Diffraction Grating Use to Reject Fluorescence from a Tunable Dye Laser K. Jain, W. T. Wozniak, and M. V. Klein

University of Illinois, Urbana, Illinois 61801. Received 14 November 1974.

Commercial cw dye lasers are finding important uses in Raman spectroscopy, especially resonance Raman spec­troscopy.1-3 However, dye lasers have a disadvantage of emitting nonlasing fluorescent light over a broad wave­length band. Moreover, those using a birefringent filter as the tuning element impose a periodic structure on this fluo­rescence. For example, the Coherent Radiation model 490 tunable dye laser gives a rather high background with sev­eral sharp fluorescence lines shifted 0-1000 cm - 1 from the exciting line. These can overwhelm the Raman signal from the sample being studied.

In this Letter we report the use of a diffraction grating to eliminate almost completely this fluorescence in Raman scattering experiments. Figure 1 shows the design of the apparatus, all components of which are clamped to a hori­zontal optical table. To reduce the power density incident on the grating, the dye laser beam is enlarged by a beam expander (Oriel Corp., model B-34-40, magnification = 10). The diffracted beam reenters the beam expander and is narrowed down to its original diameter. The beam ex­pander has a very small acceptance angle; thus it is impor­tant that the laser beam hits the mirror as close to its edge as possible and that the diffracted return beam just clears it. In our setup, the separation between the two beams near the mirror was 2-3 mm and the mirror-beam expand­er distance was about 25 cm. The beam expander can be focused so the diffracted beam emerging from it is essen­tially parallel. The distance between the grating and the beam expander is not important, for the beam is almost parallel. The diffraction grating4 has 1200 grooves/mm and is blazed at 7500 A. Its mechanical mounting is also shown in Fig. 1. The grating G, with its backing B, is ki-nematically mounted with three screws S1, S2, and S3 to mount M and held by a spring (not shown). Adjustment of S1, S2, and S3 is easily done to direct the diffracted beam back into the beam expander. The combined power loss due to the grating and the two passes through the beam ex­pander is approximately 35% at 5880 A. Since the plane of vibration of the electric vector of the light was parallel to the grating grooves, the grating efficiency should be near its maximum in the 5500-6000-A region.

Figure 2(a) shows the Raman spectrum of a gallium phosphide crystal in the 320-380-cm-1 region taken in the. backscattering geometry with VL = 17,388 cm -1 and with­out using the grating. The line at 355 cm - 1 is the polari-ton, and the shoulder at 365 cm -1 is leakage of the forbid­den TO (r) phonon.5-7 The polariton is seen in the back-scattering geometry as a result of reflection, from the pol­ished back surface of the crystal, of both the laser light and the polariton-shifted laser light. The same spectrum, taken when the incident laser beam is filtered by the grat­ing, is shown in Fig. 2(b). The strong background in Fig. 2(a) due mainly to laser fluorescence has been reduced at least by a factor of 4.

In Figs. 3(a) and 3(b) we show the Raman spectra of CC14 in a 1-mm capillary tube taken using a 90° scattering ge­ometry with and without laser beam filtering by the grat­ing. The exciting frequency in this case was 17,010 cm-1. In the figure, the lines marked A (459 cm-1), B (314 cm-1), and C (218 cm-1) are internal vibrational modes of CC14

(see Ref. 8). Lines B and C are barely visible in Fig. 3(a) where the grating was not used. With the grating, the background is reduced by a factor of 10 and the spectrum [Fig. 3(b)] is comparable with that obtained usiag other laser sources, e.g., an Ar+ ion laser.9 In Fig. 3(a) the lines marked 1, 2, 3, and 4 are sharp fluorescence lines from the dye, in this case Rhodamine 6G, usually seen in all spectra. The spacing between the lines, a property related to the bi­refringent filter itself, is approximately 140 cm-1.

An estimate of how close to the exciting line the system rejects fluorescence can be made by use of the grating equation nλ = d sin0. In this case, d = 1/2000 mm = V1.2 μm, n = 1, and cos0 is less than, but approximately equal to, one. On differentiation the result is dλ = [(dθ)/1.2](dλ in μm). dθ is roughly equal to the ratio of the final iris di­ameter (near the sample) to the grating-iris distance. For our system dθ = 2 mm/2 m = 10~3, a figure consistent with

Fig. 1. Diffraction grating with arrangement for fluorescence rejection.

Fig. 2. Raman spectrum of GaP showing the polariton (355 cm-1) and the TO (Γ) phonon (365 cm"1): (a) without grating, and (b)

with grating.

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Fig. 3. Raman spectrum of CCL4 from 200-900 cm-1: (a) without grating, and (b) with grating.

the divergence of the TEM 0 0 laser beam; and, therefore, dλ = 10 -3/1.2μmm, or 0.8 nm. This corresponds to approxi­mately 32 c m - 1 for an exciting wavelength of 5000 A.

As an experimental verification of this estimate we have been able to remove successfully any evidence of the 4889-A plasma line from an Ar+-ion laser with the grating set for 4880-A excitation. In fact, experiments with the dye laser indicate that with our system we can eliminate fluorescence to within 0.5 nm ( 20 cm - 1 ) of the exciting laser line.

This work was supported in part by the National Science Foundation under Grant GH-37757 and by the Advanced Research Projects Agency under Contract DAHC-15-73-G-10.

References 1. T. C. Strekas and T. G. Spiro, J. Raman Spectrosc. 1, 387

(1973). 2. A. L. Verma, R. Mendelsohn, and H. J. Bernstein, J. Chem.

Phys. 61, 383 (1974). 3. P. Y. Yu, Y. R. Shen, Y. Petroff, and L. M. Falicov, Phys. Rev.

Lett. 30, 283 (1973). 4. Our grating is an old Bausch & Lomb grating taken from a Spex

monochromator. The backing (B in Fig. 1) is that originally provided by Spex.

5. C. H. Henry and J. J. Hopfield, Phys. Rev. Lett. 15, 964 (1965). 6. S. Ushioda, J. D. McMullen, and M. J. Delaney, Phys. Rev. B8,

4634 (1973). 7. K. Jain and M. V. Klein, University of Illinois, unpublished re­

sults. 8. H. L. Welsh, M. F. Crawford, and G. D. Scott, J. Chem. Phys.

16, 97 (1948). 9. This system also works well as a filter for the weak lines of an

Ar+ ion laser or an Ar+-Kr+ mixed-gas laser where interference filters are not available.

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