Strongly enhanced photon collection from diamond defect centres under micro-fabricated integrated solid immersion lenses

The efficiency of collecting photons from optically active defect centres in bulk diamond is greatly reduced by refraction and reflection at the diamond-air interface. We report on the fabrication and measurement of a geometrical solution to the problem; integrated solid immersion lenses (SILs) etched directly into the surface of diamond. An increase of a factor of 10 was observed in the saturated count-rate from a single negatively charged nitrogen-vacancy (NV-) within a 5um diameter SIL compared with NV-s under a planar surface in the same crystal. A factor of 3 reduction in background emission was also observed due to the reduced excitation volume with a SIL present. Such a system is potentially scalable and easily adaptable to other defect centres in bulk diamond.Comment: 5 Pages, 5 figures (4 subfigures) - corrected typ

Theoretical and experimental studies of femtosecond streak and picosecond framing image tubes

Imperial Users onl

Improvements in Digital Holographic Microscopy

The Ph.D. dissertation consists of developing a series of innovative computational methods for improving digital holographic microscopy (DHM). DHM systems are widely used in quantitative phase imaging for studying micrometer-size biological and non-biological samples. As any imaging technique, DHM systems have limitations that reduce their applicability. Current limitations in DHM systems are: i) the number of holograms (more than three holograms) required in slightly off-axis DHM systems to reconstruct the object phase information without applying complex computational algorithms; ii) the lack of an automatic and robust computation algorithm to compensate for the interference angle and reconstruct the object phase information without phase distortions in off-axis DHM systems operating in telecentric and image plane conditions; iii) the necessity of an automatic computational algorithm to simultaneously compensate for the interference angle and numerically focus out-of-focus holograms on reconstructing the object phase information without phase distortions in off-axis DHM systems operating in telecentric regime; iv) the deficiency of reconstructing phase images without phase distortions at video-rate speed in off-axis DHM operating in telecentric regime, and image plane conditions; v) the lack of an open-source library for any DHM optical configuration; and, finally, vi) the tradeoff between speckle contrast and spatial resolution existing in current computational strategies to reduce the speckle contrast. This Ph.D. dissertation is motivated to overcome or at least reduce the six limitations mentioned above. Each chapter of this dissertation presents and discusses a novel computational method from the theoretical and experimental point of view to address each of these limitations

Achieving Emmetropia through Astigmatic Keratotomy

An estimated 1 in 6 people have a degree of the debilitating eye disorder known as astigmatism. A condition typically developed between the ages of birth and four years, corneal astigmatism is caused by irregular development of the cornea and results in blurred near and far vision. However, in addition to the onset of astigmatism early in life due to irregular development of the cornea, patients also commonly face the development of a condition known as a cataract, which is a condition common to adults over the age of 65 that is classified by a gradual expansion in opacity of the crystalline lens of the eye. Cataracts are the current leading cause of blindness worldwide and account for the majority of decreased visual acuity of patients in the United States. The concurrence of both the debilitating eye disorder astigmatism and the development of cataracts with age can lead to dramatic decreases in day-to-day function of the elderly if not combated by modern surgical practices. Cataract surgery has become one of the most common elective procedures across the globe; Medicare alone spent an estimated 60% of their funding on routine cataract surgery procedures during the 1990s. Up until the late twentieth century, nearly all cataract surgery patients had to get glasses following their operations. But with the exciting new development of a procedure called astigmatic keratotomy, there is the chance that astigmatic patients could see without compensating cylindrical lenses post surgery

Adaptive Optical Devices in Vision Science

In this thesis we investigate the use of adaptive optical devices in three different areas of vision science. These areas are defocus perception, retinal imaging and severe vision loss. Birefringent material has been utilised to produce optical components that can control the angle of refraction of incident light. Using a ferroelectric liquid crystal (FLC) the orientation of linear polarised light can be controlled. This provides us with the ability to switch between the two refractive indices of birefringent materials at very high speeds. A focus switchable lens (FSL) has been made from barium borate (BBO), and a ferroelectric liquid crystal to switch between equal and opposite defocus levels to determine the optimum focus correction by making use of the human eye's sensitivity to flicker. Flicker simulation result indicate that there is a high dependence of flicker sensitivity to the flicker frequency. High spatial frequencies also increased the ability to perceive small defocus shifts. Promising results have been obtained showing a person is able to find a point of equal defocus using flicker more accurately than they would be able to find perfect focus. The same focus switching lens system has the ability to produce fast focus switching cameras. Its potential has been analysed for the use in retinal cameras to ease the process of obtaining good quality images of the optic nerve and providing such cameras with the ability to switch focus within the depth of the optic nerve head at high speeds. Simulation results showed that two FSLs positioned within the zoom system of the imaging arm are able to create focal point shifts of very small amounts. Finally, collaborative research has been conducted in the use of a birefringent prism in conjunction with an FLC to create image jitter that can enhance visual performance in people with severe visual impairment. Image jitter created on-screen and via an optical system was tested. Patients were able to increase their reading speed and improve their ability to discriminate between happy and sad faces

Pupil wavefront manipulation for the compensation of mask topography effects in optical nanolithography

As semiconductor optical lithography is pushed to smaller dimensions, resolution enhancement techniques have been required to maintain process yields. For some time, the customization of illumination coherence at the source plane has allowed for the control of diffraction order distribution across the projection lens pupil. Phase shifting at the mask plane has allowed for some phase control as well. However, geometries smaller than the imaging wavelength introduce complex wavefront effects that cannot be corrected at source or mask planes. Three dimensional mask topography effects can cause a pitch dependent defocus (δBF), which can decrease the useable depth of focus (UDOF) across geometry of varying density. Wavefront manipulation at the lens pupil plane becomes necessary to provide the degrees of freedom needed to correct for such effects. The focus of this research is the compensation of such wavefront phase error realized through manipulation of the lens pupil plane, specifically in the form of spherical aberration. The research does not attempt to improve the process window for one particular feature, but rather improve the UDOF in order to make layouts with multiple pitches possible for advanced technology nodes. The research approach adopted in this dissertation includes rigorous simulation, analytical modeling, and experimental measurements. Due to the computational expense of rigorous calculations, a smart genetic algorithm is employed to optimize multiple spherical aberration coefficients. An analytical expression is formulated to predict the best focus shifts due to spherical aberration applied in the lens pupil domain. Rigorously simulated trends of best focus (BF) through pitch and orientation have been replicated by the analytical expression. Experimental validation of compensation using primary and secondary spherical aberration is performed using a high resolution wavefront manipulator. Subwavelength image exposures are performed on four different mask types and three different mask geometries. UDOF limiting δBF is observed on the thin masks for contact holes, and on thick masks for both one directional (1D) and two directional (2D) geometries. For the contact holes, the applied wavefront correction decreases the δBF from 44 nm to 7 nm and increases the UDOF to 109 nm, an 18% improvement. For the 1D geometries on a thick mask, the through pitch UDOF is increased from 59 nm to 108 nm, an 83% improvement. Experimental data also shows that an asymmetric wavefront can be tuned to particular geometries, providing a UDOF improvement for line ends under restricted processing conditions. The experimental data demonstrates that pupil wavefront manipulation has the capability to compensate for mask topography induced δBF. This dissertation recommends that corrective spherical aberration coefficients be used to decrease pitch dependent best focus, increase process yield, and ultimately expand the design domain over parameters such as mask materials and mask feature densities. The effect of spherical aberration applied in the pupil plane is to provide a wavefront solution that is equivalent to complex multiple-level mask compensation methods. This will allow the advantages of thicker masks to be explored for further applications in semiconductor optical lithography

Reconstructing the Image Scanning Microscopy Dataset: an Inverse Problem

Confocal laser-scanning microscopy (CLSM) is one of the most popular optical architectures for fluorescence imaging. In CLSM, a focused laser beam excites the fluorescence emission from a specific specimen position. Some actuators scan the probed region across the sample and a photodetector collects a single intensity value for each scan point, building a two-dimensional image pixel-by-pixel. Recently, new fast single-photon array detectors have allowed the recording of a full bi-dimensional image of the probed region for each scan point, transforming CLSM into image scanning microscopy (ISM). This latter offers significant improvements over traditional imaging but requires an optimal processing tool to extract a super-resolved image from the four-dimensional dataset. Here we describe the image formation process in ISM from a statistical point of view, and we use the Bayesian framework to formulate a multi-image deconvolution problem. Notably, the single-photon detector suffers exclusively from the photon shot noise, enabling the development of an effective likelihood model. We derive an iterative likelihood maximization algorithm and test it on experimental and simulated data. Furthermore, we demonstrate that the ISM dataset is redundant, enabling the possibility of obtaining reconstruction sampled at twice the scanning step. Our results prove that in ISM, under appropriate conditions, the Nyquist-Shannon sampling criterium is effectively relaxed. This finding can be exploited to speed up the acquisition process by a factor of four, further improving the versatility of ISM systems

Variable optical elements for fast focus control

In this Review, we survey recent developments in the emerging field of high-speed variable-z-focus optical elements, which are driving important innovations in advanced imaging and materials processing applications. Three-dimensional biomedical imaging, high-throughput industrial inspection, advanced spectroscopies, and other optical characterization and materials modification methods have made great strides forward in recent years due to precise and rapid axial control of light. Three state-of-the-art key optical technologies that enable fast z-focus modulation are reviewed, along with a discussion of the implications of the new developments in variable optical elements and their impact on technologically relevant applications