a New Mechanism for Optical Nonlinearity: Light - Refractive Index Changes in Amorphous Arsenic Sulfide

Amorphous semiconductors have been little studied from the point of view of nonlinear optics. However, certain aspects of their physical properties indicate that they would be useful for applications in optical communications. Broadening of the optical absorption edge suggests that resonant nonlinearities such as are observed in crystalline materials would occur over a much broader range in wavelength. Furthermore, because of structural disorder, an amorphous material can be deposited on a variety of substrates, opening the possibility for integration with other material technologies. We have measured transient refractive index changes in thin films of amorphous arsenic sulfide using a pump -probe spectroscopic technique. Calculations and experimental results indicate that the change in refractive index is not thermal in origin. We show that the dependences of this effect on excitation intensity and on excitation wavelength are consistent with the photoinduced absorption model for transient absorption changes in amorphous semiconductors. The magnitude of the measured refractive index change is quite large relative to other materials, with a nonlinear refractive index, n_2, comparable to those measured for resonant nonlinearities in crystalline semiconductors. However, we demonstrate that this mechanism for refractive nonlinearity exhibits characteristics that make it very different from those observed in crystalline materials. This effect is a hybrid nonlinearity; light which is strongly absorbed, and thus has a large effect on the material, produces a change in the refractive index at longer wavelengths where the absorption is very small. Thus the change is large, as are resonant nonlinearities in crystalline semiconductors, but it occurs in regions of low absorption, as do the nonresonant effects present in all materials. These wavelength characteristics, and the ability to deposit the material on a variety of substrates, suggest novel device applications for which amorphous semiconductors are uniquely suited. We have demonstrated the principle of such a device, making use of the already well-developed technology for fabricating waveguides in lithium niobate. Numerical calculations indicate that a practical device could be fabricated.