We have observed a darkening effect and the formation of permanent volume gratings in CdSSe semiconductor-doped glasses, both phenomena being certainly due to the same photochemical mechanism. This allowed us to reconcile contradictory results concerning the speed of their nonlinear response. We also studied the frequency dependence of X (3) below the gap and the intensity dependence of the reflectivity of these phase-conjugate mirrors. Our results can be interpreted in terms of the band-filling model with Boltzmann statistics. Finally, frequency- and. time-resolved luminescence measurements helped us to interpret various results.

New results on optical phase conjugation in semiconductor-doped glasses

The nonlinear index of refraction and the two‐photon absorption coefficient of CdS, ZnSe, and ZnS were measured using the Z‐scan technique with 100‐femtosecond pulses at 610 nm, 780 nm, and 1.27 μm. The results are in reasonable agreement with measurements made at similar wavelengths with longer pulses. A simple theoretical model accounts for the dispersion of the nonlinearities and confirms their electronic nature.

Femtosecond measurement of nonlinear absorption and refraction in CdS, ZnSe, and ZnS

We introduce a temporal delay in one beam of the two-color Z-scan apparatus, which measures nondegenerate nonlinear absorption and nondegenerate nonlinear refraction. This technique allows us to time resolve separately the sign and the magnitude of the nonlinear absorption and refraction at frequency ωp that are due to the presence of a strong excitation at frequency ωe. For example, in semiconductors we specifically measure the bound electronic, nondegenerate nonlinear refraction and nondegenerate two-photon absorption, as well as the two-photon-generated free-carrier refraction and absorption as functions of time. We demonstrate this technique on ZnSe, ZnS, and CS2, using picosecond pulses at 1.06 and 0.532 μm.

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Time-resolved Z-scan measurements of optical nonlinearities

We introduce a simple modification to the Z-scan technique that results in a sensitivity enhancement that permits measurement of nonlinearly induced wave-front distortion of ≃λ/104. This sensitivity was achieved with 10-Hz repetition-rate pulsed laser sources. Sensitivity to nonlinear absorption is also enhanced by a factor of ≃3. This method permits characterization of nonlinear thin films without the need for waveguiding.

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Eclipsing Z-scan measurement of λ/10 4 wave-front distortion

A simple dual-wavelength (two-color) Z-scan geometry is demonstrated for measuring nonlinearities at frequency ωp owing to the presence of light at ωe. This technique gives the nondegenerate two-photon absorption (2PA) coefficient β(ωp; ωe) and the nondegenerate nonlinear refractive index n2(ωp; ωe), i.e., cross-phase modulation. We demonstrate this technique on CS2 for n2 and on ZnSe where 2PA and n2 are present simultaneously.

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Measurement of nondegenerate nonlinearities using a two-color Z scan.

A two‐color ‘‘z’’ scan method is introduced to characterize the nonlinear refraction of optical materials in their absorptive region. We demonstrate the method by measuring the sign and the magnitude of the frequency nondegenerate nonlinearity in the absorptive region of semiconductor doped glasses.

Measurements of nondegenerate optical nonlinearity using a two-color single beam method

We report on time-delayed wave-mixing experiments which allowed, for the first time, the observation of ultrafast modulation due to the interference between third- and fifth-order optical polarizations. The new effect was observed in semiconductor doped glasses by controlling the relative phase of the optical fields which induce distinct wave-mixing processes. The experiments allowed us to measure the relative phase between the nonlinear susceptibilities χ (3) and χ (5) as well as their relative magnitude. In principle the interference effect may be observed in a large variety of materials and the exploitation of this phenomenon to develop a new technique for nonlinear spectroscopy is also discussed

Interference between third- and fifth-order polarizations in semiconductor doped glasses.

A procedure to improve the sensitivity of the two‐color Z‐scan technique is presented. The sensitivity of the technique as a function of the pump and the probe beam confocal parameters and their relative transverse dimensions is evaluated. The results show that, in the case of small nonlinear phase shift, the sensitivity of the measurements at the probe beam center may be a factor of 4 greater by the appropriate choice of the beams parameters. Results for arbitrary phase shift values and for detection at or out of the probe beam center are also reported.

Two‐color Z‐scan technique with enhanced sensitivity

Infrared nonlinearity of commercial Cd(S, Se) glass composites

An all-optical power-controlled switch is described. It is based on the modulation of self-diffracted beams by a control beam that modifies the third-order grating excited in a nonlinear medium. The experimental results obtained by using Cd(S, Se)-doped glasses are in agreement with the theoretical predictions. For an application, we also report the generation of dark pulses with subnanosecond width based on the studied process.

H. Ma

Papers

Measurements of nondegenerate optical nonlinearity using a two-color single beam method

Interference between third- and fifth-order polarizations in semiconductor doped glasses.

Two‐color Z‐scan technique with enhanced sensitivity

Infrared nonlinearity of commercial Cd(S, Se) glass composites

All-optical power-controlled switching in wave mixing: application to semiconductor-doped glasses.