[Paper Review] Exploiting speckle correlations to improve the resolution of wide-field fluorescence microscopy

Hasan Yılmaz, E.G. van Putten+4 more|University of Twente Research Information|Oct 8, 2014

Random lasers and scattering media28 references55 citations

TL;DR

This paper introduces Speckle Correlation Resolution Enhancement (SCORE), a method that uses speckle patterns from a gallium phosphide scattering lens to achieve sub-diffraction-limited resolution in wide-field fluorescence microscopy. By exploiting correlations in speckle illumination across multiple tilted beam angles, SCORE enables a deconvolved resolution of 130 nm over a 10×10 µm² field of view without requiring wavefront shaping or complex calibration.

ABSTRACT

Fluorescence microscopy is indispensable in nanoscience and biological sciences. The versatility of labeling target structures with fluorescent dyes permits to visualize structure and function at a subcellular resolution with a wide field of view. Due to the diffraction limit, conventional optical microscopes are limited to resolving structures larger than 200 nm. The resolution can be enhanced by near-field and far-field super-resolution microscopy methods. Near-field methods typically have a limited field of view and far-field methods are limited by the involved conventional optics. Here, we introduce a combined high-resolution and wide-field fluorescence microscopy method that improves the resolution of a conventional optical microscope by exploiting correlations in speckle illumination through a randomly scattering high-index medium: Speckle correlation resolution enhancement (SCORE). As a test, we collect two-dimensional fluorescence images of 100-nm diameter dye-doped nanospheres. We demonstrate a deconvolved resolution of 130 nm with a field of view of 10 x 10 $ ext{μm}^2$.

Motivation & Objective

• To overcome the diffraction limit in wide-field fluorescence microscopy using a simple, hardware-free method that does not require scanning probes or specialized dyes.
• To extend the field of view beyond the limited speckle-scan range typically constrained by the optical memory effect in speckle-based imaging.
• To demonstrate that speckle correlation can enhance resolution without prior characterization of the scattering medium or complex optical control.
• To achieve sub-200 nm resolution in a conventional microscope setup using only coherent illumination and a scattering layer.

Proposed method

• Illuminates a gallium phosphide (GaP) substrate with a 561 nm coherent beam, creating a speckle pattern via a 2 µm thick porous scattering layer on its surface.
• Tilts the incident beam within the optical memory effect range (Δθ ≈ 1°) to translate the speckle pattern across the sample, enabling raster scanning of the object plane.
• Uses a high-NA objective (NA = 0.95) to detect fluorescence emission with a resolution of R = λ_flu/(2NA) = 322 nm, capturing full-field images at each beam tilt.
• Employs parallel speckle detection to reconstruct a wide-field image by combining multiple speckle-scan measurements, leveraging statistical correlations in the speckle patterns.
• Applies numerical deconvolution to the reconstructed image using the known point spread function to extract the true object distribution and improve resolution.
• Relies on the correlation of speckle patterns across beam tilts to extract high-resolution information, avoiding the need for wavefront shaping or transmission matrix measurement.

Experimental results

Research questions

• RQ1 Can speckle correlations in a scattering medium be used to enhance resolution in wide-field fluorescence microscopy beyond the diffraction limit?
• RQ2 To what extent can the field of view be extended in speckle-based super-resolution imaging without compromising resolution?
• RQ3 Can high-resolution imaging be achieved without prior calibration of the scattering medium or complex optical control?
• RQ4 What is the achievable resolution in a practical implementation using a conventional microscope objective and a simple scattering lens?

Key findings

• The method achieves a deconvolved resolution of 130 nm in a 10×10 µm² field of view, significantly improving upon the conventional microscope’s resolution limit.
• Two 100-nm diameter fluorescent nanospheres separated by 146 nm center-to-center distance are clearly resolved in the SCORE image, whereas they appear as a single unresolved blob in conventional microscopy.
• The full-width-half-maximum (FWHM) of the deconvolved point spread function is 140 nm, close to the theoretical limit of 116 nm for the given illumination beam width.
• The resolution enhancement is achieved without wavefront shaping, transmission matrix measurement, or prior characterization of the scattering medium.
• The method is robust to sample drift and laser pointing noise, with the resolution limit potentially approaching 80 nm for λ_ill = 550 nm in GaP (n = 3.45).
• Theoretical analysis shows that resolution can be further improved to 64 nm by combining the illumination and detection resolution limits, as in structured illumination microscopy.