High-quality manipulable fiber-microsphere for super-resolution microscopy.

Despite the gain in resolution brought by microsphere (MS)-assisted microscopy, it has always faced several limitations, such as a limited field of view, surface defects, low contrast, and lack of manipulability. This Letter presents a new type of MS created at the tip of an optical fiber, which we call a fiber microsphere (fMS). The fMS is made from a single-mode or coreless fiber, molten and stretched, ensuring high homogeneity and a sphere diameter smaller than the fiber itself. In addition, the connection between the fMS and the fiber makes scanning the sample a simple task, offering a solution to the difficulties of handling. The fabrication procedure of the fMS and the optical system used in the study are detailed. Our measurements show a clear superiority of the fMS over the soda-lime MS in resolving power and imaging performance.

Wide-field parallel mapping of local spectral and topographic information with white light interference microscopy.

Fourier analysis of interferograms captured in white light interference microscopy is proposed for performing simultaneous local spectral and topographic measurements at high spatial resolution over a large field of view. The technique provides a wealth of key information on local sample properties. We describe the processing and calibration steps involved to produce reflectivity maps of spatially extended samples. This enables precise and fast identification between different materials at a local scale of 1 µm. We also show that the recovered spectral information can be further used for improving topography measurements, particularly in the case of samples combining dielectric and conducting materials in which the complex refractive index can result in nanometric height errors.

Depth-resolved local reflectance spectra measurements in full-field optical coherence tomography.

Full-field optical coherence tomography (FF-OCT) is a widely used technique for applications such as biological imaging, optical metrology, and materials characterization, providing structural and spectral information. By spectral analysis of the backscattered light, the technique of spectroscopic-OCT enables the differentiation of structures having different spectral properties, but not the determination of their reflectance spectrum. For surface measurements, this can be achieved by applying a Fourier transform to the interferometric signals and using an accurate calibration of the optical system. An extension of this method is reported for local spectroscopic characterization of transparent samples and in particular for the determination of depth-resolved reflectance spectra of buried interfaces. The correct functioning of the method is demonstrated by comparing the results with those obtained using a program based on electromagnetic matrix methods for stratified media. Experimental measurements of spatial resolutions are provided to demonstrate the smallest structures that can be characterized.

Modeling the specular spectral reflectance of partially ordered alumina nanopores on an aluminum substrate.

Anodizing of aluminum generates a porous alumina layer comprising cylindrical nanopores (300 nm diameter) extending essentially perpendicular to the substrate. The pore distribution over the surface exhibits a short-distance order close to hexagonal arrangement. On the contrary, long-distance order cannot be defined: the arrangement is not periodic. Visual observation of such nanoporous layers shows a reddish specular reflectance consistent with reflectance spectrum measurements. This work is a parametric study aiming at demonstrating that color effects are caused by the presence of disorder illustrated by the deviations from periodicity in terms of nanopore location and nanopore radius. Using the method of Rigorous Coupled Wave Analysis (RCWA), the reflectance spectrum has been simulated. Although our calculations were done using a simple one-dimensional (1D) model, a fair fit with experimental results is found.

Characterization of Functional Materials Using Coherence Scanning Interferometry and Environmental Chambers.

Functional materials are challenging to characterize because of the presence of small structures and inhomogeneous materials. If interference microscopy was initially developed for use for the optical profilometry of homogeneous, static surfaces, it has since been considerably improved in its capacity to measure a greater variety of samples and parameters. This review presents our own contributions to extending the usefulness of interference microscopy. For example, 4D microscopy allows real-time topographic measurement of moving or changing surfaces. High-resolution tomography can be used to characterize transparent layers; local spectroscopy allows the measurement of local optical properties; and glass microspheres improve the lateral resolution of measurements. Environmental chambers have been particularly useful in three specific applications. The first one controls the pressure, temperature, and humidity for measuring the mechanical properties of ultrathin polymer films; the second controls automatically the deposition of microdroplets for measuring the drying properties of polymers; and the third one employs an immersion system for studying changes in colloidal layers immersed in water in the presence of pollutants. The results of each system and technique demonstrate that interference microscopy can be used for more fully characterizing the small structures and inhomogeneous materials typically found in functional materials.

Fabrication of optical mosaics mimicking human corneal endothelium for the training and assessment of eye bank technicians.

The determination of endothelial cell density (ECD) is a crucial activity in eye banks for the assessment of corneal tissue quality. These cells are responsible for corneal transparency, and ECD correlates with graft survival. ECD is mainly assessed with a manual "naked-eye" procedure under a transmitted light microscope in Europe and using a specular microscope in the United States. Interbank and intrabank variability has been previously demonstrated. In order to facilitate training and continuing education of technicians and reliability assessment of eye banks' ECD determination, we use micro-optics technologies to fabricate test mosaics that exactly reproduce the image of human corneal endothelium. The description of the fabrication process is detailed, and comparisons are made between amplitude and phase mosaics.

Comparison of the behavior of a subwavelength diffractive lens in TE and TM polarization allowing some nonstandard functions.

This Letter introduces and discusses a difference in the behavior of a cylindrical diffractive lens encoded with subwavelength structures illuminated with monochromatic coherent light in the cases of TE and TM polarization. The effective medium theory is used to model with new binary phase function the diffractive lens. A new algorithm combines the finite-difference time domain for the propagation in the near field and the radiation spectrum method for the propagation in the far field. We observe the existence in the TM polarization of a second spot at half the distance of the focal length not predictable by scalar theory.

Comparison of the efficiency, MTF and chromatic properties of four diffractive bifocal intraocular lens designs.

The aim of this paper is to compare the properties of four different profiles which can be used as multifocal intraocular lens. The Hankel transform based on the theory of scalar diffraction is applied to a binary profile, a parabolic one, a parabolic profile with holes, and finally a sinusoidal one. This enables to study the various distributions of the diffractive efficiencies and the axial chromatism. The image quality is evaluated by means of simulations of the MTFs with Zemax. Finally we propose a new way to graphically synthesize all the properties of these lenses, using a radar graph.

Modeling of the angular tolerancing of an effective medium diffractive lens using combined finite difference time domain and radiation spectrum method algorithms.

A new rigorous vector-based design and analysis approach of diffractive lenses is presented. It combines the use of two methods: the Finite-Difference Time-Domain for the study in the near field, and the Radiation Spectrum Method for the propagation in the far field. This approach is proposed to design and optimize effective medium cylindrical diffractive lenses for high efficiency structured light illumination systems. These lenses are realised with binary subwavelength features that cannot be designed using the standard scalar theory. Furthermore, because of their finite and high frequencies characteristics, such devices prevent the use of coupled wave theory. The proposed approach is presented to determine the angular tolerance in the cases of binary subwavelength cylindrical lenses by calculating the diffraction efficiency as a function of the incidence angle.

Coherence scanning interferometry allows accurate characterization of micrometric spherical particles contained in complex media.

Characterizing very small particles, from a few dozen micrometers to the nanometric scale, is a very challenging application in a wide range of domains. In this work, we demonstrate, through the recovery of silica and polystyrene bead properties (i.e. their size and refractive index) that Coherence Scanning Interferometry (CSI), in addition of being contactless, non-destructive, label-free and very well spatially resolved, is a very interesting and promising tool for such complex characterization. The CSI system is used as an imaging Fourier transform spectrometer meaning that the characterizations are achieved by analyzing the interference signal in the spectral domain. Some simulations of the proposed technique are presented and show that the accuracy of such characterization, in particular the measurement of the refractive index, are closely related to the signal to noise ratio. This observation is thereafter confirmed by the experimental results of beads buried within the depth of a transparent sample. Finally, the method is theoretically tested in the case of a scattering medium in which the quality of the signal is highly degraded. In this context, a geometrical approach enabling the simulation of an interference signal from a scattering layer is first proposed and then validated by means of comparison with experimental data.

Ultrashort laser pulse splitting upon resonant reflection on a mirror-based waveguide grating.

A resonant waveguide grating based on a high reflectivity mirror causes a 2pi phaseshift of adjustable slope in the spectrum of an ultrashort light pulse, giving rise to a controllable, lossless temporal pulse splitting. This monolithic phase shifter can simply be placed on the path of the beam as a mirror. A functional element was designed and fabricated. Temporal splitting of a femtosecond laser pulse is experimentally demonstrated. The possibility of obtaining variable delay between subpulses is theoretically discussed.

Multi-level diffractive optical elements produced by excimer laser ablation of sol-gel.

Material ablation by excimer laser micromachining is a promising approach for structuring sol-gel materials as we demonstrate in the present study. Using the well-known direct etching technique, the behaviour of different hybrid organic/inorganic self-made sol-gel materials is examined with a KrF* laser. Ablated depths ranging from 0.1 to 1.5 microm are obtained with a few laser pulses at low fluence (< 1 J/cm(2)). The aim is to rapidly transfer surface relief multi-level diffractive patterns in such a substrate, without intermediate steps. The combination with the 3D profilometry technique of coherence probe microscopy permits to analyse the etching process with the aim of producing multi-level Diffractive Optical Elements (DOE). Examples of four-level DOEs with 10 microm square elementary cells are presented, as well as their laser reconstructions in the infrared.

Bridging pole and coupled wave formalisms for grating waveguide resonance analysis and design synthesis.

The algebraic polar expression of resonant reflection from a grating waveguide excited by a free space wave is formulated in terms of the physically meaningful phenomenological parameters of the coupled wave formalism. The reflection coefficient is simply represented as a circle in the complex plane which sheds light on the behaviour of the modulus and phase of anomalous reflection. Analytical expressions are derived for the phenomenological parameters that can now be calculated from optogeometrical quantities which are simple to measure. The relevance and usefulness of bridging the two formalisms is shown in the example of the design of an evanescent wave biosensor.

High-efficiency, broad band, high-damage threshold high-index gratings for femtosecond pulse compression.

High efficiency, broad-band TE-polarization diffraction over a wavelength range centered at 800 nm is obtained by high index gratings placed on a non-corrugated mirror. More than 96% efficiency wide band top-hat diffraction efficiency spectra, as well as more than 1 J/cm(2) damage threshold under 50 fs pulses are demonstrated experimentally. This opens the way to high-efficiency Chirped Pulse Amplification for high average power laser machining by means of all-dielectric structures as well as for ultra-short high energy pulses by means of metal-dielectric structures.

Spectral phase induced by the reflection on a mirror-based waveguide grating in the neighborhood of modal resonance.

A linearly polarized plane wave impinging on a mirror-based waveguide grating is shown to experience a 2pi phase shift of controllable slope in the reflection spectrum. Such a zeroth-order resonant grating effect is adequately confirmed by means of an ellipsometer. The close agreement between the analytical representation of grating coupled waveguide resonances with the experimental results confirms the relevance of the underlying phenomenological understanding of resonant gratings.