Tailored bandgaps: iterative algorithms of diffractive optics.

A diffractive optics design method based on a phase retrieval algorithm and carrier grating coding is modified to enable designing of photonic bandgap reflectances. Discrete and continuous signals are designed for a fiber grating to demonstrate the capability of the approach. The method is proved a versatile tool for synthesizing reflectance spectra of periodic structures.

Double groove broadband gratings.

Waveguiding in periodical structures of the size of the wavelength is applied to increase the functional spectral band of diffractive optics. The deviation of the effective refractive index between waveguides as a function of the wavelength is utilized to compensate the strong wavelength dependence of the efficiency of diffraction gratings.

Producing illumination-independent additively mixed colors by diffractive optics.

We apply transmission gratings under Littrow incidence to produce polychromatic colors by additive color mixing. Parametric optimization of gratings is employed to produce high efficiency. In addition, we show that the system can yield the same color from a wide variety of spectra; i.e., the system can produce metameric colors.

Electromagnetic field computation in semiconductor laser resonators.

An electromagnetic method based on rigorous diffraction theory of gratings is applied to the analysis of fields in semiconductor laser cavities. The method is based on the Fourier modal method; it is fully rigorous for infinitely periodic resonators and highly accurate for single resonators when absorbing boundary conditions are applied. Fundamental-mode intracavity and near-field distributions are evaluated for some selected geometries, and resonance frequencies are predicted.

Wideband four-level transmission gratings with flattened spectral efficiency.

Four-level transmission-type surface relief grating profiles with nearly flat efficiency over a spectral octave are designed by rigorous electromagnetic diffraction theory. Parametric optimization of the relief depths and transition points of the profile steps of these leads to efficiencies in the range 50-60% over the entire octave if the ratio of the grating period and the mean spectral wavelength is greater than ~ 3.

Giant optical activity in quasi-two-dimensional planar nanostructures.

We examine the spectral dependence in the visible frequency range of the polarization rotation of two-dimensional gratings consisting of chiral gold nanostructures with subwavelength features. The gratings, which do not diffract, are shown to exhibit giant specific rotation (approximately 10(4) degrees/mm) of polarization in direct transmission at normal incidence. The rotation is the same for light incident on the front and back sides of the sample. Such reciprocity indicates three dimensionality of the structure arising from the asymmetry of light-plasmon coupling at the air-metal and substrate-metal interfaces. The structures thus enable polarization control with quasi-two-dimensional planar objects. However, in contradiction with recently suggested interpretation of experiments on larger scale but otherwise similar structures, the observed polarization phenomena violate neither reciprocity nor time-reversal symmetry.

Electromagnetic approach to laser resonator analysis.

An electromagnetic method based on rigorous diffraction theory of gratings is introduced to analyze the modal structure of semiconductor laser cavities. The approach is based on the use of the Fourier Modal Method, the S-matrix algorithm, and the formulation of an eigenvalue problem from which the wave forms and eigenvalues of the modes can be determined numerically. The method is completely rigorous for infinitely periodic laser arrays and is applicable to individual laser resonators with the introduction of imaginary absorbing regions.

Transmission through single subwavelength apertures in thin metal films and effects of surface plasmons.

The existing analyses on extraordinary optical transmission through apertures on a metal screen have been carried out assuming perfect conductivity or by examining arrays of closely spaced holes with subwavelength dimensions. We present an electromagnetic analysis of a single hole (modeled by use of an array of distant holes) in a finitely conducting metal membrane, applying no approximations. We demonstrate that finite conductivity is not of remarkable importance with small hole-diameter-to-wavelength ratios in the absence of strong resonances. However, if the angle of incidence of a plane wave is such that surface plasmons are excited, substantial enhancement of the transmittance can be observed, and the effect of finite conductivity will no longer be negligible. Our analysis also reveals that transmission of small apertures in highly conducting membranes can be described by approximate analytical formulas if surface waves are not excited, but with poor conductors the full electromagnetic analysis should be applied.

Diffractive elements with novel antireflection film stacks.

Antireflection coatings fabricated between the substrate and the diffractive microstructure are shown to reduce Fresnel losses effectively, especially for high-index substrates used in the infrared region, if the diffractive structure and the antireflection stack are designed simultaneously. A substantial reduction of the Fabry-Perot effect caused by the high-index substrate is observed by using antireflection layers with films thicker than the normal quarter-wave films.

Simulation of light propagation by local spherical interface approximation.

A new local elementary interface approximation is introduced for the modeling of wave propagation through interfaces between homogeneous media. The incident wave and the surface profile are approximated locally by a spherical wave and a spherical surface, respectively. The wave field travels through the modulated structure according to the laws of geometrical optics, being refracted by the surface and propagating to the output plane locally as a geometric spherical wave. Diffraction theory is applied to propagate the field from the output plane onwards. We provide comparisons of the method with the thin-element approximation, the local plane-wave and interface approach, and rigorous diffraction theory using a sinusoidal surface-relief grating as an example. We illustrate the power of the new method by applying it to the analysis of a diffractive beam splitter.

Step-transition perturbation approach for pixel-structured nonparaxial diffractive elements.

An extension of an approximate step-transition perturbation method is presented that permits numerically efficient diffraction analysis of pixel-structured surface profiles in the nonparaxial domain. Comparison with the rigorous diffraction theory of gratings shows that the method is reasonably accurate provided that the pixel size exceeds approximately two wavelengths even if the structure contains isolated pixels.

Reformulation of the Fourier modal method with adaptive spatial resolution: application to multilevel profiles.

Formulation of the Fourier modal method for multilevel structures with spatially adaptive resolution is presented, using a slightly reformulated representation for the spatial coordinates. Projections to Fourier base in boundary value problem are used allowing extensions to multilayer profiles with differently placed transitions. We evade the eigenvalue problem in homogeneous regions demanded in the original formulation of the Fourier modal method with adaptive spatial resolution.

Pulse deformations at guided-mode resonance filters.

A short pulse of light incident on a waveguiding region with a periodic mixture of dielectric materials is shown to experience dramatic changes in its spatial and temporal composition. The reflected and transmitted pulse components experience lateral spread and temporal decompression, which depend on the pulse width, pulse duration, and the structural parameters of the resonant grating.