Time Resolved Measurement of Photonic Crystal Optical Nanocavity
Takasumi Tanabe, Eiichi Kuramochi, Akihiko Shinya,
Hideaki Taniyama, and Masaya Notomi
Optical Science Laboratory
A high-Q photonic crystal (PhC) nanocavity is very effective for application to all-optical switches that can operate at an ultra-low energy , because it can realize a high photon density at an extremely low input power. It is known that an ultra-high-Q can be achieved with a PhC nanocavity by employing the local width modulation of line defects . A scanning electron microscope image of such a nanocavity is shown in Fig. 1 (a). Although it is difficult to recognize, the width of the line defect is slightly modulated in the circled region. As a result, a mode-gap cavity forms in this area. Indeed, the far-field pattern shows that the light is localized when the wavelength of the input light is same as the resonance of the cavity [Fig.1(b)]. Figure 1(c) shows the transmittance spectrum measured using a wavelength tunable laser. The transmittance width is an extremely small 1.3 pm, which corresponds to a Q of 1.2 million. To achieve higher wavelength resolution, we applied a single side band modulator to sweep the frequency of the laser light with an ultra-high accuracy. We obtained the same Q value and confirmed the accuracy of the measurement .
In contrast, since an ultra-high Q cavity system has a long photon lifetime, the Q can be directly obtained in the time domain. In addition, time resolved measurement is a powerful way to characterize the dynamic behavior of the cavity system. Therefore, we combined ring-down measurement with time correlated single photon counting to obtain the optical property of the PhC cavity in the time domain . First, a rectangular pulse is employed as the input, and it is suddenly turned off at 0 ns. Then the photons that where trapped in the cavity start to decay through the output waveguide. By observing the discharging signal using time resolved measurement we obtained a photon lifetime of 1.01 ns [Fig. 2(a)]. The accuracy and the reproducibility of time domain measurement were confirmed  and the photon lifetime agrees perfectly with that obtained with spectral domain measurement.
Finally we studied the propagation of a pulse through the cavity system by using time resolved measurement. We obtained a pulse delay of 1.45 ns for an input pulse with a width of 1.9 ns [Fig. 2(b)]. This value corresponds to the record smallest group velocity of 5.8 km/s demonstrated in any dielectric slow-light material. The above result paves the way for the application of the enhancement of light and matter interaction or the development of an optical delay line with a small footprint.
 T. Tanabe, et al., Appl. Phys. Lett. 90 (2007) 031115.
 E. Kuramochi, et al., Appl. Phys. Lett. 88 (2006) 041112.
 T. Tanabe, et al., Electron. Lett. 43 (2007) 187.
 T. Tanabe, et al., Nat. Photonics 1 (2007) 49.
 T. Tanabe, et al., Opt. Express 15 (2007) 7816.
Fig. 1 (a) Scanning electron microscope image of a width-modulated line defect PhC nanocavity. (b) Far field pattern of the resonant light. (c) Transmittance spectrum.
Fig. 2 (a) Discharging waveform.
(b) Pulse response.
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