Methods such as the mirage effect and photoacoustic spectroscopy are readily adaptable to detection using Fourier transform infrared (FTIR) spectroscopy. Optical energy in the wavelength range from the ultraviolet to the IR may be used for signal excitation. The bandwidth of available modulation frequencies, over which a particular method is capable of response, is determined both by the electrical bandwidth of the transducers used for detection and the optical excitation geometry. Good depth profile resolution requires a method that exhibits high sensitivity in combination with a wide instrumental bandwidth. The deflection signal is directly proportional to the derivative of the surface deformation, which in turn is proportional to temperature. The probe beam is reflected from the sample surface and its deflection is monitored using a position-sensing detector. This remote method of detection is termed photothermal radiometry.įinally, heat generation in the sample gives rise to a thermoelastic effect that may be detected by aligning a probe beam at the inflection point of the ‘thermal bump’ that is generated using a focused excitation beam. An infrared detector may be used to measure this blackbody emission signal and to detect temperature changes in the sample caused by optical absorption. Increased blackbody emission accompanies sample heating by the excitation beam.
This phenomenon is termed the mirage effect or photothermal deflection spectrometry.įor reflective samples, temperature changes may cause variations in the sample's reflectance giving rise to photothermoreflectance spectrometry. Thermally induced variations in the gas-phase density above the sample surface also produce refractive index gradients that may be probed through the deflection of an optical probe beam aligned parallel with the sample surface. Heat conduction into the gas phase causes a modulated expansion of a layer of gas near the sample surface, producing a sound wave that can be detected using a microphone the so-called photoacoustic effect. This detection method, termed photopyroelectric spectrometry, has recently provided a rapid, sensitive method for the detection of thermal waves in very thin samples. This change in polarization may be measured as a current or a voltage. Pyroelectric thin film sensor materials exhibit a polarization change that is approximately proportional to the average change in the temperature of the pyroelectric material. New York: Dekker, courtesy of Marcel Dekker, Inc.)Ī direct detection strategy involves contacting a temperature sensor to the rear surface of the sample and monitoring the average temperature change in the sensor layer. In: Morris MD (ed.) Microscopic and Spectroscopic Imaging of the Chemical State, pp. (Reprinted from Power JF (1993) Scanning probes III: Photoacoustic and photothermal imaging. Schematic diagram showing processes accompanying heating of a sample by a modulated beam of radiation.