Sub- $\mu W$ Threshold Nano-Island Lasers

Abstract

III-V photonics is very fruitful and still promising research area for small lasers such as VCSELs and photonic crystal (PhC) lasers. For example, VCSELs have now begun to facilitate optical interconnections between racks and electronic circuit boards. However, optical devices have not replaced their electronic counterparts for inter-chip and intra-chip transmission, because even VCSELs and the conventional PhC lasers have limits on how much active volume can be reduced in size. This is, in fact, a rather general issue in III-V photonics where the uniform geometry of active medium as one of the epitaxial layers is a strong constraint in designing the architecture of the devices. If such constraint can be lifted to tailor the size of active medium, it will lead to a radical improvement in performance of nanophotonic devices, and realize ultralow threshold nanolasers at room-temperature. In this thesis, we present a new platform for laser where size and position of nano-emitters are deterministically controlled. Using a selective wet-etching technique, we remove the absorptive quantum-well (QW) background to form a single nano-island active medium ( $0.7\times0.25\times 0.02 \mu m^3$ ) inside a photonic crystal cavity. First, we report an optically pumped nano-island QW nanobeam laser, which is demonstrated in room-temperature (RT) continuous wave (CW) operation. Such RT CW operation is possible due to the ultralow threshold power of 210 nW absorbed at 980 nm and the relatively high heat conductivity of the InP material. Employing a relatively thick (420 nm) InP slab for thermal and mechanical stabilities, one needs to consider the photonic band structure where TM-like modes come close to the TE-like cavity. Here, we take advantages of 1D-like nanobeam structure to minimize the optical losses caused by TE-TM coupling. Through the 3D FDTD simulation, we also study the influence of QW etching on the optical loss in the cavity. This includes the effect of the air-gap collapsing after the QW etching. Secondly, we present our work towards achieving electrically-driven nano-island QW lasers. When employed for electrical pumping, our nano-island QW structure can offer an ideal platform to realize sub- $\mu A$ threshold nanolasers at RT because this structure has a thin air-gap that can be used as an electrical insulation between the p-doped and n-doped claddings. We show our structural design for electrical insulation that is suitable for nano-island QW structures, and demonstrate that such design can be readily fabricated. We study possible causes for air-gap collapsing, and demonstrate a method to sustain the air-gap reliably for an ideal current injection without a major leakage current. We share our stories of struggle with large contact resistance due to non-ohmic contact, and explain how we overcome such issues. With direct current, we present our clear observation of cavity modes in electroluminescence with increasing slop efficiency. Based on measured linewidth, we concluded that it has reached the transparency condition in the nano-island QW. Furthermore, driving our fabricated samples with short voltage pulses, we present our observation of cavity modes with an emphasis on circuit dynamics for accurate measurements of injected current pulses.