Thin film absorption characterization by focus error thermal lensing

Abstract

A simple, highly sensitive technique for measuring absorbed power in thin film dielectrics based on thermal lensing is demonstrated. Absorption of an amplitude modulated or pulsed incident pump beam by a thin film acts as a heat source that induces thermal lensing in the substrate. A second continuous wave collimated probe beam defocuses after passing through the sample. Determination of absorption is achieved by quantifying the change of the probe beam profile at the focal plane using a four-quadrant detector and cylindrical lenses to generate a focus error signal. This signal is inherently insensitive to deflection, which removes noise contribution from point beam stability. A linear dependence of the focus error signal on the absorbed power is shown for a dynamic range of over 10. This technique was used to measure absorption loss in dielectric thin films deposited on fused silica substrates. In pulsed configuration, a single shot sensitivity of about 20 ppm is demonstrated, providing a unique technique for the characterization of moving targets as found in thin film growth instrumentation.

Thermal lensing: outside of the lasing medium

“…As claimed by the authors, it has been by an order of magnitude smaller mass than detectable by other methods. Domené et al [89] have provided the evidence of detecting 5.6 ppm using samples of 130 nm thick Ta 2 O 5 layer. They analyzed the TLinduced distortion of the probe beam, called by them the focus error [90], by measuring the difference in intensities of the probe beam between two diagonals of a 4-quadrant detector.…”

Investigation of laser annealing mechanisms in thin film coatings by photothermal microscopy

“…The photothermal microscope used in this work is a modification of the focus error thermal lens technique previously demonstrated [8,9]. Figure 1 shows a diagram of the experimental setup.…”

“…The mean value of the signal obtained along spatial scans of 50μmx50μm and a step of 5μm was compared with the absorptance value of the same sample obtained by the PCI technique [8]. This process was carried out in two zones of the five samples, taking as uncertainty of the measurement the standard deviation of these values in each map.…”

Transparent glass slab thickness measurement by binary-spot focus signal

“…technique is adequate for samples with only one reflective surface, either measuring the position of the surface [7], surface variations with time [8] or changes in the optical path of the beam [4].…”