# Sub-Diffraction Photoacoustic Microscopy Enabled by a Novel Phase-Shifted Excitation Strategy: A Numerical Study

## TL;DR

This study proposes a new method for photoacoustic microscopy that improves resolution beyond the optical diffraction limit using a phase-shifted excitation strategy.

## Contribution

A novel phase-shifted Gaussian and donut beam excitation strategy is introduced for sub-diffraction photoacoustic imaging.

## Key findings

- The phase-shifted excitation scheme achieves a ~1.42× lateral resolution improvement over conventional methods.
- An optimal power ratio of 1.16 between the beams balances excitation confinement and side-lobe suppression.
- The method enables high-resolution imaging using low-cost continuous-wave lasers without nonlinear contrast mechanisms.

## Abstract

What are the main findings?
The proposed phase-shifted Gaussian and donut beam excitation scheme may confine the effective photoacoustic excitation region beyond the conventional optical diffraction limit.Numerical simulations show a ~1.42× lateral resolution improvement at an optimal power ratio of 1.16 between the two beams.

The proposed phase-shifted Gaussian and donut beam excitation scheme may confine the effective photoacoustic excitation region beyond the conventional optical diffraction limit.

Numerical simulations show a ~1.42× lateral resolution improvement at an optimal power ratio of 1.16 between the two beams.

What are the implications of the main findings?
It is shown that sub-diffraction photoacoustic microscopy can be achieved using frequency-domain excitation with continuous-wave lasers.The method can pave the way for cost-efficient, high-resolution biomedical photoacoustic imaging without nonlinear contrast mechanisms.

It is shown that sub-diffraction photoacoustic microscopy can be achieved using frequency-domain excitation with continuous-wave lasers.

The method can pave the way for cost-efficient, high-resolution biomedical photoacoustic imaging without nonlinear contrast mechanisms.

This numerical simulation study introduces a novel phase-shifted Gaussian and donut beam excitation strategy for frequency-domain photoacoustic microscopy, capable of achieving optical sub-diffraction-limited lateral resolution. We demonstrate that the spatial overlapping of Gaussian and donut beams with π-radian phase-shifted intensity modulation may confine the effective photoacoustic excitation region, substantially reducing the beam-waist-normalized full width at half maximum value from 1.177 to 0.828 units. This effect corresponds to a ~1.42-fold lateral resolution enhancement compared with conventional focused Gaussian beam excitation. Furthermore, the influence of the optical power ratio between the beams was systematically analyzed, revealing an optimal value of 1.16, balancing excitation confinement and side-lobe suppression. Within this framework, the presented simulation results establish a basis for the experimental realization of phase-shifted dual-beam excitation photoacoustic microscopy systems, with a potential impact on high-resolution biomedical imaging of subcellular and microvascular structures using low-cost continuous-wave optical sources such as laser diodes.