An Octave-Spanning Mid-Infrared Frequency Comb in Silicon Nanophotonic Waveguides
The paper presents a significant advancement in the generation of mid-infrared frequency combs utilizing silicon nanophotonic wire waveguides. The researchers successfully demonstrate an octave-spanning frequency comb, with a spectral range extending from 1,540 nm to 3,200 nm. This achievement is primarily facilitated by pumping a 1-cm-long, dispersion-engineered silicon wire waveguide with femtosecond (fs) pulses possessing an energy of only 15 picojoules (pJ).
Core Methodology and Device Architecture
The silicon waveguide under discussion is fabricated in a silicon-on-insulator (SOI) platform consisting of a 390-nm thick silicon device layer supported by a 2-µm buried oxide layer. The waveguide is precisely engineered with a rectangular cross-section whereby slight over-etching optimizes dispersion properties. Leveraging the high nonlinear refractive index of silicon, significant optical confinement is achieved, leading to a nonlinear parameter of 38 (W·m)-1. The authors employ a femtosecond mode-locked Ti-Sapphire laser to pump the waveguide near its zero dispersion wavelength, culminating in the broadband supercontinuum generation.
Results and Analysis
A significant outcome of this experimentation is the generation of phase-coherent supercontinuum output verified through free-running beat note measurements with narrow-bandwidth CW lasers. This coherence is an essential property for precision applications such as frequency metrology and direct frequency comb spectroscopy. Through simulations, the researchers establish that the coherence is uniform across the entire bandwidth, corroborating the experimental data. The results indicate the potential of these silicon nanophotonic structures to sustain frequency comb structures with exceptional spectral width due to their inherent dispersion properties and high-refractive index contrast.
Implications and Future Prospects
The demonstrated approach presents two major implications for the future of mid-infrared photonics. Theoretically, it underscores the viability of leveraging silicon as a medium for broadband frequency combs, which could significantly streamline and compact future optical systems compared to traditional chalcogenide-based systems. Practically, due to the inherently high nonlinearity and capacity for precise dispersion engineering of silicon waveguides, the technique paves the way for widespread CMOS-compatible photonic circuits operating in the mid-infrared spectrum.
Additionally, the potential for scaling and extending the supercontinuum generation to broader spectra—limited by the oxide layer—is particularly fascinating. With advancements in dispersion engineering and innovations such as the silicon-on-sapphire platforms, the bandwidth could be further expanded up to 8,500 nm. This expansion could be pivotal in precision measurement applications, boosting the resolution and sensitivity of systems used in various scientific and industrial fields.
In conclusion, this paper marks a significant stride toward developing efficient and compact mid-infrared frequency comb systems using silicon nanophotonics. Future research exploring improved designs, alternative platforms, and real-world applications could catalyze revolutionary changes in frequency metrology and spectroscopic methods.