Significance The laser has had a revolutionary impact on scientific research and technological applications since it was invented more than 60 years ago. Extensive theoretical and application research has been conducted, and many important developments have been reported. For example, the size of lasers has both increased and decreased. The linear dimensions of lasers have increased by more than 10 orders of magnitude, and ultra-small semiconductor lasers that are uniquely important for many applications have been developed.
Driven by Moore's law, continuous progress of microelectronics technology has resulted in unprecedented challenges and requirements. Developments in microelectronics technology have presented significant possibilities related to the transition from electronics to photonics for information transmission and processing. However, the field of integrated nanophotonics, particular in relation to lasers, still faces obstacles, including dimensions, energy consumption, and integration with silicon photonic devices. Currently, semiconductor lasers are generally more than tens of microns. To be more compatible with electronic devices, the size must be reduced by two to three orders of magnitude. According to system level analysis, the energy consumption of on-chip optical interconnection needs to be less than 10 fJ/bit, and the data transfer rate must be greater than 10 Gbit/s. Studies have shown that the power-to-bandwidth ratio decreases as the device size decreases. Silicon is an indirect bandgap semiconductor that emits light inefficiently; thus, it is necessary to integrate lasers based on other materials with silicon-based electronic chips. Nanoscale lasers have the potential to overcome the influences of mechanical strain caused by lattice mismatch and to be integrated with silicon. Therefore, the development of nanolasers is significant no matter which aspect is considered.
In addition to the on-chip interconnection required by future information technology, detection, sensing and high-definition display based on nanolasers are also important application areas. Currently, continuous miniaturization and stable operation under electrical pumping are being pursued. The emergence of novel cavity designs and gain materials have created new opportunities for nanolaser research.
Progress The emergence of semiconductor nanolasers followed naturally from the development of semiconductor lasers. Since first developed in 1962, semiconductor lasers have undergone several breakthroughs in cavity designs, and each breakthrough has led to improved performance, lower thresholds, reduced size, and the appearance of new application scenarios. The early semiconductor laser cavity based on the Fabry- Pérot etalon was naturally formed by the crystal cleavage plane. In the 1970s and 1980s, the distributed feedback laser and distributed Bragg reflection (DBR) laser with distributed feedback mechanisms were developed. These developments had a decisive influence on reducing the laser threshold, improving the monochromaticity, and increasing the modulation speed, and such developments played a critical role in the use of semiconductor lasers in the field of optical communications. The vertical cavity surface emitting laser based on the DBR structure appeared in the 1980s, followed by various microcavity concepts in the 1990s, and subsequently photonic crystal lasers. In the 21st century, the development of ever smaller lasers has led to many novel nanoscale laser designs. The typical feature of these lasers is that, in at least one dimension, the size is on the order of submicron or much shorter, representing the dawn of the nanolaser age. Nanolasers are primarily divided into two categories. One category is represented by lasers based on various nanomaterials and nanostructures, such as nanowires, nanobelts, and nanofilms (Figs. 1 and Figs. 2). In 2001, Yang's research group realized an ultraviolet laser based on ZnO nanowires at room temperature for the first time. The other category is lasers based on a plasmonic mode at the metal-dielectric interface (Figs. 4 and Figs. 5). Plasmonic devices use free electron oscillations on the metal surface to enhance light confinement, which allows the size of laser to break the diffraction limit. In 2009, three teams independently demonstrated the first plasmonic nanolasers, or spasers, with different structures based on a surface-plasmon polariton mode or a localized surface-plasmon mode.
Conclusions and Prospect Lasers with ever decreasing sizes, i.e., down to nanoscales, or nanolasers, have evolved rapidly in recent years. We briefly describe the historical background of nanolasers including various types, their basic features, possible applications, current status, existing problems, and future trends. The types of nano-cavities include nanowire cavities, whispery-gallery mode cavities, Fabry-Pérot cavities, as well as surface-plasmon polariton cavities. The types of gain media include conventional compound semiconductors as well as newly emerging materials, such as perovskites and transition-metal dichalcogenides.
