To improve the lateral resolution in microscopic imaging, microspheres are placed close to the object’ s surface in order to support the imaging process by optical near-field information. Although ...microsphere-assisted measurements are part of various recent studies, no generally accepted explanation for the effect of microspheres exists. Photonic nanojets, enhancement of the numerical aperture, whispering-gallery modes and evanescent waves are usually named reasons in context with microspheres, though none of these effects is proven to be decisive for the resolution enhancement. We present a simulation model of the complete microscopic imaging process of microsphere-enhanced interference microscopy including a rigorous treatment of the light scattering process at the surface of the specimen. The model consideres objective lenses of high numerical aperture providing 3D conical illumination and imaging. The enhanced resolution and magnification by the microsphere is analyzed with respect to the numerical aperture of the objective lenses. Further, we give a criterion for the achievable resolution and demonstrate that a local enhancement of the numerical aperture is the most likely reason for the resolution enhancement.
Interference signals in coherence scanning interferometry at high numerical apertures and narrow bandwidth illumination are spectrally broadened. This enables phase analysis within a spectral range ...much wider than the spectral distribution of the light emitted by the light source. Consequently, different surface features can be resolved depending on the wavelength used for phase analysis of the interference signals.
In addition, the surface topography itself affects the spectral composition of interference signals in different ways. Signals related to tilted surfaces or step height structures show special spectral characteristics. Thus, spectral amplitude and phase analysis enables a better understanding of the underlying physical mechanisms and gives hints how to improve the measurement accuracy.
Enhancing the lateral resolution in optical microscopy and interferometry is of great interest in recent research. In order to laterally resolve structures including feature dimensions below the Abbe ...resolution limit, microspheres in the optical near-field of the specimen are shown to locally improve the resolution of the imaging system. Experimental and simulated results following this approach are obtained by a high NA Linnik interferometer and analyzed in this contribution. They show the reconstructed surface of a 1D phase grating below the resolution limit. For further understanding of the transfer characteristics, measured interference data are compared with FEM (finite element method) based simulations with respect to the polarization dependency of the relevant image information for 1D phase gratings. Therefore, the implemented Koehler illumination as well as the experimental setup utilize polarized light.
Abstract
The lateral resolution in microscopic imaging generally depends on both, the wavelength of light and the numerical aperture of the microscope objective lens. To quantify the lateral ...resolution Ernst Abbe considered an optical grating illuminated by plane waves. In contrast, the Rayleigh criterion holds for two point sources or point scatterers separated by a lateral distance, which are supposed to emit spherical waves. A portion of each spherical wave is collected by the objective lens and results in an Airy disc corresponding to a diffraction limited intensity point spread function (PSF). If incoherent illumination is employed the intensity PSFs related to different scatterers on an object are added resulting in the well-known Rayleigh resolution criterion. In interference microscopy instead of the intensity the electric field scattered or diffracted by an object will be affected by the transfer function of the optical imaging system. For a reflective object the lateral resolution of an interference microscope can be again characterized by the Abbe limit if the object under investigation is a grating. However, if two irregularities on a flat surface are being imaged the resolution no longer obeys the Rayleigh criterion. Instead, it corresponds to an optical system with an annular aperture and thus surpasses the prediction given by the Rayleigh criterion. This holds true for both, amplitude as well as phase objects, as it will be elucidated in this study by theoretical considerations, simulation results and an experimental proof of principle.
Enhancing the lateral resolution limit in optical microscopy and interferometry is of great interest in recent research. In order to laterally resolve structures including feature dimensions below ...the resolution limit, microspheres applied in the optical near-field of the specimen are shown to locally improve the resolution of the imaging system. Experimental and simulated results following this approach obtained by a high NA Linnik interferometer are analyzed in this contribution. For further understanding of the transfer characteristics, measured interference data are compared with FEM (finite element method) based simulations with respect to the polarization dependency of the relevant image information.
Light microscopes are the most widely used devices in life and material sciences that allow the study of the interaction of light with matter at a resolution better than that of the naked eye. ...Conventional microscopes translate the spatial differences in the intensity of the reflected or transmitted light from an object to pixel brightness differences in the digital image. However, a phase microscope converts the spatial differences in the phase of the light from or through an object to differences in pixel brightness. Interference microscopy, a phase-based approach, has found application in various disciplines. While interferometry has brought nanometric axial resolution, the lateral resolution in quantitative phase microscopy (QPM) has still remained limited by diffraction, similar to other traditional microscopy systems. Enhancing the resolution has been the subject of intense investigation since the invention of the microscope in the 17th century. During the past decade, microsphere-assisted microscopy (MAM) has emerged as a simple and effective approach to enhance the resolution in light microscopy. MAM can be integrated with QPM for 3D label-free imaging with enhanced resolution. Here, we review the integration of microspheres with coherence scanning interference and digital holographic microscopies, discussing the associated open questions, challenges, and opportunities.
Um die Grenzen der optischen Messtechnik zu erweitern und feinere Strukturen messbar zu machen, wurden verschiedene Systeme in der aktuellen Forschung publiziert. Es wurde gezeigt, dass im Nahfeld ...aufgebrachte Mikrokugeln die Auflösung eines interferometrischen Messsystems verbessern und Strukturen unterhalb Abbe’s Beugungsgrenze sichtbar machen können. In dieser Studie werden Messergebnisse mit einem hochauflösenden Linnik-Interferometer unterhalb der Auflösungsgrenze gezeigt. Des Weiteren werden simulative Analysen zum Phasenverhalten im Nahfeld der Mikrokugeln vorgestellt, um die theoretische Erklärung des superauflösenden Verhaltens von Mikrokugeln und die bildgebenden Prozesse des Systems zu ermöglichen.
In order to push the limitations of optical measurement technology further and to measure finer structures, various systems have been published in current research. It has been shown that ...microspheres applied in the near-field can improve the resolution of an interferometric measuring system and make structures below Abbe’s resolution limit visible. In this study, measurement results obtained with a high-resolution Linnik interferometer show structures with lateral dimensions below the resolution limit. Furthermore, the analysis of the transfer behavior in the spatial frequency domain provides information about the mechanisms in nearfield-assisted interference microscopy by microspheres.
Optical interference microscopes are widespread in topography and roughness measurement on the micro- and nanoscale. In spite of a wide range of scientific and industrial applications, systematic ...deviations between the measured and the real surface topography occur in certain situations, e.g. at edges or steep flanks. We present a numerical model considering the properties of the measurement instrument as well as the surface structure to be measured in order to get new insights into the physical dependencies of these deviations. The computation is based on a rigorous simulation of the scattered near field combined with a Fourier optics treatment of the image formation in the measurement instrument. In this study, the near fields are calculated with an open-source finite element method (FEM) software and a commercial finite difference time domain method (FDTD) software. The numerical results are compared with an analytical Kirchhoff approach and measurements. The main intention of this paper is to introduce the modelling and point out possible fields of application. In further studies, this model could be extended to 3D and parameter dependencies of systematic deviations, such as the material of the measurement object and the NA of the measurement instrument, could be investigated.
Zusammenfassung
Um die Grenzen der optischen Messtechnik zu erweitern und feinere Strukturen messbar zu machen, wurden verschiedene Systeme in der aktuellen Forschung publiziert. Es wurde gezeigt, ...dass im Nahfeld aufgebrachte Mikrokugeln die Auflösung eines interferometrischen Messsystems verbessern und Strukturen unterhalb Abbe’s Beugungsgrenze sichtbar machen können. In dieser Studie werden Messergebnisse mit einem hochauflösenden Linnik-Interferometer unterhalb der Auflösungsgrenze gezeigt. Des Weiteren werden simulative Analysen zum Phasenverhalten im Nahfeld der Mikrokugeln vorgestellt, um die theoretische Erklärung des superauflösenden Verhaltens von Mikrokugeln und die bildgebenden Prozesse des Systems zu ermöglichen.
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