3 Department of Physics of Queens College of the City University of New York, 65-30 Kissena Blvd, Flushing, NY 11367. Micro resonators of different topologies ...
EQEC 2009
Nonlinear Optical Response of Silica and Hybrid Silica/Silicon Disc Micro Resonators C. Schmidt1, A. Chipouline1, T. Käsebier1, E.-B. Kley1, A. Tünnermann1,2, L. Deych3, and T. Pertsch1 1 Institute of Applied Physics, Friedrich-Schiller University Jena, Max-Wien-Platz 1, 07743, Jena, Germany Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Strasse 7, 07745 Jena, Germany 3 Department ofPhysics of Queens College of the City University ofNew York, 65-30 Kissena Blvd, Flushing, NY 11367 2
Micro resonators of different topologies are of interest due to their potential applications as components in future generations of optoelectronic circuits [1]. The high quality factors in combination with small mode volume allow resonators to collect high intensities at rather moderate levels of coupled light [2]. Raman scattering [3], parametric effects [4], and thermal nonlinearity [5] have been investigated both experimentally and theoretically. The thermal nonlinearity has lowest threshold and causes bistable behavior [6]. In this work a bistable operation of silica micro disc resonators and hybrid ones made of silica/silicon has been investigated experimentally. It has been found, that in a hybrid silica/silicon micro resonators a bistability effect has an opposite sign in comparison with a silica disc micro resonator. The bistable behavior of silica micro resonators [6] caused by a thermal nonlinearity exhibits transmission spectrum shift into a longer wavelength region. This corresponds to a positive numerical refractive index temperature derivative for silica. In our experiments a new type of disc micro resonators - hybrid silica/silicon ones - has been produced (see Fig. 1). It is clearly seen, that under a silica layer there is a layer of silicon, which has higher refractive index and thus the eigen mode of such structure tends to be concentrated inside the silicon layer.
Fig. 1 a) Photo of a hybrid silica/silicon disc micro resonator (inset shows profile); b) mode field distribution in a silica and c) hybrid silica/silicon disc micro resonator. Diameter 60 µm, thickness h = 1 µm,
Fig. 2 Transmission spectra of a silica and hybrid silica/silicon disc micro resonator showing anomalous bistability. D = 60 µm, λ = 1.557 µm, a) Si/SiO2 linear, b) Si/SiO2 nonlinear "normal" resonance shift, c) SiO2 linear, d) SiO2 nonlinear "abnormal" resonance shift.
The results of finite-element simulation for the fundamental mode field distribution are presented in Fig. lb,c. Bistability behaviour in such hybrid silica/silicon disc micro resonators exhibits a shift, opposite to the one observed in pure silica micro resonators that would correspond to a negative refractive index temperature derivative - see Fig. 2, blue and red arrows correspond to the scanning towards longer and shorter wavelengths respectively. But the refractive index temperature dependence in silica has qualitatively the same behaviour and can't be a reason for such anomaly. It is believed, that in silica a nonlinearity caused by a free charge generation under influence of the mode field dominates over temperature caused nonlinearity. The different signs of the temperature and free charge nonlinearities stimulate a search of the hybrid silica/silicon micro disc resonator parameters, which would make the two types of nonlinearity mutually compensated. It would, in turn, allow us to design disc micro resonators which are less sensitive to a high optical power. References [1] M. Cai, G. Hunziker, and K. Vahala, "Fiber-Optic Add-Drop Device Based on a Silica Microsphere-Whispering Gallery Mode System", IEEE Photonics Technology Lett., 11, 686 (1999). [2] B. E. Little, J. P. Laine, and H. A. Haus, "Analytical theory of coupling from tapered fibers and half-blocks into microsphere resonators," J. Lightwave Technol. 17, 704-715 (1999). [3] B. Min, T. J. Kippenberg, and K. J. Vahala, "Compact,fiber-compatible,cascaded Raman laser", Opt. Lett., 28, 1507 (2003). [4] A. Andronico, I. Favero, and G. Leo, "Difference frequency generation in GaAs microdisks", Opt. Lett., 33, 2026 (2008). [5] T. Carmon, L. Yang, and K. J. Vahala, "Dynamical thermal behavior and thermal selfstability of microcavities", Optics Express, 12, 4742 (2004). [6] C. Schmidt, A. Chipouline, T. Pertsch, A. Tünnermann, O. Egorov, F. Lederer, and L. Deych, "Nonlinear thermal effects in optical microspheres at different wavelength sweeping speeds", Optics Express, 16, 6285 (2008).
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