Self-focusing effect via Kerr nonlinearity is observed in periodically poled lithium niobate （PPLN） waveguide arrays formed by electro-optic effect. Voltage-control method is demonstrated to control the focusing and diffraction of light. Theoretical simulation results show good agreement with experimental results.
We build a frequency resolved optical gating （FROG） setup based on the second harmonic generation （SHG） FROG to characterize the mid-infrared （MIR） few-cycle laser pulse in single shot basis. Considering the extremely wide bandwidth, we use 20-μm-thick BBO crystal as the nonlinear medium, and correct the spectral response with the frequency summing efficiency. Spatial splitting is adopted to avoid additional material dispersion. In combination with a 4f imaging, this configuration enables the setup to run in single shot. With the central wavelength of 1.8 μm, the measured pulse has a duration of 9.3 fs, which corresponds to about 1.5 cycles.
A compact two-stage optical parametric chirped pulse amplifier based on photonic crystal fibre is demonstrated. A 1064-nm soliton pulse is obtained in a home-made photonic crystal fibre （PCF） with femtosecond pulse pumping and then amplified to 2 mJ in an Nd：YAG regenerative amplifier. After the amplified pulses pass through the LBO crystal, the 532-nm double-frequency light with an energy of 0.8 mJ and a duration of over 100 ps at 10-Hz repetition rate is generated as a pump source in the following two-stage optical parametric amplification （OPA）. The 850-nm chirped signal light gain from the stretcher is 1.5 × 10^4 in the first-stage OPA while it is 120 in the second-stage OPA. The total signal gain of optical parametric chirped pulse amplification （OPCPA） can reach 1.8 × 10^6.
By optimizing the phase matching condition of high harmonic generation （HHG） from a supersonic neon gas jet, the enhanced HHG in the region of 60-70 eV has been selected. Three-dimensional numerical calculation shows that plasma plays a significant role in the phase matching process of HHG in a supersonic gas jet with short medium length. Due to plasma formation, the harmonic emission decays as the laser intensity reaches over 3.5 ~ 1014 W/cm^2. The plasma induces the broadening and blue shift of the HHG spectra, which provides a method for fine-tuning the harmonic wavelength.
