Photonic crystal lasers have attracted much attention because of the feasibility of its small energy cost operation. An electrically pumped photonic crystal laser in room-temperature continuous-wave operation is strongly desired. We have developed electrically driven photonic crystal laser employing an ultracompact active region embedded with InP layer. The device exhibits threshold current of 4.8 μA and the energy cost of 4.4 fJ/bit at 10-Gbit/s modulation. In addition, we have demonstrated heterogeneous integration of photonics crystal laser on Si substrate by using buried growth on III-V template, which is directly bonded on SiO2/Si substrate.

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Nanowire lasers are promising as ultra-small, highly-efficient coherent light emitters in the context of nanophotonics, nano-optics and nanobiotechnology. Although there have been several demonstrations of nanowire lasers utilizing homogeneous bulk gain materials, it is crucial to incorporate lowdimensional quantum nanostructures into the nanowire so as to achieve superior device performance with respect to threshold current, modulation bandwidth and temperature sensitivity. The quantum dot is a useful and essential nanostructure that can meet these requirements. However, difficulties in forming stacks of quantum dots in a single nanowire hamper the realization of lasing operation. Here, we demonstrate for the first time room-temperature lasing in a single nanowire with 50 quantum dots by properly designing the nanowire cavity and tailoring the emission energy of each dot to enhance the optical gain. Our demonstration paves the way toward ultra-small lasers with extremely low-power consumption for integrated photonic systems and bio/environmental sensing.

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An overview of researches on semiconductor membrane lasers is presented. The membrane structure, which consists of very thin III-V core layers sandwiched by low index dielectric or air claddings, can enhance optical confinement to the III-V layers. This enhancement enables very low threshold current and highly efficient laser operation. An on-chip optical interconnection is one of the applications for membrane photonic integrated circuits toward large scale data center computing. Such application will require over 0.16 mW output power and 10 Gbit/s modulation with an operating current less than 1 mA. In theoretical calculations, these requirements can be met by membrane laser structures. In actual devices, a threshold current of 250 μA was achieved and a modulation efficiency of 9.9 GHz/ mA was demonstrated with InP-based membrane structure on Si substrate.

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We describe here recent progresses in two types of photonic crystal lasers, which can miniaturize conventional semiconductor lasers significantly. One is Si Raman laser based on ultrahigh-Q nanocavities, where we discuss the importance of the difference between Raman frequency and the frequency spacing of two-nanocavity modes and the effect of strain which exists in Si slab. The other is high-beam quality, high power laser based on band-edge effect of photonic crystals, where we investigate the distribution of resonant frequency of individual small areas inside the device and mutual injection locking phenomena among the neighboring small areas.

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Terahertz quantum cascade laser (THz-QCL) is a compact, high output power, narrow linewidth and low cost terahertz laser, and is very attractive for a lot of applications as a practical use terahertz light source. In this report, we demonstrate our recent research activities on the development of high performance THz-QCLs, i.e., realization of higher temperature operation of THz-QCL using an indirect injection scheme QC design, high output power operation of THz-QCLs at 77 K liquid nitrogen temperature and development of unexplored frequency THz-QCLs using nitride semiconductors. Through these studies, we also discuss on the future prospects of THz-QCLs.

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A laser oscillation assisted by localized surface plasmons of nanostructured metals is described. A random laser is readily achieved in media where metallic nanoparticles are randomly dispersed. Surprisingly, laser oscillation is feasible even in such a medium where the scattering mean free path is as long as 200 mm, indicating that plasmon-enhanced scattering is not the main origin for the laser oscillation. An experiment using nanoparticles composed of metallic core and oxide shell shows that the separation between the metal and an emission center determines the lasing threshold and suggests that the excitation rate, which is enhanced by the increased electric field due to the surface plasmons, is dominant in the low-threshold laser oscillation. We also numerically demonstrate that an energy flow in only one direction is realized for a dielectric nanosphere that contains an optical gain with a metallic semi-shell, which is one kind of optical microresonator.

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We studied on a method needed to generate mode-locked microcomb in a silica toroidal microcavity. By controlling the input power and wavelength, we obtained microcombs with arbitrary free-spectral range. We also developed a Lugiato-Lefever equation including Raman scattering effect and investigated the influence of Raman scattering upon microcomb formation. A bandwidth ranging from 1550 to 2200 nm was obtained experimentally from a continuous-wave pump due to broadband Raman gain of silica.

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