In this work, we report using an optical tweezers system to study the light-matter interaction and gradient optical forces of porous silicon nanoparticles. The particles are fabricated by first electrochemically etching a multi-layer porous film into a silicon wafer and then breaking up the film through ultrasonic fracturing. The particles have average pore diameters ranging from 20-30 nm. The fabricated batches of particles have diameters between approximately 100- 600nm. After fabrication, the particles are size-sorted by centrifugation. A commercially available optical tweezers system is used to systematically study the optical interaction with these nanoparticles. This work opens new strategic approaches to enhance optical forces and optical sensitivity to mechanical motion that can be the basis for future biophotonics applications.
We present a novel method for single frame particle image velocimetry of micron scale
spheres based on holographic video microscopy. Our approach takes advantage of the blurring
that recorded holograms suffer when a sphere moves during the exposure period of the camera.
By measuring the angular variance in intensity of the blurred hologram, we extract a modelindependent
metric for the particle velocity. We find this to be accurate for speeds that
permit characterization of other properties of the sphere, such as radius and refractive index
through Lorenz-Mie mocroscopy. Singl-frame holographic velocimetry yields information on
the dynamics of a particle, without sacrificing any other measurements.
The video stream captured by an in-line holographic microscope can be analyzed on a frame-by-frame basis to track
individual colloidal particles' three-dimensional motions with nanometer resolution, and simultaneously to measure their
sizes and refractive indexes. An efficient particle-tracking algorithm automates initial position estimation with sufficient
accuracy to enable unattended holographic particle tracking and characterization. In this work, we demonstrated this
approach to flow visualization in a microfluidic channel and also to flow cytometry of micrometer-scale colloidal
We report a simple and economical technique to create a wide variety of laser pattern for optical tweezing. The main feature of this technique is a reflective gold mirror that is mounted on a stretched latex membrane which can be vibrated with sound wave at a frequency within 100~600Hz. Due to the vibrating gold mirror, laser beam that is reflected off the mirror exhibits a wide variety of controlled patterns. With the reflected laser pattern directed into an optical microscope for optical tweezing, we were able to form different dynamic configurations of structures made of colloidal microspheres. Different formations of colloidal microspheres that correspond to the reflected laser patterns created by the sound-vibrated mirror have been observed.