Michael G. Somekh

Handbook of photonics for biomedical engineering

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Grating-coupled Otto configuration for hybridized surface phonon polariton excitation for local refractive index sensitivity enhancement

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Single shot embedded surface plasmon microscopy with vortex illumination

We demonstrate numerically through rigorous coupled wave analysis (RCWA) that replacing the prism in the Otto configuration with gratings enables us to excite and control different modes and field patterns of surface phonon polaritons (SPhPs) through the incident wavelength and height of the Otto spacing layer. This modified Otto configuration provides us the following multiple modes, namely, SPhP mode, Fabry-Pérot (FP) cavity resonance, dielectric waveguide grating resonance (DWGR) and hybridized between different combinations of the above mentioned modes. We show that this modified grating-coupled Otto configuration has a highly confined field pattern within the structure, making it more sensitive to local refractive index changes on the SiC surface. The hybridized surface phonon polariton modes also provide a stronger field enhancement compared to conventional pure mode excitation.

Sensitive detection of voltage transients using differential intensity surface plasmon resonance system

In previous work we demonstrated how a confocal microscope with a spatial light modulator in the back focal plane could perform accurate measurement of the k-vector of a propagating surface plasmon. This involved forming an embedded interferometer between light incident close to normal incidence (reference beam) and light incident at the angle to excite surface plasmons (sample beam). The signal from the interferometer was extracted by stepping the phase of the reference beam relative to the sample beam using a spatial light modulator; this requires at least 3 phase steps, which limits the speed of operation. To overcome this and extract the same information with a single measurement, we project an azimuthal varying phase between 0 and 2π in the central region of the back focal plane; corresponding to small angles of incidence. This projects a vortex beam as the reference, so that the phase of the reference beam varies with azimuthal angle. By extracting the interference signal from different portions of the reference beam, different phase steps between the reference and the sample are obtained, so all the values required for phase reconstruction can be extracted simultaneously. It is thus possible to obtain the same information with a single shot measurement, at each defocus position, without additional changes to the back focal plane illumination. Results are presented to show that the vortex illuminated sample provides similar results to the phase stepped version, whose values are, in turn, validated with ellipsometry and surface profilometry.

Application of a novel widefield surface plasmon microscope in binding assays

This paper describes theoretical and experimental study of the fundamentals of using surface plasmon resonance (SPR) for label-free detection of voltage. Plasmonic voltage sensing relies on the capacitive properties of metal-electrolyte interface that are governed by electrostatic interactions between charge carriers in both phases. Externally-applied voltage leads to changes in the free electron density in the surface of the metal, shifting the SPR position. The study shows the effects of the applied voltage on the shape of the SPR curve. It also provides a comparison between the theoretical and experimental response to the applied voltage. The response is presented in a universal term that can be used to assess the voltage sensitivity of different SPR instruments. Finally, it demonstrates the capacity of the SPR system in resolving dynamic voltage signals; a detection limit of 10mV with a temporal resolution of 5ms is achievable. These findings pave the way for the use of SPR systems in the detection of electrical activity of biological cells.

Label-Free, High Resolution, Multi-Modal Light Microscopy for Discrimination of Live Stem Cell Differentiation Status

The present work investigates the capabilities of a novel widefield surface plasmon microscope (WSPR) and its potential applications in biology. Here in particular, we concentrate on imaging interfacial interactions between specific antibodies and the proteins laminin and fibronectin, both patterned onto prefabricated Au/Cr/glass substrates. These experiments are used to demonstrate the high sensitivity, good contrast and high lateral resolution (close to a micron) of the new system. These results indicate that our microscope may form the basis for a new generation of fast assay systems.

Widefield Confocal Microscope for Surface Wave K-Vector Measurement

A label-free microscopy method for assessing the differentiation status of stem cells is presented with potential application for characterization of therapeutic stem cell populations. The microscopy system is capable of characterizing live cells based on the use of evanescent wave microscopy and quantitative phase contrast (QPC) microscopy. The capability of the microscopy system is demonstrated by studying the differentiation of live immortalised neonatal mouse neural stem cells over a 15 day time course. Metrics extracted from microscope images are assessed and images compared with results from endpoint immuno-staining studies to illustrate the system’s performance. Results demonstrate the potential of the microscopy system as a valuable tool for cell biologists to readily identify the differentiation status of unlabelled live cells.

High-contrast wide-field evanescent wave illuminated subdiffraction imaging

Optical surface waves are guided light on surface of optical structures. There are several optical structures that support optical surface waves, such as, nanostructures, gratings, optical waveguides and metamaterials. Optical surface waves have proven themselves very promising candidates for several applications including, biosensing, optical computing and optical circuitry. The characteristics of surface wave can be characterised by the wave vector (k-vector) of the surface wave. In this paper, we will discuss how a modified confocal microscope integrated with a phase spatial light modulator allows us to measure both the real part and the imaginary part (attenuation coefficient) of the surface wave k-vector. Surface plasmon resonance (SPR) excited on a uniform gold surface through Kretschmann configuration is employed as an example in this talk. Note that the system is not limited to the SPR. It is also applicable to other types of optical surface waves. We have demonstrated in our recent publication that the modified confocal not only provides the k-vector measurement both real and imaginary, it also allows us to separate different loss mechanisms in SPR. One limitation of the system was the single point detection. Here we will discuss the current stage of our development in widefield confocal surface plasmon microscope, which allows us to measure multiple points simultaneously. This has been achieved by integrating another amplitude spatial light modulator in the image plane of the objective lens. Use of orthogonal illumination patterns allows the image plane to be sequentially amplitude modulated and post-processed to recover the confocal image.

Theoretical Investigation of Surface Plasmon Resonance (SPR)-Based Acoustic Sensor

In this Letter, we show how to obtain high-contrast wide-field evanescent wave illuminated subdiffraction imaging through controlling nanoscale light-matter interaction. The light coupling, propagation, and far-field imaging processes show strong polarization selectivity and film quality dependence, which is used to improve the image-contrast-to-noise ratio (CNR) and to enlarge the field of view (FOV). We demonstrate experimentally high CNR subdiffraction imaging with lateral resolution of 122xa0nm and FOV of thousands of micrometers square.

Back Focal Plane Confocal Ptychography

We report a theoretical investigation of a surface plasmon resonance (SPR)-based acoustic sensor for optical detection of ultrasound. The structure being studied is arranged in the Krestchmann configuration and the detection is performed by observing the change of refractive index of water next to the SPR metal. The acoustic pressure is simulated using COMSOL. The simulation results illustrate an insight into mechanism of pressure variation on the surface of SPR sensor due to a constructive interference of the ultrasound. This leads to a local refractive index change of water. The local refractive index change is calculated by converting the incident pressure to water density using IAPWS-95 formulation. Then, the water density is converted to the refractive index using Lorentz-Lorenz formulation. Here we report the change in the refractive index of the water to pressure, dn/dp, which is calculated to be 1.4 x 10-10 Pa-1, which is very close to the dn/dp reported by M. W. Sigrist 1986. We also investigated the effect of temperature and wavelength on the dn/dp and found that the variation in temperature and wavelength does not show any significant effect on the dn/dp relationship. We also discuss the effect of quality factor (Q) and possible improvements to enhance the sensitivity of SPR-based acoustic sensor.