Photon-counting Raman spectroscopy of silicon nanowires

Abstract

Summary form only given. Raman scattering can be exploited for amplification in optical fiber telecommunications or, chemical identification in spectroscopy, but represents a source of detrimental noise photons for quantum communications. The spectral distribution of spontaneous Raman scattering (SpRS) can be measured in bulk samples with the free-space 90° scattering method [1]. In long fibers the SpRS spectra can be measured using a pulsed laser to achieve measurable signals [2], incompatible with the damage threshold of many on-chip devices. Measurements of stimulated Raman scattering have been performed using nonlinear pump-probe techniques [3], requiring the addition of either a highly tunable or ultra-broad bandwidth probe. Recently photon-counting techniques have been demonstrated to measure weak SpRS signals in fibers [4-6], however no direct measurements of the SpRS spectra of nanophotonic chip-devices over a broad bandwidth have been performed.In this work, we report for the first time a direct measurement of the SpRS spectra over a broad bandwidth in a photonic integrated platform using photon-counting spectroscopy. We apply it to amorphous silicon (a-Si) and crystalline silicon (c-Si) nanowire devices, where both platforms are CMOS compatible. The method, outlined in Fig. 1a), uses a CW laser to excite SpRS in the waveguide, a spatial light modulator (Finisar waveshaper) to resolve the spectral distribution, and a single photon detector to provide the sensitivity needed to measure the small signal associated with the SpRS in chip-scale waveguides. The results shown in Figure 1b) display a narrow Raman peak for c-Si at the frequency shift of ~525cm-1 with a bandwidth of <;150GHz, with typical reported values of ~100GHz. In contrast the Raman spectrum for a-Si has a 3dB bandwidth of ~2.5THz, comparable to the bandwidth of ~6THz for silica glass the standard material used in distributed telecommunication Raman-fiber amplifiers. As the SpRS spectrum is proportional to the stimulated Raman gain [6], the a-Si platform proves a promising candidate for broadband CMOS compatible chip-scale Raman amplifiers. In addition a-Si displays low two-photon absorption [7] and free carrier generation, the main limiting factors in the efficiency of c-Si Raman amplifiers [8]. In conclusion, we have reported the first demonstration of direct broadband photon-counting Raman spectroscopy in chip-scale nanophotonic devices, specifically in amorphous and crystalline silicon nanowires. Our method uses narrowband single photon sensitivity tuned over a broad spectrum to measure low SpRS signals. These results also show the promise of the a-Si platform for broadband on-chip Raman amplification.