Polarizer for infrared radiation

#The invention relates to a polarizer for electro-magnetic radiation, in particular for infrared radiation in the wavelength range from 1 to 1000 .mu.m, preferably 100 to 1000 .mu.m.

#Since Heinrich Hertz a grid of parallel-tensioned wires is used as a polarizer for electromagnetic radiation. To prevent diffraction occurring it is necessary to satisfy the condition that the spatial period g of the grid (grating) structure is selected smaller than the wavelength .lambda. used. Using the currently available smallest wire thickness of 5 .mu.m it is possible to achieve a satisfactory polarization contrast of 100 to 1 in the far-infrared spectral range (wavelength.gtoreq.100 .mu.m). Polarizer grating of this kind having round-section wires are described, for example, in J. Opt. Soc. Am. 71, 184 (1981) by J. A. Beunen et al. and in U.S. Pat. No. 2,224,214 (Brown).

#For laser applications, however, a higher contrast is required. One Hertz-type polarizer variant which is more suitable for this purpose consists of a transparent substrate on which a parallel arrangement of metal stripes is deposited which is currently photolithically possible with a spatial period of g=0.8 .mu.m. By means of such a polarizer contrasts as high as 200 to 1 can be achieved for a CO.sub.2 laser beam.

#The invention shall now be explained in more detail on the basis of the sole FIGURE showing a greatly magnified partial cross-sectional view of one preferred embodiment of the polarizer in accordance with the invention .

#On a substrate 1 which is transparent for the radiation to be polarized and which may be a Kapton or polythene film, a metal film pattern of gold stripes 2 is deposited which, in this case, can have an average width of approx. 2.2 .mu.m, and average spacing of approx. 1.8 .mu.m and a thickness of t=1.8 .mu.m. The cross-section of the stripes 2 is slightly trapezoidal.

#The spatial period g amounts to 4 .mu.m, so that the thickness t is slightly less than half the period g. It will be seen that the gaps or slots 3 between each two gold stripes 2 have roughly the same width as the coating thickness t. Also obvious are the slightly slanting edges 4 of the gold stripes stemming from production restrictions. The polarization contrast achieved with these dimensions in the far-infrared region (wavelength.gtoreq.100 .mu.m) amounts to approx. 6000 to 1 which can be enhanced even more by further increasing the coating thickness t.