Dark-field synthetic-aperture digital holographic microscopy with an enhanced numerical aperture

Synthetic-aperture digital holographic microscope (SA-DHM) has received attention in various fields due to its ability to achieve higher resolution images than single-shot optical microscopes. In SA-DHM, the numerical aperture (NA) is enhanced by capturing high spatial frequency information in the Fourier domain. This domain is divided into two regions based on the system NA: the bright-field region and the dark-field region. Specifically, the synthetic NA achievable with only bright-field measurements is limited to twice the system NA. Therefore, precise measurement of the dark-field region is essential for overcoming the limitations of conventional SA-DHMs. In most SA-DHMs, the illumination angle determines the position of the measured field in the Fourier domain, and the precise control of the illumination angle is important for high resolution. In this study, we developed a solid-state beam-steering optics using a digital micromirror device (DMD), avoiding the vibrations associated with mechanical steering optics such as galvano-mirrors. We also propose a calibration method to correct the k-vector for high-angle illumination. With our SA-DHM system, both bright-field and dark-field images are measured precisely, achieving high-resolution results with a synthetic NA that is 3.2 times larger than the system NA.