Optical alignment and spinning of laser-trapped microscopic particles

Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams or beams with helical phase structure. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 1936, when he reported a tiny torque developed in a quartz waveplate due to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating. We have observed rotation rates in excess of 350 Hz.Comment: 4 pages, 4 figure

Optical microrheology using rotating laser-trapped particles

We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.Comment: 5 pages, 4 figures. v2: bibliographic details, minor text correction

Theory of Optical Tweezers

We derive a partial-wave (Mie) expansion of the axial force exerted on a transparent sphere by a laser beam focused through a high numerical aperture objective. The results hold throughout the range of interest for practical applications. The ray optics limit is shown to follow from the Mie expansion by size averaging. Numerical plots show large deviations from ray optics near the focal region and oscillatory behavior (explained in terms of a simple interferometer picture) of the force as a function of the size parameter. Available experimental data favor the present model over previous ones.Comment: 4 pages, 3 figure

Transverse Pressure and Strangeness Dynamics in Relativistic Heavy Ion Reactions

Transverse hadron spectra from proton-proton, proton-nucleus and nucleus-nucleus collisions from 2 AGeV to 21.3 ATeV are investigated within two independent transport approaches (HSD and UrQMD). For central Au+Au (Pb+Pb) collisions at energies above $E_{\rm lab}\sim$ 5 AGeV, the measured $K^{\pm}$ transverse mass spectra have a larger inverse slope parameter than expected from the default calculations. The additional pressure - as suggested by lattice QCD calculations at finite quark chemical potential $\mu_q$ and temperature $T$ - might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au+Au (Pb+Pb) collisions. This is supported by a non-monotonic energy dependence of $v_2/$ in the present transport model.Comment: Proceedings of Strange Quark Matter 200

Coherent atomic beam splitter using transients of a chaotic system

A coherent atomic beam splitter can be realized using the transient dynamics of a chaotic system. We have experimentally observed such an effect using ultracold rubidium atoms. Our experimental results are in good agreement with numerical simulations of the Schrödinger equation for the syste

Dynamics and freeze-out of hadron resonances at RHIC

Yields, rapidity and transverse momentum spectra of $\Delta^{++}(1232)$, $\Lambda(1520)$, $\Sigma^\pm(1385)$ and the meson resonances $K^0(892)$, $\Phi$, $\rho^0$ and $f_0(980)$ are predicted. Hadronic rescattering leads to a suppression of reconstructable resonances, especially at low $p_\perp$. A mass shift of the $\rho$ of 10 MeV is obtained from the microscopic simulation, due to late stage $\rho$ formation in the cooling pion gas.Comment: Proceedings of the Strange Quark Matter 2003, eprint version differs from published versio

Phi Mesons from a Hadronic Fireball

Production of $\phi$ mesons is considered in the course of heavy-ion collisions at SPS energies. We investigate the possible difference in momentum distributions of $\phi$ mesons measured via their leptonic ($\mu^+\mu^-$) and hadronic ($K^+K^-$) decays. Rescattering of secondary kaons in the dense hadron gas together with the influence of in-medium kaon potential can lead to a relative decrease of a $\phi$ yield observed in the hadronic channel. We analyze how the in-medium modifications of meson properties affect apparent - reconstructed momentum distributions of $\phi$ mesons. Quantitative results are presented for central Pb+Pb collisions at $E_{beam}=158 GeV/A$.Comment: style Revtex4,9 pages, 5 figures. submitted to Phys. Rev.