Phase-retrieval method for measuring small contact angles of pentane on water.

Pentane drops on a water surface are predicted to have contact angles of the order of 1 degree or less in the phase of frustrated complete wetting. We have developed an optical method of measuring such small contact angles, applicable to cases where the refractive indices of the substrate and adsorbate are very similar and the fluid dynamics do not allow delay between image acquisitions, by using phase retrieval to map the surface profile of the drops. It is empirically shown that, with our method, a difference of nanometer order can be achieved for the phase-retrieved dimensions relative to their expected value. Results agree with numerical predictions by Weiss and Widom [Physica A292, 137 (2001)PHYADX0378-437110.1016/S0378-4371(00)00619-1].

High-resolution surface-plasmon resonance real-time imaging.

We use surface-plasmon resonance in a silver film to obtain high-resolution real-time images of a transparent dielectric sample in contact with it. A new aspect of the work was the use of radially polarized illumination from a LED at 530 nm to obtain speckle-free images with high spatial resolution along all orientations. The sensitivity to refractive index changes in the sample is estimated to be better than 10(-3), and the modulation transfer function out to spatial frequency 1 microm(-1) was measured.

Formation and stability of band patterns in a rotating suspension-filled cylinder.

We explore the phenomenon of segregation and pattern formation in the complex system of a rotating horizontal cylinder completely filled with a dilute suspension of non-Brownian particles. A general dimensionless analysis is presented, which reveals the importance of the different dimensionless parameters involved. A detailed account of the mechanism of segregation and formation of axial bands for the case of low viscosity fluids is given. According to the analysis the axial pressure gradient associated with an inertial-mode excitation within the bounded fluid is responsible for the formation of bands in interleaving nodal planes of the excitation. The question of stability of the band patterns is addressed and a phase diagram in the appropriate dimensionless space is presented.

Space-based intensity interferometer.

The intensity interferometer utilizes the correlations between intensity fluctuations to measure the coherence between light waves. Correlating the intensities of the waves and not their amplitudes can overcome the main disadvantage of the Michelson stellar interferometer in a space environment. It is shown that intensity interferometry should be applicable to stellar sources as weak as 8th magnitude, using techniques that are theoretically feasible today. Two approaches are proposed for a global sky observation campaign while minimizing spacecraft fuel consumption. In the second approach the computational interferometry observation method is introduced, which enables an observation of the full sky from a single orbit. Moreover, control laws were designed to relocate a spacecraft into the required baseline and to maintain it.

Simple beam combination for stellar interferometry.

In stellar interferometry, image quality improves significantly with the inclusion of more telescopes and the use of phase closure. We demonstrate, using first coherent and then partially coherent white light, a compact and efficient pairwise combination of 12 or more beams. The input beams are lined up and spread through a cylindrical lens into a comb of parallel ellipses, which interferes with a perpendicular copy of itself to form a matrix of interferograms between all pairs. The diagonal elements show interference of each beam with itself, for intensity calibration. The measured white-light visibilities were high and stable.

Measurement of the spatial response of a detector pixel.

In order to measure the internal spatial response of a pixel in a detector, it is scanned by a beam smaller than its size. This becomes difficult as the wave length grows and becomes comparable to the pixel size, such as in the infra red. To overcome this difficulty, a special phase mask which makes the beam narrower was designed, constructed, and tested successfully. The mask was made from five alternating transparent rings, where the rings had half a wave phase difference between them. The beam was scanned with and without the mask in two dimensions in fine steps by a much smaller detector and its response was taken. The spot width dropped by 19% at half its height and by 42% at tenth its height, a significant narrowing. The scan was repeated with the full detector pixel. That beam scan served as a deconvolution kernel and allowed us to find the pixel point spread function (spatial response), the pixel modulation transfer function and the optical cross talk between the pixels.

Production of radially and azimuthally polarized polychromatic beams.

We describe a system that efficiently provides radially or azimuthally polarized radiation from a randomly polarized source. It is constructed from two conical reflectors and a cylindrical sheet of polarizing film. Envisaged applications include a microscope illuminator for high-resolution surface plasmon resonance microscopy, illumination for high-resolution microlithography, and efficient coupling of a laser source to hollow optical fibers. The angular coherence function of light polarized by the device was measured to evaluate its usefulness for these applications.

Simulated annealing in ocular adaptive optics.

We present what is to our knowledge a first hardware realization of a simulated annealing algorithm in an adaptive optics system designed to image the retina of the human eye. The algorithm is applied to the retinal image itself without the need for wavefront sensors in the system. We find that this optimization algorithm can be an alternative to the traditional Hartmann-Shack sensing. We also compare the simulated annealing algorithm to the stochastic parallel gradient descent algorithm.

Comparison between sensitivities of phase and intensity detection in surface plasmon resonance.

We have experimentally and theoretically compared intensity and phase measurements of surface plasmon resonance, in order to check the widely claimed superiority of the phase method. Both experiment and theory show a slightly higher sensitivity for intensity detection. Simulations confirm that this result is generally true for a wide range of resonance conditions. The basic reason is that phase measurements must be performed by measuring light intensities in some way, and therefore both modes of operation are limited in similar ways by photon statistics. Sensitivity can only be improved by using larger light intensities.

Banding of suspended particles in a rotating fluid-filled horizontal cylinder.

Non-Brownian particles suspended at low volume concentration in a rotating horizontal cylinder filled with a low-viscosity fluid are observed to segregate into well-defined periodic axial bands. We present an experimental investigation of the dependence of the phenomenon on particle characteristics, tube diameter and length, and fluid viscosity. A theoretical explanation of the phenomenon is suggested, in which the segregation occurs as a result of mutual interaction between the particles and inertial waves excited in the bounded fluid. This leads to the result that macroscopic suspended particles accumulate in alternate nodes of the wave excitation, which is in agreement with the experiments, and leads to two degenerate band patterns for each mode. Under some conditions the observed pattern oscillates between the two possible band configurations. The mechanism underlying the oscillations is unclear. A confirmation of the theoretical approach was obtained by means of a photographic capture of the flow field resulting from the inertial waves.

How does a thin volatile film move?

When a water film evaporates from a mica substrate, an interface similar to a solidification front develops, separating two films of different thicknesses. We show experimentally that the evolution dynamics is controlled mainly by material diffusion through the vapor phase rather than by hydrodynamic flow through the film. Our results illustrate the role of different contributions to pattern formation of volatile liquid films.

Oscillatory axial banding of particles suspended in a rotating fluid.

A nonviscous fluid, completely filling a tube rotating about its horizontal axis, contains a suspension of macroscopic particles. The particles are observed to distribute themselves spontaneously in bands distributed periodically along the axis, with a band separation dependent only on the tube radius and length. In many cases, the bands oscillate periodically between two interleaving patterns. We explain this banding phenomenon as arising from the excitation of inertial standing waves in the rotating fluid.

Symbiosis of different-sized drops.

We demonstrate that the usual situation of coarsening in crystal growth or Ostwald ripening in evaporating liquid drops is not universal, and when the drops coexist with a microscopically thin continuous surface film, a different behavior is observed. The predicted behavior was investigated experimentally and supported by numerical simulations.

Improvement of optical resolution in far-field imaging by optical multiplication.

Based on the idea of the complex filters suggested by Toraldo di Francia in 1952, super-resolved imaging has been achieved by using interferometric image multiplication. The question of whether super-resolution implies a limited field of view is discussed, and an analytical criterion to decide this question is obtained. Three different imaging configurations are investigated, all of which gave a spatial resolution exceeding that corresponding to the full open aperture of the optical system. A resolution limit 55% of the Rayleigh limit was achieved for incoherently illuminated objects. One price that must be paid for the super-resolution is inefficient use of the light source. A second price is a limitation to the usable field of view.

Why is super-resolution so inefficient?

Many optical imaging systems have been designed to achieve spatial resolution exceeding the limit proposed by Abbe theory, namely lambda/2NA. These systems invariably use light inefficiently. We discuss the application of information theory to this problem, in an effort to find an upper limit to the theoretical efficiency of a super-resolving system, as a function of the degree to which its capabilities exceed the Abbe limit.

Superresolution in far-field imaging.

We demonstrate the feasibility of achieving superresolved images by using Fourier-plane phase masks and image multiplication, which together create effective point-spread functions that are not positive definite and therefore cannot be created by any single Fourier plane mask in a linear system. Three different configurations were investigated, all of which gave a spatial resolution exceeding that corresponding to the full open aperture of the optical system. One price that must be paid for the superresolution is inefficient use of the light source.

Theoretical evaluation of a four-band fiber-optic radiometer.

A theoretical simulation of a four-band fiber-optic radiometric technique is presented. This is a technique for remote, noncontact temperature measurement of a sample near room temperature, under conditions of unknown emissivity and ambient temperature. A realistic setup of a broadband IR detector, a set of three filters, an IR fiber, and a MATLAB software package for the calculations, is simulated in two steps: a calibration process and a measurement process. The results of the simulation show the limitations and advantages of the four-band radiometric technique and show the expected resolution of the sample temperature and emissivity and of the ambient temperature measurement. The theoretical resolution of the sample temperature measured by the four-band radiometric setup comes close to the resolution achieved in an equivalent single-band radiometric setup. The four-band method has an additional advantage of making it possible to calculate values of emissivity and ambient temperature.

Superresolution in far-field imaging.

Based on the idea of the complex filters suggested by Toraldo di Francia in 1952 [Nuovo Cimento Suppl. 9, 426 (1952)], superresolved imaging has been achieved by use of interferometric image multiplication. A resolution limit of 55% of the Sparrow limit was achieved for incoherent objects.