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Nanoscale Vertical Cavity Surface Emitting Laser and its Arrays

Periodic Reporting for period 1 - NOVEL (Nanoscale Vertical Cavity Surface Emitting Laser and its Arrays)

Reporting period: 2018-01-01 to 2019-12-31

Significant advances are needed to improve the speed and efficiency of future communication and computing systems. Photonic interconnect components are the only solution to offer both high speed and low power consumption. Micro-lasers (i.e. vertical-cavity surface-emitting lasers, VCSELs) have been widely deployed as optical interconnects in the last two decades. This technology has been very successful and demonstrated exponential growth in recent years mainly for 3D sensing applications, which is very different from its traditional use. However, the processing approach of this technology has been proven to be particularly challenging for miniaturization, it reduces the reliability for small devices and its high thermal resistance substantially degrades the device performance. Besides, this technology is based on a non-planar structure that is inconvenient for array fabrication as it limits the density of arrays needed for high brightness.

To address these bottlenecks and realize high-performance semiconductor light sources with CW operated current injection at room temperature, flexible and precise control of photonic confinement with a high Q-factor cavity is essential. For this purpose, lithographically defined laser diodes are introduced in which the vertical-cavity, transverse optical confinement, and electrical confinement were enabled by epitaxial growth and lithography. The objectives of this project were based on using a lithographically defined laser concept to develop a novel growth and fabrication process for GaAs-based nanoscale vertical emitting lasers (NOVEL). NOVEL employed a buried electrical- and optical-confinement method to scale the transverse cavity size down to 500 nm diameter. We have designed laser epitaxial structures, developed fabrication and characterization methods for NOVEL devices. Additionally, we established NOVEL array architectures to obtain significantly increased laser beam quality and brightness for applications in 3D sensing and LiDARs.

The fundamental optical properties and versatile advantages of NOVEL cavity structures were analyzed using numerical modeling techniques. Using this cavity approach, we investigated the effect of size on the quality factor (Q-factor) for both micro-scale and sub-wavelength sizes. The limits of emission wavelength tuning with lateral size control were analyzed. We extended the modeling study to large sizes to explore the large emission apertures in order to obtain a high-power single-mode operation.

The results are expected to have a strong impact in academia, industry, and society as it opens a way for a completely new approach enabling novel photonic technologies. The NOVEL devices, with their fundamentally new capabilities, hold special promise in a wide range of multi-disciplinary areas such as optical communication, computer science, on-chip nanophotonics, and LiDARs.
The main results of the work performed in this project can be summarized in two categories:

1) Precise control of photonic confinement for high-performance semiconductor light sources

The ability to control the photonic confinement within an optical cavity is a key element for the development of high-performance semiconductor light sources. The researcher developed a new type of optical cavity with micro- and nano-scale dimensions by employing the concept of the lithographically defined cavity (Li-cavity). The optical properties and versatile advantages of the Li-cavity were analyzed in detail with numerical simulation of practical vertical-cavity design. Numerical simulations were implemented by using the Finite Difference Time Domain (FDTD) method. It was demonstrated that i) high Q-factor is possible even for sub-wavelength size cavities based on all epitaxial GaAs/AlAs structures; ii) The Li-cavity design features large resonance wavelength tuning of with high Q-factor by adjusting its transverse size, which makes them highly appealing for wavelength-tunable light sources; iii) precise control of transverse photonic confinement provides large emission apertures to operate in single optical mode enabling high-power single-mode light sources.

2) Fabrication of micro- and nano-scale semiconductor lights sources as single emitters and arrays

The researcher performed epitaxial design, fabrication, and characterization of NOVEL structures. In order to understand the current blocking characteristics, various epitaxial design parameters have been investigated for carrier confinement. The designed vertical cavity structures were grown and characterized using spectral analysis of the epitaxial structure. Structure with a designed resonance wavelength at ~975 nm was achieved. Different current blocking techniques were studied using various dopant and annealing conditions. The carrier profile measurements were carried out using the electrochemical capacitance (ECV) profiling technique.
To investigate the optical characteristics, we developed NOVELs using e-beam lithography for sizes ranging from 0.5um to 5um. Distributed Bragg mirror (DBR) structures were formed and characterized for the top mirror of the cavity. Established characterization setup allows optical pumping of the single emitters and arrays, then emitted light is collected for concurrent analysis of output power, emission spectra, and near-field profile.
The progress in the field of semiconductor light sources will play a more significant role than today in our society and daily life through information/communication technologies and 3D sensing. The miniaturization of lasers promises on-chip optical communications and data processing speeds that are beyond the capability of electronics and today’s high-speed lasers. Lasers with very low-power consumption are one of the most important parts in creating a photonic integrated architecture. This requirement is the motivating force behind the development of small laser and nanolasers as they will enable more energy-efficient sources. The technique developed in this project has the possibility of being developed into such an energy-efficient manufacturable source. It is also expected that the NOVEL arrays demonstrated in this project will enable high brightness light sources. These laser arrays address the fundamental requirements for 3D sensing and LiDAR applications.