Photon-phonon interaction on periodic structures

Features of the interaction between optical and acoustical waves with the same wavelength and multiple of the structural layer period, propagating simultaneously through periodic structures, are analyzed. The current state of and prospects for photon-phonon crystals being used for amplification, the generation of gigahertz acoustic waves, and other nonlinear optical and acoustic effects are examined.     

Rainbow trapping using chirped all-dielectric periodic structures

We report a numerical investigation of rainbow trapping (light of different wavelengths) at different spatial locations in a newly designed two-dimensional photonic structure that is formed using chirping parameters in two-dimensional photonic crystals. Chirped parameters ensure trapping of certain light wavelengths inside these structures. To achieve broadband electromagnetic wave trapping, we properly adjust and chirp the position and dielectric filling factor of each unit cell within a photonic crystal structure. The low group velocity regions of the dielectric continuum bands at the Brillouin zone edge enable different wavelengths to be slowed and stopped along the propagation direction. The all-dielectric transparent material nature of the proposed structure realizes light trapping in different electromagnetic regions by spatially varying the effective refractive index of the structure.     

色散周期结构的辅助场时域有限差分法分析

利用辅助场时域有限差分(FDTD)方法分析色散周期结构的斜入射问题. 主要基于Floquet定理, 在FDTD迭代式中引入辅助场量, 结合变换的周期及吸收边界条件,解决了周期结构斜入射问题的电磁特性仿真. 进一步将辅助场FDTD方法引入至色散周期结构的模拟, 结合Z变换方法,给出了基于Drude色散模型的具体迭代公式. 数值计算结果表明所构建方法的有效性及普适应.     

Modeling of a high frequency ultrasonic transducer using periodic structures

Solidly mounted integrated transducers with a Bragg cell inserted between the piezoelectric film and the substrate are investigated for high frequency ultrasonic applications. A numerically stable recursive one dimensional transmission/reflection model was used to analyze the behavior of the periodic structure. This theoretical analysis includes the study of the influence of the acoustic properties of the constitutive layer, the effect of the number of cells and their arrangement. A 35 MHz integrated transducer consisting in a PZT ceramic laid down on a Au/PZT Bragg cell deposited on a porous substrate was fabricated and characterized. Both theoretical and experimental results highlight the interest of using a periodic structure for high frequency ultrasonic applications.     

Femtosecond laser microfabrication of periodic structures using a microlens array

A microlens array (MLA) was applied for femtosecond laser microfabrication. Two-dimensional periodic patterns that corresponded to the arrangement of lenses in the MLA were recorded by ablation on a glass surface. The two-dimensional periodic patterns were recorded by a single exposure without scanning of the laser beam or the sample, and worked as a grating. Optical resizing of the patterns and high efficiency of fabrication are demonstrated and discussed. PACS 42.82.Cr; 07.60.Pb; 42.62.Cf     

Highly sensitive comparative analysis of periodic structures using lateral shear interferometry

The promising character of application of large lateral shear interferometry for the comparative analysis of a pair of transmission periodic structures, one of which is the structure under study and the other is the standard structure, is shown. For increasing the sensitivity of the representation of differences in the behavior of grooves of the structure under study and the standard structure, it is proposed to use higher order diffraction interference for both structures. Results of experimental testing of the method for monitoring the quality of production of two-dimensional amplitude masks are given.     

Detection of macrodefects in spatially periodic structures using a two-mirror interferometer

A scheme for a two-mirror interferometer intended to detect macrodefects in spatially periodic structures is proposed. This scheme is shown to provide both double-beam interferograms and shear interferograms that visualize the behaviors of the function reflecting a macrodefect in the structure and of the derivative of this function.     

Wave attenuation in nonlinear periodic structures using harmonic balance and multiple scales

We study the attenuation, caused by weak damping, of harmonic waves through a discrete, periodic structure with frequency nominally within the Propagation Zone (i.e., propagation occurs in the absence of the damping). The period of the structure consists of a linear stiffness and a weak linear/nonlinear damping. Adapting the transfer matrix method and using harmonic balance for the nonlinear terms, a four-dimensional linear/nonlinear map governing the dynamics is obtained. We analyze this map by applying the method of multiple scales upto first order. The resulting slow evolution equations give the amplitude decay rate in the structure. The approximations are validated by comparing with other analytical solutions for the linear case and full numerics for the nonlinear case. Good agreement is obtained. The method of analysis presented here can be extended to more complex structures.     

Flow acoustics in periodic structures

A recent paper [A.A. Krokhin, J. Arriaga, L.N. Gumen, Speed of sound in periodic elastic composites, Phys. Rev. Lett. 91 (2004) 264302-1-4] addresses the speed of sound in periodic elastic composites (phononic crystals) with particular emphasis to the case where air bubbles are present in water and arranged periodically. In such periodically arranged mixtures, the well-known phenomena of the drop of the speed of sound may occur and applications related to, e.g., sound-beam focusing and acoustic surgery are possible [F. Cervera, L. Sanchez, J.V. Sanchez-Perez, R. Martinez-Sala, C. Rubio, F. Meseguer, C. Lopez, D. Caballero, J. Sanchez-Dehesa, Phys. Rev. Lett. 88 (2002) 023902]. In this paper, the analysis is extended theoretically to include cases where a background flow in a periodic structure is maintained. Calculations of dispersion relations and group velocities are presented in cases with one- and two-dimensional material periodicity for background flow values in the range: 0-1m/s. Materials considered in the calculations are periodic water-air mixtures. It is shown that acoustic waves couple to the group velocities only if the (acoustic) wave vector has a component along the background flow velocity direction.     

Dynamic analysis of periodic structures

The natural vibration analysis of a periodic structure with repeated identical substructures may be simplified by using some symmetrical properties of the substructure dynamic matrices, resulting in a set of linear difference equations in the displacements. These equations are readily solved for cyclic symmetric systems, simply supported systems and infinite systems. The order of the overall frequency equations is at most equal to one half of the total number of degrees of freedom retained for a single substructure regardless of the number of substructures in the system. With these natural modes, the system with general boundary conditions at end stations is analyzed by a fast converging method.     

Wave attenuation in periodic structures

A periodic structure acts as a filter for traveling waves of any kind whether they be electromagnetic, acoustic, bending or anything else. This principle has been used to reduce the vibration level in one part of a structure when it is excited at another. For example, certain frequencies from engine excitation can be attenuated in the cabin of an aircraft. In this paper we report analytical methods for predicting the attenuation of bending waves in a dissipationless beam with flexible ribs attached to it. The insertion loss (attenuation) spectra for a wide frequency range are then compared with measured results for test structures with 1 to 15 ribs attached. Some preliminary work was also done on the effect of rib spacing. The width of the attenuation band increased with increasing number of ribs with seven ribs giving almost as wide a band as it had for the infinitely long periodic beam. Since the structure has no dissipation, the attenuation is due solely to interference effects of multiple reflections.     

Backward-wave medium infrared down-conversion in proustite

Phase-matched backward-wave down-conversion has been obtained in the medium infrared using proustite (Ag₃AsS₃) as the mixing crystal. A transmission window between the two-photon lattice band and the high-frequency wing of the reststrahlen permitted 18.6 μm infrared to be generated by mixing the output of a ruby laser with stimulated Raman emission from bromoform (CHBr₃).     

Photonic pseudogaps for periodic dielectric structures

We consider the problems of existence and structure of gaps (pseudogaps) in the spectra associated with Maxwell equations and equations that govern the propagation of acoustic waves in periodic two-component media. The dielectric constant is assumed to be real and positive, and the value of = _b on the background is supposed to be essentially larger than the value of = _a on the embedded component. We prove the existence of pseudogaps in the spectra of the relevant operators. In particular, we give an accurate treatment of the term pseudogap. We also show that if the contrast _b / _a approaches infinity, then the bands of the spectrum shrink to a discrete set which can be identified with the set of eigenvalues of a Neumann-type boundary value problem and thus can be effectively calculated.     

Wave propagation along transversely periodic structures

The dispersion curves for guided waves have been of constant interest in the last decades, because they constitute the starting point for NDE ultrasonic applications. This paper presents an evolution of the semianalytical finite element method, and gives examples that illustrate new improvements and their importance for studying the propagation of waves along periodic structures of infinite width. Periodic boundary conditions are in fact used to model the infinite periodicity of the geometry in the direction normal to the direction of propagation. This method allows a complete investigation of the dispersion curves and of displacement/stress fields for guided modes in anisotropic and absorbing periodic structures. Among other examples, that of a grooved aluminum plate is theoretically and experimentally investigated, indicating the presence of specific and original guided modes.     

Focusing-to-defocusing crossover in nonlinear periodic structures

We demonstrate experimentally a transition from nonlinear beam trapping to defocusing in a two-dimensional periodic photonic structures by varying the modulation depth of the lattice. The observed effect illustrates the fundamental crossover from discrete to cw transport mechanisms. At the threshold modulation, the output beam is highly sensitive to refractive index and power variations, which can be potentially applied for high-sensitivity refractive index or temperature sensing.     

Epitaxial periodic p-n structures in PbS

The electrical and photoelectrical properites of periodic p-n structures in an epitaxial film of PbS are studied. The plane of the junctions is perpendicular to the plane of the film. The possibility of using such structures in the photodiode mode as low inertia detectors of IR radiation is shown. Qualitative agreement of the dark characteristics (volt-ampere curve, temperature dependence of resistance) of the p-n structure and photosensitive polycrystalline films was noted. This confirms the hypothesis that in sensitization of polycrystalline films, barriers of the p-n junction type are formed.