Enhanced Raman spectroscopy using collection optics designed for continuously tunable excitation

This paper presents an optical instrument to facilitate quantitative Raman measurement to be made using continuously tunable monochromatic illumination from 200 to 700 nm. The instrument includes a custom-designed F/2 achromatic catadioptric optic and is designed to couple to a F/7.5 spectrometer. The illumination optics can be interchanged to form either a point or line focus at the sample, the line focus being preferable for measurement of delicate samples. A prototype instrument has been constructed which enables samples to be easily repositioned, as is required for quantitative resonance Raman spectroscopy (RRS) study. An ultraviolet (UV) CCD is incorporated enabling operation as a UV microscope. An imaging resolution of ~7 µm has been demonstrated. Results are presented demonstrating the axial and spatial imaging performance of the instrument. Spectra illustrate the application of RRS for selectively enhancing low-concentration analytes. The enhancement as the excitation wavelength is tuned and the analyte concentration is varied, is demonstrated. The results illustrate the need for well-characterized achromatic optics when carrying out quantitative investigation using a tunable UV source laser.