Quantum limited particle sensing in optical tweezers

Particle sensing in optical tweezers systems provides information on the position, velocity and force of the specimen particles. The conventional quadrant detection scheme is applied ubiquitously in optical tweezers experiments to quantify these parameters. In this paper we show that quadrant detection is non-optimal for particle sensing in optical tweezers and propose an alternative optimal particle sensing scheme based on spatial homodyne detection. A formalism for particle sensing in terms of transverse spatial modes is developed and numerical simulations of the efficacy of both quadrant and spatial homodyne detection are shown. We demonstrate that an order of magnitude improvement in particle sensing sensitivity can be achieved using spatial homodyne over quadrant detection.Comment: Submitted to Biophys

Optical extinction, refractive index, and multiple scattering for suspensions of interacting colloidal particles

We provide a general microscopic theory of the scattering cross-section and of the refractive index for a system of interacting colloidal particles, exact at second order in the molecular polarizabilities. In particular: a) we show that the structural features of the suspension are encoded into the forward scattered field by multiple scattering effects, whose contribution is essential for the so-called "optical theorem" to hold in the presence of interactions; b) we investigate the role of radiation reaction on light extinction; c) we discuss our results in the framework of effective medium theories, presenting a general result for the effective refractive index valid, whatever the structural properties of the suspension, in the limit of particles much larger than the wavelength; d) by discussing strongly-interacting suspensions, we unravel subtle anomalous dispersion effects for the suspension refractive index.Comment: Submitted to Journal of Chemical Physics 37 pages, 4 figure

Sagnac Interferometer Enhanced Particle Tracking in Optical Tweezers

A setup is proposed to enhance tracking of very small particles, by using optical tweezers embedded within a Sagnac interferometer. The achievable signal-to-noise ratio is shown to be enhanced over that for a standard optical tweezers setup. The enhancement factor increases asymptotically as the interferometer visibility approaches 100%, but is capped at a maximum given by the ratio of the trapping field intensity to the detector saturation threshold. For an achievable visibility of 99%, the signal-to-noise ratio is enhanced by a factor of 200, and the minimum trackable particle size is 2.4 times smaller than without the interferometer

A Photometric Method for Quantifying Asymmetries in Disk Galaxies

A photometric method for quantifying deviations from axisymmetry in optical images of disk galaxies is applied to a sample of 32 face-on and nearly face-on spirals. The method involves comparing the relative fluxes contained within trapezoidal sectors arranged symmetrically about the galaxy center of light, excluding the bulge and/or barred regions. Such a method has several advantages over others, especially when quantifying asymmetry in flocculent galaxies. Specifically, the averaging of large regions improves the signal-to-noise in the measurements; the method is not strongly affected by the presence of spiral arms; and it identifies the kinds of asymmetry that are likely to be dynamically important. Application of this "method of sectors" to R-band images of 32 disk galaxies indicates that about 30% of spirals show deviations from axisymmetry at the 5-sigma level.Comment: 17 pages, 2 tables and 6 figures, uses psfig and AAS LaTex; to appear in A

The evolution of plasma parameters on a coronal source surface at 2.3 Rs during solar minimum

We analyze data from the Solar and Heliospheric Observatory to produce global maps of coronal outflow velocities and densities in the regions where the solar wind is undergoing acceleration. The maps use UV and white light coronal data obtained from the Ultraviolet Coronagraph Spectrometer and the Large Angle Spectroscopic Coronagraph, respectively, and a Doppler dimming analysis to determine the mean outflow velocities. The outflow velocities are defined on a sphere at 2.3 Rs from Sun-center and are organized by Carrington Rotations during the solar minimum period at the start of solar cycle 23. We use the outflow velocity and density maps to show that while the solar minimum corona is relatively stable during its early stages, the shrinkage of the north polar hole in the later stages leads to changes in both the global areal expansion of the coronal hole and the derived internal flux tube expansion factors of the solar wind. The polar hole areal expansion factor and the flux tube expansion factors (between the coronal base and 2.3 Rs) start out as super-radial but then they become more nearly radial as the corona progresses away from solar minimum. The results also support the idea that the largest flux tube expansion factors are located near the coronal hole/streamer interface, at least during the deepest part of the solar minimum period.Comment: 12 Figures, Accepted for publication in Ap

Properties of H II Regions in the Centers of Nearby Galaxies

As part of an optical spectroscopic survey of nearby, bright galaxies, we have identified a sample of over 200 emission-line nuclei having optical spectra resembling those of giant extragalactic H II regions. Such "H II nuclei," powered by young, massive stars, are found in a substantial fraction of nearby galaxies, especially those of late Hubble type. This paper summarizes the observational characteristics of H II nuclei, contrasts the variation of their properties with Hubble type, and compares the nuclear H II regions with those found in galaxy disks. Similarities and differences between H II nuclei and luminous starburst nuclei are additionally noted.Comment: To appear in the Astrophysical Journal. LaTex, 37 pages including 15 postscript figures. AAStex macros include

Star Formation in M51 Triggered by Galaxy Interaction

We have mapped the inner 360'' regions of M51 in the 158micron [CII] line at 55'' spatial resolution using the Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). The emission is peaked at the nucleus, but is detectable over the entire region mapped, which covers much of the optical disk of the galaxy. There are also two strong secondary peaks at ~43% to 70% of the nuclear value located roughly 120'' to the north-east, and south-west of the nucleus. These secondary peaks are at the same distance from the nucleus as the corotation radius of the density wave pattern. The density wave also terminates at this location, and the outlying spiral structure is attributed to material clumping due to the interaction between M51 and NGC5195. This orbit crowding results in cloud-cloud collisions, stimulating star formation, that we see as enhanced [CII] line emission. The [CII] emission at the peaks originates mainly from photodissociation regions (PDRs) formed on the surfaces of molecular clouds that are exposed to OB starlight, so that these [CII] peaks trace star formation peaks in M51. The total mass of [CII] emitting photodissociated gas is ~2.6x10^{8} M_{sun}, or about 2% of the molecular gas as estimated from its CO(1-0) line emission. At the peak [CII] positions, the PDR gas mass to total gas mass fraction is somewhat higher, 3-17%, and at the secondary peaks the mass fraction of the [CII] emitting photodissociated gas can be as high as 72% of the molecular mass.... (continued)Comment: 14 pages, 6 figures, Accepted in ApJ (for higher resolution figures contact the author

Quantum light depolarization: the phase-space perspective

Quantum light depolarization is handled through a master equation obtained by coupling dispersively the field to a randomly distributed atomic reservoir. This master equation is solved by transforming it into a quasiprobability distribution in phase space and the quasiclassical limit is investigated.Comment: 6 pages, no figures. Submitted for publicatio

Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies

New HI synthesis data have been obtained for six face-on galaxies with the Very Large Array. These data and reanalyses of three additional data sets make up a sample of nine face-on galaxies analyzed for deviations from axisymmetry in morphology and dynamics. This sample represents a subsample of galaxies already analyzed for morphological symmetry properties in the R-band. Four quantitative measures of dynamical nonaxisymmetry are compared to one another and to the quantitative measures of morphological asymmetry in HI and R-band to investigate the relationships between nonaxisymmetric morphology and dynamics. We find no significant relationship between asymmetric morphology and most of the dynamical measures in our sample. A possible relationship is found, however, between morphology and dynamical position angle differences between approaching and receding sides of the galaxy.Comment: 24 pages, 19 figures, AASTeX, accepted for publication in AJ, postscript figures available at ftp://culebra.tn.cornell.edu/pub/david/figures.tar.g

Properties of solar polar coronal plumes constrained by Ultraviolet Coronagraph Spectrometer data

We investigate the plasma dynamics (outflow speed and turbulence) inside polar plumes. We compare line profiles (mainly of \ion{O}{6}) observed by the UVCS instrument on SOHO at the minimum of solar cycle 22-23 with model calculations. We consider Maxwellian velocity distributions with different widths in plume and inter-plume regions. Electron densities are assumed to be enhanced in plumes and to approach inter-plume values with increasing height. Different combinations of the outflow and turbulence velocity in the plume regions are considered. We compute line profiles and total intensities of the \ion{H}{1} Ly$\alpha$ and the \ion{O}{6} doublets. The observed profile shapes and intensities are reproduced best by a small solar wind speed at low altitudes in plumes that increases with height to reach ambient inter-plume values above roughly 3-4 R_\sun combined with a similar variation of the width of the velocity distribution of the scattering atoms/ions. We also find that plumes very close to the pole give narrow profiles at heights above 2.5 R_\sun, which are not observed. This suggests a tendency for plumes to be located away from the pole. We find that the inclusion of plumes in the model computations provides an improved correspondence with the observations and confirms previous results showing that published UVCS observations in polar coronal holes can be roughly reproduced without the need for large temperature anisotropy. The latitude distributions of plumes and magnetic flux distributions are studied by analyzing data from different instruments on SOHO and with SOLIS.Comment: 11 figure