Carbon nanotubes emit intense spontaneous Raman scattering thanks to a strong electron-phonon interaction under low-dimensional quantum confinement, and they are expected to exhibit a large Raman gain. To observe the stimulated Raman scattering and Raman laser action of carbon nanotubes, we study the nanocavity-enhanced Raman scattering of semiconducting single-walled carbon nanotubes (SWCNTs) placed in high-Q silicon photonic crystal (Si PhC) nanocavities. Figure 1(a) shows a fabricated point-defect Si PhC nanocavity covered with randomly oriented SWCNTs. The experimental cavity Q value was 1100. Figure 1(b) shows the Raman spectrum of a PhC sample when it was excited with a 940-nm-wavelength laser. An intense Stokes Raman peak is seen at around 1600 cm-1, which is assigned to the G+ mode of the SWCNTs. Figures 1(c) and (d) show the Raman spectra and peak intensities of the G+ mode, respectively, as the wavelength of the excitation laser changes. Figure 1(c) also shows the PhC cavity mode at 1117 nm. When the Raman scattering wavelength of the G+ mode is adjusted to the cavity resonance, the peak Raman intensity is resonantly increased. This result shows that the resonant PhC nanocavity enhances the spontaneous Raman scattering of SWCNTs . When we took the excitation and emission extraction efficiencies of our samples into consideration, we found that the Raman intensity was two orders of magnitude larger than the intensity of SWCNTs on a flat Si film. This enhanced Raman intensity results from a 20-fold increase in the emission extraction efficiency and a 5-fold increase in the density of the optical state, both of which are provided by the resonant PhC nanocavity. Further enhancement of the Raman scattering intensity will need novel PhC nanocavities with an air slot holding many SWCNTs.
The nanocavity-enhanced spontaneous Raman scattering demonstrated here is the first step towards realizing low-threshold and nanometer-scale Raman lasers based on a high-Q Si PhC cavity and carbon nanotubes.