Generation of wavelength-tunable few-cycle pulses in the mid-infrared at repetition rates up to 10 kHz.

We demonstrate a compact and tunable mid-infrared light source that provides carrier-envelope-phase (CEP)-locked pulses at repetition rates from 500 Hz to 10 kHz. The seed pulses were generated by intra-pulse difference frequency mixing of the output of an Yb:KGW regenerative amplifier that had been spectrally broadened by continuum generation using multiple plates. Then, a two-stage optical parametric amplifier was used to obtain output energies of about 100 µJ/pulse for center wavelengths between 2.8 and 3.5 µm. Owing to the intense pulse energies, it was possible to compress the multi-cycle pulses down to two-cycle pulses using YAG and Si plates.

Modifying angular and polarization selection rules of high-order harmonics by controlling electron trajectories in k-space.

Recent advances in generation of strong laser pulses have enabled the acceleration of electrons in solids into regions far away from the band edge. Because nonlinear currents can be generated by laser-driven carriers in the non-parabolic region, tailored laser fields may allow control of optical properties of high-order harmonics (HHs). So far, investigations on laser-induced nonlinear optical phenomena have focused on the simple electron motion induced by linearly or elliptically polarized fields. However, more complex trajectories can be important for development of novel optoelectronic devices. Here, we show that a weak laser field (optical frequency is ω2) applied in a direction orthogonal to a strong main field (ω1) enhances certain HH intensity components of bulk GaSe by a factor of 100. Good agreement between the experiments and calculations shows that manipulation of the electron trajectory allows breaking inversion symmetry of the electronic states felt by the accelerated electrons and leading to a modification of selection rules for frequency-mixing processes of HHs. Owing to our usage of non-integer multiples for ω1 and ω2, it is found that the generation of HHs constitutes a novel way of ultrafast control of light polarization and optical switching.

Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses.

We have systematically investigated the spatial and temporal dynamics of crystallization that occur in the phase-change material Ge_{2}Sb_{2}Te_{5} upon irradiation with an intense terahertz (THz) pulse. THz-pump-optical-probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This fact causes the large enhancement of electric field associated with the THz pulses only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer-scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.