Coherent Multidimensional Off-resonant THz Spectroscopy on Semiconductors (original) (raw)
The work presented in this thesis is motivated by three interconnected objectives: to study new coherent sources of high-field ultra-broadband terahertz (THz) pulses for multi-octave THz spectroscopy, to develop novel techniques in the frequency range 0.1-30 THz capable of accessing single and multiple quantum coherences off-resonantly under nonperturbative conditions and to understand their dynamics on their fundamental ultrafast time scale. For the first time, the coherent generation of ultrashort MV/cm field pulses with a spectral content covering the frequency range 0.1-30 THz continuously is demonstrated in the organic crystal DSTMS by non-phase-matched difference frequency mixing within the broad spectrum of femtosecond near infrared pulses. Coherent multidimensional terahertz spectroscopy (CMTS) has developed into an important technique for, e.g, driving low-energy excitations in both bulk and nanostructured semiconductors and monitoring their coherent dynamics. A novel CMTS technique employing three phase-locked inter-delayed THz pulses is implemented. It relies on a collinear interaction of the pulses with a sample, so that different contributions to the nonlinear signal are emitted in the same direction, and thus can be measured all at once. Phase-resolved detection by electro-optic sampling allows for measuring amplitude and absolute phase of the nonlinear signal, thereby enabling to investigate the evolution of coherent interactions between quantum excitations in real time. In CMTS, the nonlinear signal is dissected into the distinct nonlinear contributions in the corresponding multidimensional frequency domain. This novel technique is applied to study the nonlinear off-resonant response of two undoped bulk semiconductors, the wide-bandgap ferroelectric lithium niobate (LiNbO3, Eg = 1000 THz) and the narrow-bandgap indium antimonide (InSb, Eg = 40 THz), in the nonperturbative regime. In LiNbO3, the nonlinear signal is generated by a femtosecond nonlinear shift current (SC), which is a distinctive characteristic of the bulk photovoltaic effect. The SC stems from the lack of inversion symmetry and the ultrafast dephasing of the field-induced interband coherent polarization due to a sufficiently high decoherence rate, which enables tunneling of electrons from the valence to the conduction band. In InSb, the nonlinear signal is caused by the coherent response on both the two-phonon and two-photon interband excitations. The impulsive generation of the two-phonon coherent polarization is enhanced by the large interband transition dipole of InSb, resulting in much larger polarization amplitudes than in the regime of linear response. In the future, CMTS could be applied for example to gain further insight in magnetic multi-quantum coherences of materials with long range magnetic order. Furthermore this novel concept can be easily implemented into two-colour nonlinear spectroscopic experiments.
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