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Experimental Observation of Inflection-Point Slow Light Modes in Photonic Crystal Coupled Waveguides

Shih-Chieh Huang1, Masao Kato2, Eiichi Kuramochi2, Chien-Ping Lee1, and Masaya Notomi2

1) Department of Electronics Engineering, National Chiao Tung University, Hsin Chu 300, Taiwan 2) NTT Basic Research Labs, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, 243-0198, Japan

E-mail: schuang.ee91g@nctu.edu.tw

Abstract: We report on the time-domain measurement of inflection-point slow light modes in a photonic crystal coupled waveguide. The S type band structure for the waveguide mode allows a unique opportunity for slow light measurement. ©2007 Optical Society of America OCIS codes: (230.7400) Waveguides, slab; (250.5300) Photonic integrated circuits

Photonic crystal waveguides have been considered a promising candidate as optical buffer devices due to

their abilities to slow down the speed of light. Recently, Mori and Baba proposed and theoretically discussed a photonic crystal coupled waveguide (PhCCW), which has a unique flat band within photonic bandgap allowing slow light propagation [1]. However, the experimental demonstration of the slow light behavior in a PhCCW has yet to be reported. In this paper, we present first the measured transmission spectrum of a properly designed PhCCW to verify the existence of that flat band region. Then, we report on the direct measurements of the retardation of propagation pulses in the PhCCW using a high speed oscilloscope. From the propagation time as a function of the waveguide length, a slow light speed of (0.017±0.002)*c was directly measured.

Fig 1(a) shows the SEM micrograph of a fabricated 2D slab PhCCW, which consists of two W1 type PhC

waveguides. The corresponding calculated band structure is shown in Fig. 1(b). A unique S-shape-like coupled defect band with clear flat band regions is created in the photonic band gap. The details of the sample fabrication and formation of defect bands can be found in [2]. A typical transmission spectrum of the fabricated PhCCWs is shown in Fig. 1(c). A sharp dip at around λ=1540 nm can be clearly seen. This indicates that the traveling light suffers a severe change in velocity, thus resulting in a large loss in power intensity. Good agreement between the measured spectrum and calculated band diagram confirms this indeed is attributed to the S-shape-like coupled waveguide modes, rather than to the band edge.

Fig. 1. (a) SEM micrograph of PhCCW with lattice constant a=438 nm. The inset shows MMI devices used in the experiments for exciting and collecting couple modes. (b) Calculated band diagram of PhCCW, where the red line represents the unique flat band of coupled modes. (c) Measured spectrum of a PhCCW with length L=200μm. A dip, marked as “A”, at around 1540 nm results from the flat band of PhCCW.

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To investigate the dynamics of light propagation in this region, we set up a time-resolved measurement system, as depicted in Fig. 2(a). Gaussian-like pulses with long pulse width (FWHM 24-28 ps) were launched at the input. Pulses passing through a branch with a tunable optical delay line were used as reference signals. All the output waveforms were finally displayed and recorded by an oscilloscope with an integrated optical module, which has a wide bandwidth of 28GHz for optical signal detection. By measuring the delays of the output pulses with respect to the reference signal, the group velocity was then determined.

Fig. 2(b), where the input pulses have a central wavelength of 1539.38nm, shows an example of the

experimental results. In this case, duration times between the output pulses and the reference pulses were 99, 108, and 132 ps for samples with length L=100, 200, and 500 μm, respectively. Plotting the duration time versus waveguide length and fitting the data with a straight line, we obtained a group velocity of (0.039±0.002)*c, where c is the speed of light in vacuum. Following this way, the frequency dependence of group velocity was investigated by varying the wavelength of the launched pulses from 1545.00 nm to 1537.30 nm. The measured results as compared to the theoretical prediction from the band structure calculation are shown in Fig. 2(c). An extremely slow group velocity of (0.017±0.002)*c was obtained at a wavelength of 1537.30 nm, which is close to the flat band of the coupled modes.

Fig. 2. (a) Diagram of the time-domain measurement system. (b) Output waveforms of time-resolved measurements recorded by an oscilloscope. In this case, the wavelength of launched pulse is 1539.38 nm. (c) Frequency dependence of the group velocities. The black line and the red dot represent the calculated and measured group velocities, respectively. In conclusion, we have designed and fabricated photonic crystal coupled waveguides, where the coupled

mode has a flat region caused by the inflection point in the band structure within the photonic band gap allowing slow light propagation. The time domain measurement of the retarded propagation pulses in PhCCWs was achieved by using a high speed oscilloscope. This is the first direct experimental demonstration of slow light due to an inflection point in the band structure of a waveguide mode. [1] D. Mori and T. Baba, "Wideband and low dispersion slow light by chirped photonic crystal coupled waveguide", Opt. Express 13, 9398(2005) [2] S. C. Huang, M. Kato, E. Kuramochi, C. P. Lee, and M. Notomi, “Time-domain and spectral-domain investigation of inflection-point slow-light modes in photonic-crystal coupled-waveguides” (submitted)

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