Feasibility study for non-intrusive ablation of prostate by electromagnetic radiation

We have developed and successfully tested models and numerical schemes for clean ablation of well-defined regions inside materials without causing damage to the surface and surrounding material, using lasers. The key to our approach is the ability is to control the beam intensity along its axis. A high intensity radiation is delivered to the beam focal area, while the intensity is kept below damage threshold along the beam. The purpose of this study was to investigate if this program can be extended to microwave electromagnetic heating of selected regions inside the dielectrics. If successful, the new technique can be used to ablate prostate cancer non-intrusively as the treatment will not require bringing the EM radiation source close to the cancerous tissue. In this study, the focal length of the lens and the beam diameter at the lens are calculated such that it can deliver high intensity electromagnetic radiation of a prescribed frequency to a given region inside the human body. However, the calculated parameters are inconvenient for an experimental implementation of the results. As the frequency is increased, experimental implementation of the technique becomes feasible, but a high frequency dose of radiation to the body results in increased damaage. Thus, the present technique and results can be used to inscribe features inside the usual dielectrics with high frequency radiation, but achieving a suimilar goal with low frequency radiation will require an alternative technique or further innovation.NRC publication: Ye

Comment on, "Breakdown of the Hellmann-Feynman theorem: degeneracy is the key"

It is clarified that the conclusions reached in Phys. Rev. B 66 033110 (2002) result from erroneous application of the Hellmann-Feynman theorem, and a result deduced there for two-fold degenerate eigenvalues is generalized.Peer reviewed: YesNRC publication: Ye

Geometrical profile of material surface ablated with high-power, short-pulse lasers in ambient gas media

Finer and cleaner features are expected in micro-machining with high power, ultrashort pulse lasers as the melt and evaporation phases are considerably reduced. However, a high-intensity optical beam propagating through a gaseous medium can cause its breakdown generating plasma, which is enhanced further by the self focusing effect of the medium. Photon-plasma scattering compensates somewhat for the self-focusing, but it also distorts the beam profile with consequent impact on the fabricated surface. Plasma also continues to supply heat beyond the pulse duration, which may cause melting and thus distort the features further. In the present article, we show that suitable parameters can be determined to reduce the distortion to the beam profile and balance self-focusing and plasma defocusing resulting in plasma filamentation. Well-shaped beam and plasma filaments, both have favourable impact on the fabricated features. The calculated surface features are compared with the experimentally machined crater profiles with good agreement.Peer reviewed: YesNRC publication: Ye

Converging lower bounds to the photonic-bandgap edges

A photonic band gap is determined by its boundaries, which are frequently computed by the Rayleigh-Ritz method, with the plane wave or the finite element basis functions. This method produces a sequence of upper bounds. Since there are no error estimates available on these approximations, the extent of the band gap is not accurately determined, particularly as this method is also known to suffer from a poor rate of convergence for the cases of interest. We adopt the method of intermediate problems to develop a procedure to calculate the lower bounds to the photonic band gap edges. The lower and the upper bounds supplement each other to determine a band gap with arbitrary accuracy, which is essential for designing the photonic band gap material. Computation of the lower bounds requires only slightly more effort than the upper bounds to produce the approximations with comparable accuracy. An alternative method to determine upper bounds is also developed in the process.Peer reviewed: YesNRC publication: Ye

Modeling of processes during pulsed-laser texturing of material surfaces

Pulsed-laser texturing of material surfaces proceeds with shallow melt resulting from the absorption of laser energy. Flow processes in the fluid determine the morphological structure of the re-solidified surface. Approximations made in the literature are improved upon here to describe the fluid flow in the melt. In particular, the pressure in the melt is included in the Navier-Stokes equation and the surface boundary condition, which is shown to have significant impact on the flow processes and thus, on the final finish of the surface. The calculations for Si surface modifications are found to be in close agreement with the experimental observations, considerably improving upon the earlier descriptionsTitle differs between author's version and publisher's versionPeer reviewed: YesNRC publication: Ye

Solution of two-temperature thermal diffusion model of laser\u2013metal interactions

The two temperature coupled equations,modelingthermal diffusion during laser-induced ablation of metals, are solved under the assumptions that the electron and the lattice heat capacities, and the thermal conductivity remain constant in the process. In view of its practical value, the solution is initially obtained for the energy sources with a Gaussian distribution. The solution is then generalized to include a larger class of source terms for comparison with other results. Present analysis is valid under less restrictive conditions than frequently imposed in the literature. In particular, the solution is valid for realistic source terms and describes the process for ultrashort to nanosecond pulse-width regimes. More general results obtained here retain the attractive features of other approximate solutions available elsewhere and reduce to them under the respective conditions. Predictions of the present model agree well with the experimental observations reported in the literature.Peer reviewed: YesNRC publication: Ye

Geometrical modeling of surface profile formation during laser ablation of materials

Recent advances in laser machining technology have made it possible to fabricate parts and features with high accuracy and precision, using high-powered, short-pulsed, Q-switched lasers. To determine the machining parameters to obtain the desired geometrical quality, an understanding of the relationship between the process parameters and the resulting surface profile is necessary. In the present study, we adopt a geometrical approach which, coupled with the material properties and machining process parameters, yields a method to determine the surface profile of the materialablated by a laser pulse. It is reasoned that the energy incident upon an infinitesimal area of the surface at a given time is transferred in the outward normal direction to the surface, and the volume of ablation, centered about the normal, is determined by the laser\u2013material interaction and the process parameters. The direction and depth of ablation determine the modified surface profile an infinitesimal time later, yielding a nonlinear partial differential equation, which is then integrated starting with the initial known surface to determine the profile at an arbitrary time. Theoretical predictions and the experimental results are compared for a test case of metals. The agreement between the two is satisfactory indicating the adequacy of the approach.Peer reviewed: YesNRC publication: Ye

Surface profile of material ablated with high-power lasers in ambient air medium

In general, material processing with high-power ultra-short-pulsed lasers yields cleaner surfaces, as long as the intensity profile of the laser beam is well shaped. However, the beam suffers distortions during propagation through ambient atmospheric media such as air. Passage through such media causes the beam to self-focus, increasing the intensity further and causing the breakdown of the gas. The resulting plasma distorts the beam\u2019s original profile and the ablatedsurface conforms to the beam profile. A numerical scheme is developed here to calculate the intensity profile of an optical beam propagating through a medium. Intensity distribution of the beam is then used to determine the profile of the processed surface by a geometrical method developed recently. The calculated profile is compared with the experimentally obtained surface with good agreement. For medium spot sizes, the self-focusing and plasma effects tend to cancel each other, maintaining the intensity profile of the beam similar to the original Gaussian distribution. For small spot sizes when the intensity is high, the plasma effects are found to distort the beam profile. This indicates that the experimental parameters can be adjusted to improve the quality of the machined surface.Peer reviewed: YesNRC publication: Ye

Effect of plasma on ultra short pulse laser material processing

Machining with high power ultrashort-pulsed lasers is becoming a preferred technique in material processing. However, the laser beam passing through a medium, e.g., air, experiences the self-focusing Kerr effect. High intensities increased further by self-focusing cause optical breakdown of the air, generating plasma. The associated diffusion compensates for the Kerr effect but it also deforms the laser beam. In the present article, properties of the plasma columns so induced by the femtosecond laser pulses are studied, which are similar to the long filaments induced with collimated ultrashort pulses. It is found that the two effects balance each other very closely for the part of the beam. Thus, placing the focal position at an appropriate position results in improved drilling and cutting, i.e., with flat bottom, parallel wall, and less dross. Theoretical calculations of the intensity profile of the optical beam propagating through air are found to be commensurate with the experimental observations.Peer reviewed: YesNRC publication: Ye

Geometrical modeling of laser ablation

Recent advances in the laser machining technology have made it possible to fabricate parts and features with high accuracy and precision, using high-powered, short-pulsed, Q-switched lasers. To determine the machining parameters to obtain desired geometrical quality, an understanding of the relationship between the process parameters and the resulting surface profile is necessary. In the present study, we adopt a geometrical approach, which, coupled with the material properties and machining process parameters, yields a model for the surface profile of the region ablated by a laser pulse. Energy incident upon an infintesimal area of the surface is transferred in the outward normal direction, and the depth of the ablation depends on the laser-material interaction and the process parameters. This determines the modified surface profile an infintesimal time later, yielding a nonlinear partial differential equation, which is then integrated to determine the profile after a given time period. Theoretical predictions and the experimental results are compared and discussed.Peer reviewed: YesNRC publication: Ye