High harmonic generation in solids attracted great attentions as a new scheme for frequency conversion. We report observation of an extremely efficient terahertz (THz) third- and fifth-harmonic generation in thin films of Cd3As2, a three-dimensional Dirac semimetal with massless electron dispersion, as it is observable with tabletop THz source at room temperature. Our THz pump-THz probe study with subcycle time resolution elucidates that the intraband current of coherently accelerated Dirac electrons is the main source of the THz harmonics as expected theoretically. The results pave the way toward novel devices for ultrafast THz electronics and photonics based on topological semimetals.
We demonstrate ultrafast superradiance showing 10-fs decay due to the nonlocal interaction between light waves and exciton waves in CuCl mesoscopic thin films. This time constant is comparable with the thermal dephasing even at room temperature. Actually, we observed photoluminescence signal from one confined mode of translational motion of the exciton surviving at room temperature. In addition, temperature dependence of the photoluminescence spectra with multiple peaks was reproduced well by theoretical calculation. This finding opens a new avenue of “thermal free photonics”, where the exciton coherence is utilized at higher temperature.
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