Raman Imaging

Raman spectroscopy imaging simultaneously identifies and localizes a number of molecular species because of Raman diffusion specificity. This allows for the characterization of vibrational, optical and electronic properties that are difficult to observe with other measurement techniques. Raman spectroscopy requires maximum measurement efficiency because the signal from Raman diffusion is much weaker than other optical characterization techniques.

A new type of Raman spectroscopy imager, RIMA^TM, has been developed by University of Montreal and Photon etc. The patented technology of the Bragg Tunable Filter (BTF) significantly reduces the acquisition time compared to currently available imagers while keeping high spatial and spectral resolutions. The standard methods, point-to-point measurements or imagers using liquid crystal tunable filters, increase substantially the acquisition time because of the downtime of mechanical displacements of the sample or the low filter transmission and polarization sensitivity. With a BTF, a single wavelength is detected at a once on the whole image. Wavelengths are scanned by changing the angle of incidence of the beam on the grating. The decision to use spectral rather than spatial scanning saves in acquisition time. The BTF has an achievable efficiency up to 80%, allowing for non-destructive molecular analysis with high sensitivity. The transmission is continuously tunable over 400 nm range with a spectral resolution of 0.2 nm.

RIMA™ was tested with a Si substrate where a pattern of Ti has been deposited. We use a single mode doubled Nd:YAG laser operating at 532 nm and a 100× Olympus microscope objective. The illumination area has a diameter of 80 μm. The power density on the sample is 3 kW·cm^-2. Figure 1 (a) shows an image at 532 nm, corresponding to the reflection of the laser. The pattern “26” showing a higher reflectance is a structure of Ti. The surrounding area with lower reflectance is the Si substrate. Figure 1 (b) shows an image at 520 cm^-1 from the laser line, corresponding to the Si Raman diffusion. We clearly observe a signal emitted from the Si substrate whereas no light is emitted from the Ti pattern. Figure 1 (c) shows the spectra at the laser wavelength and at 520 cm^-1 from it. The narrow peak centered at 520 cm^-1 coming from the Si substrate and the absence of light from the Ti patterns demonstrate that we observe the Raman diffusion from the Si substrate with good spatial and spectral resolutions of 7 cm^-1.

F1: Monochromatic images at (a) 532 nm and (b) 520 cm^-1 of a Si substrate with a pattern of Ti. 
(c) shows the spectra as a function of the wavenumber of different positions on the image.