Session 2P2a Shaping Optical Forces for Trapping and Binding Theory

Transcription

1 Session 2P2a Shaping Optical Forces for Trapping and Binding Theory Bored Helical Phases: Dynamics of Intensity Profiles and Poynting Vector Calculation upon Propagation Nathaniel P. Hermosa II (Ateneo de Manila University, Philippines); Stein C. Baluyot (Ateneo de Manila University, Philippines); Optical Trapping in Interfering Laser Beams: Principles and Applications Pavel Zemánek (Institute of Scientific Instruments of the ASCR, v.v.i., Czech Republic); T. Čižmár (Institute of Scientific Instruments of the ASCR, v.v.i., Czech Republic); M. Šiler (Institute of Scientific Instruments of the ASCR, v.v.i., Czech Republic); P. Jákl (Institute of Scientific Instruments of the ASCR, v.v.i., Czech Republic); M. Sery (Institute of Scientific Instruments of the ASCR, v.v.i., Czech Republic); Theory for Trapping Efficiency of Arbitrary Beams in Optical Tweezers Antonio A. R. Neves (Universita di Lecce, Italy); A. Fontes (Federal University of Pernambuco, Brazil); C. L. Cesar (Universidade Estadual de Campinas, Brazil); A Camposeo (Universita di Lecce, Italy); R. Cingolani (Universita di Lecce, Italy); D. Pisignano (Universita di Lecce, Italy); Dual-beam Interferometric Laser Trapping of Rayleigh and Mesoscopic Particles Vincent L. Y. Loke (The University of Queensland, Australia); Timo A. Nieminen (The University of Queensland, Australia); N. R. Heckenberg (The University of Queensland, Australia); Halina Rubinsztein-Dunlop (The University of Queensland, Australia); Giant Optical Forces and Size-selective Manipulation for Microspheres Using Evanescent Wave Excitation of Whispering Gallery Modes Jack Ng (The Hong Kong University of Science and Technology, China); Che Ting Chan (The Hong Kong University of Science and Technology, China);

2 372 Progress In Electromagnetics Research Symposium Abstracts, Hangzhou, China, March 24-28, 2008 Bored Helical Phases: Dynamics of Intensity Profiles and Poynting Vector Calculation upon Propagation Nathaniel P. Hermosa II and Stein C. Baluyot School of Science and Engineering, Ateneo de Manila University Loyola Heights, Quezon City, 1108 Philippines Abstract A modified helical phase is presented. The phases of these beams are made by boring a hole at the center of a helical phase as, { 0 0 r ri, Φ l (r, θ) = (1) lθ r > r i where r i is the cavity radius and l is the number of 2π windings around the azimuth. The beams are propagated numerically via the Split-Step algorithm introduced by Feit and Fleck. We observe that the intensity profiles of these beams vary with the cavity radius. When the ratio between the cavity radius and the beam radius (r i /r o ) is less than 0.15, the beams intensity profiles are similar to the profiles of the Laguerre-Gaussian beams. However at r i /r o greater than 0.50, the beams produce l intense distinct arms. We surmise that the number of arms is due to an integral 2π phase difference between the bore and the helical phase. In between ratios 0.15 and 0.50, the intense arms of the beams are not yet distinct and the intensity profiles still resemble distorted circles. We have experimentally verified the intensity profiles with the use of computer generated holograms. Upon propagation in free space, these beams rotate around the propagation axis while keeping their intensity profiles. The intensity profiles are the same upon scaling, up to a certain propagation distance. The rotation is a function of l and is dependent on r i. The rate of rotation with propagation distance is faster for small l and the rate decreases with increasing l. Beams with similar l but with different r i seem to have the same rotation rate. We also present calculations of the Poynting vector of these beams as they propagate. Specifically, we compute for the linear momentum of these beams in anticipation of possible applications in optical micromanipulation.

3 Progress In Electromagnetics Research Symposium Abstracts, Hangzhou, China, March 24-28, Optical Trapping in Interfering Laser Beams: Principles and Applications P. Zemanek, T. Cizmar, M. Siler, P. Jakl, and M. Sery Institute of Scientific Instruments of the ASCR, v.v.i. Academy of Sciences of the Czech Republic, Kralovopolska 147, Brno, Czech Republic Abstract The classical tool for optical micromanipulations single-beam optical tweezers has been known for more than 20 years and, apart from life sciences, it influenced also colloidal and aerosol chemistry, atomic physics, hydrodynamics, and basic physics. However, many new and exciting applications were introduced in the last five years. Some of the most remarkable examples are based on engineering of laser beam wavefront via dynamically reconfigurable phase or amplitude masks (so called holographic tweezers) or exploiting the interference of several laser beams. This last mentioned light-shaping technique is the prerequisite for various methods of sorting of colloidal particles or living cells according to their size or refractive index in optical lattices, Bessel beams, traveling standing waves; simultaneous optical confinement of thousands of objects in spatially periodic interference structures and their delivery over millimeter distances. The above list illustrates clearly the application potential of the interferometric optical micromanipulation techniques. They can be combined with microfluidic systems to sort microobjects and increase further the sensitivity and versatility of lab-on-a-chip systems dealing with very small volumes of analyte; they can be used in laboratories where non-contact manipulation with large amount of objects is needed or interactions between these objects have to be analyzed. However, more confined objects scatter more light that interferes with the trapping light pattern and can lead to lost of stability of the trapped structure. We are going to present how the trapping in spatially periodic fields, with emphasis on the standing waves trapping, depends on the object size and material, how to employ it for optical sorting with and without the fluid flow, how to use traveling periodic potential profiles for object delivery and how the number of objects influences confinement properties of the whole structure comparing to the single particle. ACKNOWLEDGMENT This work was partially supported by MEYS CR (project No. LC06007) and IRP of the ISI (AV0Z ).

4 374 Progress In Electromagnetics Research Symposium Abstracts, Hangzhou, China, March 24-28, 2008 Theory for Trapping Efficiency of Arbitrary Beams in Optical Tweezers A. A. R. Neves 1, A. Fontes 2, C. L. Cesar 3, A Camposeo 1, R. Cingolani 1, and D. Pisignano 1 1 National Nanotechnology Laboratory of CNR-INFM, Distretto Tecnologico, Scuola Superiore ISUFI Università di Lecce, via Arnesano I-73100, Lecce, Italy 2 Federal University of Pernambuco, Recife, Pernambuco, Brazil 3 CePOF, Instituto de Física, Universidade Estadual de Campinas, Brazil Abstract Today optical tweezers setup has become a highly sensitive force measurement, and in the last few years different theories were developed to determine the theoretical optical forces on a dielectric microsphere acting as a probe. Initial works separated the trapping domain in two distinct size regime, (Rayleigh and Geometrical Optics) for the particle size compared to the wavelength of the trapping beam, but for most trapping experiments the probe size is of the order of the wavelength, consequently a full range Mie regime has to be adopted. Also for a certain size ratio, of the microsphere diameter with respect to wavelength, may be such to give rise to morphology dependent resonances, which must certainly be taken into account since the scatterer cross section will change abruptly. Even though the scatterer is spherically symmetric, the trapping force is dependent on polarization and does not share the spherical symmetry of the microsphere. To account for polarizations in the high numerical aperture (NA) focal region, different approximations of the non-paraxial beam has been adopted, but only with an angular spectrum representation could the vectorial EM fields in the focal region satisfy Maxwell s equation exactly. The correct beam description represents one of the main problems for proper determination of optical forces. Recently it has been shown that with an integral relation the partial wave expansion coefficients of this focused vectorial EM fields could be determined analytically, thus gaining significant physical insight into the dependence of the optical forces through the use of the generalized Lorenz-Mie theory. Diffraction effects and aberrations due to the refractive index mismatch of the immersion oil objective are observed on the optical trapping force profiles for an arbitrary beam profile and polarization. Here we present this optical force theory, solved without approximations, providing further insight into the physics of optical trapping, in term of easily obtainable experimental parameters.

5 Progress In Electromagnetics Research Symposium Abstracts, Hangzhou, China, March 24-28, Dual-beam Interferometric Laser Trapping of Rayleigh and Mesoscopic Particles V. L. Y. Loke, T. A. Nieminen, N. R. Heckenberg, and H. Rubinsztein-Dunlop The University of Queensland, Australia Abstract There has been a significant amount of experimental work with counter-propagating, crossed-beam, and other interferometric laser trapping of neutral dielectric particles. Apart from the benefits of these configurations, such as the compensation or neutralization of scattering forces, there are a number of interesting applications. For example, the optical lattice resulting from interference can be used to sort particles of different refractive indices or sizes. The system can also be used to study thermal hopping between potential wells or to investigate Brownian motion subject to a quasi-periodic external potential. Most theoretical work on dual-beam systems has been focused on Rayleigh particles. For mesoscopic particles, where Rayleigh scattering is not applicable, we employ the generalized Lorenz Mie theory (GLMT) which is synonymous with the T-Matrix method when considering spherical particles. GLMT is an exact method which is applicable in both the Rayleigh and mesoscopic regimes. We calculate the fields and optical forces on the particles in the vicinity of the trapping region for several dual beam configurations such as counter-propagating, axially offset, and crossed beams, and with different particle sizes. We present the intensity profiles which demonstrate the interference fringes and the force vectors to predict the positions where a given particle can be localized. Surprisingly, our results show that sub-wavelength localization by interference fringes of particles that are much larger than the wavelength (or fringes) of the trapping beams is possible. The competition between such localization by interference fringes and attraction to the focus can be controlled by the beam convergence angle, or, equivalently, the numerical aperture of the focussing system.

6 376 Progress In Electromagnetics Research Symposium Abstracts, Hangzhou, China, March 24-28, 2008 Giant Optical Forces and Size-selective Manipulation for Microspheres Using Evanescent Wave Excitation of Whispering Gallery Modes Jack Ng and C. T. Chan The Hong Kong University of Science and Technology, Hong Kong, China Abstract One of the state-of-the-art techniques to determine the size of a microsphere is to utilize the extremely sharp fluorescence peaks of its WGM, where an accuracy of 0.05% has been experimentally achieved for a 5-micron-diameter sphere [2]. However, there is a desire to search for methods that are automatic and/or parallel, and therefore they have higher throughputs. Here we propose an approach base on WGM induced optical forces that will potentially allow one to select microspheres of a particular size or resonant frequency, and possibly in large quantities. Evanescent waves have the unique ability to couple to the WGMs while preserving its high-q. By exciting the WGM of a microsphere with an evanescent wave, it is shown that one can exert a giant size-selective optical force on individual microsphere (see Fig. 1), with strength proportional to Q and the size-sensitivity Q 1 (where Q is the quality factor of the WGM). For a 5 µm-diam microsphere, one can easily achieve a force exceeding 100 pn (76 nn for a perfect sphere with no absorption) at a moderate light intensity (10 4 W/cm 2 ), and with a size-sensitivity of better than 0.1%. Such resonant force is potentially useful for accurate size-selective manipulation, and for selecting microcavities that have the same size and resonant frequency (potentially with a state-of-the-art accuracy), in a fully automatic, parallel, and high throughput manner. Methodology: We calculate the optical forces that act on a high-q microsphere by an evanescent wave (or inhomogeneous plane wave). We found that evanescent waves can couple efficiently to whispering gallery modes (WGM) and the optical force is greatly enhanced at resonance. The formalism we employ is based on a multiple-scattering and Maxwell Stress Tensor formalism [1]. Figure 1: The optical force acting on a microsphere along the propagating direction of the incident wave. The parameters are incident wavelength λ = 520 nm, the dielectric constant of the sphere ε sphere = , and the sphere radius r s = µm. (a) The incident wave is a linear polarized homogeneous plane wave with a uniform intensity of I 0 = 10 4 W/cm 2. (b) The incident wave is an s-polarized evanescent wave with k 0 /k = and I 0 = 10 4 W/cm 2 at the bottom of the sphere. (c) A schematic illustration of how an evanescent wave excites a WGM. Efficient excitation. REFERENCES 1. Ng, J., Z. F. Lin, C. T. Chan, and P. Sheng, Phys. Rev. B, Vol. 72, , Mukaiyama, T., et al., Phys. Rev. Lett., Vol. 82, 4623, 1999.