Laser Heating of a Material with Time-Dependent Laser Source

Hardening depths must be determined accurately when a material is subjected to laser heat treatment. In this paper, a semi-analytical method taking into account a time-dependent pulse shape is performed. A comparison between temporal temperatures is shown for five laser shapes. The distribution temperature and surface temperature during time processing are predicted based on a significant variety of material properties. The main result obtained is that an important time shift is observed from one temperature profile to another. It can be inferred that a hardened layer may be produced by using a time-dependent pulse shape instead of a time-independent pulse.     

Holographic interferometry of oil films and droplets in water with a single-beam mirror-type scheme

Application of single-beam reflective laser optical interferometry for oil films and droplets in water detection and characterization is discussed. Oil films can be detected by the appearance of characteristic interference patterns. Analytical expressions describing intensity distribution in these interference patterns allow determination of oil film thickness, size of oil droplets, and distance to the oil film from the observation plane. Results from these analyses indicate that oil spill aging and breakup can be monitored in real time by analyzing time-dependent holographic fringe patterns. Interferometric methods of oil spill detection and characterization can be automated using digital holography with three-dimensional reconstruction of the time-changing oil spill topography. In this effort, the interferometric methods were applied to samples from Chevron oil and British Petroleum MC252 oil obtained during the Deep Water Horizon oil spill in the Gulf of Mexico.     

Angular diagram of broadband emission of millimeter-sized water droplets exposed to gigawatt femtosecond laser pulses

We report on the experiments on the interaction of gigawatt femtosecond laser pulses with suspended millimeter-sized water droplets. The transparent droplets experienced laser-induced breakdown and explosive boiling up and emitted a broadband radiation. This radiation covers the spectral range from 450 to 1100?nm and consists of the spectrum of laser pulse scattered and transformed by the droplet due to self-phase modulation and plasma emission produced in water during photoionization. The droplet emission spectrum showed remarkable broadening at all viewing angles and is maximal in the direction of the laser exit from the droplet. The enlargement of the droplet results in additional spectral spreading of the emitted radiation. The depth and amount of laser pulse spectral self-transformations upon propagation through the water droplet are simulated by means of numerical calculations.     

Cloud-droplet-size distribution from lidar multiple-scattering measurements

A method for calculating droplet-size distribution in atmospheric clouds is presented, based on measurement of laser backscattering and multiple scattering from water clouds. The lidar uses a Nd:YAG laser that emits short pulses at a moderate repetition rate. The backscattering, which is composed mainly of single scattering, is measured with a detector pointing along the laser beam. The multiple scattering, which is mainly double scattering, is measured with a second detector, pointing at a specified angle to the laser beam. The domain of scattering angles that contribute to the doublescattering signal increases monotonically as the pulse penetrates the cloud. The water droplets within the probed volume are assumed to have a constant size distribution. Hence, from the double-scatteringmeasured signal as a function of penetration depth within the cloud, the double-scattering phase function of the scattering volume is derived. Inverting the phase function results in a cloud-droplet-size distribution in the form of a log-normal function.     

Elastic light scattering from single microparticles on a femtosecond time scale

Experimentally measured and theoretically calculated elastic light-scattering spectra from single microparticles illuminated by 100-fs pulses are presented. Although in the theoretical calculation only a single incoming femtosecond laser pulse was used, the spectral behavior of scattered light shows all the features seen in the experimental spectrum from many femtosecond pulses, including morphology-dependent resonances (MDR's). The good agreement between experimental and theoretical elastic light-scattering data has stimulated a theoretical investigation of the time-dependent behavior of the elastically scattered light from a single microparticle on a femtosecond time scale. Since the spatial pulse length of the incoming laser pulse is smaller than the particle circumference, the temporal behavior of reflection, diffraction, refraction, and coupling into MDR's can be distinguished. Since the time-dependent scattering is strongly dependent on particle size, refractive index, and pulse chirp, it may be possible to encode several bits of information into a single laser pulse and therefore to increase optical data communication rates.     

Laser heating of an aqueous aerosol particle

Approximate analytical and full numerical solutions are obtained for the transient response of both a pure water and solution droplets to both short- and long-time laser heating. The differences in the temperature and size histories between pure water and solution droplets are elucidated. The validity of of the approximate analytical solution, extended from that of Armstrong ["Aerosol Heating and Vaporization by Pulsed Light Beams," Appl. Opt. 23, 148 (1984)] in pure water droplets, is evaluated by comparison to solution of the full governing equations.     

Numerical simulation on gas-droplet flow characteristics and spray evaporation process in CFB-FGD tower

Nozzle arrangement in the nozzle spray system has a significant impact on the gas-droplet flow characteristics and the temperature distribution within the circulating fluidized bed flue gas desulphurization (CFB-FGD) tower, which is critical to the SO₂ removal efficiency. The effects of spray direction, nozzle number and nozzle spray angle on gas-droplet distribution and temperature distribution inside the FGD tower are investigated with numerical simulation based on a Eulerian-Lagrangian mathematical model. An optimal nozzle arrangement scheme is proposed to improve the contact between gas and water droplets and the flue gas temperature distribution. Results show that upward spray direction is beneficial to the interaction between water droplets, improving gas-droplet flow characteristics and spray evaporation process, and water droplets number trapped by tower wall could be reduced in the water droplets evaporation. With the increase in nozzle number, it is conducive to the contact between flue gas and water droplets to increase the evaporation efficiency of water droplets, as well as the uniformity of temperature distribution inside the tower. With nozzle spray angle increases from 30° to 120°, flue gas velocity decreases, water droplets number trapped by the tower wall increases. The temperature distribution at different cross-section is the most uniform when the nozzle spray angle is 60°.     

Laser-parameter effects on the generation of ultrabroad harmonic and ultrashort attosecond pulse in a long-plus-short scheme

By numerically solving the time-dependent Schrödinger equation for helium gas in a special two-color laser field, which is synthesized by a long (9?fs) driving pulse and a short (6?fs) controlling pulse, we discuss the influence of the carrier-envelope phase, frequency, and the intensity of the controlling pulse on the generation of harmonic spectra and isolated attosecond pluses. In the cutoff region, two or three plateaus can be controlled by optimizing these laser parameters, and an ultrabroad supercontinuum harmonic spectrum with a bandwidth of 800?eV can be produced, which can support an ultrashort isolated 4.5 as pulse generation by Fourier transformation. Furthermore, using classical ionizing and returning energy maps, time–frequency analyses are presented to explain the underlying physical mechanisms.     

Thermal energy transfer by plasmon-resonant composite nanoparticles at pulse laser irradiation

Heating of composite plasmon-resonant nanoparticles (spherical gold nanoshells) under pulse laser illumination is considered. The numerical solution of the time-dependent heat conduction equation accounting for spatial inhomogeneities of absorbed laser radiation is performed. Important features of temperature kinetics and thermal flux inside nanoparticles are analyzed. Possible applications of the observed effects in nanotechnology and medicine are discussed.     

Measuring mixing dynamics of transparent fluids with electronic speckle pattern interferometry

Electronic speckle pattern interferometry (ESPI) combined with phase-shifting techniques is used for studying the mixing dynamics of transparent fluids. The potentials of the technique for studying fluid mixing are illustrated for simple examples of water flow and moving water droplets in water. With ESPI we could actually follow water droplets moving in water and water flowing in water on a television monitor at the video rate. A He-Ne laser (lambda(0) = 632.8 nm) was used as the light source, and phase stepping was applied, giving an interferometric sensitivity better than lambda(0)/10. The observed phase changes are due to changes in the refractive index caused by small temperature differences between the droplets and the surrounding water. Temperature differences of less than 0.1 K are detectable for droplets of a diameter of 4 mm.     

Nonintrusive measurements of temperature and size of single falling raindrops

A nonintrusive laser technique, based on the detection of a rainbow, is presented that permits one to determine simultaneously the temperature and size of droplets. Therefore the Airy theory for a rainbow and a calibration rainbow pattern at isothermal conditions are applied. Rainbow patterns coming from droplets in the millimeter range have been recorded on a linear CCD array. It has been found that the sphericity of the droplets plays an important role for this rainbow-based technique.     

Experimental and Theoretical Investigation on Rapid Evaporation of Ethanol Droplets and Kerosene Droplets During Depressurization

This paper reports an experimental and theoretical study of rapid evaporation of ethanol droplets and kerosene droplets during depressurization. For experimental method, an ethanol droplet or a kerosene droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature transition. And the droplet shape variation was recorded by a high speed camera. A theoretical analysis was developed based on the heat balance to estimate the droplet center temperature transition, and the evaporation model proposed by Abramzon and Sirignano was used to describe the droplet vaporization. According to the experimental data and theoretical analysis, both of the environmental pressure and the initial droplet diameter have a prominent influence on the droplet temperature transition. Comparing the evaporation processes of ethanol droplets and kerosene droplets with water droplets, the ethanol droplets have the fastest evaporation rate, followed by water, and the evaporation rates of kerosene droplets are the slowest. Also it was found that a bubble can easily emerge within kerosene droplet, and its lifetime is more than 1 s.     

Soot volume fraction and particle size measurements with laser-induced incandescence

Laser-induced incandescence from soot was analyzed with a time-dependent, numerical model of particle heating and cooling processes that includes spatial and temporal intensity profiles associated with laser sheet illumination. For volume fraction measurements, substantial errors result primarily from changes in gas temperature and primary soot particle size. The errors can be reduced with the proper choice of detection wavelength, prompt gating, and high laser intensities. Two techniques for primary particle size measurements, based on ratios of laser-induced incandescence signals from a single laser pulse, were also examined. Compared with the ratio of two integration times, the newly proposed ratio of two detection wavelengths is better suited for simultaneous volume fraction and size measurements, because it is less temperature sensitive and produces stronger signals with, however, a lower sensitivity to size changes.     

Formation of droplet networks that function in aqueous environments

Aqueous droplets in oil that are coated with lipid monolayers and joined through interface bilayers are useful for biophysical measurements on membrane proteins. Functional networks of droplets that can act as light sensors, batteries and electrical components can also be made by incorporating pumps, channels and pores into the bilayers. These networks of droplets mimic simple tissues, but so far have not been used in physiological environments because they have been constrained to a bulk oil phase. Here, we form structures called multisomes in which networks of aqueous droplets with defined compositions are encapsulated within small drops of oil in water. The encapsulated droplets adhere to one another and to the surface of the oil drop to form interface bilayers that allow them to communicate with each other and with the surrounding aqueous environment through membrane pores. The contents in the droplets can be released by changing the pH or temperature of the surrounding solution. The multicompartment framework of multisomes mimics a tissue and has potential applications in synthetic biology and medicine.     

Femtosecond laser micromachining of waveguides in silicone-based hydrogel polymers

By tightly focusing 27 fs laser pulses from a Ti:sapphire oscillator with 1.3 nJ pulse energy at 93 MHz repetition rate, we are able to fabricate optical waveguides inside hydrogel polymers containing approximately 36% water by weight. A tapered lensed fiber is used to couple laser light at a wavelength of 632.8 nm into these waveguides within a water environment. Strong waveguiding is observed due to large refractive index changes. A large waveguide propagation loss is found, and we show that this is caused by surface roughness which can be reduced by optimizing the waveguides.     

Temperature alterations of infrared light absorption by cartilage and cornea under free-electron laser radiation

Like pure water, the water incorporated into cartilage and cornea tissue shows a pronounced dependence of the absorption coefficient on temperature. Alteration of the temperature by radiation with an IR free-electron laser was studied by use of a pulsed photothermal radiometric technique. A computation algorithm was modified to take into account the real IR absorption spectra of the tissue and the spectral sensitivity of the IR detector used. The absorption coefficients for several wavelengths within the 2.9- and 6.1-microm water absorption bands have been determined for various laser pulse energies. It is shown that the absorption coefficient for cartilage decreases at temperatures higher than 50 degrees C owing to thermal alterations of water-water and water-biopolymer interactions.     

Precise wavelength control of a single-frequency pulsed Ho:Tm:YLF laser

We demonstrate wavelength control of a single-frequency diode-pumped Ho:Tm:YLF laser by referencing its wavelength to an absorption line of carbon dioxide. We accomplish this wavelength control by injection seeding with a cw Ho:Tm:YLF laser that can be tuned over or stabilized to carbon dioxide or water vapor lines. We show that the pulsed laser can be scanned precisely over an absorption line of carbon dioxide by scanning the injection seed laser wavelength. We locked the pulsed laser to within 18.5 MHz of the absorption line center by stabilizing the injection seed on the line center. The single-frequency pulsed output, intended for use as a transmitter for differential absorption lidar detection of atmospheric carbon dioxide and water vapor and for coherent detection of wind, is 100 mJ per pulse at a 5-Hz repetition rate.     

Inverse free electron lasers and laser wakefield acceleration driven by CO2 lasers

The staged electron laser acceleration (STELLA) experiment demonstrated staging between two laser-driven devices, high trapping efficiency of microbunches within the accelerating field and narrow energy spread during laser acceleration. These are important for practical laser-driven accelerators. STELLA used inverse free electron lasers, which were chosen primarily for convenience. Nevertheless, the STELLA approach can be applied to other laser acceleration methods, in particular, laser-driven plasma accelerators. STELLA is now conducting experiments on laser wakefield acceleration (LWFA). Two novel LWFA approaches are being investigated. In the first one, called pseudo-resonant LWFA, a laser pulse enters a low-density plasma where nonlinear laser/plasma interactions cause the laser pulse shape to steepen, thereby creating strong wakefields. A witness e-beam pulse probes the wakefields. The second one, called seeded self-modulated LWFA, involves sending a seed e-beam pulse into the plasma to initiate wakefield formation. These wakefields are amplified by a laser pulse following shortly after the seed pulse. A second e-beam pulse (witness) follows the seed pulse to probe the wakefields. These LWFA experiments will also be the first ones driven by a CO(2) laser beam.     

Regularized phase tracker with isophase scanning strategy for analysis of dynamic interferograms of nonwetting droplets under excitation

The surface of a nonwetting droplet is separated from a solid surface by a continuous supply of a lubricating gas film within the apparent contact region. Under certain conditions, e.g., application of an external excitation force, the gas film thickness can decrease to a level where intermolecular forces cause the droplet to wet the surface. The thickness of the lubricating film can be measured by interferometry. Externally imposed oscillations change the shape of the film, leading to dynamic interference fringes that are recorded with a high-speed CCD camera. We propose a spatiotemporal analysis of the interference patterns based on the regularized phase-tracker method. This well-known method minimizes a cost function to estimate the absolute phase of a single element in the interferogram. A proper scanning method along all elements of the interferogram is necessary to avoid phase estimation errors that will propagate throughout the entire continuous phase image of interest. The scanning method we propose traces along contours of constant phase in the interferogram and does not require segmentation of the interferogram in dark and bright fringes. Results in the form of dynamic height profiles of droplets under excitation obtained by this method are presented.     

基于外加热激励源的中红外激光辐照VO2薄膜温升研究

VO_2薄膜在中红外激光防护领域有着重要意义。为研究中红外激光辐照VO_2薄膜的温升累积效应及克服中红外激光器功率过小的困难,设计了一种外加热激励源的实验方法。首先利用ANSYS软件对中红外纳秒脉冲激光在不同激励源温度和不同功率密度下辐照VO_2薄膜的温升累积进行仿真,并做了实际验证。研究发现,随着外加激励温度的升高,可在一定时间(100s)内达到VO_2相变温度的激光功率密度逐渐减小,通过增加功率密度,也可以进一步降低相变所需的激励源温度。两者均提高了VO_2薄膜的温升速率。然后在激励源温度为40℃、中红外激光功率密度为1 W/mm~2的条件下研究了中红外纳秒脉冲激光参数对VO_2薄膜的温升累积效应,结果表明,增加激光脉宽、增加重频或光斑半径均能有效增加VO_2薄膜的温升速率。