Electrically injected vertical-cavity surface-emitting lasers with post-supported high-contrast gratings
Fig. 1. Schematics of the HCG-VCSEL with post-supported HCG.
Fig. 2. (a) L-I-V curves of an HCG-VCSEL. The inset is the lasing spot image of the HCG-VCSEL at 2 mA; (b) Spectra of the HCG-VCSEL under continuous-wave operation.
Fig. 3. Calculated small-signal modulation responses of the TM HCG-VCSEL with a λ/2-cavity and an air thickness of one-quarter wavelength beneath the HCG at different currents.
Vertical-cavity surface-emitting lasers (VCSELs) have many unique features like circular beam, low power consumption, high modulation speed, and easy fabrication of two-dimensional arrays. Now VCSELs have been widely used in optical interconnects, consumer electronics, 3D sensing, and automotive applications.
The vertical cavity of a common VCSEL is always composed of two distributed Bragg reflectors (DBRs). 20 to 40 pairs of DBR layers with several micrometer thicknesses are needed to obtain a high reflectivity of larger than 99.5%, resulting in a great challenge for material growth.
The high-contrast grating (HCG) has a typical thickness of a few hundred nanometers, and shows a broadband and high reflectivity. The HCG has been used to partly or fully replace a DBR to form an HCG-VCSEL. The HCG-VCSEL shows excellent performance like single-mode operation, single polarized output, high-speed wavelength tuning, and beam shaping.
In HCG-VCSELs, most of the HCGs are air suspended, leading to a high index contrast and also wavelength tuning. The fabrication of air suspended HCGs is challenging. A carefully chosen sacrificial layer below the HCG layer is required. Furthermore, the critical point drying is indispensable to avoid buckling of the grating after the removal of the sacrificial layer, and thus the process is complicated.
In order to solve the problems, Prof. Anjin Liu and Prof. Wanhua Zheng team from Institute of Semiconductors, Chinese Academy of Sciences, together with Prof. Dieter Bimberg in TU Berlin, have realized for the first time electrically injected VCSELs with post-supported HCGs, as shown in Fig. 1. The HCG with a thickness of 135 nm at 940 nm is employed to replace part of the top DBR. Two posts are introduced to support the air-suspended HCG to prevent the buckling of the grating. This makes the fabrication process is very simple and the critical point drying is avoided. The research results are published in Photonics Research, Volume 10, No. 5, 2022 (Jing Zhang, Chenxi Hao, Wanhua Zheng, Dieter Bimberg, Anjin Liu. Demonstration of electrically injected vertical-cavity surface-emitting lasers with post-supported high-contrast gratings[J]. Photonics Research, 2022, 10(5): 05001170).
The research team designed the epitaxial structure of the HCG-VCSEL at 940 nm. The designed HCG for the transverse electric (TE) polarization has a high reflectivity (>99.5%) from 870 nm to 1000 nm. The calculated resonant wavelength of the designed HCG-VCSEL is 941.6 nm, and the quality factor of the cavity is 1×105.
The HCG-VCSEL was fabricated with p-metal deposition, mesa etching, oxidation, n-metal deposition, electron beam lithography, inductively coupled plasma etching, and selective wet etching. The fabricated HCG-VCSEL can achieve a threshold current of about 0.65 mA and a side-mode suppression ratio of 43.6 dB at 25 °C under continuous-wave operation, as shown in Fig. 2.
Furthermore, the research team proposed the effective mode length to characterize the mode confinement in the HCG-VCSEL, and theoretically predicted the modulation performance of the HCG-VCSEL. HCG-VCSELs can have smaller effective mode lengths compared with those of conventional VCSELs.
A TM (transverse magnetic) HCG-VCSEL with a λ/2-cavity and an air thickness of one-quarter wavelength beneath the HCG can achieve an effective mode length of 1.38×(λ/n). The -3-dB frequency of the HCG-VCSEL with a λ/2-cavity can theoretically reach 46.8 GHz at 12 mA, as shown in Fig. 3, which indicates that 100 Gbit/s in the on-off keying modulation format for the HCG-VCSEL can be expected.
The corresponding author of this paper, Prof. Anjin Liu, believes that the easy design of HCG-VCSELs has great potential for applications in optical interconnects, sensing, and so on.
In the future, the output power under single-mode operation of the HCG-VCSEL will be improved by optimizing the epitaxial structure, the HCG parameters, and the fabrication process. The dynamics and polarization characteristics of the HCG-VCSEL will be further studied.
电注入、高折射率差光栅垂直腔面发射激光器
图1 HCG-VCSEL示意图，其中HCG有纳米柱支撑
图2 （a）HCG-VCSEL的L-I-V曲线，插图为HCG-VCSEL在2 mA下的激射光斑图；（b）HCG-VCSEL在连续工作下的光谱
图3 具有λ/2的腔长、HCG下空气层厚度为λ/4的TM HCG-VCSEL在不同电流下计算的小信号响应
垂直腔面发射激光器（vertical-cavity surface-emitting lasers，VCSEL）具有圆形光束、低功耗、高速调制、易于二维阵列集成等优点，广泛应用于光互连、消费电子、3D传感、汽车等领域。
传统VCSEL由2个布拉格反射镜（distributed Bragg reflector，DBR）构成垂直腔。为提供99.5%以上的高反射率，需要20~40对DBR（厚度可达数微米），对材料生长造成极大挑战。
高折射率差光栅（High-contrast gratings，HCG），厚度只有百纳米量级，同时可提供宽带高反射率。用HCG替代全部或部分DBR构建的HCG-VCSEL具有单模工作、单偏振输出、高速波长调谐、光束整形等优异特性。
在HCG-VCSEL中，大多数HCG为空气悬浮型，同时可实现波具有高折射率差的长调谐。然而，空气悬浮型HCG的制作极具挑战。一方面，HCG下面牺牲层材料的选择尤为重要。另一方面，在移除牺牲层后，需要采用临界点干燥的方法来避免光栅的坍塌，制作工艺复杂。
为解决以上问题，中国科学院半导体研究所刘安金研究员和郑婉华研究员团队，联合柏林工业大学Dieter Bimberg教授，首次实现了电注入纳米柱支撑的HCG-VCSEL，如图1所示。HCG工作波长为940 nm，厚度为135 nm，用于替代部分上DBR。同时有2根纳米柱支撑空气悬浮型的HCG，能避免光栅的坍塌。该制作工艺非常简单，避免了临界点干燥工艺。相关研究结果发表于Photonics Research2022年第5期（Jing Zhang, Chenxi Hao, Wanhua Zheng, Dieter Bimberg, Anjin Liu. Demonstration of electrically injected vertical-cavity surface-emitting lasers with post-supported high-contrast gratings[J]. Photonics Research, 2022, 10(5): 05001170）。
该研究团队设计了940 nm波段的HCG-VCSEL的外延结构。TE（transverse electric，横电场）偏振的HCG在870~1000 nm波长范围内具有超过99.5%的高反射率。设计的HCG-VCSEL的计算共振波长为941.6 nm，Q值为1.0×105。
经过p-电极沉积、台面刻蚀、氧化、n-电极沉积、电子束曝光、感应耦合等离子体刻蚀、选择性湿法腐蚀等步骤，实现了HCG-VCSEL。在室温连续电注入条件下，HCG-VCSEL的阈值电流约为0.65 mA，边模抑制比为43.6 dB，如图2所示。
此外，该团队还提出用有效模式长度表征HCG-VCSEL中的模式限制，并理论预测了其调制特性。HCG-VCSEL相比传统VCSEL具有更小的有效模式长度。具有λ/2的腔长、HCG下空气层厚度为λ/4的TM（transverse magnetic，横磁场）HCG-VCSEL可实现1.38×(λ/n)的有效模式长度，其-3 dB带宽在12 mA电流下可达46.8 GHz，如图3所示。这种HCG-VCSEL有望在开关键控调制格式下实现100 Gbit/s的速率。
论文通讯作者刘安金研究员认为，这种设计简单的HCG-VCSEL在光互连、传感等领域具有重要的应用前景。
未来，研究团队将对HCG-VCSEL外延结构、HCG的参数、工艺流程等进行优化，以提高单模输出功率。同时将进一步研究HCG-VCSEL的动态和偏振特性。