Efficient Terahertz Harmonic Generation with Coherent Acceleration of Electrons in the Dirac Semimetal Cd_{3}As_{2}.

We report strong terahertz (∼10^{12} Hz) high harmonic generation at room temperature in thin films of Cd_{3}As_{2}, a three-dimensional Dirac semimetal. Third harmonics are detectable with a tabletop light source and can be as strong as 100 V/cm by applying a fundamental field of 6.5 kV/cm inside the film, demonstrating an unprecedented efficiency for terahertz frequency conversion. Our time-resolved terahertz spectroscopy and calculations also clarify the microscopic mechanism of the nonlinearity originating in the coherent acceleration of Dirac electrons in momentum space. Our results provide clear insights for nonlinear currents of Dirac electrons driven by the terahertz field under the influence of scattering, paving the way toward novel devices for high-speed electronics and photonics based on topological semimetals.

Optical parametric amplification of carrier-envelope phase-stabilized mid-infrared pulses generated by intra-pulse difference frequency generation.

We report on a wavelength-tunable optical parametric amplifier (OPA) from 2.7 to 3.8 µm seeded with carrier-envelope phase (CEP) stabilized pulses generated by intra-pulse difference frequency generation (DFG) using a commercial Yb:KGW chirped-pulse amplifier. The Yb:KGW laser's output pulses are spectrally broadened in two-stage multi-plate pulse compression from 0.8 to 1.25 µm, which are compressed down to a sub-two-cycle duration of 6.5 fs using chirp mirrors. CEP-stabilized mid-infrared pulses are produced in intra-pulse DFG of the spectrally broaden pulses around 1.03 µm and parametrically amplified in KTiOAsO 4 crystals. The output energy and temporal duration of the OPA output pulses range from 31 to 56 µJ and from 102 to 203 fs, respectively. The root mean square value of their CEP errors is measured to be 101 mrad.

Nonlinear propagation effects in high harmonic generation in reflection and transmission from gallium arsenide.

High harmonic spectroscopy in solids is emerging as a new tool to investigate ultrafast electron dynamics in the presence of strong optical fields. However, the observed high harmonic spectra do not usually reflect the microscopic origin of high harmonic generation (HHG) because of nonlinear and/or linear propagation effects. Here, we systematically investigate the HHG in reflection and transmission from gallium arsenide exposed to intense mid-infrared optical pulses. In transmission geometry, we find that the properties of high harmonics are drastically changed by nonlinear effects during the propagation of even tens of micrometers. Especially, the nonlinear absorption and/or nonlinearly induced ellipticity of the drive pulses as well as a cascade nonlinear mixing significantly alter the high harmonic signals in the case of the transmission geometry, making an extraction of the microscopic electron dynamics of gallium arsenide difficult. On the contrary, in reflection geometry, we obtain HHG spectra that are free from propagation effects, opening a general approach for high harmonic spectroscopy.

Generation of sub-two-cycle CEP-stable optical pulses at 3.5 µm from a KTA-based optical parametric amplifier with multiple-plate compression.

We demonstrate the generation of 21 fs (1.8 optical cycle), 45-µJ, carrier-envelope phase (CEP)-stable optical pulses with an octave-spanning spectrum from 2.2 to 4.9 µm. Multi-cycle output pulses (120 fs, 149 µJ, 3.5 µm, and 300 Hz) from a KTA-based optical parametric amplifier are compressed down to the sub-two-cycle regime using YAG and Si plates. The single-shot CEP stability of the compressed pulses is measured to be 283 mrad for 500 s. This robust and compact scheme realizes a field strength of 282 MV/cm with f/11 focusing, opening a new regime of attosecond strong-field physics in solids.