Frequency-selective photoacoustic lamb wave generation using slit-patterned CSNP-PDMS transmitters with fully...

Laser-generated ultrasound (LGU) enables fully non-contact ultrasonic inspection. However, conventional pulsed-laser excitation inherently produces broadband wavefields that complicate guided-wave interpretation due to multimodal interference. In this study, a frequency-selective photoacoustic Lamb wave transducer based on slit-patterned candle soot nanoparticle-polydimethylsiloxane (CSNP-PDMS) composites is proposed and experimentally validated using a fully non-contact characterization framework. Spatially periodic slit arrays were designed according to the phase velocity dispersion characteristics of the A₀ Lamb wave mode to selectively generate narrowband guided ultrasonic waves (GUWs) with target frequencies of 350, 450, and 550 kHz. Finite element simulations and laser Doppler vibrometer (LDV)-based measurements confirmed that the proposed LGU transducers successfully confined the acoustic energy within narrow frequency bands while preserving the strong photoacoustic conversion efficiency of the CSNP-PDMS composite. Compared with a conventional bulk patch, the slit-patterned transducers generated clearly distinguishable A₀ wave packets with substantially reduced fractional bandwidths below 50%. Parametric investigations further revealed that increasing the number of active slits and laser pulse energy enhanced spectral concentration, whereas off-axis detection induced significant spectral broadening and center-frequency shifts due to phase-matching distortions. The proposed approach establishes a practical framework for tunable, fully remote, and frequency-selective guided-wave generation applicable to non-destructive testing and structural health monitoring systems.