ULTRA-SHORT PULSE NONLINEAR OPTICS - 2ND-HARMONIC GENERATION AND SUM-FREQUENCY GENERATION WITHOUT GROUP-VELOCITY MISMATCH BROADENING

The propagation of short pulses in nonisotropic nonlinear optical medi a involves new phenomena not encountered in the propagation of longer pulses. We describe two such phenomena here. Specifically, (a) we demo nstrate a new method of nonlinear frequency generation for femtosecond light pulses that alleviates group velocity mismatch broadening in th e generation of second harmonic pulses that are shorter than the input fundamental pulses. The method relies on non-collinear phase-matching and group-velocity matching. Using our method, rye generated 28 fs se cond harmonic pulses from 33 fs 800 nm fundamental pulses. (b) We show that an optical wavepacket propagating as an extraordinary wave in a non-isotropic dispersive medium experiences a rotation (or tilt) about an axis perpendicular to the propagation vector. We measured the tilt ing of a femtosecond laser wavepacket in a birefringent rutile crystal . The measurements were carried out using a linear interferometric tec hnique which, in contrast to nonlinear correlation techniques, allows measurement at very low power. A 23 fs delay between the left and righ t parts of a 30 fs, 800 nm wavepacket was measured for a 1 mm thick ru tile crystal.