bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of mass diffusivity in air using thermal wave interference detection
Authors: Lima J. A. P. , da Silva M. G. , Sthel M. S. , Cardoso S. L. , Vargas H. , Marin E. , Miranda L. C. M.
Published: January 2003

A discussion on the use of the thermal wave interference (TWI) for the monitoring of the transient of hydrocarbon in air is presented. The thermal wave signal was modeled using the logarithm-mixing model for the thermal diffusivity of a two-phase gas system in which the hydrocarbon vapor concentration in the air-filled TWI cell is a varying function of time. The time varying hydrocarbon vapor concentration was described assuming the simple Fick’s model for mass diffusion of the hydrocarbon vapor in the stagnant air column of the TWI cell. The transient TWI signal amplitude data fitting yielded two parameters, namely, the saturation concentration and the characteristic diffusion time. From the corresponding values of the diffusion time the hydrocarbon mass diffusivities were straightforwardly obtained.
Kolakoski-(2m,2n) are limit-periodic model sets
Authors: Sing Bernd
Published: January 2003

We consider (generalized) Kolakoski sequences on an alphabet with two even numbers. They can be related to a primitive substitution rule of constant length ℓ. Using this connection, we prove that they have pure point dynamical and pure point diffractive spectrum, where we make use of the strong interplay between these two concepts. Since these sequences can then be described as model sets with ℓ-adic internal space, we add an approach to “visualize” such internal spaces.
Application of photothermal effect to manufacture ultrasonic actuators (abstract)
Authors: Zhang Shu-yi , Cheng Li-ping , Shui Xiu-ji , Yu Jiong , Dong Shu-xiang
Published: January 2003

Photothermal (PT) effect has been applied to manufacture disks [A. C. Tam, a lecture at the Institute of Acoustics, Nanjing University, People’s Republic of China (1996)] and magnetic head sliders for disk drives [A. C. Tam, C. C. Poon, and L. Crawforth, Analyt. Sci. 17, s 419 (2001)]. Now we apply the PT effect to manufacture ultrasonic motors (actuators). Recently, the ultrasonic actuators with different ultrasonic modes, such as Rayleigh (surface acoustic) mode, Lamb (plate) mode, etc., have been developed. We have designed and fabricated two rotary motors driven by surface acoustic wave (SAW) with different frequencies, but lower than 30 MHz [L. P. Cheng, G. M. Zhang, S. Y. Zhang, J. Yu, and X. J. Shui, Ultrasonics 39, 591 (2002)]. On the SAW motors (actuators), two Rayleigh wave beams were generated and propagating along the surface of a 128° YK-LiNbO3 substrate in opposite directions with each other as a stator, and a plastic disk with balls distributed along the circle of the disk was as a rotor. For miniaturizing the rotary SAW motors, and increasing the rotation velocity, the SAW frequency must be increased. Then we improve the manufacturing technology of the mechanical structure by PT effect instead of the conventional mechanical processes of the stator and rotor of the motor. A new type of rotary SAW motor (actuator) has been fabricated, in which both SAW beams with opposite propagating directions are excited by two pairs of interdigital transducers with the frequency between 30–50 MHz. In the surface of the stator (128° YX-LiNbO3 substrate), a hole with the depth about 500 μm is impinged by a focused pulsed Nd:YAG laser beam (PT effect) between two SAW propagating ways on the 128° YX-LiNbO3 substrate for fixing the axis of the motor, with the frequency between 30–50 MHz. In the bottom of the rotor (plastic disk), a lot of crown (flange) blocks with the high of 20–30 μm and the diameter of also 20–30 μm can be made by the focused pulsed Nd:YAG laser a focused continuous Ar+ laser heating (PT effect) for contacting with the stator. The symmetrical frictional force pairs produced between the crown blocks of the rotor with the local supporting SAW deformation points of the stator around the circle of the rotor to drive the motor to rotate. This kind of rotary SAW actuator can be applied to drive a recording head of HDD with satisfactory performance.
Optical properties in the visible and infrared spectrum of fiber-reinforced composites determined by combined photoacoustic and IR detection (abstract)
Authors: Gibkes J. , Fotsing J. L. N. , Dietzel D. , Bein B. K. , Pelzl J.
Published: January 2003

Carbon based fiber-reinforced composites are mostly used because of their particular mechanical properties combining a low specific weight with good elastic properties, namely from stability and break resistance. This combination allows applications which are not realizable with other materials. Other advantages are a low thermal expansion and a relatively high-temperature stability, which are the important properties for heat shields, e.g., when high heat pulses have to be absorbed. Such materials can be slightly transparent, both in the visible and the IR spectrum, which makes the thermal characterization extremely difficult. For this work, a measurement system has been developed which combines IR radiometric and photoacoustic detection in two geometrical configurations: (i) In the reflection configuration of thermal waves, where modulated heating in the visible spectrum and IR detection of the thermal wave take place at the same surface, the phase retardation measured by photoacoustics depends on the absorption coefficient in the visible spectrum βVIS whereas the phase measured by IR detection additionally depends on the absorption coefficient in the IR spectrum βIR, and (ii) in the transmission configuration of thermal waves, where modulated heating and detection take place on the opposite surfaces of a sample of finite thickness, information on the integral thermal properties, namely the thermal diffusivity α=k/(ρc) and the effusivity e=(kρc)1/2 is obtained from frequency-dependent measurements of the amplitude and phase retardation of the thermal wave. Here k, ρ, and c are the thermal conductivity, mass density, and specific heat capacity. The amplitude detected by IR radiometry additionally depends on the emissivity ε, whereas the photoacoustic amplitude is mostly independent of the emissivity. By combining photoacoustic [J. Gibkes, B. K. Bein, D. Krüger, and J. Pelzl, Carbon 31, 801 (1993)] and IR detection [J. Bolte, J. H. Gu, and B. K. Bein, High Temp.—High Pressures 29, 567 (1997)] and by combining the reflection and the transmission configuration of thermal waves, a complete thermo-optical characterization of the samples has been achieved with quantitative information on βVIS, βIR, ε, e, and α. To this finality, the different frequency characteristics of IR detection and photoacoustics, which are due to the different signal generation mechanisms and equipment, have been eliminated by calibrating the detected signals with the help of signals measured by the two detection techniques for a common reference sample.
Subsurface mapping of thermal properties with optical heterodyne force microscopy (abstract)
Authors: Tomoda M. , Shiraishi N. , Inagaki K. , Kolosov O. V. , Wright O. B.
Published: January 2003

Optical heterodyne force microscopy (OHFM) is a novel local probe microscopy that combines an atomic force microscope (AFM) with a periodically modulated laser source. The amplitude and phase of the vibrations of the sample surface induced by the periodic heating can be imaged with OHFM. The thermal diffusivity, rather that just the thermal conductivity, has a significant effect on the image contrast. Surface and subsurface variations in thermal properties can be mapped to depths of the order of the thermal diffusion length, which decreases as the modulation frequency increases. The use of an incident laser beam chopped at megahertz frequencies typically allows ∼1 μm lateral or depth resolution in thermal properties. However, the thermal near field can be imaged to a lateral resolution of that of the AFM, namely a few nanometers. In this study we have detected a buried aluminum film with OHFM and done quantitative comparisons with a one-dimensional theoretical model based on the thermal wave propagation in the sample. We will show the topography and OHFM images for a 2.5×2.5 μm region obtained when scanning across a boundary between the Cr/SiO2 structure and the Cr/Al/SiO2 structure. The top layer consists of a 90 nm Cr film. For the Cr/Al/SiO2 structure only there is a buried intermediate layer, a 180 nm Al film. The films are deposited on a silica substrate. An infrared laser beam of wavelength 830 nm is chopped at ∼4 MHz to give an average incident power of 3 mW focused to a ∼3 μm diameter spot on the tip. A commercial transparent Si3N4 cantilever of spring constant ∼0.1 N m−1 and a tip radius ∼20 nm is used. Compared with the one-dimensional model of the thermoelastic response of the structure, the experimental contrast between the two regions is somewhat smaller than predicted for the amplitude images and larger than predicted for the phase images. The discrepancy may be due to a thermal boundary resistance or to three-dimensional effects not accounted for in the model.
Depth profiling the optical absorption and thermal reflection coefficient via an analysis based on the method of images (abstract)
Authors: Power J. F.
Published: January 2003

The problem of depth profiling optical absorption in a thermally depth variable solid is a problem of direct interest for the analysis of complex structured materials. In this work, we introduce a new algorithm to solve this problem in a planar layered sample which is impulse irradiated. The sample is comprised of “N” model layers of thickness Δx, of constant diffusivity α, where the conductivity varies depth wise with each layer. This derivation extends to the general case of a depth variable thermal reflection coefficient with depth variable optical source density. In such a sample, at finite time, t, past excitation, thermal energy can only significantly penetrate NL model layers NL≈4αt[−ln(ε)]/2Δx, where ε is a small error (ε⩽10−6) and a double transit through each layer is assumed. The depth profile of optical absorption in each layer, i, is approximated by δ(x−iΔx), weighted by the optical source density Si. The temperature at x=0− just inside a front medium contacting the sample is given by T(x=0,t)= ∑ i=12NLSi⋅GR(x,x0=iΔx,t)]x=0, where GR(x,x0,t) represents an effective Green’s function for optical absorption at the depth x0=iΔx in the sample. The method of images1 gives GR(x,x0=iΔx,t) in the following form: [GR(x,0Δx,t)GR(x,2Δx,t)…GR(x,2NLΔx,t)]=[A10 A12 A14 A16 …..A1,2NL0 A32 A34 A36 …..A3,2NL….0……A2NL−1,2NL][G(x−0Δx,t)G(x−2Δx,t)……G(x−2NLΔx,t)]. The G(x−nΔx,t) are shifted image fields obtained from the infinite domain Green’s function for one-dimensional heat conduction. They account for thermal wave reflection/transmission over the path length nΔx from the source (at interface i) to the surface (x=0). The Ain are lumped coefficients giving the efficiency of heat transmission from the ith source to the surface for each path order n. They are determined by a mapping procedure that identifies all propagation paths of each order, n, and computes the individual and lumped reflection coefficients. Equation (2) is written for sources placed at odd ordered model interfaces. A similar upper triangular matrix results for the placement of sources at the even ordered interfaces. Recovery of the optical absorption profile proceeds by inversion of Eq. (1) for known GR(x,iΔx,t). Determination of the kernel requires a solution of the related type II inverse problem.2,3 An evaluation of this procedure and its conditioning will be presented.
Photothermal depth profiles of mechanically and electrolytically polished NiTi shape memory alloys (abstract)
Authors: Delgadillo-Holtfort I. , Gibkes J. , Kaack M. , Dietzel D. , Bein B. K. , Pelzl J. , Buschka M. , Weinert K. , Bram M. , Buchkremer H. P. , Stöver D.
Published: January 2003

Machining of NiTi shape memory alloys (SMA) is difficult due to the required special tools, techniques, and the wear of cutting tools. Thus metal injection molding (MIM) of NiTi powders followed by polishing processes may be an alternative fabrication process for SMA components. Transient heat input across the surface and heat transport inside SMA components are important aspects for their functional efficiency. In this work the influence of polishing processes on the thermal depth profiles of SMA materials and the thermal bulk properties of MIM samples are analyzed with the help of photothermal IR radiometry. The effects of polishing have been studied both for polycrystalline nearly equiatomic NiTi alloy and MIM samples. Bulk samples, cut from a polycrystalline ingot of nearly equiatomic NiTi, had first been heat treated and flash cooled to reduce the concentration of nonequiatomic precipitations. In the second step, sample 1 was polished mechanically with a plane grinder, sample 2 was polished electrochemically in an electrolytic bath, and sample 3 was first polished electrolytically and then mechanically. The thermal depth profiles have been measured by frequency dependent photothermal radiometry using an intensity modulated argon-ion laser pump beam. The PTR amplitudes and phases have been calibrated with the signals recorded for glassy carbon. We will show the frequency variation of the inversely normalized amplitudes which correspond to the effusivity depth profile. The polished samples exhibit different depth profiles: that of electrolytic polishing (2) is most distinct from any other, while that after mechanical polishing (1) and that of combined electrolytic and mechanical polishing (3) are similar. The depth dependence of the effusivity can roughly be approximated by a three-layer model consisting of a surface layer of about 10 μm, a subsurface layer extending about 100 μm into the sample, followed by the bulk material at large penetration depths. In the case of mechanical polishing, the bulk and surface layer have similar effusivity values, whereas the effusivity of the intermediate layer is always increased. Supplementary measurements of the surface topology reveal surface roughening produced by the electrolytic treatment, which may explain the strongly decreasing effusivity near the surface. The higher effusivity in the intermediate layer of a depth of up to 100 μm may be related to local demixing of the alloy or stress release, produced by friction effects and heat generation during polishing. This work was performed in the frame of SFB 459.
Experimental and computational aspects of optical property determination of turbid media using frequency-domain laser infrared photothermal radiometry (abstract)
Authors: Nicolaides Lena , Chen Yan , Mandelis Andreas , Vitkin I. Alex
Published: January 2003

In this work, the optical and thermal properties of tissue-like materials are measured using frequency-domain infrared photothermal radiometry. This technique is better suited for quantitative multi-parameter optical measurements than the widely used pulsed photothermal radiometry (PPTR), due to the availability of two independent signal channels, amplitude and phase, and the superior signal-to-noise ratio provided by synchronous lock-in detection. A rigorous three-dimensional thermal-wave formulation with a three-dimensional diffuse and coherent photon density-wave source is applied to data from model phantoms. The combined theoretical, experimental, and computational methodology shows good promise with regard to its analytical ability to measure optical properties of turbid media uniquely, as compared to PPTR, which exhibits uniqueness problems. From data sets obtained using calibrated test phantoms, the reduced optical scattering and absorption coefficients were found to be within 20% and 10%, respectively, from the independently derived values using Mie theory and spectrophotometric measurements.
High resolution infrared “vision” of dynamic electron processes in semiconductor devices (abstract)
Authors: Malyutenko V. K.
Published: January 2003

Infrared cameras have been traditionally used in semiconductor industry for noncontact measurements of printed circuit boards (PCBs) local overheating. While an effective way to prevent defective PCB application in a “find-problems-before-your-customer-do” manner, this conventional static (25–50 frames/s) and small spatial resolution (>100 μm) approach is incapable, in principle, of explaining the physical reason for the PCB failure. What follows in this report is the demonstration of an IR camera based new approach in high-resolution dynamic study of electron processes responsible for single device performance. More specifically, time resolved two-dimensional visualization of current carrier drift and diffusion processes across the device base that happen in microsecond scale is of prime concern in the work. Thus, contrary to the conventional visualization-through-heating measurements, objective is mapping of electron processes in a device base through negative and positive luminescence (provoked by band-to-band electron transitions) and nonequilibrium thermal emission (provoked by intraband electron transitions) studies inside the region in which current flows. Therefore, the parameters of interest are not only a device thermal mass and thermal conductance, but also free carrier lifetime, surface recombination velocity, diffusion length, and contact properties. The micro-mapping system developed consists of reflective type IR microscope coaxially attached to calibrated scanning IR thermal imaging cameras (3–5 and 8–12 μm spectral ranges, HgCdTe cooled photodetectors, scene spatial resolution of some 20 μm, minimum time resolved interval of 10 μs, and temperature resolution of about 0.5 °C at 30 °C). Data acquisition and image processing (emissivity equalization, noise reduction by image averaging, and external triggering) are computer controlled. Parallel video channel equipped with a CCD camera permits easy positioning and focusing of <1×1 mm2 object along the system optical axis. To motivate this diagnostics superiority, small area light emitting diodes and stripe lasers, light and microwave modulators, thermoelectric generators, and coolers were tested with respect to impact made on visualizing of uniformity of current and heat flows, as well as their divergence in space and time.
Two-photon photothermal deflection spectroscopy of atomic sodium (abstract)
Authors: Al-Ahmed Z. , Li Y. J. , Gupta R.
Published: January 2003

Two-photon photothermal spectroscopy (with nonresonant intermediate state) is a very useful technique, because many atomic and molecular species that are usually optically inaccessible by single-photon excitation can be made accessible by two-photon excitation. Some examples of atomic species are H and O. As a first step, we are investigating photothermal signals generated in sodium vapor produced by two-photon excitation of the 52S1/2 state at 602.23 nm. Sodium vapor is contained in a glass absorption cell and N2 buffer gas at 0.66 amagat pressure is used to collisionally transfer the optical energy from Na to the rotational-translation (thermal) energy of N2 gas. A pulsed dye laser operating at 602.23 nm provides the pump beam. The absorption cell is heated in an oven to obtain about 5×10−4 Torr of Na vapor pressure. Photothermal deflection of a He–Ne laser probe beam is observed. Results of these experiments will be discussed.
Photothermal beam deflection spectroscopy using solid deflection media (abstract)
Authors: Gillikin A. M. , Palmer R. A.
Published: January 2003

Photothermal beam deflection (PTBD) is a useful tool for measuring absorbance spectra, particularly of nontransparent and poorly reflective samples. Unfortunately, the optical alignment for such measurements is often time consuming and difficult to reproduce. The desire for a permanently aligned and robust PTBD probe accessory is the driving force behind the research reported here. The use of a solid deflection medium is the key to developing such an accessory. The source of incident radiation for our PTBD accessory is a Fourier transform infrared spectrometer. The spectrometer allows us to combine infrared spectral information, and thus chemical characterization, with the localized detection capability of the PTBD technique. Various infrared transparent solids, including ZnSe and KBr, have been used as deflection media, and the resultant spectra have been compared to experimental data collected with a more traditional liquid deflection technique. The transparent solid deflection media possess reasonable values of dn/dT and provide broad, unobstructed spectral windows in the infrared region, in contrast to their liquid counterparts. The solid deflection media also provide a rigid platform on which optical devices can be mounted, creating a fixed alignment PTBD accessory with a potential for significant miniaturization. To measure a spectrum, the sample need only make good thermal contact with the inert solid deflection medium at the position of incident IR radiation. This type of device has significant potential for in-field measurements, including, ultimately, microscopic in vitro and in vivo spectrochemical analysis. Here, we present data gathered using solid deflection media for PTBD spectroscopy, and evaluate the performance of each solid and its potential for future use.
Measurement of absolute OH concentration in a flame by photothermal deflection spectroscopy (abstract)
Authors: Li Y. J. , Gupta R.
Published: January 2003

Combustion diagnostics is an important field of research. Efforts are underway in various laboratories to develop a noninvasive technique that can yield absolute concentrations (that is, without the need for calibration against a standard sample) of minority combustion species with high temporal and spatial resolution. Fluorescence techniques are plagued by the problem of quenching collisions. On the other hand, photothermal technique relies on quenching collisions to transfer optical energy to the thermal (rotational–translational) modes of the molecules. Therefore, photothermal deflection spectroscopy (PTDS) is ideally suited for a high-quenching-rate environment, such as combustion. If the quenching rates are much faster than radiative rates, as they generally are in a combustion environment, PTDS offers an opportunity to measure absolute concentration. Temporal resolution is achieved by using pulsed laser excitation and spatial resolution is achieved by crossing the pump and probe beams at a relatively large angle. Our experiments have been conducted in a methane–oxygen flame. OH molecules were excited on the Q1 (8) rotational line at 309.24 nm by radiation from a frequency-doubled pulsed dye laser. Deflection of a He–Ne laser probe beam caused by the photothermal effect was observed. Various effects, which are important in the analysis of the signal for absolute concentration measurement, for example, OH absorption and laser spectral profiles, optical saturation, and spatial profile of the pump laser are thoroughly investigated and will be discussed.
Ultrashort shear acoustic pulse excitation via laser-induced electrostrictive effect (abstract)
Authors: Gusev V. , Picart P. , Mounier D. , Breteau J. M.
Published: January 2003

In principle there are several opportunities to achieve excitation of ultrashort shear pulses by laser radiation. One of them is to use the anisotropy of material properties such as elasticity and thermal expansion (in the case of the thermoelastic mechanism of the optoacoustic transformation) or the electron–phonon deformation potential (in the case of sound excitation via electron–hole plasma photogeneration). Another is related to application of the laser-induced inverse (converse) piezoelectric effect. Laser-generated electron–hole plasma creates an electric field due to the electron–hole spatial separation (Dember field) or can screen an electric field (built-in or induced externally) pre-exsisting near the crystal surface. In piezoelectric materials the induced ultrafast transients in electric field distribution in the proximity of the crystal surface should cause excitation of ultrashort acoustic pulses of various modes. Experimentally achieved excitation of plane TA pulses in piezoelectric crystals by nanosecond laser pulses, and also excitation of both optical phonons and ultrashort electromagnetic pulses (THz radiation) by femtosecond laser pulses all support the possibility of exciting ultrashort TA, as well as LA, pulses. The idea of using the inverse piezoelectric effect is also supported by recent observations of extremely high efficiency of quasi-monochromatic LA coherent oscillations generation by laser-induced screening of built-in electric fields in strained multiple quantum wells of piezoelectric semiconductors. We analyze one more opportunity for ultrashort TA pulse excitation in optoacoustic transformation via the action of laser-induced electrostrictive stress on surface of optically transparent crystals. A simple theory is developed and numerical estimates are presented to demonstrate the possibility of exciting ultrashort acoustic pulses in optically transparent materials by laser-induced electrostrictive surface mechanical stresses. Both longitudinal and shear coherent acoustic pulses can be excited. The conditions for which the polarization of ultrashort shear pulses is controlled by the polarization of an incident laser pulse are established. The profiles and duration of longitudinal and shear acoustic strain pulses are shown to coincide with laser pulse intensity envelope. Picosecond shear pulses might be very useful in diverse applications related to measurements of shear rigidity or shear viscosity, including ultrafast solid–liquid phase transitions, ultrafast tribology, and diagnostics of liquids in confined geometry.
Excitation of evanescent acoustic waves by ultrafast laser action on crystal surface (abstract)
Authors: Saeid M. , Khater A. , Gusev V.
Published: January 2003

Following the advances in femtosecond laser technology, significant progress was achieved during the last 10 years in excitation of terahertz coherent lattice vibrations [W. A. Kütt, W. Albrecht, and H. Kurz, IEEE J. Quantum Electron. 28, 2434 (1992)]; [R. Merlin, Solid State Communications, 102, 207 (1997)]. Coherent terahertz phonons were excited in various type of crystals (in particular, in semiconductors and superconductors). However, until recently all-optical excitation and detection were reported on optical phonons only [W. A. Kütt, W. Albrecht, and H. Kurz, IEEE J. Quantum Electron. 28, 2434 (1992)]; [R. Merlin, Solid State Communications, 102, 207 (1997)]. Last year, ultrafast impulsive excitation of coherent acoustic phonon oscillations at frequencies around 4 THz was achieved [M. D. Cummings and A. Y. Elezzabi, Appl. Phys. Lett. 79, 770 (2001)]. We present here the results of theoretical analysis of acoustic signals excited by laser-induced mechanical stresses, which have spectral components around and above the cutoff frequency for propagating acoustic waves. For the analysis of both excitation process and of photo-excited wave propagation, discrete periodic structure of material should be taken into account. We are using for this purpose the simplest chain model of crystal. Analytical solutions, which reveal the role of strong acoustic velocity dispersion and the role of forbidden frequency band existence (where only evanescent modes are excited), are derived. Frequency spectra and temporal profiles of mechanical displacement are determined both at front (laser-irradiated) surface and at rear surface of photo-excited thin film. The contribution of the excited evanescent modes to the motion of the front surface is found. It is expected that evanescent modes might be very sensitive to possible surface and subsurface defects, and because of this they might be useful in nondestructive photoacoustic testing of surfaces and interfaces.
Thermoelastic contributions to picosecond-timescale reflectivity measurements in metals (abstract)
Authors: Spicer J. B. , Richardson C. J. K.
Published: January 2003

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.
Frequency-domain theory of laser infrared photothermal radiometric detection on thermal waves generated by diffuse-photon-density wave fields in turbid media (abstract)
Authors: Mandelis Andreas , Feng Chris
Published: January 2003

A three-dimensional theory of the frequency-domain thermal-wave field generated inside a turbid medium with optical and thermal properties of human tissue is presented. The optical source is treated as a three-dimensional harmonically modulated diffuse photon-density wave (DPDW) field in the diffusion approximation of the radiative transfer theory. Unlike earlier Green-function-based theoretical models, exact boundary conditions are used based on the requirement that there should be no diffuse photon intensity entering the turbid medium from the outside. Explicit analytical expressions for the DPDW field and for the dependent thermal-wave field are obtained in the spatial Hankel-transform domain. The formalism is further extended to the calculation of the infrared photothermal radiometric signal arising from the nonradiatively generated thermal-wave distribution in turbid media with instantaneous nonradiative de-excitation, as well as in media with non-zero fluorescence relaxation lifetimes. Numerical inversions have been performed and presented as examples of selected special cases of the theory. It is found that the present theory with exact DPDW-field boundary conditions is valid throughout the entire domain of the turbid medium, with the exception of the very-near-surface ballistic photon “skin layer” (7–50 μm). Photothermal radiometric signals were found to be more reliably predicted than DPDW signals within this layer, due to the depth-integration nature of this detection methodology.
Applications of laser-generated surface acoustic waves for copper film process monitoring in integrated circuit industry (abstract)
Authors: Gostein Michael , Maznev A. A. , Krastev Plamen , Mazurenko Alex
Published: January 2003

We describe applications of a compact commercial instrument for laser generation and detection of surface acoustic waves (SAWs) to problems in metal film process control for the integrated circuit (IC) industry. [M. Gostein, M. Banet, M. Joffe, A. A. Maznev, R. Sacco, J. A. Rogers, and K. A. Nelson, in Handbook of Silicon Semiconductor Metrology, edited by A. C. Diebold (Marcel Dekker, New York, 2001)] The IC industry is undergoing dramatic changes with the continued drive to reduce feature size and increase circuit speed. One of the most important of these changes is the industry-wide move to replace circuit interconnect processes based on aluminum metallization with copper-based processes. The unique process challenges of copper metallization, coupled with the increasing cost of IC manufacturing in general, have resulted in an increased need for metal film thickness measurement for process control. Laser-generated and detected surface acoustic waves provide an ideal method for nondestructively measuring film thickness on product wafers as they move through an IC factory. A patented version of the technique has been incorporated into a commercial high-throughput measurement station. The measurement station analyzes specialized test structures in the scribe lines in between IC chips on a product wafer. Here, we describe application of the technique to all stages of the copper metallization process, including measurement of seed-layer copper and its associated underlying barrier metals, measurement of electroplated copper deposited atop the seed layer, and measurement of remaining copper film thickness following a chemical–mechanical polishing step. We highlight special capabilities to measure test arrays of submicron metal lines that closely resemble actual circuit elements. In addition, we discuss characterization of the elastic properties of typical and emerging thin film materials used in the semiconductor industry, which is a necessary step in setting up the SAW instrument for measurements on IC film stacks.
Modeling of surface acoustic waves in thin film stacks with an arbitrary number of layers (abstract)
Authors: Krastev P. , Maznev A. , Gostein M. , ’t Hooft G.
Published: January 2003

An efficient mathematical algorithm for the analysis of surface acoustic waves (SAW) measurements in multilayered thin film stacks has been developed. It allows for a fast and accurate calculation of SAW velocities in stacks composed of an arbitrary number of layers with varying material properties and thicknesses. The inversion algorithm also enables fast determination of multiple stack parameters from SAW dispersion curves. The model is used in a semiconductor metrology tool designed for quality control in the manufacturing of integrated circuits. Surface acoustic waves are described by a special class of solutions of the wave equation satisfying specific boundary conditions at the surface of the media in which they propagate. In the case of a stack composed of thin films, the boundary conditions at the surface of the top layer correspond to a free surface (no stress), at the interface of the individual layers a continuity of both stress and displacement is imposed. In the case of isotropic media, or in certain cases when the wave propagates along planes of symmetry [e.g., (1,0,0) plane of a cubic crystal], the set of boundary conditions results in a system of 4N coupled linear equations for the stress and displacements, where N is the number of layers in the stack. For stacks with a large number of layers, the problem quickly becomes numerically challenging. Requirements imposed by the manufacturing process on the time for data analysis and for processing of the information require fast algorithms. Such an algorithm has been developed and is being implemented in the software to be used in in-line optoacoustic metrology systems. The algorithm is based on the concept of a response function. This function describes the response of an elastic medium to a periodic excitation on its surface. By using the response function the problem of solving a system of 4N linear algebraic equations is reduced to (N−1) matrix multiplications of 4×4 matrices and solving a 2×2 equation. In the application for which this algorithm was developed a laser beam split by a diffraction grating and projected on the surface of the stack produces the excitation of the SAW. The algorithm allows a user to analyze SAW data for a system of an arbitrary large number of layers that can be either elastically isotropic or anisotropic. It significantly accelerates the computation of SAWs, which is of prime importance for industrial applications.
Real-time imaging of surface acoustic waves propagating on opaque substrates (abstract)
Authors: Sugawara Y. , Matsuda O. , Wright O. B.
Published: January 2003

All-optical techniques are well-suited for the nondestructive imaging of acoustic waves, but until recently it was not possible to image high frequency surface acoustic wave (SAW) propagation in real time on crystals. We are at present successfully developing such a method for the imaging of SAW with frequencies in the 100 MHz to 1 GHz range, propagating on transparent substrates coated with thin opaque films, using an optical pump and probe technique with picosecond temporal and micron spatial resolutions. [O. B. Wright, Y. Sugawara, O. Matsuda, and V. E. Gusev (these proceedings)]. Although this method is very effective for the investigation of SAW and for the evaluation of the elastic properties of thin films and substrates, it is limited to samples with transparent substrates. In the present study we demonstrate how to image propagating SAW in real time on samples with opaque substrates. We present in particular results for semiconductor wafers of commercial importance such as Si. We will show an image of the surface acoustic waves excited by pulses of light, a few picoseconds long, of wavelength 415 nm focused to a ∼4 μm diameter spot on the (100) surface of a Si single crystal coated with a thin film of gold of thickness 50 nm. The image represents the optical phase difference between two probe light pulses of wavelength 830 nm spaced closely in time (∼500 ps), effectively giving an image of the out-of-plane surface particle velocity at a given moment in time. The surface acoustic wavefronts, excited every 12.5 ns, are not circular because of the cubic symmetry of Si. This method should lead to a variety of applications for characterizing the elastic and mechanical properties of thin film structures.
Photoacoustic spectroscopy of CdTe nanocrystals doped in a GeO2 glass matrix (abstract)
Authors: Tsuboya Ikumi , Arae Dai , Shen Qing , Toyoda Taro
Published: January 2003

Semiconductor particles with sizes of the order of a few nanometers in a glass matrix show very attractive properties completely different from bulk materials. They show promise both for the basic study of the three-dimensional confinement in semiconductors and for applications in the field of optoelectronics devices for switching, optical signal processing, solar energy conversion, and so on. The study of optical properties, especially optical absorption properties, is most important to understand the behavior of semiconductor nanocrystals. In general, the absorption spectra can be measured by the usual transmitted absorption measurement method, but it needs the samples to be made thin enough and have good quality surfaces by pretreatment. The photoacoustic (PA) method is one of photothermal detection, and it has proved to be useful for investigating optical absorption and thermal properties of various materials by measuring nonradiative processes. In this study, we studied the change in the electronic state due to size confinement for CdTe nanocrystals embedded in a GeO2 glass matrix by PA spectroscopy. The samples were prepared in two steps. First, base glass was produced by heating a mixture of GeO2 (0.91 mol %), NaGeO3 (0.09 mol %), and CdTe (5 wt %) powders up to 600 °C by 40 min and keeping at 600 °C for 12 h, and then heating up to 1180 °C for 5 h, keeping at 1180 °C for 4 h, and finally cooling in water. The CdTe nucleus was formed during this process. Second, the base glasses were annealed at 525 °C from 15 to 120 min to obtain various average sizes of CdTe nanocrystals. The PA intensity gradually increases towards higher energy regions at about 3.25 eV for all samples, probably due to the optical absorption of GeO2 base glasses. New peaks appeared at about 2.41 and 1.85 eV in the PA signal intensity spectra for the samples annealed for 30 and 60 min, respectively. These peaks correspond to the optical absorption of the CdTe nanocrystals in the glasses. The absorption edges and peaks shift to lower energy regions with the increase of annealing time. The average nanocrystal radii were estimated to be 3.5 and 5.5 nm in the two annealed samples from the PA spectra.
Mn ion concentration dependence of the photoacoustic and photoluminescence spectra of ZnS:Mn nanocrystals (abstract)
Authors: Toyoda Taro , Matsuzawa Juichiro , Cruz Almira B. , Shen Qing
Published: January 2003

ZnS nanocrystals doped with Mn2+ ions (ZnS:Mn) have attracted much attention in the past few years due to a change in optical properties in accordance with quantum confinement effects. ZnS:Mn nanocrystals exhibit an orange luminescence with a high quantum efficiency (∼18%) under the interband excitation of the host crystals. This spectacular result suggests that the ZnS:Mn nanocrystal system forms a new class of luminescent materials with many applications. To verify the mechanism of high quantum efficiency in ZnS:Mn nanocrystals, it is important to establish the optical absorption characters as an initial step. Few studies have been conducted on light absorption of ZnS:Mn nanocrystals directly because of the difficulty in using of conventional methods caused by strong scattering in the finely powdered structures. The optical absorption spectra of them can be obtained by applying the photoacoustic (PA) spectroscopy, which is a powerful technique for detecting small amounts of strongly scattered materials. We will show the PA difference spectra between ZnS:Mn and pure ZnS nanocrystals with different Mn ion concentrations. The peak position corresponds to the transition energy between A16 state (ground state) and E4 state, and it increases with the decrease of Mn2+ ion concentration, indicating a possibility of hybridization of the s–p states of the ZnS host and the d state of the Mn2+ ion. The intensity of optical absorption between those states increases linearly with the increase of Mn ion concentration, indicating that the optical absorption of Mn ion impurities is proportional to the concentration. We have obtained photoluminescence (PL) spectra with different Mn ion concentration. The PL peak of the Mn2+ transition in ZnS nanocrystals (the transition from T14 to A16 states) is 2.08 eV with a half width of 0.24 eV, which is slightly shifted to lower energy region when compared to the peak in the bulk sample, and is independent of the Mn ion concentration. The peak intensity of the PL spectrum shows a maximum, and decreases with higher Mn ion concentration region.
Thin transparent film characterization by photothermal reflectance (abstract)
Authors: Li Voti R. , Wright O. B. , Matsuda O. , Larciprete M. C. , Sibilia C. , Bertolotti M.
Published: January 2003

Photothermal reflectance methods have been intensively applied to the nondestructive testing of opaque thin films [D. P. Almond and P. M. Patel, Photothermal Science and Techniques (Chapman and Hall, London, 1996); C. Bento and D. P. Almond, Meas. Sci. Technol. 6, 1022 (1995); J. Opsal, A. Rosencwaig, and D. Willenborg, Appl. Opt. 22, 3169 (1983)]. The basic principle is based on thermal wave interferometry: the opaque specimen is illuminated by a laser beam, periodically chopped at the frequency f, so as to generate a plane thermal wave in the surface region. This wave propagates in the film, approaches the rear interface (film-bulk), is partially reflected back, reaches the front surface, is again partially reflected back and so on, giving rise to thermal wave interference. A consequence of this interference is that the surface temperature may be enhanced (constructive interference) or reduced (destructive interference) by simply scanning the frequency f (that is, the thermal diffusion length μ=D/πf), so as to observe damped oscillations as a function of f; in practice only the first oscillation may be clearly resolved and used to measure either the film thickness d or the film thermal diffusivity D, and this situation occurs when μ≈d. In general, photothermal reflectance does not measure directly the surface temperature variation, but rather a directly related signal determined by the thermo-optic coefficients and the sample geometry; for detection it is common to monitor the optical reflectivity variation of a probe beam normally incident on the sample. If the thin film is partially transparent to the probe, the theory becomes more difficult [O. Matsuda and O. B. Wright, J. Opt. Soc. Am. B (in press)] and one should consider the probe beam multiple reflections in the thin film. The probe modulation is optically inhomogeneous due to the temperature-induced changes in refractive index. Although in the past the complexity of the analysis has impeded research in this field, we show how a general analytical method can be used to deal with photothermal reflectance data for transparent thin films. We apply this method to a thin film of silica on a silicon substrate [O. B. Wright, R. Li Voti, O. Matsuda, M. C. Larciprete, C. Sibilia, and M. Bertolotti, J. Appl. Phys. 91 5002 (2002)].
Contributions for soil treatment: The controlled release of biological phosphate monitored by the optothermal window technique
Authors: Lima José Augusto Pedro , Massunaga Marcelo Shoey O. , Cardoso Sergio Luis , Vargas Helion , Bezerra de Melo Monte Marisa , Pereira Duarte Ana Cristina , Rothier do Amaral Manoel , de Souza-Barros Fernando
Published: January 2003

In this work we report the use of the optothermal window combined with the molybdenum colorimetric method to study the properties of phosphate coprecipitated with hydrous ferric hydroxide (ferri-oxo) aggregates. An enhancement on the sensitivity of the present method was obtained by the use of an 875 nm laser diode. The determination of phosphorous is interesting because in natural systems, cyclic processes of iron reduction and oxidation occur in the presence of phosphate compounds. The ferri-oxo aggregates formed in these cycles can sorb available soluble phosphate at rates far larger than those expected with crystalline minerals. The availability of this “mineral phosphate input” would be advantageous for certain types of agricultural activities.