We propose a modified droplet epitaxy method for fabricating self-organized GaAs/AlGaAs quantum dots (QDs) with a high As flux irradiation and a low substrate temperature. By our novel method, GaAs QDs were successfully formed, retaining their pyramidal shape, original base size and density of droplets, and preventing layer-by-layer growth. Quantum size effects of the QDs were distinctly observed by photoluminescence measurements. It was confirmed that this new modified droplet epitaxy method is promising for fabricating a high-quality GaAs/AlGaAs QD system.

Monolithic integration of III-V compound semiconductors on silicon is highly sought after for high-speed, low-power-consumption silicon photonics and low-cost, light-weight photovoltaics. Here we present a GaAs/Si direct fusion bonding technique to provide highly conductive and transparent heterojunctions by heterointerfacial band engineering in relation to doping concentrations. Metal- and oxide-free GaAs/Si ohmic heterojunctions have been formed at 300°C; sufficiently low to inhibit active material degradation. We have demonstrated 1.3 μm InAs/GaAs quantum dot lasers on Si substrates with the lowest threshold current density of any laser on Si to date, and AlGaAs/Si dual-junction solar cells, by p-GaAs/p-Si and p-GaAs/n-Si bonding, respectively. Our direct semiconductor bonding technique opens up a new pathway for realizing ultrahigh efficiency multijunction solar cells with ideal bandgap combinations that are free from lattice-match restrictions required in conventional heteroepitaxy, as well as enabling the creation of novel high performance and practical optoelectronic devices by III-V/Si hybrid integration.

Topological phonics has emerged as a novel approach to engineer the flow of light and provides unprecedented means for developing diverse photonic elements, including robust optical waveguides immune to structural imperfections. However, the development of nanoscale standing-wave cavities in topological photonics is rather slow, despite its importance when building densely-integrated photonic integrated circuits. In this Letter, we report a photonic crystal nanocavity based on a topological corner state, supported at a 90-degrees-angled rim of a two dimensional photonic crystal.A combination of the bulk-edge and edge-corner correspondences guarantees the presence of the higher-order topological state in a hierarchical manner. We experimentally observed a corner mode that is tightly localized in space while supporting a high Q factor over 2,000, verifying its promise as a nanocavity. These results cast new light on the way to introduce nanocavities in topological photonics platforms.

We developed infrared laser-evoked gene operator (IR-LEGO), a microscope system optimized for heating cells without photochemical damage. Infrared irradiation causes reproducible temperature shifts of the in vitro microenvironment in a power-dependent manner. When applied to living Caenorhabditis elegans, IR-LEGO induced heat shock-mediated expression of transgenes in targeted single cells in a more efficient and less deleterious manner than a 440-nm dye laser and elicited physiologically relevant phenotypic responses.

Topological edge states exist at the interfaces between two topologically-distinct materials.The presence and number of such modes are deterministically predicted from the bulk-band topologies, known as the bulk-edge correspondence 1 . This principle is highly useful for predictably controlling optical modes 2-5 in resonators made of photonic crystals (PhCs), leading to the recent demonstrations of micro-scale topological lasers 6-10 .Meanwhile, zero-dimensional topological trapped states in the nanoscale remained unexplored, despite its importance for enhancing light-matter interactions and for wide applications including single-mode nanolasers. Here, we report a topological PhC nanocavity with a near-diffraction-limited mode volume and its application to single-mode lasing. The topological origin of the nanocavity, formed at the interface between two topologically-distinct PhCs, guarantees the existence of only one mode within its photonic bandgap 11 . The observed lasing accompanies a high spontaneous emission coupling factor stemming from the nanoscale confinement [12][13][14][15] . These results encompass a way to greatly downscale topological photonics 2-5 .