In photothermal beam deflection spectroscopy (PTBD) 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. Recent results of PTBD experiments are presented with a high spatial resolution which is near the diffraction limit of the infrared pump beam (CLIO-FEL). We investigated germanium substrates showing restricted O+-doped regions with an infrared absorption line at a wavelength around 11.6 microns. The spatial resolution was obtained by strongly focusing the probe beam (i.e. a HeNe laser) on a sufficiently small spot. The strong divergence makes it necessary to refocus the probe beam in front of the position detector. The influence of the focusing elements on spatial resolution and signal-to-noise ratio is discussed. In future studies we expect an enhanced spatial resolution due to an extreme focusing of the probe beam leading to a highly sensitive technique for detection of sorbed species on surfaces in the far infrared region.

This paper describe recent measurements which have been performed with the CLIO FEL in the wavelength range 40 to 120µm. The FEL electron efficiency, η, is deduced from the electron energy loss. It gives an absolute measurement of the optical power produced by the electron beam, which is then compared to the extracted power of the FEL measured optically. The ratio is compared to numerical simulation taking into account the diffraction of the optical mode in the optical cavity and in the beam line.