1,447 research outputs found
Optical measurement of torque exerted on an elongated object by a non-circular laser beam
We have developed a scheme to measure the optical torque, exerted by a laser
beam on a phase object, by measuring the orbital angular momentum of the
transmitted beam. The experiment is a macroscopic simulation of a situation in
optical tweezers, as orbital angular momentum has been widely used to apply
torque to microscopic objects. A hologram designed to generate LG02 modes and a
CCD camera are used to detect the orbital component of the beam. Experimental
results agree with theoretical numerical calculations, and the strength of the
orbital component suggest its usefulness in optical tweezers for
micromanipulation.Comment: 6 pages, 7 figures, v2: minor typographical correction
Extremely asymmetrical scattering in gratings with varying mean structural parameters
Extremely asymmetrical scattering (EAS) is an unusual type of Bragg
scattering in slanted periodic gratings with the scattered wave (the +1
diffracted order) propagating parallel to the grating boundaries. Here, a
unique and strong sensitivity of EAS to small stepwise variations of mean
structural parameters at the grating boundaries is predicted theoretically (by
means of approximate and rigorous analyses) for bulk TE electromagnetic waves
and slab optical modes of arbitrary polarization in holographic (for bulk
waves) and corrugation (for slab modes) gratings. The predicted effects are
explained using one of the main physical reasons for EAS--the diffractional
divergence of the scattered wave (similar to divergence of a laser beam). The
approximate method of analysis is based on this understanding of the role of
the divergence of the scattered wave, while the rigorous analysis uses the
enhanced T-matrix algorithm. The effect of small and large stepwise variations
of the mean permittivity at the grating boundaries is analysed. Two distinctly
different and unusual patterns of EAS are predicted in the cases of wide and
narrow (compared to a critical width) gratings. Comparison between the
approximate and rigorous theories is carried out.Comment: 16 pages, 5 figure
Observation of transition from escape dynamics to underdamped phase diffusion in a Josephson junction
We have investigated the dynamics of underdamped Josephson junctions. In
addition to the usual crossover between macroscopic quantum tunnelling and
thermally activated (TA) behaviour we observe in our samples with relatively
small Josephson coupling E_J, for the first time, the transition from TA
behaviour to underdamped phase diffusion. Above the crossover temperature the
threshold for switching into the finite voltage state becomes extremely sharp.
We propose a (T,E_J) phase-diagram with various regimes and show that for a
proper description of it dissipation and level quantization in a metastable
well are crucial.Comment: 4 pages, 3 figure
Hydrogen interaction with fullerenes: From C[sub 20] to graphene
The paper presents a systematic study of the trends in the interaction of hydrogen with carbon fullerenes versus their curvature, where graphene is taken as the limit of zero curvature. The efficiency of hydrogen incapsulation in fullerenes, penetration into them, and adsorption on their surface are analyzed and discussed. The effects on magnetism are also considered; in particular, it is shown that hydrogen adsorption to some fullerenes induces magnetism to initially nonmagnetic systems. In addition, highly hydrogen-saturated fullerenes are examined and the suitability of fullerenes for hydrogen storage is discussed.Peer reviewe
Agglomeration of As Antisites in As-Rich Low-Temperature GaAs: Nucleation without a Critical Nucleus Size
To investigate the early stages of nucleation and growth of As precipitates in GaAs grown at low substrate temperature, we make use of a self-consistent-charge density-functional based tight-binding method. Since a pair of As antisites already shows a significant binding energy which increases when more As antisites are attached, there is no critical nucleus size. Provided that all excess As has precipitated, the clusters may grow in size since the binding energies increase with increasing agglomeration size. These findings close the gap between experimental investigation of point defects and the detection of nanometer-size precipitates in transmission electron microscopy.Peer reviewe
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
Mechanical Effects of Optical Vortices
We concentrate on the forces and torques exerted on transparent and absorbing particles trapped in laser beams containing optical vortices. We review previous theoretical and experimental work and then present new calculations of the effect of vortex beams on absorbing particles
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