Interaction between the mid-infrared continuous wave laser with a center wavelength of 3.8†µm and fused silica.

The absorption coefficient of fused silica for a mid-infrared (IR) laser is higher than that for a near-IR laser, but smaller than that for a far-IR laser. Therefore, the energy coupling efficiency of the mid-IR laser is higher than that for the near-IR laser, while the penetration depth is higher than that for the far-IR laser. Thus, the mid-IR laser is highly efficient in mitigating damage growth. In this study, a deuterium fluoride (DF) laser with a center wavelength of 3.8 µm was used to interact with fused silica. The temperature variation, changes in the reflected and transmitted intensities of the probe light incident on the laser irradiation area, and the vaporization and melting sputtering process were analyzed. The results demonstrate that when the laser intensity was low (<1.2 kW/cm2), no significant melting was observed, and the reflection and transmission properties gradually recovered after the end of the laser irradiation process. With a further increase in the laser intensity, the sample gradually melted and vaporized. At a laser intensity above 5.1 kW/cm2, the temperature of the sample increased rapidly and vapors in huge quantity evaporated from the surface of the sample. Moreover, when the laser intensity was increased to 9.5 kW/cm2, the sample melted and an intense melting sputtering process was observed, and the sample was melted through.

Dislocation analysis of germanium wafers under 1080 nm laser ablation.

COMSOL Multiphysics was employed to establish a dislocation model based on the Alexander and Haasen (AH) model, the heat conduction equation, and Hooke's law for calculating the dislocation distribution of germanium (Ge) under laser irradiation. The numerical simulation results were obtained. A continuous 1080 nm laser was utilized to ablate the monocrystalline Ge wafers to validate the model. The experimental results show that no surface damage appears until the irradiances go up to 234W/cm2 for 100 ms laser ablation. This is consistent with the numerical findings. The initiation times of surface damage by the experiments at 234W/cm2 and above agree well with the numerical results, which means that the model can efficiently predict the dislocation field.

Investigation on the Continuous Wave Mode and the ms Pulse Mode Fiber Laser Drilling Mechanisms of the Carbon Fiber Reinforced Composite.

The near infrared (NIR) laser drilling of a carbon fiber reinforced polymer (CFRP) composite in the continuous wave (CW) mode and the ms pulse mode was investigated by an experiment and a numerical simulation. The relationships between the laser penetrating time, entrance hole diameter, surface heat affected zone (HAZ) width, and material ablation rate and the laser irradiation time and laser peak power densities were obtained from the experiment. For the same average power density of the laser output, 3.5 kW/cm2, it was found that the ms pulse laser mode, which had a higher peak power density, had a higher drilling efficiency. When drilling the same holes, the pulse laser mode, which had the highest peak power density of 49.8 kW/cm2, had the lowest drilling time of 0.23 s and had the smallest surface HAZ width of 0.54 mm. In addition, it was found that the laser penetrating time decreased sharply when the peak power density was higher than 23.4 kW/cm2. After analyzing the internal gas pressure by the numerical simulation, it was considered that a large internal gas pressure appeared, which resulted from polymer pyrolysis, causing a large amount of the mechanical erosion of the composite material to improve the drilling efficiency. Therefore, the ms pulse laser showed its potential and advantage in laser drilling the CFRP composite.

Simultaneously extracting multiple parameters via multi-distance and multi-exposure diffuse speckle contrast analysis.

Recent advancements in diffuse speckle contrast analysis (DSCA) have opened the path for noninvasive acquisition of deep tissue microvasculature blood flow. In fact, in addition to blood flow index αDB , the variations of tissue optical absorption µa , reduced scattering coefficients [Formula: see text], as well as coherence factor ß can modulate temporal fluctuations of speckle patterns. In this study, we use multi-distance and multi-exposure DSCA (MDME-DSCA) to simultaneously extract multiple parameters such as µa , [Formula: see text], αDB , and ß. The validity of MDME-DSCA has been validated by the simulated data and phantoms experiments. Moreover, as a comparison, the results also show that it is impractical to simultaneously obtain multiple parameters by multi-exposure DSCA (ME-DSCA).

Slip-free processing of (001) silicon wafers under 1064 nm laser ablation.

Slip phenomenon on a (001) silicon surface under 1064 nm laser ablation was studied by experiments and simulations. The surface morphologies of the silicon wafers after laser irradiation were observed using an optical microscope. The slip patterns showed that slip that occurred before melting was responsible for a low-quality ablation surface. The slip damage was predicted by a three-dimensional finite element model based on heat transfer and thermoelasticity theory. The judging criterion of slip was explained in detail. The numerical results gave a better understanding of slip phenomenon in experiments. It is shown that low laser irradiances cause slip and high laser irradiances are helpful in preventing slip. The threshold irradiance is ∼1 MW/cm2. Lasers with higher irradiance are essential to obtain a slip-free ablation on a (001) silicon surface.

Surface damage induced by a combined millisecond and nanosecond laser.

The surface damage morphologies of single-crystal silicon induced by a combined pulse laser (CPL) and a single millisecond laser are investigated, respectively. The CPL includes a millisecond (ms) laser superposed by a nanosecond (ns) laser. Inspected by an optical microscope, it was found that the surface damage was more serious when the sample was irradiated by the CPL than by a single ms laser with the same incident laser energy. Besides surface cleavage, obvious ablation and fold areas were discovered by CPL irradiation. A two-dimensional spatial axisymmetric model was established to assess the difference between single ms laser and CPL irradiation and explain the generation mechanism of the different surface damage. This was attributed to the preheating effect by the ms laser and the surface damage caused by the ns laser.

Quantitative model of diffuse speckle contrast analysis for flow measurement.

Diffuse speckle contrast analysis (DSCA) is a noninvasive optical technique capable of monitoring deep tissue blood flow. However, a detailed study of the speckle contrast model for DSCA has yet to be presented. We deduced the theoretical relationship between speckle contrast and exposure time and further simplified it to a linear approximation model. The feasibility of this linear model was validated by the liquid phantoms which demonstrated that the slope of this linear approximation was able to rapidly determine the Brownian diffusion coefficient of the turbid media at multiple distances using multiexposure speckle imaging. Furthermore, we have theoretically quantified the influence of optical property on the measurements of the Brownian diffusion coefficient which was a consequence of the fact that the slope of this linear approximation was demonstrated to be equal to the inverse of correlation time of the speckle.

Interaction between a bubble and a metal target for underwater laser propulsion.

Optical beam deflection and high-speed photographic methods are employed to investigate the interaction mechanism between a laser-induced bubble and a metal target for underwater laser propulsion. A preliminary theory is proposed to reveal the step increases of the kinetic energy transferred to the target during the process of increasing the incident laser energy. This theory also helps to explain the increasing coupling efficiency with incident laser energy for underwater laser propulsion.

Tunable Plasmonic Band-Pass Filter with Dual Side-Coupled Circular Ring Resonators.

A wavelength band-pass filter with asymmetric dual circular ring resonators in a metal-insulator-metal (MIM) structure is proposed and numerically simulated. For the interaction of the local discrete state and the continuous spectrum caused by the side-coupled resonators and the baffle, respectively, the transmission spectrum exhibits a sharp and asymmetric profile. By adjusting the radius and material imbedded in one ring cavity, the off-to-on plasmon-induced absorption (PIA) optical response can be tunable achieved. In addition, the structure can be easily extended to other similar compact structures to realize the filtering task. Our structures have important potential applications for filters and sensors at visible and near-infrared regions.

Millisecond laser machining of transparent materials assisted by a nanosecond laser with different delays.

A millisecond laser combined with a nanosecond laser was applied to machining transparent materials. The influences of delay between the two laser pulses on processing efficiencies and modified sizes were studied. In addition, a laser-supported combustion wave (LSCW) was captured during laser irradiation. An optimal delay corresponding to the highest processing efficiency was found for cone-shaped cavities. The modified size as well as the lifetime and intensity of the LSCW increased with the delay decreasing. Thermal cooperation effects of defects, overlapping effects of small modified sites, and thermal radiation from LSCW result in all the phenomena.

Millisecond laser machining of transparent materials assisted by nanosecond laser.

A new form of double pulse composed of a nanosecond laser and a millisecond laser is proposed for laser machining transparent materials. To evaluate its advantages and disadvantages, experimental investigations are carried out and the corresponding results are compared with those of single millisecond laser. The mechanism is discussed from two aspects: material defects and effects of modifications induced by nanosecond laser on thermal stress field during millisecond laser irradiation. It is shown that the modifications of the sample generated by nanosecond laser improves the processing efficiency of subsequent millisecond laser, while limits the eventual size of modified region.

Quantitatively assessing flow velocity by the slope of the inverse square of the contrast values versus camera exposure time.

The slope of the inverse square of the contrast values versus camera exposure time at multi-exposure speckle imaging (MESI) can be a new indicator of flow velocity. The slope is linear as the diffuse coefficient in Brownian motion diffusion model and the mean velocity in ballistic motion model. Combining diffuse speckle contrast analysis (DSCA) and MESI, we demonstrate theoretically and experimentally that the flow velocity can be obtained from this slope. The calculation results processes of the slop don't need tedious Newtonian iterative method and are computationally inexpensive. The new indicator can play an important role in quantitatively assessing tissue blood flow velocity.

Study of underwater laser propulsion using different target materials.

In order to investigate the influence of target materials, including aluminum (Al), titanium (Ti) and copper (Cu), on underwater laser propulsion, the analytical formula of the target momentum IT is deduced from the enhanced coupling theory of laser propulsion in atmosphere with transparent overlay metal target. The high-speed photography method and numerical simulation are employed to verify the IT model. It is shown that the enhanced coupling theory, which was developed originally for laser propulsion in atmosphere, is also applicable to underwater laser propulsion with metal targets.

[The influence of temperature on the orientation of pigments in PS II].

The effect of temperature on the orientation of pigment in PSII was studied by the fluorescence excitation spectra and polarization fluorescence spectra of spinach thylakoid solution. The experimental results showed that in the temperature range of 15-45 degrees C the absorption band of chl a at 436 nm at room temperature red-shifted with increasing temperature. The excitation spectra intensity reached the maximum at the temperature of 35 degrees C, but greatly reduced with temperature at 65 and 78 degrees C. In the polarization fluorescence spectra the fluorescence peak of PSII didn't change with temperature from 15 to 45 degrees C. It was also found that the calculated fluorescence polarization degree increased with the temperature in the entire temperature range. The analysis indicated that temperature would affect the orientation of the pigments in PSII and the coupling strength between pigments so to change photosynthetic efficiency. The results will give a certain reference for the study of the energy absorption and transmission, regulation mechanism and also on solar cell materials.

Scanning laser-line source technique for nondestructive evaluation of cracks in human teeth.

This paper describes the first application of a remote nondestructive laser ultrasonic (LU) system for clinical diagnosis of cracks in human teeth, to our knowledge. It performs non-contact cracks detection on small-dimension teeth samples. Two extracted teeth with different types of cracks (cracked tooth and craze lines), which have different crack depths, are used as experimental samples. A series of ultrasonic waves were generated by a scanning laser-line source technique and detected with a laser-Doppler vibrometer on the two samples. The B-scan images and peak-to-peak amplitude variation curves of surface acoustic waves were obtained for evaluating the cracks' position and depth. The simulation results calculated by finite element method were combined with the experimental results for accurately measuring the depth of crack. The results demonstrate that this LU system has been successfully applied on crack evaluation of human teeth. And as a remote, nondestructive technique, it has great potential for early in vivo diagnosis of cracked tooth and even the future clinical dental tests.

Experimental and numerical studies for nondestructive evaluation of human enamel using laser ultrasonic technique.

In this paper, a nondestructive laser ultrasonic technique is used to generate and detect broadband surface acoustic waves (SAWs) on human teeth with different demineralization treatment. A scanning laser line-source technique is used to generate a series of SAW signals for obtaining the dispersion spectrum through a two-dimensional fast Fourier translation method. The experimental dispersion curves of SAWs are studied for evaluating the elastic properties of the sound tooth and carious tooth. The propagation and dispersion of SAWs in human teeth are also been studied using the finite element method. Results from numerical simulation and experiment are compared and discussed, and the elastic properties of teeth with different conditions are evaluated by combining the simulation and experimental results.

Formation and dynamics of a toroidal bubble during laser propelling a cavity object in water.

We captured stable self-oscillations of a toroidal bubble moving away from a laser propelled cavity object in water using a high-speed imaging system. The entire laser propelling process generates a hemispherical bubble, two toroidal bubbles, and a microbubble cluster. The hemispherical bubble is formed by laser breakdown in water. The toroidal bubbles are formed by the variation of the pressure field as a result of the propagation, reflection, and convergence of the laser plasma shockwave in the cavity.

Evaluation of third-order elastic constants using laser-generated multi-type ultrasound for isotropic materials.

Within the linear elasticity approximation the speed of a small-amplitude sound in conventional linear elasticity is determined only by the second order elastic (SOE) constants and the density of the medium. Subjecting the conveying solid to a static strain of a sufficient magnitude introduces the third-order elastic (TOE) constants in the equation of the sound speed. In this work we applied a homogeneous isotropic deformation caused by a thermal expansion of an aluminum alloy sample. Velocities of three acoustic modes: longitudinal, shear and Rayleigh waves were measured as functions of temperature within a range of 25-100 °C. Two TOE constants C111 and C112 were evaluated in an assumption that the third independent module C144 is far smaller than the former two.

Comparison between ray-tracing and physical optics for the computation of light absorption in capillaries--the influence of diffraction and interference.

Ray-tracing is the commonly used technique to calculate the absorption of light in laser deep-penetration welding or drilling. Since new lasers with high brilliance enable small capillaries with high aspect ratios, diffraction might become important. To examine the applicability of the ray-tracing method, we studied the total absorptance and the absorbed intensity of polarized beams in several capillary geometries. The ray-tracing results are compared with more sophisticated simulations based on physical optics. The comparison shows that the simple ray-tracing is applicable to calculate the total absorptance in triangular grooves and in conical capillaries but not in rectangular grooves. To calculate the distribution of the absorbed intensity ray-tracing fails due to the neglected interference, diffraction, and the effects of beam propagation in the capillaries with sub-wavelength diameter. If diffraction is avoided e.g. with beams smaller than the entrance pupil of the capillary or with very shallow capillaries, the distribution of the absorbed intensity calculated by ray-tracing corresponds to the local average of the interference pattern found by physical optics.

Numerical simulation of millisecond laser-induced damage in silicon-based positive-intrinsic-negative photodiode.

An axisymmetric mathematical model was established for millisecond-pulsed Nd:YAG laser heating of silicon-based positive-intrinsic-negative photodiode. The transient temperature fields were obtained by using the finite element method. The temperature dependences of the material parameters and the absorption coefficient were taken into account in the calculation. The results indicate that the optical absorption coefficient and the thermal conductivity are the two key factors for the temperature evolution. The diffusion of boron in the liquid phase and the introduction of deep-level defects in the depletion region of the photodiode were the two reasons for the millisecond laser-induced electrical degradation of the photodiode. The morphological damage threshold and electrical degradation threshold of the photodiode were obtained numerically. Meanwhile, the influence of the antireflection coating, the doping concentration, and the junction depth were also considered. The results show that the morphological damage threshold decreases with adding an antireflection coating, the increase of the doping concentration, and junction depth. The electrical degradation threshold increases only with the junction depth.