Transient Thermal Grating Research Articles

Finite element modeling of laser-induced three-dimensional Transient Thermal Grating in two-layered systems

The three-dimensional (3D) temperature field of laser-induced transient thermal grating (TTG) for two-layered structures such as diamond/substrate is obtained by the finite element method (FEM). The numerical results indicate that the thickness of the diamond film has a significant influence on the temperature field. On the other hand, we further find that variation of the substrate significantly affects on the temperature distribution in the diamond film, which has been ascribed to the various thermal conductivities of the substrates.

Thermal Diffusivity of Film/Substrate Structures Characterized by Transient Thermal Grating Method

Transient thermal grating method is used to measure the thermal diffusivity of absorbing films deposited on transparent substrates. According to periodically modulated dielectric constant variations and thermoelastic deformations of the thin films caused by the transient thermal gratings, an improved optical diffraction theory is presented. In the experiment, the probing laser beam reflectively diffracted by the thermal grating is measured by a photomultiplier at different grating fringe spaces. The thermal diffusivity of the film can be evaluated by fitting the theoretical calculations of diffraction signals to the experimental measured data. The validity of the method is tested by measuring the thermal diffusivities of absorbing ZnO films deposited on glass substrates.

Thermal conductivity of CVD diamond at elevated temperatures

Transient thermal grating and laser flash techniques have been used to measure in-plane ( k ∣∣) and perpendicular ( k ⊥) thermal conductivity of 0.3–0.6 mm thick polycrystalline MPCVD diamond films. A small (<20% ) anisotropy in k is revealed, and a correlation of k (8–20 W/cm K at RT) with optical absorption and hydrogen impurity concentration is established. The temperature dependence k( T) between 293 and 460 K follows the relationship k∼ T − n ( n=0.17–1.02) depending on the diamond quality.

Mask pattern transferred transient grating technique for molecular-dynamics study in solutions

We have developed a mask pattern transferred transient grating (MPT-TG) technique by using metal grating films. Transient thermal grating is generated by an ultraviolet light pattern transfer to nitrobenzene in 2-propanol solution, and the subsequent effect is detected through its diffraction to a probe beam. The thermal diffusion coefficient is obtained by the relationship between the grating periods and the signal decay lifetime, and is well in agreement with the calculated value. This technique has many advantages, such as a simple setting, an easy alignment, accurate phase control, and high stability for molecular-dynamics study in solutions.

Optimised method of thermal diffusivity determination of thin films by means of laser-induced dynamic gratings

The transient thermal grating (TTG) technique in transmitting geometry has been applied to the characterisation of thin-film thermal diffusivity. The thermal grating is initially formed in a film surrounded by two substrates The theoretical background and experimental data for diffraction active and nonactive substrates are described. A one-dimensional model of heat diffusion in a three-layer structure fits the experimental data of TTG decay well if the film thickness is less than the thermal grating period. In addition, film thickness, light absorption coefficient in a film, and thermal properties of substrates influence the TTG relaxation time. The optimum experimental conditions have been selected, and the method of thermal diffusivity characterisation of thin films has been devised.

Transient thermal gratings and carrier-induced gratings in diffusion experiments

Complications can arise during measurements of the ambipolar diffusion coefficient, D amb, of charged carriers by the transient grating method (TG) due to the concomitant decay of a transient thermal grating. The thermal diffusion coefficient, D th, of hydrogenated amorphous silicon (a-Si:H) is of the same order of magnitude as D amb, about 10 −2–1 cm 2/s. For calibration purposes we measured the thermal change of refractive index, d n/d T, by optical reflection. We monitored the temperature decay after pulsed laser excitation using transient photoreflectance (TPR). A thermal diffusion process is confirmed by a near square-root time dependence. In grating experiments, the initial temperature and its time dependence can be monitored by TPR. In order to determine D amb a good thermal connection to the substrate and a high quality material is needed.

Thermal Transient Gratings as Probes for Intrinsic Localized Modes

Recent studies on intrinsic localized modes or breathers in extended nonlinear lattices have demonstrated spontaneous thermal generation of nonlinear modes accompanied by very long relaxation time scales. The nonexponential lattice relaxation to equilibrium has been connected to local energy accumulation induced by nonlinearity while the decay law depends in general on the lattice temperature. We propose the use of transient thermal gratings as a means for experimental study of breathers at finite temperatures. We show that the thermal grating signal decays as a power law, in marked contrast to the usual exponential law followed in linear systems. In the models we study, we assume electronic decay much faster than vibrational relaxation, leading to dilferential lattice heating that results in thermal gradients.

Report on a second round robin measurement of the thermal conductivity of CVD diamond

Abstract The results of a round robin series of measurements of the thermal conductivity and thermal diffusivity of chemical-vapor-deposited (CVD) diamond are reported. The present round robin (RR2) is an outgrowth of an earlier series (RR1) aimed at understanding which measurement techniques are best suited to measuring the high conductivity of CVD diamond. RR2 includes diamond specimens of higher homogeneity, non-diamond specimens of lower conductivity as controls, and measurements by 14 laboratories using five techniques, with all techniques but one used by more than one laboratory. The data are converted to thermal conductivity at 25 °C to facilitate comparisons among specimens and laboratories. The statistical analysis excludes outliers using Chauvenet's criterion, resulting in data from typically two or three laboratories being excluded for each specimen. The analysis arrives at mean values in the range 1300–2000 W m −1 K −1 for the diamond samples, as expected because of the conditions of preparation, with uncertainties in the range 1.5–4%. For the non-diamond materials, SiC and AlN, the results are 268 W m −1 K −1 ±2.2% and 178 W m −1 K −1 ±2.5%, respectively. Comparing techniques, the d.c.-heated bar is found to be the most accurate, typically ±5% or better. Under favorable conditions, Angstrom's thermal wave method can apparently yield relative uncertainties of ±5–10%, and the mirage effect ±5–15%. Too few laboratories used the transient thermal grating and laser flash methods to make general comments on their accuracy, but one expects a comparable accuracy if used on specimens that are fine-grained and thermally isotropic. Deviations, sometimes large, from these optimal accuracies are examined, and it is suggested that certain experimental details are important for achieving accuracy. It is also found that the estimates of experimental uncertainty provided by most laboratories seriously underestimate the actual deviations of their data from the mean conductivity.

Application of the transient grating technique to the study of the energetics and dynamics of electron transfer reactions: separation of an ion pair into free ions

The enthalpy of formation of the free ions generated by the electron transfer between benzophenone in the lower triplet state and 1,4-diazabicyclo[2,2,2]octane in acetonitrile has been measured using transient thermal phase grating. This enthalpy is smaller by more than 0.14±0.08 eV than the enthalpy of formation of the geminate ion pair obtained from a previous ps thermal phase grating investigation. Therefore, the dissociation of a geminate ion pair into free ions in endothermic. However, separation is exergonic if translational entropy in taken into account. A small and positive value of C, the correction term in the Rehm—Weller equation, is obtained if this term is considered as a free energy. The principles of the transient grating technique as a tool for investigating photoinduced processes in solution are briefly reviewed.

Circular Dichroism Spectroscopy Using Coherent Laser-Induced Thermal Gratings

A new pulsed four-wave mixing technique for the detection and real-time measurement of circular dichroism (CD) in liquid samples is demonstrated. The technique is based on the formation and detection of transient thermal gratings formed by the interference of two laser beams whose polarizations are controlled and modulated using a photoelastic modulator. Through an internal heterodyne process, coherent thermal gratings interfere to greatly enhance a weak circular dichroism signal. By measuring the ratio of the difference of scattered laser light from two different polarizations, ΔS, to the average amount of scattered light, Save, one can determine the value of Δε/ε for the compound under investigation. Samples of chiral camphorquinone are used to demonstrate the technique in the ultraviolet region of the spectrum. These studies of camphorquinone, which has a value of Δε/ε ≈ 10-3 at 266 nm, produce values of ΔS/Save which are orders of magnitude larger than Δε/ε and which approach 2 (the mathematical limit for ΔS/Save). Possible extensions of the technique for measuring CD in very small sample volumes and monitoring time-dependence are discussed.

Structural limitations to local thermal diffusivities of diamond films

The photothermal displacement technique at transient thermal gratings and photothermal microscopy, both providing a spatial resolution on a micrometer scale, were used to investigate the thermal properties of crystallites and regions located between crystallites of diamond grown by microwave plasma chemical vapour deposition. The thermal properties are related to the structural properties by micro-Raman/photoluminescence spectroscopy and infrared spectroscopy. In the vicinity of a highly defective region located between crystallites, which exhibits a preferential incorporation of non-diamond carbon, silicon-vacancy complexes and hydrogen, a reduction of the thermal diffusivity by about 35% was observed. Depending upon whether this region is a grain boundary or a defect-filled microcrack, the decrease in the thermal diffusivity is caused by enhanced phonon scattering from these defects accumulated at the boundary or by a vanishing transmission probability of phonons across the crack. High thermal conductivities between 1500 W m −1K −1 and 1700 W m −1K −1 were determined within the crystallites at room temperature.

Thermal conductivity of thin diamond films grown from d.c. discharge

The in-plane thermal conductivity of thin polycrystalline diamond films produced in a d.c. discharge system has been measured by a contactless technique of transient thermal grating. Diamond films of different qualities but of the same thickness (11 μm) were deposited from CH 4/H 2 mixtures at methane contents ranging from 2% to 10%. The highest thermal conductivity k = 9.5 W cm −1 K −1 was found for the sample synthesized at the lowest CH 4 concentration (2%), the value of k smoothly decreasing to 1.2 W cm −1 K −1 with increasing methane content. The obtained results are in good agreement with previous measurements on similar films performed by the mirage technique. A correlation of thermal conductivity with film morphology and phase content analyzed by SEM and Raman spectroscopy is discussed.

Lateral structuring of III-V quantum well systems with pulsed-laser-induced transient thermal gratings

A two-dimensional quantum well structure grown by molecular beam epitaxy has been laterally structured with an interference pattern from a high-energy pulsed laser. The resulting thermal grating produces a reduction in the carrier density, which causes a lateral modulation in the band levels, with a period of 380 nm. Photoluminescence spectroscopy has been used for characterization.

Numerical simulation and error analysis for thermal diffusivity measurements using laser-induced thermal grating technique

The laser-induced thermal grating technique has been used to determine the thermal diffusivity of liquids and liquid mixtures. But the dynamic behaviour of the transient thermal grating has not yet been thoroughly investigated, and the systematic errors, which result from the departures from one-dimensional heat conduction, have scarcely been studied quantitatively. In this paper. a three-dimensional numerical simulation and results of the transient thermal grating technique are presented, which enable a good understanding of the dynamic behaviour of the transient thermal grating. The results of this simulation are important for the proper design of the experimental setup to keep the systematic errors for the diffusivity measurement small. Based on the simulation method, the systematic errors were analyzed quantitatively. Here, the following effects were studied: (I) sample thickness, (2) intersection angle, (3) absorption, (4) Gaussian beam intensity distribution and focusing of heating laser beam, and (5) heating pulse duration and laser power. This error analysis makes it possible to specify the criteria for optimum measuring conditions, to correct the measured thermal-diffusivity values for systematic errors, and to estimate the accuracy of the measurements.

Transient thermal gratings at surfaces for thermal characterization of bulk materials and thin films

Transient Thermal Gratings (TTGs) at surfaces of absorbing materials have been utilized for investigating heat diffusion in bulk materials and thin films. In this report, we describe the theoretical background of the technique and present experimental data. TTGs were excited in the surface plane by interference of two pulsed laser beams and monitored by a cw probe beam, either via temperature dependence of the reflectivity or by deflection from the displacement pattern. A theoretical model describing the thermal and thermoelastic surface response was developed, both for a homogeneous material and a multilayer structure. The potential of the technique will be demonstrated by experimental results on (i) thermal diffusivities of bulk materials, (ii) anisotropic lateral heat transport, and (iii) thermal diffusivities of metal and diamond films. Furthermore, we will show that TTGs allow thermal depth profiling of inhomogeneous materials whenever there is a vertical gradient in thermal conductivity.

Transient thermal gratings at surfaces for thermal characterization of bulk materials and thin films

Transient thermal gratings at surfaces for thermal characterization of bulk materials and thin films

Measurement of thermal diffusivity by optical excitation and infrared detection of a transient thermal grating

A contactless technique is described for measuring the thermal diffusivity in a direction parallel to the surface of a thin sample by pulsed-laser excitation and thermal-radiation detection of a thermal grating on the surface. The transient thermal grating (TTG) with periodicity Λ in the range 20–1000 μm is created with a novel interferometer which does not require a long-coherence-length laser. Fast thermometry is achieved with an infrared detector focused on a spot ∼30 μm in diameter to examine in detail the shape and the time dependence of the TTG. The performance of the system is demonstrated on a thin Ti foil with Λ ranging over an order of magnitude. Data are also presented for an 11 μm thick diamond film with a factor-of-20 higher diffusivity than for Ti. The results indicate that the system is well suited to measuring local diffusivities in regions as small as ∼100 μm in thin, free-standing films.

Lateral thermal diffusivity of epitaxial diamond films

Highly oriented heteroepitaxial diamond films grown on silicon (001) substrates were investigated by photothermal displacement spectroscopy at transient thermal gratings. Lateral thermal diffusivities were measured as a function of the height within the 90 μm thick films. A strong diffusivity variation was observed from less than 1 cm 2 s −1 near the substrate interface up to 10 cm 2 s −1 in the top growth region, which is about 80% of that for single-crystal type IIa diamond. The results are compared with those obtained from fibre-textured diamond films. The structural properties are characterized using micro-Raman spectroscopy.

The influence of thermal diffusion on laser ablation of metal films

Single-shot ablation thresholds of nickel and gold films in the thickness range from 50 nm to 7 μm have been measured for 14 ns laser pulses at 248 nm, using photoacoustic shock wave detection in air. The metal films were deposited on fused silica substrates. The ablation threshold was found to increase linearly with film thickness up to the thermal diffusion length of the film. Beyond this point it remains independent of film thickness. The proportionality between threshold fluence and thickness allows the prediction of ablation thresholds of metal films from the knowledge of their optical properties, evaporation enthalpies and thermal diffusivities. Physically it proves that ablation is driven by the energy density determined by the thermal diffusion length. A simple thermodynamic model describes the data well. Thermal diffusivities, an essential input for this model, were measured using the technique of transient thermal gratings. In addition, the substrate dependence of the ablation threshold was investigated for 150 nm Ni films.

Thermal diffusivities of thin films measured by transient thermal gratings

Transient thermal gratings have been utilized for investigating heat diffusion in thin films. The gratings were observed by either the displacement technique or by thermoreflectance. Both observation modes allow a quantitative determination of lateral diffusivities. Spatial resolution in the µm range can be obtained, even for films with large thermal diffusivities. Results of thermal diffusivity measurements are presented for thin Au-, Cr-, and Ni-films, sputtered onto the quartz substrates with different thicknesses between l00nm and 3µm.