155 research outputs found
Virtual optics and sensing of the retrieved complex field in the back focal plane using a constrained defocus algorithm
No full textThe reflected back focal plane from a microscope objective is known to provide excellent information of material properties and can be used to analyze the generation of surface plasmons and surface waves in a localized region. Most analysis has concentrated on direct measurement of the reflected intensity in the back focal plane. By accessing the phase information, we show that examination in the back focal plane becomes considerably more powerful allowing the reconstructed field to be filtered, propagated and analyzed in different domains. Moreover, the phase often gives a superior measurement that is far easier to use in the assessment of the sample, an example of such cases is examined in the present paper. We discuss how the modified defocus phase retrieval algorithm has the potential for real time measurements with parallel image acquisition since only three images are needed for reliable retrieval of arbitrary distributions
Investigating the use of a hybrid plasmonic–photonic nanoresonator for optical trapping using finite-difference time-domain method
No full textWe investigate the use of a hybrid nanoresonator comprising a photonic crystal (PhC) cavity coupled to a plasmonic bowtie nanoantenna (BNA) for the optical trapping of nanoparticles in water. Using finite difference time-domain simulations, we show that this structure can confine light to an extremely small volume of ~30,000 nm3 (~30 zl) in the BNA gap whilst maintaining a high quality factor (5400–7700). The optical intensity inside the BNA gap is enhanced by a factor larger than 40 compared to when the BNA is not present above the PhC cavity. Such a device has potential applications in optical manipulation, creating high precision optical traps with an intensity gradient over a distance much smaller than the diffraction limit, potentially allowing objects to be confined to much smaller volumes and making it ideal for optical trapping of Rayleigh particles (particles much smaller than the wavelength of light)
Surface plasmon, surface wave, and enhanced evanescent wave microscopy
No full textThis chapter gives an overview of the formation of evanescent fields excited by an objective lens. We discuss some of the different phenomena that emerge at the interface between a high index couplant and relatively low index sample. We show how surface plasmons are excited in the presence of a thin gold film. We discuss the application of surface waves microscopy with excitation of microscopic objectives and conclude that significant applications reside in the area of localized measurement of refractive index. A key challenge in surface microscopy is to maintain the high spatial resolution in the presence of waves that propagate relatively long distances along the sample surface, methods to achieve this high resolution are discussed in some detail. For cellular imaging while surface plasmons can give good images, evanescent waves generated by excitation from light incident above the critical angle can produce very high quality images of the sample surface, without needing to address the problems introduced by lateral propagation of the waves. Finally, we discuss potential new directions for imaging and localized sensing using these wavesDepartment of Electronic and Information Engineering2016-2017 > Academic research: refereed > Chapter in an edited book (author)bcw
Fluorescence lifetime imaging
No full textFluorescence lifetime imaging (FLIM) is a key fluorescence microscopy technique to map the environment and interaction of fluorescent probes. It can report on photophysical events that are difficult or impossible to observe by fluorescence intensity imaging, because FLIM is largely independent of the local fluorophore concentration and excitation intensity. Many FLIM applications relevant for biology concern the identification of Förster resonance energy transfer (FRET) to study protein interactions and conformational changes. In addition, FLIM has been used to image viscosity, temperature, pH, refractive index, and ion and oxygen concentrations, all at the cellular level. The basic principles and recent advances in the application of FLIM, FLIM instrumentation, molecular probe, and FLIM detector development will be discussed.</p
Adaptive Optics for Aberration Correction in Optical Microscopy
No full textAll forms of optical microscopy have the potential to suffer from aberrations due to misalignments in the optical system, local refractive index changes in the sample, or, in many cases, both. Aberrations produce a distorted wavefront at the focus of the imaging system leading to a non-optimum focal spot, resulting in a decrease in image resolution and hence a deterioration in image quality. The problem is particularly prevalent when imaging biological tissue using an optical sectioning microscope where the improved axial resolution over standard wide-field techniques leads the user to image deeper into their sample than ever before. The structure present in the tissue presents complex axial and lateral variations in refractive indices, inhomogeneities that increase as the thickness of tissue the light passes through increases. Adaptive optics, a technique that originated in optical astronomy, poses a powerful solution to the problem. The principle behind adaptive optics involves shaping the wavefront of the incoming light in such a way so as to overcome the distortions imposed by the sample and imaging system. Crucial to the successful implementation of adaptive optics in microscopy is the method used to determine the wavefront correction required. Here we introduce the concepts behind adaptive optics, discuss several approaches that have been taken to implement adaptive optics into microscopy, and finally provide examples of its success when applied to a variety of imaging modalities such as multiphoton microscopy, stimulated emission depletion microscopy, and selective plane illumination microscopy.</p
A General Description of the Performance of Surface Plasmon Sensors Using a Transmission Line Resonant Circuit Model
No full textWe analyze the response of surface plasmon (SP) sensors using a transmission line model. We illustrate this analysis with particular reference to a layered structure in which plasmon hybridization occurs. By applying the appropriate resonant condition to the system, we derive a circuit model which predicts the responsivity of different modes. This gives new physical insight into the sensing process. We discuss how the change in the sample region may be modeled as a change in the reactance in the equivalent circuit and from this, it follows that a single parameter can determine the change in resonance position with reactance. This approach is used to predict the response of a generic sensor to binding of an analyte and the bulk change of refractive index. This parameter arises naturally from the circuit representation in a way not readily accessible with the transfer matrix approach. The parameters can be expressed in terms of the Q of a resonant circuit and confirms the intuition that a high Q is associated with poor responsivity, however, we demonstrate that there is another circuit parameter, the resistance at resonance, that can mitigate this effect, providing a route for optimization of the sensor properties
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