We introduce tunable optofluidic microlasers based on active optical resonant cavities formed by optically stretched, dye-doped emulsion droplets confined in a dual-beam optical trap. To achieve tunable dye lasing, optically pumped droplets of oil dispersed in water are stretched by light in the dual-beam trap. Subsequently, resonant path lengths of whispering gallery modes (WGMs) propagating in the droplet are modified, leading to shifts in the microlaser emission wavelengths. We also report lasing in airborne, Rhodamine B-doped glycerolwater droplets which were localized using optical tweezers. While being trapped near the focal point of an infrared laser, the droplets were pumped with a Q-switched green laser. Furthermore, biological lasing in droplets supported by a superhydrophobic surface is demonstrated using a solution of Venus variant of the yellow fluorescent protein or E. Coli bacterial cells expressing stably the Venus protein. Our results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium.
We present dye lasing from optically manipulated glycerol-water aerosols with diameters ranging between 7.7 and
11.0 μm confined in optical tweezers. While being optically trapped near the focal point of an infrared laser, the
droplets stained with Rhodamine B were pumped with a Q-switched green laser and their fluorescence emission
spectra featuring whispering gallery modes (WGMs) were recorded with a spectrograph. Nonlinear dependence
of the intensity of the droplet WGMs on the pump laser fluence indicates dye lasing. The average wavelength
of the lasing WGMs could be tuned between 600 and 630 nm by adjusting the droplet size. These results may
lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small
perturbations in the droplet laser cavity and the gain medium.
The use of optical tweezers for the analysis of aerosols is valuable for understanding the dynamics of atmospherically
relevant particles. However to be able to make accurate measurements that can be directly tied to real-world phenomena
it is important that we understand the influence of the optical trap on those processes. One process that is seemingly
straightforward to study with these techniques is binary droplet coalescence, either using dual beam traps, or by particle
collision with a single trapped droplet. This binary coalescence is also of interest in many other processes that make use
of dense aerosol sprays such as spray drying and the use of inhalers for drug delivery in conditions such as asthma or hay
fever. In this presentation we discuss the use of high speed (~5000 frames per second) video microscopy to track the
dynamics of particles as they approach and interact with a trapped aqueous droplet and develop this analysis further by
considering elastic light scattering from droplets as they undergo coalescence. We find that we are able to characterize
the re-equilibration time of droplets of the same phase after they interact and that the trajectories taken by airborne
particles influenced by an optical trap are often quite complex. We also examine the role of parameters such as the salt
concentration of the aqueous solutions used and the influence of laser wavelength.
High density micron sized aerosols from liquid surfaces were generated using surface acoustic wave (SAW)
nebulisation. The SAWs are made from a set of interdigitated electrodes (IDT) deposited on a lithium niobate (LiNbO3)
substrate and are designed to operate around 10MHz. RF powers of ~235mW are used to achieve nebulisation. Power
below this results in droplet motion across the substrate surface. The nebulisation process generated aerosols of a narrow
size distribution with diameter ranging from 0.5-2 μm. We consider ways in which these aerosols can be loaded into
optical traps for further study. In particular we look at how SAW nebulisation can be used to load particles into a trap in
a far more robust manner than a conventional nebuliser device. We demonstrate trapping of a range of particle types and
sizes and analyse the size distribution of particles as a function of the applied frequency to the SAW device. We show
that it is simpler to load, in particular, solid particles into optical traps using this technique compared to conventional
nebulisation. We also consider the possibilities for loading nanoparticles into aerosol optical tweezers.
We demonstrate that light can be used to create microchannels in ice. We make use of free space and fiber coupled
infrared laser light to produce microchannels with diameters down to 100 microns in diameter. We demonstrate that the
channels can be created in a timescale of seconds and that by controlling the input power that they can be stabilized over
a timescale of several minutes using powers as low as 30mW. We compare the fiber coupled geometry, using both single
mode and multimode fiber and free space coupling and show that fiber coupling produces optimal results. We
demonstrate that liquid samples can be inserted into the channels and particle movement is observed using a combination
of optical and thermally induced forces. We also present data looking at droplet freezing within the microchannels. We
present preliminary results looking at dual beam coupling into such optofluidic channels and examine prospects for using
such channels as rapid microfluidic prototypes. We further discuss the possibility of using optically shaped ice channels
as a means to study aerosol nucleation processes and the ability of ice to act as a template for microfluidic devices.
In this work, three-dimensional manipulation of multiple defects and structures is performed in the framework of
holographic optical trapping approach using a spatial light modulator. A holographic optical tweezers system is
constructed using a liquid crystal spatial light modulator to generate multiple optical traps. We optimize the tweezers
setup to perform polarization-sensitive holographic optical trapping and then explore properties of optical trapping in
thermotropic liquid crystals and compare them to the case of isotropic fluids. One of the major challenges complicating
the quantitative measurements in these fluids is the anisotropic nature of the liquid crystal medium, which makes the
tight focusing of the laser beam difficult and considerably weakens optical trapping forces. Using liquid crystals with
low birefringence allows us to mitigate these artefacts. Optical trapping forces and the trap stiffness are first calibrated
for different laser powers using viscous drag forces. This is then used to probe inter-particle and defect-particle
interaction forces as well as to characterize tension of line defects in the bulk of liquid crystals.
It is shown experimentally that light emerging from two pinholes, after passing through two diffusers having same correlation properties but moving in opposite directions with same linear speed, is cross-spectrally-pure light. It is found that the visibility of the interference fringes is related to the average diffuser surface roughness
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