Raman Microprobe with Holographic Beamsplitter for Low-Frequency Operation DAVID M. PALLISTER, KEI-LEE LIU, ANURAG GOVIL, MICHAEL D. MORRIS,* HARRY OWEN, and TIMOTHY R. HARRISON Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055 (D.M.P., K.-L.L., A.G., M.D.M.); and Kaiser Optical Systems, Inc., P.O. Box 983, Ann Arbor, Michigan 48106 (H.O., T.R.H.) A Raman microprobe with a holographic beamsplitter is described. The beamsplitter is shown to have 90% laser insertion efficiency and high Raman transmission efficiency on both the Stokes and anti-Stokes side. Useful spectra are obtainable as close as __60 cm -~ from the exciting line using 4 mW power. Index Headings: Raman; Microprobe; Holographic optical element; Instrumentation. INTRODUCTION High-throughput, narrow-band holographic notch fil- ters have enabled development of compact Raman spec- trometers based on single-stage spectrographs, charge- coupled device detectors, and small lasers. 1~ Used with near-infrared diode lasers (780-785 nm) for fuorescence interference rejection, such systems are emerging4 as al- ternatives to Fourier transform Raman spectroscopy. Compact, high-throughput spectrograph systems are ideally suited for use with the Raman microprobe. Be- cause the microprobe samples a small volume, intensities are typically much smaller than those observed with con- ventional probes. 5-7This problem is exacerbated if a con- focal geometry is employed.8-~ To be effective, the ap- erture of the spatial filter must be small enough to reject most of the out-of-focus light from the laser probe. Typ- ically, attenuation is 90% or greater in a confocal mi- croscope. 12 Most Raman microprobes use the epi-illumination ge- ometry. 6 Commonly a 50/50 (reflection-transmission) beamsplitter is used, to maximize total throughput. The- oretically, 50% of the Raman scatter collected by the objective is transmitted to the spectrograph. Alterna- tively, a 20/80 or 10/90 beamsplitter can be employed. The low reflectivity attenuates the incoming laser power by a factor of 5-10, but more of the gathered Raman- scattered light is transmitted to the spectrograph. In general, Raman microprobes use 1-20 mW laser power at the sample, because high power levels would damage many materials. Where high laser power is avail- able, as with the traditional argon-ion laser, the low- reflectivity/high-transmission beamsplitter is suitable. However, a diode or other low-power laser can provide only 5-30 mW at the laser head. Typically, 25-50% of that power is lost in filters and transfer optics. With low-power lasers, insertion losses of 80-90 % at the beam- splitter are intolerable. Conventional dielectric beam- splitters have 90-95% reflectivity and 70-80% trans- mission, but have low slopes. They do not transmit Received 13 April 1992. * Author to whom correspondence should be sent. Raman-scattered light closer than 500-800 cm -1 from the exciting line efficiently. For these reasons, we have developed a Raman micro- scope which uses holographic filters as both the micro- scope beamsplitter and the spectrograph prefilter. The beamsplitter in the epi-illuminator is a modified holo- graphic notch filter, 13 which has been designed and fab- ricated for operation at a 45° angle. The holographic beamsplitter provides high Raman scatter transmission efficiency as close as _+150 cm -~ from the exciting line, and usable efficiency as close as _+60 cm -1, while simul- taneously injecting over 90 % of the incident laser power into the microscope objective. EXPERIMENTAL Raman spectra were obtained with a single-stage spec- trograph (Instruments SA HR-640) fitted with a CCD detector (Photometrics Series 200), as previously de- scribed2 ,1~ A narrow-rejection-band 633-nm notch filter (Kaiser Optical Systems, Model HSNF-633-1.0) was used as a prefilter. The microscope 13 (Olympus BH-2) was fitted with an epi-illuminator and several apochromatic objectives. 13 A holographic beamsplitter (Kaiser Optical Systems, Model HB-633-0) was mounted in a holder to fit the microscope epi-illumination system. Provision was made for fine adjustment of the beamsplitter angle around the nominal 45°. For these experiments, the Hadamard mask system was removed from the optical train. An adjustable iris was placed at the image plane of the pro- jection eye piece to allow confocal or wide field operation. A 10-mW 632.8-nm He-Ne laser (Spectra-Physics 106- 1), was used for illumination. For spectroscopy, samples of materials, as described below, were placed on standard 25 × 75 mm glass microscope slides and covered with standard 0.17-mm glass cover glasses. Spectra were ob- tained with 5 cm -1 resolution. RESULTS AND DISCUSSION The beamsplitter transmission curve at 45° incidence is shown as Fig. 1. A comparison with a commercial di- electric beamsplitter is included. The transmission curve remains smooth and monotonic, slowly increasing to- wards 85% out to beyond 3000 cm -1 from the exciting line in the Stokes and above 80% in the anti-Stokes direction. By contrast, the dielectric beamsplitter notch is wider, making spectroscopy difficult below +500 cm -1 and impossible on the anti-Stokes side. The holographic beamsplitter is optimized for use with unpolarized, Stokes-shifted light. Close to the exciting Volume 46, Number 10, 1992 0003-7028/92/4610-146952.00/0 APPLIED SPECTROSCOPY 1469 © 1992 Society for Applied Spectroscopy