Laser & Photonics Reviews

Cover image for Vol. 8 Issue 2

Early View (Online Version of Record published before inclusion in an issue)

Editor: Katja Paff

Impact Factor: 7.976

ISI Journal Citation Reports © Ranking: 2012: 2/80 (Optics); 8/128 (Physics Applied); 9/68 (Physics Condensed Matter)

Online ISSN: 1863-8899

Associated Title(s): Advanced Optical Materials, Journal of Biophotonics

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  1. 1 - 34
  1. Original Papers

    1. Switchable surface plasmon dichroic splitter modulated by optical polarization

      Seung-Yeol Lee, Hansik Yun, Yohan Lee and Byoungho Lee

      Article first published online: 16 APR 2014 | DOI: 10.1002/lpor.201400025

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      For the miniaturization of optical devices, surface plasmon polaritons (SPPs) have been widely utilized due to their outstanding confinement and field-enhancement characteristics. Analyzing a spectrum of optical signals and splitting certain regions of the spectrum range within a submicrometer-scale structure are demanded for optical integrated systems. In this paper, a novel type of dichroic surface plasmon launcher that can switch the launching direction according to incident polarization states is demonstrated. Compared to the previously reported plasmonic dichroic splitters, the proposed schemes do not use any asymmetric geometry for directional launching. Hence, the direction of guided SPPs can be interchanged according to the polarization state. Such characteristics will be helpful to design switchable plasmonic devices that can be applied to active plasmonic integrated circuits.

    2. Spontaneous scaling down of femtosecond laser-induced apertures towards the 10-nanometer level: the excitation of quasistatic surface plasmons

      Min Huang and Zhizhan Xu

      Article first published online: 6 APR 2014 | DOI: 10.1002/lpor.201300212

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      During ultrafast laser ablation on ZnO, the feature sizes of laser-induced apertures always decrease spontaneously with the pulse increasing and the crater extending, and may eventually approach an astonishing 10-nm scale, an ultradeep-subwavelength laser–solid interaction regime relating new physics in laser-induced damage. Based on the fundamental theories of plasmonics, we propose that the spontaneous scaling-down originates in the conversion of physical regimes of plasmonic interaction from the optical regime to the electrostatic regime, which arouses quasistatic SPs with interaction scales far beyond the diffraction limit and results in ultrafast, nonthermal ablation for extraordinary electrostatic field enhancement. Basically, “nanoscale” eliminates electromagnetic retardation effects greatly, brings an instantaneous respond to the incident field, and arouses electrostatic interactions with giant local-field enhancement, which may exert tremendous electrostatic forces on superficial electrons and ions and eventually lead to an “ultrafast” electrostatic nanoexplosion. Thus, the characteristics of simultaneous “nanoscale” and “ultrafast” arise spontaneously in femtosecond laser ablation.

    3. Planar bifunctional Luneburg-fisheye lens made of an anisotropic metasurface

      Xiang Wan, Xiaopeng Shen, Yu Luo and Tie Jun Cui

      Article first published online: 4 APR 2014 | DOI: 10.1002/lpor.201400023

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      Luneburg lens and Maxwell-fisheye lens are well-known microwave and optical devices with distinct focusing properties. Here, a planar bifunctional Luneburg-fisheye lens made of an anisotropic metasurface is presented, which features as a Luneburg along the horizontal optical axis, while as a fisheye along the vertical optical axis. A method to control the inhomogeneous indices of refraction along the two optical axes independently is proposed by designing an anisotropic and nonuniform metasurface, which can provide the required distributions of refractive indices approximately for Luneburg and fisheye lenses viewing from the two optical axes. Experiments in the microwave frequency range demonstrate very good performance of the planar bifunctional Luneburg-fisheye lens. The proposed method opens up an avenue to design other kinds of bifunctional devices using metasurfaces in the microwave, terahertz, and even optical ranges.

    4. Direct laser writing of symmetry-broken spiral tapers for polarization-insensitive three-dimensional plasmonic focusing

      Jiafang Li, Jiajia Mu, Benli Wang, Wei Ding, Ju Liu, Honglian Guo, Wuxia Li, Changzhi Gu and Zhi-Yuan Li

      Article first published online: 2 APR 2014 | DOI: 10.1002/lpor.201300206

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      Plasmonic focusing with metallic probes has attracted extensive studies due to its successful applications in advanced technologies such as near-field scanning optical microscopy and tip-enhanced Raman spectroscopy. Here the fabrication and characterization of a unique spiral metallic taper with polarization-insensitive three-dimensional (3D) plasmonic focusing properties are reported. Metallic probes with spiral corrugations are readily fabricated along the surfaces of the conical structures with a 3D direct laser writing method followed by a metal deposition process. With the broken structural symmetry induced by the spiral corrugations, plasmonic focusing is demonstrated under excitation of linearly polarized light with different polarization directions. Moreover, apertures with various sizes can be flexibly introduced at the apex of the conical probe structures with direct fabrication, which enables the observation of scattered light from waveguide modes, cutoff of waveguide modes and scattering from surfaces plasmons, respectively. The studies provide a novel methodology of design, realization, and application of 3D plasmonic focusing structures.

    5. Bandgap engineering of GaxZn1–xO nanowire arrays for wavelength-tunable light-emitting diodes

      Xianghui Zhang, Luying Li, Jun Su, Yumei Wang, Yuling Shi, Xiaoliang Ren, Nishuang Liu, Aiqing Zhang, Jun Zhou and Yihua Gao

      Article first published online: 2 APR 2014 | DOI: 10.1002/lpor.201300172

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      Wavelength-tunable light-emitting diodes (LEDs) of GaxZn1–xO nanowire arrays are demonstrated by a simple modified chemical vapor deposition heteroepitaxial growth on p-GaN substrate. As a gallium atom has similar electronegativity and ion radius to a zinc atom, high-level Ga-doped GaxZn1–xO nanowire arrays have been fabricated. As the x value gradually increases from 0 to 0.66, the near-band-edge emission peak of GaxZn1–xO nanowires shows a significant shift from 378 nm (3.28 eV) to 418 nm (2.96 eV) in room-temperature photoluminescence (PL) measurement. Importantly, the electroluminescence (EL) emission of GaxZn1–xO nanowire arrays LED continuously shifts with a wider range (∼100 nm), from the ultraviolet (382 nm) to the visible (480 nm) spectral region. The presented work demonstrates the possibility of bandgap engineering of low-dimensional ZnO nanowires by gallium doping and the potential application for wavelength-tunable LEDs.

    6. Manipulating transverse magnetic modes in waveguide using thin plasmonic materials

      Yadong Xu, Qiannan Wu and Huanyang Chen

      Article first published online: 1 APR 2014 | DOI: 10.1002/lpor.201300185

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      Transverse magnetic modes propagate freely for any frequency in a parallel plated waveguide. Here, it is shown that when two thin plasmonic materials are attached to the metal walls of the waveguide, a bandgap emerges due to the excited surface plasmon polaritons. As the plasmonic materials become thin enough, a critical thickness is observed. For any width larger than such a value, the bandgap is fixed, otherwise a tunable bandgap can be achieved (different widths induce different bandgaps). For application, such a tunable bandgap system can be utilized to design a band-stop filter for terahertz frequencies with a high quality factor.

    7. A nano-plasmonic chip for simultaneous sensing with dual-resonance surface-enhanced Raman scattering and localized surface plasmon resonance

      Jiao Lin, Yuan Zhang, Jun Qian and Sailing He

      Article first published online: 27 MAR 2014 | DOI: 10.1002/lpor.201400029

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      A dual-resonance surface-enhanced Raman scattering (SERS) chip which also serves as a localized surface plasmon resonance (LSPR) refractive index sensor is proposed. The dual-resonance SERS chip can simultaneously enhance excitation and Stokes lines for Raman signals detection in a broad wavelength region with virtually no limitation. Thus, it is especially useful for Raman detection at long wave numbers and hyper Raman. The great performance of this chip relies on the highly independent tunability of the two localized plasmonic resonances from the optical to the near-infrared region and the strict “hot spot” match in space for both resonant wavelengths. Furthermore, Raman signals of poly-methyl-methacrylate (PMMA) from 500 cm−1 to 3300 cm−1 are measured in the experiments and an obvious superiority can be seen compared to a single-resonance SERS chip. In an addition, by using the subradiant magnetic dipole resonance, the LSPR refractive sensor gives a high sensitivity of 577 nm/RIU and high figure of merit (FoM) of 14.2. The experimental results are consistent with the simulated results. This dual-functional sensing chip opens a route for dual-modality detection of the concentration of some specific molecules.

  2. Letter Articles

    1. Hybrid photonic circuit for multiplexed heralded single photons

      Thomas Meany, Lutfi A. Ngah, Matthew J. Collins, Alex S. Clark, Robert J. Williams, Benjamin J. Eggleton, M. J. Steel, Michael J. Withford, Olivier Alibart and Sébastien Tanzilli

      Article first published online: 27 MAR 2014 | DOI: 10.1002/lpor.201400027

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      A key resource for quantum optics experiments is an on-demand source of single and multiple photon states at telecommunication wavelengths. This letter presents a heralded single photon source based on a hybrid technology approach, combining high efficiency periodically poled lithium niobate waveguides, low-loss laser inscribed circuits, and fast (>1 MHz) fibre coupled electro-optic switches. Hybrid interfacing different platforms is a promising route to exploiting the advantages of existing technology and has permitted the demonstration of the multiplexing of four identical sources of single photons to one output. Since this is an integrated technology, it provides scalability and can immediately leverage any improvements in transmission, detection and photon production efficiencies.

  3. Review Articles

    1. Advances in vanadate laser crystals at a lasing wavelength of 1 micrometer

      Haohai Yu, Junhai Liu, Huaijin Zhang, Alexander A. Kaminskii, Zhengping Wang and Jiyang Wang

      Article first published online: 27 MAR 2014 | DOI: 10.1002/lpor.201400022

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      Sapphire, garnet and vanadate crystals are the most prominent optical materials, and vanadates play important roles in optics, especially in lasers and nonlinear optics. Neodymium-doped yttrium vanadate (Nd:YVO4) is representative and available commercially. Based on Nd:YVO4, several vanadate crystals are being developed with the goal of fulfilling the need for differential applications and improvement of certain operational aspects, such as with pulsed lasers or high-power continuous-wave lasers. In recent years, some important effects, including energy enhancement, bistability of output performance, self-Raman frequency shifting, etc., and some novel applications, such as quantum optics, pulsed lasers modulated by the two-dimensional crystals, etc., have been discovered with vanadates as gain materials. In this paper, the preparation, characterization and laser applications of vanadate laser crystals at the lasing wavelength of 1 micrometer, including YVO4, GdVO4, LuVO4, GdxY1–xVO4 and LuxGd1–xVO4 (0<x<1) doped with Nd3+ and ytterbium (Yb3+) are systematically reviewed by highlighting the most recent research progress. Their specific properties are presented, generation mechanisms of novel physical effects are discussed, new applications are given and possible future applications proposed by focusing on some potential strengths.

  4. Original Papers

    1. You have full text access to this OnlineOpen article
      Hybrid femtosecond laser microfabrication to achieve true 3D glass/polymer composite biochips with multiscale features and high performance: the concept of ship-in-a-bottle biochip

      Dong Wu, Si-Zhu Wu, Jian Xu, Li-Gang Niu, Katsumi Midorikawa and Koji Sugioka

      Article first published online: 27 MAR 2014 | DOI: 10.1002/lpor.201400005

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      True three-dimensionally (3D) integrated biochips are crucial for realizing high performance biochemical analysis and cell engineering, which remain ultimate challenges. In this paper, a new method termed hybrid femtosecond laser microfabrication which consists of successive subtractive (femtosecond laser-assisted wet etching of glass) and additive (two-photon polymerization of polymer) 3D microprocessing was proposed for realizing 3D “ship-in-a-bottle” microchip. Such novel microchips were fabricated by integrating various 3D polymer micro/nanostructures into flexible 3D glass microfluidic channels. The high quality of microchips was ensured by quantitatively investigating the experimental processes containing “line-to-line” scanning mode, improved annealing temperature (645°C), increased prebaking time (18 h for 1mm-length channel), optimal laser power (1.9 times larger than that on the surface) and longer developing time (6 times larger). The ship-in-a-bottle biochips show high capabilities to provide simultaneous filtering and mixing with 87% efficiency in a shorter distance and on-chip synthesis of ZnO microflower particles.

    2. Highly flexible broadband terahertz metamaterial quarter-wave plate

      Longqing Cong, Ningning Xu, Jianqiang Gu, Ranjan Singh, Jiaguang Han and Weili Zhang

      Article first published online: 27 MAR 2014 | DOI: 10.1002/lpor.201300205

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      Metamaterials offer exciting opportunities that enable precise control of light propagation, its intensity and phase by designing an artificial medium of choice. Inducing birefringence via engineered metamolecules presents a fascinating mechanism to manipulate the phase of electromagnetic waves and facilitates the design of polarimetric devices. Here, we present a high-efficiency, broadband, tunable and flexible quarter-wave plate based on a multilayer metamaterial. Excellent achromatic π/2 phase retardance with high transmission is observed upon terahertz propagation through the quarter-wave plate. The calculated Stokes parameter represents the output polarization state numerically, indicating an excellent broadband conversion of linearly polarized light into circularly polarized light. The metamaterial-based quarter-wave plate demonstrated in this work could be an important step forward in the development of functional terahertz polarization conversion devices for practical applications.

    3. Secure key distribution over a 500 km long link using a Raman ultra-long fiber laser

      Atalla El-Taher, Omer Kotlicki, Paul Harper, Sergei Turitsyn and Jacob Scheuer

      Article first published online: 27 MAR 2014 | DOI: 10.1002/lpor.201300177

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      In traditional communication systems the transmission medium is considered as a given characteristic of the channel, which does not depend on the properties of the transmitter and the receiver. Recent experimental demonstrations of the feasibility of extending the laser cavity over the whole communication link connecting the two parties, forming an ultra-long fiber laser (UFL), have raised groundbreaking possibilities in communication and particularly in secure communications. Here, a 500 km long secure key distribution link based on Raman gain UFL is demonstrated. An error-free distribution of a random key with an average bit-rate of 100 Hz between the users is demonstrated and the key is shown to be unrecoverable to an eavesdropper employing either time or frequency domain passive attacks.

    4. Surface plasmon resonance tunability in VO2/Au/VO2 thermochromic structure

      Huaijuan Zhou, Xun Cao, Meng Jiang, Shanhu Bao and Ping Jin

      Article first published online: 26 MAR 2014 | DOI: 10.1002/lpor.201300214

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      Nanometal–VO2 composite systems can exhibit a reversibly tunable localized surface plasmon effect under external temperature stimuli due to the temperature-sensitive dielectric function of the thermochromic VO2 thin film. In order to improve the tunability range and enhance the resonance shift magnitude (ΔλSPR), the VO2/Au/VO2 sandwich structure was deliberately designed here. The plasmonic and thermochromic characteristics of this structure were powerfully dependent on the gold deposition time (t). By varying this parameter, the resonance wavelength (λSPR) of the semiconductor phase can be regulated from the visible range (686 nm) to the near-infrared range (1482 nm) and a maximum ΔλSPR of 714 nm can be achieved at t = 3.5 min. This study may provide insights into plasmonic noble metal nanoparticles on a thermochromic VO2 matrix and offer potential applications in temperature-sensitive nanodevices.

    5. Deep-subwavelength light confinement and transport in hybrid dielectric-loaded metal wedges

      Yusheng Bian and Qihuang Gong

      Article first published online: 26 MAR 2014 | DOI: 10.1002/lpor.201300207

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      The goal of confining light at the deep-subwavelength scale while retaining moderate attenuation has been pursued for years in the field of plasmonics. However, few feasible configurations at present are excellent at balancing the tradeoff between confinement and loss. This work proposes to overcome the above limitation by using hybrid wedge structures, which consist of triangular metal wedges loaded with nanometric low/high-index dielectric claddings. Owing to the superior guiding properties of wedge plasmons in conjunction with high refractive index contrast near wedge tips, the modal sizes can be squeezed into significantly smaller spaces than those of their conventional wedge and planar hybrid counterparts, while simultaneously featuring propagation distances over tens of micrometers at telecommunication wavelengths. Studies on the evolution from a single metallic wedge to semiconductor–insulator–metal wedge(s) reveal strategies for continuous improvement of the optical performance. Discussions concerning practical issues including crosstalk and mode excitation have further elucidated their potential in building high-performance nanophotonic components.

    6. Tunable graphene-based plasmonic waveguides: nano modulators and nano attenuators

      Jieer Lao, Jin Tao, Qi Jie Wang and Xu Guang Huang

      Article first published online: 26 MAR 2014 | DOI: 10.1002/lpor.201300199

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      We propose and numerically investigate a new modulation principle and method, the gate-voltage-controlled mode-guiding switching/mode-cutoff mechanism, to achieve the modulation/attenuation function. The propagating attenuation of the graphene 2D waveguide as a function of the Fermi level of the cladding is analyzed. Different modes with low or high attenuations in a wide attenuation range have been observed. The proposed structure avoids the patterning on graphene or substrate, thus diminishing the energy scattering on the edges. It also has the advantages of large modulation depth/attenuation range, wide bandwidth, and sub-micrometer chip-length with a nanoscale lateral section, which is promising for future graphene-based integrated photonic devices.

    7. You have full text access to this OnlineOpen article
      Injection locking of mid-infrared quantum cascade laser at 14 GHz, by direct microwave modulation

      Margaux Renaudat St-Jean, Maria Ines Amanti, Alice Bernard, Ariane Calvar, Alfredo Bismuto, Emilio Gini, Mattias Beck, Jerome Faist, H. C. Liu and Carlo Sirtori

      Article first published online: 20 MAR 2014 | DOI: 10.1002/lpor.201300189

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      Compact laser sources operating in mid infrared spectral region with stable emission are important for applications in spectroscopy and wireless communication. Quantum cascade lasers (QCL) are unique semiconductor sources covering mid infrared frequency range. Based on intersubband transitions, the carrier lifetime of these sources is in the ps range. For this reason their frequency response to direct modulation is expected to overcome the limits of standard semiconductor lasers. In this work injection locking of the roundtrip frequency of a QCL emitting at 9 μm is reported. Inter modes laser frequency separation is stabilized and controlled by an external microwave source. Designing an optical waveguide embedded in a microstrip line a flat frequency response to direct modulation up to 14 GHz is presented. Injection locking over MHz frequency range at 13.7 GHz is demonstrated. Numerical solutions of injection locking theory are discussed and presented as tool to describe experimental results.

  5. Letter Articles

    1. Simultaneous brightness enhancement and wavelength conversion to the eye-safe region in a high-power diamond Raman laser

      Aaron McKay, Ondrej Kitzler and Richard P. Mildren

      Article first published online: 10 MAR 2014 | DOI: 10.1002/lpor.201400012

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      Brightness enhancement in an external cavity diamond Raman laser designed for high power conversion of a neodymium (1064 nm) laser to the eye-safe spectral region is reported. Using a multimode input beam pulsed at 36 kHz pulse repetition frequency, 16.2 W with 40% overall conversion efficiency was obtained at the second Stokes wavelength of 1485 nm. The output beam had a quality factor of inline image which is a factor of 2.7 times lower than that of the input beam, resulting in a higher overall brightness. The output power, brightness, and brightness enhancement obtained represent significant advances in performance for Raman lasers as well as other competing kHz-pulsed eye-safe technologies.

  6. Original Papers

    1. Sensitive interferometric detection of ultrasound for minimally invasive clinical imaging applications

      Amir Rosenthal, Stephan Kellnberger, Dmitry Bozhko, Andrei Chekkoury, Murad Omar, Daniel Razansky and Vasilis Ntziachristos

      Article first published online: 10 MAR 2014 | DOI: 10.1002/lpor.201300204

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      Miniaturized optical detectors of ultrasound represent a promising alternative to piezoelectric technology and may enable new minimally invasive clinical applications, particularly in the field of optoacoustic imaging. However, the use of such detectors has so far been limited to controlled lab environments, and has not been demonstrated in the presence of mechanical disturbances, common in clinical imaging scenarios. Additionally, detection sensitivity has been inherently limited by laser noise, which hindered the use of sensing elements such as optical fibers, which exhibit a weak response to ultrasound. In this work, coherence-restored pulse interferometry (CRPI) is introduced – a new paradigm for interferometric sensing in which shot-noise limited sensitivity may be achieved alongside robust operation. CRPI is implemented with a fiber-based resonator, demonstrating over an order of magnitude higher sensitivity than that of conventional 15 MHz intravascular ultrasound probes. The performance of the optical detector is showcased in a miniaturized all-optical optoacoustic imaging catheter.

    2. You have full text access to this OnlineOpen article
      Controlling the radiation direction of propagating surface plasmons on silver nanowires

      Zhuoxian Wang, Hong Wei, Deng Pan and Hongxing Xu

      Article first published online: 7 MAR 2014 | DOI: 10.1002/lpor.201300215

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      Metal nanowires supporting propagating surface plasmons (SPs) can be used as nanowaveguides and nanoantennas for light manipulation beyond the diffraction limit. Here the control of the propagation and radiation of SPs on silver nanowires is investigated. By covering an Al2O3 layer onto a silver nanowire to change the local dielectric environment, the wave vector of the propagating SPs is increased. Thus, the radiation direction of SPs into the substrate is changed according to the phase matching condition, which is experimentally shown by Fourier imaging method. The radiation angle is sensitively dependent on the Al2O3 thickness. By depositing 1 nm Al2O3, the increase of the radiation angle can be close to 1 degree. These results show that dielectric-layer-coating provides a simple and effective method to control the propagation and radiation of SPs, which will be of great importance for designing plasmonic circuits, antennas and sensors based on silver nanowires.

  7. Review Articles

    1. Ultra-low loss waveguide platform and its integration with silicon photonics

      Martijn J. R. Heck, Jared F. Bauters, Michael L. Davenport, Daryl T. Spencer and John E. Bowers

      Article first published online: 5 MAR 2014 | DOI: 10.1002/lpor.201300183

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      Planar waveguides with ultra-low optical propagation loss enable a plethora of passive photonic integrated circuits, such as splitters and combiners, filters, delay lines, and components for advanced modulation formats. An overview is presented of the status of the field of ultra-low loss waveguides and circuits, including the design, the trade-off between bend radius and loss, and fabrication rationale. The characterization methods to accurately measure such waveguides are discussed. Some typical examples of device and circuit applications are presented. An even wider range of applications becomes possible with the integration of active devices, such as lasers, amplifiers, modulators and photodetectors, on such an ultra-low loss waveguide platform. A summary of efforts to integrate silicon nitride and silica-based low-loss waveguides with silicon and III/V based photonics, either hybridly or heterogeneously, will be presented. The approach to combine these integration technologies heterogeneously on a single silicon substrate is discussed and an application example of a high-bandwidth receiver is shown.

    2. Functional organic single crystals for solid-state laser applications

      Hong-Hua Fang, Jie Yang, Jing Feng, Takeshi Yamao, Shu Hotta and Hong-Bo Sun

      Article first published online: 2 MAR 2014 | DOI: 10.1002/lpor.201300222

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      Because of long-range order and high chemical purity, organic crystals have exhibit unique properties and attracted a lot of interest for application in solid-state lasers. As optical gain materials, they exhibit high stimulated emission cross section and broad tunable wavelength emission as similar to their amorphous counterpart; moreover, high purity and high order give them superior properties such as low scattering trap densities, high thermal stability, as well as highly polarized emission. As electronic materials, they are potentially able to support high current densities, thus making it possible to realize current driven lasers. This paper mainly describes recent research progress in organic semiconductor laser crystals. The building molecules, crystal growth methods, as well as their stimulated emission characteristics related with crystal structures are introduced; in addition, the current state-of-the-art in the field of crystal laser devices is reviewed. Furthermore, recent advances of crystal lasers at the nanoscale and single crystal light-emitting transistors (LETs) are presented. Finally, an outlook and personal view is provided on the further developments of laser crystals and their applications.

  8. Letter Articles

    1. You have full text access to this OnlineOpen article
      Plasmonic lattice solitons beyond the coupled-mode theory

      Ying Xue, Fangwei Ye, Dumitru Mihalache, Nicolae C. Panoiu and Xianfeng Chen

      Article first published online: 2 MAR 2014 | DOI: 10.1002/lpor.201300202

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      The existence and properties of plasmonic lattice solitons (PLSs) supported by periodic arrays of metallic nanowires embedded into a dielectric medium with Kerr nonlinearity are studied by solving the 3D Maxwell equations, and the conclusions are compared with the results predicted by a coupled-mode theory analysis. It is found that these two methods predict markedly different characteristics for the optical power of PLSs and its dependence on the separation distance between adjacent nanowires. In particular, the coupled-mode theory is found to be valid only when the distance between nanowires is larger than some characteristic length. The compensation of modal loss by a background optical gain is also studied and it is revealed that the gain coefficient required to balance the loss is much smaller than the loss parameter of the metallic components of the plasmonic array.

    2. Time-resolved random laser spectroscopy of inhomogeneously broadened systems

      Joaquín Fernández, Sara García-Revilla, Luis D. Carlos, Edison Pecoraro, María A. Arriandiaga and Rolindes Balda

      Article first published online: 25 FEB 2014 | DOI: 10.1002/lpor.201300191

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      The understanding of energy transfer processes in biological systems occurring among optical centres which exhibit inhomogeneously broadened spectral bands is of paramount importance to determine time constants and spatial distribution of energy flow. A new time resolved-spectroscopy based on the random laser generation of the optical probes is reported. As an example, the excited state relaxation of Rhodamine B molecules in an organic-inorganic hybrid material is investigated. This kind of spectroscopy may resolve not only the spectral features of the system but also provide a high speed picture of the energy transfer and excited state relaxation of the involved optical probes. The results could be applied to other kind of efficient interacting chromophore pairs embedded in inhomogeneous scattering structures such as biological tissues.

    3. Universal method for the synthesis of arbitrary polarization states radiated by a nanoantenna

      Francisco J. Rodríguez-Fortuño, Daniel Puerto, Amadeu Griol, Laurent Bellieres, Javier Martí and Alejandro Martínez

      Article first published online: 25 FEB 2014 | DOI: 10.1002/lpor.201300184

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      Optical nanoantennas efficiently convert confined optical energy into free-space radiation. The polarization of the emitted radiation depends mainly on nanoantenna shape, so it becomes extremely difficult to manipulate it unless the nanostructure is physically altered. Here, a simple way is demonstrated to synthetize the polarization of the radiation emitted by a single nanoantenna so that every point on the Poincaré sphere becomes attainable. The nanoantenna consists of a single scatterer created on a dielectric waveguide and fed from its both sides so that the polarization of the emitted optical radiation is controlled by the amplitude and phase of the feeding signals. The nanoantenna is created on a silicon chip using standard top-down nanofabrication tools, but the method is universal and can be applied to other materials, wavelengths and technologies. This work will open the way towards the synthesis and control of arbitrary polarization states in nano-optics.

  9. Original Papers

    1. Trapped photons at a Dirac point: a new horizon for photonic crystals

      Kang Xie, Haiming Jiang, Allan D. Boardman, Yong Liu, Zhenhai Wu, Ming Xie, Ping Jiang, Quan Xu, Ming Yu and Lionel E. Davis

      Article first published online: 20 FEB 2014 | DOI: 10.1002/lpor.201300186

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      Optical cavities and waveguides are essential building blocks of many modern optical devices. They rely upon photonic bandgaps, or total internal reflections, to achieve field confinement. Here a new phenomenon is reported of wave localization that is attributable to neither of the above light guiding mechanisms. It is found that what is known as the Dirac point within a photonic band structure can play the role of a photonic bandgap with the establishment of field confinement. The new localized mode occurs at a Dirac frequency that is beyond any complete photonic bandgap, and exhibits a unique algebraic profile. The features of this new wave localization will add new capabilities and more flexibility to the design techniques of novel photonic components and photonic chip architectures.

    2. β-NaYF4:Yb3+, Er3+ upconversion microcrystals with both high emission intensity and controlled morphology

      Wu Suli, Liu Ye, Chang Jie, Ning Yanhui and Zhang Shufen

      Article first published online: 19 FEB 2014 | DOI: 10.1002/lpor.201300203

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      Upconversion emission intensity and morphology are both important features of NaYF4:Yb3+, Er3+ microcrystals. In this paper, myristic acid (MA) is selected as a novel stabilizing ligand to synthesize NaYF4:Yb3+, Er3+ through a facile hydrothermal method. Upconversion fluorescence spectra indicate that the emission intensity of product prepared with MA as ligand has increased dramatically compared with the sample prepared using commonly used oleic acid (OA) as ligand. Field emission scanning electron microscopy (FE-SEM) results display that the morphology of NaYF4:Yb3+, Er3+ obtained using MA as ligand can be tuned from nanodisk, nanorod to nanotube. Especially, nanotube with clear caves can be prepared directly with proper concentration of base (NaOH). And powder X-ray diffraction (XRD) analysis reveals a relationship between the phase transformation process and the amount of NaOH. With increasing the amount of NaOH, the transition process of cubic phase to hexagonal phase is accelerated. The effect of F and OH on the fluorescence properties of NaYF4:Yb3+, Er3+ are studied systematically. A gradual increase of the amount of NaF results in a regular increase of the emission intensity. On the contrary, the emission intensity decreases with increasing the amount of NaOH. In addition, reasonable explanations of the influence of F and OH on emission intensity are given as well.

  10. Review Articles

    1. A marriage of convenience: Hybridization of surface plasmon and dielectric waveguide modes

      Muhammad Z. Alam, J. Stewart Aitchison and Mo Mojahedi

      Article first published online: 19 FEB 2014 | DOI: 10.1002/lpor.201300168

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      Plasmonics has attracted a lot of interest in the past few years because of its unique features, especially for its ability to confine light in extremely small volumes. However, application of plasmonics is restricted by the large propagation loss associated with plasmonic waveguides. On the other hand, dielectric waveguides enjoy low loss, although the mode confinement is relatively weaker. Hybrid plasmonic waveguides (HPWGs), which combine these two guiding mechanisms, allow one to utilize the benefits of both technologies. Over the past few years there have been intense research activities around the world on this new guiding scheme. In this work the operating principle of HPWGs, various HPWG structures proposed by different research groups, and their potentail applications are reviewed.

    2. Whispering-gallery mode lasing in ZnO microcavities

      Chunxiang Xu, Jun Dai, Guangping Zhu, Gangyi Zhu, Yi Lin, Jitao Li and Zengliang Shi

      Article first published online: 13 FEB 2014 | DOI: 10.1002/lpor.201300127

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      Zinc oxide (ZnO) is considered as an ideal candidate for ultraviolet (UV) lasers due to its unique advantages of wide direct bandgap and large exciton binding energy. Recently, whispering-gallery mode (WGM) lasing has attracted considerable attention for its high quality factor and low lasing threshold. The corresponding investigations have very important significance not only for fundamental scientific research but also for the potential applications in short-wavelength optoelectronic devices. In this paper, progress in ZnO microlasers is reviewed systematically. The fabrication methods for ZnO WGM microcavities are introduced first. Then the characteristics of single-photon and multiphoton pumped WGM lasing are presented. The lasing mechanisms on excitonic, electron–hole plasma and exciton–polariton lasing are reviewed in detail. Finally, recent advances in ZnO-based microlaser devices such as heterojunction laser diodes are explored. The further research challenges and some strategies are also indicated for the promising applications.

    3. On-chip stimulated Brillouin Scattering for microwave signal processing and generation

      Ravi Pant, David Marpaung, Irina V. Kabakova, Blair Morrison, Christopher G. Poulton and Benjamin J. Eggleton

      Article first published online: 10 FEB 2014 | DOI: 10.1002/lpor.201300154

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      Demonstration of continuously tunable delay, low-noise lasers, dynamically controlled gratings, and optical phase shifting using the stimulated Brillouin scattering (SBS) process has lead to the emergence of SBS as a promising technology for microwave photonics. On-chip realization of SBS enables photonic integration of microwave photonic signal processing and offers significantly enhanced performance and improved efficiency. On-chip stimulated Brillouin scattering is reviewed in the context of slow-light based tunable delay, low-noise narrow linewidth lasers and filtering for integrated microwave photonics. A discussion on key material and device properties, necessary to enable on-chip Brillouin scattering using both the single-pass and resonator geometry, is presented along with an outlook for photonic integration of microwave signal processing and generation in other platforms.

    4. Ultrabroadband Er:fiber lasers

      Daniele Brida, Günther Krauss, Alexander Sell and Alfred Leitenstorfer

      Article first published online: 23 JAN 2014 | DOI: 10.1002/lpor.201300194

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      The state of the art of ultrafast Er:fiber technology is reviewed. Such lasers are increasingly used for generation of ultrabroadband and widely tunable pulse trains. Er:fiber sources prove to be flexible, compact and robust with important applications in fundamental and interdisciplinary sciences. After a short overview of different oscillator and amplifier designs the discussion focuses on coherent and tailored supercontinuum generation in highly nonlinear germanosilicate fibers. This approach enables a tuning range spanning from the visible to the mid infrared, synthesis of single-cycle light pulses and passive locking of the carrier-envelope phase.

    5. Optical frequency comb technology for ultra-broadband radio-frequency photonics

      Victor Torres-Company and Andrew M. Weiner

      Article first published online: 18 DEC 2013 | DOI: 10.1002/lpor.201300126

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      The outstanding phase-noise performance of optical frequency combs has led to a revolution in optical synthesis and metrology, covering a myriad of applications, from molecular spectroscopy to laser ranging and optical communications. However, the ideal characteristics of an optical frequency comb are application dependent. In this review, the different techniques for the generation and processing of high-repetition-rate (>10 GHz) optical frequency combs with technologies compatible with optical communication equipment are covered. Particular emphasis is put on the benefits and prospects of this technology in the general field of radio-frequency photonics, including applications in high-performance microwave photonic filtering, ultra-broadband coherent communications, and radio-frequency arbitrary waveform generation.

    6. Adiabatic processes in frequency conversion

      Haim Suchowski, Gil Porat and Ady Arie

      Article first published online: 20 OCT 2013 | DOI: 10.1002/lpor.201300107

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      Adiabatic evolution, an important dynamical process in a variety of classical and quantum systems providing a robust way of steering a system into a desired state , was introduced only recently to frequency conversion . Adiabatic frequency conversion allowed the achievement of efficient scalable broadband frequency conversion and was applied successfully to the conversion of ultrashort pulses, demonstrating near-100% efficiency for ultrabroadband spectrum . The underlying analogy between undepleted pump nonlinear processes and coherently excited quantum systems was extended in the past few years to multi-level quantum systems, demonstrating new concepts in frequency conversion, such as complete frequency conversion through an absorption band . Additionally, the undepleted pump restriction was removed, enabling the exploration of adiabatic processes in the fully nonlinear dynamics regime of nonlinear optics . In this article, the basic concept of adiabatic frequency conversion is introduced, and recent advances in ultrashort physics, multi-process systems, and the fully nonlinear dynamics regime are reviewed.

    7. Optical microcavities with tubular geometry: properties and applications

      Jiao Wang, Tianrong Zhan, Gaoshan Huang, Paul K. Chu and Yongfeng Mei

      Article first published online: 26 JUN 2013 | DOI: 10.1002/lpor.201300040

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      Optical mcirocavities with whispering-gallery modes (WGMs) have large potential and, in particular, those with a tubular geometry have attracted increasing attention due to their special geometry and interesting properties such as trimmed resonant modes, simplicity as fluidic channels, three-dimensionally (3D) mode confinement, unique evanescent wave, and so on. Optical microcaivities with the tubular geometry meet the challenge of assembly of conductive, semiconductive and insulating materials into a tubular geometry, thus spurring multifunctional applications to optofluidic devices, optical microdevices like microlasers, and bio/chemical sensors. Fabrication methods such as the fiber-drawing method, rolled-up nanotechnology, electrospin technique, and template-assistant method have been developed to address the various requirements. These tubular optical microcavities enable researchers to explore and construct novel optical microdevices for a wide range of potential applications. This review describes the tubular optical microcavities from the perspectives of theoretical consideration, optical characterization, and potential applications.

  11. Errata

    1. Recent advances in bioluminescence tomography: methodology and system as well as application

      C. Qin, J. Feng, S. Zhu, X. Ma, J. Zhong, P. Wu, Z. Jin and J. Tian

      Article first published online: 1 OCT 2012 | DOI: 10.1002/lpor.201270011

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