Laboratories are an essential part of undergraduate optoelectronics and photonics education. Of particular interest are the sequence of laboratories which offer students meaningful research experience within a reasonable time-frame limited by regular laboratory hours. We will present our introduction of optical tweezers into the upper-level physics laboratory. We developed the sequence of experiments in the Advanced Lab to offer students sufficient freedom to explore, rather than simply setting up a demonstration following certain recipes. We will also present its impact on our current curriculum of optoelectronics concentration within the physics program.
So often in classes that teach the non-science major students are “dog and pony” shows. The students watch demonstrations, they take notes, they supposedly “absorb” the information only to forget it after the next examination. These types of classes serve only as attempts to transfer information. But what if we give the students a toolbox that provides them with the ability to make their own observations about how light works. Then, the students are empowered to plan their experiments, manipulate the simple apparatus, make their own observations, and draw their own conclusions; more closely paralleling how scientists function. To succeed at this endeavor, we carefully designed a low cost toolbox for the students. Investigations include: Additive color mixing, digital colors and filters, shadows with colors, LED spectrum and spectra, light’s path, Polarization, Luminescence and Brightness, and simple optical instrumentation such as spectrometers. We created activities that give the students direction, but allow them freedom to explore and discover. Online forums/class discussion are also used to enhance their comprehension of their projects. Using this philosophy, we have had great success in both online and face to face classes.
College students are facing a constantly evolving educational system. Some still see mostly the traditional face to face lecture type classes where as others may never set foot on campus thanks to distance learning programs. In between they may enroll in a mix of face-to-face, two-way broadcasted interactive courses, streaming lecture courses, hybrid face-to-face/ on-line courses and the ominous MOOC! A large number of these non-traditional courses are general education courses and play an important role in developing non-science majors’ understanding of science in general, and of physics in particular. We have been keeping pace with theses modern modes of instruction by offering several on-line courses such as Physics for Computer Graphics and Animation and Light and Color. These courses cover basic concepts in light, color and optics.
We present a method to assembly colloidal particles into a three-dimensional structure utilizing the optical
micromanipulation technique. Particles are first assembled into 2-D lattices with optical tweezers. A layer by layer
approach is used to form the 3-D structure. The phase correction is applied to eliminate the diffraction from previously
assembled layers. The preliminary experimental results for the optimization of the 3-D assembly are reported.
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 <i>strongly</i> 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 <i>two</i> 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.
We report our work of binding microscopic particles into three dimension structures through a novel layer-by-layer approach. Multi-beam interferometry was used to form a two dimensional periodic optical field. Such optical landscape was used to trap colloidal particles into a single layer of <i>2-d</i> colloidal array. A second optical tweezers setup was combined to offset such single-layer <i>2-d</i> colloidal arrays in the third dimension. A strong electric field was found at the vicinity of each single layer with carefully calculated experimental parameters. Our numerical results suggest that with practical parameters, the strong electric field resulted from the diffraction of trapped particles should trap colloidal particles into a new layer of identical <i>2-d</i> arrays, thereby offering an alternative approach to optically bind microscopic particles into three dimension periodic structures. Preliminary experimental results of this method demonstrating trapping with secondary diffracted light are also presented.
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.
We present an alternative approach to image and analyze a large amount of optically-bound particles without using microscope objective lenses. The polystyrene spheres were trapped and patterned in the one dimensional optical potential traps formed by two counter-propagating laser beams. Diffraction patterns from such polystyrene sphere assembly were studied and the result are reported. This method offers a significantly larger field of view and faster analysis over the conventional high-power microscope-based imaging technique. The applications of such technique to investigate the statistic (macroscopic) behaviors of a large number of microscopic particles trapped in the optical field were proposed.
A strong electric field enhancement was found at the center of an evanescent wave centripetally-propagated along the surface of a multilayer dielectric waveguide coating on a conical prism. Behaviors similar to the cylindrical vector polarization was observed and enhanced for three orders of magnitude in the same geometry by reconfiguring the incident angles. This enhanced electric field can be utilized to trap dielectric particles with a refractive index either higher or lower than the surroundings. The enhanced optical-trap, also versatile at various incident angles, was analyzed numerically with Rayleigh scattering approximation. Simulation results for optical trapping application are presented, showing a strong gradient force on the trapped particle yet keeping radiation force negligible.
We present preliminary results from an experimental study of optically-assisted assembly. Interference patterns, formed by the intersection of two coherent laser beams, result in periodic one-dimensional potential wells ("optical trenches"). Polystyrene spheres become trapped in these potential wells and subsequently self-assemble into a two-dimensional periodic structure. The spacing between optical trenches is adjusted dynamically, which offers the freedom to dynamically control the lattice constant, offering a recipe for defect-free assembly which begins with annealing at large lattice constant and subsequent compression into a close-packed structure.