Whispering gallery modes and other cavity modes for perfect backscattering and blazing

We demonstrate the possibility to obtain perfect blazing both in Littrow and off-Littrow mountings using diffractive systems consisting of a plane metallic substrate and dielectric structures that can support cavity modes. The resonances are located at a relatively large distance between the metal and the dielectric structure, a condition that prevents the resonance increase of absorption. The high efficiency can be obtained in transverse electric or transverse magnetic polarization and at high incident angles. When cylindrical rods with circular cross-sections are used, the so-called whispering gallery modes can be used to provide the resonances, necessary for the blazing.

Total absorption of unpolarized light by crossed gratings

We present both experimental and numerical data showing the absorption of unpolarized, normally incident light by a gold crossed grating having a shallow sinusoidal profile. We show furthermore that the total absorption of unpolarized light can be achieved for an angle of incidence of 30 degrees with a crossed grating having its period adjusted appropriately from the normal incidence case to preserve the plasmonic resonance responsible for the enhanced absorptance. We contrast the process for achieving high absorptance in the principal plane of incidence aligned with the grooves of one of the gratings, with that for the principal plane at 45 degrees to each grating.

Phenomenological study of binding in optically trapped photonic crystals

We describe a phenomenological theory of the phenomenon of binding observed both experimentally and numerically when particles are trapped by an interference system in order to make a structure close to a photonic crystal. This theory leads to a very simple conclusion, which links the binding phenomenon to the bottom of the lowest bandgap of the trapped crystal in a given direction. The phenomenological theory allows one to calculate the period of the trapped crystal by using numerical tools on dispersion diagrams of photonic crystals. It emerges that the agreement of our theory with our rigorous numerical results given in a previous paper [J. Opt A8, 1059 (2006)] is better than 2% on the crystal period. Furthermore, it is shown that in two-dimensional problems and s polarization, all the optical forces derive from a scalar potential.

Photonic crystal lens: from negative refraction and negative index to negative permittivity and permeability

We consider a dielectric photonic crystal made of cylindrical holes in a high index matrix. We show that a given finite size photonic crystal can mimic a homogeneous material whose permittivity and permeability are negative. We pay attention to the limitation of the homogeneous medium model and the vital role of the truncation of the crystal.

Photonic crystal diffraction gratings

It is shown from numerical results deduced from a rigorous theory of diffraction that diffraction gratings made with two-dimensional dielectric photonic crystals may present blazing effects. Since these structures are lossless, efficiencies of 100% in the -1st order can be obtained in polarized light. Efficiency curves in Littrow mount are shown.

Integral method for echelles covered with lossless or absorbing thin dielectric layers

We make a generalization of the integral method in the electromagnetic theory of gratings to study diffraction by echelles covered with dielectric lossless or absorbing layers. Numerical examples are given that show that, as in the resonance domain, the diffraction efficiency is more complicated than being a simple product of lossless diffraction efficiency curves and plane surface reflectivity.

Diffraction efficiency of echelles working in extremely high orders

Aproposal to use a high-angle echelle in the vacuum UV in the 350th order triggered a theoretical study to determine if there were unusual obstacles to success. No serious obstacles were found except for efficiency limitations.

Echelles: scalar, electromagnetic, and real-groove properties

For lack of alternatives, echelle-grating diffraction behavior has in the past been modeled on scalar theory, despite observations that indicate significant deviations. To resolve this difficulty a detailed experimental, theoretical, and numerical study is performed for several echelles that work at low (18-13), medium (35-55), high (84-140), and very-high (to 660) diffraction orders. Noticeable deviations from the scalar model were detected both experimentally and numerically, on the basis of electromagnetic theory: (1) the shift of the observed blaze position was shown to decrease with the wavelength-to-period ratio, and it tends to zero more rapidly than the decrease of the maximum width, so that the TE- and TM-plane responses tend to merge into each other; (2) cut-off effects (Rayleigh anomalies) were found to play a significant role for high groove angles, where passing-off orders are close to the blaze order. A possibility for evaluation of the blaze angle from angular, rather than from spectral, measure nts is discussed.Several reasons for the differences between real and ideal echelles (material-index deviations, profile deformations, and groove-angle errors) are analyzed, and their effects on the performance of echelles is studied.

Lamellar metallic grating anomalies

A detailed numerical investigation of anomalies in lossy metallic lamellar gratings is presented in a large interval of a wavelength-to-period λ/d ratio. A substantial increase in absorption (a decrease in the total diffracted energy) is observed. If λ/d is small enough (within the homogenized limit), the absorption can reach almost 100%. When the groove width is large enough, the anomalies are connected with mode resonances inside the grooves.

Gratings for tunable lasers: using multidielectric coatings to improve their efficiency

The first results of a theoretical and experimental study are presented showing that a significant enhancement of the efficiency of gratings used as wavelength selectors for cw tunable lasers may be achieved with adequate dielectric coatings. It is found that, in some cases, the energy absorbed by the grating surface can be reduced by a factor >2. The output power of a cw Rh640 dye laser was practically multiplied by the same factor. It seems that holographic gratings are more adaptable than are ruled gratings to this utilization.

Equivalence of ruled, holographic, and lamellar gratings in constant deviation mountings

By use of simple theoretical considerations on the efficiency dependence of a wide class of commercially available gratings, we obtained an empiric equivalence rule. This rule makes it possible to create equivalence classes of gratings that among themselves show a variety of profiles but have, as long as only two orders are propagating, efficiency curves lying very close to each other. The equivalence rule is visualized by drawing a nomogram. The rule and the grating nomogram serve as a basis for finding new and explaining old properties of ruled, holographic, and lamellar gratings.

Efficiency optimization of rectangular groove gratings for use in the visible and IR regions

Rigorous electromagnetic theory is used to investigate the low order diffraction efficiency behavior of perfectly conducting rectangular groove plane diffraction gratings in the lambda/d domain of 0.25-1.8 of interest to spectrometry. We show how to optimize performance and make comparisons with blazed and holographic gratings. The role of finite conductivity and departure from Littrow conditions is also investigated. Comparison with experiment confirms the accuracy of the theory.

X-ray gratings: the GMS mount

Using rigorous electromagnetic theory, the interest of the specific off-plane mounting for x-ray gratings is discussed in detail. The optimization of the blaze angle for ruled gratings is considered, as well as the extension of the mounting to sinusoidal and lamellar profiles.

Grating efficiency theory as it applies to blazed and holographic gratings

Recently developed rigorous theories have been used to investigate the diffraction efficiency behavior of both blazed and holographic gratings. In order to assist designers of spectrometric systems we have covered a complete range of blaze angles for triangular grooves and modulations for sinusoidal groove shape in first and second orders. Several types of mountings are included together with the role played by finite conductivity of aluminum. Useful classifications of both types of gratings are given, as they apply from the near uv to ir regions. Comparisons showing the close agreement between theory and experiment are presented.