Materials exhibiting near zero refractive index are shown to have interesting nonlinear optical properties such as enhanced second and third harmonic generation, and large nonlinear refraction (NLR) due to their unique interplay between linear and nonlinear optical features. In particular, the NLR of highly doped semiconductors such as Indium Tin Oxide and Aluminum doped Zinc Oxide is enhanced in the near-infrared spectral regions, where the real part of the permittivity crosses zero with the advantage of having a tunable zero crossover frequency by controlling the doping level. This is also known as the epsilon near zero (ENZ) regime, where the refractive index is very small. We have used the Beam-Deflection (BD) method to directly characterize the temporal dynamics and polarization dependence of the nondegenerate (ND) NLR of doped semiconductors at ENZ. The origin of the nonlinear optical response of these materials is different than for the case of bound electronic nonlinearities which depend upon the third-order susceptibility. The ND BD technique has the potential to study the dependence on relative polarization of excitation and probe waves to accurately determine the instantaneous electronic nonlinearities separately from the non-instantaneous mechanisms such as carrier redistribution effects, however, the carrier nonlinearities are dominant in such materials. This method also reveals the effect of tuning the wavelength of excitation or probe waves through ENZ separately. BD has sensitivity to induced optical path length as small as 1/20,000 of a wavelength, which enables the possibility to resolve NLR in the presence of large nonlinear absorption backgrounds.
Sepehr Benis, Peng Zhao, David J. Hagan, and Eric W. Van Stryland, "Measurement of the ultrafast dynamics of nonlinear refraction and absorption of highly doped semiconductors at epsilon-near-zero (Conference Presentation)," Proc. SPIE 10638, Ultrafast Bandgap Photonics III, 106380X (Presented at SPIE Defense + Security: April 18, 2018; Published: 14 May 2018); https://doi.org/10.1117/12.2304668.5783273766001.
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