Journal of Low Temperature Physics, Vol. 102, Nos. 5/6, 1996 Optical Interferometry in Superfluid 3He-B H. Alles, J. P. Ruutu, A. V. Babkin, P. J. Hakonen, and E. B. Sonin* Low Temperature Laboratory, Helsinki University of Technology, 02150 Espoo, Finland (Received August 28, 1995) We report inteiferometric measurements on 0.1 ... 1 mm thick films of super- fluid 3He-B. The menisci of three different rotational states of the superfluid were observed and analyzed theoretically using two-fluid hydrodynamics." These are (i) the equilibrium vortex state in which the superfluid and the normal components corotate (solid body rotation), (ii) the vortex-free state (the Landau state), in which only the normal component rotates, and (iii) the quasistationary vortex state in which only the superfluid fi'action rotates (pure superfluid rotation). The Landau state manifested itself by a reduced parabolic meniscus at rotation speeds below the critical angular velocity s c < 0.2 rad/s for vortex formation. Transition from the Landau state to the equilibrium vortex state yielded a sudden deepening of the meniscus when f~c was exceeded. After a rapid halt of the cryostat, we observed a novel meniscus which was produced by the superfluid rotation while the normal component was at rest. The enhanced depth of this meniscus is governed by the reactive mutual friction parameter B'. By employing laser light, both for imaging and for thermomechanical excitation, we measured the response of a thin superfluid layer to a heat pulse and analyzed it within the theory of two- fluid hydrodynamics. The data were employed, using the dispersion relation for thin film oscillations, to deduce the second viscosity coefficient ~3 close to To. 1. INTRODUCTION Superfluids are known to exhibit remarkable macroscopic quantum phenomena. Unlike ordinary liquids, they show zero viscosity and are able to flow through narrow gaps and tubes without dissipation. In superfluids, as a response to rotation, a vortex lattice is created in order to allow solid body rotation on average. The existence of vortices, which carry quantized *Permanent address: Ioffe Physico-Technical Institute, St. Petersburg 19402], Russia. 411 0022-2291/96/0300-0411 $09.50/0 9 1996 Plenum PublishingCorporation