Absorption Of Beam Research Articles

Measurement of thermo-elastic deformation of an optic using a polarization-based shearing interferometer

A shearing interferometer is presented that uses polarization control to shear the wavefront and to modulate the interference pattern. The shear is generated by spatial walk-off in a birefringent crystal. By adjusting the orientation of the birefringent crystal, the components of the wavefront gradient can be independently measured to allow determination of the full wavefront vector gradient as well as reconstruction of the wavefront. Further, the monolithic nature of the crystal used for shearing allows the interferometer to be set up without need for precise alignment of any components. An algorithm incorporating homodyne detection is presented, which analyzes the modulated interferograms to determine the components of the wavefront gradient, from which the wavefront is reconstructed. The thermal deformation of a mirror subject to heating from absorption of a Gaussian pump beam was accurately observed with a sensitivity better than λ/160. We show that this sensitivity is scale invariant, and present a method to account for the nonuniform spatial frequency response of the interferometer.

Transverse blast loading of hollow beams with square cross-sections

A model of deformation of a metal hollow section beam under a uniform blast loading is developed in order to reveal the characteristic features of deformation and energy absorption of hollow section beams under such loading. It is established that as a typical structural component a hollow section distinguishes itself from its solid counterpart with two characteristic features of the response. First, a considerably larger kinetic energy is generated in the hollow section beam as the impulsive load is imparted on the upper flange of the beam having a significantly lower mass than the member. Second, a considerable proportion of the blast energy can be absorbed by the local collapse of the section. A two-phase analytical model is proposed. In the first phase, the local collapse of the thin-walled cross-section is determined by using an upper bound approach; and in the subsequent second phase, the global bending of the beam with the distorted section is analyzed by taking into account the effect of axial force. It is demonstrated that mass distribution in the hollow section is an important factor in determining the energy partitioning between the local deformation phase and global bending of the hollow beam. Reasonable agreement is obtained with the experimental data published in the literature [Jama HH, Nurick GN, Bambach MR, Grzebieta RH, Zhao XL, Steel square hollow sections subjected to transverse blast loads, Thin-Walled Structures 2012;53:109–122].

Study of effect of process parameters on titanium sheet metal bending using Nd: YAG laser

Laser Bending is a new non-contact technique of forming sheet-metal components. This method has been extended to bend Titanium components in industry. In Titanium laser bending process serious issues such as oxide film formation and subsequent deleterious changes in material properties are encountered. This paper investigates methods to minimize oxidation with minimal change in bending results for Grade-2 Titanium. Inert gas shielding is used as a means of reducing oxidation. Different gas flow conditions, nozzle positions and inert gas combinations are used to enhance the bend quality and bend angle. These process changes lead to final bending angle increase and decrease in width of Heat Affected Zone (HAZ), section thickness of sheet at HAZ and surface hardness. Also as Grade-2 Titanium is highly reflective, different coatings have been used to improve the absorption of laser beam which resulted in further increase in bending angle.

CW 레이저 조사에 의한 실리콘 웨이퍼의 손상 평가

본 연구의 목적은 CW 레이저 조사에 의한 실리콘 웨이퍼의 손상을 평가하는 것이다. 먼저, 레이저 조사에 의한 온도 및 응력 변화를 3 차원 유한요소해석 모델을 이용하여 예측하였다. 해석 결과, 93 W/㎠의 레이저 빔이 조사되었을 때, 실리콘 웨이퍼의 표면의 응력은 약 140 MPa 까지 증가하였으며 균열이 발생할 것으로 예측되었다. 레이저 강도가 더욱 증가하여 186 W/㎠ 일 때에는 실리콘 웨이퍼의 표면의 온도는 1432℃까지 증가하였으며 표면부가 용융될 것으로 예상되었다. 실험 결과, 102 W/㎠ 의 레이저 빔이 실리콘 웨이퍼 표면에 조사되었을 때 표면부에 균열이 발생하였고, 레이저 빔의 강도가 더욱 증가하여 140 W/㎠ 일때 표면부에서 용융이 발생하였다. 용융이 발생하는 레이저 빔의 강도는 유한요소해석 결과보다 낮은 값이었으며 이는 표면부에서 생성된 균열에 의해 레이저 빔의 다중반사와 다중흡수가 일어나 레이저 빔의 흡수량이 증가하였기 때문이다.

Temperature behavior of pellicles in extreme ultraviolet lithography

We present a theoretical analysis of the temperature behavior of pellicles in extreme ultraviolet (EUV) lithography. Using modified heat transfer equations and accounting for the cooling mechanism during exposure, we calculate the temperature of the pellicle due to EUV beam absorption as a function of the incident EUV power. We find the temperature rise to be less than 620 K for a 10 ms incident EUV beam with a power of 100 W. The recovery time after EUV beam exposure is also calculated to be less than 53 ms, which is significantly smaller than the time interval between the typical EUV beam exposures (∼300 ms). Thus, we can neglect the thermal heating problem for practical EUV pellicles in mass EUV processes.

Directionally selective optical limiting of femtosecond pulses in simple transparent two-layer systems

Directionally selective nonlinear transmission behavior has been observed for simple glass-based two-layer systems in Z-scan experiments employing femtosecond pulses at a wavelength of λ=800 nm. Glass sheets of 1 mm thickness with a very thin surface layer (~2 μm) containing spherical Ag nanoparticles have been studied. In these samples, irradiating from the substrate side causes a strong transmission decrease via self-focusing of the beam and subsequent two-photon absorption in the thin layer, whereas the reverse direction may pass the sample without loss. This anisotropic optical limiting is discussed in terms of a numerical model in the limit of thin samples with respect to the Rayleigh range of the laser beam.

Evidence for Point Transformations in Photoactive Molecular Crystals by the Photoinduced Creation of Diffuse Diffraction Patterns

Time-resolved diffuse X-ray scattering is one powerful method for monitoring the progression from the creation of local structural changes inside a crystalline material up to the transformation of the whole crystalline bulk. In this work, we study the mechanism of phototransformation of a molecular crystal by time-resolved diffuse X-ray scattering. Here, an optical excitation source, like a pulsed laser, initiates structural transformations which are monitored by X-ray scattering techniques. We have studied the dimerization process of the molecular switch α-styrylpyrylium (trifluoromethanesulfonate) TFMS, in particular for understanding whether cooperative effects influence the changes of the structure in the bulk and its periodicity. Upon illumination with optical light, α-styrylpyrylium TFMS instantaneously photoswitches. Depending on the optical fluence, X-ray diffuse planes are observed prior to phototransformation of the bulk. In the early stages of transformation, the analysis reveals systems of randomly distributed islands of product clusters with gradual growth in size and amount. The degree of transformation follows the optical excitation profile, i.e., the spatial absorption of the laser beam. In the present studies, no influence of cooperativity on the photodimerization process has been found.

Laser short pulse heating of metal nano-wires

Abstract Non-equilibrium energy transfer takes place in a solid substrate during a short-pulse laser irradiation and temperature field can be obtained analytically in the irradiated region. In the present study, laser short-pulse heating of metal nano-wire is considered and the analytical solution for two-dimensional axisymmetric nano-wire is presented. Since the absorption of the incident beam takes place in the skin of the irradiated surface, a volumetric heat source resembling the absorption process is incorporated in the analysis. Three different nano-wire materials are introduced in the analysis for the comparison reason. These include silver, chromium, and copper. It is found that temperature decay is gradual on the surface vicinity and temporal variation of the surface temperature follows almost the laser pulse intensity profile at the irradiated center.

Excitation of Rayleigh-Benard Convection in Homeotropic Nematic Liquid Crystal by a Gaussian Laser Beam from Above

We theoretically studied the Rayleigh-Benard convection in homeotropic nematic liquid crystal cells due to the absorption of a Gaussian light beam, which penetrates the cell normally from above. It was shown that a convection of hydrodynamic motion in nematic liquid crystals can be induced as a result of absorption Gaussian beam and consequently, can lead to hydrodynamic motion and significant director reorientation in liquid crystal cells. The results can be easily measured experimentally.

Optimized laser processing of calibration blocks for grinding burn detection with Barkhausen noise

Calibration samples for the Barkhausen noise (BN) method were produced with laser processing. A planet gear wheel used for production quality control was subjected to laser irradiation to verify the BN sensor output. Different samples were found to respond similarly to the laser processing although the laser parameters needed to be adjusted for different surface qualities separately. The surface optimization for laser processing was studied with different surface qualities of samples. The ground surface was compared with a sandblasted and vibratory ground surface. The ground and sandblasted surfaces were both amenable to the laser processing whereas the vibratory grinding process created inhomogeneous surfaces for laser beam absorption. Laser processing was found to produce uniform changes in the residual stress values in two perpendicular measuring directions. The root mean square value of the BN voltage signal exhibited linear correlations with the values of the residual stress and surface hardness.

Surface characteristics of aluminum 6061-T6 subjected to Nd:YAG pulsed-laser irradiation

The objective of this study was to investigate the surface characteristics of an aluminum 6061-T6 alloy subjected to Nd:YAG pulsed-laser irradiation. The test specimens were prepared by a mechanical polishing process using diamond paste and emery polishing paper to obtain different levels of initial surface roughness. After ten pulsed-laser shots, the surface morphology was observed via optical microscopy (OM) and scanning electron microscopy (SEM). Nano-indentation hardness testing was also conducted on the irradiated surface. The diameter of the melted zone increased with surface roughness because of the multiple reflection and absorption of the laser beam. The relative absorbance was measured as a function of the diameter of the melted zone with varied surface roughness.

Laser Bending of 5005 Aluminum Alloy Sheets

The laser bendability of thick aluminum sheets has been scarcely investigated previously; in this article this property was evaluated for sheets of 5005 aluminum alloy by means of a high power diode laser. In the experimentation, the width of the sample to bend was changed as well as laser power and scan rate. The effect of other process parameters was also investigated; the surface aspect and the sample orientation with regard to the rolling direction. Mechanical tests and surface analyses were performed before and after laser testing so as to deepen the laser-material interaction. A thermal numerical model was also implemented to study the effect of the laser power on the laser beam absorption during processing. As a result, a good bendability of the 5005 alloy sheets was observed though many parameters need to be considered for the process control. A simple process map for line bending can be extracted from experimental data only if some of those parameters are fixed; above all the sample orientation. These results are found to be significant in better understanding the bending behavior of thick aluminum sheets with high density laser power when a complex interaction between process and material parameters is present.

High-vacuum-compatible high-power Faraday isolators for gravitational-wave interferometers

Faraday isolators play a key role in the operation of large-scale gravitational-wave detectors. Second-generation gravitational-wave interferometers such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo will use high-average-power cw lasers (up to 200 W) requiring specially designed Faraday isolators that are immune to the effects resulting from the laser beam absorption–degraded isolation ratio, thermal lensing, and thermally induced beam steering. In this paper, we present a comprehensive study of Faraday isolators designed specifically for high-performance operation in high-power gravitational-wave interferometers.

Ultraschall in der Geburtshilfe: Kann der Fötus die Ultraschallwelle hören und die Hitze spüren?

Ultraschall in der Geburtshilfe: Kann der Fötus die Ultraschallwelle hören und die Hitze spüren?

On the feasibility of electron cyclotron heating of overcritical plasma in a magnetic mirror trap

The feasibility of matching electromagnetic radiation in the electron cyclotron frequency range to a dense plasma in an open magnetic trap by producing an inverted (with a minimum on the axis) plasma density profile is discussed. The use of such a profile shows promise for the implementation of efficient cyclotron heating at plasma densities above the critical density, at which the Langmuir frequency is equal to the heating radiation frequency. Examples of the magnetic field and plasma density distributions in a mirror trap are presented for which analysis of the beam trajectories shows the feasibility of efficient electron cyclotron absorption of microwave beams in overcritical plasma.

Analytical solution of non-Fourier and Fourier bioheat transfer analysis during laser irradiation of skin tissue

The thermal wave and the Pennes bioheat transfer models are solved analytically by employing the Laplace transform method for small and large values of reflection power (albedo) during laser irradiation. Most of the previous studies have been based on the infinite heat diffusion velocity, but non-Fourier thermal behavior has been observed experimentally in biological tissue. At low initial albedo values, the temperature in the skin depth that directly results from conduction heat transfer process is caused by the lengthy thermal relaxation time in skin tissue. This condition generates a big difference between the thermal wave and Pennes results at the beginning of the heating process. This difference increases under short-time heating condition and high heat flux. However, with high initial albedo, the temperature distribution in the skin depth becomes negligible because of the skin absorption of laser beams. The non-Fourier effect should be considered during laser heating with low albedo, because errors in the predicted temperature values may occur.

Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8eV spectral band. The deposited on silica substrate films of about 10μm in thickness are composed as x·GeO2-(1-x)·SiO2 with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x<1) films by a scanning focused CO2 laser beam absorption band centered at 5eV as well as two luminescence bands centered at blue (3.1eV) and UV (4.3eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO2 film luminescence related to a self-trapped exciton (STE) typical for GeO2 crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO2 crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO2 film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x<1 testifies truly amorphous state of the matter in them.

Gain spectrum of a laser-driven tripod-type atom dynamically induced in the presence of spontaneously generated coherence

We examine the absorption of a weak probe beam in a laser-driven tripod-type atom with three closely lying ground levels, where both the driving and probe lasers interact simultaneously with the three transitions. The effects of spontaneously generated coherence (SGC) are taken into account. We introduce dipole moments in the dressed-state picture and the Hamiltonian in terms of the dressed states describing the interaction between the probe and the atom. Gain spectrum under various conditions are presented and analyzed. We show that the spectral structure and the gain amplitude of the probe are strongly influenced by the effect of SGC and the frequency separation of the three closely lying ground levels. The tripod-type atomic system with quantum interference in spontaneous emission can be simulated in the dressed-state picture of a coherently driven four-level N-type system where no SGC effect exists.

Laser Beams Interaction with Polidocanol Foam: Molecular Background

The goal of this study was to investigate the laser light scattering involved in the laser irradiation process of polidocanol foam samples. Clinical experimental results proved that exposure of tissues impregnated with foaming polidocanol to laser radiation emitted at 1064 nm improved the efficacy of the treatment. Previous absorption studies on Aethoxysclerol 2% solution before and after exposure to Nd:YAG (1064 nm) laser beam have not shown important spectral modifications of it. To achieve the purpose of this work, we produced polidocanol foam using the Tessari method. The batch was passed between the two syringes ~40 times. The resulting foam was stable for 5-6 min. A 10 mm optical cell containing the foam sample was introduced into a home-made Raman spectroscopy system, in which the laser radiation used to excite the Raman emission was the second harmonic (532 nm) of a pulsed Nd:YAG laser beam. The detection was made by a high resolution spectrograph and ICCD camera. The obtained Raman spectra were more intense in foam form than in simple solution. The laser light elastic scattering produced a larger optical path of the beam in the sample, and consequently, a larger absorption of it by the foam components that enhance the laser Raman scattering. The effect of the laser light may be expanded if the polidocanol is used as foam, because then the light scattering in the tissue becomes more important and the beam absorption becomes larger.

Conversion of Concentrated Solar Thermal Energy into Chemical Energy

When a concentrated solar beam is irradiated to the ceramics such as Ni-ferrite, the high-energy flux in the range of 1500-2500 kW/m(2) is absorbed by an excess Frenkel defect formation. This non-equilibrium state defect is generated not by heating at a low heating-rate (30 K/min), but by irradiating high flux energy of concentrated solar beam rapidly at a high heating rate (200 K/min). The defect can be spontaneously converted to chemical energy of a cation-excess spinel structure (reduced-oxide form) at the temperature around 1773 K. Thus, the O(2) releasing reaction (α-O(2) releasing reaction) proceeds in two-steps; (1) high flux energy of concentrated solar beam absorption by formation of the non-equilibrium Frenkel defect and (2) the O(2) gas formation from the O(2-) in the Frenkel defect even in air atmosphere. The 2nd step proceeds without the solar radiation. We may say that the 1st step is light reaction, and 2nd step, dark reaction, just like in photosynthesis process.