The propagation of short pulses in nonisotropic nonlinear optical media involves new phenomena not encountered in the propagation of longer pulses. We describe two such phenomena here. Specifically, (a) we demonstrate a new method of nonlinear frequency generation for femtosecond light pulses that alleviates group velocity mismatch broad-ening in the 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, we generated 28 fs second 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 tilting of a femtosecond laser wavepacket in a birefringent rutile crystal. The measurements were carried out using a linear interferometric technique which, in contrast to nonlinear correlation techniques, allows measurement at very low power. A 23 fs delay between the left and right parts of a 30 fs, 800 nm wavepacket was measured for a 1 mm thick rutile crystal.
Group velocity mismatch-absent nonlinear frequency conversions for mid-infrared femtosecond pulses generation