Constructing 3D crystal templates for photonic band gap materials using holographic optical tweezers

A simple and robust method is presented for the construction of 3-dimensional crystals from silica and polystyrene microspheres. The crystals are suitable for use as templates in the production of three-dimensional photonic band gap (PBG) materials. Manipulation of the microspheres was achieved using a dynamic holographic assembler (DHA) consisting of computer controlled holographic optical tweezers. Attachment of the microspheres was achieved by adjusting their colloidal interactions during assembly. The method is demonstrated by constructing a variety of 3-dimensional crystals using spheres ranging in size from 3 µm down to 800 nm. A major advantage of the technique is that it may be used to build structures that cannot be made using self-assembly. This is illustrated through the construction of crystals in which line defects have been deliberately included, and by building simple cubic structures

Chronic Stress, Sense of Belonging, and Depression Among Survivors of Traumatic Brain Injury

To test whether chronic stress, interpersonal relatedness, and cognitive burden could explain depression after traumatic brain injury (TBI). Design : A nonprobability sample of 75 mild-to-moderately injured TBI survivors and their significant others, were recruited from five TBI day-rehabilitation programs. All participants were within 2 years of the date of injury and were living in the community. Methods : During face-to-face interviews, demographic information, and estimates of brain injury severity were obtained and participants completed a cognitive battery of tests of directed attention and short-term memory, responses to the Perceived Stress Scale, Interpersonal Relatedness Inventory, Sense of Belonging Instrument, Neurobehavioral Functioning Inventory, and Center for Epidemiological Studies Depression Scale;. Findings : Chronic stress was significantly and positively related to post-TBI depression. Depression and postinjury sense of belonging were negatively related. Social support and results from the cognitive battery did not explain depression. Conclusions : Postinjury chronic stress and sense of belonging were strong predictors of post-injury depression and are variables amenable to interventions by nurses in community health, neurological centers, or rehabilitation clinics. Future studies are needed to examine how these variables change over time during the recovery process.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72593/1/j.1547-5069.2002.00221.x.pd

Weak Lensing and CMB: Parameter forecasts including a running spectral index

We use statistical inference theory to explore the constraints from future galaxy weak lensing (cosmic shear) surveys combined with the current CMB constraints on cosmological parameters, focusing particularly on the running of the spectral index of the primordial scalar power spectrum, $\alpha_s$. Recent papers have drawn attention to the possibility of measuring $\alpha_s$ by combining the CMB with galaxy clustering and/or the Lyman-$\alpha$ forest. Weak lensing combined with the CMB provides an alternative probe of the primordial power spectrum. We run a series of simulations with variable runnings and compare them to semi-analytic non-linear mappings to test their validity for our calculations. We find that a ``Reference'' cosmic shear survey with $f_{sky}=0.01$ and $6.6\times 10^8$ galaxies per steradian can reduce the uncertainty on $n_s$ and $\alpha_s$ by roughly a factor of 2 relative to the CMB alone. We investigate the effect of shear calibration biases on lensing by including the calibration factor as a parameter, and show that for our Reference Survey, the precision of cosmological parameter determination is only slightly degraded even if the amplitude calibration is uncertain by as much as 5%. We conclude that in the near future weak lensing surveys can supplement the CMB observations to constrain the primordial power spectrum.Comment: 12 pages, 10 figures, revtex4. Final form to appear in Phys Rev

Measuring the accuracy of particle position and force in optical tweezers using high-speed video microscopy

We assess the performance of a CMOS camera for the measurement of particle position within optical tweezers and the associated autocorrelation function and power spectrum. Measurement of the displacement of the particle from the trap center can also be related to the applied force. By considering the Allan variance of these measurements, we show that such cameras are capable of reaching the thermal limits of nanometer and femtonewton accuracies, and hence are suitable for many of the applications that traditionally use quadrant photodiodes. As an example of a multi-particle measurement we show the hydrodynamic coupling between two particles

Generation of achromatic bessel beams using a compensated spatial light modulator

We report the creation of white-light, achromatic Bessel beams using a spatial light modulator and a prism to compensate for the dispersion. Unlike the Bessel beam created by a refractive axicon, this achromatic beam has a radial wavevector and hence an intensity cross-section which is independent of wavelength. The technique also lends itself to the generation of higher order Bessel beams with an on-axis optical vortex and associated orbital angular momentum