Compact optical device achieves super-resolution imaging beyond the diffraction limit

Researchers from the University of Science and Technology of China (USTC) have unveiled a planar optical device that significantly enhances the capabilities of dark-field microscopy, achieving super-resolution imaging beyond the diffraction limit. The work was led by Prof. Zhang Douguo and has been published in the Proceedings of the National Academy of Sciences.

Dark-field microscopy is a powerful technique used to visualize unstained samples by illuminating them with light at oblique angles, resulting in high-contrast images of weakly scattering objects. However, traditional dark-field microscopy is limited by the diffraction barrier and often requires complex, bulky setups with precise alignment. Super-resolution imaging techniques, which can overcome this barrier, are typically expensive and difficult to operate. The need for a simpler, more accessible solution has long been a challenge in the field.

The study introduces a planar photonic device that integrates a scattering layer, a one-dimensional photonic crystal (1DPC), and a metallic film to generate dark-field speckle patterns. This compact device can be easily integrated into conventional microscopes, eliminating the need for complex optical systems or precise alignment.

The key innovation lies in the use of the 1DPC, which acts as a momentum-space filter to produce hollow cones of speckle patterns. These patterns serve as an illumination source, enabling high-contrast imaging with a 1.55-fold improvement in spatial resolution compared to traditional methods.

The researchers demonstrated the device’s capabilities by imaging various samples, including polystyrene beads, nanowires, and biological specimens. Using a Blind-SIM reconstruction algorithm, they successfully resolved adjacent beads with a center-to-center distance as small as 340 nm, which is beyond the diffraction limit. The device also supports optical surface imaging by generating evanescent speckles when the incident wavelength is tuned, further expanding its applications.

The experimental setup involved a standard upright microscope with a X40 objective lens (NA 0.6) and a coherent laser source coupled into multimode fibers. The speckle patterns were dynamically changed by vibrating the fibers, allowing the capture of multiple frames for image reconstruction. The results showed that the proposed technique not only enhances resolution but also maintains high contrast, even for large field-of-view imaging.

This research represents a significant leap forward in microscopy technology. The compact planar device offers a practical and accessible solution for super-resolution imaging, making high-contrast, label-free microscopy available to a broader range of researchers and clinicians. By simplifying the setup and eliminating the need for complex alignment, this innovation could democratize access to advanced imaging techniques.