Mechanical Evidence of the Orbital Angular Momentum to Energy Ratio of Vortex Beams

We measure, in a single experiment, both the radiation pressure and the torque due to a wide variety of propagating acoustic vortex beams. The results validate, for the first time directly, the theoretically predicted ratio of the orbital angular momentum to linear momentum in a propagating beam. We experimentally determine this ratio using simultaneous measurements of both the levitation force and the torque on an acoustic absorber exerted by a broad range of helical ultrasonic beams produced by a 1000-element matrix transducer array. In general, beams with helical phase fronts have been shown to contain orbital angular momentum as the result of the azimuthal component of the Poynting vector around the propagation axis. Theory predicts that for both optical and acoustic helical beams the ratio of the angular momentum current of the beam to the power should be given by the ratio of the beam’s topological charge to its angular frequency. This direct experimental observation that the ratio of the torque to power does convincingly match the expected value (given by the topological charge to angular frequency ratio of the beam) is a fundamental result

Demonstration of Ion Trap Principles

Particle trapping is a state-of-the-art technology, which already a powerful tool for scientists working with micro- and nano-components. Much interest now revolves around length scales where quantum mechanical effects become pronounced. Quantum mechanics forms our only framework for understanding many problems in solid-state physics (e.g., magnetism), and is playing an ever more important role in applied chemistry, biochemistry and many other areas. Trapping technologies provide a test bed for systematic exploration of fundamental paradigms, offering enhancements to our understanding of key mechanisms and, perhaps, opportunities for quantum information technology. We have assembled a Newtonian Lab demonstration trap, demonstrating key principles of an ion trap, as a first step toward more advanced particle-trapping technology. This system utilizes a low-frequency alternating voltage to trap charged micro-particles. We have confirmed that trapping has occurred, by scattering visible laser beams off the trapped particles. Our next step is to explore designs for a hybrid combination of high-frequency optical tweezers with the sort of low-frequency electrostatic trap we have demonstrated, with the goal of stabilizing particles trapped in low-pressure atmospheres, where it may be possible to achieve cooling towards the quantum mechanical ground state of at least one degree of freedom

Computer-Generated Holographic Optical Tweezer Arrays

Holographic techniques significantly extend the capabilities of laser tweezing, making possible extended trapping patterns for manipulating large numbers of particles and volumes of soft matter. We describe practical methods for creating arbitrary configurations of optical tweezers using computer-generated diffractive optical elements. While the discussion focuses on ways to create planar arrays of identical tweezers, the approach can be generalized to three-dimensional arrangements of heterogeneous tweezers and extended trapping patterns.Comment: 8 pages, 9 Postscript figures, REVTeX source. For related materials, see http://griergroup.uchicago.edu/~grier

Optical Cloaking by Aberation Correction

Light incident on a material is scattered and then continues its propagation in seemingly random directions. If one can force light to pass through a material and not scatter, however, then one could “see” through the material. This scattering of light can be described as aberration within the light. A technique used for “Aberration Correction” is adding phase-shifts to regions of light allowing for all wave fronts of light to interfere in a constructive manner. This is accomplished in the use of a Spatial Light Modulator (SLM). The SLM, an array of linearly aligned crystals, allow for added phase shifts to light incident on the SLM. By shifting the phase of light, it is possible to allow light to pass through some material without having the light be scattered by the material. This case allows for one to “see” through the material, on account of the light passing through the material rather than being scattered by it. This technology has potential to be used for non-invasive surgeries as well as being a strong starting point for research into optical cloaking. If a procedure for allowing light to pass through a material is developed, then the procedure could be used for the purpose of Optical Cloaking. By expanding the region in which one “sees” through a material so that one encloses the entire material, one would cloak the entire material rather than “see” through some region of it. This procedure would have applications in both medical and military technology

Light sheet microscopy with acoustic sample confinement

Contactless sample confinement would enable a whole host of new studies in developmental biology and neuroscience, in particular, when combined with long-term, wide-field optical imaging. To achieve this goal, we demonstrate a contactless acoustic gradient force trap for sample confinement in light sheet microscopy. Our approach allows the integration of real-time environmentally controlled experiments with wide-field low photo-toxic imaging, which we demonstrate on a variety of marine animal embryos and larvae. To illustrate the key advantages of our approach, we provide quantitative data for the dynamic response of the heartbeat of zebrafish larvae to verapamil and norepinephrine, which are known to affect cardiovascular function. Optical flow analysis allows us to explore the cardiac cycle of the zebrafish and determine the changes in contractile volume within the heart. Overcoming the restrictions of sample immobilisation and mounting can open up a broad range of studies, with real-time drug-based assays and biomechanical analyses.</p

Video recording true single-photon double-slit interference

As normally used, no commercially available camera has a low-enough dark noise to directly produce video recordings of double-slit interference at the photon-by-photon level, because readout noise significantly contaminates or overwhelms the signal. In this work, noise levels are significantly reduced by turning on the camera only when the presence of a photon has been heralded by the arrival, at an independent detector, of a time-correlated photon produced via parametric down-conversion. This triggering scheme provides the improvement required for direct video imaging of Young's double-slit experiment with single photons, allowing clarified versions of this foundational demonstration. Further, we introduce variations on this experiment aimed at promoting discussion of the role spatial coherence plays in such a measurement. We also emphasize complementary aspects of single-photon measurement, where imaging yields (transverse) position information, while diffraction yields the transverse momentum, and highlight the roles of transverse position and momentum correlations between down-converted photons, including examples of "ghost" imaging and diffraction. The videos can be accessed at http://sun.iwu.edu/~gspaldin/SinglePhotonVideos.html online.Comment: 7 pages, 8 figure