An optical tweezers (OT) system uses focused laser light to contain and manipulate nano-scale to micro-scale particles.
Trap stiffness is the quantitative measurement of the ability to trap a particle. For some techniques, this measurement
depends on an accurate knowledge of the particle's position in time. A position sensing detector (PSD) is used to track
particle motion by detecting laser light from the trapping region. The PSD outputs voltages corresponding to the x- and y-coordinates of particle motion, providing a means of knowing the location of the particle in time. An OT system
requires a calibration to convert the measured voltages into accurate distances. This process is time-consuming and
frequently needs to be repeated, however, with the growing availability of computer-aided data acquisition and control, the complete process can now be automated, reducing time spent by researchers and increasing level of accuracy of future measurements. We have developed a program written in LabVIEW that will, after initialization, 1) via image processing, calibrate the pixel size of the camera, 2) calibrate the optical tweezer position detector by controlling a motorized mirror to move a trapped bead through a detection laser with simultaneous position detector signal measurements, 3) re-align the trap beam and the detection beam by motorized mirror control, 4) measure position data for the same trapped particle being illuminated by the detection beam, and 5) analyze the position signal via the power spectrum method and equipartition method to give two trap stiffness values for comparison.
Previous automated calibration methods require additional and sometimes costly equipment as well as some precalibration of stage motion or pixel size. Here, the user only needs to input the known size of the bead (provided by the manufacturer) into the program, insert their prepared slide into their microscope, input some parameters and make selections, and click "start" in order to achieve experimental values of the camera and position detector calibrations, as well as trap stiffness. We intend to implement many other calibration techniques that require additional equipment, but have designed this initial system for use in a standard position-detection-capable OT setup as long as it has a digital camera and motorized mirror that can be controlled with LabVIEW.
We examine the enhancement of optical trapping forces due to plasmon resonances of nanoshells. Nanoshells are nanoscale
particles with a dielectric core and metallic coating that exhibit tunable plasmon resonances. Theory predicts that
the optical trapping force may be three to fifty times larger for trapping-laser wavelengths near resonance than for
wavelengths far from resonance . The resonance absorption of nanoshells can be tuned by adjusting the ratio of the
radius of the dielectric core, r<sub>1</sub>, to the total radius, r<sub>2</sub> . Using back focal plane detection, we measure the trap stiffness
of optical tweezers, from lasers at 973 nm and 1064 nm, for single trapped nanoshells with several different r<sub>1</sub>/r<sub>2</sub> ratios.
Enhanced trapping strengths are not found through these measurements done with single wavelength optical traps. A
tunable-wavelength laser trap will enable more conclusive results.
We investigate near-resonant trapping of Rayleigh particles in optical tweezers. Although optical forces due to a near-resonant laser beam have been extensively studied for atoms, the situation for larger particles is that the laser wavelength is far from any absorption resonance. Theory predicts, however, that the trapping force exerted on a Rayleigh particle is enhanced, and may be three to fifty times larger for frequencies near resonance than for frequencies far off resonance. The ability to selectively trap only particles with a given absorption peak may have many practical applications.
In order to investigate near-resonant trapping we are using nanoshells, particles with a dielectric core and metallic coating that can exhibit plasmon resonances. The resonances of the nanoshells can be tuned by adjusting the ratio of the radius of the dielectric core, r<sub>1</sub>, to the overall radius, r<sub>2</sub>, which includes the thickness of the metallic coating. Our nanoshells, fabricated at Rice University, consist of a silica core with a gold coating. Using back focal plane detection, we measure the trap stiffness of a single focus optical trap (optical tweezers), from a diode laser at 853 nm for nanoshells with several different r<sub>1</sub>/r<sub>2</sub> ratios.