A novel and simple method for improving the detection limit of conventional Raman spectra using a micro-Raman system and picoliter volumes is presented. A micro-cavity in a reflecting metal substrate uses various mechanisms that collectively improve the entire Raman spectrum from the sample. A micro-cavity with a radius of several micrometers acts as a very effective device that provides multiple excitation of the sample with the laser and couples the forward-scattered Raman photons toward the collection optics in the back-scattered Raman geometry. One of the important features of the micro-cavity substrate is that it enhances the entire Raman spectrum of the molecules under investigation and maintains the relative intensity ratios of the various Raman bands. This feature of maintaining the overall integrity of the Raman features during signal enhancement makes the micro-cavity substrate ideal for forensic science applications for chemical detection of residual traces and other applications requiring low sample concentrations. The spectra measured in these cavities are also observed to be highly reproducible and reliable. A simple method for fabricating micro-cavity substrates with precise sizes and shapes is described. It is further shown that micro-cavities coated with nanofilms of gold take advantage of both surface-enhanced Raman scattering (SERS) and micro-cavity methods and also significantly improve sample detection limits.
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