1.School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China 2.College of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61604073, 61805119, 11404170) and the Natural Science Foundation of Jiangsu Province, China (Grant Nos. BK20160839, BK20180469)
Received Date:22 June 2019
Accepted Date:29 July 2019
Available Online:01 November 2019
Published Online:20 November 2019
Abstract:Metallic semiconductor nanolaser, as an ultra-small light source, has been increasingly attractive to researchers in last decade. It can have wide potential applications such as in photonic integrated circuits, on-chip interconnect, optical communications,etc. One obstacle to miniaturization of the laser size is that the loss increases rapidly with the cavity volume decreasing. In previous studies, a type of Fabry-Perot cavity with capsule-shaped structure was investigated and demonstrated both numerically and experimentally, showing that its cavity loss is reduced dramatically in contrast to the scenario of conventional rectangular cavities. However, when the cavity size is reduced down to nanoscale, capsule-shaped structure surfers high loss. To overcome this difficulty, in this paper, a novel type of double-concave cavity structure for metallic semiconductor nanolaser in a 1.55 μm wavelength range is proposed and numerically studied. The proposed structure consists of InGaAs/InP waveguide structure encapsulated by metallic clad, and has a cylindrical reflection end face and concave curved sidewalls. The cylindrical reflection end face can push the resonant mode into the cavity center and reduce the optical field overlap with metallic sidewalls, which can reduce the metallic loss. The curved-sidewalls topologically reduce the electric field component perpendicular to the sidewalls, and thus reducing the plasmonic loss. By optimizing the waist width of the double-concave cavity structure, the radiation loss can be effectively reduced, resulting in the improvement of cavity quality factor and the decrease of threshold current. Finite-difference time-domain simulations are conducted to investigate the properties of the proposed cavity structures such as resonant mode distribution, cavity quality factor, confinement factor, threshold gain and threshold current in this paper. The numerical results show that the double-concave cavity laser with cavity volume as small as 0.258 λ3 increases 24.8% of cavity quality factor and reduces 67.5% of threshold current, compared with the conventional capsule-shaped one, demonstrating an effective improvement of metallic nanolaser. With those advantages, the proposed structure can be used for realizing the ultra-small metallic semiconductor nanolasers and relevant applications. Keywords:semiconductor laser/ microcavity/ surface plasmon polaritons
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2.原理和设计本文提出的双凹型金属半导体纳米激光器谐振腔结构带有圆柱形的反射端面和内凹的弯曲侧壁. 侧壁的曲线独立可调, 可设计成一次函数、二次函数(如抛物线型等)、三角函数(如余弦函数)等. 以抛物线型为例, 其谐振腔结构示意图如图1所示. 本文针对1.55 μm波长的通信波段进行设计, 谐振腔的半导体材料有源层是厚度为300 nm的InGaAs, 其下层为500 nm厚的p掺杂InP包层, 上层为500 nm厚的n掺杂InP包层, 并使用100 nm厚的n掺杂InGaAs作为接触层. 半导体侧壁及反射端面由厚度为30 nm的绝缘体材料SiO2覆盖, 整个谐振腔由100 nm厚的金属Ag包裹. 图 1 双凹型金属半导体纳米激光器谐振腔示意图 (a)结构示意图; (b)俯视图 Figure1. Schematic of double-concave cavity of metallic semiconductor nanolaser: (a) The structure; (b) top view of the double-concave cavity.
图4描述了Q值最大处的三种双凹腔结构的谐振模式(TE模式)在经过腔中心的xy、yz和xz截面上的归一化电场强度分布图. 所有谐振腔结构的几何参数详见表2. 图4(a)—(c)所示的是CSC型腔结构, 谐振模式主要集中在半导体层, 但是在侧壁部分(特别是腔反射端面附近)有较大的SPPs模式分布, 金属损耗较大. 图4(d)—(l)所示的是三种双凹腔结构的谐振模式, 其归一化的金属损耗相比于CSC型腔结构均未改善, 这与图3(c)的结论一致. 图4(g)—(i)和4(j)—(l)所示的是分别是抛物线型和余弦函数型谐振腔结构, 它们相比于图4(d)—(f)所示的一次函数型谐振腔结构具有更加平滑的弯曲侧壁, 因此具有相对较低的金属损耗. 值得注意的是, 如上文所述, 由于双凹腔型结构具有更低的辐射损耗, 图4(d)—(l)所示的谐振模式依然具有更高的品质因子. 图 4 不同谐振腔结构的谐振模式(TE模式)的归一化电场强度|E|2在穿过腔中心的xy、yz、xz平面的分布图 (a)— (c)为胶囊型腔; (d)— (f)为一次函数型腔; (g)— (i)为抛物线型腔; (j)—(l)为余弦函数型腔. 所有腔的几何参数详见表2 Figure4. Normalized electric field intensity distribution |E|2 of the resonant mode (TE mode) in the xy-, yz- and xz-planes crossing the cavity center: (a)–(c) The capsule-shaped cavity; (d)–(f) the linear-function-shaped cavity; (g)–(i) the parabola-shaped cavity; (j)–(l) the cosine-shaped cavity. All the geometric parameters of the cavities are listed in Table 2 in detail.
参数
胶囊型
一次函数型
抛物线型
余弦函数型
L/nm
700
700
700
700
W/nm
520
520
520
520
W0/W
1.00
0.75
0.8
0.80
L/R
1.43
1.43
1.43
1.43
V/λ3
0.267
0.258
0.257
0.258
λ/nm
1564
1552
1550
1551
Q
141
174
175
176
Г
0.460
0.441
0.440
0.445
gth/cm–1
2190
1870
1850
1830
Ith/μA
800
290
280
260
表2四种谐振腔的金属半导体纳米激光器的几何参数和数值仿真结果 Table2.Geometric parameters and simulation results of the metallic semiconductor nanolasers with four types of cavities.
为了进一步研究该新型双凹腔侧壁的内凹程度对金属半导体纳米激光器性能的影响, 我们根据数值仿真得到的Q值和谐振模式分布, 利用(2)式—(4)式, 计算了基于上述三种腔结构的金属半导体纳米激光器的限制因子、阈值增益和阈值电流随束腰宽度W0的变化关系, 见图5. 这里, 体积V定义为金属半导体腔的总体积(包括绝缘体层和金属层). 由图5(a)可见, 当W0/W < 1时, 随着束腰宽度W0减小, 三种双凹腔结构的限制因子总体均呈现下降趋势, 但是在W0/W = 0.8和0.85附近取得局域极大值, 这是束腰半径减小引起的金属损耗增大和辐射损耗减小共同导致的结果. 由图5(b)和图5(c)可见, 阈值增益与阈值电流随着束腰宽度W0的减小, 呈现出先增大后减小的趋势, 一次函数型和另两种双凹型腔结构分别在W0/W = 0.75和0.8处取得最小值. 并且, 抛物线型和余弦函数型腔结构在W0/W ∈ [0.7, 0.85]的较大束腰宽度范围内, 都比CSC型腔结构具有更小的阈值, 因而对于半导体微加工的制造公差具有较好的容忍度, 有较好的应用价值. 图 5 三种曲线侧壁双凹型谐振腔(L = 700 nm, W = 520 nm, L/R = 1.43)的金属半导体纳米激光器的限制因子Γ、阈值增益gth和阈值电流Ith与W0/W的关系 (a) Γ; (b) gth; (c) Ith Figure5. The confinement factor Γ, threshold gain gth and threshold current Ith of the metallic semiconductor nanolasers with three double-concave cavities with curved sidewalls (L = 700 nm, W = 520 nm, L/R = 1.43) as functions of the W0/W: (a) Γ; (b) gth; (c) Ith.