Mark J. Kendrick

Wavelength dependence of optical tweezer trapping forces on dye-doped polystyrene microspheres

We present an experimental and numerical study of the wavelength dependence, near resonance, of the optical tweezer trap stiffness on three different dye-doped 1 μm polystyrene spheres with peak absorptions at λ=625, 775, and 840 nm. Experimentally, an increase in the trap stiffness of ~35% on the red side of resonance was observed for the dye-doped spheres relative to polystyrene spheres without dye. Numerical simulations for spheres of different sizes, between 20 nm and 1 μm, and for absorption strengths corresponding to peak extinction coefficient values between 0.0027 and 0.081 were also conducted. Numerical results showed a maximum increase in the trap stiffness of ~35%, which is consistent with experimental results.

Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: Numerical modeling of time-resolved photocurrent

We present a model that describes nanosecond (ns) time-scale photocurrent dynamics in functionalized anthradithiophene (ADT) films and ADT-based donor-acceptor (D/A) composites. By fitting numerically simulated photocurrents to experimental data, we quantify contributions of multiple pathways of charge carrier photogeneration to the photocurrent, as well as extract parameters that characterize charge transport (CT) in organic films including charge carrier mobilities, trap densities, hole trap depth, and trapping and recombination rates. In pristine ADT films, simulations revealed two competing charge photogeneration pathways: fast, occurring on picosecond (ps) or sub-ps time scales with efficiencies below 10%, and slow, which proceeds at the time scale of tens of nanoseconds, with efficiencies of about 11%–12%, at the applied electric fields of 40–80 kV/cm. The relative contribution of these pathways to the photocurrent was electric field dependent, with the contribution of the fast process increasing wi...

Numerical modeling of time-resolved photocurrent in organic semiconductor films

We performed numerical simulations of transient photocurrents in organic thin films, in conjunction with experiments. This enabled us to quantify the contribution of multiple charge generation pathways to charge carrier photogeneration, as well as extract parameters that characterize charge transport, in functionalized anthradithiophene (ADT-TES-F) films prepared using two different deposition methods: drop casting on an untreated substrate and spin casting on a pentauorobenzenethiol (PFBT)-treated substrate. These deposition methods yielded polycrystalline lms with considerably larger grain sizes in the case of the spin cast lm. In both drop cast and spin cast films, simulations revealed two competing charge photogeneration pathways: fast charge generation on a picosecond (ps) or sub-ps time scale with efficiencies below 10%, and slow charge generation, on the time scale of tens of nanoseconds, with efficiencies of 11-12% in drop cast and 50-60% in spin cast films, depending on the applied electric field. The total charge photogeneration efficiency in the spin cast sample was 59-67% compared to 14-20% in the drop cast sample, whereas the remaining 33-41% and 80-86%, respectively, of the absorbed photon density did not contribute to charge carrier generation on these time scales. The spin cast film also exhibited higher hole mobilities, lower trap densities, shallower traps, and lower charge carrier recombination, as compared to the drop cast lm. As a result, the spin cast lm exhibited higher photocurrents despite a considerably lower lm thickness (and thus reduced optical absorption and cross section of the current flow).

pH/ion sensitive nanoprobes with optical tweezers

We present fluorescence-based pH/ion nanosensors, positioned and manipulated using holographic optical tweezers, with simultaneous fluorescence read-out, within a microfluidic device and within a biological cell.

Optical tweezers with optically resonant particles

Optical tweezers are typically used on transparent dielectric particles. Particles with optical resonances should experience a larger trapping force near resonance. We present a numerical and experimental study of trapping force on such particles.

Wavelength Dependence of Optical Tweezer Trapping Forces on Resonant Particles

Optical tweezers are typically used with transparent dielectric particles. Particles with optical resonances should experience a larger trapping force near resonance. We present a numerical and experimental study of the trapping forces on such particles.

Optical tweezers with resonant particles

Optical tweezers are typically used on transparent dielectric particles. Particles with optical resonances would experience larger trapping forces and allow trapping of smaller particles. We present a study of increased trapping forces on such particles.

Fluorescent and photoconductive properties of anthradithiophene and pentacene derivatives

We present optical, fluorescent and photoconductive properties of high-performance anthradithiophene and functionalized pentacene derivatives. Fluorescence emission with lifetimes around or longer than ~10 ns is observed at visible and near-infrared wavelengths, depending on the molecule.

Optical field enhancement In tweezer trapping

Near-resonant light can be used to enhance optical trapping of particles with wavelength dependent optical responses. We present results from our study of optically enhanced forces acting on dielectric and metal nanoparticles in tweezers trapping.