the crystal. The explanation is that the light generates free charge carriers, which are then accelerated by the built-in electric field to generate the current. One normally expects that the generation of free carriers is only possible if the photon energy is above the band gap, which is in LiNbO3 in the ultraviolet spectral region. Nevertheless, we were able to observe the photogalvanic effect in LiNbO3 with THz radiation, which has photon energies 500 to 1000 times lower than the band gap. The explanation for this counter-intuitive result is that the high electric fields allow tunneling from the valence to the conduction band. Intersubband transitions are transitions between confined states in low-dimensional semiconductor structures. Our GaAs/AlGaAs sample had a transition frequency in the MIR range. With resonant MIR pulses we could observe Rabi oscillations, i.e., coherent oscillations of the excited-state population. An additional electric field leads to a decrease of the overlap between ground- and excited state wavefunctions and thus to a decrease of the transition dipole moment. In contrast to this effect, we observed that the electric field of a nonresonant THz pulse resulted in an absorption increase. Additionally, despite the decrease of the transition diole moment the Rabi frequency remained unchanged. The results can be reproduced with calculations of a two-level system without rotating-wave approximation and including radiative coupling.
Projektbezogene Publikationen (Auswahl)
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High-field terahertz bulk photovoltaic effect in lithium niobate, Phys. Rev. Lett. 112, 146602 (2014)
C. Somma, K. Reimann, C. Flytzanis, T. Elsaesser, and M. Woerner
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Terahertz radiative coupling and damping in multilayer graphene, New J. Phys. 16, 013027 (2014)
P. Bowlan, E. Martinez-Moreno, K. Reimann, T. Elsaesser, and M. Woerner
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Ultrafast terahertz response of multi-layer graphene in the nonperturbative regime, Phys. Rev. B 89, 041408(R) (2014)
P. Bowlan, E. Martinez-Moreno, K. Reimann, T. Elsaesser, and M. Woerner
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Nonresonant coherent control: Intersubband excitations manipulated by a nonresonant terahertz pulse. Phys. Rev. B 92, 085306 – Published 17 August 2015
G. Folpini, D. Morrill, C. Somma, K. Reimann, M. Woerner, T. Elsaesser, and K. Biermann
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Ultra-broadband terahertz pulses generated in the organic crystal DSTMS. Opt. Lett.40,14, pp. 3404-3407 (2015)
C. Somma, G. Folpini, J. Gupta, K. Reimann, M. Woerner, and T. Elsaesser
