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Nanoparticles are known to sinter at much lower temperatures than the corresponding bulk or micro size particles. A laser-assisted sintering process is considered in this study to sinter Ag nanoparticles by dispensing Ag paste onto an indium tin oxide-coated Si substrate. The Gaussian beam of a CO2 laser source is propagated through axicon and biconvex lenses, and the resulting hollow beam is focused on the Ag paste with a hollow parabolic mirror. A Bessel-Gaussian irradiance distribution is obtained at the focal plane of the parabolic mirror due to the interference of the hollow laser cone. The Fresnel diffraction approximation is considered to determine the phasor of the laser and an analytical approach is implemented to calculate the irradiance distribution of the Bessel-Gaussian beam. This irradiance distribution is utilized as a heat source in a heat conduction model and the temperature distribution is analyzed for thin Ag films formed during the laser sintering of Ag nanoparticles. An analytical expression is obtained for the temperature distribution by solving the heat conduction equation using Fourier transform for finite media. The widths of the deposited Ag lines are predicted from the temperature profiles and the model predictions compare well with the experimental results. The isotherms are found to be geometrically noncongruent with convex and concave tips depending on the locally maximum and minimum irradiances of the Bessel-Gaussian beam, respectively. The convex and concave tips, however, appear in the same isotherm for sufficiently high substrate speed relative to the laser beam.

Multiple modes of atomization in electrosprays are affected by viscosity, surface tension and electrical conductivity of the semiconductor nanosuspensions. While the effect of gravity is dominant in the dripping mode, the electric field degenerates the electrospray mechanism into a microdripping mode that can potentially allow the deposition of semiconductor nanodots on a substrate. Drop size and frequency of droplet formation are obtained as functions of non-dimensional parameters, which agree well with experimental data. The analysis shows that it is possible to produce the desired size and frequency of ejection of monodisperse droplets by manipulating the electrode voltage for any nanosuspension.

Nanoelectrospray laser deposition (NELD) of nanoparticles (NPs) on various substrates has attracted considerable attention as a fast, cost-effective, and scalable technique for precise control of heating time and zone. In this work, NELD-assisted sintering of titanium dioxide (TiO2) NPs on borosilicate glass and quartz substrates is addressed. A [Formula: see text] CO2 laser was used for patterning and sintering titania nanoparticles in ambient air. The effects of laser dose and deposition process parameters on the morphological, structural, and optical characteristics of the sintered TiO2 patterns were characterized using optical microscopy, scanning electron microscopy, and x-ray diffraction. The results point out that the anatase phase was preserved after laser sintering, without the appearance of any TiO2 rutile traces. We show that the improvement in the morphological properties of TiO2 patterns is due to the laser sintering of a dense layer of ceramic with enhanced interconnectivity and connection between single nanoparticles. A theoretical model was developed to select the temperature required to sinter TiO2 nanoparticles and to correlate it with the laser power and scanning speed to prevent cracking on the substrate and sintered nanoparticles and also to get transparent TiO2 films. An optical transmittance of [Formula: see text] was achieved. The experimental data were in accordance with the theoretical model, predicting the success of the model.

An analytical solution of a biharmonic equation is presented in axisymmetric toroidal coordinates for Stokes flow due to surface tension gradient on the free surface of sessile drops. The stream function profiles exhibit clockwise and counter-clockwise toroidal volumes. The ring or dot formed by the downward dividing streamlines between these volumes predicts the experimentally deposited particle ring or dot well. This finding suggests that the downward dividing streamline can be taken to be a reasonable indicator of where deposition occurs. Different light patterns directed at different locations of the droplet can give rise to a single spot or ring. A relationship between the positions of the light intensity peak and possible locations of particle deposition is analysed to demonstrate that the streamlines can be generated on-demand to achieve particle deposition at predetermined locations on the substrate. Toroidal corner vortices called Moffatt eddies have appeared in other corner flows and develop in this optical Marangoni flow as well near the contact line.

Metallization plays a very important role in fabricating low cost and high efficiency silicon solar cells. Silver (Ag) metallization of industrial crystalline silicon (c-Si) solar cells comprises a large share of the total cell production cost. However, in amorphous silicon based (a-Si:H) c-Si heterojunction solar cell (SHJ) fabrication, the metallization process costs more compared to the metallization process for traditional crystalline silicon (c-Si) solar cells since it requires expensive, low temperature Ag pastes [1]. These specialized, low temperature Ag pastes are required because the passivation quality at the a-Si:H(i)/c-Si interface degrades if annealed at temperatures above 200°C. This temperature limitation not only increases the fabrication cost, but also results in poor sintering of the metal contacts which increases the resistive loss [2]. In this work, we introduce a novel Ag metal contact printing technique for SHJ solar cells using a Ag nanoparticle ink and an in-line laser sintering process with the goal of reducing the bulk resistivity of the printed Ag without requiring any ex situ annealing. In this process, the Ag nanoparticle ink is dispensed through a very small diameter needle on the SHJ substrate with simultaneous sintering using a focused continuous carbon dioxide (CO 2 ) laser beam. We have been able to print Ag lines with width as low as 50 µm and bulk resistivity as low as 8 µΩ·cm using this printing technique.

Pursuing ever-smaller feature size in laser-based lithography is a research topic of vital importance to keep this technique competitive with other micro-/nano-fabrication methods. Features smaller than the diffraction-limited spot size can be obtained by "thresholding", which utilizes the deterministic nature of damage threshold with ultrashort laser pulses and is achieved by precisely tuning pulse energies so that only the central portion of the focal spot produces permanent modification. In this paper, we examine the formulation commonly used to describe thresholding and show that the relationship between feature size (r) and laser fluence (F) is invariant with respect to the nature of laser absorption. Verified by our experiments performed on metal, semiconductor, and dielectric samples, such invariance is used to predict the smallest feature size that can be achieved for different materials in a real-world system.

Particle transport through Marangoni convection inside a sessile droplet can be controlled by the UV light distribution on the surface. The photosensitive solution changes the surface tension gradient on the droplet surface and can induce clockwise and counter-clockwise circulations depending on the incident light distribution. In this paper, the stream function in the sessile drop has been evaluated in toroidal coordinates by solving the biharmonic equation. Multiple primary clockwise and counter-clockwise circulations are observed in the droplet under various concentric UV light profiles. The downward dividing streamlines are expected to deposit the particles on the substrate, thus matching the number of deposited rings on the substrate with the number of UV light rings. Moffatt eddies appear near the contact line or centerline of the droplet either due to a sharp change in the UV light profile or because the illuminated region is away from them.

Time-resolved, single-shot measurements are performed to determine the reflectance, transmittance, and absorptance in ultrafast laser interaction with polypropylene for a wide range of laser pulse energies. An ellipsoidal mirror is used to collect the majority of the reflected light, enabling the detection of plasma emission starting at about 40 ns after the incident pulse. The measured transmittance is explained by a model that takes into account different effective absorption channels, and the non-linear absorption coefficient is estimated, which suggests that the non-linear absorption originates from the two-step or two-photon absorption through overtone. The results are useful for selecting laser parameters in the processing of polymeric materials.

Polymers are important materials for both industrial and scientific applications. However, it is challenging to efficiently process polymers with an ultrafast laser due to their low melting point, high bandgap, and different absorption mechanics with different laser parameters. It is common practice in industries to use different kinds of lasers and, therefore, different laser parameters, such as pulse duration, wavelength, pulse energy, etc. In particular, the effect of pulse duration during ultrafast laser interaction with polymers is significant as the absorption mechanism can be different with different pulse durations. In this study, the effect of pulse duration is investigated during the ultrafast laser interaction with transparent polypropylene (TPP), which is an important polymeric material widely used in many industrial applications. This study is based on the experimental measurement of the optical properties of TPP during ultrafast interaction, where optical properties, i.e., reflectance, transmittance, and absorptance, are determined by performing time-resolved measurements in single-pulse configurations. This experiment is carried out by collecting in situ data of the reflection and transmission of each laser pulse in an ellipsoidal reflector-based experimental setup, which enables the collection of both specular and diffusive reflection with nearly full coverage, and absorption is calculated from the experimental results. It is found that TPP undergoes a dramatic morphological change with different pulse durations ranging from 167 fs to 1 ps, which is correlated with the change of optical properties during the ultrafast laser interaction with TPP for different pulse durations. This result will be useful for controlling the processing of polymers with ultrafast lasers for industrial applications.

Opto-thermal interaction between a laser beam and a nanosuspension droplet is studied in this paper to analyze the thermal response of the droplet at different laser intensities depending on the optical properties of the nanosuspensions. A pulsed Nd:YAG laser beam is used to irradiate four types of droplet, 20 wt % silver nanoparticle suspension, 10 wt % and 5 wt % germanium nanoparticle suspension and 8 mM SDS solution, in an electrospray laser deposition experiment to observe the geometrical changes in their shape. A high-speed camera captured the images of the droplets, showing the decrease in their diameter and inception time for droplet slow evaporation as well as the pre-explosion time and an explosion regime map of four different explosion patterns: shallow, deep, directional and isotropic explosions depending on the laser peak intensity for 20 wt % silver nanoparticle suspension microdroplet. The other three types of suspension, however, do not undergo slow evaporation or experience mid-air explosion. A theoretical conduction model has been developed to predict the explosion of the droplet by finding its bubble formation region inside the droplet. These results are corroborated by experiments that the explosion characteristics depend on the optical properties of the suspension such as reflectance, absorption coefficient and refractive index. The temperature distribution and the predicted bubble formation region for the four types of suspension are investigated, and only the silver nanoparticle suspension of 20 wt % with low reflectance and high absorption coefficient is predicted to have bubble formation patterns within the droplet, similar to the four explosion patterns in the experiment. The conduction model agrees well with the experimental results and shows that it can predict the threshold laser peak intensity and explosion patterns of a droplet a priori at which explosion occurs.

Material processing is a type of laser applications that requires good control of lasermatter interaction under the condition of high average laser power and high peak intensity. In this talk, I will present examples of applying optical sensing in ultrafast laser processing of polymetric materials to identify optimal processing parameter and achieve closed-loop operation.

A semi-analytical solution for the concentration of photosensitive suspension is developed in a hemispherical droplet illuminated with ultraviolet (UV) laser. A biharmonic equation in stream function is analytically solved using toroidal coordinates, which is used to solve the transport equation for concentration. Flow pattern and photosensitive material concentration are affected by the peak location of the UV light intensity, which corresponds to the surface tension profile. When the laser beam is moved from the droplet center to its edge, a rotationally symmetric flow pattern changes from a single counter clockwise circulation to a circulation pair and finally to a single clockwise circulation. This modulation in the orientation of circulation modifies the concentration distribution of the photosensitive material. The distribution depends on both diffusion from the droplet surface and the Marangoni convection. The region beneath the droplet surface away from the UV light intensity peak has low concentration, while the region near the downward dividing streamline has the highest concentration. When the UV light peak reaches the droplet edge, the concentration is high everywhere in the droplet.

Nanoscale patterns on rigid or flexible substrates are of considerable interest in modern nanophotonics and optoelectronics devices. Subwavelength structures are produced in this study by using a laser beam and microdroplets that carry nanoparticles to the deposition substrate. These droplets are generated from an aqueous suspension of nanoparticles by electrospray and dispensed through a conical hollow laser beam so that laser-droplet interactions occur immediately above the substrate surface. Each microdroplet serves the dual role as a nanoparticle carrier to the substrate and as a superlens for focusing the laser beam to a subwavelength diameter. This focused beam vaporizes the droplet and sinters the nanoparticles on the substrate. The deposition of subwavelength nanostructures and thin films on a silicon wafer are demonstrated in this paper. This process may be applied to produce novel nanophotonics and electronics devices involving a variety of subwavelength patterns including an ordered array of semiconductor nanoparticles.

Ultrafast lasers have been used for high-precision processing of a wide range of materials, including dielectrics, semiconductors, metals and polymer composites, enabling numerous applications ranging from micromachining to photonics and life sciences. To make ultrafast laser materials processing compatible with the scale and throughput needed for industrial use, it is a common practice to run the laser at a high repetition rate and hence high average power. However, heat accumulation under such processing conditions will deteriorate the processing quality, especially for polymers, which typically have a low melting temperature. In this paper, an analytical solution to a transient, two-dimensional thermal model is developed using Duhamel's theorem and the Hankel transform. This solution is used to understand the effect of laser parameters on ultrafast laser processing of polypropylene (PP). Laser cutting experiments are carried out on PP sheets to correlate with the theoretical calculation. This study shows that, in laser cutting, the total energy absorbed in the material and the intensity are two important figures of merit to predict the cutting performance. Heat accumulation is observed at low scanning speeds and high repetition rates, leading to significant heat-affected zone and even burning of the material, which is supported by experimental data and modelling results. It is found that heat accumulation can be avoided by a proper choice of the processing condition.

We report on the experimental and theoretical studies of ultrafast laser-induced optical breakdown on the surface of fused silica to elucidate the mechanism of damage formation and sub-optical-cycle dynamics in material processing using single and a burst of two femtosecond laser pulses. Ionization pathways, including photo-ionization (PI) and avalanche ionization (AI), are investigated by using single-beam and double-beam laser damage threshold measurements, which are used to analyze electron dynamics and extract the avalanche coefficient. The relationship between damage size and laser fluence is interpreted as a result of a combination of PI and AI. Electrical field rather than laser intensity is the fundamental influential factor in PI, and AI is found to play a significant role in creating the free electron density needed for optical breakdown. These findings are verified by a double-pulse delay-scan experiment where two cross-polarized pulses are used to induce damage with delay within a few optical cycles. Variation of the damage diameter is observed within one optical cycle, which is explained by the periodic change of polarization in the combined electric field. This finding shows the potential of controlling laser induced damage by tuning the temporal overlap of a burst of ultrashort laser pulses.

Pursuing ever-smaller feature size in laser-based lithography is a research topic of vital importance to keep this technique competitive with other micro-/nano-fabrication methods. Features smaller than the diffraction-limited spot size can be obtained by “thresholding”, which utilizes the deterministic nature of damage threshold with ultrashort laser pulses and is achieved by precisely tuning pulse energies so that only the central portion of the focal spot produces permanent modification. In this paper, we examine the formulation commonly used to describe thresholding and show that the relationship between feature size (r) and laser fluence (F) is invariant with respect to the nature of laser absorption. Verified by our experiments performed on metal, semiconductor, and dielectric samples, such invariance is used to predict the smallest feature size that can be achieved for different materials in a real-world system.

In this paper, an additive manufacturing process has been used to deposit nanoparticles on a substrate. In this innovative technique called the nano-electrospray laser deposition process, droplets of various nanosuspensions are dispensed onto a silicon substrate where subwavelength structures and bouncing droplets have been observed. An analytical model is presented for determining the temperature distribution in the substrate by considering the microdroplet as a ball lens. This lens continuously changes the focus of the laser beam as the droplet travels toward the substrate. The laser is either defocused or focused on the substrate forming locally decreased or enhanced heating near the center of the laser beam depending on the distance of the droplet from the substrate. It is found that the enhancement in heating differs for the nanosuspensions since their optical properties are different. The subwavelength structures determined from the post-pulse temperature qualitatively match with the experimental results. The steady end-period temperature is also compared with the experimentally observed temperature for bouncing droplets and the temperatures are in good agreement.

Silver nanoparticle-based electrodes were studied extensively in recent years as an electrode material for wearable and flexible electronics due to their stability and conductivity. A wet chemical deposition technique is considered as a low-cost scalable technique. The current wet chemical-based nanoparticle deposition techniques include electrospray deposition, drop casting, spin coating, and the inkjet printing process. These techniques generally require a separate postdeposition annealing step. This can be a problem for substrates with a low melting point. In addition, some of the above-mentioned methods require physical contact, which increases the probability of cross-contamination. In this research, we present a technique that combines electrospray and laser radiation to deposit and sinter nanoparticles simultaneously on a rigid or flexible substrate. In this process, the microdroplets of aqueous silver nanoparticle suspension ejected in what is known as the microdripping mode from a metallic capillary nozzle, which can be controlled by an electric potential. A conical hollow laser beam is used to vaporize the liquid and sinter the nanoparticles at desired locations on a substrate. This is a promising technique compared to the traditional methods to fabricate conductive micropatterns due to its simplified one step deposition, suppressed cross-contamination, and applicability to various surfaces. Thin-film micropatterns of silver nanoparticles were fabricated using a Nd:YAG laser with powers from 5 to 13 W. The correlation between the grain size distribution, composition, and electrical resistivity was studied using a scanning electron microscope, energy-dispersive x ray, and four-point probe analysis. The results are comparable to the conventional thermal sintering method.

Polymers were one of the first materials to be processed by ultrafast lasers. However, the nature of absorption for near-infrared laser beams is not fully understood, and therefore it remains challenging to process polymeric materials with high energy efficiency. In this study, the pulse-to-pulse evolution of optical properties (reflectance, transmittance, and absorptance) of polypropylene (PP), which is an important polymeric material widely used in many industrial applications, is determined by performing time-resolved measurements for a wide range of pulse energies. The goal is to differentiate between linear and nonlinear absorption in different laser-matter interaction regimes and select the processing condition that yields the highest energy efficiency. The experiment is performed by recording the reflection and transmission of each laser pulse in an ellipsoidal mirror-based setup, which enables the collection of scattering reflection with nearly full coverage. Absorption is calculated from the experimental data, and a model consisting of linear and nonlinear absorption is used to analyze the results. It is found that PP undergoes a dramatic morphological change from pulse to pulse, which is accompanied by changes in optical properties, that is, the tuning of the laser condition to fully utilize the laser energy. Their results could help increase energy efficiency in ultrashort-pulsed laser processing of polymers toward the high-throughput operation.

Laser-microdroplet interactions influence the quality of nanoparticle deposition on a substrate. When a microdroplet and its impinging spot are heated, the microdroplet can evaporate gently, boil immediately after impingement, or bounces back inhibiting the deposition process. The interaction between a laser and droplets carrying semiconductor and metal nanoparticles is studied for different laser powers. The results indicate that the laser is refocused by the droplets, and deposition of nanoparticles and formation of nanofeatures are achieved under certain conditions. On the other hand, when the laser power exceeds a critical value, heating up the substrate at a specific temperature, microdroplets bounce back from the substrate, except for the cases of liquids with low reflectance coefficient and high absorption coefficient, where a new laser-spraying regime is observed.

In the processing of laser materials, the scanning area is limited by the extent to which the laser focus can be maintained within a plane. In order to increase the processing area, one has to enlarge the size of the laser focal spot, and, therefore, reduce the peak intensity and spatial resolution. In this paper, the authors report on a method that extends the processing area by controlling the wavefront of a laser beam using adaptive optics. By adding Zernike modes to the laser beam wavefront, the axial (along the beam propagation direction) location of the focal spot can be tuned within a range larger than the confocal length (two times the Rayleigh length), while the lateral spot size (and, thus, peak intensity and resolution) is maintained throughout the tuning range. A wave propagation model that simulates the propagation of a wavefront-controlled beam is used to compare with the experimental results, which are also confirmed by Zemax calculation. Laser trepanning of circular holes with consistent size on a wide polypropylene sheet is demonstrated. This work shows the potential of tailoring the phase of a laser beam for improving the accuracy and throughput in large-area laser processing of materials.

A measurement system is developed utilizing electromagnetic compatibility test equipment for the study of induced current in conductive materials subjected to radio frequency (RF) magnetic field strengths similar to the 1.5 T magnetic resonance imaging (MRI) B1 magnetic field at ∼65 MHz. The intent of developing such a system was to produce μT range RF magnetic fields in the laboratory to facilitate characterization of induction in conductive materials with modified surface electromagnetic properties to address unintended eddy current issues like Joule heating caused by implanted devices during MRI. A Helmholtz coil (HHC) is used as the RF magnetic field source, and the radiated field is monitored using a receiving loop antenna positioned coaxially outside the HHC. The measurement system operates in continuous wave and pulsed wave modes. Analytical models of the system were derived, which calculate the spatial distribution of RF magnetic flux and the induced current within a coaxially located sample in the transmission path between the HHC and receiving (R/C) loop from output voltage measurements at a single coaxial position. Induced currents were evaluated at multiple flux densities and at different frequencies, showing direct proportionality over the flux densities tested. Induced current results recorded in samples of different sizes and electrical conductivities (ranging from 0.1 to 5.8 × 107 (Ω m)−1 produced changes, matching trends predicted by conductive, closed-loop antenna theory. Induced currents were also used with simultaneous temperature rise measurements to characterize the effective surface conductivity for wire with non-uniform properties at 65 MHz.

Material processing by ultrafast lasers is an attractive technology for high-precision fabrication, such as cutting, drilling, and surface modification, of a wide range of material, including dielectrics, semiconductor, metals, and polymer composites. However, it is still challenging to apply ultrafast laser processing in many applications because some key processes, such as absorption and heat accumulation, are not fully understood, especially for polymeric materials, which have a low melting temperature and, therefore, are more vulnerable to thermal damage. In this study, an analytical solution to a transient, two-dimensional thermal model is derived using Duhamel's theorem and Hankel’s transform method to understand the effect of laser parameters during ultrafast laser interactions with polypropylene (PP), which is a material widely used in many industrial applications. To correlate with theoretical calculation, laser cutting experiments are carried out on PP sheets. This study found that the total energy absorbed in the material and the laser intensity are two important factors to estimate the laser processing performance. In addition, time-resolved measurements are performed by using fast photodiodes and an oscilloscope to understand the dynamics of ultrafast laser interactions during the laser cutting process. Transmitted and reflected signals are monitored and analyzed to extract information on nonlinearity and the absorption coefficient.

The authors report on the theoretical and experimental studies of laser-induced optical breakdown on the surface of fused silica to elucidate the influence of time delay and spatial separation between two ultrashort pulses on the position and size of the modification. Carriers involved in the damage formation including free electrons in the conduction band and self-trapped excitons (STEs) are investigated. The relationship between damage morphology and time delay shows that the seeding carriers (free electrons and STEs) generated from the first pulse are found to play a significant role for the second pulse—which is temporally and spatially separated from the first pulse—in creating the critical electron density needed for an optical breakdown. Consequently, processing outcomes, such as accuracy (position of the hole) and resolution (size of the hole), depend on the interplay of various laser-induced physics that can be tailored for specific goals. As a demonstration, laser lithography with resolution below the diffraction limit is achieved by exploiting multipulse induced physics. This work is a step toward repeatable laser processing of dielectrics beyond the diffraction limit and provides insights into ultrafast laser-matter interaction under the condition of an extremely high pulse repetition rate.

We demonstrate a method of laser ablation with reduced feature size by using a pair of ultrashort pulses that are partially overlapped in space. By tuning the delay between the two pulses, features within the overlapping area are obtained on the surface of fused silica. The observed dependence of the feature position on delays longer than the free-carrier lifetime indicates an ionization pathway initiated by self-trapped excitons. This method could be used to enhance the resolution of laser-based lithography.

A set of experiments were conducted looking at how characteristics of loop elements in an array affect their detected induced current and subsequent Joule heating when located in a radio frequency magnetic field. These experiments included evaluations of the observed steady state temperature rise versus the radius of the loop, the location of the loop with respect to the RF source, and the effect on steady state temperature rise when other loops were brought into close proximity. Loop elements were then combined into a radially-symmetric, five-element array which was used to create a coarse magnetic image of a simple piece of aluminum sheet. After normalization, the temperature rise versus loop size results showed a direct linear relationship which enabled prediction of the lower element size threshold based on the properties of existing thermosensing layer materials used in other bolometer applications and the typical performance of analog measurement equipment. Offset sample test results show how the RF field varied linearly across a given plane perpendicular to the RF source transmission axis, increasing as the distance between the sample and one side of the Helmholtz coil is reduced. Separation distance test results also reveal the RF magnetic field cancellation effects due to mutual coupling as the separation between closed-loop elements are brought into close proximity. Lastly, the excellent agreement achieved between the captured image and the actual object show promise in creating an imaging system with significant resolution and contrast differentiation.

Nanoparticles of various materials are known to possess excellent mechanical, chemical, electrical, and optical properties. However, it is difficult to deposit and transform nanoparticles into large two-dimensional and threedimensional structures in a controlled manner. A laser-based new additive manufacturing process is presented for depositing nanoparticles using an electrospray technology. This process is versatile and scalable, and uses less materials and energy. In this process, aqueous microdroplets of nanoparticle suspension are injected into a hollow laser beam that vaporizes the water, sinters the nanoparticles and deposits nanoparticles on rigid or flexible substrates. Nanoparticles of silver have been deposited on silicon wafer, card stock and polyimide film, and high precision deposition has been observed to occur under a particular microfluidic regime called microdripping mode. This process can promote roll-toroll manufacturing of a variety of energy and electronic devices such as conformal solar cells, sensors, and actuators.

An analytic solution is obtained for three-dimensional quasi-steady state temperature distribution during laser heating of moving thin wires. The wire moves at a constant speed through a vacuum chamber, which is back-filled with an inert gas such as argon, and a laser beam of rectangular cross-section is incident on the wire. The ambient gas provides a convection heat transfer mechanism, which yields a Biot number, Bi, for the heating process to determine whether the temperature distribution would be uniform or nonuniform in the cross-section of the wire. Generally, the criterion of Bi less than 0.1 is applied to assume spatially uniform temperature distribution in a solid. The temperature distribution is determined for different Bi numbers and the variation of the temperature in the azimuthal direction is analyzed. The method of solution involves the Fourier transform in the azimuthal direction and the Hankel transform in the radial direction for a three-dimensional quasi-steady state heat conduction equation containing an advection term that accounts for the motion of the wire. The thermal and optical properties of the material is assumed to be constant in the temperature range of this study. The heat loss due to radiation heat transfer between the wire surface and the surrounding environment is neglected due to the small laser-heated surface area. Using this model, the temperature profile is studied for different process parameters such as the incident laser power, laser beam profile, Biot number, and wire speed.

The propagation of Gaussian beams is analyzed for an acousto-optic deflector by varying the refractive index in two-dimensions with a row of phased array piezoelectric transducers. Inhomogeneous domains of phase grating are produced by operating the transducers at different phase shifts, resulting in two-dimensional index modulation of periodic and sinc function profiles. Also different phase shifts provide a mechanism to steer the grating lobe in various directions and, therefore, the incident angle of the laser beam on the grating plane is automatically modified without moving the beam. Additionally, the acoustic frequency can be varied to achieve the Bragg condition for the new incident angle of the laser beam so that the diffraction efficiency of the deflector is maximized. The Gaussian beam is expressed in terms of planes and the second order coupled mode theory is implemented to analyze the diffraction of the beam. The diffraction efficiency is found to be nearly unity for optimal operating parameters of the acousto-optic device.

Abstarct Laser diffusion has previously been studied to incorporate dopants in semiconductors and to carburize steel for surface hardening without melting the substrate, among others. The optical and electromagnetic properties of materials can also be modified by this diffusion method to tailor the material response at different frequencies of the electromagnetic spectrum. Platinum atoms have been diffused into titanium and tantalum sheets by a laser chemical vapor diffusion method using a metallorganic compound of platinum and laser beams of different polarizations. Thermal decomposition of the precursor at the laser-heated spot on the surface of the substrate generates platinum atoms that diffuse into the substrate, producing laser-platinized samples. The transmittances of the samples are determined by measuring the strength of the transmitted magnetic field oscillating at 63.86 MHz. The laser-platinized samples produced by linearly polarized lasers exhibit higher transmittances than the samples obtained by using azimuthally polarized lasers.

Laser diffusion is generally used to modify the metallurgical composition at the surface of materials for improving the mechanical properties. Platinum has been diffused into titanium and tantalum sheets in this study, and the concentrations of Pt in the substrates are determined. The concentration of Pt is higher at lower scanning speeds due to higher surface temperature and longer diffusion time than in the case of higher scanning speeds. Additionally, the samples treated with a linearly polarized laser beam exhibit slightly higher concentration of Pt. The enhanced diffusion in the case of linearly polarized laser treatment can be attributed to controlled excitation of the local vibration modes of the atoms in the substrate. The reflec- tivity of the samples are also measured at the wavelength of 1,064 nm and compared with theoretical results.

Semiconductor and metal nanoparticles are known to possess excellent mechanical, electrical, and optical properties, but harnessing those novel properties for practical applications is impeded by the difficulty in depositing the nanoparticles into large two-dimensional thin films or three-dimensional structures. A new laser-assisted manufacturing process is developed for patterning and sintering nanoparticles to deposit semiconductor and conductor thin films. This technique has the ability to deposit various materials on rigid and flexible substrates such as paper and plastics for manufacturing flexible electronics.

Typical laser systems produce Gaussian laser beams that may not be suitable for high precision materials processing. This study considers laser microvia drilling of multilayer polymeric substrates for high density interconnects of microelectronics devices. Closely spaced microvias reduce the interconnect distance between the processors to meet the ever increasing demand for transferring large volumes of data at high rates. CO2 lasers of 9.3 μm wavelength is commonly used for drilling microvias in current polymeric substrates because of their higher absorption coefficient at this wavelength than at 10.6 μm wavelength of conventional CO2 lasers. High absorption coefficient provides a volumetric heating mechanism of shallow depth to enable surface-controlled vaporization of the polymeric materials. Gaussian or top-hat laser beams generally leave carbonized polymeric residue at the bottom corner and on the side wall of the microvias, and this residue hinders the subsequent microsoldering of electronic devices to the interconnects. The formation of the residue can be reduced using M-beams for microvia drilling. A thermal model is developed to determine the intensity distribution of the M-beam. To achieve this M-beam from a Gaussian beam, a lens system is designed using the Fresnel diffraction model. Drilling experiments have been conducted using an M-beam and the shape and size of the microvia are found to match the theoretical predictions very well.Typical laser systems produce Gaussian laser beams that may not be suitable for high precision materials processing. This study considers laser microvia drilling of multilayer polymeric substrates for high density interconnects of microelectronics devices. Closely spaced microvias reduce the interconnect distance between the processors to meet the ever increasing demand for transferring large volumes of data at high rates. CO2 lasers of 9.3 μm wavelength is commonly used for drilling microvias in current polymeric substrates because of their higher absorption coefficient at this wavelength than at 10.6 μm wavelength of conventional CO2 lasers. High absorption coefficient provides a volumetric heating mechanism of shallow depth to enable surface-controlled vaporization of the polymeric materials. Gaussian or top-hat laser beams generally leave carbonized polymeric residue at the bottom corner and on the side wall of the microvias, and this residue hinders the subsequent microsoldering of electronic devices...

Phased array ultrasonic transducers enable modulating the focal position of the acoustic waves, and this capability is utilized in many applications, such as medical imaging and non-destructive testing. This type of transducers also provides a mechanism to generate tilted wavefronts in acousto-optic deflectors to deflect laser beams for high precision advanced laser material processing. In this paper, a theoretical model is presented for the diffraction of ultrasonic waves emitted by several phased array transducers into an acousto-optic medium such as TeO2 crystal. A simple analytic expression is obtained for the distribution of the ultrasonic displacement field in the crystal. The model prediction is found to be in good agreement with the results of a numerical model that is based on a non-paraxial multi-Gaussian beam (NMGB) model.

An uncooled SiC-based electro-optic device is developed for gas sensing applications. P-type dopants Ga, Sc, P and Al are incorporated into an n-type crystalline 6H-SiC substrate by a laser doping technique for sensing CO2, CO, NO2 and NO gases, respectively. Each dopant creates an acceptor energy level within the bandgap of the substrate so that the energy gap between this acceptor level and the valence band matches the quantum of energy emitted by the gas of interest. The photons of the gas excite electrons from the valence band to the acceptor level, which alters the electron density in these two states. Consequently, the refractive index of the substrate changes, which, in turn, modifies the reflectivity of the substrate. This change in reflectivity represents the optical signal of the sensor, which is probed remotely with a laser such as a helium-neon laser. Although the midwave infrared (3-5 mm) band is studied in this paper, the approach is applicable to other spectral bands.

An n-type 4H–SiC substrate has been doped with gallium using a continuous wave Nd:YAG laser to heat the sample to high temperatures but below the peritectic temperature of SiC. Mathematical models have been presented for the temperature and Ga concentration distributions in the sample. The Ga atoms, which are produced due to the thermal decomposition of a metallorganic precursor, diffuse into the sample by the solid-phase diffusion process at high temperatures. This process is modeled by considering the temperature-dependent diffusion coefficient and the Ga concentration profile was measured by the secondary ion mass spectrometry (SIMS). The concentration of Ga (6.25 × 1020 cm−3) at the substrate surface was found to exceed the solid solubility limit (1.8 × 1019 cm−3) of Ga in SiC. Comparing the SIMS data to the results of the diffusion model, the activation energy, pre-exponential factor and diffusion coefficient of Ga were determined for different doping conditions. Four doped samples were produced by scanning the samples with a laser beam for different number of passes. The sample prepared with four passes showed the highest diffusion coefficient of 5.53 × 10−7 cm2/s with activation energy 1.84 eV and pre-exponential factor 1.05 × 10−2 cm2/s. The diffusion coefficient is five orders of magnitude higher than the typical diffusion coefficient of Ga in SiC. This indicates that the laser doping process enhances the diffusion coefficient of dopant significantly.

A theoretical approach for estimating solutions to Maxwell’s equations for structures with spatially-varying electromagnetic properties is presented for conductive media containing surfaces modified with functionally graded, heterogeneous electrical conductivity. The basis of the approach is an equivalent depth technique that replaces a graded conductivity region consisting of a phase mixture with a series of thin layers with uniform, multi-phase properties locally matching the effective mixture properties of the graded region. Radio frequency field propagation within each layer is determined as if it had existed within a constant conductivity medium but its depth is electromagnetically equivalent to the replaced graded region existing prior to the layer. The equivalent depth approach was applied to planar, thin foil, and cylindrical media to enable comparison with experimental results. Model predictions were compared with total transmission results for Pt-doped titanium thin foils and steady-state temperature rise in closed wire loops made from Sn-modified copper wire. For the thin foil case, the model-predicted total transmissivity shows good agreement with trends in the experimental results due to property changes in the modified surface layers. In the cylindrical wire case, similar agreement between the predicted effective conductivity values for the modified layers and experimental results was observed. Thus, the equivalent depth approach is an effective method for estimating solutions to Maxwell’s equations in complex media and a useful tool for predicting the performance of tailored surface conductivity modifications.A theoretical approach for estimating solutions to Maxwell’s equations for structures with spatially-varying electromagnetic properties is presented for conductive media containing surfaces modified with functionally graded, heterogeneous electrical conductivity. The basis of the approach is an equivalent depth technique that replaces a graded conductivity region consisting of a phase mixture with a series of thin layers with uniform, multi-phase properties locally matching the effective mixture properties of the graded region. Radio frequency field propagation within each layer is determined as if it had existed within a constant conductivity medium but its depth is electromagnetically equivalent to the replaced graded region existing prior to the layer. The equivalent depth approach was applied to planar, thin foil, and cylindrical media to enable comparison with experimental results. Model predictions were compared with total transmission results for Pt-doped titanium thin foils and steady-state temper...

Many from within manufacturing industry consider superplastic forming (SPF) to be ‘high tech’, but it is often criticized as too complicated, expensive, slow and, in general, an unstable process when compared to other methods of manipulating sheet materials. Perhaps, the fundamental cause of this negative perception of SPF, and also of diffusion bonding (DB), is the fact that the current process of SPF/DB relies on indirect sources of heating to produce the conditions necessary for the material to be formed. Thus, heat is usually derived from the electrically heated platens of hydraulic presses, to a lesser extent from within furnaces and, sometimes, from heaters imbedded in ceramic moulds. Recent evaluations of these isothermal methods suggest they are slow, thermally inefficient and inappropriate for the process. In contrast, direct heating of only the material to be formed by modern, electrically efficient, lasers could transform SPF/DB into the first choice of designers in aerospace, automotive, marine, medical, architecture and leisure industries. Furthermore, ‘variable temperature’ direct heating which, in theory, is possible with a laser beam(s) may provide a means to control material thickness distribution, a goal of enormous importance as fuel efficient, lightweight structures for transportation systems are universally sought. This paper compares, and contrasts, the two systems and suggests how a change to laser heating might be achieved.

Superplastic forming (SPF) has traditionally relied on hot platen presses and furnaces as the principal heat sources to raise materials to superplastic forming temperatures. However, recent research, in the UK and the US, has concluded that such indirect heating methods are slow, expensive, and can only provide a single temperature to the work piece, which can be undesirable. In contrast, LISTechnology Limited (laser induced superplasticity technology) has been created to provide an alternative technology that can directly heat materials to be superplastically formed fast, at low cost and with the potential to control thickness distribution during forming through differential heating. The first ‘laser cell’ for components formed from single sheet titanium is currently being built to demonstrate how direct heating of SPF materials with a laser will significantly increase material heat up rates, compared with current methods, whilst the low thermal mass of the cell will allow rapid cooling to below oxidation temperatures, thus significantly reducing manufacturing cycle times. Furthermore, the cell will only utilise thermally stable, inert ceramic dies within which the titanium will be formed, these being contained within a sealed argon environment, thereby offering the possibility of alpha case free, rapid forming at high temperatures.

The process of Superplastic Forming (SPF) has produced a vast number of components, from simple shapes to those with complex geometric features, all to a dimensional accuracy difficult to obtain by traditional sheet metal methods. When used in conjunction with Diffusion Bonding (DB), extraordinarily lightweight, stiff, strong and fatigue enduring components and structures can be manufactured that are virtually impossible to produce by any other means. However, SPF/DB is often criticised as too complicated, expensive, slow and unstable, a reputation that has contributed to its use being largely confined to manufacturing when no other means of production can be found. Perhaps, the fundamental cause of this negative perception of SPF/DB is the fact that derivatives of mid 18th century hydraulic presses are used, virtually universally, that provide only a single temperature to the work piece, which is heated primarily through conduction, a form of heating which is slow and expensive. By comparison, modern, electrically efficient, lasers, if used to directly heat the work piece, could transform SPF/DB into a mainstream process and the first choice of designers. This paper compares, and contrasts, the two systems.

Thermal barrier coatings are generally designed to possess very low thermal conductivity to reduce the conduction heat transfer from the coating surface to the metal turbine blade beneath the coating. In high-temperature power generation systems, however, a considerable amount of radiative heat is produced during the combustion of fuels. This radiative heat can propagate through the coating and heat up the metal blade, and thereby reduce the effectiveness of the coating in lowering the thermal load on the blade. Therefore, radiative properties are essential parameters to design radiative barrier coatings. This article presents a combined radiation and conduction heat transfer model for the steady-state temperature distribution in semitransparent yttria-stabilized zirconia (YSZ) coatings. The results of the model show a temperature reduction up to 45 K for YSZ of high reflectance (80%) compared to the YSZ of low reflectance (20%). The reflectivities of YSZ and metal blade affect the temperature distribution significantly. Additionally, the absorption and scattering coefficients of YSZ, the thickness of the coating, and the thermal conductivities of YSZ and metal blade affect the temperature distribution.