We have demonstrated chemical transformation in single microscopic-sized aerosol droplets localised in optical tweezers. Droplets in situ are measured during chemical transformation processes of solvent exchange and solute transformation through an ion exchange reaction. Solvent exchange between deionised water and heavy water in aerosol droplets is monitored through observation of the OH and OD Raman stretches. A change in solute chemistry of aerosol is achieved through droplet coalescence events between calcium chloride and sodium carbonate to promote ion exchange. The transformation forming meta-stable and stable states of CaCO3 is observed and analysed using Gaussian peak decomposition to reveal polymorphs.
Standard Raman spectroscopy (SRS) is a noninvasive technique that is used in the biomedical field to discriminate between normal and cancer cells. However, the presence of a strong fluorescence background detracts from the use of SRS in real-time clinical applications. Recently, we have reported a novel modulated Raman spectroscopy (MRS) technique to extract the Raman spectra from the background. In this paper, we present the first application of MRS to the identification of human urothelial cells (SV-HUC-1) and bladder cancer cells (MGH) in urine samples. These results are compared to those obtained by SRS. Classification using the principal component analysis clearly shows that MRS allows discrimination between Raman spectra of SV-HUC-1 and MGH cells with high sensitivity (98%) and specificity (95%). MRS is also used to distinguish between SV-HUC-1 and MGH cells after exposure to urine for up to 6 h. We observe a marked change in the MRS of SV-HUC-1 and MGH cells with time in urine, indicating that the conditions of sample collection will be important for the application of this methodology to clinical urine samples.
Two counter-propagating Bessel beams are used to create an optical trap to confine polydisperse aerosol droplets. A
single arm can be used to optically guide droplets over macroscopic distances. Two opposing beams create a trapping
region to optically confine particles over distances of 4mm. Droplets are optically trapped in the surrounding rings and
the central core and are characterised using light scattering techniques. The elastically scattered fringe spacing from the
532nm trapping beam and from a 633nm probe beam are used to independently size droplets using Mie theory, as well as
assessing the size from glare spots.
The dual beam fiber trap is an important tool in the field of optical micromanipulation. The characteristics of these traps
are governed by the fibers used. Photonic crystal fibers have emerged in recent years and may be engineered to have vastly
different properties to conventional fibers. In particular, endlessly single-mode photonic crystal fiber (ESM-PCF) will
guide any wavelength of light in a single-mode and is commercially available in core diameters up to 35 μm. By utilising
these unique properties, we show that it is possible to create novel dual beam ESM-PCF traps for micron size particles.
Firstly, we characterise an ESM-PCF trap when using a near-infrared laser coupled into a 25 μm core fiber when trapping
a sample in a square capillary. We calculate the trap stiffness for polymer micro-spheres and show that aside from the
expected confining potential, it is possible to create line and repulsive potentials. Interference effects due to the capillary
are observed. Secondly, we create a dual wavelength standing wave trap which can selectively move two sizes of particles
in an optical conveyor belt. Finally, we use a supercontinuum source to create the first white light dual beam fiber trap and
show that the low coherence length of the source results in interference free potentials. Overall PCF has great promise for
future studies.
The interaction between light and particles in an optical trap provides a tool for physicists to study
Brownian motion and dynamics for objects in the micron to nano-sized regime. In this paper, we
describe how the forward scattering light field from micro-particles trapped in an optical vortex
beam provides a means of studying the helicity and coherence of the trapping beam. Hence, optical
trapping may be implemented to study of the relative phase and spatial coherence of two points in
trapping light fields with arbitrary wavefronts.
We demonstrate the use of supercontinuum radiation to provide enhanced guiding distances of microscopic particles
compared to the standard continuous wave or femtosecond lasers. Our technique relies on the chromatic aberration of the
lens used to form an elongated focal region within which guiding takes place. The resulting beam profile has been
modelled and shows that for a Gaussian input beam, the intensity profile after the lens can be considered as a sum of
Gaussians, one for each wavelength but with varying focal position due to dispersion. Our experimental investigations
compare radiation from continuous wave (bandwidth <1nm) and femtosecond pulsed (bandwidth > 100nm) lasers as
well as supercontinuum radiation (bandwidth > 450nm) and show good agreement with theory.
We demonstrate that counter-propagating light fields have the ability to create self-organized one-dimensional optically bound arrays of microscopic particles, where the light fields adapt to the particle locations and vice versa. We are able to create chains of up to 9 particles with only modest laser power. We outline the experimental observation of this phenomenon examining the effect of laser wavelength (780nm and 1064nm) and particle size (1, 2.3 and 3 micron diameter sphere sizes) on the interparticle separation. We develop a theoretical model to describe this situation making use of the beam propagation method to calculate the fields. Using the fields we are able to calculate the gradient and scattering forces experienced by the particles. Equilibrium positions in these forces indicate the predicted positions of the spheres. We find good agreement between the theory and experimental data for two and three particles, if the scattering force is assumed to dominate the axial trapping of the particles. We discuss the limitation of the model when dealing with spheres size of the order of the wavelength of light involved and also the experimental uncertainties relating to the measurement of the laser beam waist separations. The extension of these ideas to two and three dimensional optically bound states is also discussed.
Optical guiding of micron-sized particles is shown using both Gaussian and zeroth-order Bessel light beams. Axial and transverse forces for guiding in both beams are calculated. Experiments show that the Bessel beam allows for extended guiding distances compared to a Gaussian beam, at the expense of guiding velocity.
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