Reflected Laser Light Research Articles

LASER INDUCED NONLINEAR OPTICAL RESPONSE OF CARBON NANOTUBES DEPOSITED ON A DIELECTRIC SUBSTRATE

In the present paper, the nonlinear response of carbon nanotubes has been investigated by using the dielectric constants of these nanotubes evaluated by using the Maxwell-Garnett Effective Medium Theory. SWNT’s deposited on the tip of a dielectric substrate show a drastic change in reflectance and transmittance of the incident laser light due to change in their nonlinearity and optical constants. This property may be further used for developing all-optical switching devices.

Laser plasma physics in shock ignition – transition from collisional to collisionless absorption

Shock Ignition is considered as a relatively robust and efficient approach to inertial confinement fusion. A strong converging shock, which is used to ignite the fuel, is launched by a high power laser pulse with intensity in the range of 1015 − 1016 W/cm2 (at the wavelength of 351 nm). In the lower end of this intensity range the interaction is dominated by collisions while the parametric instabilities are playing a secondary role. This is manifested in a relatively weak reflectivity and efficient electron heating. The interaction is dominated by collective effects at the upper edge of the intensity range. The stimulated Brillouin and Raman scattering (SBS and SRS respectively) take place in a less dense plasma and cavitation provides an efficient collisionless absorption mechanism. The transition from collisional to collisionless absorption in laser plasma interactions at higher intensities is studied here with the help of large scale one-dimensional Particle-in-Cell (PIC) simulations. The relation between the collisional and collisionless processes is manifested in the energy spectrum of electrons transporting the absorbed laser energy and in the spectrum of the reflected laser light.

Spatial analysis of the back reflected laser light during CO2-laser cutting of metal sheets

During laser material processing such as laser welding or cutting, it is known that the whole laser power is not only absorbed in or transmitted through the interaction zone, but a significant percentage is also reflected back to the laser source. Some publications analyzing the laser welding process showed a clear correlation between the reflected laser power and the result of the process. In this report, the authors analyze the information content of back reflected laser light during the high-power CO2-laser cutting of metal sheets. For this purpose, laser reflections are detected coaxially via a scraper-mirror online during laser cutting with a high time resolution. For different materials (mild steel, stainless steel, aluminum) and thicknesses (1 and 3 mm), a detailed spatial analysis on where the laser beam is reflected from (cutting front or workpiece surface) will be given. The spatial analysis of the reflected laser light shows that only 10% is reflected from the cutting front in the kerf. The main part (>90%) is reflected from the overlap zone of the laser beam with the workpiece surface outside the cutting kerf. Therefore, the signal strongly depends on the surface roughness of the treated material. It will be shown that, as expected, reflective surfaces with little roughness show higher CO2-laser reflections than rough surfaces with a higher level of roughness. Furthermore, the fraction of back reflected laser light increases with higher cutting feed rate which can be explained with a larger overlapping zone of the CO2-laser beam with the workpiece surface in front of the cutting kerf. In addition, the back reflection signal also strongly depends on the position of the focal plane with respect to the workpiece surface. This relationship can be used to measure the absolute position of the focal plane in-process as well as detecting an undesired focus shift during laser cutting.

Preliminary study for non – invasive optical detection of squamous and basal cell carcinomas

BackgroundThe early detection of skin cancer may highly increase the chances of its healing. One of the non-invasive methods of such detection based on the Oblique- Incidence Diffuse Reflectance (OIDR) measurements of the reflected diode laser light from the skin. In this research we designed and implemented the OIDR reflectometry measuring system with a 650 nm diode laser source to aid physicians in diagnosing both squamous cell carcinomas (SCC) and basal cell carcinomas(BCC).MethodThe laser is delivered obliquely to the skin surface by an optical fiber fitted through a tube holder of CCD camera. The diffused reflected laser light from the skin is captured by the CCD camera and sent to a computer, which is supplied by a specially prepared Matlab program to analyze these images in order to decide in a time whether the lesion is malignant or benign. Fifty cases were diagnosed under supervision of the consultant section of The Governmental Specialized Marjan Teaching Hospital – MOH – Iraq.ResultThe fifty diagnosed cases by this technique, the results were 90% accurate.ConclusionThe method of laser oblique-incidence diffuse reflectance (OIDR) combined with using the developed algorithms that have high classification rates may prove useful in the clinic as the process is fast, noninvasive and accurate.

Dynamics of Relativistic Laser-Plasma Interaction on Solid Targets

A novel time-resolved diagnostic is used to record the critical surface motion during picosecond-scale relativistic laser interaction with a solid target. Single-shot measurements of the specular light show a redshift decreasing with time during the interaction, corresponding to a slowing-down of the hole boring process into overdense plasma. On-shot full characterization of the laser pulse enables simulations of the experiment without any free parameters. Two-dimensional particle-in-cell simulations yield redshifts that agree with the data, and support a simple explanation of the slowing-down of the critical surface based on momentum conservation between ions and reflected laser light.

Nonequilibrium and Relaxation in Deeply Supercooled Liquid of Isopropylbenzene Obtained through Glass Transition from Vapor-Deposited Glass

Change in the light interference in film samples of isopropylbenzene was studied in the supercooled liquid (SCL) state. Samples were originally formed as glass by vapor deposition on a cold substrate and the intensity of laser light reflection from them was monitored as the temperature was raised with a constant rate up to the region of the SCL state above the glass-transition temperature. Two types of periodic changes in the light intensity were observed in the SCL state. One was attributed to the interference condition change accompanying the structural relaxation from the low-density SCL to the equilibrium SCL state, and the other was due to the gradual expansion of the high-density SCL. Analysis of the latter change revealed that the initial density of the high-density glass was larger than that estimated in our previous paper.

Thermal imaging technique to characterize laser light reflection from thermoplastics

Characterization of laser light reflection during the laser transmission welding (LTW)33Abbreviations: LTW—laser transmission welding, CB—carbon black, IR—infrared, NPFD—normalized power flux distribution of thermoplastics is especially important for applications in which non-zero laser incidence angles are used. At higher laser incidence angles, reflection increases and has the potential to burn surrounding features of the part to be welded. This study presents and validates a technique for laser reflection measurement. Reflected energy is absorbed by a black plastic plate (containing carbon black, which is the absorber of the reflected energy). The surface temperature of the plate is measured by an infrared (IR) camera. The distribution of reflected power required to generate this temperature profile is estimated using a simple heat transfer model. The technique was validated by irradiating the black plate by the laser directly, while observing the time-varying temperature distribution of the plate by the IR camera. In this case, good agreement was observed between the estimated total power and the actual laser input power. Good agreement also existed between the estimated power distribution and that determined experimentally via a knife edge based beam profiling technique. The thermal imaging technique was subsequently used to measure the magnitude and distribution of laser light reflection from unreinforced nylon 6.

Use of thermal imaging to characterize laser light reflection from thermoplastics as a function of thickness, laser incidence angle and surface roughness

Laser light reflection during the laser transmission welding (LTW) of thermoplastics has the potential to overheat and/or cause unintentional welding of adjacent features of the part being welded. For this reason, and in order to assess how much light is being absorbed by the transparent part (after measurement of the light transmitted through the transparent part), it is important to be able to quantify the magnitude and distribution of reflected light. The magnitude and distribution of the reflected light depends on the total laser input power as well as its distribution, the laser incidence angle (angle between the normal to the transparent part surface and the laser beam), the laser light polarization as well as the surface and optical properties of the transparent part. A novel technique based on thermal imaging of the reflected light was previously developed by the authors. It is used in this study to characterize the magnitude and distribution of reflected light from thermoplastics as a function of thickness (1–3.1mm), laser incidence angle (20–40°) and surface roughness (0.04–1.04μm). Results from reflection tests on nearly polished nylon 6 (surface roughness between 0.04 and 0.05μm) have shown that, for the various thicknesses tested (1–3.1mm), the total reflection was larger than the specular top surface reflection predicted via the Fresnel relation. From these observations, it is conjectured that, in addition to top surface reflection, the bulk and/or bottom surface also contribute to the total reflection. The results also showed that reflection decreased slightly with increasing thickness. As expected, for the p-polarized light used in this study, the reflection decreased with increasing angle of incidence for the range of angles studied. It was also found that when the surface roughness was close to zero and when it was close to the wavelength of the input laser beam (i.e. 940nm), the reflectance values were close and reached a minimum between these two roughness values.

A comparison of light spot hydrophone and fiber optic probe hydrophone for lithotripter field characterization

The performance of a newly developed light spot hydrophone (LSHD) in lithotripter field characterization was compared to that of the fiber optic probe hydrophone (FOPH). Pressure waveforms produced by a stable electromagnetic shock wave source were measured by the LSHD and FOPH under identical experimental conditions. In the low energy regime, focus and field acoustic parameters matched well between the two hydrophones. At clinically relevant high energy settings for shock wave lithotripsy, the measured leading compressive pressure waveforms matched closely with each other. However, the LSHD recorded slightly larger |P_| (p < 0.05) and secondary peak compressive pressures (p < 0.01) than the FOPH, leading to about 20% increase in total acoustic pulse energy calculated in a 6 mm radius around the focus (p = 0.06). Tensile pulse durations deviated ~5% (p < 0.01) due to tensile wave shortening from cavitation activity using the LSHD. Intermittent compression spikes and laser light reflection artifacts have been correlated to bubble activity based on simultaneous high-speed imaging analysis. Altogether, both hydrophones are adequate for lithotripter field characterization as specified by the international standard IEC 61846.

Formation of organized nanostructures from unstable bilayers of thin metallic liquids

Dewetting of pulsed-laser irradiated, thin (&amp;lt;20 nm), optically reflective metallic bilayers on an optically transparent substrate with a reflective support layer is studied within the lubrication equations model. A steady-state bilayer film thickness (h) dependent temperature profile is derived based on the mean substrate temperature estimated from the elaborate thermal model of transient heating and melting/freezing. Large thermocapillary forces are observed along the plane of the liquid-liquid and liquid-gas interfaces due to this h-dependent temperature, which, in turn, is strongly influenced by the h-dependent laser light reflection and absorption. Consequently the dewetting is a result of the competition between thermocapillary and intermolecular forces. A linear analysis of the dewetting length scales established that the non-isothermal calculations better predict the experimental results as compared to the isothermal case within the bounding Hamaker coefficients. Subsequently, a computational non-linear dynamics study of the dewetting pathway was performed for Ag/Co and Co/Ag bilayer systems to predict the morphology evolution. We found that the systems evolve towards formation of different morphologies, including core-shell, embedded, or stacked nanostructure morphologies.

Measurement and Decapsulation of IC Molding Resin Using YAG Laser for Failure Analysis

We are now developing a laser decapsulation technique of IC molding resin for failure analysis. Laser machining shortens decapsulation process time and reduces adverse environment effects compared to conventional etching methods. However, pure laser decapsulation cannot avoid the damage of IC chips. Therefore, it is necessary to leave molding resin on the chip surface. In this paper, the decapsulation experiment results using a YAG laser are described. In addition, to estimate residual molding thickness, the method of measuring laser light-reflection intensity is proposed. The relation between molding thickness and light-reflection intensity was examined by irradiating a low-output processing laser. And the results of measurement and decapsulation experiments using an experimental system are also described.

Reflection of femtosecond laser light in multipulse ablation of metals

The shot-to-shot reflectance of high-intensity laser light is studied as a function of both the number of laser shots and laser fluence in multipulse ablation of a metal when the irradiated surface undergoes structural changes from an initially smooth surface to a deep crater. Our study shows that the reflectance of the irradiated surface significantly decreases due to the high intensity of laser pulses and the laser-induced surface structures in ablation regimes typically used for femtosecond laser processing of materials. The high-intensity effect dominates in the reflection reduction at low numbers of laser shots when laser-induced surface structures do not cause the reflectance to decrease noticeably. With increasing the number of laser shots, the structural effect comes into play, and both high-intensity and structural effects quickly reduce the reflectance of the sample to a low value.

Laser plasma interaction studies in the context of shock ignition—Transition from collisional to collisionless absorption

The shock ignition concept implies laser pulse intensities higher than 1015 W/cm2 (at the wavelength of 351 nm), which is the commonly accepted limit where the inverse bremsstruhlung absorption dominates. The transition from collisional to collisionless absorption in laser plasma interactions at higher intensities is studied in the present paper with the help of large scale one-dimensional particle-in-cell simulations. The initial parameters are defined by the hydrodynamic simulations corresponding to recent experiments. The simulations predict that a quasi-steady regime of laser plasma interaction is attained where the total laser energy absorption stays on the level of ∼65% in the laser intensity range 1015–1016 W/cm2. However, the relation between the collisional and collisionless processes changes significantly. This is manifested in the energy spectrum of electrons transporting the absorbed laser energy and in the spectrum of the reflected laser light.

Nonlinear absorption of intense short pulse laser over a metal surface embedded with nanoparticles

AbstractThe anomalous absorption of laser, incident at an arbitrary angle of incidence on a metal surface embedded with nanoparticles, is studied. The electrons inside a nanoparticle resonantly absorb laser energy when the laser frequency equals the frequency of surface charge oscillations of the nanoparticle. A monolayer of nanoparticles of radius rnp0 ≈ 50 A with inter-particle separation d ~ 10rnp0 can cause up to 40% reduction of the reflection of p-polarized laser light. The absorption coefficient increases with the angle of incidence and has a sharp peak at a resonant frequency width of about 1%. At high laser power, even if the nanoparticles are initially off resonant with the laser, the particle heating and subsequent expansion reduces the resonance frequency, and the resonance absorption is realized after a time delay. The delay is found to be directly proportional to the cluster size and inversely proportional to the laser intensities.

Sensitivity of Depth-Integrated Satellite Lidar to Subaqueous Scattering

A method is presented for estimating subaqueous integrated backscatter using near-nadir viewing satellite lidar. The algorithm takes into account specular reflection of laser light, laser scattering by wind-generated foam as well as sun glint and solar scattering from foam. The formulation is insensitive to the estimate of wind speed but sensitive to the estimate of transmittance used in the atmospheric correction. As a case study, CALIOP data over Tampa Bay were compared to MODIS 645 nm remote sensing reflectance, which previously has been shown to be nearly linearly related to turbidity. The results indicate good correlation on nearly all CALIOP cloud-free dates during the period 2006 through 2007, particularly those with relatively high atmospheric transmittance. The correlation decreases when data are composited over all dates but is still statistically significant, a possible indication of variability in the biogeochemical composition in the water. Overall, the favorable results show promise for the application of satellite lidar integrated backscatter in providing information about subsurface backscatter properties, which can be extracted using appropriate models.

Early season remote sensing of wheat nitrogen status using a green scanning laser

In-season, spatially variable nitrogen (N) fertilizer applications in agricultural systems can help to maximize crop N use efficiency and minimize N losses via hydrological leaching, runoff, and atmospheric volatilization. N fertilizer management often relies upon measurements of crop spectral reflectance using ground-based optical on-the-go sensors or hand-held chlorophyll meters. However, soil background reflectance can confound on-the-go sensing, especially during early crop growth stages, and hand-held chlorophyll meters are impractical for spatially explicit mapping at the field scale. Scanning laser technology is available that measures the intensity of the reflected laser light plus height information within a mm-scale ground instantaneous field of view at a very high sampling rate (up to 50,000 points s −1 in this study). We examined the ability to quantify foliar N status of spring wheat ( Triticum aestivum L.) using a green (532 nm) terrestrial laser scanner during an early stem extension growth stage (Zadoks growth stage 3.2). Laser data were processed by (1) removing soil background returns based on laser-determined height information, (2) standardizing green laser intensity based on white-reference panel readings, and (3) filtering noisy laser returns from leaf edges based on a laser return intensity threshold value. The return intensity of the reflected green laser light more accurately ( r 2 = 0.68, RMSE = 0.30 μg g −1) predicted foliar N concentration than conventional chlorophyll meter readings ( r 2 = 0.36, RMSE = 0.41 μg g −1) and spectral indices measured by a ground optical on-the-go sensor ( r 2 < 0.41, RMSE > 0.39 μg g −1). The results indicate that laser scanners are useful for measuring the N status of wheat during early growth stages, and provide justification for incorporating laser scanner based measurements into developing spatially-explicit estimates of foliar N during this critical growth period. Further research is needed to evaluate the operational practicality of a green scanning laser from a moving platform.

Optical fiber corrosion sensor based on laser light reflection

The development of an optical fiber corrosion sensor based on the principle of lightreflection is presented in this paper. The sensor consists of an optical fiber reflection sensorand a tube/film subassembly formed by welding a sacrificial metallic film to a steel tube.One side of the sacrificial metallic film is finely polished and is isolated from theenvironment while the other side is exposed to the corrosive environment. The corrosionpits initiated at the exposed film surface slowly penetrate the sacrificial film as theexposure time increases. The corrosion pits that reach the polished surface reducethe surface reflectivity of the polished surface. This decrease in reflectivity isdetected by the optical fiber reflectivity sensor. To interrogate multiple sensorsat one time, a sensor multiplexing scheme was implemented. The principles ofoperation, packaging, and characterization of the corrosion sensors are presented.

Hybrid ion acceleration with ultrathin composite foils irradiated by high intensity circularly-polarized laser light

A complete analytical description of ion acceleration in the laser radiation-pressure regime is presented. The combined effects of hot electron and light-pressure phenomena are used to qualitatively and quantitatively describe most recent experimental results in this regime. An essential part of the developed model is exhibited in the calculation of nonlinear laser light reflection and transmission properties, as well as in the spectral characterization of the laser light after interaction. The validity of the analytical model is supported by recent experimental results and by particle-in-cell simulations.

Signal overlap in the monitoring of laser welding

Laser weld monitoring is usually based on the feedback from three photodiodes which are intended to provide independent information about the thermal condition of the melt (the T signal), the radiation from the plume of a heated gas above the melt (the P signal) and the amount of reflected laser light (the R signal). This work demonstrates that, in fact, the plume of the hot gas above the weld pool contributes a large part of the thermal signal, which has hitherto been assumed to come only from the melt itself. It is suggested that the correlation between the T and P signals is so strong that a T–P signal would be more useful than the raw T signal in identifying the fluctuations in infrared radiation from the melt pool.

Single-layered PDLC films for electrically variable laser light reflection application

Electrically controllable two-beam reflection of coherent light double-passed through the microscale polymer-dispersed liquid crystal (PDLC) single layer is reported. By PDLC medium, the intensities of two reflected beams can be reciprocally changed by applying a suitable voltage in the range 0–100 V. The optical phase shift electrically induced for coherent light propagating in two passes through the single-layered PDLC film, as well as the additional phase shift externally introduced by the substrate of the PDLC cell, are responsible for the electrically controllable reflection in a pair of two beams with complementary voltage-dependent intensity behaviors.