Ann Roberts

Quantitative optical phase microscopy.

We present a new method for the extraction of quantitative phase data from microscopic phase samples by use of partially coherent illumination and an ordinary transmission microscope. The technique produces quantitative images of the phase profile of the sample without phase unwrapping. The technique is able to recover phase even in the presence of amplitude modulation, making it significantly more powerful than existing methods of phase microscopy. We demonstrate the technique by providing quantitatively correct phase images of well-characterized test samples and show that the results obtained for more-complex samples correlate with structures observed with Nomarski differential interference contrast techniques.

Refractive Index Measurement in Viable Cells Using Quantitative Phase-Amplitude Microscopy and Confocal Microscopy

The refractive index (RI) of cellular material provides fundamental biophysical information about the composition and organizational structure of cells. Efforts to describe the refractive properties of cells have been significantly impeded by the experimental difficulties encountered in measuring viable cell RI. In this report we describe a procedure for the application of quantitative phase microscopy in conjunction with confocal microscopy to measure the RI of a cultured muscle cell specimen.

Plasmonic lenses formed by two-dimensional nanometric cross-shaped aperture arrays for Fresnel-region focusing.

We present the experimental demonstration of what are to our knowledge the first two-dimensional planar plasmonic lenses formed by an array of spatially varying cross-shaped apertures in a metallic film for Fresnel-region focusing. The design utilizes localized surface plasmon resonances occurring inside the apertures, accompanied by an aperture geometry dependent phase shift, to achieve the desired spatial phase modulation in the transmitted field. The performance of lenses with different design configurations was evaluated using a confocal scanning optical microscope, and the effects of diffraction on the optical response of these microscale devices are discussed.

Electromagnetic theory of diffraction by a circular aperture in a thick, perfectly conducting screen

A rigorous electromagnetic theory of the diffraction of radiation by a circular aperture in a thick screen is developed. In particular, the case of an incident plane wave is considered, and the effects of varying the thickness of the screen and of varying the wavelength, polarization, and angle of incidence of the incident wave on the reflection and transmission properties of the screen are investigated.

Quantitative phase tomography

We describe the application of a new technique for the simultaneous determination of three-dimensional absorption and refractive index distributions using a combination of quantitative phase-amplitude microscopy and tomographic reconstruction techniques. We briefly review the phase-amplitude microscopy technique and present experimental results in which we have successfully reconstructed the refractive index profile of two different optical fibres.

Plasmonic quarter-wave plate

Here we present a strategy for designing wave plates utilizing resonances of subwavelength apertures in metallic films. Specifically, we show that it is possible to tune the geometry in a periodic array of cross-shaped apertures in a silver film to produce a quarter-wave plate at a particular wavelength in the near-infrared. This is achieved by introducing an asymmetry into the lengths of the arms of the crosses.

The Dark Side of Plasmonics

Plasmonic dark modes are pure near-field modes that can arise from the plasmon hybridization in a set of interacting nanoparticles. When compared to bright modes, dark modes have longer lifetimes due to their lack of a net dipole moment, making them attractive for a number of applications. We demonstrate the excitation and optical detection of a collective dark plasmonic mode from individual plasmonic trimers. The trimers consist of triangular arrangements of gold nanorods, and due to this symmetry, the lowest-energy dark plasmonic mode can interact with radially polarized light. The experimental data presented confirm the excitation of this mode, and its assignment is supported with an electrostatic approximation wherein these dark modes are described in terms of plasmon hybridization. The strong confinement of energy in these modes and their associated near fields hold great promise for achieving strong coupling to single photon emitters.

Refractive-index profiling of optical fibers with axial symmetry by use of quantitative phase microscopy

The application of quantitative phase microscopy to refractive-index profiling of optical fibers is demonstrated. Phase images of axially symmetric optical fibers immersed in index-matching fluid are obtained, and the inverse Abel transform is used to obtain the radial refractive-index profile. This technique is straightforward, nondestructive, repeatable, and accurate. Excellent agreement, to within approximately 0.0005, between this method and the index profile obtained with a commercial profiler is obtained.

Bandpass grids with annular apertures

A rigorous theory is developed for describing the diffraction of a plane wave by a doubly periodic array of annular apertures in a thick, perfectly conducting screen. Coaxial waveguide modes are used to describe the fields within each aperture, while the fields above and below the grid are written as plane wave expansions. Appropriate boundary conditions are applied at the upper and lower surfaces of the screen, and the method of moments is used to determine mode and wave amplitudes. Such a structure was found to exhibit excellent bandpass characteristics which can be adjusted by changing the size of the apertures and the thickness of the screen. >

Nondestructive imaging of a type I optical fiber Bragg grating

Nondestructive images of refractive-index variation within a type I fiber Bragg grating have been recorded by the differential interference contrast imaging technique. The images reveal detailed structure within the fiber core that is consistent with the formation of Talbot planes in the diffraction pattern behind the phase mask that had been used to fabricate the grating.