Dielectric photonic crystal with a near-zero effective refractive index in the microwave range

The microwave propagation in a 3D dielectric photonic crystal at the frequency range from 8.5 to 12GHz has been investigated. A frequency range where effective refractive index n _ef of the crystal approaches zero is found. In the frequency range where n _ef <0.2, themicrowave beam becomes collimated, sharpened, and free of diffraction noise. An excitation of a surface wave, propagating along the external surface of the structure, is revealed in same frequency range. This property of the photonic crystal structure can be used to screen and mask objects scanned by external radiation.     

Microwave Photonic Crystal from Spirals

Features of microwave propagation through a two-dimensional periodic structure of cylindrical spiral coils were studied in the frequency range from 8.5 to 12 GHz. The frequency dependence of the effective refractive index n_ef of the structure is measured. The features of measurements are noted and the structure reflectance is estimated in various regions of the frequency range under study. In the frequency range where n_ef ~ 0, a surface wave propagating around the structure is detected.     

Effective dielectric constant of two-dimensional photonic crystals in optical bands

By using the Fourier expansion method, we have developed an approach to calculate the effective dielectric index of a two-dimensional photonic crystal. The approach is very general: it can take into account various Bravais lattice structure as well as arbitrary spatial variation of the dielectric index. It has been found that near a nondegenerate frequency ω_n(Γ) at Γ point, the transverse magnetic (TM) mode is ordinary, as it is independent of the propagation direction, whereas in general the transverse electric (TE) mode depends on the propagation direction, it is extraordinary. Therefore, a two-dimensional photonic crystal can always be described by an effective dielectric index for TM mode near the nondegenerate frequency ω_n(Γ). However, the TE mode is much more complicated unless the lattice structure is highly symmetric. Moreover, a two-dimensional square photonic crystal has been identified as an effective birefringent crystal having two negative refractive indexes from the perspective of Snell's law.     

Light-induced acoustic effect in LiNbO3：Fe：Ce crystals

The phenomena of acoustic emission in LiNbO3：Fe：Ce crystals have been observed in the process of light-induced quasi-breakdown. It is found that the ultra-high frequency acoustic signal introduced into the crystal is modulated by the low frequency acoustic waves. Its frequency increases with the increase of the intensity of incident light and its jump period of breakdown is the same as that of the photovoltaic current Ic, the change of light-induced refractive index △n and the diffracted light intensity L. This phenomenon has been analysed in this paper, which is caused by the inverse piezoelectric strain effect of the jump of space charge field during the quasi-breakdown.     

Nonlinear absorption and refractive index of Brillouin scattered mode in piezoelectric semiconductor plasmas by an applied magnetic field

With the aid of hydrodynamic model, a detailed analytical investigation is made of the stimulated Brillouin scattering (SBS) of the Stokes component of the scattered wave in piezoelectric-doped semiconductor plasma subjected to a magnetostatic field. The origin of the SBS process lies in the third-order nonlinear optical susceptibility arising due to the induced nonlinear current density and acoustic perturbations internally generated due to crystal properties such as piezoelectricity and electrostriction. Using the coupled mode theory of plasmas the effective refractive index and absorption coefficient are determined via the effective susceptibility. The influence of piezoelectricity, magnetostatic field and doping concentration has been explored. The analysis has been applied to both noncentrosymmetric and centrosymmetric crystals. Numerical estimates are made for n-type InSb crystal duly irradiated by a frequency doubled 10.6 μm CO₂ lasers. Results are found to be well in agreement with available literature. The analysis establishes that a large nonlinear refractive index and small absorption coefficient can easily be obtained under moderate excitation intensity in piezoelectric doped magnetized semiconducting crystal, which proves its potential as candidate material for the fabrication of cubic nonlinear devices.     

Assessment of damage ranges by measurements of the refractive index in ion implanted LiNbO3 and LiTaO3

Ion implantation in LiNbO₃ and LiTaO₃ produces radiation damage by nuclear collisions. The amorphisation of the lattice reduces the refractive index of the material. In the case of fast ion bombardment with helium this damage layer is buried below the surface. The refractive index profile which then exists is suitable for an optical waveguide on the surface which supports several modes. Analysis of the refractive index profile yields the damage distribution in the crystal and this in turn can be compared with theoretical estimates of the damage production and ion ranges.

Our analyses show that in the high energy range from 0.5 to 2.0 MeV the depth of the damage is predominantly controlled by the electronic stopping.     

Near-infrared sensitive organic–inorganic photorefractive device

Organic–inorganic hybrid structure, assembled by Rh-doped Bi₁₂TiO₂₀ crystal and liquid crystal (LC) layer, operating at near-infrared range is proposed and demonstrated. Due to the photorefractive properties of inorganic substrate, light illumination caused a space charge field which acts as a driving force for LC molecules re-alignment and subsequent refractive index modulation. All optically controlled phase retardation ability has been demonstrated supporting possibilities for further infrared applications.     

Dispersion of light in opal photonic crystal

The selective reflection spectra of films prepared from the photonic crystal, i.e., synthetic opal, are investigated. It is revealed that the refractive index changes at the boundaries of the photonic band gap. The wave vector is renormalized as a result of the interaction of the light wave with the periodic structure of the crystal, and the dependence of the frequency on the photon wave vector deviates from a linear behavior. The band gap determined from these data is approximately equal to 1.7 × 10¹⁴ Hz.     

Dispersion characteristic of the phononic crystal and a superlens based on it

A phononic crystal in the form of a periodic rhombic structure with a large filling factor s=0.86, made up of steel cylinders in the air, is investigated. Under excitation of the crystal from the depth of the structure by a point acoustic radiator, the dispersion characteristic of the crystal is obtained and the band structure is constructed in the upper (reduced) Brillouin zone which has negative dispersion while the crystal has negative refraction. The second derivative of the dispersion dependence differs from zero. The resolution of the flat lens was as high as λ/3 near the edges of the band. Structures like this may be effective nonlinear converters of acoustic waves.     

An electromagnetic parameters extraction method for metamaterials based on phase unwrapping technique

Abstract

A new method of extracting effective permittivity and permeability of metamaterials has been developed in this paper. Scattering parameters are used to represent effective wave impedance and refractive index of the metamaterials. Considering analytical continuation of effective refractive index of the metamaterials in the upper half of a complex angular frequency plane, a phase unwrapping technique has been implemented to solve branch ambiguity of the refractive index caused by complex inverse cosine function. An empirical formula to estimate the starting frequency in the phase unwrapping technique has been developed to improve the efficiency of the parameter retrieval procedure. Numerical results including five slabs of rods and split-ring resonators (SRRs) and a structure composed of two SRRs and four capacitively loaded strips are given to demonstrate good accuracy, high efficiency, and noise insensitivity of the proposed retrieval method.     

高掺镁铌酸锂晶体的生长和倍频性能

我们通过测定MgO在同成分LiNbO₃中的有效分凝系数、相位匹配温度与MgO浓度之间的关系,找到了使Mg:LiNbO₃晶体的相位匹配温度达到最高的掺MgO配方,并克服了Mg:LiNbO₃晶体在高掺杂生长时易出现生长条纹和脱溶等问题,从而生长出了抗光折变能力强,光学均匀性良好的Mg:LiNbO₃晶体。用于连续泵浦Nd:YAG声-光调Q腔内倍频时,获得了平均功率最高达2瓦的二次谐波输出。 关键词：     

A maze structure for sound attenuation

Sonic crystals are periodic arrangement of scatterers made of material with low acoustic impedance or sound hard materials [1]. Sonic crystals have numerous applications such as green belts and sound barriers. Here we showed that a typical maze structure at children playground can attenuate noise effectively for frequencies ranging from 12.5 Hz to 20,000 Hz. The original designer for the maze structure probably does not have that in mind. The maze structure can be viewed as a sonic crystal structure with sound attenuation characteristics. We found that the maze was able to attenuate noise up to 17.9 dBA for frequency range below 1000 Hz and 23 dBA for higher frequency range up to 20,000 Hz. The maze structure was able to mitigate noise at a wide range of frequencies in addition to the center frequency (f_c$f_c$) of 478 Hz which was estimated based on the Bragg’s Law. The periodic effects of the maze was also proven by numerical studies. Our results demonstrated that the maze structure commonly found in children playgrounds was able to attenuate noise covering the whole human hearing range.     

Studies on the optical and dielectric properties of a zinc thiourea chloride NLO single crystal

Single crystals of a nonlinear optical material, zinc thiourea chloride were grown by the slow evaporation technique. The crystal structure and lattice parameters of the grown crystal were determined by the single crystal X-ray diffraction studies. The single crystal XRD revealed that the material crystallized in a orthorhombic crystal system. Optical studies were carried out and it was found that the tendency of transmission observed from the specimen, with respect to the wavelength of light, is practically more suitable for opto-electronic applications. The optical band gap is found to be 4.30 eV. Optical constants such as the band gap, refractive index, reflectance, extinction coefficient and real (?_r) and imaginary (?_i) components of the dielectric constant and electric susceptibility were determined from the UV–vis–NIR spectrum. The dielectric constant and dielectric loss of zinc thiourea chloride were measured in the different frequency range from 50 Hz to 5 MHz at different temperatures. Further, electronic properties, such as valence electron plasma energy, Penn gap, Fermi energy and electronic polarizability of the grown crystal have been estimated.     

Large range of omni-directional reflection in 1D photonic crystal heterostructures

A simple design of one-dimensional omni-directional reflector based on photonic crystal heterostructures structure has been proposed. The proposed structure consists of a periodic array of alternate layers of SiO₂ and Te as the materials of low and high refractive indices, respectively. The structure considered here has three stacks of periodic structures having five layers each. The lattice period of successive stack is increased by a certain multiple (say gradual constant, δ) of the lattice period of the just preceding stack. For numerical computation, the method of transfer matrix method (TMM) has been employed. It is found that such a structure has wider reflection bands in comparison to a conventional dielectric PC structure and the width of the omni-directional reflection (ODR) bands can be enlarged by increasing the value of the gradual constant δ.     

Studies on second harmonic generation efficiency of organic material l-arginine maleate dihydrate

A crystalline organic nonlinear optical l-arginine maleate dihydrate has been grown by solution growth technique. Single crystal X-ray diffraction study confirms that the grown crystal belongs to triclinic system having non-centrosymmetric space group P₁. The optical studies were carried out to estimate the transmission range, band gap, extinction coefficient, refractive index and reflectance of the grown crystal. The fundamental solid state parameters were determined from dielectric studies to analyze the polarizability and second harmonic generation efficiency of the grown crystal. Activation energy of the grown crystal was found to be 0.218 eV from ac conductivity measurements.     

Acoustic wave focusing by two-dimensional lattice of cylinders in air

A sound source (3 cm in diameter) image in air, formed in the far wave zone behind a two-dimensional periodic lattice, was experimentally obtained. The lattice consisted of plane parallel rows of steel cylinders with a diameter of 1.58 cm, forming an acoustic crystal with hexagonal structure, The crystal lattice constant (a = 2.14 cm) is smaller than the emission wavelength in air (∼ 3.4 cm). The relations between the emission wavelength and lattice parameters were selected according to model calculation in the second transmission band of the crystal at its negative refractive index n = −0.7. The lateral size of the focused (over the sound pressure distribution) image of the sound source is close to the emission wavelength. A distinctive feature of the experiment is the formation of such a sharply focused image under conditions of an extremely small output aperture (the beam diameter at the acoustic crystal output did not exceed 2.5 acoustic wavelengths in air). It can be considered that the flat acoustic lens is realized. Possible explanations of the observed effect are discussed.     

Negative refraction and left-handed electromagnetism in microwave photonic crystals

We demonstrate the negative refraction of microwaves in a metallic photonic crystal prism. The spectral response of the photonic crystal prism, which manifests both positive and negative refraction, is in complete agreement with band-structure calculations and numerical simulations. The validity of Snell's law with a negative refractive index is confirmed experimentally and theoretically. The negative refraction observed corresponds to left-handed electromagnetism that arises due to the dispersion characteristics of waves in a periodic medium. This mechanism for negative refraction is different from that in metamaterials.     

Characteristics of paraxial propagation of a super Lorentz—Gauss SLG01 mode in uniaxial crystal orthogonal to the optical axis

Analytical propagation expression of a super Lorentz-Gauss（SLG） 01 mode in uniaxial crystal orthogonal to the optical axis is derived.The SLG 01 mode propagating in uniaxial crystal orthogonal to the optical axis mainly depends on the ratio of the extraordinary refractive index to the ordinary refractive index.The SLG 01 mode propagating in uniaxial crystals becomes an astigmatic beam.The beam spot of the SLG 01 mode in the uniaxial crystal is elongated in the x-or y-direction,which is determined by the ratio of the extraordinary refractive index to the ordinary refractive index.With the increase of the deviation of the ratio of the extraordinary refractive index to the ordinary refractive index from unity,the elongation of the beam spot also augments.In different observation planes,the phase distribution of an SLG 01 mode in the uniaxial crystal takes on different shapes.With the variation of the ratio of the extraordinary refractive index to the ordinary refractive index,the phase distribution is elongated in one transversal direction and is contracted in the other perpendicular direction.This research is beneficial to the practical applications of an SLG mode.     

Relative performances of effective medium formulations in interpreting specific composite thin films optical properties

Several powerful effective medium formulations/approximation (EMA) and associated theories with different origins and concepts have been discussed and utilized here in order to model the experimental refractive index evolutions of ZrO₂-SiO₂ and Gd₂O₃-SiO₂ composite films with respect to their compositional mixings. Amongst these formulations, the Böttcher's generalized theory has been noticed to have more versatility and can simulate varieties of experimental observations incorporating a form factor parameter to account for the grain structure and morphology to a great extent. The refractive index modeling results of most of the available theories were compared with respect to their functional evolutions and limitations. It was noticed that at higher silica fractions (>20%) in our composite films, the effective experimental refractive index parameters have remained close to the most modeling results and Böttcher's expression has shown to fit the observable parameters very accurately. However, under low silica compositions (<20%) the refractive index values of the composite films depicted different functional evolutions. Such deviations have been attributed to the various morphological, grain structure and band gap supremacies observed in these specific composite films which are not accounted by the effective medium formulations and approximations. These observations are well supported by the atomic force microscopy results.     

Nonlinear diffraction in gratings based on polymer–dispersed TiO 2 nanoparticles

In this letter we report a simple technique to produce volume holographic gratings based on photopolymerizable composites containing TiO₂ nanoparticles. Diffraction gratings with high refractive index modulation amplitude (up to 1.25 × 10⁻²) have been formed due to the periodic distribution of high refractive index nanoparticles in a low refractive index polymer matrix. The diffraction efficiency increases strongly on increasing the nanoparticle concentration. Taking the mixture with 10 wt.% TiO₂ nanoparticles, gratings with high diffraction efficiency, low level of scattering and high transparency in the visible-wavelength range have been obtained. This will ultimately lead to different applications of diffractive optical elements based on nanocomposites. The dependence of the gratings’ diffraction efficiency on the intensity of probe laser pulses at 1064 nm has been explored. It is shown that the nonlinear response of the gratings is attributed mainly to the nonlinear properties of the TiO₂ nanoparticles embedded in the polymer matrix. The mechanism of the grating formation and the reasons for the nonlinear behavior of the diffraction efficiency are discussed.