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 [1], 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 [2]. 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 [3].
 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 [4].  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 [5] 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.

[1] T. Tanabe, et al., Appl. Phys. Lett. 90 (2007) 031115.
[2] E. Kuramochi, et al., Appl. Phys. Lett. 88 (2006) 041112.
[3] T. Tanabe, et al., Electron. Lett. 43 (2007) 187.
[4] T. Tanabe, et al., Nat. Photonics 1 (2007) 49.
[5] 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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