We present a review of the development of a compact and high-power broadband terahertz (THz) source optically excited by a femtosecond photonic crystal fiber (PCF) amplifier.The large mode area of the PCF and the stretcher-free configuration make the pump source compact and very efficient.Broadband THz pulses of 150 μW extending from 0.1 to 3.5 THz are generated from a 3-mm-thick GaP crystal through optical rectification of 12-W pump pulses with duration of 66 fs and a repetition rate of 52 MHz.A strong saturation effect is observed,which is attributed to pump pulse absorption;a Z-scan measurement shows that three-photon absorption dominates the nonlinear absorption when the crystal is pumped by femtosecond pulses at 1 040 nm.A further scale-up of the THz source power is expected to find important applications in THz nonlinear optics and nonlinear THz spectroscopy.
The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using the scalar diffraction theory.The simulation shows that the transmitted terahertz waveforms compress with increasing propagation distance,and the multi-frequency focusing phenomenon at different focal points is observed.Additionally,the distribution of terahertz fields in a plane perpendicular to the axis is also discussed,and it is found that the diffraction not only induces focusing on-axis but also inhibits focusing at off-axis positions.Therefore,the Fresnel lens may be a useful alternative approach to being a terahertz filter.Moreover,the terahertz pulses travelling as a basic mode of a Gaussian beam are discussed in detail.
The carrier dynamics and terahertz photoconductivity in the n-type silicon (n-Si) as well as in the p-type Silicon (p-Si) have been investigated by using femtosecond pump-terahertz probe technique. The measurements show that the relative change of terahertz transmission of p-Si at low pump power is slightly smaller than that of n-Si,due to the lower carrier density induced by the recombination of original holes in the p-type material and the photogenerated electrons. At high pump power,the bigger change of terahertz transmission of p-Si originates from the greater mobility of the carriers compared to n-Si. The transient photoconductivities are calculated and fit well with the Drude-Smith model,showing that the mobility of the photogenerated carriers decreases with the increasing pump power. The obtained results indicate that femtosecond pump-terahertz probe technique is a promising method to investigate the carrier dynamics of semiconductors.
