Interferometric optical tweezers

Summary form only given. Optical trapping of micron-size dielectric microspheres using a single beam gradient force was first demonstrated by Ashkin in 1986. Since then, extensive research and development of this technique have turned it into a practical device (known as optical tweezers), which has been used in a wide variety of biological and biomedical applications. In this paper, we report the first experimental observation, we believe, of trapping and manipulation of dielectric particles by a set of two-beam interference fringes.

Optical trapping of micron-size, dielectric micros pheres using a single beam gradient force ( Fig. la) was first demonstrated by Ashkin 1 in 1986. Since then, extensive research and development of this technique has turned it into a practical device (known as optical tweezers) which has been used in a wide variety of bio logical and biomedical applications. 2 In conventional optical tweezers, a water-immersed or oil-immersed microscope objective lens with a high numerical aper ture (NA ~ 1.25) is usually used to focus the trapping laser beam to a micron-size spot to achieve a sufficiently strong axial trapping force. 3 This imposes some restric tion on the geometry of the sample cell, since the work ing distance of a high NA objective lens is limited to a few tens of microns below a cover glass (which is matched to the objective lens). High numerical aperture, however, is not required for lateral stability; even weakly focused beams can produce relatively strong lateral trap ping force. 3 For example, gradual accumulation and selfalignment of dielectric microspheres into a two-dimen sional lattice-like mosaic pattern generated by the interference of two or more beams have been reported. 4 Recently, we have successfully demonstrated 5 for the first time the trapping and manipulation of dielectric particles (2.8 mm diameter latex microspheres suspend ed in water inside a thin sample cell) by a set of twobeam interference fringes (Fig. lb) using a 20x objective lens (NA=0.4). The interferometric fringes at the sample plane are swept by retro-reflecting one of the beams from a moving mirror that is attached to a piezoelectric transducer. When the fringes (with total optical power of a few milliwatts) are swept at a speed of 5-10 mm/sec, a sample particle that is trapped in the bright fringes moves along with the fringes until it reaches the edge of the focal spot, where it is stopped by the optical poten tial barrier introduced by the focused beam spot. Visual ly, the boundary of the bright spot acts like a wall against which the impinging particle bounces. The par ticle keeps striking the "wall" as long as the fringes con tinue to sweep. When the sweeping direction of the fringes is reversed, the particle moves toward the other direction and repeats the action described above at the opposite edge of the spot.
An alternative, and simpler, approach to two-beam interference, generates a similar (fringe-like) pattern using a single beam to project a reduced image of a Ronchi ruling at the focal plane of the microscope objective (Fig. 1c). In our experiment, we illuminated (with a single beam) a Ronchi ruling (250 linepairs/inch) placed at the back focal plane of a 20x objec tive lens (Fig. 1c), and projected at its front focal plane, a set of fringes, i.e., alternating bright and dark lines, with line-width on the order of 5 µm. In this approach, the fringes are shifted by translating the Ronchi ruling along the direction perpendicular to the rulings. A sequence of three selected frames from a video record illustrating the trapping and sweeping of a particle from the left to the right is shown in Figure 1d.
From a practical point of view, the second approach is perhaps more favorable for the following reasons. First, it requires only a single beam and the configura tion is simpler and more compact. Secondly, it can be easily modified for two-dimensional micro-manipula tion of two (or more) particles independently. For example, one can project several micron-size cross-hair patterns, using each to trap and manipulate individual particles and to control the relative position of two or more particles. The cross-hairs together with the actua tors (for very high precision x,y,z control) can be inte grated into a compact device by Micro-Opto-Electro-Mechanical (MOEM) technology. This can potentially reduce the size, the complexity, and the cost of optical tweezers for biological and biomedical applications.

Optical Patterning of Three-Dimensional Spatio-Tensorial Micro-Structures in Polymers Celine Fiorini, Jean-Michel Nunzi, Fabrice Charra, and Paul
Raimond, CEA Saclay, Gif-sur-Yvette, France ne challenging requirement for the design of devices for photonic applications is to achieve com plete manipulation of molecular order.
The great latitude and flexibility of optical methods offers interesting prospects for material engineering using light-matter interactions. Efficient spatial modu lation of polymer macroscopic properties is usually achieved using holographic recording of an interference pattern between intense light-waves. For second-order optical nonlinear processes, a full control of the molecu lar orientation is mandatory. However, patterning with polarized monochromatic beams results only in molec ular alignment. We report on a new, purely optical tech nique based on a non-classical holographic process with coherent mixing of dual-frequency fields. It enables effi cient and complete three-dimensional spatio-tensorial control of polymer micro-structures.
Stolen and Tom have shown in glass optical fibers 1 that the coherent superposition of two beams at funda mental and second harmonic frequencies results in a polar field E = Eω+E2ω.
Indeed, the temporal average of the field cube E 3 is non-zero. 2 In organics, this results in a selective excitation of the molecules oriented in a giv en direction and sense, depending on the local polarity of the dual-frequency field E. [2][3] The process is depicted in the upper part of Figure 1. Its peculiar physical origin is a simultaneous one-and two-photon absorption on the same excited electronic level. 3 The orientation-selec tive excitation relaxes thermally inside the polymer. Using appropriate molecules it results in a quasi-perma nent molecular angular redistribution. 4 A spatially modulated x (2) -susceptibility is thus recorded. Poling efficiency depends both on the relative phase and the relative intensities between the writing beams at funda- Note that the geometry of this latter case can also be obtained using octupolar dye molecules and a combination of writing beams with parallel polarizations. 5 mental and second harmonic frequencies. 4 The tensori al properties of the photoinduced x (2) describes the inplane polar geometry of the pattern. The lower part of Figure 1 illustrates x (2) tailoring using appropriate mol ecular geometry or appropriate combinations of polar ized light. 5 Tailoring of the modulation periodicity of the molecular order over the propagation length is achieved by varying the writing fields , wavevector mis match. For normal incidence of both the writing fields at fundamental and second harmonic frequencies, we get exactly the period for phase-matched frequencydoubling. The conversion efficiency then varies qua¬ dratically with the propagation length. This offers an interesting route for the development of low power fre quency doubling for compact blue laser devices.
We have demonstrated that, due to coherent interac tions, the coupling of two beams at fundamental and second harmonic frequencies permits a polar organiza tion of molecules in periodic structures. One break through with this all-optical technique is that it enables the complete three-dimensional control of the materi al's spatial and tensorial properties, without sophisticat ed electrode shaping or multilayer deposition tech niques. More importantly, this patterning technique is based on self-organization of the molecules at a micronscale level. It opens new perspectives in the field of smart materials for photonic devices technology.