Double-resonant extremely asymmetrical scattering of electromagnetic waves in periodic arrays separated by a gap

Two strong simultaneous resonances of scattering--double-resonant extremely asymmetrical scattering (DEAS)--are predicted in two parallel, oblique, periodic Bragg arrays separated by a gap, when the scattered wave propagates parallel to the arrays. One of these resonances is with respect to frequency (which is common to all types of Bragg scattering), and another is with respect to phase variation between the arrays. The diffractional divergence of the scattered wave is shown to be the main physical reason for DEAS in the considered structure. Although the arrays are separated, they are shown to interact by means of the diffractional divergence of the scattered wave across the gap from one array into the other. It is also shown that increasing separation between the two arrays results in a broader and weaker resonance with respect to phase shift. The analysis is based on a recently developed new approach allowing for the diffractional divergence of the scattered wave inside and outside the arrays. Physical interpretations of the predicted features of DEAS in separated arrays are also presented. Applicability conditions for the developed theory are derived.Comment: 8 pages, 5 figure

Extremely asymmetrical scattering of electromagnetic waves in gradually varying periodic arrays

This paper analyses theoretically and numerically the effect of varying grating amplitude on the extremely asymmetrical scattering (EAS) of bulk and guided optical modes in non-uniform strip-like periodic Bragg arrays with stepwise and gradual variations in the grating amplitude across the array. A recently developed new approach based on allowance for the diffractional divergence of the scattered wave is used for this analysis. It is demonstrated that gradual variations in magnitude of the grating amplitude may change the pattern of EAS noticeably but not radically. On the other hand, phase variations in the grating may result in a radically new type of Bragg scattering - double-resonant EAS (DEAS). In this case, a combination of two strong simultaneous resonances (one with respect to frequency, and another with respect to the phase variation) is predicted to take place in non-uniform arrays with a step-like phase and gradual magnitude variations of the grating amplitude. The tolerances of EAS and DEAS to small gradual variations in the grating amplitude are determined. The main features of these types of scattering in non-uniform arrays are explained by the diffractional divergence of the scattered wave inside and outside the array.Comment: 13 pages, 10 figure

Non-steady-state extremely asymmetrical scattering of waves in periodic gratings

Extremely asymmetrical scattering (EAS) is a highly resonant type of Bragg scattering with a strong resonant increase of the scattered wave amplitude inside and outside the grating. EAS is realized when the scattered wave propagates parallel to the grating boundaries. We present a rigorous algorithm for the analysis of non-steady-state EAS, and investigate the relaxation of the incident and scattered wave amplitudes to their steady-state values. Non-steady-state EAS of bulk TE electromagnetic waves is analyzed in narrow and wide, slanted, holographic gratings. Typical relaxation times are determined and compared with previous rough estimations. Physical explanation of the predicted effects is presented.Comment: 7 pages, 3 figures. This paper is freely available online at http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-6-268 which includes multimedia files not included in this preprint versio

Grazing-angle scattering of electromagnetic waves in gratings with varying mean parameters: grating eigenmodes

A highly unusual pattern of strong multiple resonances for bulk electromagnetic waves is predicted and analysed numerically in thick periodic holographic gratings in a slab with the mean permittivity that is larger than that of the surrounding media. This pattern is shown to exist in the geometry of grazing-angle scattering (GAS), that is when the scattered wave (+1 diffracted order) in the slab propagates almost parallel to the slab (grating) boundaries. The predicted resonances are demonstrated to be unrelated to resonant generation of the conventional guided modes of the slab. Their physical explanation is associated with resonant generation of a completely new type of eigenmodes in a thick slab with a periodic grating. These new slab eigenmodes are generically related to the grating; they do not exist if the grating amplitude is zero. The field structure of these eigenmodes and their dependence on structural and wave parameters is analysed. The results are extended to the case of GAS of guided modes in a slab with a periodic groove array of small corrugation amplitude and small variations in the mean thickness of the slab at the array boundaries.Comment: 16 pages, 6 figure

Extremely asymmetrical scattering in gratings with varying mean structural parameters

Extremely asymmetrical scattering (EAS) is an unusual type of Bragg scattering in slanted periodic gratings with the scattered wave (the +1 diffracted order) propagating parallel to the grating boundaries. Here, a unique and strong sensitivity of EAS to small stepwise variations of mean structural parameters at the grating boundaries is predicted theoretically (by means of approximate and rigorous analyses) for bulk TE electromagnetic waves and slab optical modes of arbitrary polarization in holographic (for bulk waves) and corrugation (for slab modes) gratings. The predicted effects are explained using one of the main physical reasons for EAS--the diffractional divergence of the scattered wave (similar to divergence of a laser beam). The approximate method of analysis is based on this understanding of the role of the divergence of the scattered wave, while the rigorous analysis uses the enhanced T-matrix algorithm. The effect of small and large stepwise variations of the mean permittivity at the grating boundaries is analysed. Two distinctly different and unusual patterns of EAS are predicted in the cases of wide and narrow (compared to a critical width) gratings. Comparison between the approximate and rigorous theories is carried out.Comment: 16 pages, 5 figure

Labile carbon addition affects soil organisms and N availability but not cellulose decomposition in clear-cut Norway spruce forests

We assessed the effects of sucrose addition on the biological and chemical properties of organic soil in clear-cut Norway spruce forests managed with or without wood-ash fertilization and mechanical site preparation. Sucrose addition increased the abundances of enchytraeids and tardigrades and soil moisture percentage in the clear-cut areas. Sucrose also increased nematode abundance in the non-fertilized plots. Sucrose reduced the pool of waterextractable NH4-N in the soil in the first year, but increased it in the second year. Sucrose addition did not affect the decomposition rate of cellulose strips. The biomass of ground vegetation was not affected by sucrose. Carbohydrate addition seems to enhance N immobilization in clear-cut areas in the short term, and it is suggested that aims at reducing N loss from disturbed forest soil do not necessarily accelerate carbon loss from the forest ecosystem.peerReviewe

Optical measurement of torque exerted on an elongated object by a non-circular laser beam

We have developed a scheme to measure the optical torque, exerted by a laser beam on a phase object, by measuring the orbital angular momentum of the transmitted beam. The experiment is a macroscopic simulation of a situation in optical tweezers, as orbital angular momentum has been widely used to apply torque to microscopic objects. A hologram designed to generate LG02 modes and a CCD camera are used to detect the orbital component of the beam. Experimental results agree with theoretical numerical calculations, and the strength of the orbital component suggest its usefulness in optical tweezers for micromanipulation.Comment: 6 pages, 7 figures, v2: minor typographical correction