一维亚波长周期结构物成像的光子学研究

当用波长为λ的单色平行光垂直照射到光栅常数d<λ的亚波长周期结构衍射物时,会产生隐失波。由于快速衰减,这种亚波长周期物是不能成像的。但是,通过对亚波长周期结构物进行适当的编码后可得到均匀波,使这种携带了隐失波信息的均匀波通过经特别设计的光学系统并被放大到CCD所能识别的像素大小后,再进行必要的解码以滤掉编码波,即可得到原亚波长周期结构衍射物的像,最终达到超分辨的目的。基于这种新颖的成像技术,用常规仪器就实现了对亚波长周期结构物的成像。在运用光子学方法对实验和成像过程进行较为详尽的理论分析的同时,对编码器、解码器的位置以及它们相对物光栅的取向给出了设计性研究,对滤波器的选择给予了必要的说明。实验结果验证了该设计理论的正确性。     

Optical Trapping with Focused Surface Waves

Near-field optical trapping can be realized with focused evanescent waves that are excited at the water–glass interface due to the total internal reflection, or with focused plasmonic waves excited on the water–gold interface. Herein, the performance of these two kinds of near-field optical trapping techniques is compared using the same optical microscope configuration. Experimental results show that only a single-micron polystyrene bead can be trapped by the focused evanescent waves, whereas many beads are simultaneously attracted to the center of the excited region by focused plasmonic waves. This difference in trapping behavior is analyzed from the electric field intensity distributions of these two kinds of focused surface waves and the difference in trapping behavior is attributed to photothermal effects due to the light absorption by the gold film.     

Focusing of evanescent vector waves

We study focusing of two and three-dimensional evanescent vector waves, with a particular emphasis on identifying suitable intensity structures for applications in optical data storage. For two-dimensional evanescent waves large transverse spatial wave vectors result in purely circularly polarized evanescent states. We suggest that these may have applications in all-optical data storage through the inverse Faraday effect. On the other hand, for three-dimensional evanescent waves longitudinally polarized modes are observed to give the most tightly focused spot, and this may be utilized to confine light behind a solid immersion lens.     

采用消逝波干涉的二维表面微光阱阵列

提出了一种采用两套超大红失谐消逝波干涉和一束蓝失谐消逝波光场来实现原子二维表面微光阱阵列和原子有效强度梯度冷却的新方案,得到了二维表面微光阱阵列的光强分布和光学势分布.研究发现,二维表面微光阱阵列中微光阱的光学势能够有效地囚禁从标准磁光阱中释放的冷原子,并且被囚禁的冷原子能在蓝失谐消逝波光场的作用下产生有效的强度梯度Sisyphus冷却,对⁸⁷Rb原子而言,原子温度能被冷却到2.56μK.该方案在冷原子物理、原子光学和量子光学领域中有着广阔的应用前景. 关键词： 消逝波干涉 微光阱阵列 原子囚禁 强度梯度冷却     

The role of propagating and evanescent waves in scanning near-field optical microscopy

Theoretical study on the image formation in scanning near-field optical microscopy is carried out in the framework of the direct moment method. Information brought, respectively, by the propagating and evanescent components in the optical near field that is collected by a scanning fiber tip with a sub-wavelength aperture is numerically and systematically analyzed in the light of the resolution achieved by the microscope. The analyses reveal that resolutions beyond the diffraction limit can be achieved even in the absence of the evanescent waves. That is, it is incorrect or at least incomplete to believe that a microscope that collects only the propagating waves is limited by the diffraction. Our studies show that a scanning near-field optical microscope can achieve resolutions beyond the diffraction limit by collecting only the propagating waves.     

Multilevel phase Fresnel zone plate lens as a near-field optical element

We propose and develop a new solid immersion lens (SIL), which is called the multilevel phase Fresnel zone plate lens (FZPL) for the near-field (evanescent wave) microscopy. The simple analysis is presented by using the scalar diffraction theory. The outstanding advantages of this FZPL are that it both focuses incident waves and produces evanescent waves. A FZPL can effectively concentrate the high angle rays important for the high resolution in comparison with the conventional SILs. The optical system equipped with the FZPL is not only simple in the assembly but also effective in making an optical head unit.     

Linearly polarised three-colour lasing emission from an evanescent wave pumped and gain coupled fibre laser

A novel Whispering--Gallery--Mode (WGM) fibre laser, emitting linearly polarised three-colour light, is demonstrated by pumping and gain coupling with evanescent waves. The pump light is longitudinally coupled into a bare optical fibre immersed in a dye solution of lower refractive index. The dye molecules around the bare fibre are excited by the evanescent waves of pump light when they propagate along the fibre in a total internal reflection. When the pump beam within the fibre is a meridian beam, the WGM lasing emission from the fibre laser is a linearly polarised transverse electric wave, while it is a mixed wave of the linearly polarised transverse electric and magnetic waves if the pump beam is a skew beam within the fibre. Because the excited molecules are located within the evanescent field of WGM, a good spatial overlap between the dye gain and the evanescent field leads to a high pumping efficiency and a longer gain distance along the fibre. Once the bare fibre is inserted into three glass capillaries filled with Rhodamine 6G, 610 and 640 dye solutions, respectively, WGM laser oscillations at the wavelengths of 567--575, 605--614 and 656--666~nm occur simultaneously, and a linearly polarised three-colour lasing emission is achieved in a single optical fibre.     

Transmission enhancement by conversion of evanescent waves into propagating waves

The convolution between spatial modes of two different parts of an optical system can convert evanescent waves into propagating waves. This principle is applied to different optical systems for analyzing various effects in transmission enhancements experiments. We discuss here the differences between the present principle which is related to broadening of resonances and the near-field optical microscopy based on a tunneling effect by a tip detector. The present analysis is applied in particular to two systems: a) transmission enhancement in one slit by coupling the transmitted radiation with transversal Fabry–Pérot electromagnetic (EM) modes, and b) transmission enhancement by coupling between a metallic film with arrays of holes and surface plasmons (SP). The present approach gives more information on transmission enhancement phenomena than that obtained by conventional treatments and can also solve certain disagreements between different theories. The differences between the present process of converting evanescent waves into propagating waves, and that related to the new development of getting a super-resolution by an hyperlens are discussed. PACS 41.20.Jb; 73.20.Mf; 42.79.Dj     

Near-field characteristics of highly non-paraxial subwavelength optical fields with hybrid states of polarization

The vectorial structure of an optical field with hybrid states of polarization(So P) in the near-field is studied by using the angular spectrum method of an electromagnetic beam. Physical images of the longitudinal components of evanescent waves are illustrated and compared with those of the transverse components from the vectorial structure. Our results indicate that the relative weight integrated over the transverse plane of the evanescent wave depends strongly on the number of the polarization topological charges. The shapes of the intensity profiles of the longitudinal components are different from those of the transverse components, and it can be manipulated by changing the initial So P of the field cross-section. The longitudinal component of evanescent wave dominates the near-field region. In addition, it also leads to three-dimensional shape variations of the optical field and the optical spin angular momentum flux density distributions.     

Near-Field Scanning Optical Microscopy Combined with a Tapping Mode Atomic Force Microscope

Tapping mode atomic force microscopy is used to control the tip-sample distance in near field scanning optical microscopy (NSOM), which gives both topographic and near-field images simultaneously. The evanescent waves are scattered by a vibrating silicon-nitride tip in the proximity of sample surfaces and are detected through a microscope objective. This NSOM allows the observation of opaque samples with reflection illumination. A glass grating of 1-μm pitch and an InP grating of 0.5-μm pitch are observed with a lateral resolution of 100 nm.Presented at 1996 International Workshop on Interferometry (IWI ‘96), August 27-29, Saitama, Japan     

Multi-Slit Diffraction of Evanescent Electromagnetic Waves

The well-known Fraunhofer multi-slit diffraction is described as the multi-slit interference modulated by the singleslit diffraction, namely the multiplication between the single-slit diffraction factor and the multi-slit interference factor. By considering the simplified argument we show that the multi-slit diffraction of evanescent waves which are in the near-field region also has the interference and diffraction effects, and that this two-fold effect can be expressed as the convolution of the diffraction factor and the interference factor. Our conclusion could be helpful to understand the contribution of evanescent waves to the optical responses of sub-wavelength structures such as slits and grooves.     

Evanescent waves of an annular left-handed material lens

The imaging system formed by an annular left-handed material （LHM） lens as well as the evanescent waves in the lens are simulated numerically with a finite-difference time-domain （FDTD） method. For b - a 〉 λ （a and b are respectively the inner and outer radii of the annular lens, and λ is the wavelength）, when a point source is placed at an internal grid point, we demonstrate that the evanescent waves are produced around the internal interface, and cannot propagate outwards. As for b - a 〈λ ）,, the evanescent waves appear around both the internal and the external interfaces, which remarkably implies the coupling between the two interfaces. Hence it can be inferred that the evanescent waves around the external interface participating in the super-resolution imaging result from the coupling of the evanescent waves around the interface. Moreover, the partly uncomprehended properties of the evanescent waves in the LHM slab are also disclosed. It is conducive to understanding the evanescent waves in the LHMs further.     

Tunneling of evanescent waves into propagating waves

The tunneling of evanescent waves into propagating waves is related to the convolution of the high spatial frequencies of the source with those of the detectors. Such an approach is demonstrated by treating the evanescent waves which are diffracted from very narrow apertures in a plane screen (with dimensions much smaller than the wavelength) and are converted to propagating waves by tip detectors. The mechanism responsible for the conversion of evanescent waves into propagating waves is explained and a general formula for the conversion of evanescent waves into propagating waves is derived. PACS 42.25.Fx; 42.30.Kq; 42.25.Bs     

Near-field amplitude and phase measurements using heterodyne optical feedback on solid-state lasers

Heterodyne optical feedback on a solid-state laser is experimentally investigated as an efficient tool to characterize coherently near-field evanescent waves. A well-known topography of evanescent field is obtained via a total internal reflection of the light beam emitted by a class B Yb:Er glass laser. A subwavelength size optical fiber tip is scanned to locally probe the resulting evanescent wave in the near field. After a frequency shifting using a pair of acousto-optic modulators, the collected light is optically reinjected to excite the relaxation oscillations of the laser. The resulting dynamical response simultaneously allows very sensitive measurements of the amplitude and the phase of the evanescent wave. Extension of these preliminary results to near-field optical microscopy is suggested and discussed.     

Optimum coupling and efficiency of dye lasers

Evanescent waves and fields play an important role in microwaves, quantum mechanics, optics and elastic waves. Because electromagnetic waves in waveguides have dispersion characteristics similar to those of a unidimensional de Broglie wave, there is a close analogy between quantum mechanical tunneling and the transition through an attenuating sector of a waveguide. Microwave “evanescent mode” filters, quantummechanical tunneling resonance and optical frustrated total internal reflection filters are compared in light of this analogy. The flowgraph technique and “unit real” function representation are shown to be effective in discussing the interdependence of incident and reflected waves in various locations in multilayered structures. In electrognetic and elastic waves, the role of evanescent waves is significant in the case of incident beams of limited extent. The role of equivalent circuits of transverse resonance is discussed in the context of oscillatory natural modes determining the character of propagating fields.     

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.     

Near-Field Characterization of Optical Micro/Nanofibres

Near-field scanning optical microscopy is used to investigate the waveguiding properties of optical micro/nanofibres （MNFs） by means of detecting optical power carried by evanescent waves. Taper drawn silica and tellurite MNFs, supported on low-index substrates, are used to guide a 532-nm-wavelength light beam for the test. Modification of the single-mode condition of the MNF in the presence of a substrate is observed. Spatial modulation of the longitudinal field intensity （with a 195-nm period） near the output end of a 760-nm-diameter silica MNF is well resolved. Energy exchange through evanescent coupling between two parallel MNFs is also investigated.     

Optical forces acting on a nanoparticle placed into an interference evanescent field

We describe a general way how to calculate analytically optical forces acting on Rayleigh particles or colloids placed into interference field made by evanescent waves. In this paper we focus on a configuration with three interfering waves and we present a comprehensive analysis of optical trap positions, depths, and forces depending on the configuration and polarisation of the incident waves. Particle behaviour is predicted including optical sorting according to the particle refractive index.     

Superresolution of three-dimensional optical imaging by use of evanescent waves

We simulate a three-dimensional optical diffraction tomography experiment in which superresolution is achieved by illuminating the object with evanescent waves generated by a prism. We show that accounting for multiple scattering between the object and the prism interface is mandatory to obtain superresolved images. Because the Born approximation leads to poor results, we propose a nonlinear inversion method for retrieving the map of permittivity of the object from the scattered far field. We analyze the sensitivity to noise of our algorithm and point out the importance of using incident propagative waves together with evanescent waves to improve the robustness of the reconstruction without losing the superresolution.