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Papers by David Andrews

Proceedings of SPIE, Nov 1, 2002

Conference Digest. 2000 International Quantum Electronics Conference (Cat. No.00TH8504)

In several studies of second harmonic emission from isotropic suspensions it has been shown that ... more In several studies of second harmonic emission from isotropic suspensions it has been shown that the unusually strong signal detected from some particles, notably purple membrane material, is attributable to optical coherence within the separate particles of the suspension. As such, the emission displays an amalgam of the characteristics associated with full coherence (second harmonic generation) and incoherence (hyper-Rayleigh scattering). The principle of local additivity for the hyperpolarisabilities associated with different optical centres or chromophores within such systems leads to intriguing possibilities for materials strongly pumped by an ultrafast source. The key feature is the relationship between the hyperpolarisabilities of optical centres in their ground and electronic excited states under conditions of resonance with the applied radiation, a result which is derived from a full quantum electrodynamical treatment of the interaction. As a consequence of this relationship, the effective second order susceptibility of each domain or particle proves to be very strongly influenced by the instantaneous degree of optical excitation, and the harmonic signal acquires a temporal signature which faithfully registers the dynamics of optical excitation and decay. Thus, where a significant degree of optical excitation is established in such a system by a primary laser source, studying the characteristics of the second harmonic generated by a probe beam offers the means for direct measurement of the excited state population dynamics. It is shown that the effect is strongly dependent on the nature of the excited state damping, whose correct representation in the expressions for hyperpolarisability is crucial for analysis of the results.

2003 European Quantum Electronics Conference. EQEC 2003 (IEEE Cat No.03TH8665), 2003

ABSTRACT

Complex Mediums V: Light and Complexity, 2004

SPIE Proceedings, 2005

We examine novel features that might emerge from the interaction of Laguerre-Gaussian beams with ... more We examine novel features that might emerge from the interaction of Laguerre-Gaussian beams with liquid crystals. We study the response of nematic liquid crystal media to the throughput of twisted laser light. Specific attention is focused on the spatial evolution of the director orientation angle.

SPIE Proceedings, 2010

Optical binding is a phenomenon that is exhibited by micro-and nano-particulate systems, suitably... more Optical binding is a phenomenon that is exhibited by micro-and nano-particulate systems, suitably irradiated with offresonance laser light. Recent quantum electrodynamical studies have shown that the optomechanical effect owes its origin to a radiative intervention with the Casimir-Polder dispersion force. The potential energy surfaces for optically induced inter-particle coupling reveal unexpected features of considerable intricacy, and when several particles are present, the effect can result in the formation of geometrically varied non-contact assemblies. In general, previous studies have been restricted to considering only the dynamic electromagnetic coupling between particles, where the latter are considered to be non-polar and centrosymmetric. However, when optical binding between non-polar particles takes place, other forms of interaction need to be entertained - more especially so, since any presence of a permanent dipole moment necessarily also admits a non-zero hyperpolarizability. Consequently, amongst the static contributions to the interaction between any pair of particles, a coupling between the electric dipole of one and the hyperpolarizability of the other must also be considered. In this paper we study these static contributions to the overall optical binding, comparing their effect with other inter-particle interactions, particularly the prominent electric dipole-dipole coupling. The results suggest that static coupling between polar particles can significantly modify the observed optical binding.

Optical Devices and Diagnostics in Materials Science, 2000

ABSTRACT

Nonlinear Spectroscopy, 2002

SPIE Proceedings, 2008

ABSTRACT Optical binding is a phenomenon that is exhibited by micro-and nano-particles systems, s... more ABSTRACT Optical binding is a phenomenon that is exhibited by micro-and nano-particles systems, suitably irradiated with off-resonance laser light. When several particles are present, the effect commonly results in the formation of particle assemblies. In the optically induced potential energy surfaces responsible for such assembly formation, the location and intensity of local energy maxima and minima depend on the particle configurations with respect to the input beam polarization and Poynting vector. This paper reports the results of recent quantum electrodynamical studies on the energy landscapes for systems of three and more particles; the analysis of local minima allows determination of the energetically most favorable positions, and it shows how the addition of further particles subtly modifies each energy landscape. The analysis includes the identification and characterization of potential points of stability, as well as the forces and torques that the particles experience as a consequence of the throughput electromagnetic radiation. As such, the development of theory represents a rigorous and general formulation paving the way towards a fuller comprehension of nanoparticle assembly based on optical binding.