Gerard M. O’Connor

Temperature dependence of the photoluminescence intensity of ordered and disordered In0.48Ga0.52P

The integrated photoluminescence (PL) intensities of both ordered and disordered epilayers of InGaP grown on GaAs have been measured as a function of temperature. The highest PL efficiency occurs in the most disordered sample. We find that the PL intensities can drop from 2 to almost 4 orders of magnitude between 12 and 280 K. The samples show an Arrhenius behavior characterized by two activation energies. Below 100 K the activation energies lie in the region of 10–20 meV. Above 100 K the activation energy is approximately 50 meV except in the most disordered sample where it increases to 260 meV. We conclude that the low‐temperature PL efficiency is most likely controlled by carrier thermalization from spatial fluctuations of the band edges followed by nonradiative recombination. At higher temperatures the PL efficiency is dominated by a nonradiative path whose characteristic activation energy and transition probability depend upon the degree of sublattice ordering.

Nanocrystalline structure of nanobump generated by localized photoexcitation of metal film

The extreme cooling rates in material processing can be achieved in a number of current and emerging femtosecond laser techniques capable of highly localized energy deposition. The mechanisms of rapid solidification of a nanoscale region of a metal film transiently melted by a localized photoexcitation are investigated in a large-scale atomistic simulation. The small size of the melted region, steep temperature gradients, and fast two-dimensional electron heat conduction result in the cooling rate exceeding 1013 K/s and create conditions for deep undercooling of the melt. The velocity of the liquid/crystal interface rises up to the maximum value of ∼80 m/s during the initial stage of the cooling process and stays approximately constant as the temperature of the melted region continues to decrease. When the temperature drops down to the level of ∼0.6Tm, a massive homogeneous nucleation of the crystal phase inside the undercooled liquid region takes place and prevents the undercooled liquid from reaching th...

Laser direct-write technique for fabricating microlens arrays on soda-lime glass with a Nd:YVO4 laser.

A one-step direct-write technique for fabricating spherical microlenses on soda-lime glass substrates is described. Using a Q switched Nd:YVO(4) laser combined with a galvanometer system, square and triangular microlens arrays were fabricated. The focal length of microlenses is measured using direct and nondirect methods. Values around 118 and 125 µm were obtained for the microlens focal length of square and triangular arrays, respectively. A noncontact profilometer is used for determining the surface roughness of square and triangular arrays. Results are compared with that of glass substrate.

Fabrication and characterization of microlens arrays on soda-lime glass using a combination of laser direct-write and thermal reflow techniques

We describe a hybrid technique for fabricating microlens arrays on soda-lime glass substrates composed by a direct-laser write and a post thermal treatment. In particular we use a nanosecond Q-Switch Nd: YVO4 laser and a mufla Heraeus furnace working in the range of 620 °C−670 °C. An improvement in the quality of the microlens arrays were obtained as temperature increases, reducing their optical aberrations, surface roughness and increasing their resemblance. In particular at 670 °C we obtain high quality microlens array with diameter 47.89 ± 6.65 μm; focal length 510 ± 10 μm; focal spot size 2.82 ± 0.02 μm; a root mean square of the total aberration λ/28 ± λ/77, strehl ratio 0.9475 ± 0.0352 and depth of focus 16.438 ± 5.762 μm. Our results show the reliability of the combination of the Laser-direct writing technique with thermal treatment for fabricating high quality microlens arrays.

Directional ion emission from thin films under femtosecond laser irradiation

Thin films of nickel have been irradiated using femtosecond laser pulses in vacuum. Subsequent emission of plasma ions is diagnosed using an ion probe. Angular distributions of the emitted ions are presented for a range of target film thicknesses. Data are compared to the Anisimov model of plasma expansion [S. I. Anisimov, D. Bauerle, and B. S. Luk’yanchuk, Phys. Rev. B 48, 12076 (1993)]. The tendency of the ions to be ejected at small angles to the normal of the target surface is explained in terms of the initial conditions of the plume. Results are explained in terms of the initial shape and adiabatic index of the plasma.

Laser-based microstructuring of surfaces using low-cost microlens arrays

Since frictional interactions in microscopically small components are becoming increasingly important for the development of new products for all modern technology, we present a laser-based technique for micro-patterning surfaces of materials using low-cost microlens arrays. By combining a laser direct-write technique on soda-lime glass and a thermal treatment, it was possible to obtain high quality microlens array elements using a low cost infrared laser widely implemented in industry which makes this technique attractive in comparison with other more expensive methods. The main advantage of using microlens arrays for micropatterning surfaces is the possibility of fabricating a large number of identical structures simultaneously, leading to a highly efficient process. In order to study the capabilities of the microlenses fabricated for microstructuring materials, identical structures and arrays of holes were fabricated over a variety of materials, such as stainless steel, polymer, and ceramic. The minimum diameter of the individual microstructure generated at surface was 5 µm. Different nanosecond lasers operating at infrared and green wavelengths were used. The topography and morphology of the elements obtained were determined using confocal microscopy.

Estimating spot size and relating hole diameters with fluence and number of shots for nanosecond and femtosecond laser ablation of polyethylene terephthalate

The relationship between focused spot size, pulse energy, and ablated hole diameter was explored using 266 nm nanosecond pulses and 775 nm femtosecond pulses on polyethylene terephthalate. The spot size of a Gaussian laser beam can be deduced from the diameters of holes machined with single shots at different pulse energies but the results can be influenced by such issues as the thickness of the material and the range of fluence chosen. These limitations of the method were investigated. Multiple shot craters were also measured but gave an overestimation of spot size, particularly in the femtosecond case. A model was developed to predict the diameter for a given energy and number of shots. Tests of the model gave results that agreed well with the predicted values. It was found that for the same total energy smaller holes can be obtained by decreasing the pulse energy and increasing the number of shots. An effective spot size for multiple shots was determined.

Laser microfabrication of a microheater chip for cell culture outside a cell incubator

Microfluidic chips have demonstrated their significant application potentials in microbiological processing and chemical reactions, with the goal of developing monolithic and compact chip-sized multifunctional systems. Heat generation and thermal control are critical in some of the biochemical processes. The paper presents a laser direct-write technique for rapid prototyping and manufacturing of microheater chips and its applicability for lab-on-a-chip cell culture outside a cell incubator. The aim of the microheater is to take the role of conventional incubators for cell culture for facilitating microscopic observation and/or other online monitoring activities during cell culture and provides portability of cell culture operation. Microheaters (5mm×5mm) have been successfully fabricated on soda-lime glass substrates covered with aluminium layer of thickness 120nm. Experimental results show that the microheaters exhibit good performance in temperature rise and decay characteristics, with localized heating at targeted spatial domains. These microheaters were suitable for a maximum long-term operation temperature of 120°C and validated for operation at 37°C for 48h. Results demonstrated that the microheaters are suitable for the culture of immortalised cell lines. The growth and viability of SW480 colon adenocarcinoma cells cultured the developed microheater chip were comparable to the results obtained in a conventional cell incubator.

Combined anodizing and picosecond laser treatment to control the corrosion rate of biodegradable magnesium alloy AZ31

The corrosion rate of magnesium alloys is generally too high for biodegradable implant applications. This work explored combinations of anodizing and picosecond laser surface treatments to modify the corrosion response of magnesium alloy AZ31. Anodizing of the AZ31 in NaOH solutions produced porous oxide layer structures. Shallow laser treatment of these anodized surfaces, using low pulse powers, resulted mainly in oxide ablation and impaired corrosion resistance. Higher pulse power, resulting in rapid melting and resolidification into the substrate, provided an improved corrosion response. The refined grain structure produced is approximately only 5 µm deep and therefore has minimal influence on bulk mechanical properties. It is therefore a suitable process for surface modifications on small medical device structures. Controlling the initial point of degradation has been demonstrated by the use of selective laser treatment of the AZ31 surface.

Parallel Direct Simulation Monte Carlo of Two-Phase Gas-Droplet Laser Plume Expansion from the Bottom of a Cylindrical Cavity into an Ambient Gas

A combined computational model for simulation of the expansion of two-phase laser plumes is developed. The model includes a two-dimensional thermal model of the irradiated target, Direct Simulation Monte Carlo method for flow of multi-component gas mixture in the plume, and a Lagrangian scheme for tracking of trajectories of individual sub-micron droplets generated on the irradiated surface. The model is implemented in a parallel computational code and applied for simulations of the plume expansion into an ambient gas, which is induced by a nanosecond laser pulse irradiating the bottom of a cylindrical cavity on the target surface. Simulations reveal the significant physical effects of the ambient gas chemical composition on the motion of laser ablated submicron debris in the vicinity of the target.