Holography with guided optical waves

Waveguide holograms are recorded in a storage material layer covering a planar dielectric waveguide with a guided mode as reference wave. They are also read-out with a guided mode. In the storage material, assumed to have a lower refractive index than the waveguiding film, the fields of the reference and of the read-out wave are evanescent waves. We describe the preparation of the epoxy resin (Araldit) waveguides on glass substrates, their coating with a dichromated gelatin (DCG) layer as storage material, the processing of the DCG, and the recording and reconstruction techniques. Waveguide holograms of good quality with a size of several square centimeters have been realized for the first time. The holograms were recorded and read-out with wavelength λ=488 nm. They have a length of about 25 mm and a width of 17 mm. To obtain holograms that wide a special prism coupler had to be used which allowed to couple in a laser beam of elliptical cross section, and thus to excite a guided reference or read-out mode with a width of 17 mm.     

Subwavelength lithography using metallic grating waveguide heterostructure

We present subwavelength periodic gratings achieved by employing a metallic grating waveguide heterostructure (MGWHS). The mask can be designed to make one of its diffraction order (±mth) waves resonate with the surface plasmon wave supported by the MGWHS. With a finite-difference time-domain method, we numerically demonstrate that one-dimensional periodic structure of about 60 nm feature, which is far beyond the diffraction limit, can be patterned with the interference of the 3rd diffraction order waves of the mask at a wavelength of 546 nm. The technique can also be extended to two-dimensional patterns using circularly polarized incidence and for the incidence with an angle θ.     

Holographic phase conjugation through a sub-wavelength hole

Holographic phase conjugation is analyzed as a method to create a photo-refractive lens with high numerical aperture. For this purpose a sub-wavelength hole is drilled into a metal surface directly on top of an iron-doped lithium niobate crystal. An interference pattern generated by the light coming from this point source and a plane reference wave is recorded. By using the phase-conjugated reference wave for read-out, a light wave being focused onto the former point source is reconstructed. In principle, a focusing system close to the theoretical diffraction limit could be implemented by this method. The performance of this arrangement is mainly determined by properties of the lithium niobate crystal, especially the crystal symmetry. Experimentally, the tight holographic focusing is demonstrated. The focus width of the reconstructed wave is shown to be below 1.2 μm, which is our spatial resolution. The diffraction efficiency obtained, however, is just 3×10⁻⁵ compared to 3×10⁻² in the plane-wave case. This can be explained by experimental reasons, the inhomogeneous light intensity and limitations originating from the crystal symmetry. We estimate that the diffraction efficiency for phase conjugation through a sub-wavelength hole can be improved by three to four orders of magnitude by addressing the above-mentioned issues.     

Novel spatial solitons in light-induced photonic bandgap structures

The study of wave propagation in periodic systems is at the frontiers of physics, from fluids to condensed matter physics, and from photonic crystals to Bose-Einstein condensates. In optics, a typical example of periodic system is a closely-spaced waveguide array, in which collective behavior of wave propagation exhibits many intriguing phenomena that have no counterpart in homogeneous media. Even in a linear waveguide array, the diffraction property of a light beam changes due to evanescent coupling between nearby waveguide sites, leading to normal and anomalous discrete diffraction. In a nonlinear waveguide array, a balance between diffraction and self-action gives rise to novel localized states such as spatial “discrete solitons” in the semi-infinite (or total-internal-reflection) gap or spatial “gap solitons” in the Bragg reflection gaps. Recently, in a series of experiments, we have “fabricated” closely-spaced waveguide arrays (photonic lattices) by optical induction. Such photonic structures have attracted great interest due to their novel physics, link to photonic crystals, as well as potential applications in optical switching and navigation. In this review article, we present a brief overview on our experimental demonstrations of a number of novel spatial soliton phenomena in light-induced photonic bandgap structures, including self-trapping of fundamental discrete solitons and more sophisticated lattice gap solitons. Much of our work has direct impact on the study of similar discrete phenomena in systems beyond optics, including sound waves, water waves, and matter waves (Bose-Einstein condensates) propagating in periodic potentials.      

Diffraction gratings of isotropic negative-phase velocity materials

Diffraction of electromagnetic plane waves by the gratings made by periodically corrugating the exposed planar boundaries of homogeneous, isotropic, linear dielectric-magnetic half-spaces is examined. The phase velocity vector in the diffracting material can be either co-parallel or anti-parallel to the time-averaged Poynting vector, thereby allowing for the material to be classified as of either the positive- or the negative-phase velocity (PPV or NPV) type. Three methods used for analyzing dielectric gratings — the Rayleigh-hypothesis method, a perturbative approach, and the C formalism — are extended here to encompass NPV gratings by a careful consideration of field representation inside the refracting half-space. Corrugations of both symmetric as well as asymmetric shapes are studied, as also the diversity of grating response to the linear polarization states of the incident plane wave. The replacement of PPV grating by its NPV analog affects only nonspecular diffraction efficiencies when the corrugations are shallow, and the effect on specular diffraction efficiencies intensifies as the corrugations deepen. Whether the type of the refracting material is NPV or PPV is shown to affect surface wave propagation as well as resonant excitation of surface waves.     

Atom-optics holograms

The temporal evolution of atomic wave packets interacting with object and reference electromagnetic waves is investigated, and an analytical solution for the off-resonant density matrix is presented. It is shown that, under certain physical conditions, the diffraction of an ultracold atomic beam by an inhomogeneous laser field can be interpreted as if the beam passes through a three-dimensional hologram. We show that high diffraction efficiencies can be realized if one restricts the extent of the atomic hologram in the time domain rather than in space. The hologram, thus, can work in a pulsed regime pumping atoms from the beam or from the initial wave packet into the reconstructed matter wave. The suggested regime is well compatible with the Raman cooling methods and the recent realization of an atom laser, which are capable of repeatedly reproducing coherent, or almost coherent, atomic wave packets necessary for the actual implementation of the reading beam. It is found that the diffraction efficiency of such a hologram may reach 100% and is determined by the duration of laser pulses. On this basis, a new method for the reconstruction of the object image with matter waves is offered. The latter may have useful practical applications, ranging from atom lithography, to the manufacturing of microstructures, and quantum microfabrication.     

Directional properties of an array of sound receivers positioned in an impedance screen recess

Expressions for calculating the directional characteristics of an array of sound receivers positioned in a waveguide with impedance walls are obtained from the solution to the problem on the diffraction of a plane sound wave by the waveguide open end with impedance flanges. The waveguide can be of a finite length, and, in this case, it can be considered as an open cavity in an impedance screen. The solution of the integral equation for the sound pressure distribution over the opening area is reduced to the solution of an infinite system of algebraic equations for the coefficients of the field expansion in normal waveguide waves. Examples of calculated directional characteristics are presented for arrays with receivers positioned at different distances from the opening and for different values of the impedances of the waveguide walls and flanges.     

Operator method in electromagnetic waves diffraction by semi-infinite diaphragm system in rectangular waveguide problem

Conclusions The investigations carried out show that the electromagnetic waves diffraction problem by semi-infinite diaphragm system in rectangular waveguide may be solved with the help of the operator method. This approach allows to investigate the main characteristics of these electrodynamic systems and to establish the fact of total reflection regions in semi-infinite diaphragm system and its connection with the “fading” regions in corresponding infinite structures. This physical fact is common for these objects. It is obviously that any rejection filters may be designed with the help of semi-infinite diaphragm system. However, it is necessary to have accuracy and sufficiently simple method for designing these devices, especially in millimetre wave region. The characteristics analysis method for semi-infinite and finitely element systems may be a subject of further investigations.     

Guiding properties of a smoothly inhomogeneous layer and diffraction excitation of ionospheric waveguides

The author investigates the diffraction of a spherical wave by a smoothly inhomogeneous layer in which a waveguide zone is situated behind a barrier of finite weight. An expression is derived for layer-guided waves generated outside the layer by energy transfer between the waveguide and the external homogeneous half space. Certain properties of these waves are investigated. It is shown that, since the losses associated with energy leakage from the waveguide across the barrier are more significant than the initial wave amplitude at large distances, the role of principal (optimal) mode passes from one mode to another at a lower level in the potential well (waveguide) with increasing distance. The problem of diffraction excitation of ionospheric waveguides in ultralong-range propagation of short radio waves is considered as an example. The results are discussed and compared with published experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 12, pp. 1728–1737, December, 1978.The author is grateful to V. S. Buldyrev for valuable comments, as well as to V. N. Mirolyubov for several useful consultations.     

Simultaneous diffraction of two waves at a reflection volume hologram

Based on the theory of coupled waves the simultaneous diffraction of two waves at a plane dielectric reflection volume hologram is described. By analogy to the transmission volume grating the intensities of the output beams strongly depend on the phase shift between the incident waves and the phase shift of the grating with respect to the interference structure of the read-out waves. This gives the possibility of transferring the incident energy from one wave into the other and can be used for the modulation or deflection of the reconstructed beams. The separate determination of the different kinds of phase shifts by intensity or phase measurements is discussed.     

Dispersion properties and field structures of eigenmodes of nonuniform plasma waveguides in a longitudinal magnetic field

We study the field structure and dispersion properties of a hybrid eigenmode guided by a nonuniform magnetized plasma waveguide. It is shown that the rotational and quasi-potential waves contribute to the formation of such a mode in the whistler frequency range. Depending on the plasma density, the rotational component of the hybrid mode is determined by either waves with complex transverse wave numbers or whistler waves, or by true surface waves. In the presence of an axial nonuniformity of the plasma in a channel, the transverse field structure of the propagating mode changes, which is stipulated by changes in both the values of transverse wave numbers and their dependence on the radial coordinate. It is found that the spectrum of axial wave numbers of eigenmodes of a plasma waveguide undergoes a pronounced condensation when smoothing the waveguide walls. The damping of the hybrid mode of a nonuniform waveguide due to electron collisions is found and it is shown that collisional losses determine the damping of waves trapped in the waveguide in the experiments on ionization self-channeling of whistler waves. We have found the effect of “displacing” the strong field from the inner core to the background outer region of the waveguide with increasing plasma density on its axis and broadening background region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 607–617, July 2006.     

Tuning fork enhanced interferometric photoacoustic spectroscopy: a new method for trace gas analysis

A photoacoustic trace gas sensor based on an optical read-out method of a quartz tuning fork is shown. Instead of conventional piezoelectric signal read-out, as applied in well-known quartz-enhanced photoacoustic spectroscopy (QEPAS), an interferometric read-out method for measurement of the tuning fork’s oscillation is presented. To demonstrate the potential of the optical read-out of tuning forks in photoacoustics, a comparison between the performances of a sensor with interferometric read-out and conventional QEPAS with piezoelectric read-out is reported. The two sensors show similar characteristics. The detection limit (L) for the optical read-out is determined to be L _opt=(2598±84) ppm (1σ) compared to L _elec=(2579±78) ppm (1σ) for piezoelectric read-out. In both cases the detection limit is defined by the thermal noise of the tuning fork.     

Cure monitoring using ultrasonic guided waves in wires

The possibility of using ultrasonic guided waves for monitoring the cure process of epoxy resins is investigated. The two techniques presented use a wire waveguide which is partly embedded in the resin. The first technique is based on the measurement of attenuation due to leakage of bulk waves into the resin surrounding the waveguide. The second technique measures the reflection of the guided wave that occurs at the point where the waveguide enters the resin. Both the attenuation and the reflection coefficient change significantly during cure, and the numerical methods to relate these to the material properties of the curing resin are presented in this paper. The results from the modeling are experimentally verified and show good agreement. The applicability of each testing method is discussed, and typical cure-monitoring curves are presented.     

基于Rayleigh-Bloch模式的单层结构弯曲声波导

基于一维声栅中的Rayleigh-Bloch(RB)模式基本特点,设计了一种单层结构弯曲声波导.利用有限元方法从时域和频域两方面验证了弯曲声波导的有效性,RB模式波可以沿着波导的弯曲界面传播.研究发现,由于采用了环形结构基本单元,在该波导中存在两种传播模式,分别对应能量局域在单元间(模式-1)和单元内部(模式-2)两种情况.其中,模式-2声传输效果更佳,几乎可实现无损传输.时域研究中分别采用了调制脉冲和高斯脉冲两种信号形式,分析了它们在弯曲声波导中的传输过程.由于波导中只允许RB模式波传播,因此对于宽频信号来说,可起到滤波的效果.并且,不同模式(频率)的信号会出现在声波导的不同位置,所得结果对于声波定向传输、声探测与识别等研究具有理论与应用价值.     

Evanescent Plane Waves in Multilayer Structures

We have considered evanescent plane waves in structures with a layer of a substance with ε, μ < 0 and with a layer of a well-reflecting metal, ε < 0, μ ≥ 1. Waves with increased amplitude as compared with the initial wave have been found to occur, due to which evanescent waves with wave number as in the initial wave but with increased amplitude arise behind these layers. A composite material with ε, μ < 0 at optical frequencies are proposed. Surface waves on a metal layer are considered in detail. It is shown that surface waves with a sufficiently arbitrary wave number can be excited. It is also shown that, on very thin layers, surface waves with wave number exceeding ten times that of a homogeneous plane wave in vacuum can be excited. Propagation losses are calculated. For a silver layer, the wave path can be from 30 up to 100 wavelengths. Practical use in developing techniques for optical transformations of short-wave surface waves in 2D space, similar to those in 3D space, are pointed out.     

On switching behavior of ferrite substrate with magnetostatic and spin wave exchange term

Due to the generation of magnetostatic and spin waves, switching phenomena of ferrite substrate with a normal magnetic biasing field is presented. Generally below the X-band of microwave range, the Pozar’s quasi TEM waves (extraordinary waves) were studied, but for the study of X-band there should be an inclusion of spin wave exchange term (ωr) in the magnetostatic wave analysis which depends upon the static internal field (Hex). This term is included in analysis because the wavelength of microwave approach is the inter-atomic distance of ferrite material. In this work we synthesize LiTi ferrite through Solid State Reaction Technique (SSRT) and obtained electric and magnetic properties for the analysis. Absorbing and transmission power coefficients have been calculated to obtain the power loss and transmitted power through the substrate, respectively. The absorbing power coefficient verifies the switching behavior of substrate for certain range of applied external magnetic field (H _o) which depends on the resonance line width parameter (ΔH) of ferrite material.     

Surface waves guided by magnetic metamaterials

We consider electromagnetic waves guided by slabs of anisotropic resonant magnetic metamaterials in free space and in metal waveguides. It is shown that wave propagation is possible in a waveguide below cutoff with an insert of such a metamaterial. The dependence of the dispersion properties of waves on the geometric sizes of the insert and the waveguide are studied. The obtained results can be used for miniaturization of electrodynamic systems. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 618–625, July 2006.     

Cascade processes in a plasma oscillator

A theoretical and numerical analysis is made of the dynamics of nonlinear electron-beam scattering of a wave reflected by the emitting device of a plasma oscillator. It is shown that a counterpropagating plasma wave can interact nonlinearly with other waveguide modes of the system and with charge-density beam waves, leading to changes in the operation of the oscillator. It is established by means of a numerical simulation that the generation efficiency is reduced as a result of scattering of the counterpropagating wave and stimulated emission of a strong-potential plasma wave with phase velocity v _ph=ω/k _z≪c. Zh. éksp. Teor. Fiz. 112, 1299–1311 (October 1997)     

Waveguide Filter on Base of Dielectric the Bragg Structure for Millimeter Waves Applications

It is considered and offered the waveguide band elimination filter with the working mode H₁₀ that can be used in millimeter waves devices such as up, down- converters, mixer, etc. The characteristics of the filter was calculated by using analogy in electromagnetic the waveguide propagation and optical the wave propagation in the medium with the specific refractive index N. This conception can be applied for party-filled waveguide in the case when the Brillouin decomposition of the waveguide waves into plane waves is correct and the transformation of the incident (propagating) mode into other the mode types is absent.In this work were calculated the spectrum energy transmittance T(f) in the filter stop band and the wavelength ₀ in the center of this band using the characteristic equation obtained in the result of theoretical consideration. Theoretical results are compared with experimental measurements.     

Exact solution of stationary holographic four-wave mixing in photorefractive crystals: two-level model

Based on the two-level model of recording and read-out of dynamic holographic gratings, precise expressions for wave amplitudes, diffraction efficiency and the gain coefficient representing the generalization of the corresponding formula by Kukhtarevet al. have been obtained. This model allows one to take into account the effect of the read-out beam on the holographic grating and to solve a problem of four-wave mixing in transmission geometry with wave depletion and arbitrary value of the spatial phase shift between the holographic grating and the interference pattern.