Tuning-fork-enabled heterodyne fiber-optic acoustic probing for nanoscale subsurface imaging

This is the authors' abstract. We don't add key points for this paper.

High-resolution imaging of buried structures is essential for understanding material properties and device functionality in micro- and nanoscale systems, yet nondestructive subsurface imaging remains challenging owing to the effects of electromagnetic and acoustic dispersion and diffraction. Here, we proposed a feedback-controlled acoustic atomic force microscopy (AFM) scanning platform based on a tapered fiber Bragg grating (FBG) probe integrated with a quartz tuning fork form. Localized heterodyne acoustic signals are detected and optically demodulated through the tapered FBG probe, enabling sensitive subsurface feature detection. Experiments show that the signal-to-noise ratio of the detected difference-frequency signals in the kilohertz range varies from 2.51 dB to 39.32 dB, and a maximum subsurface image contrast of 36.82 dB is achieved. It demonstrates that subsurface imaging is realized through a sample containing internal grid structures with a 2.7 μm overlayer, resolving a grid period of 1.51 μm and a height of 190 nm, which are in good agreement with actual values. Compared with conventional subsurface AFM techniques, the proposed system realizes fully fiber-integrated excitation, sensing, and demodulation, providing a mechanically robust and alignment-free approach for subsurface characterization and offering a promising way toward compact photonic scanning probes for the detection of concealed microstructures.