Functional organic single crystals for solid‐state laser applications

In the article by Hong‐Hua Fang et al. (doi: http://dx.doi.org/10.1002/lpor.201300222 ), published in Laser Photonics Rev. 8, 687–715 (2014), the names of two authors in references were wrongly spelt and citations in the text were missing. This erratum is published to correct this.     

Functional organic single crystals for solid‐state laser applications

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.     

布拉格反射波导半导体激光器的研究

高功率半导体激光器在固体或光纤激光器泵浦、材料加工、医疗、传感、空间通讯和国防上有着极其重要的应用,但传统半导体激光器面临垂直发散角大、椭圆光斑的难题,限制了其直接应用。为了降低激光器的垂直发散角,本项目采用布拉格反射波导结构,利用光子带隙导引替代传统的全反射进行光场限制,优化设计了多种布拉格反射波导激光器结构,并制备了高性能的激光器器件。首先,采用传输矩阵理论和布洛赫波近似的方法计算了布拉格反射波导的模式色散关系,发现通过控制腔模光场分布,可实现不同远场的激光输出。接着,针对布拉格波导光子带隙导引机制,深入研究了四分之一波长布拉格反射波导激光器、单边布拉格反射波导激光器的光场特性,弄清了影响此类激光器远场的本质因素,最终设计并验证了一种布拉格反射波导双光束激光器,激光器在垂直方向可输出两个对称的、近圆形光束,单光束垂直和侧向发散角半高全宽分别低至7.2°和5.4°。另外,通过调控光缺陷层,使激光器工作在受抑隧穿光子带隙导引机制下,实现了超窄的单光束激光输出,激光器单管连续输出功率超过4.6 W,垂直发散角最低降至4.9°(半高全宽)和9.8°(95%功率)。这种高功率、窄的圆形光束输出可以大幅降低半导体激光器的应用成本,提高泵浦或光纤耦合效率,具有广阔的应用前景。     

Photonic liquid crystal fibers — a new challenge for fiber optics and liquid crystals photonics

The paper reviews and discusses the latest developments in the field of the photonic liquid crystal fibers that have occurred for the last three years in view of new challenges for both fiber optics and liquid crystal photonics. In particular, we present the latest experimental results on electrically induced birefringence in photonic liquid crystal fibers and discuss possibilities and directions of future developments.     

Micro‐ and submicrometric waveguiding structures in optical crystals produced by ion beams for photonic applications

Energetic ion beams with diverse energies, species and beam dimensions have been extensively utilized to modify the properties of materials to achieve versatile applications in many aspects of industry, agriculture and scientific research. In optics, the ion‐beam technology has been applied to fabricate various micro‐ and submicrometric guiding structures on a wide range of optical crystals through the efficient modulation of the refractive indices or structuring of the surface, realizing various applications in many branches of photonics. The ion‐beam fabricated optical waveguides and other photonic structures have shown good guiding performance as well as properties related to the materials, suggesting promising potential for many aspects of photonics. This paper gives the state‐of‐the‐art review of fabrication, characterization and application on the ion‐beam‐processed micro‐ and submicrometric photonic structures by highlighting the most recent research progress. A brief prospect is presented by focusing on a few potential spotlights.     

Photonic crystals for applications in photoelectrochemical processes: Photoelectrochemical solar cells with inverse opal topology

Photonic crystal structures, that present strong light localization effects near photonic band gap frequency regions, can be very useful to maximize chemical processes of phototoactive materials. One example is the use of photonic crystals to improve solar energy harvesting in photoelectrochemical solar cells. Here, we describe the optical monitoring synthesis of macroporous materials, with inverse opal topology, made of transition metal and rare-earth oxide nanoparticles. Through the optical properties we can obtain information concerning both infiltration and over layer growth. Finally, we report on the efficiency improvement of photoelectrochemical cells when titania inverse opal topology is used.     

Numerical studies of 2D photonic crystals: Waveguides,coupling between waveguides and filters

In photonic crystals, light propagation is forbidden in a certain wavelength range, the bandgap. In a two-dimensional crystal composed of parallel high-refractive index rods in a low-index background a line defect can be formed by removing a row of these rods, which can act as a waveguide for frequencies in the bandgap of the crystal. In order to get more insight into the main features of such waveguides we have studied a number of properties, using simulation tools based on the finite difference time domain method and a finite element Helmholtz solver. We show conceptually simple methods for determining the bandgap of the crystal as well as the dispersion of a waveguide for wavelengths in this bandgap. For practical applications, it is also important to know how much light can be coupled into the waveguide. Therefore, the coupling of light from a dielectric slab waveguide into the photonic crystal waveguide has been examined, showing that a coupling efficiency of up to 83% can be obtained between a silicon oxide slab and a waveguide in a crystal of silicon rods. Finally, calculations on an ultra-compact filter based on reflectively terminated side-branches of waveguides (similar to tuned stubs in microwave engineering) are shown and discussed.     

Stimulated globular scattering of laser radiation in photonic crystals: Temperature dependences

The characteristics of stimulated globular scattering such as the frequency shift, threshold, and conversion efficiency are studied in photonic crystals (synthetic opal matrices and opal nanocomposites) at different temperatures. The results are compared with the study of stimulated Raman scattering in calcite single crystals. In both cases, a decrease in temperature from +20° C to −196° C resulted in an increase in the energy of stimulated scattering energy and its redistribution into the higher-order components.     

飞秒激光加工铌酸锂晶体：原理与应用

由于具有超短的脉冲宽度和极高的峰值强度,飞秒激光微加工是一种有效的材料加工方法, 已广泛应用于光子集成器件的加工。铌酸锂晶体具有优异的电光、非线性光学和压电特性,是集成 光学和导波光学中常见的材料。本文综述了飞秒激光对铌酸锂晶体的处理,重点介绍了飞秒激光加 工的物理原理及其制备的铌酸锂基光子器件的最新进展。飞秒激光技术使铌酸锂晶体在微纳光子学 领域具有广阔的应用前景。     

二氧化硅胶体球自组装光子晶体的研究

光子晶体由于具有光子带隙和光子局域等一系列优异的光学特性而受到了人们广泛的关注。由于采用胶体颗粒自组装法制备光子晶体制备工艺简单,所需要的费用也较低,因此已成为制备可见光至红外波段三维光子晶体的一种简便有效的方法。采用垂直沉积法制得了三维光子晶体薄膜,并用扫描电子显微镜和紫外-可见光-近红外分光光度计对其显微结构和光学特性进行了详细的研究。结果表明,自组装薄膜在三维方向上都具有有序结构,其密排面平行于载波片的表面。制备的光子晶体薄膜具有明显的光子带隙特性,带隙中心波长为956nm。研究了带隙中心波长同入射线与密排面法线夹角之间的变化关系,其结果与理论值吻合得很好。     

Research and production of laser crystals in China

A general review of the progress in the research and production of laser crystals in China is presented. In the past quarter century, a considerable foundation and strength has been built up in this field and through further effort China's laser crystals will soon reach world level.     

The single-longitudinal-mode operation of a ridge waveguide laser based on two-dimensional photonic crystals

An electrically driven, single-longitudinal-mode GaAs based photonic crystal (PC) ridge waveguide (RWG) laser emitting at around 850 nm is demonstrated. The single-longitudinal-mode lasing characteristic is achieved by introducing the PC to the RWG laser. The triangle PC is etched on both sides of the ridge by photolithography and inductive coupled plasma (ICP) etching. The lasing spectra of the RWG lasers with and without the PC are studied, and the result shows that the PC purifies the longitudinal mode. The power per facet versus current and current-voltage characteristics have also been studied and compared.     

Ultra‐short and ultra‐intense X‐ray free‐electron laser single pulse in one‐dimensional photonic crystals

The propagation within a one‐dimensional photonic crystal of a single ultra‐short and ultra‐intense pulse delivered by an X‐ray free‐electron laser is analysed with the framework of the time‐dependent coupled‐wave theory in non‐linear media. It is shown that the reflection and the transmission of an ultra‐short pulse present a transient period conditioned by the extinction length and also the thickness of the structure for transmission. For ultra‐intense pulses, non‐linear effects are expected: they could give rise to numerous phenomena, bi‐stability, self‐induced transparency, gap solitons, switching, etc., which have been previously shown in the optical domain.     

On‐chip single photon sources using planar photonic crystals and single quantum dots

We review the basic light‐matter interactions and optical properties of chip‐based single photon sources, that are enabled by integrating single quantum dots with planar photonic crystals. A theoretical framework is presented that allows one to connect to a wide range of quantum light propagation effects in a physically intuitive and straightforward way. We focus on the important mechanisms of enhanced spontaneous emission, and efficient photon extraction, using all‐integrated photonic crystal components including waveguides, cavities, quantum dots and output couplers. The limitations, challenges, and exciting prospects of developing on‐chip quantum light sources using integrated photonic crystal structures are discussed.     

Angular‐ and polarization‐independent structural colors based on 1D photonic crystals

Wide‐angle, polarization‐independent structural reflective colors from both directions based on a one‐dimensional photonic crystal are demonstrated. Our device produces a distinct and saturated color with high angular tolerant performance up to ±70° for any polarization state of an incident light wave, which is highly desirable for a broad range of research areas. Moreover, the purity of the color and luminous intensity of the proposed device are improved as compared to conventional colorant‐based color filters and colloidal glasses. The present approach may have the potential to replace existing color filters and pigments and pave the way for various applications, including color displays and image sensor technologies.

     

Periodic,quasi‐periodic,and random quadratic nonlinear photonic crystals

Quadratic nonlinear photonic crystals are materials in which the second order susceptibility χ⁽²⁾ is spatially modulated while the linear susceptibility remains constant. These structures are significantly different than the more common photonic crystals, in which the linear susceptibility is modulated. Nonlinear processes in nonlinear photonic crystals are governed by the phase matching requirements, which are determined by the reciprocal lattice of these crystals. Therefore, the modulation of the nonlinear susceptibility enables to engineer the spatial and spectral response in various three‐wave mixing processes. It enables to support the efficient generation of new optical frequencies at multiple directions. We analyze three wave mixing processes in nonlinear photonic crystals in which the modulation is either periodic, quasi‐periodic, radially symmetric or even random. We discuss both one‐dimensional and two‐dimensional modulations. In addition to harmonic generations, we outline several new possibilities for all‐optical control of the spatial and polarization properties of optical beams in specially designed nonlinear photonic crystals.     

On‐axis shaping of second‐harmonic beams

We report complete spatial shaping (both phase and amplitude) of the second‐harmonic beam generated in a nonlinear photonic crystal. Using a collinear second‐order process in a nonlinear computer generated hologram imprinted on the crystal, the desired beam is generated on‐axis and in the near field. This enables compact and efficient one‐dimensional beam shaping in comparison to previously demonstrated off‐axis Fourier holograms. We experimentally demonstrate the second‐harmonic generation of high‐order Hermite–Gauss, top hats and arbitrary skyline‐shaped beams.

     

Continuous‐wave vertically emitting photonic crystal terahertz laser

Compact semiconductor light sources with high performance continuous‐wave (CW) and single mode operation are highly demanded for many applications in the terahertz (THz) frequency range. Distributed feedback (DFB) and photonic crystal (PhC) quantum cascade (QC) lasers are amongst the leading candidates in this field. Absorbing boundary condition is a commonly used method to control the optical performance of a laser in double‐metal confinement. However, this approach increases the total loss in the device and results in a large threshold current density, limiting the CW maximum output power and operating temperature. In this letter, a robust surface emitting continuous‐wave terahertz QC laser is realized in a two‐dimensional PhC structure by a second order Bragg grating extractor that simultaneously provides the boundary condition necessary for mode selection. This results in a 3.12 THz single mode CW operation with a 3 mW output power and a maximum operation temperature (T_max) of 100 K. Also, a highly collimated far‐field pattern is demonstrated, which is an important step towards real world applications.     

Self‐Collimation in Photonic Crystals: Applications and Opportunities

A comprehensive review considering recent advances in self‐collimation and its applications in optical integration is covered in the current article. Self‐collimation is compared to the conventional technique of photonic bandgap engineering to control the light propagation in photonic crystal‐based structures. It is fully discussed how the self‐collimation phenomenon can be tailored to be independent of the incident angle and polarization. This adds substantial flexibility to the structure to overcome light coupling challenges and simultaneously aids in the omission of bulk and challenging elements, including polarizers and lenses from optical integrated circuits. Additionally, designed structures have the potential to be rescaled to operate in any desired frequency range thanks to the scalability rule in the field of electromagnetics. Moreover, it is shown that one can boost the coupling efficiency by applying an anti‐reflection property to the structure, which provides not only efficient index matching but also the matching between external waves with uniform amplitude and Bloch waves with periodic amplitude.     

Modelling issues for waveguide photonic crystals and photonic microstructures: A personal view

This paper is primarily an attempt to highlight areas of photonic crystal and microstructure research where there is a need for computer-based modelling. No attempt is made to assess the relative merits of different computational methods. Instead, attention is focussed mainly on engineering issues that, in the author's view, are likely to be important in predicting and analysing the properties of complex device structures built up from photonic crystal building blocks.