Single‐mode distributed feedback laser operation with no dependence on the morphology of the gain medium

Organic distributed feedback (DFB) lasers can be useful photonic tools for biological applications where the roles of organic materials are important, because highly coherent single mode emission with broad tuning range can be obtained. However, the formulaic structures of organic lasers, and the uses of gain media as resonators themselves, are not suitable for inducing laser emission from irregular shaped gain media, such as dye‐staining cells and tissues. Here, we report a reusable photonic template comprising an exceedingly thin and discrete titanium dioxide (TiO₂) layer on a one‐dimensional (1D) quartz grating to induce single mode DFB lasing from a variety of states of optical gain media. Using the same template, the external gain media of optically thick and thin casted film, liquid, and a free‐standing thick film reveal single mode lasing with reliable performance. Numerical simulations demonstrate that the 25‐nm thick TiO₂ disconnected grating lines support a spatially confined DFB mode in the vertical direction, even under no index difference between superstrate and substrate. Additionally, not using the typical waveguide gain layer promises high sensitivity and detection limit in refractometric sensing. These results suggest that the photonic structure may serve as a versatile sensing platform for bioapplications.     

TiO<Subscript>2</Subscript> microstructures by inversion of macroporous silicon using atomic layer deposition

An approach is presented which is capable of fabricating arbitrarily shaped three-dimensional microstructures. Two methods—namely, macroporous silicon and atomic layer deposition—are combined to realize structures in the micrometer and submicrometer range. Using TiO₂ as an example, the fabrication of single hollow objects as well as complex network structures is shown. The scalability and the wide range of applicable materials are the key points of this method for future applications.     

Characterization of corrugated feed horns at 216 and 300 GHz

We present measurements of the near-field beam profiles of two corrugated feed horns with center frequencies of 216 and 300 GHz. Both co-polar and cross-polar components of the electric field have been probed across the operation band of the horns. A full-wave numerical near-field transformation, based on a vector Kirchhoff integral, is implemented to transform the measured field to the horn aperture for a comparison with the ideal aperture field. From the overlap integral of the transformed and the ideal field, we conclude that the field radiated by these horns is the HE₁₁ mode, to a purity of better than 99%.     

Corrugated waveguide band edge filters for CMB experiments in the far infrared

Millimetre wave corrugated waveguide-horn structures are used as both single-moded and multi-moded bolometer feeds in a number of cosmic microwave background (CMB) experiments (e.g. PLANCK, Archeops, QUaD). Such horns tend to be employed over a relatively wide bandwidth and for single-moded horns the waveguide acts as the high pass filter. In this paper we report on our investigation on how the waveguide details determine the exact location of the low frequency band edge of such corrugated horns. A sharp step-like band edge, below which there is negligible propagation, is ideally required. Furthermore any leakage below the expected cut-off, possible in corrugated guides, could lead to non-idealised cross-polar effects. Typically deeper corrugations are required in the waveguide filter than at the horn aperture for wide bandwidth operation, thus necessitating a transition section over which the corrugation depth smoothly varies. An electromagnetic mode matching technique and a surface impedance hybrid mode model are used to compute the horn transmission characteristics. We have also undertaken laboratory measurements of the band edge of prototype corrugated horns in order to test the models.     

Corrugated waveguide band edge filters for CMB experiments in the far infrared

Millimetre wave corrugated waveguide-horn structures are used as both single-moded and multi-moded bolometer feeds in a number of cosmic microwave background (CMB) experiments (e.g. PLANCK, Archeops, QUaD). Such horns tend to be employed over a relatively wide bandwidth and for single-moded horns the waveguide acts as the high pass filter. In this paper we report on our investigation on how the waveguide details determine the exact location of the low frequency band edge of such corrugated horns. A sharp step-like band edge, below which there is negligible propagation, is ideally required. Furthermore any leakage below the expected cut-off, possible in corrugated guides, could lead to non-idealised cross-polar effects. Typically deeper corrugations are required in the waveguide filter than at the horn aperture for wide bandwidth operation, thus necessitating a transition section over which the corrugation depth smoothly varies. An electromagnetic mode matching technique and a surface impedance hybrid mode model are used to compute the horn transmission characteristics. We have also undertaken laboratory measurements of the band edge of prototype corrugated horns in order to test the models.     

A novel prism beam-shaping laser diode bar end-pumped TEM<Subscript>00</Subscript> mode Nd:YVO<Subscript>4</Subscript> laser

A beam-shaping diode end-pumped TEM₀₀ mode CW Nd:YVO₄ laser is presented. A special beam-shaping element made up of isosceles right-angled prism pieces is adopted to realize beam symmetric. Two cylinder lenses are used to couple the shaped beam into a 3 × 3 × 9 mm Nd:YVO₄ crystal with 0.3 at % neodymium doping. By using this laser system, we have achieved 6.1 W CW laser operated in single transverse mode at 1064 nm with 95.2% reshaping efficiency and 25.6% optical-optical conversion efficiency.     

Pulse shaping control of alignment dynamics in N2

Control on the alignment transients of impulsively aligned ensembles of N₂ molecules has been demonstrated by the use of laser pulses shaped by a spatial light modulator. An alignment experiment has been inserted in the feedback loop of an evolutionary algorithm that found optimum pulse shapes for a set of criteria. Optimum pulse shapes for the maximization of total alignment and for the control of certain aspects of the revival structures are given. The physical mechanisms responsible for the control are analysed with the help of single‐parameter control schemes and numerical simulations, which allowed us to explore the low‐temperature region. This approach sheds light on the role played by different control mechanisms for the alignment dynamics of a molecular ensemble. Copyright © 2006 John Wiley & Sons, Ltd.     

Acoustic particle velocity horns

The paper considers receiving acoustic horns designed for particle velocity amplification and suitable for use in vector sensing applications. Unlike conventional horns, designed for acoustic pressure amplification, acoustic velocity horns (AVHs) deliver significant velocity amplification even when the overall size of the horn is much less than an acoustic wavelength. An AVH requires an open-ended configuration, as compared to pressure horns which are terminated at the throat. The appropriate formulation, based on Webster's one-dimensional horn equation, is derived and analyzed for single conical and exponential horns as well as for double-horn configurations. Predicted horn amplification factors (ratio of mouth-to-throat radii) were verified using numerical modeling. It is shown that three independent geometrical parameters principally control a horn's performance: length l, throat radius R(1), and flare rate. Below a predicted resonance region, velocity amplification is practically independent of frequency. Acoustic velocity horns are naturally directional, providing maximum velocity amplification along the boresight.     

Linear Dispersion Relation of Asymmetric Modes in the Axisymmetric Slow-Wave Structure

The linear dispersion relation of asymmetric electromagnetic modes in the corrugated-wall waveguide is derived and calculated numerically. The numerical analyses indicate that the effects of lower asymmetric modes on the main interaction mode TM_m must be considered and mode selection also must be solved in order to realize the single asymmetric mode operation of the oversized structures.     

Quasi-optical waveguide-mode converters using a pair of phase correction mirrors

This paper describes a quasi-optical method for the conversion of modes transmitted through highly oversized circular waveguides. A waveguide-mode is radiated once from a waveguide cut in the form of a radiation beam, which is then properly shaped by two curved mirrors and directed back into the waveguide. The curved mirror shapes are iteratively and automatically determined for given propagation distances using the design technique for phase correction mirrors. The proposed method gives favorable results in designing a waveguide expander/reducer, a TE₀₁-TE₀₂ mode converter, and a TE₀₁-HE₁₁ mode converter.     

神光-Ⅲ主机黑腔腔形研究与设计

利用三维视角因子软件IRAD3D研究了神光-Ⅲ主机靶丸表面辐射驱动时变对称性和驱动强度随黑腔腔形和排布方式的变化。模拟结果表明:采用椭球形黑腔能显著减小黑腔壁面积从而减小腔壁能量消耗并提高靶丸消耗份额;不同腔形和排布方式下均可通过调节腔长使勒让德不对称性模P2时间积分量较小;采用双环辐照和椭球腔形都利于缩短腔长;四环排布利于调节P4分量,但环向不对称性M4较差;椭球腔双环排布能兼顾极向不对称性P2,P4和环向不对称性M4,且靶丸表面X光驱动强度相对柱腔双环提高25%~27%。     

Lasing of a single crystal Nd fluoride fiber

We present laser action in a single crystal fluoride fiber obtained by the micro-pulling-down technique. The 700 μm diameter and 1 cm long Nd:LiYF₄ fiber was pumped by a beam shaped diode bar emitting at 806 nm. A peak power of 300 mW was achieved with a slope efficiency of 12%. When considering the pump power fraction absorbed by the laser mode, a slope efficiency of 37% was achieved. To the best of our knowledge, this is the first report of a Nd: LiYF₄ fiber laser.     

Probing crystallographic orientation of a single GaN nanotube using polarized Raman imaging

We report the study of crystallographic orientation of a highly anisotropic square‐shaped single wurtzite GaN nanotube (NT) probed by polarized Raman imaging. The polarization‐dependent Raman intensities are collected in two mutually orthogonal directions with respect to the NT long axis. Variation in intensities of symmetry allowed A₁(LO) mode of GaN with two different polarization directions reveals the possible crystalline orientations in the NT. The polarized Raman spectral imaging illustrates the fact of inhomogeneous crystalline orientations along the edges of the NT facets owing to its finite wall thickness. The deviation of polarization selection rule for GaN is attributed to the geometrical anisotropy of the NT. Electron microscopic structural data confirms the spectral imaging analyses. Copyright © 2013 John Wiley & Sons, Ltd.     

Nanointegration of ZnO with Si and SiC

The study is dedicated to some aspects of the controlled heteroepitaxial growth of nanoscaled ZnO structures and an investigation of their general and dimension mediated properties. ZnO nanostructures were synthesized by optimized MOCVD process via two growth approaches: (i) catalyst free self-organized growth of ZnO on Si substrates and (ii) ZnO heteroepitaxy on p-type hexagonal 4H-SiC substrates. The SiC substrate was prepared by sublimation epitaxy and served as a template for the ZnO epitaxial growth. The epitaxial growth of n-ZnO on p-SiC resulted in a regular matrix of well-faceted hexagonally shaped ZnO single crystals. The achievement of ZnO integration with Si encompasses controlled growth of vertically oriented nanosized ZnO pillars. The grown structures were characterized by transmission electron microscopy and microphotoluminescence. Low concentration of native defects due to a stoichiometry balance, advanced optical emission, (excitonic type near-band-edge emission and negligible defect related luminescence) and continuous interfaces (epitaxial relationship ZnO_[0 0 0 1]/SiC_[0 0 0 1]) are evidenced. The ZnO nanopillars were further probed as field emitters: the grown structures exhibits advanced field emission properties, which are explained in term of dimensionality and spatial uniformity of the nanopillars. The present results contribute to understanding and resolving growth and device related issues of ZnO as a functional nanostructured material.     

全光纤多种子激光脉冲产生系统

报道一种可精确同步输出宽带啁啾脉冲、纳秒级整形脉冲以及窄带光参量啁啾放大抽运脉冲的全光纤多种子激光脉冲产生系统.采用掺镱光纤锁模激光器和掺镱单纵模光纤振荡器作为系统的光源,将掺镱光纤锁模激光器输出的超短脉冲分束啁啾展宽至0.9ns后,一路进行放大产生10μJ量级的宽带啁啾脉冲为高能拍瓦激光器系统提供种子光脉冲,另一路通过1.2nm滤波产生140ps的基元脉冲,通过光纤堆积器产生可编程整形的2.3ns脉冲,再经过放大达到10μJ量级,提供任意整形的压缩脉冲.同时,将部分掺镱光纤锁模激光器输出的超短脉冲进行光电转换并锁相后,产生与锁模超短脉冲高精度同步的电脉冲用于触发幅度调制器,将掺镱单纵模光纤振荡器输出的连续光削波放大,产生光参量啁啾放大抽运脉冲.该系统能够根据物理实验的需要,非常灵活地在输出各种脉冲之间做出选择.     

有效抑制光子晶体加载矩形谐振腔中模式竞争的方法

研究了光子晶体加载矩形谐振腔,针对传统光子晶体加载无法完全抑制模式竞争的问题,提出了特性阻抗和介质加载方法进行改进,完全实现了谐振腔的高次模式的单模工作,为抑制高次模式工作的谐振腔中模式竞争的问题提供了一种新方法.在此基础之上,采用解析方法以及高频仿真软件HFSS,设计了一个采用TM₅₃₀高次模式工作的传统光子晶体加载的谐振腔,进一步采用了该新方法对其进行改进,并深入分析了采用这种方法抑制模式竞争的物理原因.计算结果表明,新方法能有效抑制模式竞争,实现谐振腔的高次工作模式的单模工作. 关键词： 光子晶体 谐振腔 模式竞争 特性阻抗     

Wideband slow light achievement in MIM plasmonic waveguide by controlling Fano resonance

In this paper, we have investigated the characteristics of an asymmetric shaped Fano line in a metal–insulator–metal (MIM) plasmonic waveguide side coupled to two resonating stub structures. The spectral properties of Fano resonance are quite distinct due to the destructive interference between a two propagating plasmon modes. Two structural parameters are carefully adjusted: physical separation between both the resonating stubs and length of resonating stubs. By tailoring the separation between both the resonating structures, coupling between both the plasmon modes is controlled, and hence asymmetric nature of Fano line can be shaped accordingly. Resonance condition of Fano line can be tuned by scaling the length of stubs. A strong red shift in resonating wavelength with varying degree of asymmetry is observed, when length of resonating structures is increased. The sharp resonant peak, due to an asymmetric shaped Fano resonance is generally accompanied by large dispersion that results in reduction of group velocity of light near Fano resonance. By controlling the coupling between resonating stub, or by scaling the length of lower resonating stub, large value of group index (n_g = 75) and delay bandwidth product (DBP = 0.2533) is obtained. The structure can be modified to suit different applications in optical buffers, optical switches and nonlinear optics devices.     

Study of optical output couplers for submillimeter wavelength backward-wave oscillators (BWO's)

The machining of slow wave structures for high frequency BWO's is extremely difficult beyond l THz. Recently a microfabrication technique using photolithography and ion-beam assisted etching has been used to construct a prototype BWO operating at 200 to 265 GHz.The output coupler for such tubes remains a problem. Waveguides do not exist or are very lossy at the frequencies of interest (300 to 2000 GHz).This paper discusses several scaled experiments of optical output couplers for submillimeter BWO's. Various designs of planar antennas (Vivaldi horns) and lens-feed systems (hyperhemispherical lens) were constructed and tested between 20 and 100 GHz using a spectrum analyzer. The lens system was also tested at 337 GHz using a CO₂ pumped FIR laser.     

Raman studies of InAs/In0.53Ga0.47As single quantum wells grown on InP substrate by M.B.E.

We analyzed using the Raman technique a series of single quantum wells of InAs/In _0.53Ga_0.47As at different thicknesses of InAs layer grown on a (100) InP substrate by MBE. These high lattice mismatch systems are particularly interesting for potential applications in the mid-IR wavelength range. The well thickness was between 6 and 12 monolayers. The In _0.53 Ga_0.47As grown on an InAs layer is subject to a tensile biaxial strain and the InAs to a compressive one. In the Raman spectra we observed an intense narrow line corresponding to the LO phonon of the InAs layer between a GaAs-like LO mode and a smaller InAs-like LO phonon typical of In_0.53Ga_0.47As. With the increase of the well thickness the experimental energy shift of the LO phonon of the InAs layer decreases, indicating a smaller strain, whereas the GaAs-like LO phonon of the alloy remains constant and the intensity ratio of these two modes becames smaller. The dominant and sharp features of the InAs LO and GaAs-like LO characterize the good quality of our structures. With the increase of the InAs layer thickness we also observed the appearance and the intensity rise of a weak peak around the frequency of the InAs TO mode. This peak could be associated with the TO mode that is forbidden in our scattering geometry. We believe that this is indicative of a slight deterioration of the structural perfection of the sample with the increase of the well thickness. To our knowledge, this is the first study of vibrational properties of InAs/In _0.53 Ga_0.47As single quantum wells grown on InP substrates.     

RECIPROCITY IN SIMULATIONS OF BOLOMETRIC DETECTORS IN TRANSMITTING MODE

Bolometric detectors used in radio-astronomical instruments such as the ESA Planck Surveyor, operate as incoherent receivers of electromagnetic radiation. Meanwhile, the optical (quasi-optical) parts of the instruments (feed horns, telescope mirrors) are conventionally designed in transmitting mode when a coherent source of radiation (supposedly, equivalent to the bolometric detector) replaces the detector and radiates the electromagnetic waves through the optical system to the sky. The rules of reciprocity in such a replacement of a detector by an “equivalent transmitter” located in a lossy (open) structure (a bolometric cavity coupled via the feed horn with the outer space) are not obvious and usually ignored. A conventional simplistic approach used in this problem may cause errors in the design of such systems. By considering simple, analytically solvable models that simulate bolometric detectors surrounded by some structures, we find the rules of reciprocity providing the necessary equivalence in replacing the receiver by an appropriate transmitter as needed for the rigorous simulations of infrared and submillimetre-wave bolometric systems.