Broadband resonant cavity inside a two-dimensional sonic crystal

A rectangular cavity inside a two-dimensional sonic crystal was theoretically and experimentally characterized by examining its response to a cylindrical source emitting narrow-band filtered noise bursts with central frequencies ranging from 2 to 12 kHz. A broadband intensity resonance was observed for frequencies within the full band-gap region of the sonic crystal (5.5–6.5 kHz). Unlike ordinary resonances, this broadband resonance depends on the reflection properties of the sonic crystal forming the surrounding walls rather than on the geometry of the cavity.     

Optimized focusing properties of photonic crystal slabs

We propose an optimization procedure for focusing operation in finite two dimensional photonic crystal slabs. The device consists of a triangular lattice air holes etched in a semiconductor matrix at a nanometer scale to operate at 1.55 μm. To reach simultaneously an effective refractive index equal to −1 along with a very high transmission coefficient whatever optical wave incidence, the parameters as the lattice period and/or filling factor are precisely adjusted depending on the slab thickness. The method relies on Fabry-Perot resonances engineering in the air/crystal/air cavity constituting the lens.     

Localized modes in a defectless photonic crystal waveguide at terahertz frequencies

We propose an idea to excite localized modes in a photonic crystal (PC) waveguide without ruining the discrete translational symmetry of the lattice. This can be done by arranging dispersive elements having negative permittivity over a desired frequency range into a periodic structure. We demonstrate numerically the realization of a cavity mode inside the air region of a geometrical defectless two-dimensional square-lattice PC consisting of polaritonic cylinders placed in air matrix. The corresponding waveguide structure in the form of a PC fiber supports the cavity mode as a guided mode to propagate along the guiding direction at very small propagation constant with near zero group velocity. These localized modes can be recognized as localized defectless modes inside the structure with four-fold symmetry.     

Transmission and radiation of acoustic oblique incident through tube arrays based on phononic crystals theory

In a utility boiler, the heat exchanger’s structure is similar to a two-dimensional phononic crystal. Based on the phononic crystal theory, this paper studies sound propagation through tube arrays as a function of the incident sound direction and the surroundings temperature. We carried out both the computational and experimental work for particular values of the pitch and diameters in the tube arrays and studied the band-gap diagram and insertion loss spectra for different angles of incidence. The first band gap is found to correspond to Bragg’s Law while the second band gap moves to lower frequencies as the angle increases. Simulations indicate also that the uneven temperature field influences the insertion loss spectrum. Results of experiments and calculations confirm that, for a particular tube array, the most important factors influencing sound propagation are incidence angle and the surrounding temperature. For the acoustic source in tube arrays, the acoustic radiation have relation with the frequency whether in the acoustic bang gap or not. The results should provide a basis for further work: both on sound source localization and low frequency sonic cleaning in large tube arrays.     

Shape-optimal design of graded index sonic crystal formations using natural cubic splines

Noise reduction through upward refraction can be achieved by artificial means, using a graded index sonic crystal. In addition to upward refraction, it will be shown that these periodically spaced cylinder formations can simultaneously benefit from band-gap phenomena. The aim of this paper is to present a method to optimise the broadband noise reducing performance of graded index sonic crystals, in a frequency range from well below to well above the lowest band-gap frequency. A design technique based on the creation of complex cylinder formations has been explored, in which the effective propagation speed is spatially varied using natural cubic splines. Sets of complex barrier shapes are compactly described by re-locating a number of control points in a two-dimensional cartesian plane and connecting the control points by (curved) line segments. In addition to the cluster shape, a complex graded index sonic crystal structure was formed by varying the lattice constant and the cylinder radius, where the cylinder radius was varied as a function of height. All these parameters were optimised with a multi-objective genetic algorithm, for structures based on horizontally oriented acoustically hard cylinders, located above a perfectly reflecting ground plane. A four-lane outdoor situation, with a traffic scenario consisting of 95% light and 5% heavy duty vehicles driving at 70 km/h has been studied in a two-dimensional domain. For such a configuration we obtained a spatially averaged mean reduction of 4.2–5.3 dBA, with structures covering an effective cross-sectional area of 1 m². It was found that the insertion loss among the studied traffic lanes was reasonably constant. In addition, it was found that the low-frequency performance of the studied structures is enhanced by incrementing the barrier-height while increasing the number of scatterers as a function of height.     

Effects of wave propagation anisotropy on the wave focusing by negative refractive sonic crystal flat lenses

<正>In this study,wave propagation anisotropy in a triangular lattice crystal structure and its associated waveform shaping in a crystal structure are investigated theoretically.A directional variation in wave velocity inside a crystal structure is shown to cause bending wave envelcpes.The authors report that a triangular lattice sonic crystal possesses six numbers of a high symmetry direction,which leads to a wave convergence caused by wave velocity anisotropy inside the crystal.However,two of them are utilized mostly in wave focusing by an acoustic flat lens.Based on wave velocity anisotropy,the pseudo ideal imaging effect obtained in the second band of the flat lens is discussed.     

光子晶体光纤带隙及模场的一种综合模拟分析方法

提出直接由MATLAB读入光纤端面图实现复杂结构光子晶体光纤(PCF)模型的快速建立,并综合平面波法(PWM)和频域有限差分(FDFD)法,模拟分析带隙型光子晶体光纤(PBG-PCF)的带隙和模场分布.利用PWM计算得到了在PBG-PCF中传输的光波频率及模式有效折射率范围;基于FDFD法在给定波长及模式有效折射率范围情况下,模拟得到了PBG-PCF中可能存在的模场分布及其他特性.以市售的PBG-PCF为例,验证了数值模拟的正确性,随后系统地分析了结构参数(晶格结构、"原子"占有率、背景材料折射率及"原子 关键词： 光子晶体光纤(PCF) 光子带隙(PBG) 平面波法(PWM) 频域有限差分法(FDFD)     

Matter-wave gap solitons in atomic band-gap structures

We demonstrate that a Bose-Einstein condensate in an optical lattice forms a reconfigurable matter-wave structure with a band-gap spectrum, which resembles a nonlinear photonic crystal for light waves. We study in detail the case of a two-dimensional square optical lattice and show that this atomic band-gap structure allows nonlinear localization of atomic Bloch waves in the form of two-dimensional matter-wave gap solitons.     

Modal selective tuning in a photonic crystal cavity

We present a way to selectively tune the properties of the degenerated modes confined in a single point defect two-dimensional photonic crystal cavity based on a triangular lattice of air holes. We investigate the dependence of the modal properties of the resonator on the position of the first neighbor holes, showing that it is possible to finely tune the resonant frequency of only one of these two modes and to increase the quality factor of the mode that has no frequency shift. This is achieved by controlling the wavevector components inside the cavity. This approach is a viable strategy for the development and the optimization of several innovative devices based on bi-modal cavity arrays, such as arrays of integrated optical filters and optical read-out sections for biosensing applications.     

Strong exciton-photon coupling in an organic single crystal microcavity

We demonstrate strong exciton-photon coupling of Frenkel excitons at room temperature in a microcavity composed of a melt grown thin film anthracene single crystal and two distributed Bragg reflectors. Angle-resolved reflectivity and normal incidence photoluminescence under weak excitation are observed. The microcavity spectrum is a function of the anisotropy of the crystalline material and the strong exciton-photon coupling of the excitonic resonances to the cavity photon. The photoluminescence spectrum is found to be completely polarized along the crystal axes.     

Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity

We propose a new scheme for locking the frequency of a laser to a resonant reference cavity. A linear polarizer or Brewster plate is placed inside the reference cavity, so that the reflected light acquires a frequency-dependent elliptical polarization. A simple polarization analyzer detects dispersion shaped resonances which can provide the error signal for electronic frequency stabilization without any need for modulation techniques.     

Matryoshka locally resonant sonic crystal

The results of numerical modeling of sonic crystals with resonant array elements are reported. The investigated resonant elements include plain slotted cylinders as well as their various combinations, in particular, Russian doll or Matryoshka configurations. The acoustic band structure and transmission characteristics of such systems have been computed with the use of finite element methods. The general concept of a locally resonant sonic crystal is proposed that utilizes acoustic resonances to form additional band gaps that are decoupled from Bragg gaps. An existence of a separate attenuation mechanism associated with the resonant elements that increases performance in the lower frequency regime has been identified. The results show a formation of broad band gaps positioned significantly below the first Bragg frequency. For low frequency broadband attenuation, a most optimal configuration is the Matryoshka sonic crystal, where each scattering unit is composed of multiple concentric slotted cylinders. This system forms numerous gaps in the lower frequency regime, below Bragg bands, while maintaining a reduced crystal size viable for noise barrier technology. The finding opens alternative perspectives for the construction of sound barriers in the low frequency range usually inaccessible by traditional means including conventional sonic crystals.     

A sonic crystal diode implementation with a triangular scatterer matrix

This paper investigates a nonreciprocal sound transmission effect provided by a triangular lattice two-dimensional sonic crystal made of rods in a triangular cross-section. This sonic crystal (SC) device works as a frequency selective acoustic diode operating at a frequency of 8950 Hz. The scatterer matrix of the sonic crystal diode prototype was composed of triangular shaped wood rods that break the symmetry of the spatial inversion and provide nonreciprocal wave transmission with a contrast rate of 89% in experiments. This acoustic diode device can provide a high contrast, narrow band, one-way sound transmission for acoustic wave control applications.     

平均折射率为零的光子晶体中缺陷模频率特性的实验研究

利用传输线技术制备了左手材料,将左手材料与正常材料交替排列组合成平均折射率为零的一维光子晶体.该光子晶体在特定频段具有光子带隙,带隙不随晶格尺度和入射角的变化而改变.通过掺杂技术破坏光子晶体的周期性,可在禁带中引入缺陷模,这种结构的光子晶体可用于实现滤波器小型化和超强耦合.研究表明,通过调节缺陷的厚度可以控制缺陷模的频率,这为调节频率提供了一种方法.实验与仿真结果相符. 关键词： 左手材料 复合左右手传输线 光子晶体     

Magic Wavelength Measurement of the ~(87)Sr Optical Lattice Clock at NIM

We report on the magic wavelength measurement of our optical lattice clock based on fermion strontium atoms at the National Institute of Metrology(NIM).A Ti:sapphire solid state laser locked to a reference cavity inside a temperature-stabilized vacuum chamber is employed to generate the optical lattice.The laser frequency is measured by an erbium fiber frequency comb.The trap depth is modulated by varying the lattice laser power via an acousto-optic modulator.We obtain the frequency shift coefficient at this lattice wavelength by measuring the differential frequency shift of the clock transition of the strontium atoms at different trap depths,and the frequency shift coefficient at this lattice wavelength is obtained.We measure the frequency shift coefficients at different lattice frequencies around the magic wavelength and linearly fit the measurement data,and the magic wavelength is calculated to be 368554672(44) MHz.     

内插扩张室声子晶体管路带隙特性研究

声子晶体管路的带隙特性,可以实现管路系统在特定频率下的噪声控制.利用二维模态匹配法推导出单个内插扩张室元胞的传递矩阵,结合Bloch定理,得到声子晶体管路的能带结构计算方法;验证了二维方法在计算能带结构时的准确性.研究发现,内插扩张室声子晶体管路存在布拉格带隙和局域共振带隙.进一步研究了晶格常数以及内插管长度对能带结构的影响,结果表明,晶格常数主要控制布拉格带隙,而内插管长度对局域共振带隙有较大的影响,并研究了两种参数变化下的带隙耦合.研究结果可以为管路降噪设计提供新的思路.     

Using Josephson vortex lattices to control terahertz radiation: tunable transparency and terahertz photonic crystals

The Josephson vortex (JV) lattice is a periodic array that scatters electromagnetic waves in the THz-frequency range. We show that JV lattices can produce a photonic band-gap structure (THz photonic crystal) with easily tunable forbidden zones controlled by the in-plane magnetic field. The scattering of electromagnetic waves by JVs results in a strong magnetic-field dependence of the reflection and transparency. Fully transparent or fully reflected frequency windows can be conveniently tuned by the in-plane magnetic field. These proposals are potentially useful for controllable THz filters.     

Positioning photonic crystal cavities to single InAs quantum dots

We have investigated the optical properties of planar photonic crystal cavities formed by removing a single hole from a two-dimensional square lattice of air holes etched through a thin GaAs slab. We have demonstrated cavity resonances with quality factors (Q’s) as high as 8500, using an internal light source provided by an ensemble of InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE). The high-Q modes are confined to a very small mode volume, V = 0.7(λ/n)³, making them attractive to study in the context of cavity quantum electrodynamics with single QDs, where a high is needed to observe the strong coupling between an electronic state of the dot and the optical cavity mode. To this end, we have developed an accurate and robust alignment technique that positions a photonic crystal cavity to a single QD with 25 nm resolution. We present the details of this new technology and demonstrate its effectiveness by strategically positioning a number of QDs within photonic crystal cavities at points where the electric field intensity is high.     

Nonlinear sensitivity enhancement with one-dimensional photonic bandgap microcavity arrays

It is shown that an array of microcavities, or defects, in a photonic band-gap lattice can dramatically increase nonlinear sensitivity compared with that of bulk material. These defects open transmission resonances within the bandgap. Nonlinear induced detuning of these resonances can result in a greater than tenfold increase in the nonlinear phase shift relative to that of a bulk material of the same thickness and give a better figure of merit than operating at the band edge of a photonic lattice without defects.     

Light diffraction features in an ordered monolayer of spheres

The structure and optical diffraction properties of monolayers of monodisperse spheres crystallized on transparent dielectric substrates are studied. Two types of diffraction phenomena are considered: surface light diffraction on the lattice of spheres and waveguide resonances in the monolayer plane. For experimental study of these phenomena, optical retroreflection and transmission spectra are measured as functions of the light incidence angle and azimuthal orientation of the incidence plane. The monolayer structures determined by scanning electron microscopy and light diffraction methods are in quantitative agreement. It is concluded that one-dimensional Fraunhofer diffraction is applicable to describe surface diffraction in the hexagonal lattice of spheres. In the case of oblique light incidence, anisotropy of diffraction and transmission spectra depending on the light incidence plane orientation with respect to the sphere lattice and linear polarization of incident light is detected. Waveguide resonances of the planar two-dimensional photonic crystal are approximated within the light diffraction model in the “empty” hexagonal lattice. The best approximation of the waveguide resonance dispersion is achieved using the effective refractive index, depending on the wavelength. Surface diffraction suppression by waveguide resonances of the photonic crystal is demonstrated. Surface diffraction orders are identified as diffraction at singular points of the Brillouin zone of the planar twodimensional photonic crystal.