Abstract
We report a systematic investigation of both three-photon absorption (3PA) spectra and wavelength dispersions of Kerr-type nonlinear refraction in wide-gap semiconductors. The Z-scan measurements are recorded for both ZnO and ZnS with femtosecond laser pulses. While the wavelength dispersions of the Kerr nonlinearity are in agreement with a two-band model, the wavelength dependences of the 3PA are found to be given by (3Ephoton/Eg-1)5/2(3Ephoton/Eg)-9. We also evaluate higher-order nonlinear optical effects including the fifth-order instantaneous nonlinear refraction associated with virtual three-photon transitions, and effectively seventh-order nonlinear processes induced by three-photon-excited free charge carriers. These higher-order nonlinear effects are insignificant with laser excitation irradiances up to 40 GW/cm2. Both pump-probe measurements and three-photon figures of merits demonstrate that ZnO and ZnS should be a promising candidate for optical switching applications at telecommunication wavelengths.
Highlights
As a wide and direct band gap (~ 3.3 eV) semiconductor, zinc oxide (ZnO) is one of the materials that have tremendous potentials for blue-ultraviolet light emitters and detectors, transparent high-power electronics, and piezoelectric transducers
There have been reports on the nonlinear optical properties of ZnO and zinc sulfide (ZnS), including two-photon absorption (2PA), Kerr-type refractive nonlinearity, and nonlinear processes induced by 2PA-excited charge carriers [5,6,7]
We have measured the spectra of 3PA coefficients and Kerr-type nonlinear refraction from 720 nm to 950 nm in ZnO and ZnS crystals at laser excitation irradiances up to 40 GW/cm2
Summary
As a wide and direct band gap (~ 3.3 eV) semiconductor, zinc oxide (ZnO) is one of the materials that have tremendous potentials for blue-ultraviolet light emitters and detectors, transparent high-power electronics, and piezoelectric transducers. There have been reports on the nonlinear optical properties of ZnO and ZnS, including two-photon absorption (2PA), Kerr-type refractive nonlinearity, and nonlinear processes induced by 2PA-excited charge carriers [5,6,7]. In this paper we report a systematical investigation into both 3PA and Kerr-type refractive nonlinearity (n2) in wide-gap semiconductors, ZnO and ZnS. By using both theoretical simulation and experimentally measured pump-probe data, in Section 4, we give out the justification why higher-order nonlinear mechanisms like the fifth-order nonlinear refraction (n4) and 3PA-excited charge carrier effects can be ignored under our experimental condition. The three-photon figures of merit are assessed for ZnO and ZnS
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