We report a new highly efficient source of frequency-tunable (0.5-3.5 THz) narrow-bandwidth terahertz wave packets with up to 1 mW average power, based on parametric down-conversion in quasi-phase-matched GaAs. Different lasers were employed as a pump source, including femtosecond OPA/DFG system (wavelength range 2-4μm), Tm-fiber femtosecond laser (wavelength ~2μm), and near-degenerate synchronously-pumped picosecond OPO system with extra- and intracavity THz generation. We prove experimentally that the optical-to-terahertz conversion efficiency is fluence-dependent, with the scaling factor being the same for femtosecond (optical rectification) and picosecond (difference frequency generation) pump pulses, with optical-to-terahertz conversion efficiency on the order of 0.1% per μJ.
High average power single-mode fiber lasers have attracted significant attention as alternatives to conventional solidstate lasers owing to their relative high brightness, compactness and robustness. Likewise the turn-key operation of industrially qualified ultrafast fiber oscillators is well established. In recent years the convergence of reliable ultrafast fiber oscillators, high brightness pump diodes and high power fiber amplifiers has enabled ultrafast fiber lasers to surpass ultrafast solid-state lasers in terms of average power. While fiber lasers have generally not been able to match the ultrashort pulse energies produced by solid-state lasers, careful management of nonlinearities can overcome the conventional B-integral limit of π thereby permitting stable operation of practical ultrafast fiber lasers with pulse energies approaching the milli-Joule level. Here we review modes of nonlinear propagation in fibers which have enabled increases in ultrashort pulse energies from nano-Joule to milli-Joule levels, namely: solitons, similaritons and cubicons. As an example of a practical high energy ultrafast fiber laser, we demonstrate a cubicon Yb fiber chirped pulse amplification system producing 550 fs pulses with 50 μJ at >15 W.
We describe a pulse shaping technique which uses second harmonic generation with Fourier synthetic quasi-phase- matching gratings. We demonstrate both amplitude and phase tailoring by generating a picosecond square-like pulse as well as trains of femtosecond pulses with a terahertz-range repetition rate from either a transform-limited or chirped pump pulse.
J-aggregates of tiacarbocyanine derivative are investigated in frozen solution at low temperatures by optical spectroscopy and spectral hole burning. It is found that absorption and luminescence bands have inhomogeneous widths of 220 and 130 cm-1 at 5 K respectively used in contrast to pseudoisocyanine (PIC) J-bands are separated by Stokes shift of 100 cm-1. Moreover, J-aggregate fluorescence spectrum depends drastically on the temperature and initial dye concentration. Temperature broadening of spectral hole, burnt in the J-absorption band differs qualitatively from that, burnt in the monomeric band. It differs also, but quantitatively, from broadening of the spectral hole in J-band of PIC. These observations are discussed in terms of nonradiative relaxation of excitons in the system under investigation.
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