Nanoscale lasers and optical amplifiers are the core devices for future optoelectronic integration on the chip and are critical to the information science and technology of tomorrow's supercomputers and "on-chip data centers." In particular, if these nanoscale devices can be fabricated on silicon substrates, they will lead the revolutionary development of on-chip optical interconnects, which has become one of the focuses of international academia and technology industry in recent decades.

Professor Ning Cunzheng, an expert in the "Thousand Talents Program" Department of Electronics, Tsinghua University, has been engaged in the research of semiconductor luminescence physics, nanophotonics and extreme miniaturization of semiconductor fabrication and characterization for many years. He has made for the first time in the world electro-injection nanometer lasers with sizes smaller than half wavelength, And the first to realize the continuous mode operation of room temperature injection of metal-injected nanolasers, which is a pioneering leader in the field of nanometer laser technology. Professor Ning Cunzheng's group has long been engaged in the research on the physics and application of micro-nano-optoelectronic material devices, exploring the limitations of miniaturization of laser and optical amplifiers, and developing new nano-optoelectronic materials for final optoelectronic integration and its application in future computer chips Forefront exploration.

On July 17, Sun Yong, an assistant researcher with Professor Ning Jurun's group, published a paper titled "Giant optical gain in a single-phase erbium-doped silicate nanowire" crystal erbium chloride silicate nanowire, reported for the first time an optical net gain greater than 100 dB / cm achieved in a single erbium-doped nanowire waveguide. This research result breaks through the limitation that the optical gain in traditional erbium-doped materials is only a few dB / cm, laying an important foundation for the realization of nano-scale high-gain optical amplifiers on silicon-based optoelectronic integrated chips.

Erbium-doped fiber amplifier is an indispensable key device in all-optical network and high-speed information system in the future. The advent of erbium-doped fiber amplifier is a revolutionary technological breakthrough in the field of optical fiber communications, making long-distance, high-speed and high-capacity optical fiber communication possible. However, in typical erbium-doped materials the optical gain per centimeter is limited to a few dB due to the lower erbium ion concentration. Therefore, erbium-based materials based lasers and amplifiers are too large to be used for system integration on future photonic chips. In recent decades, scientists have been trying to find another way, to study erbium compounds containing high concentrations of erbium, trying to increase the optical gain by increasing the concentration of erbium. However, academics have found that erbium compounds grown by thin film epitaxy have poor optical properties, resulting in too short fluorescence lifetime, and at the same time they also cause fluorescence quenching due to the high concentration of erbium. No optical net gain has been reported yet. How to extend the successful technology paradigm in long-distance optical communication to the field of photonic integrated chips is an important issue to be solved urgently and has become a research focus in recent decades.

Figure 1 Nanowire waveguide to achieve a schematic diagram of optical amplification (left), nanowire scanning electron micrograph (right)

In recent years, Professor Ning Cunzheng successfully developed a novel single-crystal erbium compound nanowire-erbium chloride silicate (ECS) grown on a silicon substrate. Unlike conventional erbium-doped materials, this is an erbium-containing compound with erbium concentrations 2-3 orders of magnitude higher than usual and without the usual erbium ion polymerization in the doped materials resulting in fluorescence quenching. In order to obtain erbium-containing materials with high optical gain, it is often necessary to satisfy both the conditions of high erbium concentration and good crystal quality. In order to overcome this technical problem, after several years' hardworking, members of the research team finally obtained nearly flawless high erbium concentration nanowires with single crystal quality. Professor Ning Cunzheng, led by young teachers Sun Hao and others, overcome many bottleneck difficulties in the single nanowire waveguide precise test at the submicron scale. For the first time, the intrinsic absorption coefficient of a material was accurately measured on a single nanowire and finally an optical net gain of up to 100 dB / cm was obtained, much higher than reported for other erbium-containing materials.

Figure 2 Nanowire signal enhancement and optical net gain test results, illustrations for testing system photos

The result of this study is of great importance to the study of fundamental physical properties of micro / nano structure materials and the application of devices. First of all, the gain characteristics of erbium-doped materials have laid the physical foundation for the future development of photonic integrated active devices on silicon substrates with an optical communication wavelength of 1.5 μm, such as nanoscale lasers and optical amplifiers. According to estimates, such ultra-high gain Enough to make sub-millimeter-sized lasers and amplifiers on the chip with this material. Second, the combination of long coherence lifetime of the erbium-containing material and its narrow spectral width make it attractive for quantum information system applications. In addition, the material can be used in applications such as solar cells, solid-state lighting, biofluorescence Marking and other fields. At the same time, this study is of great significance to the study of nanowire structures of other rare earth elements with similar crystal quality.

The research work is the result of the cooperation between Tsinghua University and Arizona State University in the United States. The key work is completed domestically. Sun Hao, an assistant researcher in Department of Electronics, is the author of the dissertation and Professor Ning Cunzheng is the author of the dissertation. Other collaborators include Dr. Yin Lei Jun of Arizona State University and Dr. Liu Zhicheng and engineer Zheng Yi Ze of Tsinghua Task Force.

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