HIGH-SENSITIVITY POLARIZATION SPECTROSCOPY USING TWIN-PHOTON BEAMS

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The present research investigated the application of the twin-photon beams to the signal-to-noise ratio enhancement for polarization spectroscopy.1. A LBO crystal was pumped by the 454-nm Ar laser to generate twin photon beams by the type II angle-phase matched parametric down conversion. The power of twin beams was obtained to be about 20 pW when the pump power was 140 mW.2. The twin beams were separated by a polarizing beam splitter, and their intensities were detected by avalanche photodiodes (APD). The APD,cooled below 100 K,was operated in the linear mode with amplification factor of 500. In our photon counting system with the APD the overall efficiency for converting the incident photon to the photoelectron was below 10%. Thus we measured the photocurrent using an analog current meter instead of counting of photon numbers.3. We measured the intensity difference between the twin beams. (The average value of 300 measurements was obtained, the integration time was 0.62 s for each measurement). The fluctuation of the intensity difference was reduced by a factor of 17% compared to that for the non-correlated beams. Thus it was shown that the twin-photon beams was appropriate for noise reduction for the polarization spectroscopy.