  In this work, we systematically investigated the ultrafast optical properties
of aluminum (Al) thin films on silicon cantilevers using a microscopic
femtosecond optical pump-probe technique to explore the effect of light
irradiation upon cantilevers while considering radiation pressure and
photothermal effects. The ultrafast laser pulses used for the study were less
than 30 fs in pulse duration and 830 nm in wavelength, and the photon energy
(1.49 eV) of the light pulses is close to the interband transition threshold
(ITT) of Al. Therefore, the change in ITT due to the strain of the cantilevers
induced by the light irradiation is detected through the change in the
transient reflectivity, which is dominated by the thermalized (Th) electron
signal. We uncovered the position dependency of the transient reflectivity
change and the Th electronic signal amplitude of the 100 nm-thick Al films on
160 $\mu$m, 200 $\mu$m and 240 $\mu$m-length cantilevers, and these results are
in excellent agreement with two temperature model-based curve fits.
Furthermore, to understand the effect of light irradiation, we derived
equations for the position-dependent radiation pressure effect and the
photothermal effect, and demonstrated that thermal expansion-induced changes in
ITT dominate the position dependence of the signal intensity. Our findings
offer avenues for exploring strain effects on ultrafast properties and
applications for ultrafast scanning probe microscopy.
