Weiqiang Mu

Surface-enhanced Raman scattering from silver-coated opals.

We describe surface-enhanced Raman scattering measurements from a benzenethiol monolayer adsorbed on a silver-coated film that is, in turn, deposited on an artificial opal, where the latter is a close-packed three-dimensional dielectric lattice formed from polystyrene spheres. Data for a range of sphere sizes, silver film thicknesses, and laser excitation wavelengths are obtained. Enhancement factors can be in the range of 10(7). To partially explain these large enhancements, we have performed model finite-difference time domain simulations of the position-dependent electric fields generated at the opal surfaces for several experimentally studied laser wavelengths and sphere diameters.

One-dimensional long-range plasmonic-photonic structures

We have fabricated line gratings from periodically etched fused silica on which a thin silver film is deposited that is in turn covered with a silica index-matched fluid. This dielectrically symmetric geometry supports an independent long-range surface plasmon-polariton (LRSPP) and a short-range surface plasmon polariton, and the associated plasmonic band structure has been probed. Coupling to external light is achieved via the patterned grating, and an ultrasharp LRSPP linewidth of 4 nm is observed. The experimental results are compared with finite-difference time-domain simulations.

Highly directional fluorescence emission from dye molecules embedded in a dielectric layer adjacent to a silver film

We report measurements on the angular radiation patterns from dye molecules embedded in a polymethyl methacrylate thin film spin-coated on a thin silver film. We systematically studied the influence of the thickness of the silver films and the thickness of the dielectric layers on the radiation pattern. We present the detailed radiation patterns over a large angular range showing highly polarized fluorescence emission coupled into the surface plasmon modes or waveguide modes. We also studied the influence of the polarization of the excitation beam on the radiation patterns. The experimental data are compared with numerical simulations using an asymptotic approach based on the Lorentz reciprocity theorem.

Dynamic control of defects in a two-dimensional optically assisted assembly

In this paper we demonstrate controlled loading of a closely packed array of optical traps. We also describe the technical advantages of our method of filling the trap array (which makes use of an independent, steerable trap created by a separate objective lens), as well of our specific implementation of array generation by multi-beam interference. Microscopic polystyrene spheres are trapped and subsequently assembled into sites on a two-dimensional optical lattice, which is formed from the interference of two pairs of coherent laser beams via an optical setup that allows for simple, continuous variation of lattice parameters over a very wide range. Individual particles in the initial assembly are dynamically manipulated with the independent laser beam, which offers the freedom to generate either defect-free lattices or a lattice with designer defects. As examples we demonstrate the assembly of a defect-free square lattice and a lattice with a single vacancy.

Force measurement on microspheres in an optical standing wave

We have measured the optical forces on isolated particles trapped in an optical lattice generated by the interference of two coherent laser beams. Two independent methods are employed here that are based on the equipartition theorem and hydrodynamic drag. The optical force on a particle in an optical lattice depends strongly on the ratio of the particle diameter to the period of the lattice. Based on the observed size dependence, we developed an approach that allows tunable, size-dependent force selection of a subset of particles from an ensemble containing mixed particles.

Angular radiation pattern of electric dipoles embedded in a thin film in the vicinity of a dielectric half space

We report measurements on the angular radiation pattern from an array of dye molecules embedded in a polymethylmethacrylate film deposited on a dielectric hemispherical lens. The radiation pattern is both highly structured and directed, with most of the power being radiated into the media having the higher refractive index. We also present a simulation of the far-field radiation pattern of a dipole embedded in a thin dielectric layer, which apparently has not been investigated before. The simulation matches the experimental results rather well.

Long-range surface plasmon polaritons propagating on a dielectric waveguide support

In the traditional long-range surface plasmon geometry, an ultrathin metal film is sandwiched between two layers having identical dielectric constants. Here we demonstrate the long-range surface plasmon polariton (LRSPP) properties for a new structure where a thin layer with a dielectric constant exceeding that of the surroundings is inserted within the sandwich, provided the layer thickness d satisfies the condition k(⊥)d=mπ where k(⊥) is the component of the guide wavevector perpendicular to the layer and m is an integer. The resulting plasmon modes have smaller losses and nearly the same phase velocity as the original LRSPP. This provides a strategy to support silver films having thicknesses of 10s of nanometers to create plasmonic devices for sensor applications.

Enhanced particle transport in an oscillating sinusoidal optical potential

We have studied the delivery of a colloidal particle in the presence of an oscillating, spatially periodic, optical potential. The average particle velocity relative to the fluid velocity in this potential depends greatly on the oscillation amplitude and frequency. The results of both our simulations and experiments show that for some combinations of these parameters, the average particle transportation velocity can be enhanced due to the synchronization of the particles movement with the oscillating potential.

Force measurement and optical assisted particle separation in an optical standing wave

We have measured the optical force on isolated particles trapped in an optical lattice generated by the interference of two coherent laser beams by a method based on the equipartition theorem and by an independent method based on hydrodynamic-drag. The experimental results show that the optical force on a particle in this type of optical lattice depends strongly on the ratio of the particle diameter to the period of the lattice. By tuning this ratio, the force due to the optical lattice can be made to vanish. We also formed optical lattices involving two independent standing waves with different spatial periods formed by tightly focusing four laser beams which are pair wise coherent. By shifting the relative phases of the interfering beams we can advance the two waves in opposite directions. Depending on the spacing and the translation speed of the two interference patterns, appropriately sized particles can be translated in opposite directions; using this approach we succeeded in separating two different sizes of particles in the presence of a simulated fluid flow.

Optical separation of particles based on a dynamic interferometer

We demonstrate the sequential spatial separation of a solution consisting of a mixture of two microspheres with different diameters using a dynamic optical interferometery scheme. Two coherent lasers beams are focused together through an objective lens to form an in-plane standing wave. By linearly increasing the phase of one of incoming beams relative to the other, the optical lattice is translated. The optical forces on particles with different sizes depends on the spacing of the standing wave relative to the particle diameter; therefore, by adjusting the spacing of the standing wave so as to minimize the interaction of particles of one size with the optical lattice, all other particles can be swept out by the translating potential wells that are associated with the intensity maxima of the standing wave, while the selected particles remain trapped in the overall center of the Gaussian beam envelope of the optical lattice. Here, we demonstrate the selectivity of this optical conveyor belt by dragging smaller particles out to one side of an ensemble while simultaneously keeping the larger ones trapped. The Brownian dynamics of particles translated in an optical lattice and measurements of the associated optical force are also presented.