Quadratic medium yields two-dimensional spatial solitons
Strong nonlinear coupling between fundamental and harmonic fields in a quadratic medium can produce two-dimensional spatial solitons, report researchers from the Center for Research in Electro-Optics and Lasers (CREOL, Orlando, FL) led by William Torruellas. In type II second-harmonic generation (SHG), two input fields with frequency w interact in a nonlinear medium to form a second-harmonic of frequency 2w. Normally, natural birefringence causes spatial displacement or `walk-off` of the interacting beams, and SHG efficiency is saturated by diffraction. When the three fields involved in type II SHG are nearly phase-matched above a threshold power in potassium titanyl phosphate (KTP), however, strong nonlinear coupling compensates for diffraction and walk-off, spatially locking the interacting fields.
A flashlamp-pumped Nd:YAG laser was passively modelocked and Q-switched to produce 15-ps Gaussian pulses at 1.064 µm. An extracavity electro-optic pulse selector allowed individual pulses to be focused onto the 1-cm-long KT¥crystal (JTT International, Orlando, FL). Beyond a threshold power of 10 GW/cm2, self-focusing induced by second-order interaction strictly compensated for diffraction and walk-off of all three fields involved. The beams were trapped into spatial soliton-like beams, with an output beam waist of 12.5 µm. To date, the only other method of producing stable two-dimensional spatial solitons in a condensed media is via the photorefractive effect (see p. 40).