The silicon platform has been widely applied in optoelectronics and sensing to attain versatile functions. Epi-mode quantitative phase imaging (QPI) is promising for non-destructive and high-throughput material metrology. However, spatial resolution and its influencing factors have not been fully explored in epi-mode QPI. Here, we propose to explore the spatial phase resolution limit by studying the influencing factors, including object shape, object size, and phase noise. These factors are related to the signal-to-noise ratio (SNR). The phase object distribution is solved from the transfer-matrix model combined with the consideration of the limited aperture of the imaging system. The study may inspire researchers to further enhance the resolvability of QPI by improving image SNR.
Quantitative phase profilometry (QPP) has been recently developed for mapping the thickness profiles of thin films. However, the insufficient signal-to-noise ratio (SNR) has hindered its applicability on thinner structures, e.g., monolayer graphene. Here, we propose and implement a “phase cavity” concept in QPP to amplify the measured phase signal to achieve high contrast and accurate thickness profiling of thin structures. We achieved phase amplification by modulating the oxide film thickness on the silicon substrate and optimizing the illumination wavelength. We obtained the exact thickness profile of monolayer graphene and confirmed it with Raman spectroscopy and atomic force microscopy.
In quantitative phase imaging, spatial resolution and its influencing factors have not been fully explored. Here, we propose to define phase resolution based on the Sparrow limit and investigate the effect of phase inequivalence between adjacent object points. To simulate the measured object phase distribution, the analytical solution to the complex scattered field from a thin phase object is first obtained by solving the inhomogeneous wave equation in the wavevector space. Our theory shows that the phase resolution is not only related to the illumination wavelength and the numerical aperture of the imaging system, but also the object size and the phase detection signal-to-noise ratio. We have validated our findings by simulating phase images of different point arrays and two-point objects under different noise levels.
Laser-based wide-field coherent imaging methods suffer from low image contrast due to the speckle noise as well as poor lateral resolution using collimated illumination. To improve the image contrast and spatial resolution for label-free cell imaging applications, we propose a new dynamic speckle illumination scheme using perfect optical vortex (POV) beams that can provide finer speckles with more uniform distributions. The low spatial coherence from the POV speckle field has significantly improved the signal-to-noise ratio (SNR) and the image contrast, thus contributing to a high spatial resolution that matches the diffraction limit in our cell imaging experiments. Importantly, the depth-resolved imaging capability has been realized which has allowed us to visualize fine subcellular structures at different focal planes.
We propose quantitative phase microscopy (QPM) with second-harmonic generation (SHG) and structured illumination, namely S2QPM, that can achieve fourfold resolution enhancement compared with normal incident illumination. The wave propagation and reconstruction model in S2QPM are provided from solving the inhomogeneous nonlinear Helmholtz equation in the k-space. Our derived physical model is accurate and does not involve far-field approximations. To validate our theoretical model, a Siemens star phase object has been reconstructed under simulated experimental conditions. We envision that S2QPM may potentially open more exciting applications in label-free bioimaging and material characterization. .
KEYWORDS: Thin films, Microscopes, Metrology, Thin film solar cells, Thin film devices, Atomic force microscopy, Transmission electron microscopy, Phase measurement, Laser interferometry, Transistors
A transmission-mode high sensitivity quantitative phase microscope (QPM) is developed for profiling transparent thin film structures. The QPM system is implemented with a common-path interferometry design and a high well-depth camera, which has allowed us to achieve an optical path length difference sensitivity of around 50 picometers. A frame averaging method can be used to further improve the sensitivity. To account for multiple interference within thin films, a transmission matrix model is developed to achieve accurate height profile reconstruction. With the correction model, the profiling accuracy can be improved from 20.6% to 4.0% for a MoS2 thin film with a thickness of around 25 nm.
We propose a new label-free widefield super-resolution imaging technique which involves structured illumination and second harmonic generation (SHG). In an interferometric setup with a reference beam and a sample beam, the use of structured light effectively doubles the region of detectable spatial frequencies. This procedure utilizes both, intensity and phase information of the sample. By solving the wave equations for SHG fields, the images can then be reconstructed in Fourier space with a spatial resolution up to four times the Abbe diffraction limit.
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