Microsphere-assisted super-resolved Mirau digital holographic microscopy for cell identification

Morphometric effects of particulate air pollution on an optically trapped single red blood cell

Particulate air pollution is associated with excess deaths and increases in hospital admissions because of cardiovascular and respiratory diseases. Several scientific studies and assessments have linked particulate pollution to a variety of health problems. In this paper, we provide a single cell in vitro analysis for the effect of the particles, which can enter into blood stream, on red blood cells (RBCs). The RBCs under experiment are incubated with [Formula: see text] particle as the most abundant air pollutants in big cities. The self-referencing digital holographic microscopy (DHM) in Gates' arrangement as a vibration-immune methodology is considered here for live visualization and quantitative analysis of the cells. DHM is a label-free and noninvasive method, therefore, suitable for quantitative and morphometric imaging of biological specimens in arbitrary time scales and at video rates. Single RBCs are immobilized by a blinking multiple optical trapping system integrated to the DHM system. Through post-process numerical reconstruction of the recorded digital holograms, the morphology changes of the pollution-exposed RBCs are tracked and expressed in terms of volume and several statistical morphometry parameters.

Reflectance mapping with microsphere-assisted white light interference nanoscopy

The characterisation of novel materials presents a challenge that requires new and original developments. To face some of these demands for making measurements at the nanoscale, a new microsphere-assisted white light interference nanoscope performing local reflectance mapping is presented. This technique presents the advantages of being non-destructive, full-field and label-free. A 145 μm diameter microsphere, glued to the end of an optical fiber, is inserted inside the white light interference microscope to improve the lateral resolution from 940 nm to 520 nm. The acquisition and the Fourier transform processing of a stack of interference images superimposed on the virtual image produced by the microsphere allows the extraction of the local reflectance over a wavelength range of 460 nm to 900 nm and a field of view of 8 μm in diameter. The enhancement in the lateral resolution of the reflectance is demonstrated through the spectral distinction of neighboring ripples on a laser-textured colored stainless-steel sample that cannot be resolved without the microsphere, on regions with a surface of 279 × 279 nm2 horizontally spaced 279 nm apart. Future improvements could potentially lead to a lateral resolution of reflectance measurement over a 100 nm diameter area in air, paving the way to sub-diffraction reflectance mapping.

Effect of boundary conditions in modeling of microsphere-assisted imaging

Exploring the performance of label-free imaging relies heavily on adequate physical models and accurate numerical simulations. A particularly challenging situation is imaging through contact microspheres, which have demonstrated resolution values exceeding the diffraction limit. Here an ab initio modeling of microsphere-assisted imaging is reported and its results are analyzed. The key part of modeling is solving the light scattering problem, which requires handling a rather large computational domain and broad angle illumination made up of multiple mutually incoherent plane waves. To account for plane wave incidence, two simulation approaches are developed that differ only by boundary conditions-quasiperiodic and absorbing. The algorithms to find images in both approaches are discussed and the simulation results are compared for free space and microsphere-assisted imaging. It is shown that while the super-resolution in microsphere-assisted imaging can be demonstrated using both approaches, the latter allows a large reduction in the computational resources. This significantly extends the capability of the simulations, enabling a rigorous exploration of novel imaging regimes.

A dataset of digital holograms of normal and thalassemic cells

Digital holographic microscopy (DHM) is an intriguing medical diagnostic tool due to its label-free and quantitative nature, providing high-contrast images of phase samples. By capturing both intensity and phase information, DHM enables the numerical reconstruction of quantitative phase images. However, the lateral resolution is limited by the diffraction limit, which prompted the recent suggestion of microsphere-assisted DHM to enhance the DHM resolution straightforwardly. The use of such a technique as a medical diagnostic tool requires testing and validation of the proposed assays to prove their feasibility and viability. This paper publishes 760 and 609 microsphere-assisted DHM images of normal and thalassemic red blood cells obtained from a normal and thalassemic male individual, respectively.

Simple high-resolution 3D microscopy by a dielectric microsphere: a proof of concept

We present a simple high-resolution approach for 3D and quantitative phase imaging (QPI). Our method makes the most of a glass microsphere (MS) for microscopy and a glass plate for lateral shearing self-referencing interferometry. The single MS serves all the functions of a microscope objective (MO) in digital holographic microscopy (DHM) while offering the advantages of compactness, lightness, and affordability. A proof-of-concept experiment is performed on a standard diffraction grating, and various effective parameters on the imaging performance are investigated. The results are validated by atomic force microscopy and Mirau-DHM, and 3D morphometric information of the sample under inspection is obtained. The technique is then applied for 3D quantitative measurement and visualization of a human red blood cell, proving the principle of our easy-to-implement and vibration-immune arrangement for high-contrast label-free QPI of biological samples, and its utility in cell morphology, identification, and classification.

Microsphere-assisted super-resolved hyperspectral microscopy

Hyperspectral microscopy (HSM) combines conventional microscopy with basic hyperspectral imaging and results in 3D microscopic spatio-spectral information. The combination comes along with a variety of applications, such as detection and classification of different material properties through spectral fingerprints, which otherwise cannot be detected with a color camera alone. However, similar to other microscopies, the resolution of HSM is limited by diffraction. In recent years microsphere (MS)-assisted microscopy has attracted intensive attention for resolution enhancement. Here, we extend the MS idea into HSM. We show that MS-HSM reveals extra hyperspectral information about the specimen which is lost without it. Moreover, the insertion of MS results in super-resolved images. The resolution enhancement and obtaining the extra hyperspectral information can be tuned by the MS size.

Roadmap on Label-Free Super-Resolution Imaging

Label-free super-resolution (LFSR) imaging relies on light-scattering processes in nanoscale objects without a need for fluorescent (FL) staining required in super-resolved FL microscopy. The objectives of this Roadmap are to present a comprehensive vision of the developments, the state-of-the-art in this field, and to discuss the resolution boundaries and hurdles which need to be overcome to break the classical diffraction limit of the LFSR imaging. The scope of this Roadmap spans from the advanced interference detection techniques, where the diffraction-limited lateral resolution is combined with unsurpassed axial and temporal resolution, to techniques with true lateral super-resolution capability which are based on understanding resolution as an information science problem, on using novel structured illumination, near-field scanning, and nonlinear optics approaches, and on designing superlenses based on nanoplasmonics, metamaterials, transformation optics, and microsphere-assisted approaches. To this end, this Roadmap brings under the same umbrella researchers from the physics and biomedical optics communities in which such studies have often been developing separately. The ultimate intent of this paper is to create a vision for the current and future developments of LFSR imaging based on its physical mechanisms and to create a great opening for the series of articles in this field.

Dielectric microspheres enhance microscopy resolution mainly due to increasing the effective numerical aperture

Microsphere-assisted microscopy utilizing a microsphere in immediate proximity of the specimen boosts the imaging resolution mainly as a result of an increase in the effective numerical aperture of the system.

Addressing the imaging limitations of a microsphere-assisted nanoscope

In the past decade, microsphere-assisted nanoscopy has been developed rapidly to overcome the diffraction limit. However, due to the limited size and high surface curvature of microspheres, the magnified imaging still suffers from problems like limited view scope, imaging distortion, and low contrast. In this paper, we specialize in the imaging mechanism of microspheres and find irradiance as the key factor for microsphere imaging quality. Utilizing a modified optical tweezer system, we achieve precise manipulation of microspheres and further propose a high-quality large-field magnified imaging scheme. The results show that the imaging area of 5 µm microspheres can reach 16×12 µm² with the minimum identifiable feature of 137 nm. This scheme provides a new solution for extending the measuring scope of microsphere-assisted nanoscope, and will certainly promote the application of this technology in practice.

Microlens-assisted microscopy for biology and medicine

The addition of dielectric transparent microlens in the optical scheme is an effective and at the same time simple and inexpensive way to increase the resolution of a light microscope. For these purposes, spherical and cylindrical microlenses with a diameter of 1-100 μm are usually used. The microlens focuses the light into a narrow beam called a photonic nanojet. An enlarged virtual image is formed, which is captured by the objective of the light microscope. In addition to microscopy, the microlenses are successfully applied to amplify optical signals, increase the trapping force of optical tweezers and are used in microsurgery. This review considers the design and principle of microlens-assisted microscopes. Taking into account the advantages of the super-resolution optical methods for research in life science, the examples of the use of the microlenses in biomedical practice are discussed in detail.

Microsphere-assisted Fabry-Perot interferometry: proof of concept

We propose a microsphere-assisted Fabry-Perot interferometry (MAFPI) for microstructure measurement. We stretch the single-mode fiber and combine it with microspheres of different sizes and refractive indices, which can form super-focused spots with different characteristics, that is, a photonic nanojet phenomenon. As a proof of principle, we performed scanning imaging of optical discs and holographic gratings by MAFPI. The optical disc image obtained by MAFPI is consistent with the result obtained by a scanning electron microscope, and the obtained grating image is consistent with the actual result.

3D imaging using scanning diffractometry

Imaging of cells is a challenging problem as they do not appreciably change the intensity of the illuminating light. Interferometry-based methods to do this task suffer from high sensitivity to environmental vibrations. We introduce scanning diffractometry as a simple non-contact and vibration-immune methodology for quantitative phase imaging. Fresnel diffractometry by a phase step has led to several applications such as high-precision measurements of displacement. Additional scanning may lead to 3D imaging straightforwardly. We apply the technique to acquire 3D images of holographic grating, red blood cell, neuron, and sperm cell. Either visibility of the diffraction fringes or the positions of extrema may be used for phase change detection. The theoretical analysis through the Fresnel diffraction from one-dimensional phase step is presented and the experimental results are validated with digital holographic microscopy. The presented technique can be suggested to serve as a robust device for 3D phase imaging and biomedical measurements.

Microsphere-assisted super-resolved Mueller matrix microscopy

Mueller matrix microscopy (MMM) is a powerful approach to probe microstructural and optical information of many important specimens (e.g., tissue and bacteria), which otherwise cannot be obtained directly from intensity or spectral images. Achieving high lateral resolution in MMM, similar to other microscopy approaches, remains a challenge. Here, we extend the idea of microsphere (MS) -assisted microscopy into MMM toward resolution-enhanced polarimetric imaging. The goal is achieved by insertion of a transparent MS in the working distance of the imaging microscope objective in the optical train of an MMM system. We experimentally show that an MS close to the sample in MMM may increase the resolution beyond the intrinsic diffraction limit of the system by redirecting the higher spatial frequencies of the sample into the acceptance cone. In order to be a case in point, the experiment is conducted on a standard holographic diffraction grating with 1 µm line-width, which is beyond the diffraction limit of a 10× objective. Two-dimensional images of the Mueller matrix and some of the widely used quantitative polarimetric parameters of the sample are calculated and compared in the two cases before and after insertion of MS. The proposed arrangement is easy to implement and has the potential to serve as a high-resolution polarimetric microscope for visualizing the polarization characteristics of the microscopic objects.

Microsphere-Toward Future of Optical Microscopes

Optical microscope is one of the most widely used imaging tools for its great flexibility, reliable design, and low cost. Optical microsphere nanoscope (OMN) is invented as a method that can greatly enhance the observation power of conventional optical microscopes. In this perspective, the promising outlook for this approach is briefly discussed. There exists a great freedom to apply this method in various applications. OMN has been successfully commercialized. Our past experience and strategies are summarized in this perspective, which serves as a good reference for the future technology entrepreneurs. Based on our story and model, the factors for success are listed. It can be used to evaluate other commercialization projects and find out the directions that require further improvement.

Microdeformation of RBCs under oxidative stress measured by digital holographic microscopy and optical tweezers

This paper utilized digital holographic microscopy and optical tweezers to study microdeformation of red blood cells (RBCs) dynamically under oxidative stress. RBCs attached with microbeads were stretched by dual optical tweezers to generate microdeformation. Morphology of RBCs under manipulation were recorded dynamically and recovered by off-axis digital holographic microscopy method. RBCs treated with H₂O₂ at different concentrations were measured to investigate the mechanical properties under oxidative stress. Use of optical tweezers and off-axis digital holographic microscopy enhanced measuring accuracy compared with the traditional method. Microdeformation of RBCs is also more consistent with the physiological situation. This proposal is meaningful for clinical applications and basic analysis of Parkinson's disease research.

Illumination conditions in microsphere-assisted microscopy

White-light microsphere-assisted microscopy is a full-field and label-free imaging promising technique making it possible to achieve a subdiffraction lateral resolution. However, performance of this technique depends not only on the geometrical parameters but also on the illumination conditions of the optical system. In the present work, experimental measurements and computer simulations have been performed in air in order to determine the influence of the two diaphragm apertures of the Köhler arrangement and the spectral width of the light source on both the depth-of-focus of the microsphere and the optimization of the imaging contrast. Furthermore, the super-resolution phenomenon is demonstrated and the cumulated optical aberrations are shown through the measurement of the optical transfer function for the different arrangements of the illumination part.

Quantitative phase imaging system with slightly-off-axis configuration and suitable for objects both larger and smaller than the size of the image sensor

We propose a quantitative phase imaging system with exact slightly-off-axis configuration and suitable for objects both smaller and larger than the size of the effective recording region of the image sensors. In this system, the object is illuminated by a convergent spherical beam and a specially designed aperture filter is placed on the spatial frequency plane of the object wave; at the same time, a point source emitting from the edge of the aperture is taken as the reference beam, so that the optimal frequency condition for reconstruction of slightly-off-axis digital holograms can be always guaranteed for both large and small objects as well as different magnification (or the field of view) configurations. At the same time, a 1x2 single-mode optical fiber splitter is used for generating the reference and the illumination beams. Benefited from such fiber-based slightly-off-axis design, the proposed system provides a low-cost way to convert a regular microscope into a slightly-off-axis holographic one for microbiological specimens with a high spatial resolution.

Coherence scanning and phase imaging optical interference microscopy at the lateral resolution limit

To get physical insight into the 3D transfer characteristics of interference microscopy at high numerical apertures we study reflecting rectangular grating structures. In general, the height obtained from phase information seems to be reduced, whereas height values resulting from coherence scanning sometimes seem to be systematically overestimated. Increasing the numerical aperture of an interference microscope broadens the spectra of the resulting interference signals, thus offering a broad variety of wavelength contributions to be analyzed. If phase analysis of a measured far-field interference wavefront is performed at very short wavelengths the periodical profiles obtained from coherence scanning and phase shifting analysis differ only by the measured amplitude. However, at longer wavelength there is a 180° phase shift of the measured profiles obtained from phase analysis compared to coherence peak analysis. Increasing the evaluation wavelength improves the lateral resolution since the long wavelength contributions are related to electromagnetic waves of high angles of incidence. This behavior is to the best of our knowledge not documented in literature so far. It was first observed experimentally and could be confirmed by simulation results obtained from either Kirchhoff diffraction theory or an extended Richards-Wolf model developed in our group. Compared to original input profiles used for the simulation the profiles obtained from phase evaluation correspond quite well at longer wavelength, whereas the results obtained from coherence peak analysis are typically inverted with respect to height.

Full field-of-view digital lens-free holography for weak-scattering objects based on grating modulation

Grating-based single-shot digital lens-free holography with spatial spectral multiplexing is proposed to realize full field-of-view (FOV) imaging for weak-scattering objects. Multiple object waves are generated by a one-dimensional grating that is placed in near contact with the object to avoid the cross talk among different diffraction orders during reconstruction. A multiplexed off-axis hologram is created by interference between the object waves and reference wave and captured by an image sensor in one shot. Multiple imaging areas corresponding to the captured object waves can be simultaneously retrieved during reconstruction. A formula which guarantees full FOV imaging without cross talk or information loss is presented. The imaging experiments of a USAF resolution target are presented to demonstrate the feasibility of this method.

Double-channel angular-multiplexing polarization holography with common-path and off-axis configuration

We propose a double-channel angular-multiplexing polarization holographic imaging system with common-path and off-axis configurations. In the system, its input plane is spatially divided into three windows: an object window and two reference windows, and two orthogonal linear polarizers are attached, respectively, on the two reference windows; a two-dimensional cross grating is inserted between the input and output planes of the system. Thus the object beam passing through the object window and the two orthogonal polarized reference beams passing through the two reference windows can overlap each other at the output plane of the system and form a double-channel angular-multiplexing polarization hologram (DC-AM-PH). Using this system, the complex amplitude distributions of two orthogonal polarized components from an object can be recorded and reconstructed by one single-shot DC-AM-PH at the same time. Theoretical analysis and experimental results demonstrated that the system can be used to measure the Jones matrix parameters of polarization-sensitive or birefringent materials.

Microsphere-assisted phase-shifting profilometry

In the present work, we have investigated the combination of a superresolution microsphere-assisted 2D imaging technique with low-coherence phase-shifting interference microscopy. The imaging performance of this technique is studied by numerical simulation in terms of the magnification and the lateral resolution as a function of the geometrical and optical parameters. The results of simulations are compared with the experimental measurements of reference gratings using a Linnik interference configuration. Additional measurements are also shown on nanostructures. An improvement by a factor of 4.7 in the lateral resolution is demonstrated in air, thus giving a more isotropic nanometric resolution for full-field surface profilometry in the far field.

Modern imaging: introduction to the feature issue

This special issue of Applied Optics contains selected papers reflecting the various disciplines that are needed for the design, implementation and advancement of imaging technology and systems, and it highlights the state-of-the-art research developments in the areas of modern imaging use.