Femtosecond off-axis digital holography for monitoring dynamic surface deformation

Abstract

We present a femtosecond off-axis digital holography for investigating the dynamic reversible surface change of a metal film induced by femtosecond laser pulses with fluences near the ablation threshold. A reflection Michelson interferometer (RMI) and a transmission Mach-Zehnder interferometer (TMZI) are integrated in the same setup for recording digital holograms. The RMI is used to measure the laserinduced surface deformation of the metal film, while the TMZI is used to analyze the refraction index change of the metal film induced by the femtosecond laser pulses. Experimental results show that both surface modification and refraction index change of chromium metal film can be observed when femtosecond laser pulses are below and above the ablation threshold. Based on the experimental results, the physical processes of the metal induced by femtosecond laser pulses are given qualitatively.

Femtosecond laser-induced thermal lens effect in chromium film

“…(9), we can see that an increase of the power or a decrease of the radius of the pump beam directly results in an increase of the phase shift of the TL effect. The phase shift induced by femtosecond laser pulses has a linear relation with the laser power, which has been proved in our previous work [10]. It is also of interest to study how the contrast ratios of the TL induced by femtosecond laser pulses changes with different laser powers.…”

“…It is a reasonable assumption according to the discussion in our previous work [10]. Then the phase shift can be expressed as…”

“…For example, in this experiment, the sample is a thin Cr film on a glass substrate. Figure 2(b) shows the various possible physical phenomena when the femtosecond laser pulses excite the Cr film on the glass substrate: (I) air refractive index gradient changes or plasma may be produced by femtosecond laser pulses [27], (II) the Cr film absorbs the energy, and the laser-excited material might be molten and thermal expansion gives rise to surface deformation, (III) the absorption-induced increased temperature might induce the refractive index changes of Cr film [10], or (IV) the deposited heat might induce the refractive index change of glass. In short, the relative contributions of these physical effects result in the phase shift of the probe beam.…”

“…7(b) are simulated assuming that the maximum phase shift is a constant in Eq. (10). In fact, the maximum phase shift induced by the femtosecond laser changes with the various radii and power levels of the pump laser.…”

“…It is reasonable to regard the reversible dark-center phenomenon as the result of diffraction of the phase element induced by the femtosecond laser. However, it is not exactly accurate to explain the phenomenon by using a binary phase plate (0 and ϕ) because the phase shift induced by femtosecond laser pulses is a continuous distribution [10]. We posit that the phenomenon of reversible dark-center diffraction induced by femtosecond laser pulses is attributed to the laser-induced thermal lens (TL) effect, which occurs as the result of heat dissipation in materials following light absorption by a pump Gaussian laser beam [11].…”