Theoretical and experimental studies on broadband photoacoustic response of surface plasmon sensing

The surface plasmon (SP) sensing technique demonstrates high sensitivity and a broad bandwidth of measuring photoacoustic (PA) pressure transients. In this work, we further present a systematic investigation on PA response characteristics of the recently developed SP-based ultrasonic detector, where the ensemble of surface plasmon polaritons (SPPs) at the metal-dielectric interface is approximated as an equivalent acoustic detector. Relying on the intrinsically ultrafast temporal response (∼140 fs) and highly localized evanescent field (optical penetration depth of ∼185 nm) of the SPPs, the SP sensing can respond ultrasounds with the gigahertz frequency band theoretically, which, however, is far higher than the bandwidth in practical PA detection. We reveal that, due to acoustic interference, the finite lateral probing dimension in the SP sensor imposes an ultimate constraint on the accessible ultrasonic cutoff frequency, representing good agreement with the experimental results by acquiring PA impulses from an optically absorbing graphene film using our SP sensor. The theoretical framework enables analyzing the SP response characteristics of ultrasonic/PA pressure transients, which, therefore, offers guidelines for configuring the SP sensor with adequate sensitivity and bandwidths to access various biomedical PA applications, including volumetric imaging and spectroscopic analysis.