Surface plasmon microscopy (SPM) is widely applied for label-free detection of changes of refractive index and concentration, as well as mapping thin films in near field. Traditionally, the SPM systems are based on the detection of light intensity or phase changes. Here, we present two kinds of surface plasmon holographic microscopy (SPHM) systems for amplitude- and phase-contrast imaging simultaneously. Through recording off-axis holograms and numerical reconstruction, the complex amplitude distributions of surface plasmon resonance (SPR) images can be obtained. According to the Fresnel’s formula, in a prism/ gold/ dielectric structure, the reflection phase shift is uniquely decided by refractive index of the dielectric. By measuring the phase shift difference of the reflected light exploiting prism-coupling SPHM system based on common-path interference configuration, monitoring tiny refractive index variation and imaging biological tissue are performed. Furthermore, to characterize the thin film thickness in near field, we employ a four-layer SPR model in which the third film layer is within the evanescent field. The complex reflection coefficient, including the reflectivity and reflection phase shift, is uniquely decided by the film thickness. By measuring the complex amplitude distributions of the SPR images exploiting objective-coupling SPHM system based on common-path interference configuration, the thickness distributions of thin films are mapped with sub-nanometer resolution theoretically. Owing to its high temporal stability, the recommended SPHMs show great potentials for monitoring tiny refractive index variations, imaging biological tissues and mapping thin films in near field with dynamic, nondestructive and full-field measurement capabilities in chemistry, biomedicine field, etc.
We present a quasicommon-path digital holographic microscopy with phase aberration compensation, which is based on a long-working distance objective and can be used for the quantitative characterization of microstructure specimens. The quasicommon-path arrangement makes the holographic system very compact and stable. Meanwhile, the object and reference beams all travel along the same path, which can effectively eliminate the system aberration, and the mirror in the reference arm can be adjusted precisely for the phase tilt compensation. In the experiment, a wafer with orderly patterns and unified height of 180 nm is measured, and its three-dimensional surface topography is obtained. A long-term system stability of 1.39 nm is achieved in measurement with the proposed method.
Biological cells are usually transparent with a small refractive index gradient. Digital holographic interferometry can be used in the measurement of biological cells. We propose a dual-wavelength common-path digital holographic microscopy for the quantitative phase imaging of biological cells. In the proposed configuration, a parallel glass plate is inserted in the light path to create the lateral shearing, and two lasers with different wavelengths are used as the light source to form the dual-wavelength composite digital hologram. The information of biological cells for different wavelengths is separated and extracted in the Fourier domain of the hologram, and then combined to a shorter wavelength in the measurement process. This method could improve the system’s temporal stability and reduce speckle noises simultaneously. Mouse osteoblastic cells and peony pollens are measured to show the feasibility of this method.
Short-coherence in-line phase-shifting digital holographic microscopy based on Michelson interferometer is proposed to measure internal structure in silicon. In the configuration, a short-coherence infrared laser is used as the light source in order to avoid the interference formed by the reference wave and the reflected wave from the front surface of specimen. At the same time, in-line phase-shifting configuration is introduced to overcome the problem of poor resolution and large pixel size of the infrared camera and improve the space bandwidth product of the system. A specimen with staircase structure is measured by using the proposed configuration and the 3D shape distribution are given to verify the effectiveness and accuracy of the method.
A dual-wavelength common-path digital holographic microscopy is presented to simultaneously improve the phase measurement accuracy and stability. Two laser beams with different wavelength are reflected by the front and back surface of a parallel glass plate to form the composite hologram in the lateral shearing region, and a shorter synthetic wavelength Λ289nm is obtained by calculating the arctangent and product of the two reconstructed complex amplitudes. Thus, phase speckle noise can be reduced in the dual-wavelength numerical reconstruction process, and the phase measurement accuracy and stability can be improved. The experiment results of the peony pollens specimen show the feasibility of the proposed configuration.
In traditional dual-wavelength digital holographic microscopy (DHM), a synthetic wavelength is obtained by using two lasers with different wavelengths, and the measurement range of the samples’ step height can be expanded from nanometers to micrometers. However, the measurement accuracy reduces along with the expansion of the measuring range, and significant noise is simultaneously introduced in this process. For cases where the sample’s step height is smaller than the wavelength of the illumination light, the measurement accuracy is very important. We present a new approach for dual-wavelength DHM. The synthetic wavelength is shorter than either of the two lasers, and thus higher measurement accuracy can be achieved. The numerical simulation and experiment results show the feasibility of this technique.
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