Nondegenerate nonlinear refraction (ND-NLR) in semiconductors is greatly enhanced over the degenerate case and exhibits strong nonlinear dispersion, which provides the potential to greatly modify the refractive dispersion. This nondegenerate enhancement arises from the resonance of the small photon energy with the intraband self-transition, and the larger photon energy with the interband transition. Our earlier theory predicts the dispersion of ND-NLR in semiconductors from a Kramers-Kronig transformation of a nonlinear absorption spectrum, which considers nondegenerate two-photon absorption (ND-2PA), electronic Raman, and quadratic Stark effect. Experimentally, the dispersion of ND-NLR is measured using our Beam-Deflection technique, and the coefficient, n2, is determined over a broad spectral range with various degrees of nondegeneracy. In the extremely nondegenerate case, n2 is greatly enhanced near the onset of ND-2PA, which rapidly switches sign to negative near the bandgap over a very narrow wavelength range. The data suggests larger figure-of-merits by using ND-NLR for all-optical switching. Near and within the ND-2PA regime, strongly dispersive ND-NLR can significantly alter the dispersion of a material, and provides the possibility to optically modify the group index, and group velocity dispersion (GVD) properties. From their irradiance dependence, the nonlinear group index, n2,g, and the nonlinear GVD factor, D2, are calculated from the first and second order derivatives of n2, which show even greater nondegenerate enhancement. Also the nonlinear group index is maximized where there is no two-photon absorption. Potential applications, including nondegenerate all-optical switching based on n2,g, and ultrafast all-optical pulse shaping based on D2, are discussed.
Peng Zhao, David Hagan, and Eric Van Stryland, "Extremely nondegenerate nonlinear refraction and dispersion in semiconductors (Conference Presentation)," Proc. SPIE 10638, Ultrafast Bandgap Photonics III, 106380W (Presented at SPIE Defense + Security: April 17, 2018; Published: 14 May 2018); https://doi.org/10.1117/12.2304820.5783273767001.
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