Optical polarimetry for fundamental Physics

Abstract:

The research project aims at developing an experimental method for measuring small birefringences, both in transmission and reflection, based on a polarimeter with heterodyne detection. Our attention is mainly aimed at designing the best optics for the future third generation gravitational wave detector, Einstein Telescope. At the same time we will be trying to develop an ultra-sensitive polarimetric technique capable of detecting vacuum magnetic birefringence.

The originality of the proposed instrument resides in a novel way of modulating the ellipticity 𝜓 induced by the birefringence: for transmission two half-wave plates, HWP's, co-rotating at a frequency v_p are placed before and after the birefringent region, making the polarisation rotate at a frequency 2v_p causing an ellipticity modulation at v_𝜓= 4v_p while keeping the polarisation fixed on the output analyzer. As the measurement noise decreases as 1/n, high modulation frequencies are pursued.

Figure 1: Polarimeter for measuring the birefringence of samples in transmission.

A magnetic field (black vertical arrows) is used for calibration/verification that the polarimeter is functioning correctly. In fact the magnetic field generates a known birefringence (Cotton-Mouton effect) in gases including air.

Figure 2: Polarimeter for measuring the birefringence of samples in reflection. This scheme is obtained simply by reversing the beam entrance of the scheme in Figure 1.

The polarimeter schemes are capable of determining the 2D birefringence maps of substrate materials such as crystalline silicon or sapphire and of reflective coatings of mirrors. Substrates and coating birefringences distort the laser wave-front and also cause a reduction in sensitivity of interferometers such as gravitational wave detectors.

The next step will be to install a high finesse Fabry-Perot cavity (without the HWPs) to meausure the birefringence noise generated by the cavity mirrors. New promising coating materials, such as crystalline coatings, will be tested. If these new materials will result in a low enough birefringence noise, a measurement of vacuum magnetic birefringence can be attempted again.

Risultati attesi:

We expect to reach an optical path difference sensitivity of ∆D ≈ 1e-12 m both in transmission and in reflection. A series of 2D birefringence maps of different substrates of cristalline silicon grown by different companies will therefore be measured and studied. Priority will be for this material given that this crystalline silicon is the preferred one for the Einstein Telescope cryogenic interferometer. Hopefully a crystal with a birefringence within the Einstein Telescope requirements of ∆n ≤ 1e-8 will be found. We also expect to show results for 2D birefringence maps for different coating growths to understand the origin of the undesired stress induced birefringence.