[Paper Review] Doppler dual-comb coherent Raman spectromicroscopy

Florian M. Schweizer, Hannah Terrasa+1 more|arXiv (Cornell University)|Mar 24, 2026

Spectroscopy Techniques in Biomedical and Chemical Research0 citations

TL;DR

The paper introduces time-domain coherent Raman spectroscopy using two frequency combs generated by the Doppler effect from a single ultra-broadband laser, enabling background-free, fast, and high-resolution imaging via cross-phase modulation and down-conversion. It demonstrates millisecond-scale acquisition and ~280 nm diffraction-limited resolution on an ~8 μm PMMA bead.

ABSTRACT

Chemical imaging enabled by Raman processes is crucial to investigating biological and chemical samples in a label-free manner. Stimulated Raman spectroscopy (SRS) overcomes the key limitation associated with low signal levels in spontaneous Raman spectroscopy, however, at the expense of probing only narrow Raman bands. Time-domain implementation of coherent anti-Stokes Raman spectroscopy (CARS) by dual frequency combs can achieve broad Raman bandwidths; nevertheless, its execution is demanding due to strenuous temporal-synchronization of two independent ultrashort laser sources. Here, we introduce time-domain coherent Raman spectroscopy utilizing two frequency combs generated by the Doppler effect from a single ultra-broadband laser source. In contrast to CARS, in our approach, the interference of impulsively launched vibrations by two broadband frequency combs (τ ~ 6 fs) periodically modulates the Kerr nonlinear response of the medium, leading to cross-phase modulation (XPM) experienced by both the combs. This phase modulation leads to spectral broadening and periodic modulation in the anti-Stokes region of the combs. Down-conversion by a factor of ~ 10-8 in the frequency of the vibrations enabled by the dual-comb approach empowered us to use photon-counting methodology in the anti-Stokes region. This makes our technique extremely versatile, background-free, sensitive and fast (millisecond acquisition times), in probing a range of samples from wide bandgap dielectrics and liquids to individual micro-particles with nondestructive pulse energies (~ 100 pJ) incident on the sample. Owing to the higher-order nonlinearity involved in the XPM process, we achieved ~ 2.5 times improvement in diffraction-limited spatial resolution (~ 280 nm) in ultra-broadband chemical imaging of a ~ 8 μm bead of poly-methyl-methacrylate.

Motivation & Objective

• Address the limitation of narrow Raman bands in spontaneous Raman spectroscopy by achieving broad Raman bandwidths.
• Develop a time-domain coherent Raman technique using Doppler-generated dual frequency combs from one laser source.
• Demonstrate cross-phase modulation-based spectral broadening and anti-Stokes modulation for sensitive imaging with photon-counting detection.
• Showcase fast acquisition, low pulse energies, and applicability to diverse samples from liquids to micro-particles.

Proposed method

• Generate two frequency combs via the Doppler effect from a single ultra-broadband laser source (~6 fs pulses).
• Utilize impulsively launched vibrations to induce cross-phase modulation that broadens the spectrum and modulates the anti-Stokes region.
• Down-convert vibrational frequencies by ~10^-8 to enable photon-counting in the anti-Stokes region.
• Leverage higher-order nonlinearity in XPM to achieve improved spatial resolution (~280 nm) in ultra-broadband imaging.
• Operate with nondestructive pulse energies around ~100 pJ on the sample.
• Provide background-free, rapid imaging with millisecond-scale acquisition times.

Experimental results

Research questions

• RQ1 Can Doppler-generated dual-comb schemes enable broadband, background-free coherent Raman spectroscopy?
• RQ2 What are the limits on spatial resolution and sensitivity using XPM-based Raman spectromicroscopy?
• RQ3 How does down-conversion of vibrational frequencies affect detection strategy (photon counting) and noise performance?
• RQ4 Is the approach broadly applicable to solids, liquids, and micro-particles with low pulse energies?

Key findings

• Demonstrated millisecond acquisition times for coherent Raman imaging.
• Achieved ~2.5x improvement in diffraction-limited spatial resolution (~280 nm) for an ~8 μm PMMA bead.
• Implemented background-free detection via photon-counting in the down-converted anti-Stokes region.
• Utilized ~100 pJ pulse energies to probe wide-bandgap dielectrics, liquids, and micro-particles.
• Showed that XPM-induced spectral broadening enhances Raman signal processing in a dual-comb setup.