Photothermal beam deflection (PTBD) spectroscopy using an FEL as a pump source


Photothermal beam deflection (PTBD) spectroscopy using an FEL as a pump source

Foerstendorf, H.; Seidel, W.; Glotin, F.; Prazeres, R.; Ortega, J. M.

The PTBD technique is based on the theory of photothermal spectroscopy which describes the conversion of absorbed energy of a light beam incident on a sample into heat by nonradiative de-excitation processes [1]. The distribution of this induced, exponentially decaying, thermal field is given by the solution of the heat equation with a source term of a Gaussian beam [2]. In typical PTBD experiments the magnitude of the signal is proportional to the slope of the induced displacement of the sample surface. Additionally, it can be shown that there is a direct proportionality between the observed signal and the absorption coefficient of the material under investigation [2]. Therefore, a direct access to absorption spectra is provided [3]. In PTBD spectroscopy generating and detection of thermal waves occur generally in the sub-millimeter length scale. Therefore, PTBD provides spatial information about the surface of the sample and permits imaging and/or microspectrometry.
We investigated distinct patterns of O+-implanted and untreated regions at the surface of germanium substrates serving as model systems. This was achieved by special stainless steal masks in front of the substrate during the implantation processes. The different areas of the surfaces can be distinguished by optical absorption (i.e. the amplitude of the deflection signal) at λ = 11.6 µm of the germaniumoxide produced during ion implantation.
The dimensions of the masks were fully recovered by recording one dimensional absorption profiles of the substrate’s surfaces using a high resolution positioning system. Since the minimum dimensions of the implanted pattern was 30 µm the spatial resolution obtained is near the diffraction limit of the infrared pump beam.

References
[1] A. Rosencwaig and A. Gersho, J. Appl. Phys. 47 (1976) 64.
[2] M.A. Olmstead et al., Appl. Phys. A 32 (1983) 141.
[3] W. Seidel et al., Eur. Phys. J - Appl. Phys. 25 (2004) 39.

  • Poster
    WIRMS 2005 Int. Workshop on Infrared Microscopy and Spectroscopy with Accelerator Based Sources, 26.-30.06.2005, Rathen, Deutschland

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