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

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.