Principle of birefringence - Graphic of the rotation process. Linearly polarized light enters a quarter-wave plate, is converted to circularly polarized light and then passes through a birefringent particle (vaterite). Angular moment is transferred to the particle causing it to rotate. The resulting light exiting the particle is elliptically polarized.

Optical Trapping & Holography Research

An optical trap (or optical tweezer) is a tool capable of manipulating microscopic particles using the inherent momentum of light. There are multiple options for controlling the trapping beam(s): our lab manipulates the trapping beam(s) using both fast steering mirrors (FSMs) and spatial light modulators (SLMs). Current research focuses on a means for building more sensitive trapping systems and the use of optical trapping beams for micropatterning. Trapping is useful in a variety of fields that range from biology, physics, and chemistry to materials science. For this reason, increasing optical trapping sensitivety,and therefore force measurment capability, will aide in the more accurate study of biomolecular interactions. Recently we have investigated control over the rotation of birefrigant particles.

We have aslo used an optomagnetic trap to attain near holonomic control over a specially fabricated Janus microparticle. The optical field was used to control the three postional degrees of freedom and constrain one degree of rotational freedom, while the the magnetic filed was used to control the remaining two degrees of rotational freedom. This level of control enables applications ranging from biophysical force and torsion measurments to microfluidics and self-assembly. The figures below display the optomagnetic setup and the controlled manipulation of the Janus particles. Image (a) shows the simultaneous movement and rotation of a single particle using the optomagnetic trap, while images (b-d) show the rotation of the particle in the out of the plane direction.
Simultaneous optical and magnetic control of a dot Janus particle is demonstrated. The magnetic field setup (left) required for ‘dot’ Janus optical and magnetic manipulation. The solenoids apply the magnetic field required for rotation while the optical trap positions the particle in three dimensions (a) Overlay of seven images as dot Janus particle is spatially moved around a predefined circle (dotted line) by a holographic optical trap and rotationally oriented by a 0.1 Hz rotating magnetic field. For visual aid, illustrations of the particle orientation are provided with the arrows indicating the direction of the magnetic field at each instance. (b–d) Sequence of images showing rotation of Janus particle around f axis while optically trapped. A particle with small debris was intentionally used to better visualize the orientation of the Janus particle in f.
Another application of the optical trapping system has been for three-dimensional parallel micropatterning using a spatil light modulator (SLM). The SLM enables a user to display phase holograms which direct incident laser light in prescribed intensity distributions. These distributions can contain multiple foci which provides the means to pattern in parallel.
Scehmatic diagram demonstrating how phase information displayed on the SLM is propagated through the Fourier lens to produce a desired four beam intenisty distribution. This distribution was used to simultaneously build pyramidal microstructers in a serial fashion (see video).
© 2010 University of Rochester - Last Modified: Thursday, May 13, 2010