Absorption Of Optical Energy Research Articles

Thermocavitation in gold-coated microchannels for needle-free jet injection

Continuous-wave lasers generated bubbles in microfluidic channels are proposed for applications such as needle-free jet injection due to their small size and affordable price of these lasers. However, water is transparent in the visible and near-IR regime, where the affordable diode lasers operate. Therefore, a dye is required for absorption, which is often unwanted in thermocavitation applications, such as vaccines or cosmetics. In this work, we explore a different mechanism of the absorption of optical energy. The microfluidic channel wall is partially covered with a thin gold layer, which absorbs light from a blue laser diode. This surface absorption is compared with the conventional volumetric absorption by a red dye. The results show that this surface absorption can be used to generate bubbles without the requirement of a dye. However, the generated bubbles are smaller and grow slower when compared to the dye-generated bubbles. Furthermore, heat dissipation in the glass channel walls affects the overall efficiency. Finally, degradation of the gold layer over time reduces the reproducibility and limits its lifetime. Further experiments and simulations are proposed to potentially solve these problems and optimize the bubble formation. Our findings can inform the design and operation of microfluidic devices used in phase transition experiments and other cavitation phenomena, such as jet injectors or liquid dispensing for bio-engineering.

Theoretical Comparison of Optothermal Absorption in Transmissive Metalenses Composed of Nanobricks and Nanoholes

Background: Optical components with high damage thresholds are very desirable in intense-light systems. Metalenses, being composed of phase-control nanostructures with peculiar properties, are one of the important component candidates in future optical systems. However, the optothermal mechanism in metalenses is still not investigated adequately. Methods: In this study, the optothermal absorption in transmissive metalenses made of silicon nanobricks and nanoholes is investigated comparatively to address this issue. Results: The geometrical dependencies of nanostructures’ transmittance, phase difference, and field distribution are calculated numerically via simulations. To demonstrate the optothermal mechanism in metalenses, the mean absorption efficiencies of the selected unit-cells, which would constitute metalenses, are analyzed. The results show that the electric field in the silicon zone would lead to an obvious thermal effect, and the enhancement of the localized electric field also results in the strong absorption of optical energy. Then, two typical metalenses are designed based on these nanobricks and nanoholes. The optothermal simulations show that the nanobrick-based metalens can handle a power density of 0.15 W/µm2, and the density of the nanohole-based design is 0.12 W/µm2. Conclusions: The study analyzes and compares the optothermal absorption in nanobricks and nanoholes, which shows that the electric-field distribution in absorbent materials and the localized-field enhancement are the two key effects that lead to optothermal absorption. This study provides an approach to improve the anti-damage potentials of transmissive metalenses for intense-light systems.

Synthesis and Photoluminescence of Nanostructures Based on Zinc, Cadmium, and Manganese Sulfides in a Polyacrylate Matrix

The photoluminescence of nanostructures based on zinc, cadmium, and manganese sulfides is studied in relation to the conditions of synthesis and doping in a (poly)methyl methacrylate (PMMA) medium. Photoluminescence excitation is defined by interband electronic transitions in the semiconductor structure, by the absorption of optical radiation energy at structural defects in the crystals, and by energy transfer to the excited energy levels of Mn2+ ions. The luminescence signal results from recombination processes at the levels of structural defects mainly at the surface of particles and from 4T1 → 6A1 electronic transitions between the intrinsic energy levels of Mn2+ ions. Based on variations observed in the photoluminescence and photoluminescence excitation spectra of PMMA/(Zn, Cd, Mn)S composites, some inferences about the structure of the particles are made. It is shown that their photoluminescence is influenced by the distribution of Mn2+ ions in the structure of the layers and at the surface of particles.

Синтез и фотолюминесценция наноразмерных структур на основе сульфидов цинка, кадмия и марганца в полиакрилатной матрице

Photoluminescence of nanoscale structures based on zinc, cadmium and manganese sulfides depending on the conditions of synthesis and doping in the medium (poly)methylmethacrylate (PMMA). Photoluminescence excitation is associated with interband transitions of electrons in the semiconductor structures, absorption of optical radiation energy by defects in the crystal structure, as well as with the transfer of energy to the excited energy levels of Mn2+ ions. Luminescence occurs as a result of recombination processes at the levels of defects in the structure of the surface of particles and 4T1 → 6A1 transitions between the proper energy levels of Mn2+ ions. Based on changes in the photoluminescence spectra and photoluminescence excitation of PMMA/(Zn,Cd,Mn)S compositions, assumptions are made about the structure of particles. It is shown that their photoluminescence is affected by the distribution of Mn2+ ions in the structure of layers and on the surface of particles.

Theory of dielectric loss in Graphene-on-substrate: A tight- binding model study

Graphene-on-substrate exhibits interesting dielectric behaviour due to screening of coulomb interaction induced by many body effects. In this communication we attempt to study the dielectric loss property of graphene within tight-binding model approach. The Hamiltonian consisting of electron hopping upto third-nearest-neighbour's with impurities in two in equivalent sub-lattices. The graphene-on-substrate raises the energy +Δ at one sub lattice and reduces energy -Δ at other sub lattice. Further we introduced coulomb interaction between π - electrons at the two sub lattices separately with the same effective coulomb interaction. We calculate polarization function Π(q, ω) which is a two particle Green's function arising due to charge-charge correlation by using Zubarev's Green's function technique. Finally we calculate dielectric function of graphene i.e. ε(q, ω) =1+Π(q,ω) at arbitrary wave vector q and frequency ra. The dielectric loss in graphene calculated from the imaginary part of dielectric function which is a measure of absorption spectrum. Only a few Fragmentary theoretical attempts have been made to utilize the full frequency and wave vector dependent dielectric function. We compute numerically the frequency dependent dielectric loss function for 100x100 momentum grid points. We observe a low energy Plasmon resonance peak and a high energy flat peak arising due to absorption of optical energy at substrate induced gap. With increase of small Plasmon wave vector, both low and high energy peaks approach each other. The dielectric loss at low energies exhibits a parabolic curve, but it exhibit a clear peak on introduction of higher order electron hopping's. The Coulomb interaction suppresses induced gap in graphene and decreases the optical energy absorption spectra. The increase of substrate induced gap shifts the high energy flat peak to higher energies and enhances the dielectric loss throughout the frequency range. Finally the effect of doping on dielectric loss is investigated and compared with the experimental results.

Light-induced thermal convection for size-based microparticle sorting

Separating and sorting of micrometer-sized particles in liquids are important subjects in current analytical chemistry, colloidal chemistry, environmental chemistry, and biological technology. In this paper, we propose and demonstrate a microparticle separating and sorting method based on light-induced thermal convection (LITC). The thermal convection generated as a result of frequency-dependent absorption of optical energy plays an important role in microparticle trapping and manipulation. By using the LITC, we can separate and sort microparticles of different sizes. Manipulation of microparticle sorting is noncontact, which contributes to keeping the natural characteristics of the microparticles. In addition, compared with traditional sorting methods, the microparticle sorting method based on the LITC effect is much easier to implement and convenient to manipulate. This possibility adds a light-induced thermal convection effect as an additional degree of freedom to microparticle separating and sorting.

Effect of heat source on the growth of dendritic drying patterns

Shining a tightly-focussed but low-powered laser beam on an absorber dispersed in a biological fluid gives rise to spectacular growth of dendritic patterns. These result from localized drying of the fluid because of efficient absorption and conduction of optical energy by the absorber. We have carried out experiments in several biologically relevant fluids and have analysed patterns generated by different types of absorbers. We observe that the growth velocity of branches in the dendritic patterns can decrease below the value expected for natural drying.

Rapid Simulation of Laser Processing of Discrete Particulate Materials

The objective of this paper is to develop a computational model and corresponding solution algorithm to enable rapid simulation of laser processing and subsequent targeted zonal heating of materials composed of packed, discrete, particles. Because of the complex microstructure, containing gaps and interfaces, this type of system is extremely difficult to simulate using continuum-based methods, such as the Finite Difference Time Domain Method or the Finite Element Method. The computationally-amenable model that is developed captures the primary physical events, such as reflection and absorption of optical energy, conversion into heat, thermal conduction through the microstructure and possible phase transformations. Specifically, the features of the computational model are (1) a discretization of a concentrated laser beam into rays, (2) a discrete element representation of the particulate material microstructure and (3) a discrete element transient heat transfer model that accounts for optical (laser) energy propagation (reflection and absorption), its conversion into heat, the subsequent conduction of heat and phase transformations involving possible melting and vaporization. A discrete ray-tracking algorithm is developed, along with an embedded, staggered, iterative solution scheme, which is needed to calculate the optical-to-thermal conversion, particle-to-particle conduction and phase-transformations, implicitly. Numerical examples are given, focusing on concentrated laser beams and the effects of surrounding material conductivity, which draws heat away from the laser contact zone, thus affecting the targeted material state.

A brief account of nanoparticle contrast agents for photoacoustic imaging.

Photoacoustic imaging (PAI) is a hybrid, nonionizing modality offering excellent spatial resolution, deep penetration, and high soft tissue contrast. In PAI, signal is generated based on the absorption of laser-generated optical energy by endogenous tissues or exogenous contrast agents leading to acoustic emissions detected by an ultrasound transducer. Research in this area over the years has shown that PAI has the ability to provide both physiological and molecular imaging, which can be viewed alone or used in a hybrid modality fashion to extend the anatomic and hemodynamic sensitivities of clinical ultrasound. PAI may be performed using inherent contrast afforded by light absorbing molecules such as hemoglobin, myoglobin, and melanin or exogenous small molecule contrast agent such as near infrared dyes and porphyrins. However, this review summarizes the potential of exogenous nanoparticle-based agents for PAI applications including contrast based on gold particles, carbon nanotubes, and encapsulated copper compounds.

BiotecVisions 2012, November

BiotecVisions 2012, November

Temperature amplification during laser heating of polycrystalline silicon microcantilevers due to temperature-dependent optical properties

The absorption of optical energy and subsequent thermal transport are investigated experimentally and numerically for 500 μm long, 250 μm wide, and 2.25 μm thick polycrystalline silicon microcantilevers irradiated by an 808 nm continuous-wave laser. Temperature profiles were measured using Raman thermometry at 314 and 532 mW of laser power, and the microcantilever peak temperature was measured for laser powers up to 719 mW. A modular technique for multilayered structures is used to calculate the optical absorption in polysilicon microcantilevers, and the thermal response is then calculated with a two-dimensional, finite difference model. Very good agreement is obtained between the measured and calculated temperature profiles and peak temperatures versus laser power. The abrupt increase or amplification of peak temperature for laser powers between 415 and 440 mW is shown to be a result of peaks in the temperature-dependent optical absorptance.

Seismic excitation using a high power CO2 laser

Lasers provide one method of remotely exciting propagating waves in soil. The absorption of optical energy results in plasma formation, a dynamic change near the soil surface, and excitation of propagating waves. Experiments were performed using a high power CO2 pulsed laser to determine the pressure waveforms generated in desert sand and caliche clay. The laser energy ranged from 400–750 J with a pulse duration of 35 μs. The resulting pressure pulse was observed for duration of at least 400 μs and had a peak pressure of 100 kPa. The duration increased at greater depths, while the peak pressure and energy in a pulse decreased rapidly at these depths. Surface velocity reached a peak of 35 mm/s as acquired on geophones placed in proximity to the laser plasma event. This system, in combination with a laser Doppler vibrometer, has the potential to be a useful component of non‐invasive mine detection equipment.

Conversion of CO2to CO with CH4over Ni/SiO2Catalyst: Photoacoustic Measurements of Reaction Rate

The catalyzed reaction of carbon dioxide with methane to produce hydrogen and carbon monoxide, CO2 + CH4 → 2CO + 2H2 (∆H=59.1 kcal/mol), has received increasing attention from both an environmental and an industrial perspective. The reaction is attractive because the greenhouse gases (CO2 and CH4) can be converted into a synthesis gas with a low H2/CO ratio, which is suitable for the syntheses of valuable oxygenated chemicals such as aldehydes, methyl alcohol, acetic acid, and dimethyl ether. Ni, Pt, and Ru based catalysts are known to be effective for the CO2-CH4 reaction. One of the major problems is the deactivation of catalysts due to the formation of coke on the catalysts during the reaction. The noble metal based catalysts are less sensitive to coking compared to nickel based catalysts. Nevertheless, it is still worthwhile to develop the nickel-based catalysts resistant to the formation of coke because the noble metals are expensive and of limited availability. The reforming of methane with carbon dioxide over nickel based catalysts has been studied by many investigators. However, there are few reports about fundamental kinetic parameters at present and there are some disagreements on the reaction mechanism including the reaction orders and the activation energy. The catalytic research involves the measurement of reaction rate and the investigation of factors influencing the reaction rate. An accurate description of the reaction rate can be obtained from a rate law which is essential in a reactor design. Although it is difficult to determine the accurate mechanism from an empirical kinetic expression, such empirical kinetic expression may be helpful to understand the catalytic reaction mechanism. The most important consideration in the kinetic experiments is to measure the concentration of reactants or products of interest as a function of time. To measure the concentration, absorption spectroscopy techniques have been generally used. However, their sensitivities may be diminished due to the difficulty in precise measurements of the difference between the incident and the transmitted intensities of radiation. The photoacoustic spectroscopy (PAS) being based on a direct measurement of acoustic waves generated owing to the absorption of optical energy provides highly selective detection and extremely low molecular gas level detection, which enable us to obtain a signal with enough intensity for even time-resolved experiments. Further advantages of this technique can be found in the capability of the elimination of any sophisticated sampling techniques which are inevitable in the conventional techniques. The photoacoustic spectroscopy is thus believed to be a suitable technique for the in situ experiment of a catalytic reaction because it can directly measure the absorption characteristics of the samples of interest. In the previous works, we have shown that the photoacoustic spectroscopy is applicable to the kinetic studies of CO oxidation and water-gas shift reaction over catalysts. In this work, the temporal change in the concentration of CO2 in the catalytic CO2-CH4 reaction was measured in situ as a function of time by using a CO2 laserbased photoacoustic spectroscopy with a suitable difference photoacoustic cell. The measurements were governed at a relatively low reactor pressure so that the technique provided precise kinetic data for the initial reaction stage characterized by high reaction rates. The kinetic data obtained were compared with those of other investigators.

Thermoelastic contributions to picosecond-timescale reflectivity measurements in metals (abstract)

The increase in temperature at the surface of a metal during absorption of optical energy is a dynamic, multistep process. This process has been studied in a variety of metallic systems because it provides direct insight into the fundamental interaction between conduction electrons and the rest of the material structure. In particular, high-time-resolution, ultrafast measurements of thermal and elastic transients have proven to be useful for investigating the structure, thermophysical properties, and elastic properties of thin films [C. A. Paddock and G. L. Eesley, J. Appl. Phys. 60, 285 (1986); C. Thomsen, H. T. Grahn, H. J. Maris, and J. Tauc, Phys. Rev. B 34, 4129 (1986); O. B. Wright, Phys. Rev. B 49, 9985 (1994)]. During ultrafast optical excitation of metals, the rapid deposition of energy causes the absorbing electrons to enter a superheated state, dispersing beyond the excitation volume before energy is deposited to the lattice. The rate of energy transfer between electrons and phonons is described using the electron–phonon coupling parameter; the same parameter that figures prominently in descriptions of classical superconductivity processes [P. B. Allen, Phys. Rev. Lett. 59, 1460 (1987)]. For experimental observations on long time scales (>1 ns), thermal transport measurements are assumed to be independent of elastic deformations in the material, and the analysis of the thermal and elastic responses can be separated. This can also be done for ultrafast signals, for which reflectivity measurements taken immediately after excitation can be modeled using a completely thermal analysis if reduced values for the electron–phonon coupling parameter are used. These reduced values are typically interpreted when strong electronic signals are not observed, and often vary substantially [G. Tas and H. J. Maris, Phys. Rev. B 49, 15046 (1994); V. E. Gusev and O. B. Wright, Phys. Rev. B 57, 2878 (1998); J. L. Hostetler, A. N. Smith, D. M. Czajkowsky, and P. M. Norris, Appl. Opt. 38, 3614 (1999)] from accepted values that are experimentally determined from measurements using the cooling rate of superheated electrons [S. D. Brorson, A. Kazeroonian, J. S. Moodera, D. W. Face, T. K. Cheng, E. P. Ippen, M. S. Dresselhaus, and G. Dresselhaus, Phys. Rev. Lett. 64, 2172 (1990)] or superconductivity. In this work, we show that these reduced values for the electron–phonon parameter tend to lower estimates of thermal diffusion at longer times producing significant deviations from measured results. By including elastic contributions to the modeled signal at all times, agreement with measured signals can be obtained for short and long times using values for materials parameters that are in agreement with those obtained from other techniques.

Ultrafast optoacoustic wave generation effects on picosecond-timescale reflectivity measurements in metal thin films

The increase in temperature at the surface of a metal during absorption of optical energy is a dynamic, multistep process. High-time-resolution, ultrafast measurements of thermal and elastic transients have proven to be useful for investigating the structure, thermophysical properties, and elastic properties of thin films [Paddock and Eesley, J. Appl. Phys. 60, 285–290 (1986)]. During ultrafast optical excitation of metals, the rapid deposition of energy causes the absorbing electrons to enter a superheated state, dispersing beyond the excitation volume before energy is deposited to the lattice with the rate of energy transfer between electrons and phonons being described using the electron–phonon coupling parameter. Typically, ultrafast reflectivity measurements taken immediately after excitation can be modeled using a completely thermal analysis if reduced values for the electron–phonon coupling parameter are used. These reduced values often vary substantially [Gusev and Wright, Phys. Rev. B 57, 2878–2888 (1998)] from accepted values that are experimentally determined from measurements using the cooling rate of superheated electrons. By including elastic contributions to the modeled signal at all times, agreement with measured signals can be obtained for short and long times using values for materials parameters that are in agreement with those obtained from other techniques.

NUMERICAL ANALYSIS OF HEAT TRANSFER IN A PHOTOVOLTAIC PANEL, I: INDOOR CASES

This paper presents a numerical analysis of heat transfer in a photovoltaic (PV) panel for indoor cases. A mathematical model was first described from energy conservations, in which the effects of thermal storage, the absorption of optical energy, as well as the thermal contact resistances at interfaces between materials in the PV panel were taken into consideration. Based on this model, for indoor cases, numerical analysis of heat transfer was then carried out by using a fractional scheme under the assumptions of constant physical properties, and constant efficiency of the optical energy conversion for the panel working under indoor conditions. It was found that the thermal contact resistances at interfaces play a profound role of the temperature field in a PV panel. The effect of thermal storage is rather significant for the evolution of average surface temperature and average temperature of solar cells. However, the extinction coefficient of the transparent glass cover shows just a slight influence on temperature field.

Formation of multidomain structures in the interaction of elliptic light beams with a gas mixture

It is shown that the absorption of optical energy by vibrational levels of a gas mixture can lead to the formation of several high-temperature domains along the region of propagation of a light pulse. Point and distributed models of the interaction of laser radiation with a gas are analyzed. The control of parameters can be used to control the position of domains and their number. For practical applications (e.g., data recording in the bulk of a medium), the existence of two or more high-temperature regions in combination with the dynamics of chemical reactions may have both positive and negative effects.

Modeling and recording characteristics of a dye-doped polymer bilayer film

The optical, thermal, and structural behavior of a laser-irradiated dye-in-polymer bilayer film consisting of an elastic and a plastic layer was investigated by a computer modeling and recording experiment. The results of numerical calculations describe absorption of optical energy in the recording layers, the heat flow, and the resultant bump formation throughout the multilayer. Recording performance of the material is strongly affected by the dye concentration and the thickness of the plastic layer. The calculated and measured properties of the media are compared, and the relationship between the dye concentration, thickness of plastic layer, and thermal, structural, and recording performance are described.

Thermal and structural analysis of a polymer bilayer film as an erasable optical material

The present study is primarily concerned with the absorption of optical energy, temperature distributions and the resultant recording performances of a laser irradiated dye-in-polymer bilayer film consisting of an elastic and a plastic layer. From the results of numerical calculations and recording experiments, it is found that the recording performance of the material is strongly influenced by the dye concentration and the thickness of the plastic layer. An optimum dye concentration in the plastic layer ensures the maximum expansion of the recording layer by softening the plastic layer. An optimum thickness of the plastic layer gives maximum reflectance and height of structurally stable bumps. These optimum parameters lead to an optimal design of the bilayer as an optical recording material.

Nonlinear absorption of optical radiation energy by microparticles in a transparent dielectric

Nonlinear absorption of energy and thermal processes during interaction of optical radiation pulses with microparticles in a transparent dielectric are theoretically studied, taking into account temperature dependence of the thermal and optical parameters.