Abstract
Gaseous Cotton-Mouton effect (discoveredin1905) means the birefringence of light passing through gaseous medium in the presence of external transverse magnetic field. Its electric analog is the Kerr effect. The Cotton-Mouton effect is propotional to pressure (P), square of external magnetic field (B^2), effective length of magnetic field region (LB), and inverse of the wave-length of the light(λ^-1). The 2 different indices of refraction of gas for polarization components of incident linear polarized light parallel and perpendicular to the transversemagnetic field imply 2 different speeds of light. The transmitted light polarization components thus obtain phase difference (propotional to the difference of the 2 indices of defraction Δn) and the polarization becomes elliptical. The Cotton-Mouton effects for different gases can be found by measuring the ellipticity of the transmitted light and distinguished by the Cotton-Mouton coefficients (CCM). For common gases this effect is too small to be measured (Δn ~ 10^-11 - 10^-16). That's why we can't find systemetic and complete report of the Cotton-Mouton effect of all gases under different temperature, pressure, wave-length and external field strength for over 1 century. The Q & A(Quantum electro-dynamics test and search for Axion) experiment[1,2] is capable of providing precision measurement of the Cotton-Mouton effect under different pressures. It was originally aiming at the detection of the vacuum birefringence and the search for axion and the interaction of (pseudo-)scalar particle with two photons. Basically it explores the ellipticity of vacuum birefringence and polarization rotation of vacuum dichroism generated by transverse magnetic field(LB ~ 0.6 m, B ~ 2.2 T) onto a 1064 nm Nd-YAG laser beam using a pair of cross-orientated polarizers (with an extinction ratio up to 92.87 dB). Technically it adopts a suspended 3.45 m high-finesse (F ~ 30000) Fabry-Perot interferometer (with X-pendulum-double-pendulum system for seismic noise isolation) to extend the effective length of magnetic field region by bouncing the light back and forth between cavity mirrors for amplifying the signal for detection. A quarter/variable wave plate is applied to compensate the cavity mirror birefringence and increase the system's total extinction ratio and angular detection sensitivity. By employing double-modulation and phase-lock detection techniques in signal acquiring, data-taking was successful for over 900 hours in 48 days (1152 hours), including the Cotton-Mouton effect and the Verdet effect (i.e. Faraday rotation in the presence of axial magnetic field) of N2, O2, CO2, Ar and Kr of pressure ranging from 0.5 to 300 Torr, and polarization rotation in vacuum (3 mTorr) for 378 hours and ellipticity in vacuum for 156 hours. The Cotton-Mouton coefficients(CCM in m^-1 T^-2) are N2: (-1.74±0.13)×10^-7; O2: (-1.53±0.09)×10^-6; CO2: (-3.85±0.24)×10^-7; Ar: (3.94±0.31)×10^-9; Kr: (7.57±1.16)×10^-9. The Verdet coefficients (Cυ in m^-1 T^-1) are N2: (2.93±0.62)×10^-4; O2: (8.43±0.17)×10^-4; CO2:(4.46±0.90)×10^-4; Ar: (4.54±1.08)×10^-4; Kr: (1.02±0.20)×10^-3. The current sensitivity is 0.93 μrad/Hz^1/2 for polarization rotation, and1.02 μrad/Hz^1/2 for ellipticity. Compared to the intergration time 19.2 hours and sensitivity 1.4 μrad/Hz^1/2 for polarization rotation in 2006, an improvement factor of 19.7 in intergration time and 30% in sensitivity of polarization rotation are achieved. Besides the points mentioned above, this thesis also reports on more detailed models and signal analyses in this double modulation scheme, as well as the measurement of extinction ratio of polarizers/analyzers, the vacuum pressure and the measurement of magnetic field of the rotating magnet around the optical path. In the end, we give an outlook for future development of this experiment.