Determination of excited-state polarizabilities of Cr3+-doped materials by degenerate four-wave mixing

Baryscan: a sensitive and user-friendly alternative to Z scan for weak nonlinearities measurements

A new and efficient technique for measuring weak optical nonlinearities is reported. Like Z scan, its implementation is basic, both experimentally and theoretically, but leads to an improved sensitivity of λ/5.10⁴, which represents, to date, one of the highest observed enhancements. With this technique, which is based upon the use of a position sensitive detector, nonlinear properties are deduced by monitoring the barycentric position of a truncated pump-probe laser beam as the sample is moved along the optical axis. The technique is experimentally validated by measuring the pump-induced refractive index change and the underlying polarizability variation resulting from the excitation of the Cr(3+) ions in ruby.

Nondegenerate two-wave mixing in GdAIO(3):Cr(3+)

We have measured the nearly degenerate two-wave mixing in GdAlO(3):Cr(3+) crystals. In contrast to that in other chromium-doped materials, the signal in this sample arises from a mixed phase absorption grating where the absorption contribution is intensity dependent. The real (n(2)') and the imaginary (n(2)'') parts of the nonlinear index of refraction, the saturation intensity, and the excited-state lifetime are determined. The ratio r = n(2)''/n(2)' changes linearly with the intensity. Owing to saturation, the grating presents anharmonicities that give rise to self-diffracted beams (forward four-wave mixing). For comparison, results are also presented for ruby and alexandrite.

Dispersion effects in four-wave mixing measurements of ions in solids

We have performed nondegenerate four-wave mixing measurements on Cr(3+) ions in three crystals: BeAl(2)O(4), Gd(3)Sc(2)Ga(3)O(12), and LaMgAl(11)O(19). Two laser beams of the same frequency were used to resonantly pump one of the absorption bands of Cr(3+), and probe beams with three different wavelengths ranging from 442 to 1064 nm were used to generate a signal from the laser-induced population grating. The observed four-wave mixing signal strength was found to increase as the wavelength of the probe beam was decreased. A model was utilized to explain these results based on the change in polarizability of the Cr(3+) ions when they are in the metastable state versus the ground state, and it was found that the dispersion effects can be described by a single-oscillator model with the lowest-energy charge-transfer transition of Cr(3+) making the dominant contribution to the signal.