Micro-Optical Lenslets by Photo-Expansion in Chalcogenide Glasses

A detailed study of the wavelength dependence of the formation of lenslets by photo-expansion in chalcogenide glasses is reported. Photo-expansion in chalcogenide glasses offers a one-step fabrication process to record surface structures such as gratings and microlenses. The process is purely optical and requires no fixing and etching to get the desired surface structure. Two competing effects, namely large volumes accessed by low energy photons and large magnitudes of photostructural changes due to high energy photons, provide the possibility of using different fabrication techniques to obtain lenslets by the photoexpansion process. Strongly absorbed 514.5 nm light is used to record micro-optical lenslets in As2S3 glass and up to 10% relative volume changes are observed. The low power density requirements at this wavelength are used to demonstrate a parallel lithographic fabrication technique for recording lenslets with high repeatability and throughput, and with excellent control over curvature and dimensions. The magnitude of surface dilation is maximized at the photon energy that combines the advantage of short wavelength exposures in producing large percentage volume changes, with the advantage of low energy photons in irradiating large volumes of the glass matrix. A variety of wavelengths within the Urbach tail range of the optical absorption edge are employed to find the optimal wavelength for large absolute volume changes in As2S3 glass. Lenslets as high as 8 m are fabricated with focused light exposures from a dye laser operating at 584.5 nm. Lenslets are structurally characterized with alphastep scanners and an atomic force microscope (AFM) and are optically characterized by testing the collimation of 1550-nm light emerging from a single-mode fiber with these lenslets.