The displacement-thickness theory of trailing edge noise

The problem of estimating the sound generated by turbulent boundary layer flow over the edge of a rigid half-plane is re-examined. A theory is proposed which is strictly valid at low Strouhal numbers based on boundary layer width, wherein the flow inhomogeneities are specified in terms of the fluctuations in the boundary layer displacement thickness. This enables account to be taken of changes in the properties of the turbulence as it translates past the edge, which are shown to result in the appearance of an acoustic dipole whose axis is aligned with the mean flow, and which supplements the radiation field predicted by conventional methods [1,2]. Detailed comparison is made with acoustic and surface pressures which are calculated according to the evanescent wave theory of edge noise [3–5].     

Verification of evanescent coupling from subwavelength grating pairs

Near-field evanescent wave coupling of various subwavelength grating pairs, using a 1.55 μm infrared semiconductor laser has been demonstrated for use as an optical MEMS sensor. Subwavelength grating pairs were fabricated on both glass and silicon substrates. When coupled together the effective grating period is not subwavelength and can exhibit several diffraction orders. The 1.55 μm infrared source was incident on the coupled pairs and the first-order output intensity was recorded and compared with the output intensity from simulated results. This demonstrated evanescent wave coupling concept can be applied to MEMS systems with nanometer gap separations (e.g., pressure sensors, biosensors, and accelerometers) to allow for subnanometer displacement detection.     

InGaAsP/InP evanescent mode waveguide optical isolators and their application to InGaAsP/InP/Si hybrid evanescent optical isolators

We theoretically investigated InGaAsP/InP evanescent mode waveguide optical isolators and proposed their application to InGaAsP/InP/Si hybrid evanescent optical isolators. InGaAsP/InP evanescent optical isolators are composed of semiconductor optical amplifier (SOA) waveguides having InGaAsP multiple quantum well (MQW) active layer and upper InGaAsP waveguide layer with ferromagnetic layer. Optical isolation is obtained for evanescent optical mode in the InGaAsP waveguide layer. InGaAsP/InP/Si hybrid evanescent optical isolators are theoretically proposed based on the idea of InGaAsP/InP evanescent optical isolators. InGaAsP/InP/Si hybrid evanescent optical isolators are composed of ferromagnetic metal loaded silicon evanescent waveguides with wafer-bonded InGaAsP/InP optical gain material. The optical isolation and propagation loss are discussed with the structure of silicon evanescent waveguides, and optical isolation of 8.0 dB/mm was estimated. The concept of semiconductor evanescent mode optical isolators is feasible with InP based photonic integrated circuits and advanced silicon photonics.     

用于微型生化分析的光探测系统

针对微生化芯片样品剂量少和与微流体芯片集成等要求,设计了两种在芯片内用分光光度法对混合后液体的吸收光谱进行探测的方法。实验利用510～840 nm的可见光光源,采用了直接探测法和消逝波探测法对样品进行探测。通过两种方法的比较证明：直接探测法具有原理简单、消耗样品量少和结构容易实现等优点;消逝波探测法具有需要样品量极少、灵敏度高和易于集成的优点。     

Characterization of strong electromagnetic field confinement on gold nanostructures by apertureless scanning near-field optical microscopy

We report on the detection of the optical near field of a 1D gold particle array by using an apertureless scanning near-field optical microscope. The strong near-field confinement measured above the grating proves unambiguously the near-field origin of the detected optical signal. Comparing the experiment with theory leads us to assign the optical near field to the first diffracted order of the grating, which is evanescent.     

生物传感中平板光波导模式研究

为提高生物传感探测的灵敏度和响应速度,基于平面光波导理论,运用消逝场原理在生物传感领域的应用,在相同条件下比较了平面波导单模和多模2种模式下消逝场区域能量的大小,并且用VC和Matlab工具模拟论证单模能量高于多模。理论计算表明：单模和多模的荧光效率分别为5.83%和1.75%,在此计算中单模的荧光效率比多模的高。因此,在生物传感及其相关应用中,选用平板光波导的单模模式具有一定的优越性,为实际设计提供了理论依据。     

Spatial separation of the traveling and evanescent parts of dipole radiation

Electric dipole radiation consists of traveling and evanescent plane waves. When radiation is detected in the far field, only the traveling waves will contribute to the intensity distribution, as the evanescent waves decay exponentially. We propose a method to spatially separate the traveling and evanescent waves before detection. It is shown that when the radiation passes through an interface, evanescent waves can be converted into traveling waves and can subsequently be observed in the far field. Let the radiation be observed under angle theta(t) with the normal. Then there exists an angle theta(ac) such that for 0 < or = theta(t) < theta(ac) all intensity originates in traveling waves, whereas for theta(ac) < theta(t) < pi/2 only evanescent waves contribute. It is shown that with this technique and under the appropriate conditions almost all far-field power can be provided by evanescent waves.     

负折射率材料对隐失波的幅度增益

分析了隐失波透过负折射率材料制成的透镜时,可以实现相位的调整,从而实现波的聚焦的原理;另外利用叠加原理(干涉法)及场量匹配法分析了负折射率材料如何实现对隐失波的振幅的放大,得到负折射率材料对隐失波有幅度增益,行波和隐失波都能聚焦成像的结论;并利用传输线理论提出隐失波透过负折射率材料平板时可等效地形成一个LC振荡电路,从而实现对隐失波的幅度增益.     

Theory of atomic diffraction from evanescent waves

We review recent theoretical models and experiments dealing with the diffraction of neutral atoms by a reflection grating, formed by a standing evanescent wave. We analyze diffraction mechanisms proposed for normal and grazing incidence, point out their scopes and confront the theory with experiment. Received: 12 June 1999 / Published online: 8 September 1999     

Boundary layer attenuation of higher order modes in waveguides

The attenuation of higher order modes in rectangular and circular tubes is treated here by using results for the boundary layer admittance for the respective normal modes. Comparison with results available in the literature for propagating modes is given. Results for evanescent modes and at the cut-off frequencies are discussed. Finally, the well-known Kirchhoff theory is extended to obtain a test of validity for the proposed calculations.     

一种新型的振荡场光化学传感器

利用法布里珀罗振荡模构造了一种新型的光化学传感器。与传统的以表面等离子模和光导模为探针的迅衰场传感器不同 ,这种新型结构中被传感介质处于法布里珀罗腔的振荡场中 ,其能量在全部入射光能中的比重占得很大 ,使得这种传感器具有很高的灵敏度。实验证实这类传感器对葡萄糖溶液浓度和折射率的测量精度皆可达 10 - 5数量级以上     

Heterodyne interferometer for displacement measurement with amplitude quadrature and noise suppression

The high precision displacement measurement in nanoscale is crucial to many applications. We present a heterodyne interferometry with differential phase to amplitude conversion scheme for displacement measurement in nanoscale. In this approach, the differential phase introduced by the displacement is converted into the amplitudes of heterodyne signals in quadrature. Meanwhile, the heterodyne signals in phase quadrature are also achieved so that the displacement can be determined from the amplitude ratio of the quadrature signals, and the direction of displacement can be determined from the phase quadrature. Since the differential phase to quadrature amplitude conversion is achieved through the optical addition and subtraction by polarization tuning, which are based on differential detection concept. Thus the proposed method benefits from the features of differential detection with common phase noise and correlated amplitude noise rejection and that of quadrature detection with real time and wide dynamic range of phase measurement. To demonstrate the capability of proposed method in differential phase measurement, we measure the displacement drove by a commercially available PZT pusher and found close agreement between the experiment and the theory. The experimental evidence of noise suppression is also found with spectral measurements, which demonstrates the resolution of displacement measurement at 60 pm and minimum detectable differential phase of 5.6 × 10⁻⁶ rad/ over 50 kHz.     

光纤倏逝波生物传感器及其应用

以光纤倏逝波原理，研制成功一台光纤倏逝波生物传感器。该传感器对纯净的Cy5荧光染料溶液的检测灵敏度达0．01μmo1／1；对浓度为0．01μmo1／1的嗜肺军团菌核酸样品的测量信噪比为4．61。所有检测结果与商品化生物芯片扫描仪同时检测得到的结果具有很好的一致性。     

Evanescent light–atom interaction detected by optogalvanic effect

An evanescent light penetrating an atomic vapour near a dielectric surface could be a probe for many atom-boundary phenomena. We show the possibility of very sensitive detection of the resonant atom–light interaction near the surface by using the optogalvanic effect for an evanescent wave. We observe a narrowing of the profile of the detected atomic line, and we point out some properties of the optogalvanic effect in the evanescent wave.     

Evanescent Bessel beam generation via surface plasmon resonance excitation by a radially polarized beam

A simple setup for generating evanescent Bessel beams is proposed. When a radially polarized beam is strongly focused onto a dielectric-metal interface, the entire beam is p-polarized with respect to the dielectric-metal interface, enabling excitation of surface plasmons from all directions. The angular selectivity of surface plasmon excitation mimics the function of an axicon, leading to an evanescent nondiffracting Bessel beam. The created evanescent Bessel beam may be used as a virtual probe for near-field optical imaging and sensing applications.     

光纤倏逝波型石英增强光声光谱技术

采用块状光学准直聚焦透镜组的传统石英增强光声光谱(QEPAS)技术存在体积难以缩减,结构稳定性不佳,无法适应空间狭小、振动复杂的特殊环境等缺点.基于此,将光纤倏逝波技术与QEPAS技术相结合,提出了一种新型微纳结构光纤QEPAS痕量气体检测技术.实验中,为了提高QEPAS系统信号幅值,优化了石英音叉与激光束的空间位置、激光波长调制深度,同时对比了两种不同共振频率的石英音叉,最终采用共振频率较低的30.720 kHz石英音叉作为声波探测元件,获得的检测极限为6.25×10~(-4)(体积分数),归一化噪声等效吸收系数为4.18×10~(-7)cm~(-1).W·Hz~(-1/2).     

光波导振荡场传感器

针对消逝场传感器的不足,提出了一种传感介质处于波导芯层,且以超高阶导模为探针的光波导振荡场传感器.由于其波导芯层处于功率密度极高的振荡场区域,而超高阶导模又具有对芯层参数极为灵敏的性质,因此这种新型传感器的灵敏度大大增强.实现了超越消逝场传感器灵敏度极限值几个量级的位移传感器.     

Flow Injection Small-volume Fiber-optic pH Sensor Based on Evanescent Wave Excitation and Fluorescence Determination

A small-volume fiber-optic pH sensor (FOEWS) based on evanescent wave excitation is developed and evaluated. The sensor is simply fabricated by inserting a decladded optical fiber into a transparent capillary tube. A microchannel between the optical fiber and the capillary inner wall was formed and acted as flow cell for solution flowing through. The pH-sensitive fluorophore of fluorescein can be excited by the evanescent wave field produced on the fiber core surface to produce emission fluorescence. pH value was then sensed by its enhancing effect on the emission fluorescence intensity. The response range of the sensor is from pH 2.09 to pH 8.85 and the linear range is from pH 3.25 to 8.85. The proposed sensor has a small detection volume of 2.5 μL and a short response time of 8 s. It has been applied to measure pH values of real water samples and was in good agreement with the results obtained by commercial pH meter.     

Refractive index and temperature sensing characteristics of an optical fiber sensor based on a tapered single mode fiber/polarization maintaining fiber

An evanescent field optical fiber sensor based on a short section of polarization maintaining fiber spliced with a tapered single mode fiber is proposed and experimentally investigated. We mainly focus on the refractive index(RI) and temperature sensing characteristics of this compact device. The transmission spectrum of the resonance wavelength,induced by the interference between the excited low order cladding modes and core modes,shows the quadratic function relationships with RI and linear relationships with temperature. Thus,the proposal of this simple-to-fabricate,compact,and low cost sensor shows its possible potential in the sensitive detection field.     

Traversal time for Dirac particles through a potential barrier

A traversal time that has no problem of superluminality was advanced for particles to tunnel through potential barriers in the non‐relativistic quantum theory in a previous paper by C.‐F. Li and Q. Wang, Physica B 296 (2001) 356. This time is generalized in this paper to Dirac's relativistic quantum theory. Both evanescent and propagating cases are considered. It is shown that the traversal time in the evanescent case has much the same properties as in the non‐relativistic quantum theory and thus has no problem of superluminality. It also gets rid of the problem of superluminality in the propagating case. Comparisons with the dwell time, the group delay, and the velocity of monochromatic front are also made.