Broadband plasmon waveguide resonance sensing for photoacoustic spectroscopic analysis

Sensitive detection of incident acoustic waves over a broad frequency band offers a faithful representation of photoacoustic pressure transients of biological microstructures. Here, we propose a plasmon waveguide resonance sensor for responding to the photoacoustic impulses. By sequentially depositing Au, MgF₂, and SiO₂ films on a coverslip, a composite waveguide layer produces a tightly confined optical evanescent field at the SiO₂-water interface with extremely strong electric field intensity, enabling the retrieval of photoacoustic signals with an estimated noise-equivalent-pressure (NEP) sensitivity of ∼92 Pa and a -6-dB bandwidth of ∼208 MHz. An ultraviolet spectroscopically resolved photoacoustic detection system integrating our sensor allows for label-free spectral measurements of human glioma xenografts from mice brains ex vivo, in which photoacoustic measurement at the frequency domain differentiates the glioma from a healthy tissue that agrees with standard H&E-staining histologic examinations. We expect that our sensitive broadband sensor could potentially empower photoacoustic histopathological assessments of neoplasms.