Friday, December 2, 2016

Direct imaging of a digital-micromirror device for configurable microscopic optical potentials

G. Gauthier, I. Lenton, N. McKay Parry, M. Baker, M. J. Davis, H. Rubinsztein-Dunlop, and T. W. Neely

Programmable spatial light modulators have significantly advanced the configurable optical trapping of particles. Typically, these devices are utilized in the Fourier plane of an optical system, but direct imaging of an amplitude pattern can potentially result in increased simplicity and computational speed. Here we demonstrate high-resolution direct imaging of a digital micromirror device (DMD) at high numerical apertures (NAs), which we apply to the optical trapping of a Bose–Einstein condensate (BEC). We utilize a (1200×19201200×1920) pixel DMD and commercially available 0.45 NA microscope objectives, finding that atoms confined in a hybrid optical/magnetic or all-optical potential can be patterned using repulsive blue-detuned (532 nm) light with 630(10) nm full width at half-maximum resolution, within 5% of the diffraction limit. The result is near arbitrary control of the density of the BEC without the need for expensive custom optics. We also introduce the technique of time-averaged DMD potentials, demonstrating the ability to produce multiple gray-scale levels with minimal heating of the atomic cloud, by utilizing the high switching speed (20 kHz maximum) of the DMD. These techniques will enable the realization and control of diverse optical potentials for superfluid dynamics and atomtronics applications with quantum gases. The performance of this system in a direct imaging configuration has wider application for optical trapping at non-trivial NAs.


Tunable Fano resonant optical forces exerted on a graphene-coated dielectric particle by a Gaussian evanescent wave

Yang Yang, Xiaofu Zhang, Anping Huang and Zhisong Xiao

In this paper, we investigate the optical forces exerted on a graphene-coated dielectric particle by the Gaussian beam transmitted through the prism setup systematically. It is shown that the optical force spectra exhibit significant Fano resonance under the excitation of a Gaussian evanescent wave. The magnitude and morphology of Fano resonance can be modulated effectively by the graphene coating. Also, the modification on the threshold of the Fermi energy of graphene could help to regulate the trapping behavior efficiently. The proposed work may provide a new avenue in the specific optical tweezers and nano-optics.


Real-time force measurement in double wavelength optical tweezers

Sławomir Drobczyński and Kamila Duś-szachniewicz
In optical tweezers, the trap stiffness varies across the sample area. To avoid this problem, the force measurement is often performed in a fixed place where the trap stiffness is well determined. However, for some experiments, bringing the sample to the fixed position is problematic. In this paper, we describe a precise and fast procedure for mapping the trap stiffness over the whole sample area. Such a map allows development of a real-time procedure for force measurement at any point of the sample area. The presented method is particularly suitable for measuring forces, for example, in living cells samples.


Wednesday, November 30, 2016

Contact electrification of individual dielectric microparticles measured by optical tweezers in air

Haesung Park and Thomas W. LeBrun

We measure charging of single dielectric microparticles after interaction with a glass substrate using optical tweezers to control the particle, measure its charge with a sensitivity of a few electrons and precisely contact the particle with the substrate. Polystyrene (PS) microparticles adhered to the substrate can be selected based on size, shape, or optical properties and repeatedly loaded into the optical trap using a piezoelectric (PZT) transducer. Separation from the substrate leads to charge transfer through contact electrification. The charge on the trapped microparticles is measured from the response of the particle motion to a step excitation of a uniform electric field. The particle is then placed onto a target location of the substrate in a controlled manner. Thus, the triboelectric charging profile of the selected PS microparticle can be measured and controlled through repeated cycles of trap loading followed by charge measurement. Reversible optical trap loading and manipulation of the selected particle leads to new capabilities to study and control successive and small changes in surface interactions.


Optical manipulation using highly focused alternate radially and azimuthally polarized beams modulated by a devil’s lens

Zhirong Liu and P. H. Jones

We propose and demonstrate a novel high numerical aperture (NA) focusing system composed of an annular beam with alternate radially and azimuthally polarized rings, focused by a devil’s lens (DL), and further investigate its radiation forces acting upon a Rayleigh particle both analytically and numerically. Strongly focused cylindrical vector beams produce either dark-centered or peak-centered intensity distributions depending on the state of polarization, whereas the DL produces a series of foci along the propagation direction. We exploit these focusing properties and show that by selecting an appropriate truncation parameter in front of the focusing lens, the proposed optical focusing system can selectively trap and manipulate dielectric micro-particles with low or high refractive indices by simply adjusting the radius of the pupil or the beam. Finally, the stability conditions for effectively trapping and manipulating Rayleigh particles are analyzed. The results obtained in this work are of interest in possible applications in optical confinement and manipulation, sorting micro-particles, and making use of a DL.


Fast and Accurate Algorithm for Repeated Optical Trapping Simulations on Arbitrarily Shaped Particles Based on Boundary Element Method

Kai-Jiang Xu, Xiao-Min Pan, Ren-Xian Li, Xin-Qing Sheng

In optical trapping applications, the optical force should be investigated within a wide range of parameter space in terms of beam configuration to reach the desirable performance. A simple but reliable way of conducting the related investigation is to evaluate optical forces corresponding to all possible beam configurations. Although the optical force exerted on arbitrarily shaped particles can be well predicted by boundary element method (BEM), such investigation is time costing because it involves many repetitions of expensive computation, where the forces are calculated from the equivalent surface currents. An algorithm is proposed to alleviate the difficulty by exploiting our previously developed skeletonization framework. The proposed algorithm succeeds in reducing the number of repetitions. Since the number of skeleton beams is always much less than that of beams in question, the computation can be very efficient. The proposed algorithm is accurate because the skeletonization is accuracy controllable.


Tuesday, November 29, 2016

The RNA helicase Mtr4p is a duplex-sensing translocase

Eric M Patrick, Sukanya Srinivasan, Eckhard Jankowsky & Matthew J Comstock

The conserved Saccharomyces cerevisiae Ski2-like RNA helicase Mtr4p plays essential roles in eukaryotic nuclear RNA processing. RNA helicase activity of Mtr4p is critical for biological functions of the enzyme, but the molecular basis for RNA unwinding is not understood. Here, single-molecule high-resolution optical trapping measurements reveal that Mtr4p unwinds RNA duplexes by 3′-to-5′ translocation on the loading strand, that strand separation occurs in discrete steps of 6 base pairs and that a single Mtr4p molecule performs consecutive unwinding steps. We further show that RNA unwinding by Mtr4p requires interaction with upstream RNA duplex. Inclusion of Mtr4p within the TRAMP complex increases the rate constant for unwinding initiation but does not change the characteristics of Mtr4p's helicase mechanism. Our data indicate that Mtr4p utilizes a previously unknown unwinding mode that combines aspects of canonical translocating helicases and non-canonical duplex-sensing helicases, thereby restricting directional translocation to duplex regions.


Ripplon laser through stimulated emission mediated by water waves

Samuel Kaminski, Leopoldo L. Martin, Shai Maayani & Tal Carmon

Lasers rely on stimulated electronic transition, a quantum phenomenon in the form of population inversion. In contrast, phonon masers1, 2, 3 depend on stimulated Raman scattering and are entirely classical. Here we extend Raman lasers1, 2, 3 to rely on capillary waves, which are unique to the liquid phase of matter and relate to the attraction between intimate fluid particles. We fabricate resonators that co-host capillary4 and optical modes5, control them to operate at their non-resolved sideband and observe stimulated capillary scattering and the coherent excitation of capillary resonances at kilohertz rates (which can be heard in audio files recorded by us). By exchanging energy between electromagnetic and capillary waves, we bridge the interfacial tension phenomena at the liquid phase boundary to optics. This approach may impact optofluidics by allowing optical control, interrogation and cooling6 of water waves.


Lab on Fiber Technology for biological sensing applications

Patrizio Vaiano, Benito Carotenuto, Marco Pisco, Armando Ricciardi, Giuseppe Quero, Marco Consales, Alessio Crescitelli, Emanuela Esposito, Andrea Cusano

This review presents an overview of “Lab on Fiber” technologies and devices with special focus on the design and development of advanced fiber optic nanoprobes for biological applications. Depending on the specific location where functional materials at micro and nanoscale are integrated, “Lab on Fiber Technology” is classified into three main paradigms: Lab on Tip (where functional materials are integrated onto the optical fiber tip), Lab around Fiber (where functional materials are integrated on the outer surface of optical fibers), and Lab in Fiber (where functional materials are integrated within the holey structure of specialty optical fibers).
This work reviews the strategies, the main achievements and related devices developed in the “Lab on Fiber” roadmap, discussing perspectives and challenges that lie ahead, with special focus on biological sensing applications.


Multibuilding Block Janus Synthesized by Seed-Mediated Self-Assembly for Enhanced Photothermal Effects and Colored Brownian Motion in an Optical Trap

Kanokwan Sansanaphongpricha, Michael C. DeSantis, Hongwei Chen, Wei Cheng, Kai Sun, Bo Wen, Duxin Sun

The asymmetrical features and unique properties of multibuilding block Janus nanostructures (JNSs) provide superior functions for biomedical applications. However, their production process is very challenging. This problem has hampered the progress of JNS research and the exploration of their applications. In this study, an asymmetrical multibuilding block gold/iron oxide JNS has been generated to enhance photothermal effects and display colored Brownian motion in an optical trap. JNS is formed by seed-mediated self-assembly of nanoparticle-loaded thermocleavable micelles, where the hydrophobic backbones of the polymer are disrupted at high temperatures, resulting in secondary self-assembly and structural rearrangement. The JNS significantly enhances photothermal effects compared to their homogeneous counterpart after near-infrared (NIR) light irradiation. The asymmetrical distribution of gold and iron oxide within JNS also generates uneven thermophoretic force to display active colored Brownian rotational motion in a single-beam gradient optical trap. These properties indicate that the asymmetrical JNS could be employed as a strong photothermal therapy mediator and a fuel-free nanoscale Janus motor under NIR light.