Jacek Mazur

Photothermal methods for determination of thermal properties of bulk materials and thin films

Information on the thermal properties of materials is very important both in fundamental physical research and in engineering applications. The development of materials with desirable heat transport properties requires methods for their experimental determination. In this paper basic concepts of the measurement of parameters describing the heat transport in solids are discussed. Attention is paid to methods utilizing nonstationary temperature fields, especially to photothermal methods in which the temperature disturbance in the investigated sample is generated through light absorption. Exemplary photothermal measuring techniques, which can be realized using common experimental equipment, are described in detail. It is shown that using these techniques it is possible to determine the thermal diffusivity of bulk transparent samples, opaque and semi-transparent plate-form samples, and the thermal conductivity of thin films deposited on thick substrates. Results of the investigation of thermal diffusivity of the ground in the polar region, which is based on the analysis of the propagation of the thermal wave generated by sun-light, are also presented. Based on chosen examples one can state that photothermal techniques can be used for determination of the thermal properties of very different materials.

Determination of thermal conductivity of thin layers used as transparent contacts and antireflection coatings with a photothermal method

A photothermal experiment with mirage detection was used to determine the thermal conductivity of various thin films deposited on semiconductor substrates. The first type consisted of conducting oxide films: ZnO and CdO deposited on GaSb:Te, while the other contained high dielectric constant HfO(2) layers on Si. All films were fabricated using a magnetron sputtering technique. Experimental results showed that the value of the thermal conductivity of ZnO and CdO films is lower than the value obtained for HfO(2). Thermal conductivities of investigated thin films are about 2 orders of magnitude lower than those corresponding to bulk materials.

Low loss silica-titania waveguide films

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Photothermal measurements for plates

Basing on a 1D model of heat propagation in gas-sample-gas structure, a method for determining thermal properties of plat-like samples is proposed. The method assumes photodeflection detection of photothermal signal. In the method, a signal frequency dependence is measured. It is shown that in high frequency limit, the signal depends only on thermal properties of gas. Excluding the gas contribution to the measured dependence a signal part connected with the sample can be calculated. Fitting theoretical curves to obtained data, one can estimate thermal diffusivity of the sample. Methods for determining thermal diffusivity of gas and distance between a probe beam and the sample surface are also proposed. A good agreemnt between theory and experiment is obtained.

Photothermal cell for measurement of thermal parameters

An experimental setup with well defined boundary conditions is proposed for photothermal measurement of thermal parameters of opaque, plate-like samples. One dimension mode of temperature field in layered structure and simple-single ray description of photodeflection detection is used for theoretical description of experimental results.

Solid state photoacoustics with laser pulse: mathematical model and experimental data filtering

The first part of this paper presents theoretical analysis of the response of typical photoacoustic cell excited with a laser pulse absorbed by the sample located inside the cell and causing changes of temperature in the whole system. Pressure response curves provide information about optical and thermal properties of the sample. System of 1D heat conduction differential equations for each region of the cell has been solved. Changes of solid sample temperature, resulting from absorption of a laser beam warm up gas surrounding the sample. Changes of gas temperature are transferred into changes of its pressure, in result of thermodynamic processes in the gas filling photoacoustic cell and are detected with a microphone in a typical photoacoustic experiment. In the second part of the article a review of application of wavelet analysis for PT signal filtering is presented. The wavelet method is compared with Fourier transform and signal subtraction and seems to be very promising.

Photothermal Determination of the Thermal Conductivity of Superhard Thin Films

The nondestructive method is proposed for determination of the thermal conductivity of transparent, thermally thin coating on opaque, thermally thick substrates. The sample is heated by intensity modulated, unfocused laser beam. The photodeflection detection is used for signal registration and the “mirage” signal dependence on modulation frequency is measured.. The method is based on 1-D model of heat propagation in layered structure. The light interference in coating layer is taken into account. Two geometries of photothermal experiments are considered. The modulated light may illuminate the coated surface of sample or the opposite one. It is shown that the ratio of signals measured in these geometries does not depend on parameters of the probe beam. The proposed method is verified experimentally for silicon wafers coated with DLC. Obtained results are very promising.