occurs in any medium in the presence of crossed transverse electric and magnetic fields. This anisotropy has indeed been predicted in the 70’s [24] and has only been detected recently on light absorption by a crystal [25]. 

## II. OPTICAL SET-UP 

The light source is a commercial NPRO Nd:YAG laser (Lightwave Electronics, now JDSU, NPRO-126) emitting 130 mW of cw light at λ = 1064 nm. It is tunable thanks to a Peltier cell controlling the crystal temperature and to a piezo-electric actuator that constrains the crystal. 

The core of the experimental set-up consists of a plane square optical cavity (see Fig. 1) made with 4 identical commercial mirrors from Layertec GmbH. These spherical mirrors with a 500 mm radius of curvature are arranged on a square with an arm length L 0 = 400 mm, so that the beam waist on the mirrors is w = 0 . 7 mm. Due to non-normal incidence on the mirrors, a square cavity is strongly birefringent, so that in our case the resonance frequencies of the two polarization eigenmodes are well separated. We can thus adjust the incident polarization to be orthogonal to the cavity plane (s-polarization), so that only the high finesse mode is fed. The value of this finesse is F = Δ ν FSR / Δ ν c ≃ 50000, corresponding to a photon lifetime in the cavity τ = 42 µ s (see Fig. 2). Here Δ ν FSR = 188 MHz is the free spectral range and Δ ν c is the cavity linewidth. 

The laser beam is matched to the cavity transverse mode TEM 00 by two pairs of cylin- drical lenses, in order to take into account the astigmatism of the laser and that of the cavity. The position of the 4 lenses is adjusted by sweeping the laser frequency and minimizing the amplitude of higher order transverse modes. About 99% of the intracavity power resonates in the TEM 00 mode. Light is injected into the cavity both clockwise (cw) and counter- clockwise (ccw), and 4 large area homemade photodetectors monitor the light transmitted and reflected by the cavity in each direction. 

At this stage of the development, the cavity is not yet in a vacuum chamber but in an air- tight plexiglas box. A second wooden box backed with an acoustic isolating foam surrounds the first one to provide passive thermal insulation and airborne acoustic noise reduction. As far as solidborne noise is concerned, the whole experiment takes place on a Newport RS4000 optical table sustained by 4 PL2000 pneumatic isolators. 

