Mid-infrared microlenses fabricated by the melting method

It is shown that by using the melting method it is possible to fabricate microlenses that work with mid-IR radiation. The fabrication method is described, and the characteristics of the polymeric material and the fabricated lenses are shown.     

两种离子交换型变折射率微透镜

分别求得制作自聚焦平面微透镜的开孔式和掩盘式离子交换的相应扩散方程的解,导出掩盘式微透镜的最佳折射率分布,给出微透镜的成像矩阵,制得掩盘式平面微透镜阵列并给出其成像照片及基本参数.     

Printing of polymer microlenses by a pyroelectrohydrodynamic dispensing approach

The investigation of a method for fabricating microlenses by a nozzle-free inkjet printing approach is reported. The new method, based on a pyroelectrohydrodynamic mechanism, is also able to dispense viscous liquids and to draw liquid phase drops directly from the reservoir. Specifically, by dispensing optical grade polymer dissolved in different solvent mixtures, microlenses were printed with a pattern defined directly through this deposition method. The reliability of the microlenses and the tunability of their focal properties were demonstrated through profilometric and inteferometric analyses.     

Design of low-loss tapered waveguide by applying photonic-crystal-based microlenses in telescopic structure

In this paper, the design of low-loss tapered waveguides by applying photonic-crystal-based microlenses is presented. The microlenses are realized by using two-dimensional photonic crystal (PhC) structures with a hexagonal periodic lattice of air-holes. In order to make the PhC structures behave as homogeneous and isotropic microlenses, the PhC structures are designed to operate in the low-frequency domain with light wavelength sufficiently larger than the PhC lattice constant. The effective indices of the PhC are calculated and the structures are properly arranged in microlenslike shapes. By monolithically integrating the effective microlenses in the tapered area between the two optical waveguides with different core dimensions, the wavefront of the eigenmode can be coupled from the larger core to the smaller core effectively and also the radiation loss in the tapered area between the two connected optical waveguides can be effectively suppressed. The results show that the PhC-based microlens in the telescopic structure can serve as an optical element in the tapered waveguide with the advantages of low loss, high efficiency and compactness.     

Focusing performance of the closed-boundary cylindrical microlenses analyzed by the boundary element method

In this paper, we investigate the focusing performance of closed-boundary cylindrical microlenses (CBCMs) based on rigorous electromagnetic theory and the boundary element method. The CBCMs with different incident angles, different quantization-level numbers, different microlens diameters, different f-numbers, and different polarizations of incidence are studied. Several focusing performance measures, such as the focal spot size, the diffraction efficiency, the real focal position, and the normalized transmitted power, are presented. It provides very useful information in designing the CBCMs in micro-optical systems.     

Coherent thermal emission by microstructured waveguides

Following Greffet et al. [Coherent emission of light by thermal sources. Nature 2002;416:61-4], we study in this article, the possibility to engineer thermal coherent sources with waveguides. The idea is to rule a grating on a waveguide made of a slice of germanium deposited on a participating media such as glass. The guided waves, thermally excited are coupled to the far field by the grating and increase the system emissivity in certain directions and wavelengths. We numerically compute the diffraction of a plane wave by the grating by a rigorous coupled waves algorithm (RCWA). The reflected, transmitted and absorbed energy calculated allows to obtain the system emissivity by means of the Kirchhoff law.     

Packing density of conventional waveguides and photonic crystal waveguides

We compare coupling between parallel waveguides within one-dimensional photonic crystals and coupling between conventional waveguides. We consider the situation in which coupling between the waveguides is minimized, so that light in the waveguides propagates essentially independently. Subject to this condition, we compare the minimum mutual distance between conventional planar waveguides and waveguides in one-dimensional photonic crystals. We find that the packing densities of the conventional and periodic structures are comparable.     

Dual-gated BN-sandwiched multilayer graphene field-effect transistor fabricated by stamping transfer method and self-aligned contact

To fabricate a BN-sandwiched multilayer graphene field-effect transistor, we developed a self-aligned contact scheme in combination with optimized stamping processes for the stacking of two-dimensional (2D) materials. By using a self-aligned contact method during device fabrication, we can skip the dry-etch process which requires an exact etch-stop at the surface of the graphene layer and is not easy to control. In the structure of a dual-gate transistor, successful device operation at low temperature with and without magnetic fields proves that the self-alignment contact can be an effective tool for reliable device fabrication using 2D materials.     

Optical and structural characterization of a self-aligned single electron transitor structure by cathodoluminescence microscopy

We report about optical and structural investigations of a self-aligned single electron transistor (SET) structure using cathodoluminescence-(CL) and transmission electron microscopy (TEM). The SET structures were fabricated by MBE growth of GaAs/AlAs on different prepatterned GaAs (1 0 0) substrates. This technique for the in situ formation of nanoscopic semiconductor heterostructures is presently a widely used and promising approach for the fabrication of low-dimensional systems like quantum wires and quantum dots (QD). The active region of the SET structure consists of a GaAs/AlGaAs-QD formed by thickness modulation of a single quantum well (SQW) during the MBE growth. The position and the size of the QD is defined by the design of the substrate pattern. The thickness modulation of the GaAs-SQW is evidenced by TEM investigations. The lateral confinement potential given by the thickness modulation of GaAs-SQW is directly imaged by CL microscopy.     

Linear and nonlinear waveguides induced by optical vortex solitons

We study, numerically and analytically, linear and nonlinear waveguides induced by optical vortex solitons in a Kerr medium. Both fundamental and first-order guided modes are analyzed, as well as cases of effective defocusing and focusing nonlinearity.     

Proton-exchanged optical waveguides fabricated by glutaric acid

In this paper, we first report that a new proton source, glutaric acid, has been used to fabricate optical waveguides in Z-cut lithium niobate crystals. The relationship was experimentally established between proton-exchanged (PE) waveguide parameters and fabrication conditions. It is shown that this new organic acid can be used to obtain deep PE waveguides in fast diffusion speed (0.275 μm²/h at 221°C) and with low loss (0.2 dB/cm). It provides an alternative approach for fabricating PE waveguides in lithium niobate substrate.     

Magnetic behaviour of arrays of Ni nanowires by electrodeposition into self-aligned titania nanotubes

Arrays of Ni nanowires electrodeposited into self-aligned and randomly disordered titania nanotube arrays grown by anodization process are investigated by X-ray diffraction, SEM, rf-GDOES and VSM magnetometry. The titania nanotube outer diameter is about 160 nm, wall thickness ranging from 60 to 70 nm and 300 nm in depth. The so-obtained Ni nanowires reach above 100 nm diameter and 240 nm length, giving rise to coercive fields of 98 and 200 Oe in the perpendicular or parallel to the nanowires axis hysteresis loops, respectively. The formation of magnetic vortex domain states is also discussed.     

Two-dimensional X-ray waveguides: fabrication by wafer-bonding process and characterization

The fabrication of two-dimensionally confining X-ray waveguides enables the generation of nanoscopic X-ray beams. First applications of such waveguides for lens-less holographic imaging have already been demonstrated, but were limited by the fabrication methods and the design. To overcome these limitations, we present here the fabrication process for a second generation of X-ray waveguide with air or vacuum as guiding channel, based on e-beam lithography, ion etching and subsequent wafer bonding. This is a first step towards waveguides fulfilling requirements of high transmission and high confinement, since the process can be scaled down to smaller channel dimensions from the present structures. We address the structuring method used and present results of first X-ray characterization at synchrotron beamlines, under two entirely different beam settings, corresponding to the coupling of a coherent beam and an incoherent beam. PACS 41.50.+h; 42.82.Cr; 07.85.Qe     

Image formation by phase coincidences in optical waveguides

A thick dielectric slab of uniform refractive index, acting as a multimode optical waveguide, can produce real optical images provided its length L is chosen so that the phase changes suffered along L by the various propagating modes differ by integer multiples of φ or of 2φ. For slab of thickness W, refractive index n, and light of wavelength λ, this condition is satisfied if L ? 4hnW²/λ, where h = 1, 2, 3, …. The formation of the images is described theoretically and demonstrated experimentally.     

微细笔直写聚合物条形光波导的研究

提出一种采用微细笔直写技术直写聚合物光波导的简便方法，利用该技术成功直写出了氟化聚酰胺条形光波导，并着重研究了气体压力和直写速度对波导宽度的影响规律。实验结果表明：直写获得的条形波导的边缘整齐、表面光滑，而且不存在小孔等缺陷或杂质。当其他工艺参数不变的情况下，波导宽度随着气体压力的增加而增大，随着直写速度的增大而减小。微细笔直写获得的条形光波导成功地实现了通光，在波长为1550nm处的最小传输损耗为0.59dB/cm。     

Acoustical scattering by arrays of cylinders in waveguides

Multiple scattering of acoustic waves in a planar horizontal waveguide by finite-length cylinders is considered. Cylinder height equals the waveguide depth, and both are vertically constrained by the pressure-release boundaries. An analytically exact solution is obtained via normal mode expansion method in conjunction with the concept of the T matrix. The problem is decomposed into an infinite number of two-dimensional multiple scattering problems, modulated by waveguide mode shapes. Examples are presented for an isovelocity waveguide in which the medium is uniform and the waveguide depth is constant. It is found that, in numerical computations, including one or two evanescent modes captures the essence of the evanescent modes. Multiple scattering in the waveguide is compared with the corresponding two-dimensional case. It is concluded that, in low frequencies, the wave patterns in the two cases are very similar, with a shift in the frequency. The similarity diminishes when there are multiple propagating modes. Despite the mode mixing, some key features in the scattering as observed in the two-dimensional problem remain observable in the waveguide case.     

Characterization of polymer strip waveguides by leaky-mode spectroscopy

Strip waveguides, recorded by UV lithography into photopolymer films were characterized as leaky guides. The effective modes indices of the guides have been measured by the prism coupling method. It is demonstrated that application of the effective index method on leaky strip guides allows the determination of the optical pattern parameters throughout an exposure or annealing procedure.     

Reflectionless evanescent-wave amplification by two dielectric planar waveguides

Utilizing the underlying physics of evanescent-wave amplification by a negative-refractive-index slab, it is shown that evanescent waves with specific spatial frequencies can also be amplified without any reflection simply by two dielectric planar waveguides. The simple configuration allows one to take advantage of the high resolution limit of a high-refractive-index material without contact with the object.     

Improvements in diffraction efficiency of gratings and microlenses with continuous relief structures

The fabrication of diffractive optical elements by laser-beam writing or gray-tone technology leads to continuous-relief phase elements. The diffraction efficiency of such elements is limited by the resolution of the process. In this paper, we compare the continuous-relief elements with the multilevel elements fabricated by binary technology. In particular, we will show that for similar sampling resolution of the ideal phase function, the continuous profiles have higher efficiencies than the multilevel profiles if the designed phase modulation is 4π and more.     

Quasi-solitonic behavior of self-written waveguides created by photopolymerization

We investigated the condition of unique self-written channel and multichannel propagation inside bulk photopolymerizable materials. Light was introduced in the medium by a single-mode optical fiber. At a very low beam power of 5 muW , a unique uniform-channel waveguide without any broadening was obtained by polymerization. When the input power is increased to 100 muW , the guide becomes chaotic and multichannel. We connected two fibers separated by a 1-cm distance. The results open the door to studies of the optical and electro-optical properties of photopolymerized guides doped by nonlinear optical chromophores and to possible applications in integrated optical devices.