arXiv subjects

Yan Feng

Publications and source records attributed to Yan Feng.

At least 109 records · Page 6Linked to original sources

arXiv2018

Fraunhofer diffraction at the two-dimensional quadratically distorted (QD) Grating

A two-dimensional (2D) mathematical model of quadratically distorted (QD) grating is established with the principles of Fraunhofer diffraction and Fourier optics. Discrete sampling and bisection algorithm are applied for finding numerical solution of the diffraction pattern of QD grating. This 2D mathematical model allows the precise design of QD grating and improves the optical performance of simultaneous multiplane imaging system.

physics.optics↗

arXiv2017

Magnetometry using fluorescence of sodium vapor

Magnetic resonance of sodium fluorescence is studied with varying laser intensity, duty cycle, and field strength. A magnetometer based on sodium vapor cell filled with He buffer gas is demonstrated, which uses a single amplitude-modulated laser beam. With a 589 nm laser tuned at D1 or D2 line, the magnetic field is inferred from the variation of fluorescence. A magnetic field sensitivity of 150 pT per Hz square root is achieved at D1 line. The work is a step towards sensitive remote magnetometry with mesospheric sodium.

physics.optics↗

arXiv2015

Doppler-free intermodulated fluorescence spectroscopy of $^4He$ $2^3P-3^{1,3}D$ transitions at 588 nm with a one-watt compact laser system

We have demonstrated Doppler-free intermodulated fluorescence spectroscopy of helium $2^3P-3^{1,3}D$ transitions in an rf discharged sealed-off cell using a compact laser system at 588 nm. An external cavity diode laser at 1176 nm was constructed to seed a Raman fiber amplifier. Laser power of more than one watt at 588 nm was produced by frequency doubling of the fiber amplifier output using a MgO:PPLN crystal. A doubling efficiency of 23 % was achieved. The power-dependent spectra of the $2^3P-3^3D$ transitions were investigated. Furthermore, the Doppler-free spectrum of the spin-forbidden $2^3P-3^1D$ transitions was observed for the first time. Our results are crucial towards precision test of QED atomic calculations, especially for improving the determination of the helium $3^1D-3^3D$ separation.

physics.atom-ph↗

arXiv2014

Superdiffusion of 2D Yukawa liquids due to a perpendicular magnetic field

Stochastic transport of a two-dimensional (2D) dusty plasma liquid with a perpendicular magnetic field is studied. Superdiffusion, which is a type of non-Fickian transport, is found to occur especially at higher magnetic fields with $β$ of order unity. Here, $β= ω_c / ω_{pd}$ is the ratio of the cyclotron and plasma frequencies for dust particles. The mean-square displacement ${\rm {MSD}} = 4 D_αt^α$ is found to have an exponent $α> 1$, indicating superdiffusion, with $α$ increasing monotonically to $1.1$ as $β$ increases to unity. The 2D Langevin molecular dynamics simulation used here also reveals that another indicator of random particle motion, the velocity autocorrelation function (VACF), has a dominant peak frequency $ω_{peak}$ that empirically obeys $ω_{peak}^2 = ω_c^2+ ω_{pd}^2/4$.

physics.plasm-ph↗

arXiv2013

Longitudinal viscosity of 2D Yukawa liquids

The longitudinal viscosity $η_l$ is obtained for a two-dimensional (2D) liquid using a Green-Kubo method with a molecular dynamics simulation. The interparticle potential used has the Debye-Hückel or Yukawa form, which models a 2D dusty plasma. The longitudinal $η_l$ and shear $η_s$ viscosities are found to have values that match very closely, with only negligible differences for the entire range of temperatures that is considered. The bulk viscosity $η_b$ is determined to be either negligibly small or not a meaningful transport coefficient, for a 2D Yukawa liquid.

physics.plasm-ph↗

arXiv2012

Energy Transport in a Shear Flow of Particles in a 2D Dusty Plasma

A shear flow of particles in a laser-driven two-dimensional (2D) dusty plasma are observed in a further study of viscous heating and thermal conduction. Video imaging and particle tracking yields particle velocity data, which we convert into continuum data, presented as three spatial profiles: mean particle velocity (i.e., flow velocity), mean-square particle velocity, and mean-square fluctuations of particle velocity. These profiles and their derivatives allow a spatially-resolved determination of each term in the energy and momentum continuity equations, which we use for two purposes. First, by balancing these terms so that their sum (i.e., residual) is minimized while varying viscosity $η$ and thermal conductivity $κ$ as free parameters, we simultaneously obtain values for $η$ and $κ$ in the same experiment. Second, by comparing the viscous heating and thermal conduction terms, we obtain a spatially-resolved characterization of the viscous heating.

physics.plasm-ph↗

arXiv2012

Observation of temperature peaks due to strong viscous heating in a dusty plasma flow

Profound temperature peaks are observed in regions of high velocity shear in a 2D dusty plasma experiment with laser-driven flow. These are attributed to viscous heating, which occurs due to collisional scattering in a shear flow. Using measurements of viscosity, thermal conductivity, and spatial profiles of flow velocity and temperature, we determine three dimensionless numbers: Brinkman Br = 0.5, Prandtl Pr = 0.09, and Eckert Ec = 5.7. The large value of Br indicates significant viscous heating that is consistent with the observed temperature peaks.

physics.plasm-ph↗

arXiv2012

Waves and instability in a one-dimensional microfluidic array

Motion in a one-dimensional (1D) microfluidic array is simulated. Water droplets, dragged by flowing oil, are arranged in a single row, and due to their hydrodynamic interactions spacing between these droplets oscillates with a wave-like motion that is longitudinal or transverse. The simulation yields wave spectra that agree well with experiment. The wave-like motion has an instability which is confirmed to arise from nonlinearities in the interaction potential. The instability's growth is spatially localized. By selecting an appropriate correlation function, the interaction between the longitudinal and transverse waves is described.

physics.flu-dyn↗

arXiv2012

Frequency-dependent shear viscosity of a liquid 2D dusty plasma

The viscoelasticity of a two-dimensional liquid strongly-coupled dusty plasma is studied experimentally, without macroscopic shear. Positions and velocities of the dust particles, measured by video microscopy, are used as the inputs to the generalized Green-Kubo relation to obtain the complex viscosity $η(ω)$. The real part of $η(ω)$ (which corresponds to dissipation) diminishes gradually with frequency, while the imaginary part (which corresponds to elasticity) is peaked at a frequency below the 2D dusty plasma frequency. The viscoelastic approximation is found to accurately describe the 2D experimental results for $η(ω)$, yielding the Maxwell relaxation time $τ_M = 0.10 s$. Results for $η(ω)$ are compared to 2D molecular dynamics Yukawa simulations and to a previous experiment that was performed using an oscillating macroscopic shear.

physics.plasm-ph↗

arXiv2011

Green-Kubo relation for viscosity tested using experimental data for a 2D dusty plasma

The theoretical Green-Kubo relation for viscosity is tested using experimentally obtained data. In a dusty plasma experiment, micron-size dust particles are introduced into a partially-ionized argon plasma, where they become negatively charged. They are electrically levitated to form a single-layer Wigner crystal, which is subsequently melted using laser heating. In the liquid phase, these dust particles experience interparticle electric repulsion, laser heating, and friction from the ambient neutral argon gas, and they can be considered to be in a nonequilibrium steady state. Direct measurements of the positions and velocities of individual dust particles are then used to obtain a time series for an off-diagonal element of the stress tensor and its time autocorrelation function. This calculation also requires the interparticle potential, which was not measured experimentally, but was obtained using a Debye-Hückel-type model with experimentally determined parameters. Integrating the autocorrelation function over time yields the viscosity for shearing motion amongst dust particles. The viscosity so obtained is found to agree with results from a previous experiment using a hydrodynamical Navier-Stokes equation. This comparison serves as a test of the Green-Kubo relation for viscosity. Our result is also compared to the predictions of several simulations.

physics.plasm-ph↗

arXiv2011

Accurate particle position measurement from images

The moment method is an image analysis technique for sub-pixel estimation of particle positions. The total error in the calculated particle position includes effects of pixel locking and random noise in each pixel. Pixel locking, also known as peak locking, is an artifact where calculated particle positions are concentrated at certain locations relative to pixel edges. We report simulations to gain an understanding of the sources of error and their dependence on parameters the experimenter can control. We suggest an algorithm, and we find optimal parameters an experimenter can use to minimize total error and pixel locking. Simulating a dusty plasma experiment, we find that a sub-pixel accuracy of 0.017 pixel or better can be attained. These results are also useful for improving particle position measurement and particle tracking velocimetry (PTV) using video microscopy, in fields including colloids, biology, and fluid mechanics.

physics.plasm-ph↗

arXiv2011

Rapid heating and cooling in two-dimensional Yukawa systems

Simulations are reported to investigate solid superheating and liquid supercooling of two-dimensional (2D) systems with a Yukawa interparticle potential. Motivated by experiments where a dusty plasma is heated and then cooled suddenly, we track particle motion using a simulation with Langevin dynamics. Hysteresis is observed when the temperature is varied rapidly in a heating and cooling cycle. As in the experiment, transient solid superheating, but not liquid supercooling, is observed. Solid superheating, which is characterized by solid structure above the melting point, is found to be promoted by a higher rate of temperature increase.

physics.plasm-ph↗

arXiv2011

Identifying anomalous diffusion and melting in dusty plasmas

Anomalous diffusion in liquids and the solid-liquid phase transition (melting) are studied in two-dimensional Yukawa systems. The self-intermediate scattering function (self-ISF), calculated from simulation data, exhibits a temporal decay, or relaxation, with a characteristic relaxation time. This decay is found to be useful for distinguishing normal and anomalous diffusion in a liquid, and for identifying the solid-liquid phase transition. For liquids, a scaling of the relaxation time with length scale is found. For the solid-liquid phase transition, the shape of the self-ISF curve is found to be a sensitive indicator of phase. Friction has a significant effect on the timing of relaxation, but not the melting point.

physics.plasm-ph↗

arXiv2011

Viscosity calculated in simulations of strongly-coupled dusty plasmas with gas friction

A two-dimensional strongly-coupled dusty plasma is modeled using Langevin and frictionless molecular dynamical simulations. The static viscosity $η$ and the wave-number-dependent viscosity $η(k)$ are calculated from the microscopic shear in the random motion of particles. A recently developed method of calculating the wave-number-dependent viscosity $η(k)$ is validated by comparing the results of $η(k)$ from the two simulations. It is also verified that the Green-Kubo relation can still yield an accurate measure of the static viscosity $η$ in the presence of a modest level of friction as in dusty plasma experiments.

physics.plasm-ph↗

arXiv2011

Errors in particle tracking velocimetry with high-speed cameras

Velocity errors in particle tracking velocimetry (PTV) are studied. When using high-speed video cameras, the velocity error may increase at a high camera frame rate. This increase in velocity error is due to particle-position uncertainty, which is one of two sources of velocity errors studied here. The other source of error is particle acceleration, which has the opposite trend of diminishing at higher frame rates. Both kinds of errors can propagate into quantities calculated from velocity, such as the kinetic temperature of particles or correlation functions. As demonstrated in a dusty plasma experiment, the kinetic temperature of particles has no unique value when measured using PTV, but depends on the sampling time interval or frame rate. It is also shown that an artifact appears in an autocorrelation function computed from particle positions and velocities, and it becomes more severe when a small sampling-time interval is used. Schemes to reduce these errors are demonstrated.

physics.plasm-ph↗

arXiv2010

Viscoelasticity of 2D liquids quantified in a dusty plasma experiment

The viscoelasticity of two-dimensional liquids is quantified in an experiment using a dusty plasma. An experimental method is demonstrated for measuring the wavenumber-dependent viscosity, $η(k)$, which is a quantitative indicator of viscoelasticity. Using an expression generalized here to include friction, $η(k)$ is computed from the transverse current autocorrelation function (TCAF), which is found by tracking random particle motion. The TCAF exhibits an oscillation that is a signature of elastic contributions to viscoelasticity. Simulations of a Yukawa liquid are consistent with the experiment.

cond-mat.soft↗

arXiv2010

Evolution of shear-induced melting in dusty plasma

The spatiotemporal development of melting is studied experimentally in a 2D dusty plasma suspension. Starting with an ordered lattice, and then suddenly applying localized shear, a pair of counter-propagating flow regions develop. A transition between two melting stages is observed before a steady state is reached. Melting spreads with a front that propagates at the transverse sound speed. Unexpectedly, coherent longitudinal waves are excited in the flow region.

cond-mat.soft↗

arXiv2008

Solid superheating observed in two-dimensional strongly-coupled dusty plasma

It is demonstrated experimentally that strongly-coupled plasma exhibits solid superheating. A 2D suspension of microspheres in dusty plasma, initially self-organized in a solid lattice, was heated and then cooled rapidly by turning laser heating on and off. Particles were tracked using video microscopy, allowing atomistic-scale observation during melting and solidification. During rapid heating, the suspension remained in a solid structure at temperatures above the melting point, demonstrating solid superheating. Hysteresis diagrams did not indicate liquid supercooling in this 2D system.

cond-mat.soft↗