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Alex Sushkov Dima Budker Valeriy Yashchuk (UC Berkeley)

Kerr Effect-based Measurement of the Electric Field. Alex Sushkov Dima Budker Valeriy Yashchuk (UC Berkeley). Kerr Effect. Kerr effect : appearance of birefringence in an initially isotropic medium, induced by an applied external electric field:  n = n || -n  = KE 2

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Alex Sushkov Dima Budker Valeriy Yashchuk (UC Berkeley)

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  1. Kerr Effect-based Measurement of the Electric Field Alex Sushkov Dima Budker Valeriy Yashchuk (UC Berkeley)

  2. Kerr Effect • Kerr effect: appearance of birefringence in an initially isotropic medium, induced by an applied external electric field:n = n||-n= KE2 • For input light polarized at 45o to E, the induced ellipticity: = Ln/ = (L/)KE2 • K is related to hyperpolarizability  :P = E +  E3/6 + …

  3. Why do it? • Measure the magnitude of an electric field. • Measure the spatial and temporal dependence of an electric field. • Investigate the structure of liquid helium: - pair and higher correlations of atoms in the liquid, - Kerr virial coefficients, - effect of temperature and superfluidity.

  4. Experimental Setup ArXiv: physics/0403143

  5. Some apparatus details • Cryogenics: Janis pumped helium cryostat with optical access, Tmin = 1.5 K. • High-voltage cables: carbon steel wire insulated with teflon heat-shrink. • Breakdown in LHe: - breakdown in LHeII at 70 kV/cm - no breakdown in LHeI observed up to 130 kV/cm. • Polarimeter: - 785 nm diode laser, - electro-optic modulator, - lock-in detection at 50 kHz, - room-T sensitivity 10-7 rad/Hz1/2.

  6. Results: LN2 • Measured Kerr constant for LN2 at 73 K:K = (4.380.15)10-18 (cm/V)2 • Published value:(K.Imai et. al., Proceedings of the 3rd Int. Conf. On Prop. and App. Of Diel. Mat., 1991 Japan)K = 4.310-18 (cm/V)2 (no error given)

  7. Results: LHe Measured Kerr constant for LHe at 1.5 K: K = (1.43 0.02(stat) 0.04(sys))10-20 (cm/V)2

  8. T-dependence: LHe Within the experimental error, the LHe Kerr constant does not depend on temperature in the range 1.5 K to 2.2 K.

  9. Superfluidity: absence of temperature dependence means that superfluidity does not affect the Kerr constant of LHe. He atom hyperpolarizability: = (44.20.8) atomic units (Tammer et al.) KLHe  (1.100.02)10-20 (cm/V)2 Van der Waals complexes:polarizability anisotropy of complexes of two or more He atoms contribute 23% to KLHe. Theoretical interpretation

  10. Measuring the electric field in the neutron EDM experiment • Electric field requirements: • E = 50 kV/cm. • 1% uniformity over the cell volume. • < 1% drift over 500 seconds. • diagnostics for the HV test set-up. • Signal to be measured: assuming sample length L = 30 cm, the Kerr ellipticity is  210-5rad. • Sensitivity: with sensitivity 10-7 rad/Hz1/2, we get E/E  510-3 in a 1 second measurement. • Challenge:achieve this sensitivity at 300mK.

  11. A cold magnetometer • Goal:shot-noise limited sensitivity ofB ~310-13 Gauss/Hz1/2

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