Optirs & Laser Technolog.v, Vol. 28, No. 8, pp. 561-563, 1996

Copyright f 1996 Elsevier Science Ltd

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Simple depolarizers for spectrophotometric measurements of anisotropic samples M. EL SHERIF, M. S. KHALIL, S. KHODEIR, N. NAGIB

Spectrophotometric measurements on optically anisotropic samples are considerably affected by the partial polarization of light in the instrument. It is shown that by depolarizing the incident beam, more realistic results are obtained even for strong directional anisotropic materials. Two types of depolarizers were investigated. Copyright @ 1996 Elsevier Science Ltd.

KEYWORDS: spectrophotometry, polarization, anisotropic materials

Introduction Experimental details

The presence of a pronounced component of polarized light in spectrophotometers that is wavelength dependent has been observed by several authorsi-4. However eliminating or decreasing the polarization effect experimentally in such measurements still needs more effort. The polarized component is usually initiated at the light source and is enhanced by polarization sensitive optical elements. If the sample is isotropic, the measurements are not affected by polarization, provided that the light beam is strictly parallel, falls normal to the sample and the detector is polarization insensitive. In fact, very accurate and reproducible transmission data were obtained for isotropic samples in a spectrophotometer utilizing polarized light5. The extent to which the measurements on anisotropic samples are affected by polarization will depend in the first place upon the anisotropic characteristics of the sample. In this respect, measurement on dichroic samples exhibit strong directional sensitivity in the presence of a polarized component2. The mathematical treatment of the interaction between the polarized component and optical elements can be found in Refs 6 and 7.

Transmittance measurements were carried out on a calcite Nicol prism polarizer using a UV-3101PC scanning spectrophotometer from Schimadzu, Japan. This anisotropic sample served two purposes, the first is that the spectral transmittance of the Nicol prism is known in general*. The second, is that only the extraordinary rays will be transmitted. Thus, although calcite is a birefringent material, the prism will show strong directional characteristics for any incident polarization state similar to dichroic samples. The Nicol prism was introduced in the path of the sample beam with its transmission axis set at angles O”, 45” and 90” with the horizontal direction and the transmittances were recorded in the spectral range 400-700 nm.

In the present work, the possibility of eliminating the instrumental polarization by introducing a new simple depolarizing element in the light beam prior to interaction with the sample was investigated. The function of a depolarizer is to produce a random distribution of different polarization states which converts light to a state very close to unpolarized light.

The authors are in the National Institute for Standards, PO Box 136 Giza, Code no. 12211, Giza, Egypt. Received 13 November 1995. Revised 20 March 1996.

Two different types of depolarizer were used to investigate their efficiencies. An identical pair of each type was used with one depolarizer in the path of the sample beam before the Nichol prism and the other in the path of the reference beam of the spectrophotometer. The first type (D,) is a conventional wedge-type depolarizer made of calcite crystal with wedge angle about 1”. The second type (Dz) was prepared as follows: high quality Muscovite mica sheets of thicknesses 15-35 urn were cut into small pieces of surface areas nearly 1 mm2 and were mixed with fine crystallites of calcite of linear dimensions about 0.1 to 0.2 mm in a cementing material (optical Araldite). The mixture was then embedded between two thin glass plates. The mean value of the refractive index of the cementing material in the studied spectral region (400-700 nm) is N 1.52 which is close to the mean values of the refractive indices of mica and calcite. Light scattering by the depolarizer is accordingly highly reduced.

561

562 Depolarizers for spectrophotometric measurements of anisotropic samples: M. El Sherif et al.

Results and discussion

Ideally, the transmittance of a Nicol prism for an incident unpolarized light is independent of the orientation of the prism and increases smoothly with increasing X in the visible spectrum, as represented in Fig. 1 by heavy black dots taken from Ref. 8. However, even in the highest quality prisms, the transmittance is not necessarily the same when the prism is rotated through 180”. This is due to the oblique faces of the prism and also due to possible manufacturing errors.

Figure 1 shows the transmittances of the prism at the selected orientations. The differences between the recorded transmittances can only be understood from the viewpoint of the presence of instrument polarization effects possessing azimuthal preference at 0”. At this orientation, the transmittance exceeds the highest possible theoretical value of 50% all over the whole spectrum. Also, the increase in the transmittance with X is more pronounced at this orientation than expected for such prismss. The transmittance curve at 45” is close to the known characteristic curve of the prism. The irregularities in the transmittance (curves at 0” and 90“) are due to Wood anomalies. In particular, those at N 460 nm and N 550 nm were previously reported at nearly the same positions 1,2,4. Essentially, they result from bands of abnormally high or low intensity, in the spectrum of light diffracted from a grating, which cause fluctuations in the degree of polarization3 by up to 30%.

Figures 2 and 3 shows the recorded transmittance of the Nicol prism at the same orientations after inserting the two pairs of identical depolarizers of type Dt and D2, respectively. The three curves on each figure approached each other and the transmittance dropped to a value which is very close to the known data of the Nicol prism. The insertion of the depolarizers increased the transmittance at 90” and decreased the transmittance at 0”. The persistence of Wood anomalies (although severely attenuated), the differences between the transmittances at the three orientations and the exchange in positions of the curves at the short wavelength side, indicate that the depolarization process is not complete. At the same time, the transmittances at different orientations are -5-10% less than expected for the prism. This can only be explained as a result of scattering effects by the depolarizers. As the acceptance angle of the Nicol prism for the e-ray is considerably small, scattered rays at angles exceeding a certain value (about 10’) will be absorbed by the blackened faces of the prism. A correction can be made to account for the scattering effects. This reduction in intensity is not expected for ordinary samples. If the transmittance is of a complex structure or the anisotropy arises from internal stresses, the interpretation of the results is a tedious work as it will not be easy to find an intermediate orientation where polarization effects are minimum. The use of depolarizers in this case is more adequate.

In order to assess the efficiencies of the depolarizers, the degree of polarization’ P of the instrument was calculated before and after inserting each of the depolarizers. The results are shown in Fig. 4. It indicates that the polarization of the light beam emerging from each of the depolarizers (especially for the

1 :F-r_r-11: . . . . . . . . . . . . d

1 -------

20 ------------\ \ .x> .___--, _--___I

0 ~~~~I”,“~-(~‘~‘~,t,,~(/~~~~’ 400 450 500 550 600 650 700

Wavelength (nm)

Fig. 1 Transmittance of the Nicol prism with its transmission axis oriented at o”, 45” and 90”. Heavy black dots represent its typical transmittance for unpolarized light8

o L-... ~~ 1 _ _ .i _~A _1 400 450 500 550 600 650 700

Wavelength (nm)

Fig. 2 Transmittances of the Nicol prism after inserting the depolarizer D,

“0 s .E 20

e t.!

I-

0 -- LIIIIL.__~, I 400 450 500 550 600 650 700

Wavelength (nm)

Fig. 3 Transmittances of the Nicol prism after inserting the depolarizer D2

>-.___.___-- _o,~-_r--~*---------, / 1 400 450 500 550 600 650 700

Wwelength (nm)

Fig. 4 Degree of polarization of the instrument: (a) before inserting a depolarizer; (b) after inserting D,: and (c) after inserting D,

Depolarizers for spectrophotometric measurements of anisotropic samples: n/l. El Sherif et al. 563

depolarizer D2) is drastically reduced throughout the spectrum. Negative values of P were reported previously in a different situation’. The degree of polarization with the depolarizer Dr , although greater than that of DZ, is practically independent of wavelength.

this work and was found to be highly efficient over an extended spectral range. At the same time, it has the advantages of low cost, simplicity of preparation and the possibility of being manufactured to any desired size.

References Conclusions

Polarization effects in optical instruments can be greatly reduced by depolarizing the light beam prior to the interaction with the sample. Although the depolarization process may be incomplete, the result is highly representative for the actual transmittance. An averaging process of the final data is then adequate and gives a definite and more realistic transmittance of the sample. A new depolarizing element (Dl) is presented in

6 7

8

Hills, M.E., Olsen A.L., Nichols, L.W. iippl Op/, 7 (1968) 1437 Grum, F., Costa, F. Appl Opr, 13 (1974) 2228 Zwinkels, J.C., Dodd, C.X. Appl Op/, 28 (1989) 2381 El Sherif, M. J Eng Appl Sci, 43 (1996) (in Press) Mielenz, K.D., Eckerle, K.L., Madden, R.P., Reader, J. Appl Opt, 12 (1973) 1630 Howell, B.J. Appl Opt, 18 (1979) 809 Shurcliff, W.A. Polurixd Light. Harvard University Press. Cambridge. MA (1962) Bennett, J.M., Bennett, H.E. Handbook o/‘Optics, W.G. Driscoll and W. Vaugham (eds), McGraw-Hill Book Company (1978)

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