Ultrathin materials for wide bandwidth laser ultrasound generation: titanium dioxide nanoparticle films with adsorbed dye

Abstract

Materials that convert the energy of a laser pulse into heat can generate a photoacoustic wave through thermoelastic expansion with characteristics suitable for improved sensing, imaging, or biological membrane permeation. The present work involves the production and characterization of materials composed of an ultrathin layer of titanium dioxide (<5 μm), where a strong absorber molecule capable of very efficiently converting light into heat (5,10,15,20-tetrakis(4-sulfonylphenyl)porphyrin manganese(III) acetate) is adsorbed. The influence of the thickness of the TiO₂ layer and the duration of the laser pulse on the generation of photoacoustic waves was studied. Strong absorption in a thin layer enables bandwidths of ∼130 MHz at −6 dB with nanosecond pulse laser excitation. Bandwidths of ∼150 MHz at −6 dB were measured with picosecond pulse laser excitation. Absolute pressures reaching 0.9 MPa under very low energy fluences of 10 mJ cm⁻² enabled steep stress gradients of 0.19 MPa ns⁻¹. A wide bandwidth is achieved and upper high-frequency limits of ∼170 MHz (at −6 dB) are reached by combining short laser pulses and ultrathin absorbing layers.