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Through theoretical modeling, we find that the dynamics of photogenerated carriers play a very important role in shaping the temporal waveform of terahertz (THz) radiation pulses emitted from biased low-temperature (LT)-grown GaAs antenna. Our modeling gives successful analyses for the sharp and short, slow and long negative parts of temporal THz waveforms. By including intraband, carrier relaxation effects in the modeled mobility, we find an obvious dependence of the THz conversion efficiency on the material of THz emitter and experimental parameters such as the optical duration, the center wavelength, and the fluence of the laser pulses. Our research also shows that electron-hole and electron-electron collisions in LT-GaAs contribute to the saturation phenomenon with an increase of laser fluence. OCIS codes: 320.7090, 140.3070, 260.2110, 300.6470.

We report an all-normal-dispersion, low-repetition-rate, high-energy, twin-pulse, passively mode locked ytterbium-doped fiber laser. The mode-locking mechanism of the laser is based on nonlinear polarization evolution and strong pulse shaping with a cascade long-period fiber grating bandpass filtering in a highly chirped pulse. The laser generates a highly stable twin-pulse group with 248 ps and 296 ps duration simultaneously and maximum pulse energy of 26.8 nJ-each pulse at a 2.5445 MHz repetition rate. Energy quantization is observed, which demonstrates the nonparabolic nature of these pulses. The laser can also work in third-harmonic mode locking with 17.8 nJ energy (at a repetition rate of 7.65 MHz and pulse width of 780 ps).