Silicon Nitride Microresonator Raman Lasers

TL;DR

By integrating ultra-high-Q thin-film silicon nitride (SiN) microresonators with silica cladding, the authors realize stimulated Raman scattering (SRS) enabled Raman lasing on a CMOS-compatible platform. They engineer waveguide confinement and dispersion to suppress Kerr nonlinearities and maximize cladding Raman gain, achieving lasing thresholds as low as $1.8\ ext{mW}$ and intrinsic quality factors exceeding $10^7$, with broadband tunability driven by silica's wide Raman gain (over $120\ ext{cm^{-1}}$). The work demonstrates chip-scale Raman lasers in SiN with milliwatt thresholds and tunable outputs, expanding the nonlinear optics capabilities of SiN for spectroscopy, optical communications, and quantum photonics.

Abstract

Silicon nitride (SiN) has emerged as a promising platform for integrated nonlinear photonics because of its low propagation loss, wide transparency window, and CMOS compatibility. Nonlinear processes arising from photon-electron interactions, such as Kerr frequency comb generation and second harmonic generation, have been extensively explored. In contrast, photon-phonon interaction-based nonlinearities, such as stimulated Raman scattering, remain largely unexplored in this integrated platform, despite their potential for broadband frequency conversion. Here, we demonstrate efficient Raman lasing in ultra-high-Q SiN microresonators by harnessing the strong intracavity field enhancement and engineering the optical mode to overlap with the Raman-active silica cladding. Through dispersion engineering and waveguide geometry optimization, we suppress competing Kerr nonlinearities while enhancing Raman gain, achieving lasing with sub-2 mW thresholds. We further investigate the trade-off between optical confinement and quality factor, revealing its impact on the overall nonlinear efficiency. Moreover, we also demonstrate broadband tunability of the Raman shift exceeding 120 inverse centimeters, enabled by the wide Raman gain spectrum of silica, offering new flexibility in designing integrated tunable Raman lasers. These results position SiN as a viable platform for chip-scale Raman lasers, expanding the nonlinear optics toolbox of the SiN platform and enabling compact, power-efficient light sources for applications in spectroscopy, optical communications, and quantum photonics.

Figures (4)

• Figure 1: Raman lasing in silicon nitride microresonators (a) The pump light field can be concentrate in the silica cladding around a thin-film silicon nitride waveguide and resonantly enhanced in a microresonator, faciliating Raman lasing. The inset shows the mode profile of a SiN waveguide, emphasizing the overlap between the optical field and the silica cladding, with the waveguide core region highlighted in white. (b) Schematic of broadband Raman lasing enabled by the wide Raman gain bandwidth of silica. (c) Simulated effective $\mathrm{TE_{00}}$ mode area of silicon nitride waveguides, used to estimate the nonlinear Raman effects in the cladding material. Unit: $\mu$m$^2$. The blue dashed line indicates the waveguide geometries with GVD=0. (d) Calculated Raman lasing threshold power as function of intrinsic $Q$ for microresonators with different $A_{eff}$.
• Figure 2: High-$Q$ silicon nitride microresonators (a) Optical microscopy image of a SiN microring resonator with a 220 $\mu$m radius. (b) Scanning electron microscopy (SEM) image of the cross-section of a 340-$nm$-thick SiN waveguide cladded in SiO$_2$. (c) Bird-view SEM image of a SiN waveguide after the etching process. Scale bars, 300 $nm$ (b, c). (d,e) Measured transmission spectra of SiN microring resonators with different waveguide widths: (d) 1.2 $\mu$m and (e) 3 $\mu$m. (f,g) Measured integrated dispersion of the fundamental TE mode. Insets shows the corresponding field distribution for the fundamental TE mode. The grey circles are for the high-order TE mode. The normal dispersion and the absence of avoided mode crossing prevent Kerr comb generation from competing with the nonlinear Raman scattering. (h, i) (left) Measured typical transmission spectra at 1560.8 $nm$ (h) and 1567.5 $nm$ (i) exhibiting high $Q_\text{int}$: (h) 12.3 million and (i) 40.2 million. (right) Histrogram of measured $Q_{\text{int}}$ between 1560 $nm$ and 1630 $nm$ for microring resonators with different waveguide widths: (h) 1.2 $\mu$m and (i) 3 $\mu$m.
• Figure 3: Raman lasing in SiN microresonators (a), (c) Extracted (dots) and simulated (solid lines) coupling quality factor as a function of wavelength for microresonators with waveguide dimensions of 340 $nm$$\times$1.2 $\mu$m (a) and 340 $nm$$\times$3 $\mu$m (c). Dashed lines indicate the critical-coupling condition. Different colors represent different bus-to-ring coupling designs. (b), (d) Measured Raman lasing threshold power for pumping different resonances for the two microresonator geometries. Despite having a larger effective area, the multi-mode microresonators exhibit significantly higher $Q$, resulting in improved lasing performance with lower threshold power compared to the single-mode devices. (e, f) Measured Raman lasing spectra by pumping SiN microresonators (waveguide cross-sectional dimension: 340 $nm$$\times$3 $\mu$m) under different bus-to-ring coupling conditions: (e) critical-coupling and (f) over-coupling. Insets show the corresponding lasing threshold and slope efficiency. The critically coupled device exhibits the lowest threshold (1.8 $mW$), while the over-coupled device achieves a higher slope efficiency (50%).
• Figure 4: Raman laser tuning dynamics (a) Measured Raman lasing spectra by pumping a single-mode microresonator (waveguide dimension: 340 $nm$$\times$1.2 $\mu$m) at different wavelengths. (b) Measured Raman shift frequency as a function of the pump wavelength. Four distinct Raman frequencies are observed, with each of the Raman lasing frequencies remaining pinned within a specific pump tuning range. (c) Measured Raman lasing spectra as a function of Raman shift frequency. The broad bandwidth of the silica Raman gain enables Raman lasing over a large Raman shift frequency range.