George I. Stegeman

Self-focusing and self-defocusing by cascaded second-order effects in KTP

We monitor the induced phase change produced by a cascaded chi((2)):chi((2)) process in KTP near the phase-matching angle on a picosecond 1.06-microm-wavelength beam using the Z-scan technique. This nonlinear refraction is observed to change sign as the crystal is rotated through the phase-match angle in accordance with theory. This theory predicts the maximum small-signal effective nonlinear refractive index of n(eff)(2) congruent with +/-2 x 10(-14) cm(2)/W (+/-1 x 10(-11) esu) for an angle detuning of +/-5 degrees from phase match for this 1-mm-thick crystal with a measured d(eff) of 3.1 pm/V. For a fixed phase mismatch, this n(eff)(2) scales linearly with length and as d(eff)(2) however, for the maximum n(eff)(2) the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near pi/4.

χ(2) cascading phenomena and their applications to all-optical signal processing, mode-locking, pulse compression and solitons

Cascading is the process by which the exchange of energy between optical beams interacting via second order nonlinearities (χ(2)) leads to various effects such as nonlinear phase shifts, the generation of new beams, all-optical transistor action, the formation of soliton-like (solitary) waves, etc. Here we review the fundamentals of the processes and discuss experimental verification of the effects and various related applications.

Large nonlinear phase shifts in second-order nonlinear-optical processes

We show that processes such as second-harmonic generation and subsequent downconversion, and parametric mixing in general, can lead to large field-dependent phase shifts for the input beams under a variety of conditions.

Calculations of nonlinear TE waves guided by thin dielectric films bounded by nonlinear media

The dispersion relations for TE polarized waves guided by thin dielectric (planar integrated optics) films, surrounded on one or both sides by media with intensity-dependent refractive indexes, are solved numerically and interpreted. Comparisons are made between these nonlinear guided waves and the usual integrated optics modes associated with linear media. For media characterized by self-defocusing nonlinearities, increasing guided wave power leads to a decrease in the guided wave effective index and to mode cutoffs characterized either by finite or diverging total guided wave powers. For self-focusing media, the effective index increases with increasing power. New wave solutions are obtained with power thresholds and anomalous power-dependent field distributions. These characteristics are illustrated by numerical calculations for a specific material system and possible applications to optical devices are discussed.

The nonlinear optical properties of AlGaAs at the half band gap

We report experimental values for the nonlinear optical coefficients of AlGaAs, in the half-band-gap spectral region. The dispersion of the nonlinear refractive-index coefficient, n/sub 2/, is measured for both TE- and TM-polarized light. We observe n/sub 2/(TE)>n/sub 2/(TM) and a ratio of cross-phase modulation to self-phase modulation (TE) of /spl sim/0.95, as predicted from band structure calculations. The spectral dependence of the two- and three-photon absorption coefficients are also measured. Finally, the implications for all-optical switching and spatial soliton propagation are discussed.

All‐optical switching devices based on large nonlinear phase shifts from second harmonic generation

We show that the large nonlinear phase shifts obtained from phase‐mismatched second harmonic generation can be used to implement all‐optical switching devices such as a nonlinear Mach–Zehnder interferometer and a nonlinear directional coupler.

Optical spatial solitons: historical perspectives

Optical spatial solitons are self-trapped optical beams that exist by virtue of the balance between diffraction and nonlinearity. They propagate and interact with one another while displaying properties that are normally associated with real particles. Solitons, in general, manifest themselves in a large variety of wave/particle systems in nature: practically in any system that possesses both dispersion (in time or space) and nonlinearity. Solitons have been identified in optics, plasmas, fluids, condensed matter, particle physics, and astrophysics. Yet over the past decade, the forefront of soliton research has shifted to optics. In the paper, we describe the historical evolution of spatial solitons from speculative creatures to one of the most fascinating features optics has to offer.

Self‐Trapping of Optical Beams: Spatial Solitons

Although people have always been fascinated by visual manifestations of nonlinear wave phenomena, such as tsunamis and tidal waves, the first scientifically documented report of a self‐trapped wave did not come until 1834, when a Scottish scientist, John S. Russell, observed a “rounded smooth and well defined heap of water” propagating in a narrow and shallow canal “without change of form or diminution of speed.” The water was calm on both sides of this unusual wave, and Russell noted that it had the form of a “solitary elevation.”

Ultrafast all‐optical switching in semiconductor nonlinear directional couplers at half the band gap

Efficient ultrafast all‐optical switching in nonlinear directional couplers made of AlGaAs and AlGaAs/GaAs quantum wells near half the band gap is reported. The switching is limited by multiphoton absorption which is dominated by three‐photon absorption in this spectral range. The three‐photon absorption in the quantum well nonlinear directional coupler is stronger than that of bulk AlGaAs. Autocorrelations of the output pulses in the bar and cross states confirm pulse breakup through nonlinear coupling, and illustrate the effects of multiphoton absorption. All sets of experimental data are fitted well by a theoretical model.

Large nonlinear phase modulation in quasi-phase-matched KTP waveguides as a result of cascaded second-order processes

Nonlinear spectral modulation and broadening have been observed in quasi-phase-matched KTP waveguides near 850 nm, indicative of self-phase modulation that is due to cascaded second-order processes. Numerical beam-propagation simulations indicate nonlinear peak phase shifts larger than π for 600 W of peak power in a 2.8-mm-long guide.