Dennis R. Alexander

Theoretical determination of net radiation force and torque for a spherical particle illuminated by a focused laser beam

Series expressions for the net radiation force and torque for a spherical particle illuminated by an arbitrarily defined monochromatic beam are derived utilizing the spherical‐particle/arbitrary‐beam interaction theory developed in an earlier paper. Calculations of net force and torque are presented for a 5‐μm‐diam water droplet in air optically levitated by a tightly focused (2 μm beam waist diameter) TEM00‐mode argon‐ion (λ=0.5145 μm) laser beam for on and off propagation axis, and on and off structural resonance conditions. Several features of these theoretical results are related to corresponding experimental observations.

Fifth‐order corrected electromagnetic field components for a fundamental Gaussian beam

Fifth‐order corrected expressions for the electromagnetic field components of a monochromatic fundamental Gaussian beam (i.e., a focused TEM00 mode laser beam) propagating within a homogeneous dielectric media are derived and presented. Calculations of relative error indicate that the fifth‐order Gaussian beam description provides a significantly improved solution to Maxwell’s equations in comparison with commonly used paraxial (zeroth‐order) and first‐order Gaussian beam descriptions.

Internal and near‐surface electromagnetic fields for a spherical particle irradiated by a focused laser beam

Theoretical expressions for the internal and external electromagnetic fields for an arbitrary electromagnetic beam incident upon a homogeneous spherical particle are derived, and numerical calculations based upon this theoretical development are presented. In particular, spatial distributions of the internal and near‐surface electric field magnitude (source function) for a focused fundamental (TEM00 mode) Gaussian beam of 1.06 μm wavelength and 4 μm beam waist diameter incident upon a 5‐μm‐diam water droplet in air are presented as a function of the location of the beam focal point relative to the sphere center. The calculations indicate that the internal and near‐surface electric field magnitude distribution can be strongly dependent upon relative focal point positioning and may differ significantly from the corresponding electric field magnitude distribution expected from plane‐wave irradiation.

Laser spark ignition and combustion characteristics of methane-air mixtures

Ignition breakdown kernels of methane-air mixtures initiated by laser-induced sparks and by conventional electric sparks arc compared during initial stages. Experiments were conducted using a four-stroke (Otto-cycle) single-cylinder typical high-pressure combustion chamber. The piston is cycled in the cylinder by using an electric motor driven hydraulic ram. An cxcimer laser beam, either produced from krypton fluoride gas (λ = 248 nm) or argon fluoride gas (λ = 193 nm), or a Nd:YAG laser beam (λ = 1064 nm) is focused into a combustion chamber to initiate ignition. Conventional electric spark ignition is used as a basis for comparison between the two different ignition methods and the resultant early breakdown kernel characteristics. A streak camera is used to investigate and record the initial stages of kernel formation. Both a breakdown and a radial expansion wave of the ignition plasma are observed for certain laser ignition conditions of methane-air mixtures under typical internal combustion (IC) engine conditions. Results indicate that only certain wavelengths used for producing laser ignition produce a radial expansion wave. Laser ignition kernel size is calculated and laser-supported breakdown velocity is calculated by using Raizers theory and is compared with measured results. Laser ignition results in a 4–6 ms decrease in the time for combustion to reach peak pressure than is obtained when using electric spark ignition in the same combustion chamber and under the same ignition conditions.

Internal fields of a spherical particle illuminated by a tightly focused laser beam: Focal point positioning effects at resonance

The spherical particle/arbitrary beam interaction theory developed in an earlier paper is used to investigate the dependence of structural resonance behavior on focal point positioning for a spherical particle illuminated by a tightly focused (beam diameter less than sphere diameter), linearly polarized, Gaussian‐profiled laser beam. Calculations of absorption efficiency and distributions of normalized source function (electric field magnitude) are presented as a function of focal point positioning for a particle with a complex relative index of refraction of n=1.33+5.0×10−6i and a size parameter of α≊29.5 at both nonresonance and resonance conditions. The results of the calculations indicate that structural resonances are not excited during the on‐center focal point positioning of such a tightly focused beam but structural resonances can be excited by proper on‐edge focal point positioning. Electric wave resonances were found to be excited by moving the focal point from on‐center towards the edge of the...

Formation of multiscale surface structures on nickel via above surface growth and below surface growth mechanisms using femtosecond laser pulses

The formation of self-organized micro- and nano-structured surfaces on nickel via both above surface growth (ASG) and below surface growth (BSG) mechanisms using femtosecond laser pulse illumination is reported. Detailed stepped growth experiments demonstrate that conical mound-shaped surface structure development is characterized by a balance of growth mechanisms including scattering from surface structures and geometric effects causing preferential ablation of the valleys, flow of the surface melt, and redeposition of ablated material; all of which are influenced by the laser fluence and the number of laser shots on the sample. BSG-mound formation is dominated by scattering, while ASG-mound formation is dominated by material flow and redeposition. This is the first demonstration to our knowledge of the use of femtosecond laser pulses to fabricate metallic surface structures that rise above the original surface. These results are useful in understanding the details of multi-pulse femtosecond laser interaction with metals.

Extraordinary shifts of the Leidenfrost temperature from multiscale micro/nanostructured surfaces.

In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost temperature, significant enhancement of the heat transfer in the film boiling regime was also observed for the laser-processed surfaces; water droplet evaporation times were reduced by up to 33% for a surface temperature of 500 °C.

All-Optical Network Coding

We investigate the application of network coding to all-optical networks from both the algorithmic and infrastructural perspectives. We study the effectiveness of using network coding for optical-layer dedicated protection of multicast traffic that provides robustness against link failures in the network. We present a heuristic for solving this problem and compare it with both inefficient optimal methods and non-network-coding approaches. Our experiments show that our heuristic provides near-optimal performance while significantly outperforming existing approaches for dedicated multicast protection. We also propose architectures for specialized all-optical circuits capable of performing the processing required for network coding and show how these devices can be effectively deployed in an all-optical multicast network.

Investigation of femtosecond laser assisted nano and microscale modifications in lithium niobate

A study of the physicochemical modifications at micro and nano scales as a result of femtosecond laser processing is essential to explore the viability of this process to write surface and subsurface structures in transparent media. To this end, scanning probe and transmission electron microscopy and spectroscopy techniques were used to study these modifications in lithium niobate. A variable power Ti:Sapphire system (800nm,300fs) was used to determine the ablation threshold of (110) lithium niobate, and to write these structures in the substrate for subsequent analysis. Higher processing energies were used to amplify the laser-induced effects for a clear understanding. Evidences of a number of simultaneously occurring mechanisms such as melting, ablation, and shockwave propagation are observed in the scanning electron microscope (SEM) micrographs. X-ray diffraction (XRD), Auger and electron dispersive spectroscopy (EDS) studies indicate loss of lithium and oxygen from the immediate surface of the process...

Influences on Concentration Measurements of Liquid Aerosols by Laser-Induced Breakdown Spectroscopy

Results are presented for laser-induced breakdown spectroscopy (LIBS) as a method for measuring salt concentrations in seawater aerosol droplets. An excimer laser, operating at high pulse energy with KrF gas (λ = 248 nm) produces laser-induced breakdown plasmas in an aerosol spray. Emission lines of Na and H are monitored with an optical multichannel analyzer to characterize the plasma spatially and temporally. Studies of temporally resolved atomic line emissions from the plasma determine the optimum time for gating of the detector to be 2–4 μs after the excimer laser pulse arrives in the probe volume. Spatially resolved measurements of atomic emission line intensities are studied by positioning a stream of monodisperse droplets at various locations relative to the measurement probe volume. The electron temperature of the plasma is estimated to be 12,600 ± 4600 K, averaged over 1700 measurements during a 100-ns interval 2 μs after breakdown. Calibration curves are presented relating the Na(I) 589-nm to Hα 656.3-nm intensity ratio as a function of Na concentration, ranging from 100 to 10,000 ppm. Limits of detection for Na by the current method under the experimental conditions are estimated to be approximately 165 ppm for monodisperse sprays and 925 ppm for one case involving a polydisperse spray. Droplet diameter strongly influences the observed emission intensity ratio.