We have developed a method that employs nanocapsules, optical trapping, and single-pulse laser photolysis for
delivering bioactive molecules to cells with both high spatial and temporal resolutions. This method is particularly
suitable for a cell-culture setting, in which a single nanocapsule can be optically trapped and positioned at a pre-defined
location next to the cell, followed by single-pulse laser photolysis to release the contents of the nanocapsule onto the
cell. To parallelize this method such that a large array of nanocapsules can be manipulated, positioned, and photolyzed
simultaneously, we have turned to the use of spatial light modulators and holographic beam shaping techniques. This
paper outlines the progress we have made so far and details the issues we had to address in order to achieve efficient
parallel optical manipulations of nanocapsules and particles.
We make use of a spatial light modulator to implement a phase-shifting interferometric method to determine the
topological charge of multiple singularities embedded in the transverse phase of singular beams. This method
allows us to discern between closely spaced singular points and elucidate the dynamics of optical vortices as their
charge is increased continually. The transverse phase of beams with a determined phase profile are analyzed
used this technique, yielding the precise location of multiple singularities as well as the value of their topological
charge. We use apply this method to accurately map the phase and study the transit of vortices across fractional
Bessel beams during their continuous order upconversion.
Droplet microfluidics is an emerging area in miniaturisation of chemical and biological assays, or "lab-on-a-chip"
devices. Normally consisting of droplets flowing in rigid microfluidic channels they offer many advantages over
conventional microfluidic design but lack any form of active control over the droplets. We present work, using
holographic beam shaping, that allows the real time reconfigurability of microfluidic channels allowing us to redirect,
slow, stop, and merge droplets with diameters of approximately 200 microns. A single beam is be sufficient to perform
simple tasks on the droplets but by using holographic beam shaping we can produce multiple foci or continuous patterns
of light that enable a far more versatile tool.
We present results describing the behavior of optically trapped airborne particles, both solid and liquid. Using back focal
plane interferometry we measure characteristic power spectra describing the position fluctuations within the trap. We
show it is easy to transfer between an over and under damped regime by either varying the trapping power or the
distance into the medium the beam is focused. The results assist in the understanding of airborne tweezers and it is hoped
having under damped systems could lead to exploring analogies in many areas of fundamental physics.
The Brownian dynamics of an optically trapped water droplet is investigated across the transition from over to
under-damped oscillations. The spectrum of position fluctuations evolves from a Lorentzian shape typical of overdamped
systems (beads in liquid solvents), to a damped harmonic oscillator spectrum showing a resonance peak.
In this later under-damped regime, we excite parametric resonance by periodically modulating the trapping
power at twice the resonant frequency. We also derive from Langevin dynamics an explicit numerical recipe
for the fast computation of the power spectra of a Brownian parametric oscillator. The obtained numerical
predictions are in excellent agreement with the experimental data.
We discuss the application of optical trapping techniques to droplets, both in air (aerosols) and in fluid (emulsions). We show the holographic optical manipulation of aerosols and how this can be used to transfer orbital angular momentum to airborne particles. We demonstrate new types of traps for aerosols in the form of dual beam fibre traps and compare the trapping efficiency of IR and visible lasers. We discuss some of the interesting dynamics that can be observed when trapping airborne particles and how this appears to differ from conventional liquid based devices. We also examine how holographic optical trapping can be used to facilitate droplet manipulation in another liquid phase. We conclude with a discussion of the difficulties associated with trapping particles in air and possible solutions and well as look at some of the anticipated applications of such work, in particular in digital microfluidics.
We demonstrate the use of holographic optical tweezers for the optical trapping and manipulation of arrays of airborne water droplets (aerosols). Making use of a phase-only spatial light modulator we present evidence of stable, interactive manipulation of both single and multiple aerosol droplets, of the order of 10 microns in diameter, and also their controlled coagulation. We discuss the advantages, disadvantages, and limitations of using a spatial light modulator for droplet manipulation including the implications of the update speed of the device (a Holoeye LC-R2500 SLM), diffraction efficiency, and droplet growth and evaporation due to laser intensity variations. We will examine the generic difficulties of trapping in air, working in the absence of inertial damping. Finally we will discuss the applications of the above work in fields such as atmospheric chemistry and microfluidic microchemical reactors whilst presenting preliminary results on fusion of two or more droplets of differing phases.