The aim is to develop a rapid and direct method for measuring the bulk viscosity of a liquid as a function of temperature. Brillouin scattering of a laser beam in fresh water and salt water at different temperatures has been studied. The results show that there exists a close temperature-dependent relationship among the Brillouin frequency shift, the Brillouin linewidth, and the bulk viscosity of water. Thus the bulk viscosity of water can be determined directly from Brillouin-scattering measurements. The method has a high signal-to-noise ratio and high accuracy.
A mechanically-tunable random laser based on a waveguide-plasmonic scheme has been investigated. This laser can be constructed by spin coating a solution of polydimethylsiloxane doped with the rhodamine 6G organic dye and silver nanowires onto a silicone rubber slab. The excellent overlap of the plasmon resonance peak of the silver nanowires with both the pump wavelength and the photoluminescence spectrum provides the low threshold and tuning properties of the random laser. The random laser wavelength can be tuned from 558 to 565 nm by stretching the soft substrate, which causes reorientation and breakage of the silver nanowires. The polarization state of the random laser can also be changed from random polarization to partial polarization by stretching. The laser performance remains unchanged after the stretching and restoration experiments. These results not only enable easy realization of an ultrathin flexible plasmonic random laser but also provide insights into the mechanisms of three-dimensional plasmonic feedback random lasers.
A plasmonic random laser is fabricated using gold-silver bimetallic porous nanowires with abundant nanogaps that provide strong feedback or gain channels for coherent lasing from dye molecules. The strong confinement of the nanogaps allows the bimetallic porous nanowire-based random laser, which is pumped by ns pulses, to operate with a very low threshold and extremely low concentrations of Rhodamine 6 G (as low as 0.067 mM). This random laser can be used as a pump source for another coherent random laser based on oxazine. These results provide a basis for studies of coherent random lasing pumped by another random laser.
We proposed a method based on microwave magnetic dipole transitions to prepare samples of atoms with well defined position and velocity. Each microwave pulse corresponds to a position measurement for the atoms and two pulses separated by a given delay result in a velocity measurement. The method gives velocity sensitivity approaching that obtained with Raman transitions but it is easier to implement. Moreover, it has the advantages that it also selects in position and has less demanding experimental requirements. The method can be demonstrated in a magneto-optical trap. An important tool in atomic physics is the capability of velocity measurements. The simplest way to measure velocity is by time of flight, something that is heavily used, for example, to determine the momentum distribution in a Bose-Einstein condensate [1-3] or to reconstruct a molecular wavefunction by looking at the expanding products after dissociation [4, 5]. More precise measurements of velocity exploit the Doppler effect, since it introduces a frequency shift proportional to the velocity [6]. The careful measurement of velocity (or velocity changes) offers the possibility to measure accelerations [7, 8], rotations [9], fundamental constants [10, 11], and it is used to prepare atomic samples with a given velocity distribution [12]. Raman transitions between hyperfine levels are generally used for fine velocity selection [6]. Microwave magnetic dipole (M1) transitions are not used for that purpose because the momentum of a microwave photon is orders of magnitude smaller than that of an optical pho-ton. Still M1 transitions are simpler to implement and have longer coherence times compared to Raman transitions [13]. We present a way to use M1 transitions for fine velocity selection that reaches competitive sensitivity with respect to Raman transitions and has position sensitivity as well [14], something useful to prepare atomic samples with well defined phase space distributions [14]. Consider atoms moving vertically in a static magnetic field B ST (z) = ηz along the vertical z-axis. By turning on a microwave magnetic field we drive transitions between magnetic sensitive states only for atoms located at a particular position. Two position measurements separated by a time interval ∆t give the selection in velocity along the z-axis. Each position measurement corresponds to a microwave π-pulse followed by a cleaning laser pulse that removes the atoms not excited by the transition. The position measurement is analogous to that used in Nuclear Magnetic Resonance (NMR) [15], the difference being that now the atoms continue moving due to the low densities used in laser cooling and a second position measurement provides the velocity selection. The Hamiltonian of the system (alkali atom + magnetic field) along the z-axis and in the dipole and long-wavelength approximations is [14] H(t) = 1 2M P 2 z + M g 0 Z + H A − µ · [ηZz + B p (t)] , (1) where M is the mass of the atom, g 0 = 9.8 m/s 2 is the gravitational acceleration, H A is the hyperfine Hamil-to...
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