Laser-induced ultrasound via broadband absorption in Al-nanoparticle-embedded nanoporous alumina

To address the limitations of single-wavelength absorption at the resonant peak in metal-based photoacoustic materials, a composite structure of aluminum nanoparticle-nanoporous anodic alumina oxide array (AAO-Al NPs) with broadband absorption characteristics is proposed. The combined effect of local surface plasmon resonance from Al nanoparticles of varying sizes and surface plasmon polarization resonance from the Al film significantly enhances light absorption to 93.5% in the 400–1100 nm wavelength range. By comparing the performance of devices with different pore sizes and thicknesses, it was found that larger pore sizes and thinner films enhance the photoacoustic signal strength. Under excitation by a 532 nm pulsed laser with a fluence of 20.5 mJ/cm2, an AAO-Al NPs-PDMS photoacoustic transducer with a 200 nm pore size and a 15 μm thickness, featuring a 120 nm Al film deposited on one side of the AAO surface, generated a positive sound pressure of 2.3 MPa, a −6 dB bandwidth of 20.9 MHz, and a photoacoustic conversion efficiency of 0.12%. Moreover, the 1064 nm wavelength can also effectively excite acoustic signals from the sample. These results strongly suggest that self-assembled metal nanoparticles in nanoporous AAO arrays are promising for broadband laser-excited photoacoustic ultrasound applications.