Theory of Far Infrared Detection Using Nonlinear Optical Mixing

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Abstract
The possibility of far infrared detection using nonlinear optical mixing with lasers is discussed. For the detection of the mixed wave a visible spectrometer and a photo-multiplier whose response time is very short (-8 sec) should be used. The noise sources are entirely different from those of conventional far infrared detectors and the most serious one is laser background noise. The use of polarizers is proposed for the elimination of the noise of this type. A preliminary experiment using Glan-Thompson polarizers and a grating spectrometer showed that, when a ruby laser and a 0.5 cm thick crystal of CdS are used, the minimum detectable power of 10-5∼10-8 W can be expected, corresponding to far infrared wavelength of 1000∼100 µ. This value would be further improved by using better or more polarizers, a higher dispersion spectrometer or more efficient mixing materials. The production of far infrared by beating two lasers and detecting by the mixing with one of them in the same material is also considered. A detection free of serious noise is shown to be possible only for crystals having point symmetry of 32, 3 m and 6̄ m 2.