Sean E. Kirkwood

Microscope-based label-free microfluidic cytometry.

A microscope-based label-free microfluidic cytometer capable of acquiring two dimensional light scatter patterns from single cells, pattern analysis of which determines cellular information such as cell size, orientation and inner nanostructure, was developed. Finite-difference time-domain numerical simulations compared favorably with experimental scatter patterns from micrometer-sized beads and cells. The device was capable of obtaining light scattering patterns from the smallest mature blood cells (platelets) and cord blood hematopoietic stem/progenitor cells 
(CD34 + cells) and myeloid precursor cells. The potential for evaluation of cells using this label-free microfluidic cytometric technique was discussed.

Mechanism for femtosecond laser pulse patterning of self- assembled monolayers on gold-coated substrates

Self-assembled monolayer (SAM) patterning on gold thin films was performed using 800 nm, 118 fs laser pulses. SAM removal was ablative and was observed at fluences near the multishot ablation threshold for the thin gold film. Line widths six times smaller than the 2 e-folding intensity beam diameter were observed demonstrating sub-diffraction limited patterning with femtosecond lasers. Similar experimental results in air and N2 indicated that the removal process does not involve oxidation of the gold-sulfur bond as was claimed in the literature.

Simulation of femtosecond laser ablation of silicon

Femtosecond laser ablation is an important process in micromachining and nanomachining of microelectronic, optoelectronic, biophotonic and MEMS components. The process of laser ablation of silicon is being studied on an atomic level using molecular dynamics simulations. We investigate ablation thresholds for Gaussian laser pulses of 800 nm wavelength, in the range of a few hundred femtoseconds in duration. Absorption is modelled via linear and 2-photon absorption processes into a hot electron bath which then transfers energy into the crystal lattice. The simulation box is a narrow column approximately 5.4 nm x 5.4 nm x 81 nm with periodic boundaries in the x and y transverse directions and a 1-D heat flow model at the bottom coupled to a heat bath to simulate an infinite bulk medium corresponding to the solid bulk material. A modified Stillinger-Weber potential is used to model the silicon atoms. The calculated thresholds are compared to various reported experimental values for the ablation threshold of silicon. We provide an overview of the code and discuss the simulation techniques used.

Nanomilling surfaces using near-threshold femtosecond laser pulses

We have produced crater depths of less than 10 nanometers using 100-1000 pulses of a near-infrared femtosecond laser (800 nm, 125 fs) on a copper thin film surface. By determining the single-shot ablation threshold, incubation coefficient and surface reflectivity, the femtosecond laser pulse parameters for surface nanomilling are established close to the multiple-pulse ablation threshold limit for a copper thin film. Photomultiplier measurements of a copper emission line were used as a real time monitor of the nanomilling process for which photons were detected only once every several shots. The results are consistent with a model that ablation occurs in bursts every several shots after a number of intervening incubation energy storage shots.

Quantitative emission from femtosecond microplasmas for laser-induced breakdown spectroscopy

An ongoing study of the scaling of Laser-Induced Breakdown Spectroscopy (LIBS) to microjoule pulse energies is being conducted to quantify the LIBS process. The use of microplasmas for LIBS requires good understanding of the emission scaling in order to maximize the sensitivity of the LIBS technique at low energies. The quantitative scaling of emission of Al, Cu and Si microplasmas from 100 μJ down to 100 nJ is presented. The scaling of line emission from major and minor constituents in Al 5052 alloy is investigated and evaluated for analytical LIBS. Ablated crater volume scaling and emission efficiency for Si microplasmas are investigated.

Light scattering characterization of single biological cells in a microfluidic cytometer

The characterization of single biological cells in a microfluidic flow by using a 2D light scattering microfluidic cytometric technique is described. Laser light is coupled into a microfluidic cytometer via an optical fiber to illuminate a single scatterer in a fluidic flow. The 2D light scattering patterns are obtained by using a charge-coupled device (CCD) detector. The system is tested by using standard polystyrene beads of 4 μm and 9.6 μm in diameter, and the bead experimental results agree well with 1D Mie theory simulation results. Experiments on yeast cells are performed using the microfluidic cytometer. Cell results are studied by finite-difference time-domain (FDTD) method, which can simulate light scattering from non-homogeneous cells. For example, a complex biological cell model with inner mitochondrial distribution is studied by FDTD in this paper. Considering the yeast cell size variations, the yeast cell 2D scatter patterns agree well with the FDTD 2D simulation patterns. The system is capable of obtaining 2D side scatter patterns from a single biological cell which may contain rich information on the biological cell inner structures. The integration of light scattering, microfluidics and fiber optics described here may ultimately allow the development of a lab-on-chip cytometer for label-free detection of diseases at a single cell level.

Direct writing of self-assembled monolayers on gold coated substrates using a CW argon laser

The ability to engineer surface properties such as hydrophobicity, charge, and adhesion at the micrometer scale is the key to developments in emerging technologies (e.g. bio-sensors, and barrier-free microfluidic systems). Development of a methodology to manipulate surface properties of a self-assembled monolayer of alkanethiol on a gold film was the objective of this paper. This system is broadly studied and widely believed to serve as the platform of choice to develop a variety of biological technologies. The proposed approach is unique in that it eliminates the need for photolithography, is non-contact, and can be extended to other systems such as SAMs on silicon wafers or polymeric substrates. For this study, an initial hydrophobic monolayer of l-hexadecanethiol on a 300 /spl Aring/ gold sputtered film is used. Localized regions are then desorbed in a nitrogen atmosphere by scanning the focal spot of a 488 nm CW Argon ion laser beam. The beam with a Gaussian spatial profile was scanned at a rate slower than the heat diffusion rate along the surface. After completing the scans, the sample is dipped into a dilute solution of 16-mercaptohexadecanoic acid and a hydrophilic monolayer self-assembles along the previously irradiated regions. The resultant lines are viewed by wetting with tridecane.

Development of an optomicrofluidic flow cytometer for the sorting of stem cells from blood samples

In this paper, we report the preliminary development of a fiber coupled microfluidic flow cytometer with its potential application of sorting the very small embryonic like (VSEL) stem cells out of a mixture of platelets and VSEL stem cells. The identification of a VSEL stem cell from a platelet is based on the large difference of their abilities to scatter light. A simple cytometer prototype was built by cutting the fluidic and other channels into a polymer sheet and bonding it with epoxy between two standard glass slides. Standard photolithography was used to expose an observation window over the upper coated glass to reduce background scattered light. Liquid sample containing micro-particles (such as cells) is injected into the microfluidic channel. Light from a 532-nm CW diode laser is coupled into the optical fiber that delivers the light to the detection region in the channel to interrogate the flowing-by micro-particles. The scattering light from the interrogated micro-particle is collected by a photodiode placed over the observation window. The device sorts the micro-particle into the sort or waste outlet depending on the level of the photodiode signal. We used fluorescent latex beads to test the detection and sorting functionalities of the device. It was found that the system could only detect about half of the beads but could sort almost all the beads it detected.

Femtosecond interaction processes near threshold : Damage and ablation

Femtosecond laser ablation is an important process in micromachining and nanomachining of microelectronic, optoelectronic, biophotonic and MEMS components. It is also important in the damage of optical components and materials. A thorough understanding of all aspects of femtosecond matter interaction processes in the near-threshold regime is required if one wants to have complete control of these processes. Two aspects of the interaction process for metals and semiconductors are examined in detail in the present paper, namely the effect of a more complete model for the temperature dependent electron thermal conductivity in metals and the avalanche ionization process in semiconductors. These are included in two temperature and molecular dynamics modeling calculations respectively. The proper inclusion of these processes allows the model calculations to better reproduce published experimental measurements for copper and silicon.

The interaction of low-intensity femtosecond laser pulses with a copper foil

Summary form only given.The results of experimental and theoretical research on interaction of femtosecond laser pulses at a wavelength 800 nm and 400 nm with a copper foil are presented. Ti:Sapphire laser intensity on foil surface was in the rang of 1012-3*1014 W/cm2 in these experiments that were carried out at University of Alberta (Canada). The calculations of femtosecond laser pulses interaction with a matter and study of the foil blow-off are performed with using of ID ERA code developed in RFNC-VNIITF. The hydrodynamic and electron conductivity with degeneracy effects were taken into account. Wide-range equations of state for solid, gas and plasma were used in ERA-code simulations. Two- temperature case was considered in these calculations by taking into account of energy exchange between electrons and phonons (ions). Laser energy absorption was calculated from Helmholtz equation with complex dielectric permittivity. The scattering of electrons on phonons and free electrons was taken into account in calculation of dielectric permittivity. The results of ERA code calculations are in a good agreement with data on laser light reflection that have been obtained earlier in experiments on interaction of 150-fs laser pulses with copper foils at University of Alberta. ERA code simulations of foil dynamic under action of 100-fs laser pulse with wavelength of 800 nm and intensity of 3*1013W/cm2 were performed as with low-intensity nanosecond prepulse as without it. It is shown that nanosecond prepulse at intensity of 108 W/cm2 - 109 W/cm2 have no significant influence on copper foils.