Tip-enhance Raman spectroscopy (TERS)

RAMAN

Raman scattering spectra provide a unique fingerprint to identify almost any chemical. This is extremely useful for identifying materials that not only pose a health or safety hazard, but are chemically similar to other, non-dangerous materials. Additionally, the method can be used for standoff inspection in situations where access is limited or otherwise prevented due to safety concerns.

The Need

The primary disadvantage of using Raman spectroscopy is the extremely small scattering cross-section compared to Rayleigh or fluorescence scattering. High power lasers and/or long integration times are often required to overcome this limitation. For applications, where eye safety, physical space, power requirements and time are a concern, alternative methods must be found.

RMD’s solutions

To meet this challenge, scientists at RMD have explored the use of Raman scattering for standoff detection. Applications include detection of explosive compounds and chemical pollutants.

Ultraviolet: We have used deep-UV Raman spectroscopy for standoff identification of IEDs. It offers several advantages, but the primary one is that the scattering cross section dramatically increases with shorter wavelength. While ultraviolet is attractive, there are challenges due to limited sources and detector choices. Consequently, we are exploring the use of new receiver options to help promote the use of deep-UV Raman.

Surface plasma enhancement: Surface Enhanced Raman Spectroscopy (SERS) has been employed to increase the signal strength of Raman cross section signatures. SERS combines modern laser spectroscopy with the optical properties of metallic nanostructures. RMD’s scientists have explored this technique to help develop a compact Raman system that can be used for space missions to find chemical signatures that indicate signs of life in the universe.

Standoff Raman Lidar detection: While there are a number of techniques available to remotely sense atmospheric pollutants, many of these techniques have significant performance limitations. The inherent advantage of the Raman Lidar system is that the transmitter and receiver can be co-located, eliminating the need for a remotely located receiver or the use of retro-reflectors.

This greatly simplifies the placement and set up of the remote emissions monitor and provides simplified calibration steps using atmospheric oxygen and nitrogen. The pollution profile can be resolved in both time and space, enabling a snapshot of the emissions plume to precisely identify the source of excess pollution. RMD has successfully demonstrated the capabilities of Raman LIDAR for remote sensing of automobile emissions, showing that it offers chemical-specific information that scattering alone cannot provide.

Raman spectroscopy signatures of three explosive-related compounds.