Slow Light Propagation in Large-scale Photonic Crystal Coupled Resonator Waveguides

Slow Light Propagation in Large-scale Photonic Crystal Coupled Resonator Waveguides
　
Eiichi Kuramochi, Takasumi Tanabe, Hideaki Taniyama, and Masaya Notomi
Optical Science Laboratory
　　A periodic chain of optical resonators (coupled resonator optical waveguide: CROW) is expected to be a promising candidate as a slow light medium [1]. In the last annual report, we mentioned that CROWs composed of ultrahigh quality factor (Q, ～10⁶) Si photonic crystal (PhC) resonators [2] could couple more than 60 resonators with a very low propagation loss, which is a unique advantage of the PC-CROW [3]. We also reported the dispersion of the PC-CROWs, which corresponded to a very low group velocity (v_g) [3]. In this work, we greatly improved the passband spectrum of PC-CROW to realize distortion-free short pulse propagation because a short pulse occupies a wide frequency range.
　　Figure 1 is a schematic of the improved PC-CROW structure. We employed an inline coupling structure to maximize the coupling between the CROW and external waveguides. In addition, the resonator interval was apodized for the same purpose. The passband exhibited close to the ideal low loss, and was wide and flat as shown in Fig. 2. We performed a time domain pulse propagation experiment using a short pulse (FWHM: 16 ps, wavelength: ～1563 nm). The pulse was transmitted through the CROW and was delayed by 35 ps (N=30) and 75 ps (N=60), respectively (N: resonators in CROW), which agreed well with the v_g of 0.0085c (c: light speed in a vacuum) evaluated by dispersion measurement (Fig. 3). The delay was several times larger than the original pulse width, which is unachievable with existing slow light media. Moreover, the pulse distortion and the ringing after-pulse were well suppressed. The results [4] clearly demonstrated the feasibility of a PC-CROW as a slow light medium for high-speed optical communication.
　　This work was partly supported by CREST of the Japan Science and Technology Agency.
[1] A. Yariv, et al., Opt. Lett. 24 (1999) 711.
[2] E. Kuramochi, et al., Appl. Phys. Lett. 88 (2006) 041112.
[3] E. Kuramochi, et al., CLEO/QELS2007, Baltimore, U.S.A., May 2007, 2.
[4] E. Kuramochi, et al., CLEO/QELS2008, San Jose, U.S.A., May 2008, 3.

Fig. 1. (a) In-line coupling structure. (L_cc: resonator interval; L_cw: the distance between the external waveguide and thecenter of the outermost resonator.) (b) PC-CROW structure studied here. The lattice constant, hole radius, and slab thickness were 420, 110, and 205 nm, respectively.Holes (A/B) were shifted (8/4 nm) away from the line defect.

Fig. 1. Transmission spectra of long PC-CROWs
(N: number of resonators). Fig. 2. Time-resolved data of optical pulses after passing through sample recorded with oscilloscope.

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