Pulsewidth control of intense sub-picosecond laser using autocloned photonic crystal

A two-dimensional, autocloned, photonic crystal is used to control the pulse duration of a sub-picosecond Ti:sapphire laser for material processing applications. The photonic crystal consists of a periodic stack of 99.5-nm-thick Ta2O5 layers and 137.5-nm-thick SiO2 layers with a periodic surface corrugation deposited on a SiO2 substrate. The photonic crystal size is 10 ×10 mm2. It can withstand intense sub-picosecond laser pulses. Photonic crystals behave as a negative group velocity dispersion (GVD) device in a low incidence angle range and also as a positive GVD device in a high incidence angle range. By simply changing the incidence angle of the laser, pulsewidth can be controlled. With a 200-fs up-chirped pulse, the transmitted pulsewidth is compressed down to 121 fs at an incidence angle of 10°. At an incidence angle of more than 20°, pulsewidth is steeply stretched mainly because of the reduction in transmission spectral band in the photonic crystal. A down-chirped 200-fs pulse is stretched up to 232 fs because of the negative GVD of the photonic crystal at a normal incidence, whereas at a high incidence angle of 54°, pulse is compressed down to 111 fs because of the positive GVD of the photonic crystal. An autocloned photonic crystal can be used as a very simple GVD device to control an intense sub-picosecond laser pulse.