Strong Optomechanical Coupling in a Bi-layer Photonic Crystal


Young-Geun Roh, Takasumi Tanabe, Akihiko Shinya, Hideaki Taniyama,
Eiichi Kuramochi, Shinji Matsuo*, Tomonari Sato*, and Masaya Notomi
Optical Science Laboratory, *NTT Photonics Laboratories

 Mechanical interaction between light and matter is one of the most fundamental problems in physics. Recently, coupling phenomena between optical and mechanical resonant modes in photonic structures has attracted strong interests. In 2006, our group theoretically predicted that a bi-layer photonic crystal (PhC) with a thin air gap can generate strong radiation force [1]. In this work, we have realized a bi-layer PhC for the first time and have demonstrated strong optomechanical coupling in it [2].
 A periodic hole pattern arranged in a square lattice was fabricated in two vertically separated slabs using electron beam lithography, dry-wet etching, and CO2 supercritical drying (Fig. 1). The period, hole diameter, slab thickness, and gap thickness are 750 nm, 540 nm, 200 nm, and 200 nm, respectively. We performed reflectance spectrum measurements of the bi-layer PhC formed in the lateral area of 30×20 µm, which revealed photonic bandedge resonant modes around 1567 nm and 1457 nm. These modes exhibit opposite mode symmetry along the vertical direction. Figure 2 shows reflectance spectra obtained by wavelength scan around symmetric mode (1567 nm), which exhibit input power dependence and optical bistability. We concluded that the observed redshift is explained by a slab displacement (i.e. gap decrease) induced by attractive radiation force for the symmetric resonant mode. The thermo-optically induced shift is excluded because we observed opposite blueshift for the antisymmetric mode (1457 nm). We estimated that each slab was displaced by 0.9 nm at Pin=13 mW when the redshift was 1.8 nm. This corresponds to attractive force of 10.8 nN, and thus F/U (i.e. generated force per unit energy in cavity) can be estimated 0.4 µN/pJ, which is a very large value among various optomechanical systems. Thanks to this large optomechanical coupling, we have successfully observed tiny Brownian motion of mechanical modes of bi-layer PhCs [shown in Fig. 3(a)] in RF spectra of the reflected light beam [Fig. 3(b)].

[1] M. Notomi et al., Phys. Rev. Lett. 97 (2006) 023903.
[2] Y. Roh et al., Phys. Rev. B 81 (2010) 121101(R).

Fig. 1. SEM image of fabricated bi-layer PhC.
Fig. 1. (a) Power dependence of reflectance spectra.
(b),(c) Simultaneous scan of each resonant mode
Fig. 2. (a) Fundamental vibration mode of bi-layer
PhC. (b) RF spectrra of reflected beam

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