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High-quality factor crystalline silicon WGM microresonators for near and mid-IR wavelengths
I. Bilenko1,2, A. Shitikov1,2, T. Tebeneva3, O. Benderov3, A. Rodin3, N. Kondratiev1, V. Lobanov1, A. Voloshin1
1Russian Quantum Center, coherent microoptics and radiophotonics, Moscow, Russian Federation
2Lomonosov Moscow State University, Physics, Moscow, Russian Federation 3MIPT, Laboratory of applied infrared spectroscopy, Dolgoprudny, Russian Federation
In the past decades, optical whispering gallery mode (WGM) resonators with high-quality factor Q become powerful tools in a wide range of applications in optics, photonics, and radiophotonics. Different materials and production techniques are used for the WGMs preparation. Silicon is a major semiconductor material for modern microelectronic, so samples of high purity and homogeneity are easily available. However, interest to silicon WGMs was limited because it is not transparent for the visible wavelengths and exhibit strong free electron and two-photon absorption (FEA and TPA) on the 1.5 ^m telecom wavelengths.
In our previous work, we experimentally demonstrated ultrahigh Q above 109 at 1.5 p,m - two orders of magnitude higher than ever achieved before [1]. It became possible due to the application of undoped material with very low residual conductivity resulting in negligible FEA and polishing technique that prevents excess surface absorption. Measurements were made using low incident power in order to work below TPA limitations.
In present work, we report results of a different kind of the material comparison made on the 1.5 mm telecom wavelengths and also first successful attempts to observe high-quality resonances at wavelengths below 2 p,m - band where TPA in pure silicon is depressed.
All measurements were made by means of the tunable semiconductor lasers. We used original semispherical coupler for the WGM excitation. Diameters of the resonators vary from 2.4 to 2.6 mm, curvature radii - from 0.5 to 1 mm. A large number of modes (fundamental and nonfundamental) were observed and characterized for each sample.
We observe a correlation between the achievable value of Q and the conductivity of the materials. As far as a bulk absorption evaluated from the conductivity and proved experimentally for our samples is too low to affect the Q, we regard these results as evidence of a new surface absorption mechanism to be investigated.
Also, we managed to resolve WGM resonances linewidth corresponded to Q > 107 at 2.3 p,m wavelength (see Fig.1) limited by our present measurement technique. It is planned to continue material characterization, to apply the ringdown method of Q measurement on 2.3 p,m and to expand the measurements to longer wavelengths.
Summarizing, we demonstrated that the silicon high-Q WGM resonators can be effectively used both in near and mid-IR wavelength. The last ones are of great importance as, unlike other materials used for WGM resonators namely, CaF2 and MgF2, silicon has low bulk absorption at mid-IR along with very high Kerr nonlinearity. This opens the possibility to use it for Kerr comb sources [2], laser stabilization for heterodyne radiometry [3] and optics to/from THz frequency converters [4].
This work was supported by the Russian Science Foundation (project 17-12-01095).
Fig. 1.
References
[1] A. E. Shitikov, I. A. Bilenko, N. M. Kondratiev, V. E. Lobanov, A. Markosyan, and M. L. Gorodetsky, "Billion Q-factor in silicon WGM resonators," Optica 5, 1525 (2018).
[2] N. G. Pavlov, S. Koptyaev, G. V. Lihachev, A. S. Voloshin, A. S. Gorodnitskiy, M. V. Ryabko, S. V. Polonsky, and M. L. Gorodetsky, "Narrow-linewidth lasing and soliton Kerr microcombs with ordinary laser diodes," Nat. Photon. 12, 694 (2018).
[3] A. Rodin, A. Klimchuk, A. Nadezhdinskiy, D. Churbanov and M. Spiridonov, "High resolution heterodyne spectroscopy of the atmospheric methane NIR absorption", Opt. Exp., Vol. 22, Issue 11, pp.13825-13834 (2014).
[4] A. B. Matsko, D. V. Strekalov, and N. Yu. "Sensitivity of terahertz photonic receivers," Phys. Rev. A 77, 043812 (2008).
