Faraday and Cotton-Mouton effects of helium atλ=1064nm

Abstract

We present measurements of the Faraday and the Cotton-Mouton effects of helium gas at λ = 1064 nm. Our apparatus is based on an up-to-date resonant optical cavity coupled to longitudinal and transverse magnetic fields. This cavity increases the signal to be measured by more than a factor of 270 000 compared to the one acquired after a single path of light in the magnetic field region. We have reached a precision of a few percent both for Faraday effect and Cotton-Mouton effect. Our measurements give for the first time the experimental value of the Faraday effect at λ= 1064 nm. This value is compatible with the theoretical prediction. Concerning Cotton-Mouton effect, our measurement is the second reported experimental value at this wavelength, and the first to agree at better than 1σ with theoretical predictions.

First results from the new PVLAS apparatus: A new limit on vacuum magnetic birefringence

“…The integration time was T = 4 hours. Given that F = 6.7 × 10 5 , λ = 1064 nm, B 2 dl = 10.25 T 2 m, from the amplitude of the He peak at 32 µbar, the value of ∆n u for helium results ∆n (He,PVLAS) u = (2.2 ± 0.1) × 10 −16 T −2 atm −1 , in perfect agreement with other published values [6,28,29]. It must be noted that this value is obtained from a single low pressure point.…”

“…The magnetic birefringence of gases also depends linearly on pressure. In Table V the equivalent partial pressures P eq which would mimic a vacuum magnetic birefringence for various gases [6,[28][29][30] are reported. The vacuum system must maintain these species well below their vacuum equivalent pressures.…”

“…The lowest pressure of helium used was P (He) = 32 µbar. Considering that the unitary birefringence (B = 1 T and pressure = 1 atm) of helium due to the CottonMouton effect is ∆n (He) u = (2.1 ± 0.1) × 10 −16 T −2 atm −1 [28,29], the birefringence induced @ B = 2.5 T and P = 32 µbar is ∆n (He) = 3.9 × 10 −20 . In figure 2 the Fourier transform of the measured ellipticity signal ψ(t) is shown.…”

Vacuum Magnetic Birefringence

“…[5], one has to consider systematic effects that mimic the CM effect we want to measure. We thus derive a more general expression taking into account the symmetry properties of Y j towards experimental parameters:…”

“…[3,4,5]. A linearly polarized Nd:YAG laser beam (λ = 1064 nm) goes through an acousto-optic modulator (AOM) used in double passage for an adjustment of the laser frequency.…”