Kerr effect in liquid helium at temperatures below the superfluid transition

The electro-optical Kerr effect induced by a slowly-varying applied electric field in liquid helium at temperatures below the $\lambda$-point is investigated. The Kerr constant of liquid helium is measured to be $(1.43\pm 0.02^{(stat)} \pm 0.04^{(sys)})\times 10^{-20}$ (cm/V)$^2$ at $T=1.5$ K. Within the experimental uncertainty, the Kerr constant is independent of temperature in the range $T=1.5$ K to 2.17 K, which implies that the Kerr constant of the superfluid component of liquid helium is the same as that of normal liquid helium. Our result also indicates that pair and higher correlations of He atoms in the liquid phase account for about 23% of the measured Kerr constant. Liquid nitrogen was used to test the experimental set-up, the result for the liquid nitrogen Kerr constant is $(4.38\pm 0.15)\times 10^{-18}$(cm/V)$^2$. The knowledge of the Kerr constant in these media allows the Kerr effect to be used as a non-contact technique for measuring the magnitude and mapping out the distribution of electric fields inside these cryogenic insulants.