Dynamic Release of Trapped Light from an Ultrahigh-$Q$ Nanocavity via Adiabatic Frequency Tuning

Adiabatic frequency shifting is demonstrated by tuning an ultrahigh-$Q$ photonic crystal nanocavity dynamically. By resolving the output temporally and spectrally, we showed that the frequency of the light in the cavity follows the cavity resonance shift and remains in a single mode throughout the process. This confirmed unambiguously that the frequency shift results from the adiabatic tuning. We have employed this process to achieve the dynamic release of a trapped light from an ultrahigh-$Q$ cavity and thus generate a short pulse. This approach provides a simple way of tuning $Q$ dynamically.

Figure 1

(a) Experimental setup. (b) A scanning electron microscope image of a width-modulated line-defect silicon PhC-NC, where the neighboring air holes are shifted 3, 6, and 9 nm. The structure of this nanocavity is identical to that described in Ref. [9]. The PhC-NC is coupled to input and output W1.05 (105% width of W1) waveguides through W0.98 (98% width of the W1) line defects. The length of a W0.98 line defect is $12a$, where $a=420\mathrm{nm}$ is the lattice constant. The W1 waveguide has a width of $a\sqrt{3}$. The target hole diameter $d$ is 216 nm and the slab thickness $t$ is 204 nm.

Figure 2

(a) 2D spectrogram map of time-resolved emission spectra when no pump pulse is applied. (b) Emission spectra when a 0.76-pJ pulse is applied at 0 ps. (c) (left) Time-resolved transmission spectra, which are the collected transmittance waveforms of the PhC-NC at different input wavelengths. Carrier lifetime is 2 ns, which is obtained from the slow red-shift after application of the pump. (right) Spectrum obtained by taking a cross-sectional image along the wavelength axis of the 2D map at $t=0$ and 200 ps as shown in (1) and (2).

Figure 3

(a) Operating principle of the dynamic tuning of $Q$. Schematic frequency band diagram in the spatial coordinates in the high $Q$ mode. The light blue area is the frequency regime where propagation is allowed. The yellow area is the mode-gap region of a line defect. (b) Low-$Q$ mode. (c) Mode profile calculated with the 3D finite difference time domain method for a PhC-NC coupled to the input and output waveguides. The structure is the same as that in Fig. 1b except that $d=0.48a$. WG: waveguide. (d) The refractive index at the cavity is modulated with a Gaussian distribution (FWHM: $6a$). (e) Refractive index modulation versus $Q_c$. The inset shows the definitions of $Q_c$ and $Q_i$. $Q_i$ does not change significantly within this range of modulation.

Figure 4

(a) On-to-off step input (dotted line). Recorded output from the PhC waveguide, which is the light coupled from the PhC-NC, with and without a pump pulse. The refractive index is modulated by $\sim 0.01\%$. The exponential fit after the pump is 0.07 ns, which corresponds to a $Q$ of $8.6\times {10}^4$. (b) Spectrally resolved waveforms obtained by taking line plots along the temporal axis of Figs. 2a, 2b.