Direct Visualization and Determination of the Multiple Exciton Generation Rate.

Multiple exciton generation (MEG) takes place in competition to other hot carrier cooling processes. While the determination of carrier cooling rates is well established, direct information on MEG dynamics has been lacking. Here, we present a methodology to obtain the MEG rate directly in the initial ultrafast transient absorption dynamics. This method is most effective to systems with slow carrier cooling rates. Perovskite quantum dots exhibit this property and are used to illustrate this approach. They show a delayed carrier concentration buildup following an excitation pulse above the MEG threshold energy, which is accompanied by a faster carrier relaxation, providing a direct evidence of the MEG process. Numerical modeling within a simple framework of two competing cooling mechanisms allows us to extract the MEG rate and carrier energy cooling rates for this material. The presented methodology could provide new insights in carrier generation physics and valuable information for MEG investigations.

Fast-frame single-shot pump-probe spectroscopy with chirped-fiber Bragg gratings.

To acquire single-shot pump-probe waveforms for each laser pulse at a high repetition rate and high signal-to-noise ratio, we combined the photonic time-stretch technique and time-encoding method using a chirped-fiber Bragg grating (CFBG) and a grating-pair pulse compressor. By changing the pre-chirping of the probe pulse, a variable time window of the pump-probe traces from 1.4 to 17 ps was demonstrated. The use of a CFBG improved the signal-to-noise ratio of the waveforms by minimizing the loss of probe pulses due to the transmission through a long fiber. These techniques are promising, for example, in applications in multi-timescale pump-probe spectroscopy of irreversible phenomena.

Efficient carrier multiplication in CsPbI3 perovskite nanocrystals.

The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties-these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.

High-efficiency terahertz pulse generation via optical rectification by suppressing stimulated Raman scattering process.

We experimentally demonstrate high-efficiency terahertz pulse generation via optical rectification in LiNbO3. The spectral broadening of an excitation pulse via the stimulated Raman scattering process coincides with high-efficiency terahertz pulse generation, which enhances undesired stretching of the excitation pulse owing to the very high group velocity dispersion in LiNbO3. We avoid this by the bandwidth control of the excitation pulse and achieve the highest reported efficiency of 0.21% for energy conversion into a THz pulse.

Generation of ultrashort optical pulses in the 10 fs regime using multicolor Raman sidebands in KTaO3.

We demonstrated Fourier synthesis of a new type of multiple coherent anti-Stokes Raman scatterings in KTaO(3) crystal. The signals with a wavelength range of 500-750 nm were generated by two femtosecond pulses whose frequency difference was set to be a two-phonon Raman frequency. Angle dispersion of the signals was compensated into one white-continuum beam by spherical mirrors and a prism. The spectral phase, measured by spectral phase interferometry for direct electric-field reconstruction, was compensated for by carefully aligning the angle and the position of the optical components so that 13 fs isolated pulses were generated. This result shows the robustness of our scheme to produce ultrashort optical pulses.