Showing papers by "David McGloin published in 2007"
TL;DR: It is demonstrated that optical spin-to-orbital angular momentum conversion can occur in a homogeneous and isotropic medium and that the orbital rotation speeds of trapped particles are altered because of this conversion as predicted by theory.
Abstract: As a fundamental property of light, the angular momentum of photons has been of great interest. Here, we demonstrate that optical spin-to-orbital angular momentum conversion can occur in a homogeneous and isotropic medium. This Letter presents both theoretical and experimental studies of this conversion in a tightly focused beam and shows that the orbital rotation speeds of trapped particles are altered because of this conversion as predicted by theory.
524 citations
TL;DR: This paper elucidates through both theory and experiment the behavior of silica microspheres moving under the influence of the periodic optical field provided by a Bessel beam, and compares two different computational models based on Mie scattering and geometrical ray optics to find good qualitative agreement.
Abstract: Spatially periodic optical fields can be used to sort dielectric microscopic particles as a function of size, shape or refractive index. In this paper we elucidate through both theory and experiment the behavior of silica microspheres moving under the influence of the periodic optical field provided by a Bessel beam. We compare two different computational models, one based on Mie scattering, the other on geometrical ray optics and find good qualitative agreement, with both models predicting the existence of distinct size-dependent phases of particle behavior. We verify these predictions by providing experimental observations of the individual behavioral phases.
89 citations
TL;DR: The ability of optical vortex traps to position trapped droplets precisely while excluding surrounding aqueous droplets from entering the trap, thereby preventing unwanted cross contamination by other nearby droplets is demonstrated.
Abstract: This paper describes the use of an optical vortex trap for the transport and fusion of single femtoliter-volume aqueous droplets. Individual droplets were generated by emulsifying water in acetophenone with SPAN 80 surfactant. We demonstrate the ability of optical vortex traps to position trapped droplets precisely while excluding surrounding aqueous droplets from entering the trap, thereby preventing unwanted cross contamination by other nearby droplets. Additionally, the limitation of optical vortex traps for inducing droplet fusion is illustrated, and a remedy is provided through modulation of the spatial intensity profile of the optical vortex beam. Spatial modulation was achieved by translating the computer-generated hologram (CGH) with respect to the input Gaussian beam, thereby shifting the location of the embedded phase singularity (dark core) within the optical vortex beam. We present both simulated and experimentally measured intensity profiles of the vortex beam caused by translation of the CGH. We further describe the use of this technique to achieve controlled and facile fusion of two aqueous droplets.
84 citations
TL;DR: The Brownian dynamics of an optically trapped water droplet are investigated across the transition from over- to underdamped oscillations, and the power spectra of position fluctuations are in excellent agreement with the obtained analytical solutions of a parametrically modulated Langevin equation.
Abstract: The Brownian dynamics of an optically trapped water droplet are investigated across the transition from over- to underdamped oscillations. The spectrum of position fluctuations evolves from a Lorentzian shape typical of overdamped systems (beads in liquid solvents) to a damped harmonic oscillator spectrum showing a resonance peak. In this later underdamped regime, we excite parametric resonance by periodically modulating the trapping power at twice the resonant frequency. The power spectra of position fluctuations are in excellent agreement with the obtained analytical solutions of a parametrically modulated Langevin equation.
67 citations
TL;DR: In this paper, the ability of holographic light fields to alter the normal growth patterns of filamentous fungi has been investigated, and the merits of using discrete and continuous light fields produced using a spatial light modulator are discussed.
Abstract: We report on the ability of holographic light fields to alter the normal growth patterns of filamentous fungi. The light fields are produced on a microscopic scale by borrowing methods from the field of optical tweezers, but without the aim of directly trapping or manipulating objects. Extended light fields are shown to redirect and constrict hyphal tip growth, and induce hyphal branching in a highly reproducible manner. The merits of using discrete and continuous light fields produced using a spatial light modulator are discussed and the use of three-dimensional 'pseudowalls' of light to control the growth patterns is reported. We also demonstrate the dependence of hyphal tip growth on the wavelength of light, finding that less power is needed at shorter wavelengths to effect changes in the growth dynamics of fungal hyphae.
16 citations
TL;DR: Degenerate four-wave mixing is demonstrated using an artificial Kerr medium and is evidenced by directly observing the phase conjugation of a vortex signal beam.
Abstract: Degenerate four-wave mixing is demonstrated using an artificial Kerr medium and is evidenced by directly observing the phase conjugation of a vortex signal beam. The nonlinear susceptibility is produced by a refractive index grating created in a suspension of dielectric microscopic particles optically confined in the intensity grating distribution of two interfering laser beams.
15 citations
2 citations
05 Sep 2007
TL;DR: In this paper, the application of optical trapping techniques to droplets, both in air (aerosols) and in fluid (emulsions), is discussed, and the authors demonstrate new types of traps for aerosols in the form of dual beam fibre traps and compare the trapping efficiency of IR and visible lasers.
Abstract: We discuss the application of optical trapping techniques to droplets, both in air (aerosols) and in fluid (emulsions). We show the holographic optical manipulation of aerosols and how this can be used to transfer orbital angular momentum to airborne particles. We demonstrate new types of traps for aerosols in the form of dual beam fibre traps and compare the trapping efficiency of IR and visible lasers. We discuss some of the interesting dynamics that can be observed when trapping airborne particles and how this appears to differ from conventional liquid based devices. We also examine how holographic optical trapping can be used to facilitate droplet manipulation in another liquid phase. We conclude with a discussion of the difficulties associated with trapping particles in air and possible solutions and well as look at some of the anticipated applications of such work, in particular in digital microfluidics.
1 citations
05 Sep 2007
TL;DR: In this article, the Brownian dynamics of an optically trapped water droplet is investigated across the transition from over to under-damped oscillations, and an explicit numerical recipe is derived for the fast computation of the power spectra of a Brownian parametric oscillator.
Abstract: The Brownian dynamics of an optically trapped water droplet is investigated across the transition from over to
under-damped oscillations. The spectrum of position fluctuations evolves from a Lorentzian shape typical of overdamped
systems (beads in liquid solvents), to a damped harmonic oscillator spectrum showing a resonance peak.
In this later under-damped regime, we excite parametric resonance by periodically modulating the trapping
power at twice the resonant frequency. We also derive from Langevin dynamics an explicit numerical recipe
for the fast computation of the power spectra of a Brownian parametric oscillator. The obtained numerical
predictions are in excellent agreement with the experimental data.
1 citations
TL;DR: In this article, the use of light from a thermal source for optical trapping and guiding of dielectric microscopic particles is reported, where the light source is filtered for increased spatial coherence and subsequently shaped into a white light Bessel beam for reduced spot size and increased optical gradient.
Abstract: We report the use of light from a thermal source for optical trapping and guiding of dielectric microscopic particles. The light source is filtered for increased spatial coherence and subsequently shaped into a white light Bessel beam for reduced spot size and increased optical gradient.
1 citations
17 Jun 2007
TL;DR: In this paper, the use of Laguerre-Gaussian beams for the optical manipulation and fusion of single femtoliter-volume aqueous droplets, a technique developed to facilitate new applications in droplet microfluidics.
Abstract: This paper describes the use of Laguerre-Gaussian beams for the optical manipulation and fusion of single femtoliter-volume aqueous droplets, a technique developed to facilitate new applications in droplet microfluidics. In addition, this paper describes the possibility of extending this work by using more complex holographic fields generated by computer controlled spatial light modulators (i.e. dynamic holographic optical tweezers) which should lead to the control of larger arrays of droplets. Finally this paper discusses possible applications in the fields of digital microfluidics, with specific reference to the manipulation of cellular material within femtolitre test tubes.
17 Jun 2007
TL;DR: In this paper, the use of holographic tweezers to shape light fields by creating beam with optical singularities (optical vortices) is examined, and the transfer of orbital angular momentum from a light beam to an optically trapped airborne particle is also studied.
Abstract: Holographic optical tweezers were used to trap small arrays of particles. The way in which such devices can be used to create optical microreactors through droplet coagulation is also shown. The use of holographic tweezers to shape light fields by creating beam with optical singularities (optical vortices) is examined. The transfer of orbital angular momentum from a light beam to an optically trapped airborne particle is also studied.