Controlling the refractive index and third-order nonlinearity of polyimide/Ta2O5 nanolaminates for optical applications

Abstract

In this study, the authors investigated third-order optical nonlinearity in polyimide/Ta2O5 nanolaminates deposited by atomic layer deposition. Third harmonic signal measurements were done with a multiphoton microscope at an excitation wavelength of 1.55 μm, laser pulse duration of 150 fs, and estimated pulse energy of 1.2 nJ. Third-order optical nonlinearity is an essential property in optical signal applications for telecommunication. Transparency at telecommunication wavelengths and a high refractive index are desired for a material. Polyimide is optically transparent, enabling light guidance through the material. The refractive index of the material can be fine-tuned by combining polyimide with a substantially higher refractive index material—in this case, Ta2O5. The layer thicknesses in nanolaminates were varied, and the third harmonic generation was compared to plain polyimide and Ta2O5 reference films. Third harmonic generation in the nanolaminates decreased slightly and refractive index increased with increasing Ta2O5 content. Normalized third-order nonlinear susceptibilities, χ(3), calculated for the nanolaminates were between the values of Ta2O5 and polyimide and increased with increasing polyimide content.