Raman spectroscopy is a useful technique in the identification and characterisation of compounds, but in terms of
sensitivity its application is limited. With respect to this the discovery of the surface-enhanced Raman scattering (SERS)
phenomenon has proved monumental, and much research has been carried out over the past 30 years developing the
technique.
Pterins are biological compounds that are found in nature in colour pigmentation and in mammalian metabolic pathways.
Moreover, they have been identified in abnormal concentrations in cancer patients, suggesting potential applications in
cancer diagnostics.
SERS is an ideal technique to identify these compounds, and both nanoparticle suspensions and pulsed laser deposited
nanoparticle substrates have been used to examine the spectra of xanthopterin, both in aqueous solution and in different
pH environments.
Conical refraction produces the well-known ring profile when circularly polarised light is incident on a biaxial crystal.
Conical diffraction of linearly polarised light in a biaxial crystal produces a beam with a crescent-shaped intensity
profile. Rotation of the plane of polarisation of the incident light produces the unique effect of spatially moving the
crescent-shaped beam around a ring. We use this effect to trap microspheres and white blood cells and to position them
at any angular position on the ring. Continuous motion around the circle is also demonstrated by rotating the input linear
polarisation. The ability to spatially locate a beam and an associated trapped object simply by varying the polarisation of
light suggests that this optical process should find application in the manipulation and actuation of micro- and nano-scale
physical and biological objects.
Internal conical refraction leads to the formation of zero (J0) and first order (J1) Bessel beams in superposition. The
(J0) beam retains the input circular polarisation and the (J1) has opposite polarisation but with a single phase change
around the beam axis giving it &barh; optical angular momentum per photon. This results in the conical beam having ½ &barh;
net optical angular momentum per photon. This provides a simple system in which a beam of 0, ½ and &barh; optical
angular momentum can be easily generated and selected with use of only a circular polariser. In the far field the
characteristic Bessel beam structures are formed and can be made non-diverging with use of a lens. We report the
formation of non-diverging Bessel beam of core diameter (a) of 5.7μm over a maximum non-diverging core length of
1(±.05)mm. However due to the fine structure of the conical beam at its beam waist position two cores are produces
and are of opposite phase.
For many applications of pulsed laser ablation it is necessary to have an understanding of the expansion dynamics of the
ablation plume both in vacuum and in low pressure gases. Knowledge of the ablation plume hydrodynamics can also
contribute to the understanding of the laser ablation process. In this paper we will consider some of the existing
theoretical models of laser ablation plume expansion and draw some conclusions as to which model is most appropriate
for the low temperature plasmas which arise in pulsed laser deposition.
For ablation plumes which are significantly ionised, Langmuir probes have proved to be a relatively simple and
inexpensive tool for measuring the plume shape, ion energy distribution and electron temperature. We describe some
recent work on the development of Langmuir probes for laser ablation plume diagnosis. Typically in laser ablation
plasma the flow velocity is supersonic, which complicates the interpretation of the I-V probe characteristic. We describe
some new work on the behaviour of a flat probe lying parallel to the plasma flow. For nanosecond ablation of silver, we
also show how a planar Langmuir probe can be used to obtain a fairly comprehensive description of the expansion
dynamics of the ionised part of the ablation plume, including plume shape, ion energy distribution and electron
temperature.
A 5-fold enhancement in the luminescence of CdTe nanocrystal quantum dots (QDs) is observed when they are placed in proximity to a nanostructured Au film deposited by pulsed laser deposition technique. No enhancement is observed with a nanostructured Ag film. The enhancement is due to the interaction of the QDs excitons with the localized surface plasmons (LSP). The Au surface plasmon (SP) frequency is closer to the QDs emission frequency than Ag LSP frequency and this accounts for the differences in observed behavior. As the SP-QD interaction strongly depends on the geometric structure and shape of the metal nanoparticles, a comparison with QDs deposited on a film of Au colloidal nanoparticles is presented. In the case of QDs placed directly on the Au colloids the luminescence quenching is much stronger and with a spacer layer a 3.5-fold enhancement over the bare QDs luminescence is observed.
A time-resolving Langmuir probe has been used to study the plasma plumes produced by ablation of silver with 200 femtosecond laser pulses at fluences of 1-12 J cm-2 at a central wavelength of 775 nm. Initial results have shown that surface contamination, and subsequent recontamination, can significantly influence the time of flight (TOF) signals obtained using the Langmuir probes. Surface conditioning techniques have been developed to overcome these influences. The TOF signals have been used to establish that the threshold fluence for the laser produced plasma in silver, under the present operating conditions, occurs at 1.04 J cm-2. The angular dependence of the magnitude of the ion yields and energies, at the time when the ion flux is maximized, agree with the predictions of Anisimov’s self-similar isentropic model of the plasma expansion.
Thin films of wurtzite Ga1-xMnxN have been grown by pulsed laser deposition from a range of Mn-doped-ceramic targets (x = 0.005-0.10). The effect of varying the substrate temperature, background nitrogen pressure and deposition time on the Mn concentration in the deposited films has been studied. Film thickness and surface roughness were monitored during deposition by in situ optical reflectometry. X-ray diffraction measurements showed that under optimal deposition conditions, the films were single phase and epitaxial. Room temperature ferromagnetism was observed over the whole range of Mn concentration, though it was observed that the moment per Mn ion increases as the concentration is reduced.
Thin films of ZnO and GaN have been deposited by pulsed laser deposition in atmospheres of oxygen and nitrgoen respectively. A time-of-flight ion probe and optical spectroscopy were used to study the interaction of the ablation plasma with the background gas. The deposition rate was measured using in situ optical reflectivity, and the thin film quality was assessed using x-ray diffraction and photoluminescence. By correlating the plasma measurements and the thin film characterization it was possible to identify the plasma regime required for the deposition of good quality films.
The optimized optical design of GaN resonant cavity light emitting didoes (RCLEDs) emitting at 510nm for maximum extraction efficiency into numerical apertures (NAs) of 1.0 (total emission) and 0.5 (typical plastic optical fiber NA) are determined using a modeling tool based on the simulation of dipole emission in a multilayer structure. The optimization is performed for a metal-AlGaN/GaN DBR cavity structure as functions of the aluminum fraction in the DBR and the internal quantum well (QW) emission linewidth. The optimum number of DBR pairs is shown to depend on both these parameters together and the emission NA, and varies between 3 and 14. The maximum calculated extraction efficiency for a metal-AlN/GaN cavity structure, assuming a QW emission linewidth of 30nm, is 0.18 (0.055) into an emission NA of 1.0 (0.5). The position of the QW relative to the metal mirror is shown to be the crucial device parameter in determining the extraction efficiency of the RCLED. Simulations show farfield measurements should provide information on the position of the QWs in the cavity. The reduction in the spectral emission linewidth of the RCLED due to the cavity is also modeled.
It is suggested that the measurement of the angular distribution of light transmitted through a thick slab of diffusively scattering material may provide information about the nature of the local scattering events. Monte Carlo simulations indicate that the angular distribution depends on ℓ/ℓ* where ℓ and ℓ* are the true and transport mean free paths, respectively, and this dependence is confirmed analytically using a modified diftusion model. Preliminary experimental results for liquid and solid foams are explicable in terms of strong forward scattering in the individual scattering events.
In situ optical reflectometry at 670 nm is used to monitor the pulsed laser deposition of thin films and multilayers of copper and cobalt. By comparing measured reflectivity with a Fresnel calculation, which includes a Bruggemann description of the dielectric function for partially dense material, the nature of the early stages of the deposition of copper on cobalt can be discerned. It seems that the deposition begins with the nucleation of a partially dense layer, and the formation of fully dense material commences after a deposition corresponding to about 0.3 nm of Cu. Thereafter the thickness of the fully dense layer increases while a partially dense surface layer is maintained. The experiment demonstrates that optical reflectometry can be an effective thickness monitor for the deposition of ultrathin metal films and multilayers and also yields some new information on the nature of the growth.
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