Biologically enabled sub-diffractive focusing.

Evolution shows that photonic structures are a constituent part of many animals and flora. These elements produce structural color and are useful in predator-prey interactions between animals and in the exploitation of light for photosynthetic organisms. In particular, diatoms have evolved patterned hydrated silica external valves able to confine light with extraordinary efficiency. Their evolution was probably guided by the necessity to survive in harsh conditions of sunlight deprivation. Here, we exploit such diatom valves, in conjunction with structured illumination, to realize a biological super-resolving lens to achieve sub-diffractive focusing in the far field. More precisely, we consider a single diatom valve of Arachnoidiscus genus which shows symmetries and fine features. By characterizing and using the transmission properties of this valve using the optical eigenmode technique, we are able to confine light to a tiny spot with unprecedented precision in terms of resolution limit ratio, corresponding in this case to 0.21λ/NA.

Exploiting lens aberrations to create electron-vortex beams.

A model for a new electron-vortex beam production method is proposed and experimentally demonstrated. The technique calls on the controlled manipulation of the degrees of freedom of the lens aberrations to achieve a helical phase front. These degrees of freedom are accessible by using the corrector lenses of a transmission electron microscope. The vortex beam is produced through a particular alignment of these lenses into a specifically designed astigmatic state and applying an annular aperture in the condenser plane. Experimental results are found to be in good agreement with simulations.

Bidirectional optical sorting of gold nanoparticles.

We present a generic technique allowing size-based all-optical sorting of gold nanoparticles. Optical forces acting on metallic nanoparticles are substantially enhanced when they are illuminated at a wavelength near the plasmon resonance, as determined by the particle's geometry. Exploiting these resonances, we realize sorting in a system of two counter-propagating evanescent waves, each at different wavelengths that selectively guide nanoparticles of different sizes in opposite directions. We validate this concept by demonstrating bidirectional sorting of gold nanoparticles of either 150 or 130 nm in diameter from those of 100 nm in diameter within a mixture.

Algorithm-based continuous pulse duration tuning and performance control of a mode-locked laser diode.

A control algorithm is presented that addresses the stability issues inherent to the operation of monolithic mode-locked laser diodes. It enables a continuous pulse duration tuning without any onset of Q-switching instabilities. A demonstration of the algorithm performance is presented for two radically different laser diode geometries and continuous pulse duration tuning between 0.5 ps to 2.2 ps and 1.2 ps to 10.2 ps is achieved. With practical applications in mind, this algorithm also facilitates control over performance parameters such as output power and wavelength during pulse duration tuning. The developed algorithm enables the user to harness the operational flexibility from such a laser with 'push-button' simplicity.

Optical eigenmodes; exploiting the quadratic nature of the energy flux and of scattering interactions.

We report a mathematically rigorous technique which facilitates the optimization of various optical properties of electromagnetic fields in free space and including scattering interactions. The technique exploits the linearity of electromagnetic fields along with the quadratic nature of the intensity to define specific Optical Eigenmodes (OEi) that are pertinent to the interaction considered. Key applications include the optimization of the size of a focused spot, the transmission through sub-wavelength apertures, and of the optical force acting on microparticles. We verify experimentally the OEi approach by minimising the size of a focused optical field using a superposition of Bessel beams.

Numerical investigation of passive optical sorting of plasmon nanoparticles.

We explore the passive optical sorting of plasmon nanoparticles and investigate the optimal wavelength and optimal beam shape of incident field. The condition for optimal wavelength is found by maximising the nanoparticle separation whilst minimising the temperature increase in the system. We then use the force optical eigenmode (FOEi) method to find the beam shape of incident electromagnetic field, maximising the force difference between plasmon nanoparticles. The maximum force difference is found with respect to the whole sorting region. The combination of wavelength and beam shape study is demonstrated for a specific case of gold nanoparticles of radius 40 nm and 50 nm respectively. The optimum wavelength for this particular situation is found to be above 700 nm. The optimum beam shape depends upon the size of sorting region and ranges from plane-wave illumination for infinite sorting region to a field maximising gradient force difference in a single point.

Optical path clearing and enhanced transmission through colloidal suspensions.

We utilize advanced laser fields to clear a path through a dynamic turbid medium, a concept termed "Optical path clearing (OPC)." Particles are evacuated from a volume of the medium using the gradient and/or scattering forces due to an applied laser field with a suitably tailored spatial profile. Our studies encompass both an analytical model and proof-of-principle experiments where paths are cleared in dense bulk colloidal suspensions. Based on our results we suggest that high-performance and high efficiency OPC will be achieved by multiple-step clearing using dynamic laser fields based on Airy or inverted axicon beams.

Propagation characteristics of Airy beams: dependence upon spatial coherence and wavelength.

We generate a broadband "white light" Airy beam and characterize the dependence of the beam properties on wavelength. Experimental results are presented showing that the beam's deflection coefficient and its characteristic length are wavelength dependent. In contrast the aperture coefficient is not wavelength dependent. However, this coefficient depends on the spatial coherence of the beam. We model this behaviour theoretically by extending the Gaussian-Schell model to describe the effect of spatial coherence on the propagation of Airy beams. The experimental results are compared to the model and good agreement is observed.

In-fiber common-path optical coherence tomography using a conical-tip fiber.

Common-path optical coherence tomography (CPOCT) is known to reduce group velocity dispersion and polarization mismatch between the reference and the sample arm as both arms share the same physical path. Existing implementations of CPOCT typically require one to incorporate an additional cover glass within the beam path of the sample arm to provide a reference signal. In this paper, we aim to further reduce this step by directly making use of the back-reflected signal, arising from a conical lens-tip fiber, as a reference signal. The conical lens, which is directly manufactured onto the optical fiber tip via a simple selective-chemical etching process, fulfils two functions acting as both the imaging lens and the self-aligning reference plane. We use a Fourier-domain OCT system to demonstrate the feasibility of this technique upon biological tissue. An in-fiber CPOCT technique may prove potentially useful in endoscopic OCT imaging.

Guided neuronal growth using optical line traps.

Optically guided neuron growth is a relatively new field where the exact mechanisms that initiate growth are not well understood. Both Gaussian light beams and optical line traps have been purported to initiate neuronal growth. Here we present a detailed study using optical line traps with symmetric and asymmetric intensity profiles which have been previously reported to bias the direction of neuronal growth. In contrast to these previous studies, we show similar levels of growth regardless of the direction of the intensity variation along the line trap. Furthermore, our experimental observations confirm previous suggestions that the filopodia produced from neuronal growth cones can be affected by laser light. We experimentally observe alignment of filopodia with the laser field and present a theoretical model describing the optical torques experienced by filopodia to explain this effect.

Optical micromanipulation using supercontinuum Laguerre-Gaussian and Gaussian beams.

We characterize a single beam supercontinuum "white light" trap and determine the trap stiffness in the transverse trapping plane. We realize a holographic white light trapping system using a spatial light modulator, and explore the generation of a dual beam trap and characterize its performance. We also demonstrate optical trapping and rotation of particles using a supercontinuum vortex beam. It is shown that orbital angular momentum can be transferred to spheres trapped in a supercontinuum vortex. Quantified rotation rates are demonstrated.

Optical deflection and sorting of microparticles in a near-field optical geometry.

Near-field optical micromanipulation permits new possibilities for controlled motion of trapped objects. In this work, we report an original geometry for optically deflecting and sorting micro-objects employing a total internal reflection microscope system. A small beam of laser light is delivered off-axis through a total internal reflection objective which creates an elongated evanescent illumination of light at a glass/water interface. Asymmetrical gradient and scattering forces from this light field are seen to deflect and sort polystyrene microparticles within a fluid flow. The speed of the deflected objects is dependent upon their intrinsic properties. We present a finite element method to calculate the optical forces for the evanescent waves. The numerical simulations are in good qualitative agreement with the experimental observations and elucidate features of the particle trajectory. In the size range of 1 microm to 5 microm in diameter, polystyrene spheres were found to be guided on average 2.9 +/- 0.7 faster than silica spheres. The velocity increased by 3.0 +/- 0.5 microms(-1) per microm increase in diameter for polystyrene spheres and 0.7 +/- 0.2 microms(-1) per microm for silica. We employ this size dependence for performing passive optical sorting within a microfluidic chip and is demonstrated in the accompanying video.

The resolution of optical traps created by Light Induced Dielectrophoresis (LIDEP).

Light induced dielectrophoresis (LIDEP) is a variant of the dielectrophoresis (DEP) mechanism that has been used for some time to manipulate particles in a microfluidic environment. Rather than relying on lithographically created contacts to generate the required electrical fields, the electrical contacts in LIDEP are created through the selective illumination of a photoconductor. The key question we address is how microscopic traps created via LIDEP compare to optical traps based on the gradient force, in terms of power required and trap stiffness achieved, as well as the size resolution of such a trap. We highlight the complex interplay between optical power and resolution with electrical parameters, such as the electrical resistance and applied AC Voltage. We show that for a spotsize of five micrometres and larger, particles can indeed be trapped with low power. We use trap stiffness per mW to compare LIDEP with an optical trap and show that our system is 470+/- 94 times stiffer per mW than a conventional optical trap, with no loss of resolution. We also discuss the difficulties of achieving trapping at smaller spot sizes, and that the submicron resolution possible with gradient force trapping is very difficult to realise with LIDEP.

Optical impedance of metallic nano-structures.

Impedance matching refers to the suppression of reflected radiation from an interface and is a concept that applies right across the electromagnetic spectrum. In particular it has come to prominence in relation to the propagation of light in metallic structures and associated meta-materials. Whilst established for microwaves and electrical circuits, this concept has only very recently been observed in the optical domain, yet is not well defined or understood. We present a framework to elucidate the concept of optical impedance. We describe using a scattering matrix approach the characteristic, iterative, image and wave impedances of an optical system. With a numerical model, we explore each form of impedance matching in metal-dielectric structures. Thin gold layers may extend the concept of Brewster's angle to normal incidence and s polarization. Optical impedance for recently realized metallic gold nanopillars which has shown negative permeability is also explored and we show that current measurements are inconclusive to robustly state its characteristic impedance is matched to the vacuum.

Dual beam fibre trap for Raman micro-spectroscopy of single cells.

Raman spectroscopy permits acquisition of molecular signatures from both cellular and sub-cellular samples. When combined with optical trapping we may interrogate an isolated cell reducing extraneous signals from the local environment. To date, experimental configurations have employed combinations of the single beam optical tweezers trap and Raman spectroscopy, using either the same beam or separate beams for Raman interrogation and trapping. A key problem in optical tweezers is the ability to hold and manoeuvre large cells. In this paper, we use a dual beam fibre trap to hold and manoeuvre cells combined with an orthogonally placed objective to record Raman spectra. The dual beam trap, due to its divergent light fields, offers an as yet unexploited ability to hold and move large cellular objects with reduced prospects of photodamage. We additionally show how this system permits us to move large primary human keratinocytes (approximately 30 microns in diameter), such that we may record Raman spectra from local parts of a trapped cell with ease. Finally, we develop a rudimentary microfluidic system used to generate a flow of cells. Using our dual beam trap, combined with this flow system, we hold and acquire Raman spectra from individual cells chosen from a sample of HL60 human promyelocytic leukemia cells.

Development of a graded index microlens based fiber optical trap and its characterization using principal component analysis.

We demonstrate a miniaturized single beam fiber optical trapping probe based on a high numerical aperture graded index (GRIN) micro-objective lens. This enables optical trapping at a distance of 200µm from the probe tip. The fiber trapping probe is characterized experimentally using power spectral density analysis and an original approach based on principal component analysis for accurate particle tracking. Its use for biomedical microscopy is demonstrated through optically mediated immunological synapse formation.

[The loss of endothelial cells after perforating keratoplasty--clinical study]. / Pierderea de celule endoteliale dupa keratoplastia perforanta--studiu clinic.

PURPOSE OF THE STUDY: To determine endothelial cell loss rate after penetrating keratoplasty. To determine for low long this rate remain high. MATERIAL AND METHOD: We have three groups of patients. First group consists of 15 patients without ophthalmic pathology. The second group consists of 15 patients, after cataract surgery. The Third group consists of 20 patients after penetrating keratoplasty, 10 of them with edematous keratoplasty and 10 with keratoconus or central leucoma. Every patient was examined with direct specular microscopy, after 1, 3, 6, 9, 12, 15 and 18 months. RESULTS: In the first group we can't determine an endothelial cell loss. In the second group the endothelial cell loss was 4% per month, 5.9% per month, then the value become normal. In the third endothelial cell was continuous within all intervals. CONCLUSIONS: After penetrating keratoplasty there is a significant endothelial cell loss that continues over 18 month.

Modal characterization using principal component analysis: application to Bessel, higher-order Gaussian beams and their superposition.

The modal characterization of various families of beams is a topic of current interest. We recently reported a new method for the simultaneous determination of both the azimuthal and radial mode indices for light fields possessing orbital angular momentum. The method is based upon probing the far-field diffraction pattern from a random aperture and using the recorded data as a 'training set'. We then transform the observed data into uncorrelated variables using the principal component analysis (PCA) algorithm. Here, we show the generic nature of this approach for the simultaneous determination of the modal parameters of Hermite-Gaussian and Bessel beams. This reinforces the widespread applicability of this method for applications including information processing, spectroscopy and manipulation. Additionally, preliminary results demonstrate reliable decomposition of superpositions of Laguerre-Gaussians, yielding the intensities and relative phases of each constituent mode. Thus, this approach represents a powerful method for characterizing the optical multi-dimensional Hilbert space.

Measurement of the restoring forces acting on two optically bound particles from normal mode correlations.

Optical binding along the axis of two counterpropagating laser fields may be used to organize microparticles into longitudinal, spatially separated, arrays. Here we investigate correlations between the displacements of two optically bound microparticles from their equilibrium positions due to noise. Measurement of the decay time of the correlation functions of the center of mass and relative normal modes is shown to provide an in situ method to determine the optical restoring forces acting on the bound particles, thereby providing a test of our physical understanding of longitudinal optical binding.

Fluorescence spectroscopy of an in vitro model of human cervical precancer identifies neoplastic phenotype.

The clinical diagnosis of cervical neoplasia by spectroscopic methods is potentially a reliable, fast and cost-effective alternative to the conventional smear test. However, it is currently limited by significant inter-patient variation in the spectroscopic properties of the cervix. Characterisation of suitable in vitro models of the spectroscopic changes that take place during neoplastic progression may prove to be a significant step towards the successful development of reliable in vivo systems. In this study, we used organotypic epithelial raft culture as an in vitro model of cervical tissue to analyse changes in the fluorescence properties of surface squamous epithelium that are associated with the development of neoplastic disease. Collagen plugs lined by primary human keratinocytes (PHKs) were used to model the normal cervical epithelium, and plugs lined by cells of the SiHa line were used as a model of neoplastic cervical tissue. Fluorescence emission spectra of these rafts were recorded at excitation wavelengths in the 250-330 nm range, complementing previous work published at longer wavelengths. Normalised, truncated emission spectra were analysed using multivariate principal component analysis. We successfully distinguished between in vitro models of normal and neoplastic cervical tissue and demonstrated a differential effect of acetic acid, which enhances the discrimination of normal from neoplastic tissue. Identification of these differences between in vitro organotypic epithelial rafts may ultimately aid the discrimination of cervical lesions in vivo.