Abstract
Laser frequency combs, with their unique combination of precisely defined spectral lines and broad bandwidth, are a powerful tool for basic and applied spectroscopy. Here, we report offset-free, mid-infrared frequency combs and dual-comb spectroscopy through supercontinuum generation in silicon-on-sapphire waveguides. We leverage robust fabrication and geometrical dispersion engineering of nanophotonic waveguides for multi-band, coherent frequency combs spanning 70 THz in the mid-infrared (2.5 μm–6.2 μm). Precise waveguide fabrication provides significant spectral broadening with engineered spectra targeted at specific mid-infrared bands. We characterize the relative-intensity-noise of different bands and show that the measured levels do not pose any limitation for spectroscopy applications. Additionally, we use the fabricated photonic devices to demonstrate dual-comb spectroscopy of a carbonyl sulfide gas sample at 5 μm. This work forms the technological basis for applications such as point sensors for fundamental spectroscopy, atmospheric chemistry, trace and hazardous gas detection, and biological microscopy.
Highlights
Spectroscopy has been a primary scientific tool for studying nature, leading to seminal advances in astronomy, quantum physics, chemistry, and biology
Frequency comb lasers combine the above qualities in addition to a broad spectrum of precisely defined optical lines that can be absolutely referenced to radio frequencies (RF) or atomic frequency standards.[1,2,3]
We model the supercontinuum generation by solving the generalized nonlinear Schrodinger equation using a split-step Fourier method.[46]
Summary
Spectroscopy has been a primary scientific tool for studying nature, leading to seminal advances in astronomy, quantum physics, chemistry, and biology. We engineer supercontinuum in silicon-on-sapphire (SoS) waveguides to realize versatile and coherent mid-IR frequency combs with tunable spectral shape and coverage. This platform is introduced for mid-IR nonlinear photonics in an earlier work of Singh et al.[36] We build upon this initial demonstration to fully realize the strength of the SoS platform in terms of geometrical engineering of the waveguides by changing the strip waveguide dimensions and introducing a new waveguide cross section, called “notch waveguides.”. The motivation to use a nanophotonic platform lies in the connection between geometric control of the waveguides and the group-velocity-dispersion (GVD), which allows the unique tailoring of the nonlinear light generation with the application-defined power, spectral shape, and bandwidth. The user-controlled and engineered multiband spectra would benefit applications where parallel multi-comb operation is desired, such as point sensors for real-time in situ chemical synthesis monitoring, near-field microscopy, and remote sensing
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