Thesis (Ph. D.)--Ohio State University, 2009. / Title from first page of PDF file. Includes vita. Includes bibliographical references (p. 136-142).

This thesis describes the experimental and theoretical work that investigated the transfer of orbital angular momentum from light to matter. This was achieved by combining two established areas of laser physics which were "optical tweezers" and Laguerre-Gaussian laser modes. The optical tweezers are essentially a tightly focussed laser beam from a high numerical aperture microscope objective lens, which traps particles in three dimensions just below the beam focus. By incorporating a Laguerre- Gaussian laser mode into the tweezers system, the trapping efficiency was doubled. These improved optical tweezers have been successfully demonstrated both theoretically and experimentally. In addition to the spin angular momentum which is associated with the polarisation state, the Laguerre-Gaussian laser modes also possess orbital angular momentum. The "optical spanners" utilised this property by transferring orbital angular momentum from the laser beam to the trapped particle, causing it to rotate whilst being held in the optical trap. This effect was theoretically modelled and experimentally observed. Using the optical spanners, the spin angular momentum of the laser was used to directly cancel the orbital angular momentum in the beam, which was observed as a cessation in rotation of the trapped particle. This demonstrated the mechanical equivalence of the spin and orbital components of angular momentum in a light beam, and gave experimental evidence for the well defined nature of the orbital angular momentum present in Laguerre-Gaussian laser modes.

Since its inception in 1970, optical manipulation has evolved into a versatile tool across many fields of science. Notably, the now widely employed optical tweezers invented in 1986 is a good example, which is in essence a strongly focused fundamental Gaussian beam. Although the optical tweezers remained as an important tool in optical manipulation, the shaped structured light such as an optical vortex beam also provides unusual light patterns and promotes exciting discoveries. This thesis is devoted to some unsolved theoretical aspects of optical manipulation. Since optical force acting on a micro-particle is typically on the order of pN and seldom larger than nN, it is a common belief that optical force is relevant in particle manipulation only when all other forces are comparable or smaller than the optical force. In chapter 2, surprisingly we showed that this is not always the case. Here, we find that under appropriate condition, optical vortices can make a sphere orbit around the beam center owing to the non-conservative optical force. If the sphere is attached to a mechanical spring, the spring can be stretched significantly even when the mechanical spring is orders of magnitude stronger than the optical force. Since its inception in 1970, optical manipulation has evolved into a versatile tool across many fields of science. Notably, the now widely employed optical tweezers invented in 1986 is a good example, which is in essence a strongly focused fundamental Gaussian beam. Although the optical tweezers remained as an important tool in optical manipulation, the shaped structured light such as an optical vortex beam also provides unusual light patterns and promotes exciting discoveries. This thesis is devoted to some unsolved theoretical aspects of optical manipulation. Since optical force acting on a micro-particle is typically on the order of pN and seldom larger than nN, it is a common belief that optical force is relevant in particle manipulation only when all other forces are comparable or smaller than the optical force. In chapter 2, surprisingly we showed that this is not always the case. Here, we find that under appropriate condition, optical vortices can make a sphere orbit around the beam center owing to the non-conservative optical force. If the sphere is attached to a mechanical spring, the spring can be stretched significantly even when the mechanical spring is orders of magnitude stronger than the optical force

Studies of light-matter interactions in organic semiconductors and in optical tweezer trapping of nanoparticles are presented. In the research related to organic semiconductor materials, a variety of novel materials and their composites have been characterized, and physical mechanisms behind their optoelectronic properties have been established. Three novel functionalized hexacene derivatives were deemed sufficiently stable to enable characterization of these materials in devices. From dark current and photocurrent measurements of the hexacene thin-films, it was determined that all three derivatives are photoconductive in the near-infrared, and space charge limited mobility values were obtained. In addition, physical mechanisms behind charge transfer, charge carrier photogeneration, and charge transport in small-molecule donor/acceptor composite films have been systematically studied. In these studies, it was determined that the charge transfer from the donor to the acceptor molecule can result in either an emissive charge transfer exciton (exciplex) or a non-emissive charge transfer exciton formation, depending on the energy difference between LUMO of the donor and the acceptor. However, the most dramatic trends in photoluminescent and photoconductive properties of the donor/acceptor composites were correlated with the separation between the donor and acceptor molecules at the donor/acceptor interface. In particular, composite films with larger separations exhibited electric field-assisted charge transfer exciton dissociation, which contributed to nanosecond time-scale photocurrents under a 500 ps pulsed photoexciation. Large donor/acceptor separation also resulted in reduced charge carrier recombination, which led to a factor of 5-10 increase in continuous wave photocurrents in certain donor/acceptor composites, as compared to those in pristine donor films. In the optical tweezer based studies, work towards the development of high spatial resolution optical tweezer controlled nanoprobes is presented. In particular, the possibility of exploiting the optical resonance of a particle to increase the optical tweezer forces acting on it within the trap has been investigated. Such an increase in the force would improve the potential spatial resolution of an optical tweezer controlled probe. Experimental results and numerical simulations on micron sized resonant dielectric particles showed a small increase in the optical forces that confine such particles within the trap, when tweezer trapping is conducted at wavelengths on the red-side of the optical resonance. Preliminary work on optical tweezer controlled ion/pH sensitive probes and on surface charge measurements is also reported. / Graduation date: 2012

Raman microscopy is a tool used by physicists to collect molecular information from a wide variety of samples. Biophysicists have increasingly made use of Raman microscopy in combination with optical tweezers to identify the molecular makeup of structures inside cells. There are high levels of background and noise in Raman spectra from cells, however, that obscure low intensity scattering peaks and prevent complete molecular characterization. We have designed and built a Mechanical Modulation Raman Microscope(MMRM) that is capable of background subtraction and noise reduction for Raman spectra from cells in vivo. There are two mechanisms of modulation: (1) three-axis stage modulation for objects fixed to the coverslip and (2) separate optical trap modulation for objects in solution. In both cases, objects of interest are modulated in and out of the Raman excitation volume while spectra are collected. Difference spectra are created by subtracting the spectrum without the object from the spectrum including the object. These difference spectra are averaged over the number of cycles of modulation. With the mechanical modulation technique, the background in Raman spectra is removed, and the signal-to-noise ratio is improved by two orders of magnitude. This technique was applied to fission yeast cells. Mechanical modulation Raman spectra of exponentially growing cells and starved cells were collected in three dimensions, and spatial differences were observed in the molecular composition for different metabolic states of individual yeast cells. / text

Thesis (Ph.D.) - University of St Andrews, April 2010.

Rheology is a discipline of continuum mechanics that is concerned with the mechanical properties of matter as it flows. Key to the study of rheology is the concept that materials do not behave as Newtonian liquids of as heterogenous, homogenous mateirials, but as a combination of the two. This combination and blurring of the line between liquid and solid peoperties is knows as viscosity. Furthermore, the viscosity of a material or liquid will not necessarily remain constant when it is subjected input forces or stresses at different frequencies. This consideration brings with it the idea of viscoelasticity which can account for the variations in the characteristics of a sample medium.Magnetic tweezers are tools that allow examination of and investigation into the viscoelastic properties of a sample on the mesoscopic scale. Magnetic matter can be inserted into and bound onto a sample. This magnetic matter can then be manipulated using an external magnetic or electromagnetic field. Calibrated magnetic tweezers apparatus can be used to investigate the mechanical and viscoelastic properties of a material with the novel application of time-variant forcesand stresses. The resultant behaviour, or response, of the sample can be observed using a microscope and analysed further.Fibrin is the highly extensible, fibre-like protein that makes up blood clots. Its particularly high levels of extensibility combined with interesting material properties such as viscoelasticity can strain-hardening make it an ideal test sample for magnetic tweezers experiments. The high elastic limit of fibrin ensures that plastic deformation does not usually occur under the range of input forces and stresses exerted by magnetic tweezers. This allows non-destructive and repeatable tests to be performed.Magnetic tweezers have been developed and used in a series of experiments on fibrin to produce a viscoelastic characterisation of the fibrous networks. The key results in this work are the design of high-sensitivity apparatus for the experiments and associated techniques for high-frequency analysis, use of the tweezers with a high-speed CCD attached to a microscope and the analysis of the viscoelastic properties of fibrin over a several decades of frequency.

No description available.

This thesis describes the construction of an Optical Tweezer apparatus to be used in conjunction with a confocal Raman spectrometer. The tweezer utilizes an infrared (λ=1064 nm) laser directed into an inverted microscope with NA=1.4 oil immersion 100x objective lens that strongly focuses the laser light into a sample to function as a single-beam gradient force trap. The long term goal of this research program is to develop a single molecule Raman tweezers apparatus that allows one to control the position of a Raman nanoplasmonic amplifier. This thesis describes the construction of the Raman tweezer apparatus along with several Raman spectra obtained from optically trapped samples of polystyrene fluorescent orange, amine-modified latex beads. In addition, I explored the Raman spectra of bulk cytochrome c mixed with or injected onto Ag aggregates for SERs enhancement.

Longitudinal optical binding refers to the light induced self organisation of micro particles in one dimension. In this thesis I will present experimental and theoretical studies of the separation between two dielectric spheres in a counter-propagating (CP) geometry. I will explore the bistable nature of the bound sphere separation and its dependency on the refractive index mismatch between the spheres and the host medium, with an emphasis on the fibre separation. The physical under pining principle of longitudinal optical binding in the Mie regime is the refocusing effect of the light field from one sphere to its nearest neighbour. In a second set of experiments I developed means to visualise the field intensity distribution responsible for optical binding using two-photon fluorescence imaging from fluorescein added to the host medium. The experimental intensity distributions are compared to theoretical predictions and provide an in situ method to observe the binding process in real time. This coupling via the refocused light fields between the spheres is in detailed investigated experimentally and theoretically, in particular I present data and analysis on the correlated behaviour of the micro spheres in the presence of noise. The measurement of the decay times of the correlation functions of the modes of the optically bound array provides a methodology for determining the optical restoring forces acting in optical binding. Interestingly micro devices can be initiated by means of the light-matter interaction. Light induced forces and torques are exerted on such micro-objects that are then driven by the optical gradient or scattering force. I have experimentally investigate how the driving light interacts with and diffracts from the motor, utilising two-photon imaging. The micromotor rotation rate dependence on the light field parameters is explored and theoretically modelled. The results presented will show that the model can be used to optimise the system geometry and the micromotor.