Coherent Raman spectroscopy platform for real-time standoff and portable molecular sensing

Background

Sensitive and selective molecular spectroscopy plays a crucial role across fields ranging from environmental monitoring and industrial quality control to security screening and forensic analysis. In particular, the ability to detect trace amounts of chemicals or explosives from a safe distance has become increasingly important for military, homeland security, and public‐safety applications. Traditional spectroscopic methods, such as spontaneous Raman scattering or infrared absorption, often lack the combination of sensitivity, speed, and standoff capability required for many real-world scenarios. Consequently, there is a strong demand for rapid, noncontact techniques that can deliver real-time chemical identification with high specificity.

Conventional approaches suffer from inherently weak signal levels, requiring long acquisition times or high laser powers that compromise sensitivity and portability. Nonresonant background contributions can obscure faint molecular signatures, while two-dimensional raster scanning strategies limit temporal resolution and hinder real-time imaging of dynamic samples. Moreover, the reliance on bulky, high-energy ultrafast laser systems and complex optical setups drives up cost and footprint, making field deployment challenging. Fiber-based delivery schemes further introduce dispersion, alignment issues, and signal losses, degrading overall signal-to-noise ratios and restricting effective standoff distances for reliable detection.

Technology

The system employs co-aligned pump and Stokes near-IR femtosecond pulses with a picosecond narrowband probe to excite molecular vibrations, suppress nonresonant background, and deliver millisecond spectral resolution. A cylindrical lens shapes the beams into a line at focus, imaged onto a spectrometer slit with perpendicular scanning to assemble a full hyperspectral 3D cube faster than raster methods. For standoff operation at several meters, directional CARS emission is collected by telescope optics into a CCD spectrometer. A portable fiber-based variant uses photonic crystal fiber for four-wave mixing to generate all wavelengths, deliver time-delayed pulses via a fiber tip, and return the signal to a miniature fiber spectrometer.

The picosecond probe delay suppresses nonresonant background and greatly improves signal-to-noise ratio compared with spontaneous Raman. High-power near-IR pump/Stokes beams paired with a narrowband probe yield clear, fast CARS spectral lines and allow measurement of vibrational lifetimes for added specificity. Line-shaped illumination and one-dimensional scanning accelerate data acquisition by orders of magnitude versus 2D raster imaging. Directional emission extends detection range to meters, while the fiber-based design reduces footprint and cost relative to bulk ultrafast laser setups. Together, these features enable real-time, remote, and portable molecular analysis for security, environmental monitoring, and biomedical applications.

By comparing the CARS spectra of tissue of interest with the spectra of specific pathological tissue and normal tissue, the device is able to realize real-time biopsy for obtaining complete surgical margin control during removal of cancer. During Mohs surgery to remove skin cancer, CARS device can be utilized to greatly simplify the biopsy procedure and provide results in milliseconds. The device can also be used to detect precancerous conditions.

Benefits

• Enhanced sensitivity (orders-of-magnitude higher than spontaneous Raman)
• High molecular specificity via vibrational fingerprinting
• Real-time detection in milliseconds
• Remote standoff sensing over meter-scale distances
• Rapid hyperspectral imaging using line-scanning
• Portable fiber-based near-field detection
• Improved signal-to-noise ratio by eliminating nonresonant background
• Directional CARS emission for efficient long-distance collection

Applications

• Standoff explosives detection
• Portable chemical threat screening
• Industrial process quality control
• Environmental pollutant monitoring
• Forensic substance identification

Additional info

A time-delayed CARS platform employs three collinear femtosecond near-IR pump/Stokes pulses and a picosecond probe offset by picoseconds to suppress nonresonant background, generating millisecond-resolved molecular spectra. A cylindrical lens shapes a focal line for rapid 1D hyperspectral scanning into a 3D data cube. For remote sensing, a telescope collects directional CARS at meter-scale; a portable variant uses PCF-based four-wave mixing and fiber delivery/collection to a miniature spectrometer.