Silicon-chip-based mid-infrared dual-comb spectroscopy (1610.01121v2)

Abstract: On-chip spectroscopy that could realize real-time fingerprinting with label-free and high-throughput detection of trace molecules is one of the 'holy grails" of sensing. Such miniaturized spectrometers would greatly enable applications in chemistry, bio-medicine, material science or space instrumentation, such as hyperspectral microscopy of live cells or pharmaceutical quality control. Dual-comb spectroscopy (DCS), a recent technique of Fourier transform spectroscopy without moving parts, is particularly promising since it measures high-precision spectra in the gas phase using only a single detector. Here, we present a microresonator-based platform designed for mid-infrared (mid-IR) DCS. A single continuous-wave (CW) low-power pump source generates two mutually coherent mode-locked frequency combs spanning from 2.6 $\mu$m to 4.1 $\mu$m in two silicon micro-resonators. Thermal control and free-carrier injection control modelocking of each comb and tune the dual-comb parameters. The large line spacing of the combs (127 GHz) and its precise tuning over tens of MHz, unique features of chip-scale comb generators, are exploited for a proof-of-principle experiment of vibrational absorption DCS in the liquid phase, with spectra of acetone spanning from 2870 nm to 3170 nm at 127-GHz (4.2-cm$^{-1}$) resolution. We take a significant step towards a broadband, mid-IR spectroscopy instrument on a chip. With further system development, our concept holds promise for real-time and time-resolved spectral acquisition on the nanosecond time scale.

• The paper presents a silicon-on-insulator dual-comb spectrometer that enables rapid, sensitive mid-IR vibrational absorption measurements.
• The methodology employs mutually coherent microresonator combs from a single CW pump, achieving 127 GHz resolution with precise thermal and voltage tuning.
• The results pave the way for real-time, high-throughput trace molecule detection and integrated lab-on-chip spectroscopy systems.

Overview of Silicon-Chip-Based Mid-Infrared Dual-Comb Spectroscopy

This research paper presents an innovative approach to achieving mid-infrared (mid-IR) dual-comb spectroscopy (DCS) using a silicon-on-insulator chip platform. Offering a powerful technique for molecular fingerprinting, this approach potentially paves the way for high-throughput detection of trace molecules in the liquid phase. This work stands as a significant stride towards integrating broadband mid-IR spectroscopy onto a chip, employing two mutually coherent mode-locked frequency combs that cover wavelengths from 2.6 µm to 4.1 µm, enabling the precise acquisition of acetone spectra at 127 GHz resolution.

Technical Advances

The silicon-based dual-comb spectrometer described in the paper employs microresonators to generate two frequency combs using a single continuous-wave (CW) pump laser. This setup is innovative in its use of a single detector to measure the optical domain absorption spectra, translating them effectively into the RF domain. Utilizing microresonators for the generation of frequency combs allows significant flexibility in the mid-IR range where the molecular absorption strengths are considerably higher than in other regions, enabling high-sensitivity detection.

Key features of the described systems include:

• Mutual Coherence and Control: The use of a shared CW pump laser enforces mutual coherence between the two combs, while thermal control and free-carrier injection allow for their independent mode-locking and tuning. The large line spacing inherent to the chip-scale comb generators (127 GHz) supports accurate signal resolution.
• Tunability: The comb parameters can be finely tuned over tens of MHz. This capacity is due largely to the careful thermal tuning of resonances and the application of reverse-bias voltage to PIN diodes surrounding the resonators.
• Proof-of-Principle Experiment: The paper successfully demonstrates vibrational absorption DCS with acetone, acquiring spectra with a high signal-to-noise ratio through rapid acquisition methods.

Results and Implications

The mid-IR dual-comb spectroscopy demonstrated here offers several advantages:

• High Acquisition Rates: The synchronization of the dual-comb generation allows for fast acquisition times significantly less than those available with current Michelson-based Fourier transform spectrometers.
• Spectral Coverage: With the presented system, the spectral bandwidth covers critical ranges for vibrational spectroscopy on nanosecond time scales. This interface with the fundamental CH, NH, and OH stretching modes is crucial for broad chemical application.
• Potential Expansion: Future enhancement could include integrated microheaters for refined control and expanded RF detection capabilities, potentially boosting the system's spectral range.

The potential applications of this technology are expansive, spanning fields such as chemistry, biomedicine, material science, and industrial process control. With advancements in integration techniques and mid-IR detector capabilities, this chip-based platform could be developed into a comprehensive spectroscopy solution, capable of real-time and time-resolved spectral acquisition at unmatched speeds.

Conclusion and Future Prospects

This paper presents a detailed exposition of the development of a CMOS-compatible mid-IR dual-comb spectroscopic system that enables rapid and sensitive vibrational absorption measurements in liquid-phase studies. As the authors suggest, enhancing the RF bandwidth and integrating additional components like quantum cascade lasers could make this platform a pervasive tool for real-time vibrational sensing and molecular fingerprinting. Further research and development could elevate it to a robust laboratory-on-chip capable of advancing spectroscopy to new heights in both practical and theoretical undertakings.