Digital Holographic Microscopy

Roadmap on computational methods in optical imaging and holography [invited]

Computational methods have been established as cornerstones in optical imaging and holography in recent years. Every year, the dependence of optical imaging and holography on computational methods is increasing significantly to the extent that optical methods and components are being completely and efficiently replaced with computational methods at low cost. This roadmap reviews the current scenario in four major areas namely incoherent digital holography, quantitative phase imaging, imaging through scattering layers, and super-resolution imaging. In addition to registering the perspectives of the modern-day architects of the above research areas, the roadmap also reports some of the latest studies on the topic. Computational codes and pseudocodes are presented for computational methods in a plug-and-play fashion for readers to not only read and understand but also practice the latest algorithms with their data. We believe that this roadmap will be a valuable tool for analyzing the current trends in computational methods to predict and prepare the future of computational methods in optical imaging and holography.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00340-024-08280-3.

3D integral imaging of acoustically trapped objects

3D imaging provides crucial details about the objects and scenes that may not be obtained via 2D imaging methods. However, there are several applications in which the object to be 3D-imaged requires to be immobilized. The integrated digital holographic microscopy (DHM) and optical trapping (OT) system is a useful solution for such a task, but both DHM and OT are mostly suitable for microscopic specimens. Here, for the first time to the best of our knowledge and as an analogy to the DHM-OT system, we introduce integral imaging (InIm) and acoustic trapping (AT) integrated system for 3D imaging of immobilized mesoscopic and macroscopic objects. Post-processing of InIm data enables reconstructing the scene at any arbitrary plane, therefore, it re-focuses any particular depth of the object, which is a curtail task, especially when the object is trapped by AT. We demonstrate the capability of our system by simultaneous trapping and 3D imaging of single and multiple irregularly shaped objects with mm sizes.

Waveguide holography for 3D augmented reality glasses

Near-eye displays are fundamental technology in the next generation computing platforms for augmented reality and virtual reality. However, there are remaining challenges to deliver immersive and comfortable visual experiences to users, such as compact form factor, solving vergence-accommodation conflict, and achieving a high resolution with a large eyebox. Here we show a compact holographic near-eye display concept that combines the advantages of waveguide displays and holographic displays to overcome the challenges towards true 3D holographic augmented reality glasses. By modeling the coherent light interactions and propagation via the waveguide combiner, we demonstrate controlling the output wavefront using a spatial light modulator located at the input coupler side. The proposed method enables 3D holographic displays via exit-pupil expanding waveguide combiners, providing a large software-steerable eyebox. It also offers additional advantages such as resolution enhancement capability by suppressing phase discontinuities caused by pupil replication process. We build prototypes to verify the concept with experimental results and conclude the paper with discussion.

Digital holographic microscopy of spiropyran-based dynamic materials

Spiropyran (SP)-based dynamic materials undergo structural changes in response to external stimuli. In this paper, we show that digital holographic microscopy (DHM) is an effective candidate for characterisation of SPs (embedded in polymer matrices) and for monitoring of their dynamical changes. The polymer matrices are polylactic acid (PLA) and poly(methyl methacrylate) (PMMA) films, which are decorated with SPs and immobilised on graphene quantum dots (GQDs). GQDs are modified by benzylamines prior to the loading of SP species because of the enhancement of hydrophobic characteristics. UV irradiation is used as the external stimulus and the dynamical changes of the samples before and after UV irradiation are measured. DHM is arranged on a novel self-referencing setup, which substantially reduces the sensitivity of DHM to environmental vibrations. Morphometric information for characterisation of the samples is obtained by analysis of the recorded digital holograms. The experimental results demonstrate the potential of the presented technique to serve as an alternative technique for surface measurement methodologies such as atomic force microscope and stylus profiler for surface characterisation of similar materials.

Label-free high-resolution white light quantitative phase nanoscopy system

We present a high-resolution white light quantitative phase nanoscopy (WLQPN) system that can be utilized to visualize nanoparticles and subcellular features of the biological specimens. The five-phase shifting technique, along with deconvolution, is adopted to obtain super-resolution in phase imaging. The phase shifting technique can provide full detector resolution, making it beneficial as compared to the well-known Fourier analysis method. The Fourier transform method requires minimum angle of sin - 1 3 f x λ , where f x is maximum achievable spatial frequency. It limits the highest achievable resolution to much below the actual diffraction limit of the system. Thus, to obtain a high-resolution phase map of the biological specimen, a two-step process is adopted. First, the phase map is recovered using the five-phase shifting algorithm, with full detector spatial resolution. Second, the complex field is obtained from the recovered phase map and further processed using the Richardson Lucy total variation deconvolution algorithm to obtain super-resolution phase images. The present technique was tested on 1951 USAF resolution chart, 200 nm polystyrene beads and Escherichia coli bacteria using a 50×, 0.55NA objective lens. The 200 nm polystyrene beads are visually resolvable and subcellular features of the E. coli bacteria are also observed, suggesting a significant improvement in the resolution.

Microbial Motility at the Bottom of North America: Digital Holographic Microscopy and Genomic Motility Signatures in Badwater Spring, Death Valley National Park

Motility is widely distributed across the tree of life and can be recognized by microscopy regardless of phylogenetic affiliation, biochemical composition, or mechanism. Microscopy has thus been proposed as a potential tool for detection of biosignatures for extraterrestrial life; however, traditional light microscopy is poorly suited for this purpose, as it requires sample preparation, involves fragile moving parts, and has a limited volume of view. In this study, we deployed a field-portable digital holographic microscope (DHM) to explore microbial motility in Badwater Spring, a saline spring in Death Valley National Park, and complemented DHM imaging with 16S rRNA gene amplicon sequencing and shotgun metagenomics. The DHM identified diverse morphologies and distinguished run-reverse-flick and run-reverse types of flagellar motility. PICRUSt2- and literature-based predictions based on 16S rRNA gene amplicons were used to predict motility genotypes/phenotypes for 36.0-60.1% of identified taxa, with the predicted motile taxa being dominated by members of Burkholderiaceae and Spirochaetota. A shotgun metagenome confirmed the abundance of genes encoding flagellar motility, and a Ralstonia metagenome-assembled genome encoded a full flagellar gene cluster. This study demonstrates the potential of DHM for planetary life detection, presents the first microbial census of Badwater Spring and brine pool, and confirms the abundance of mobile microbial taxa in an extreme environment.

A Practical Guide to Analyzing and Reporting the Movement of Nanoscale Swimmers

The recent invention of nanoswimmers-synthetic, powered objects with characteristic lengths in the range of 10-500 nm-has sparked widespread interest among scientists and the general public. As more researchers from different backgrounds enter the field, the study of nanoswimmers offers new opportunities but also significant experimental and theoretical challenges. In particular, the accurate characterization of nanoswimmers is often hindered by strong Brownian motion, convective effects, and the lack of a clear way to visualize them. When coupled with improper experimental designs and imprecise practices in data analysis, these issues can translate to results and conclusions that are inconsistent and poorly reproducible. This Perspective follows the course of a typical nanoswimmer investigation from synthesis through to applications and offers suggestions for best practices in reporting experimental details, recording videos, plotting trajectories, calculating and analyzing mobility, eliminating drift, and performing control experiments, in order to improve the reliability of the reported results.

3D monitoring of the surface slippage effect on micro-particle sedimentation by digital holographic microscopy

In several phenomena in biology and industry, it is required to understand the comprehensive behavior of sedimenting micro-particles in fluids. Here, we use the numerical refocusing feature of digital holographic microscopy (DHM) to investigate the slippage effect on micro-particle sedimentation near a flat wall. DHM provides quantitative phase contrast and three-dimensional (3D) imaging in arbitrary time scales, which suggests it as an elegant approach to investigate various phenomena, including dynamic behavior of colloids. 3D information is obtained by post-processing of the recorded digital holograms. Through analysis of 3D trajectories and velocities of multiple sedimenting micro-particles, we show that proximity to flat walls of higher slip lengths causes faster sedimentation. The effect depends on the ratio of the particle size to (1) the slip length and (2) its distance to the wall. We corroborate our experimental findings by a theoretical model which considers both the proximity and the particle interaction to a wall of different hydrophobicity in the hydrodynamic forces.

Heterodyne phase shifting method in scanning probe microscopy

The present paper describes a novel implementation of the continuous phase shifting method (PSM), named heterodyne holography, in a scanning probe microscope configuration, able to retrieve the complex scattered field in on-axis configuration. This can be achieved by acquiring a continuous sequence of holograms at different wavelengths in just a single scan through the combination of scanning interference microscopy and a low-coherent signal acquired in the frequency domain. This method exploits the main advantages of the phase shifting technique and avoids some limits relative to off-axis holography in providing quantitative phase imaging.

Measurement of morphology thickness and refractive index in a melanoma A375 cell line using digital holographic microscopy

Digital holographic microscopy (DHM) is a technique that has high potential for analyzing biological samples and has been successfully applied to the study of cells and cell lines providing information about important parameters such as refractive index, morphology, and dry mass, among others; it has also found applicability to study the effects of therapeutic treatments. Finding the size and shape of cells is important since they tend to change in the presence of some pathologies. In this research work, we obtain the morphology thickness and refractive index of the A375 melanoma cell line through a slight tilting of the cell in a DHM setup. Further, the development of a novel mathematical expression based on this tilt and in the optical phase difference is presented. We show images of melanoma cells with the refractive index information included, and their morphology thickness as rendered from the holographic phase maps recorded with DHM.

Error diffusion method with optimized weighting coefficients for binary hologram generation

The error diffusion method can effectively reduce quality degradation by propagating thresholding errors to neighboring pixels in the conversion of a gray-scale hologram to a binary hologram. In previous works, the four weighting coefficients in error diffusion are mostly set as the Floyd-Steinberg coefficient, which was determined empirically and originally proposed for photograph processing. In this work, we point out that the Floyd-Steinberg coefficients can be suboptimal for hologram error diffusion binarization. Furthermore, the weighting coefficients are optimized for each different hologram adaptively. Compared with conventional coefficients, our optimized coefficients can better preserve the fidelity of a reconstructed image after a hologram is binarized.

Multi-step phase aberration compensation method based on optimal principal component analysis and subsampling for digital holographic microscopy

Digital holographic microscopy (DHM) is a well-known powerful technique allowing measurement of the spatial distributions of both the amplitude and phase produced by a transparent sample. Nevertheless, in order to improve the transverse resolution of the DHM system, a microscope objective has to be introduced in the object beam path, which inevitably leads to phase aberration in the object wavefront. In recent decades, a multitude of techniques have been proposed to compensate for this phase aberration, and the principal component analysis (PCA) technique has proven to be one of the most promising approaches due to its high compensation accuracy, low computational complexity, and simplicity to implement. However, when it comes to high-order phase aberration, which is common for a mal-aligned DHM system, the PCA technique usually performs poorly since it is unable to fit the cross-terms of the standard Zernike polynomials. To address this problem, here we propose a multi-step phase-aberration-compensation method based on optimal PCA and sub-sampling where PCA is first applied to remove the non-cross-aberration terms, followed by sub-sampled fitting for the remaining cross-aberration correction. The key advantage of our approach is that it can handle both the conventional objective phase curvature and high-order aberrations such as astigmatism and anamorphism with very little computational overhead. Simulation and experimental results demonstrate that our method outperforms state-of-the-art approaches, and the compensation results are consistent with those obtained from the double-exposure method.

Strain Multiplexed Metasurface Holograms on a Stretchable Substrate

We demonstrate reconfigurable phase-only computer-generated metasurface holograms with up to three image planes operating in the visible regime fabricated with gold nanorods on a stretchable polydimethylsiloxane substrate. Stretching the substrate enlarges the hologram image and changes the location of the image plane. Upon stretching, these devices can switch the displayed holographic image between multiple distinct images. This work opens up the possibilities for stretchable metasurface holograms as flat devices for dynamically reconfigurable optical communication and display. It also confirms that metasurfaces on stretchable substrates can serve as platform for a variety of reconfigurable optical devices.

Quantitative phase imaging for medical diagnosis

Optical microscopy is an indispensable diagnostic tool in modern healthcare. As a prime example, pathologists rely exclusively on light microscopy to investigate tissue morphology in order to make a diagnosis. While advances in light microscopy and contrast markers allow pathologists to visualize cells and tissues in unprecedented detail, the interpretation of these images remains largely subjective, leading to inter- and intra-observer discrepancy. Furthermore, conventional microscopy images capture qualitative information which makes it difficult to automate the process, reducing the throughput achievable in the diagnostic workflow. Quantitative Phase Imaging (QPI) techniques have been advanced in recent years to address these two challenges. By quantifying physical parameters of cells and tissues, these systems remove subjectivity from the disease diagnosis process and allow for easier automation to increase throughput. In addition to providing quantitative information, QPI systems are also label-free and can be easily assimilated into the current diagnostic workflow in the clinic. In this paper we review the advances made in disease diagnosis by QPI techniques. We focus on the areas of hematological diagnosis and cancer pathology, which are the areas where most significant advances have been made to date. [Image adapted from Y. Park, M. Diez-Silva, G. Popescu, G. Lykotrafitis, W. Choi, M. S. Feld, and S. Suresh, Proc. Natl. Acad. Sci. 105, 13730-13735 (2008).].

A general model for resolution of digital holographic microscopy

For digital holographic microscopic imaging, the resolution in the reconstructed image is one of the most important parameters. To optimize the lateral resolution, a general model for the resolution of digital holographic microscopy (DHM) is proposed in this work, in which the effects of the sizes of each pixel, total area of the charge coupled device (CCD) and the microscopic objective lens are taken into account. Comparison between our model and others was carried out by calculating the point spread function (PSF) of DHM at different reconstruction distances and with different fill factors. It is shown that the effect of fill factors on the resolution of DHM becomes significant when the reconstruction distance is long. For high resolution DHM imaging the influence of fill factors must be taken into account when estimating the resolution of the reconstructed image. Furthermore, It is also demonstrated that the sidelobe of PSF can be cut effectively choosing appropriate values of the fill factors. Finally, the reconstructions of polyethylene microspheres have been implemented to demonstrate the theoretical analysis. These results obtained are helpful for estimation of the resolution and design of the DHM systems.