2.3. Measurement Setup for Laser-Induced Damage Threshold

The output pressure (P) from thin-film photoacoustic transmitters increases with incident optical fluence (F), Grüneisen coefficient (Γ) and optical absorption (A). It can be represented as the following:

where c, τL and η are the sound speed, the laser pulse duration, and the fraction of thermal energy sustained within heat absorbers after laser pulse duration, respectively [6]. Here, 1 − η means the fraction of thermal energy transferred to the surrounding PDMS. Although the output pressure can be enhanced with the input optical fluence and absorption by the nanocomposite, the laser-induced damage threshold (F_th) determines an upper limit of such incident optical fluence. A higher optical input (F ≥ F_th) causes physical damage to the transmitter. Hence, for a given optical absorption, the higher the damage threshold is, the stronger will be the maximum-available output pressure achieved by the transmitters.

The mechanical robustness of each transmitter was assessed by measuring the laser-induced damage threshold. Figure 4 shows the experimental setup. The same laser source already described in the previous setup was used to excite transmitters through a circular aperture with a 3-mm diameter. A digital microscope (Dino-Lite Pro, AnMo Electronics Corporation, Hsinchu, Taiwan) was fixed to capture optical images from the transparent side of the transmitters. We followed the measurement steps previously reported in [6], by increasing the incident laser fluence (mJ·cm⁻²) until a physical damage occurred.

Experimental setup to measure the laser-induced damage threshold of CNSPs–PDMS transmitters.