Abstract
An adaptive spatial filter is used as an optical novelty filter to detect
photothermal mirror (PM) signals in high absorbing materials using continuous wave
laser excitation. The optical novelty filter uses an optical beam-fanning limiter
based on single domain barium titanate (BaTiO3), cut and poled 45°
relative to the c-axis. The optical novelty filter approach relaxes the
requirement for high sample surface smoothness because the effect aperture adapts to
the surface, reducing the stationary background from the optical signal and provides
a means of developing the photothermal mirror signal. Time-dependent probe laser
phase shifts due to photothermal surface deformation pass through the optical
novelty filter and are detected as an intensity increase over the stationary or
“mundane” signal. Experimental studies are performed using four well-characterized
metals using both the conventional photothermal mirror and optical novelty filter
apparatuses in order to understand the complicated signal behavior. Signal behavior
is analyzed in different excitation intervals using pseudo-chopped sample excitation
with different duty cycles. Optical novelty filter signals show fast response for
changes in the spatial beam profile followed by long relaxation time. Reasons for
the optical novelty filter response are described.
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