Photorefractive effect yields stable, two-dimensional solitons

A steady-state, two-dimensional soliton has been generated in photorefractive material strontium barium niobate (SBN) by researchers at Princeton University (Princeton, NJ) in collaboration with the University of Arkansas (Fayetteville, AK), Hughes Research Laboratories (Malibu, CA), and the University of L’Aquila (L’Aquila, Italy). Because they are based on the photorefractive effect rather than the optical Kerr effect, these solitons are two-dimensional; conventional Kerr solitons are stable in only one dimension.

An optical signal from an argon-ion laser was launched in the nonlinear crystal at the same time as an external field of 1 to 2 kV/cm was applied. The 1-µW, 514-nm soliton beam maintained a constant diameter of approximately 10 µm along the 5.5-mm length of the SBN crystal (Deltronic Crystal Industries, Dover, NJ). It is stable and unaffected by perturbations in the refractive index.

The wavelength-dependent mechanism is driven by the choice of photorefractive material; allowable soliton wavelengths are those at which the crystal is photosensitive. The group has demonstrated the process at wavelengths throughout the visible spectral region. Required power for the optical signal (1 µW or less) is sufficiently low that diode lasers may be used in the future, according to group leader Mordecai Segev.

The soliton acts as a graded-index waveguide within the crystal. It is capable of directing a more powerful, photorefractively insensitive, nonsoliton beam of a different wavelength without degradation. The group has successfully guided a 1319-nm, 1-W optical signal using this method. Possible applications include beam steering, optical wiring, and optical interconnects.