光子晶体在半导体激光器中的应用

LIU Guang-yu,ZHANG Yan,PENG Biao

摘要：摘摘要：光子晶体（PC）在过去的二十年备受的研究者的关注。他们人为制造了周期性介电结构。周期性介电结构具有光子带隙（PBG）和被称为光子带隙的材料。本文主要介绍了一维与二维光子晶体对半导体激光器中的作用。

关键词：光子晶体;半导体激光器不

引言：

光子晶体的概念在1987年被第一次提出了，具有比如自发发射的抑制、光的控制等许多有趣的特性。。在信息社会中，半导体激光器是重要的光电子器件。光子晶体也被引入到了激光器结构中，以改善激光器的性能。光子晶体可以控制光的传播以提高在激光器中的光限制和反馈。

一维光子晶体

最简单的光子晶体是一维光子结晶。在特定频率范围内，光子晶体可以完美的反射任何极化任何方向的入射光波。传统的双层不同折射率材料交替构造的多层膜结构就是典型的一维光子晶体。分布式布拉格反射器在垂直腔面激光器发射（VCSEL）中就是一个良好的应用。

除了分布式布拉格反射器，一维光子晶体也可以在VCSEL中以另一种方式被应用。带有顶镜的垂直谐振器的一维光子晶体膜被进行了研究。这些结构使得封装式的垂直腔面发射激光器的制造成为可能，同时，它还可以被设计成平面内发射型。通过这种混合方法，在制造经典的VCSEL和光子晶体激光器中遇上的工艺困难将会小很多，因为一个完整的布拉格堆叠被替换为单个光子晶体膜，并且需要在光子晶体层不需要处在有源增益层中。这种方法的一个简单的例子是使用一个简单的光子晶体膜（PCM）作为波长选择性发射面：当光波照射在一个PCM的平面（垂直或倾入射）上是，锐利的共振反射光谱就可以观察到。图2给出了PCM的结构。在PCM设计中，三个参数需要定义：晶格常数（A），填充因子（FF）和PCM厚度（h），为了开始PCM的设计，A是任意的、我们将它设置为1微米; 当lambda;= 1550nm时，整个设计结构被重新缩放至操作的初始值. 在PCM结构的横向侧被设定为约30微米，因此，它包括在x方向的几个光栅周期。嵌入在两个布拉格反射镜典型的微腔通常具有2 000和10000的范围内的品质因子，此外，这些反射镜具有反射率在97％和100％的范围内。

在另一篇文章中，JM分析在这些膜结构中形成的表面垂直发射共振模式，并揭示仅有由带有空气孔的晶格薄膜即可产生一个非常有效的垂直腔而不需要上方和下方的多层反射镜。这意味着，垂直空腔可以简单地通过具有周期性图案的平板结构实现，从而使得简单的生产表面大阵列的微型激光器成为可能，每个激光器的辐射波长由局部孔大小和间隔来确定。

3．二维光子晶体

典型的光子晶体激光器的光谐振腔是通过使用二维光子晶体作为平面的光局域和使用薄介电平板波导作为垂直光局域方式形成。二维光子晶体是三角晶格组成的气孔。缺陷腔通过去除晶格的中心孔而形成。

我们以O. J. Paint的工作为例。图3显示了高Q光学微腔是通过在半个波长厚度的多量子阱波导结构中蚀刻三角形阵列的空气孔而。通过在二维光子晶体中引入缺陷形成腔局域的光学模式，其模态的体积可以被限制在2至3（lambda;/ 2n）3的范围内。

他们通过二维光子晶体中移除某一空气孔而形成缺陷，从而获得模式体积小于0.03mu;m3的的高Q微腔激光器。缺陷模式的能量在如此微小体积内被局域，从而极大的增强了自发辐的射率，为缺陷光子晶体在低阈值的激光器和高调制率的发光二极管中运用创造巨大前景。由于其几何形状的灵活性，光子晶体形成的纳米光学结构还有大量的可发展性。

可以采用光刻方法以改变光子晶体的几何形状，以便调谐器件特性。

缺陷空腔不仅可以通过从二维光子晶体薄板移除空气孔来形成，还通过调整空气孔半径和晶格常数来实现。图4显示了这种类型的微腔。

在空气中的InGaAsP膜就是一个标准的平面光子晶体（PPSC）谐振腔。该三角光子晶体晶格周围空腔的几何形状是R / A= 0.3和D / A= 0.75，其中，r是光子晶体的孔的半径，d是厚度膜的，并且A是晶格的周期性。较小的孔半径rdef/ A=0.15从光学空腔引入。这种单一的缺陷PPC腔被通常会有两个简并的，具有较好的品质因素的偶极子共振模式。

早期的光子晶体激光器通常是光泵浦如JKHwang等的工作。他们报道在室温下连续工作的光泵2- D光子晶体激光器。他们提出并展示了脉冲模式工作周期高达10％的二维 PBG激光器。通过晶片融合技术薄平板PBG激光器结构被夹在空气和和钻孔氧化铝层之间。该二维 PBG激光是建立在薄的InGaAsP平板波导结构之上。在垂直方向上强的光学限制是通过具有高折射率的InGaAs / A1z03上全内反射（或空气）实现。在水平面，二维三角光子晶体反射镜是由钻孔形成。它的晶格常数为450nm和孔的半径为135纳米。通过一个光斑尺寸10微米的，波长为980nm的GaAs激光器去对该激光空腔进行光学泵浦。对于发出的光是由相同的光学元件的在顶部收集。所收集的输出功率的激光波长与入射泵浦功率是表示入射泵浦能量的9.2 mW.约10％的激射阈值被吸收并用于载流子产生。在报告中，他们第一次在室温下联系工作的光子晶体激光器。

随着技术的发展，电泵浦光子晶体激光器得以实现。第一个在室温下连续工作的二维光子晶体二极管激光器是由Dai Ohnishi小组所报道。 Fig.5示出了装置结构的示意图。这种激光可以实现大面积上的单模共振模式，这在传统激光器中不可能的。在工作中，他们优化了外延层的组合物来更好的载流子限制和澄清在该光子晶体和空气孔的直径之间的关系与激光的阈值电流来估计优化的阀值电流。

一个韩国小组通过微米尺寸的光子晶体谐振器引入电接点解决了这一问题。他们在实验上示范了在室温下连续工作的，电驱动的，单模式的，低阈值电流约为260微安光子带隙激光器。电驱动装置的驱动是该装置朝向一个实际应用的关键步骤。

要能合理设计电接点的在光子晶体谐振腔中的位置需要我们队三角晶格光子晶体腔的工作模式分布有所了解。三个潜在的候选方案，每一个中心节点都被认为是因为引入了小型中央后为电接触并没有显着降低模式的品质因子。小中心柱用可以同时用作电线，一个模式选择器，以及散热器。在光子晶体单元谐振器的中心安置一个亚微米尺寸的半导体小柱体（图6）。半导体板坯的厚度为282.5纳米。电子从上部电极侧向供给，而空穴通过柱底直接注射。在六个应力补偿的InGaAsP量子阱中，该载流子和空穴重新结合产生了波长在1.5微米的荧光（EL）峰。通过冷腔的单模式的Q因子测量（自以约为225微安的透明电流相关联的光谱线宽度）, 估计它的Q值大于2500。单极模式的体积大约为5.87times;10-2mu;m3。实际模式在非简并单极模式下，通过测量与基于实际结构参数的计算的比较证实。作为一个小步骤朝向无阈值激光或单光子源，该波长大小的光子晶体激光器可能会感兴趣的光子晶体，腔量子电动力学和量子信息的社区。

除了Hatice Altug所描述单个器件光子晶体谐振器阵列。他们的工作在设计，制造和实验示范的二维耦合光子晶体谐振器阵列（二维 CPCRA），表现出超平坦的色散频带（很小的群速度）以上的波矢量的整个范围，并在所有的晶体方向。这降低耦合的灵敏度和减少的光学畸变通过这样的结构脉冲传播。

二维 CPCRA的是由正方晶格光子晶体的周期性改性孔构造。图7示出了在自由直立膜通过电子束光刻，干法和湿法蚀刻的组合制作在SOI晶片一个CPCRA的SEM照片。这种结构可以被看作是通过除去单个空气孔形成的单缺陷的光子晶体腔的二维阵列。

他们制作的InP材料纳腔体系等阵列。活性区包含四个InGaAsP量子阱（量子阱）与1560处具有峰值的可致发光发射波长。所述联接的PC纳腔阵列15微米尺寸激光器在光学脉冲泵浦垂直于结构在室温下。用一个40倍具有0.6的数值孔径显微镜物镜在808纳米的二极管激光聚焦在样品上观察以15微米尺寸的光斑。泵脉冲是20纳秒长，用1％的占空比，以减少结构的加热。发射的光从样品的顶部收集。从耦合纳腔阵列激光器观察单模激射，如图8所示。

外文文献出处：Laser Technology and Applications LIU Guang-yu，ZHANG Yan et，Application of Photonic Crystals in Semiconductor Lasers[J]2007

附外文文献原文

Application of Photonic Crystals in Semiconductor Lasers

1 Introduction

Photonic crystals, which have many interesting characteristics like the inhibition of spontaneous emission, the controlling of light and so on, were first proposed in1987. In the information society, semiconductor lasers are important optoelectronic devices. Photonic crystals are introduced into the laser structures to improve the laser performance. PCs can control the propagation of light to improve the optical confinement and feedback in lasers.

1. 1-D Photonic Crystal

The simplest photonic crystal is the 1-D photonic crystal. A photonic crystal can be a perfect mirror for light from any direction with any polarization within a specified frequency range. The traditional multilayer film consisting of alternating layers of materials with two different dielectric constants is one example. Distributed Bragg reflector (DBR) applied in the verticalcavity surface-emitting lasers (VCSELs) is a wellknown application.

Besides DBR, 1-D photonic crystal can also be applied in VCSELs in another way. Vertical resonators with a top mirror constituted of 1-D photonic crystal membrane on top of a Bragg stack are investigated.These structures allow the fabrication of compact vertical- cavity surface emitting lasers, which can be designed, in addition, for in- plane emission. With this hybrid approach, fabrication problems related to both classical VCSELs and photonic crystal lasers may be significantly relaxed, given that a full Bragg stack is replaced by a single photonic crystal membrane and that the photonic crystal is not formed in the active gain layer. A simple illustration of this approach is the use of a plain photonic crystal membrane (PCM) as a wavelength selective transmitter reflector: when light is shined on a PCM in an out-of-plane (normal or

oblique) direction, sharp resonances in the reflectivity spectrum can be observed. Fig.2 shows the PCM structures. In the PCM design, three parameters need to be defined: the lattice constant (A), the filling factor (FF)and the PCM thickness(h,).In order to begin with the PCM design, A is `arbitrarily' set to 1 um; this initial value wil

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Application of Photonic Crystals in Semiconductor Lasers

1 Introduction

Photonic crystals, which have many interesting characteristics like the inhibition of spontaneous emission, the controlling of light and so on, were first proposed in1987. In the information society, semiconductor lasers are important optoelectronic devices. Photonic crystals are introduced into the laser structures to improve the laser performance. PCs can control the propagation of light to improve the optical confinement and feedback in lasers.

1. 1-D Photonic Crystal

The simplest photonic crystal is the 1-D photonic crystal. A photonic crystal can be a perfect mirror for light from any direction with any polarization within a specified frequency range. The traditional multilayer film consisting of alternating layers of materials with two different dielectric constants is one example. Distributed Bragg reflector (DBR) applied in the verticalcavity surface-emitting lasers (VCSELs) is a wellknown application.

Besides DBR, 1-D photonic crystal can also be applied in VCSELs in another way. Vertical resonators with a top mirror constituted of 1-D photonic crystal membrane on top of a Bragg stack are investigated.These structures allow the fabrication of compact vertical- cavity surface emitting lasers, which can be designed, in addition, for in- plane emission. With this hybrid approach, fabrication problems related to both classical VCSELs and photonic crystal lasers may be significantly relaxed, given that a full Bragg stack is replaced by a single photonic crystal membrane and that the photonic crystal is not formed in the active gain layer. A simple illustration of this approach is the use of a plain photonic crystal membrane (PCM) as a wavelength selective transmitter reflector: when light is shined on a PCM in an out-of-plane (normal or

oblique) direction, sharp resonances in the reflectivity spectrum can be observed. Fig.2 shows the PCM structures. In the PCM design, three parameters need to be defined: the lattice constant (A), the filling factor (FF)and the PCM thickness(h,).In order to begin with the PCM design, A is `arbitrarily' set to 1 um; this initial value will be modified later when the whole designed structure is re-scaled to operate atlambda;＝1 550 nm .The

lateral size of the PCM structure is set to about 30 um,in such a way that it includes several grating periods in the x direction. Typical micro- cavities embedded in two Bragg mirrors have quality factors in the range of 2 000 and 10 000. Moreover, these mirrors have reflectivity inthe range of 97%and 100%.

In another paper, J. M. Pottage analyzed the surface emitting resonances supported by such films and showed that a lattice of air holes creates a highly effective vertical cavity without the need for multilayer mirrors above and below the layer. This means that vertical cavities can be simply realized by periodic patterning in the plane, permitting simple production of large arrays of surface emitting microlasers, each emission

wavelength being determined by the local hole size and spacing.

3 2- D Photonic Crystal

Typical photonic crystal lasers optical resonant cavity is formed by using 2-D photonic crystals for in-plane localization and a thin dielectric slab waveguide for vertical confinement. The 2-D photonic crystal is a triangular lattice consisting of air holes. The defect cavity is formed by removing the central holes from the lattice.

L.ets take the O. J. Painters work for an example.Fig.3 shows the high-Q optical microcavities which areformed by etching a triangular array of air holes into a half-wavelength thick multiquantum-well waveguide.Defects in the 2-D photonic crystal are used to support highly localized optical modes with volumes ranging from 2 to 3(lambda;/2n)³.

They used a 2- D photonic crystal to localize light into a single defect by removing an air a high-Q microcavity laser with a hole, thus forming modal volume less They used a 2- D photonic crystal to localize light into a single defect by removing an air a high-Q microcavity laser with a hole, thus forming modal volume lessthan 0.03 }m3. The confinement of the defect mode energy to this tiny volume, and the predicted enhancement of the spontaneous emission rate make the defect cavity a very interesting device for low threshold of lasers, and high modulation rate of light-emitting diodes. Nano-optic structures formed from photonic crystals also hold a great deal of promise due to the flexibility in their geometries.

Lithographic methods may be employed to alter thephotonic crystal geometry so as to tune device charactemstic s.

Defect cavity can be formed not only by removing air hole from 2-D photonic crystal slab, but also by tuning of the air hole radius and the lattice constant. Fig.4shows such kinds of micro- cavities.

The planar photonic crystals (PPCs) cavities are defined in an InGaAsP membrane suspended in air. The geometry of the triangular photonic crystal lattice surrounding cavities is r/a=0.3 and d/a=0.75, where r is the radius of photonic crystal holes, d is the thickness of the membrane, and a is the periodicity of the lattice.A smaller hole with radius r_def/ a=0.15 is introduced form an optical cavity. Such a single-defect PPC cavity is known to support two doubly degenerate dipole modes with modest quality factors.

Early photonic crystal lasers were optically pumped such as the work reported by J.K.Hwang etc. They reported continuous room- temperature operation of optitally pumped 2- D photonic crystal lasers. They proposed and demonstrated wafer-fused 2-D PBG-bandgap lasers operating in a pulsed mode with duty cycles up to 10%. The thin- slab PBG laser structure is sandwiched between air and a drilled aluminum oxide la

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