PH.23 MICROSYSTEMS TECHNOLOGY LABORATORIES ANNUAL RESEARCH REPORT 2009 PHOTONICS PHOTONICS MATERIALS Cavity-enhanced Photosensitivity in Chalcogenide Glass J. Hu, M. Torregiani, F. Morichetti, N. Carlie, L. Petit, A. Agarwal, A. Melloni, K. Richardson, L. C. Kimerling Sponsorship: DOE Chalcogenide glasses, namely the amorphous compounds of sulfur, selenium and/or tellurium, have emerged as a promising material candidate for nonlinear optics in recent years due to their high Kerr nonlinearity and low two-photon absorption (TPA), giving rise to a superior nonlinear igure of merit (FOM) compared to conventional semiconductor materials such as silicon. Here nonlinear absorption, infrared photosensitivity, and thermal stability characteristics of the glass material are tested in a cavity-enhanced setting. The chalcogenide glass resonators are patterned by lift-off entirely using a 500-nm CMOS line [1], [2]. In the nonlinear optical measurements, TM polarization light from a tunable laser is irst ampliied using an erbium-doped iber ampliier (EDFA) and then end-coupled into the bus- waveguide through a silica optical iber. Figure 1 plots the TM transmission spectra near a resonant peak of a 30-µm radius micro-disk at different input power levels. Corresponding nonlinear absorption in the optical resonator can be calculated using the generalized coupling matrix method based on the resonant peak extinction ratio change. The absorption value does not exhibit measurable increase within the accuracy of our testing setup, suggesting a TPA coeficient α 2 < 1.5 × 10 -13 m/W, almost two orders of magnitude smaller than that of silicon. Further, the resonant peak shape is signiicantly altered at input power levels higher than 17 dBm, a combined consequence of the index-trimming effect and thermo-optic instability. To further investigate the index-trimming effect due to 1550-nm light illumination, we perform pump-probe measurements to evaluate the refractive index change in As 2 S 3 under non-resonant and resonant pumping conditions,. The resonant peak shift is negligible when the pump beam is not aligned with the resonant wavelength. However, signiicant resonant wavelength red shift is observed as the pump beam with an input power > 14 dBm is tuned to the resonant wavelength, which unequivocally conirms the cavity-enhancement due to infrared photosensitivity. To summarize, we have performed systematic cavity- enhanced optical characterizations of As 2 S 3 chalcogenide glass ilms using planar micro-disk resonators. No TPA is observed in the nonlinear measurements and the TPA coeficient is calculated to be < 1.5 × 10 -13 m/W. We also characterize refractive index-trimming induced by resonant illumination of light at 1550 nm. A power- density threshold of < 0.1 GW/cm 2 is measured in the unannealed As 2 S 3 ilms. At high input power (~ 10 mW coupled power in the bus waveguide), the thermo-optic effect and the cavity-enhanced photosensitivity lead to resonant peak shape distortion and cavity instability. 1552.9 1552.95 1553 1553.05 1553.1 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 Wavelength (nm) T r a n s m i s s i o n ( d B ) -10.9 dBm -5.9 dBm -0.9 dBm 4.1 dBm 9.1 dBm 11.6 dBm 14.1 dBm 15.1 dBm 16.1 dBm 17.1 dBm 18.1 dBm 19.1 dBm 20.1 dBm 21.1 dBm 22.1 dBm 23.1 dBm FIGURE 1: (a) Transmission spectra of a micro-disk resonator near a resonant peak measured at diferent input power levels; the arrows indicate the temporal sequence of measurements. [3] FIGURE 2: Transmission spectra recorded in the pump- probe measurements showing signiicant resonant peak shift when the pump beam is tuned to the resonant wavelength, indicating refractive index trimming due to 1550-nm infrared light illumination; the arrows indicate the temporal sequence of measurements. REFERENCES [1] J. Hu, V. Tarasov, N. Carlie, N. Feng, L. Petit, A. Agarwal, K. Richardson, and L. C. Kimerling, “Si-CMOS-compatible lift-of fabrication of low-loss planar chalcogenide waveguides,” Opt. Express vol. 15, pp. 11798- 11807, 2007. [2] J. Hu, N. Carlie, N. Feng, L. Petit, A. Agarwal, K. Richardson, and L. C. Kimerling, “Planar waveguide-coupled, high- index-contrast, high-Q resonators in chalcogenide glass for sensing,” Opt. Lett. vol. 33, pp. 2500-2502, 2008. [3] J. Hu, M. Torregiani, F. Morichetti, N. Carlie, L. Petit, A. Agarwal, A. Melloni, K. Richardson, and L.C. Kimerling, “Cavity- enhanced photosensitivity in chalcogenide glass,” OSA Integrated Photonics and Nanophotonics Research and Applications Topical Meeting, 2009.