Photonic nanojet‐mediated SERS technique for enhancing the Raman scattering of a few molecules

We report a two‐step enhancement of Raman scattering signal (η) of a few dye molecules. In the first step, high‐quality surface‐enhanced Raman scattering (SERS) substrates have been used. The SERS substrates were fabricated by direct current sputtering of Au followed by thermal annealing. The role of thermal annealing of the SERS substrates and numerical aperture of Raman microscopic objective lens on the enhancement has been studied for optimizing the enhancement in the SERS technique. In the second step, the value of η obtained with conventional SERS technique has been improved significantly with the help of photonic nanojet (PNJ) of an optical microsphere (PNJ‐mediated SERS technique). The signal to noise ratio and reproducibility of the experimental results have been found to be very high. Based on our theoretical simulations on PNJ, a few suitable parameters have been proposed for obtaining better enhancement using this technique. To the best of our belief, this report will enable the SERS community to improve η value with ease. Copyright © 2016 John Wiley & Sons, Ltd.     

二维异质结光子晶体中含近邻点缺陷的弯曲波导的可调谐滤波特性

应用时域有限差分(FDTD)法,对光在二维异质结光子晶体中含两个近邻点缺陷直角弯曲波导的传输特性进行了数值模拟研究。计算结果表明,具有特定本征共振频率的光子晶体微腔(点缺陷),可将在其邻近波导中传播的相同频率成分的光波"引入"到微腔中。"引入"的频率依赖于异质结光子晶体弯曲波导和微腔的参量,从而能够实现调变"引入"频率。通过改变点缺陷周围介质材料(液晶)的折射率或相关介质柱半径,可分别实现约31 nm或12 nm的波长调谐范围。计算结果为光通信中的调谐、上/下载滤波器,提供了一条新的设计思路和依据。     

两种单光纤光镊捕获效果的数值仿真与实验研究

采用一种基于时域有限差分(FDTD)的数值算法,仿真计算了抛物线形和大锥角形两种新型单光纤光镊的出射光场,并在稳态场下通过对麦克斯韦应力张量积分求得介质球在两种光场中受到的光阱力,得到大锥角型光纤端产生的光阱力较大的结论;讨论了不同介质球大小、折射率,光纤探针形状对光阱力的影响.在实验中这两种光纤探针都实现了对水中酵母菌细胞的捕获,且采用流体力学法对抛物线形和大锥角形二种新型单光纤光镊产生的光阱力进行了标定.结果表明:基于FDTD数值仿真方法计算受力与实验结果一致,并且这种计算光纤光镊产生的光阱力的方法简单.适用;且抛物线形和大锥角形光纤探头都具备构成单光纤光镊的条件.     

Polarization‐controlled surface plasmon holography

The ability of generating arbitrary surface plasmon (SP) profiles in a controllable manner is of particular interest in designing plasmonic imaging, lithography and forcing devices. During the past decades, holography has gained enormous interest and achievements in free‐space three‐dimensional displays. Here, by applying a two‐dimensional version of holography, we experimentally demonstrate a generic method to control the SP profiles. Through controlling the orientation angles of two separated slits under circular polarization incidence, the amplitude and phase of the excited SPs can be freely manipulated, which allows direct generation of the desired SP profiles. A series of controllable SP holography schemes are theoretically and experimentally demonstrated, where the holographic SP profiles with high imaging quality can be dynamically modulated by varying the circular polarization handedness or orientation angle of linear polarization. The universality and simplicity of the proposed design strategies would offer promising opportunities for practical plasmonic applications.

     

Photonic Jet by a Near‐Unity‐Refractive‐Index Sphere on a Dielectric Substrate with High Index Contrast

Photonic jets are normally generated in transmission mode and are represented as a spatially localized high‐intensity region on the shadow side of a particle‐lens, with a background to medium refractive index contrast of 1.3–1.7 while illuminated by a plane wave. Here, a photonic jet is discovered in the opposing plane wave propagation direction and lies in the area in the upper boundary of a near‐unity refractive index sphere on a high refractive index dielectric substrate. The redistribution of the power flow is inhibited during the reflected wave passing the near‐unity refractive index sphere. This has led to a unique effect on the focus position and shape of the produced photonic jet in reflection mode which can be maximally maintained near the sphere regardless of the modulation of the refractive index for the dielectric substrate material.     

Quantum Dots Luminescence Collection Enhancement and Nanoscopy by Dielectric Microspheres

In recent years, dielectric microspheres have been used in conjunction with optical microscopes to beat the diffraction limit and to obtain superresolution imaging. The use of microspheres on quantum dots (QDs) is investigated, for the first time, to enhance the light coupling efficiency. The enhancement of the QD luminescence collection in terms of extraction and directionality is demonstrated, as well as the enhancement of spatial resolution. In particular, it is found that a dielectric microsphere, placed on top of an epitaxial QD, increases the collected radiant energy by about a factor of 42, when a low numerical aperture objective is used. Moreover, if two or more QDs are present below the microsphere, the modification of the far field emission pattern allows selective collection of the luminescence from a single QD by simply changing the collection angle. Dielectric microspheres present a simple and efficient tool to improve the QD spectroscopy, and potentially QD-based devices.     

基于二维光子晶体点缺陷可调谐光功率分配器

在二维矩形阵列结构硅光子晶体中去除中间一排硅柱形成线波导,在线波导右侧引入点缺陷。利用时域有限差分法(FDTD)模拟仿真以及数值分析研究线波导中光耦合特性,计算出两个通道的分光比,发现改变光子晶体的温度可以明显改变这两个通道的光功率比。基于此结构,提出了一种新的光功率分配器,可以获得大范围的光功率比值,从1∶1～90∶1都可以通过改变光子晶体的温度来实现,并且当温度从0℃～200℃就可以实现这一功能,最后设计了一款三通道可调谐光功率分配器,通过调节两个点缺陷区域内温度来实现光功率的分配。     

近场光镊技术研究新进展

光镊技术,又称光学捕获技术,它是利用光的辐射压力来捕获和操纵包括电介质颗粒、生物细胞及生物大分子在内的微小粒子的。近场光镊技术利用近场光学倏逝场随距离急剧衰减的特征,可显著地降低捕获粒子的尺寸,实现纳米捕获。追踪了近场光镊技术的最新进展,包括全内反射相干倏逝场、近场光学镀膜光纤探针尖、激光照明金属探针尖和聚焦倏逝场用于近场光学捕获,并对其进行了比较,分析了它们存在的主要问题和未来发展方向。     

A MEMS Tunable Optical Ring Resonator Filter

In this work, a new architecture for a ring resonator tunable optical filter is proposed. In this architecture, two rings are connected to each other by a directional coupler and tuning is achieved through the control of the separation between the coupler two parallel guides using a micro-electro-mechanical systems (MEMS) actuator. The optical performance of the filter is tested using the FDTD and an analytical model is developed to predict its performances. A good agreement between the analytical and numerical results is obtained. The MEMS design is also presented and verified using an finite element method (FEM) analysis.     

光子晶体光纤及其在光通信中的应用

简述三类光子晶体光纤(Photonic crystal fibers,PCF)的结构、导光机制及特性,介绍了PCF的研究现状和在光通信中的应用,并探讨了PCF的应用前景。     

Applications of Nonlinear Effects in Highly Nonlinear Photonic Crystal Fiber to Optical Communications

Unique dispersion characteristics and enhanced nonlinearity make the small-core photonic crystal fiber (PCF) an ideal candidate for nonlinear optical devices to telecommunication applications. Some technical reasons behind great research interest of highly nonlinear PCFs in optical communication components are reviewed. Nonlinear effects in highly nonlinear PCFs and their research progress are presented. Several typical applications including WDM sources, optical amplification, optical switching, wavelength conversion, optical regeneration and all-optical demultiplexing etc are introduced, together with state-of-the art performances. Some new possible applications and future prospects are discussed.     

Intensity‐Enhanced Apodization Effect on an Axially Illuminated Circular‐Column Particle‐Lens

A particle can function as a refractive lens to focus a plane wave, generating a narrow, high intensive, weak‐diverging beam within a sub‐wavelength volume, known as the ‘photonic nanojet’. It is known that apodization method, in the form of an amplitude pupil‐mask centrally situated on a particle‐lens, can further reduce the waist of a photonic nanojet, however, it usually lowers the intensity at the focus due to blocking the incident light. In this paper, the anomalously intensity‐enhanced apodization effect was discovered for the first time via numerical simulation of focusing of the axially illuminated circular‐column particle‐lenses, and a greater than 100% peak intensity increase was realised for the produced photonic nanojets.     

Subdiffraction‐limited chemical imaging of patterned phthalocyanine films using tip‐enhanced near‐field optical microscopy

A tip‐enhanced near‐field optical microscope, based on a shear‐force atomic force microscope with plasmonic tip coupled to an inverted, confocal optical microscope, has been constructed for nanoscale chemical (Raman) imaging of surfaces. The design and validation of the instrument, along with its application to near‐field Raman mapping of patterned organic thin films (coumarin‐6 and Cu(II) phthalocyanine), are described. Lateral resolution of the instrument is estimated at 50 nm (better than λ/10), which is roughly dictated by the size of the plasmonic tip apex. Additional observations, such as the distance scaling of Raman enhancement and the inelastic scattering background generated by the plasmonic tip, are presented. Copyright © 2016 John Wiley & Sons, Ltd.     

纳米尺度的光学成像与纳米光谱———近场光学与近场光学显微镜的进展

近场光学是指当光探测器及探测器－样品间距均小于辐射波长条件下的光学现象．利用近场光学扫描显微镜和近场光谱仪，不但能够以突破衍射极限的超高分辨率在纳米尺度实现光学成像，而且还可获得纳米微区的光谱信息．文章介绍近场光学的原理及其在凝聚态物理领域中的应用与进展，并给出了我们的初步结果     

光子晶体耦合波导实现光开关效应的研究

提出了一种基于光子晶体耦合波导实现光开关效应的方法:将线缺陷引入二维光子晶体以形成两平行的邻近波导,两邻近波导及其中间的两排介质柱构成了光开关模型耦合区;利用平面波展开法计算了不同介质填充率情况下的色散特性.结果发现:减小介质填充率可以实现波导耦合长度的减小;分段调整中间介质柱的填充率和选择不同的耦合搭配长度,定向耦合...     

Optical trapping and manipulation of micrometer and submicrometer particles

Subwavelength features in conjunction with light‐guiding structures have gained significant interest in recent decades due to their wide range of applications to particle and atom trapping. Lately, the focus of particle trapping has shifted from the microscale to the nanoscale. This few orders of magnitude change is driven, in part, by the needs of life scientists who wish to better manipulate smaller biological samples. Devices with subwavelength features are excellent platforms for shaping local electric fields for this purpose. A major factor that inhibits the manipulation of submicrometer particles is the diffraction‐limited spot size of free‐space laser beams. As a result, technologies that can circumvent this limit are highly desirable. This review covers some of the more significant advances in the field, from the earliest attempts at trapping using focused Gaussian beams, to more sophisticated hybrid plasmonic/metamaterial structures. In particular, examples of emerging optical trapping configurations are presented.

     

Theoretical Design of a Pump‐Free Ultrahigh Efficiency All‐Optical Switching Based on a Defect Ring Optical Waveguide Network

A theoretical design of a defect ring optical waveguide network is proposed to construct a pump‐free ultrahigh efficiency all‐optical switch. This switch creates ultrastrong photonic localization and causes the nonlinear dielectric in the defect waveguide to intensely respond. At its ON state, this material defect without Kerr response helps to produce a pair of sharp pass bands in the transmission spectrum to form the dual channel of the all‐optical switch. When it is switched to its OFF state, the strong Kerr response induced refractive index change in the high nonlinear defect waveguide strongly alters the spectrum, leading to a collapse of the dual channels. Network equation and generalized eigenfunction method are used to numerically calculate the optical properties of the switch and obtain a threshold control energy of about 2.90 zJ, which is eight orders of magnitude lower than previously reported. The switching efficiency/transmission ratio exceeds 3× 10¹¹, which is six orders of magnitude larger than previously reported. The state transition time is nearly 108 fs, which is approximately two orders of magnitude faster than the previously reported shortest time. Furthermore, the switch size can be much smaller than 2.6 µm and will be suitable for integration.     

Real‐Time Operation of Photovoltaic Optoelectronic Tweezers: New Strategies for Massive Nano‐object Manipulation and Reconfigurable Patterning

Optical and optoelectronic techniques for micro‐ and nano‐object manipulation are becoming essential tools in nano‐ and biotechnology. Among optoelectronic manipulation platforms, photovoltaic optoelectronic tweezers (PVOTs) are an emergent technique that are particularly successful at producing permanent nanoparticle microstructures. New strategies to enhance the capabilities of PVOT, based on real‐time operation, are investigated. This optoelectronic platform uses z‐cut LiNbO₃:Fe substrates under excitation by a Gaussian light beam. Unexpected results show that during illumination, metallic particles previously deposited on the substrate are ejected from the light spot region. This behavior differs from the trapping phenomenon observed in previous work on PVOT operation, using a sequential method in which illumination is prior to particle manipulation. To discuss the results, a novel mechanism of charge exchange between particles and the ferroelectric substrate is proposed. Applications of this repulsion behavior are investigated. On the one hand, either particle repulsion or trapping in the illuminated region can be obtained by simply light switching on/off. On the other hand, by moving the light spot, different kinds of arbitrarily shaped tracks along the light path, either empty or filled with particles, are obtained. The results demonstrate new key capabilities of PVOT, such as pattern drawing, erasure, and reconfiguration.     

Raman to the limit: tip‐enhanced Raman spectroscopic investigations of a single tobacco mosaic virus

The development of fast identification techniques of viruses is an ongoing important research topic. Conventional virus detection and identification is generally based on various different microbiological methods. However, these techniques are not suitable for the analysis of single virus particles. Therefore, our goal is to establish tip‐enhanced Raman scattering (TERS), providing vibrational spectroscopic information with a spatial resolution less than 50 nm, to characterize single viruses at a molecular level. Here we report, to the best of our knowledge for the first time, about TERS spectra of a tobacco mosaic virus, showing the great capability of this technique. However, the application of the TERS technique for a rapid and direct detection of different species of single viruses is under development, which is useful for a wide range of analytical fields. Copyright © 2008 John Wiley & Sons, Ltd.     

Ultrafast on‐Chip Remotely‐Triggered All‐Optical Switching Based on Epsilon‐Near‐Zero Nanocomposites

On‐chip‐triggered all‐optical switching is a key component of ultrahigh‐speed and ultrawide‐band information processing chips. 1 - 4 This switching technique, the operating states of which are triggered by a remote control light, paves the way for the realization of cascaded and complicated logic processing circuits and quantum solid chips. Here, a strategy is reported to realize on‐chip remotely‐triggered, ultralow‐power, ultrafast, and nanoscale all‐optical switching with high switching efficiency in integrated photonic circuits. It is based on control‐light induced dynamic modulation of the coupling properties of two remotely‐coupled silicon photonic crystal nanocavities, and extremely large optical nonlinearity enhancement associated with epsilon‐near‐zero multi‐component nanocomposite achieved through dispersion engineering. Compared with previous reports of on‐chip direct‐triggered all‐optical switching, the threshold control intensity, 560 kW/cm², is reduced by four orders of magnitude, while maintaining ultrafast switching time of 15 ps. This not only provides a strategy to construct photonic materials with ultrafast and large third‐order nonlinearity, but also offers an on‐chip platform for the fundamental study of nonlinear optics.