Damage in a Thin Metal Film by High-Power Terahertz Radiation

We report on the experimental observation of high-power terahertz-radiation-induced damage in a thin aluminum film with a thickness less than a terahertz skin depth. Damage in a thin metal film produced by a single terahertz pulse is observed for the first time. The damage mechanism induced by a single terahertz pulse could be attributed to thermal expansion of the film causing debonding of the film from the substrate, film cracking, and ablation. The damage pattern induced by multiple terahertz pulses at fluences below the damage threshold is quite different from that observed in single-pulse experiments. The observed damage pattern resembles an array of microcracks elongated perpendicular to the in-plane field direction. A mechanism related to microcracks’ generation and based on a new phenomenon of electrostriction in thin metal films is proposed.

Figure 1

(a) Experimental layout. (b) The terahertz electric field waveform in the time domain and (c) its Fourier-transformed spectrum. The electric field waveform and spectrum are obtained in free-space electro-optical sampling (EOS) carried out in a gallium phosphide crystal (GaP).

Figure 2

SEM images of through holes and their edges in aluminum film produced by single terahertz pulses: (a)–(c) $F=140\mathrm{mJ}/{\mathrm{cm}}^2$; (d)–(f) $F=300\mathrm{mJ}/{\mathrm{cm}}^2$. The arrows indicate the direction of the terahertz electric field.

Figure 3

Determination of the damage threshold in aluminum film irradiated with a single terahertz pulse; experimental data are shown with points approximated with a linear function (black solid line).

Figure 4

SEM images of aluminum film damages induced by multiple terahertz pulses at different fluences: (a)–(c) $F=80\mathrm{mJ}/{\mathrm{cm}}^2$, $N=600$; (d)–(f) $F=240\mathrm{mJ}/{\mathrm{cm}}^2$, $N=60$. Here $F$ is the fluence at the center of the focal spot, and $N$ is the number of pulses. The arrows indicate the direction of the terahertz electric field.

Figure 5

SEM images of damage in aluminum film produced by (a) single and (b) multiple laser pulses: (a) $F=60\mathrm{mJ}/{\mathrm{cm}}^2$, $N=1$; (b) $F=50\mathrm{mJ}/{\mathrm{cm}}^2$, $N=100$. The vector of the electric field of the incident laser beam $E$ is oriented along a small axis of the focal spot (white arrow). $N$ is the number of pulses.