Coupled Resonator Waveguides Formed by Ultrahigh-Q Si Photonic Crystal Nano-resonators
Eiichi Kuramochi, Takasumi Tanabe, Hideaki Taniyama, and Masaya Notomi
Optical Science Laboratory
@A periodic chain of optical resonators (coupled resonator optical waveguide: CROW) has been expected as a promising candidate of a slow light media . An ultrahigh intrinsic quality factor (Q) of a resonator is required to achieve ultraslow group velocity (vg) and low propagation loss simultaneously. Recently, we have demonstrated that such CROW is achievable on a Si photonic crystal (PC) platform besides an advantage of ultra-small footprint (`3Êm@single resonator.)
@Figure 1(a) is a microscope image of a PC-CROW fabricated by electron beam lithography (the number of the resonator: N=3.) A locally width-modulated line-defect resonator  realized ultrahigh experimental Q (1.2~106). Sharp peaks which corresponded to CROW modes were clearly observed in transmittance measurements (Fig. 1(b).) Figure 2 shows dispersion of CROW modes when the interval of the resonator (LCC) was 7a (a: lattice constant=420nm.) Surprisingly, the dispersion of the coupled resonator modes were fitted well by almost equal theoretically derived cosine-function, which corresponded to very small coupling coefficient k (`7~10-4), when N was changed from 5 to 60. The passing of light over 60 PC resonators was achieved for the first time which successfully demonstrated advantage of ultrahigh-Q. The dispersion was controlled well by LCC (Fig. 3) and the lowest k (3.3~10-4) corresponded to very slow vg (`5~10-3c; c: the speed of light in vacuum.)
@This work is encouraging possibility of a PC-based CROW as a slow light media.
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 E. Kuramochi, et al., Appl. Phys. Lett. 88 (2006) 041112.
 T. Tanabe, et al., Opt. Express. 15 (2007) 7826.
Fig. 1. (a) Microscope image of a PC-CROW (N=3, LCC=9a.) The white octagons show modified holes which creates ultrahigh-Q resonators (The parameter is described in Ref. 3.) (b) A transmission spectrum of a PC-CROW (LCC=7a, N=15).
Fig. 2. Dispersion characteristics of PC-CROWs (LCC=7a).
Fig. 3. Dispersion characteristics of PC-CROWs (N=10).