A Two-Dimensional Photonic Crystal Nanocavity on Gold: Towards the Ultimate Nanolaser

Se-Heon Kim, KNI Prize Postdoctoral Fellow, Department of Electrical Engineering, Kavli Nanoscience Insitute - Caltech

ABSTRACT: Two-dimensional (2-D) photonic crystal (PhC) slab nanocavities have become an important tool to store and manipulate light and to increase various types of photon-matter interactions, including weak- and strong-coupling for cavity quantum electrodynamics (cQED) experiments. A PhC semiconductor nanolaser - an ultra-small semiconductor laser utilizing a PhC nanocavity as the feedback mechanism - has attracted much attention due to its potential promise of thresholdless lasing and ultra-high-speed modulation of lasers. Since the first demonstration of optically pumped PhC nanolaser in 1999 (at Caltech), there has been considerable improvements in its design, both in terms of high Q-factors and small mode volumes. It is interesting to note that there has been no substantial modification to the originally proposed air-suspended slab geometry, since the total internal reflection-like confinement in the direction perpendicular to the slab is a key to maximize Q factors. However, the use of air cladding - both an electrical and a thermal insulator - poses severe constraints on the laser's practical applications, such as current injection operation and room-temperature continuous wave lasing.

In this talk, I propose and demonstrate a novel PhC nanolaser, in which a flat metal surface is brought into contact with the PhC nanocavity. The metal can serve as both an electrical-current pathway and a heat sink. On the other hand, it can also be used as a reflector to limit optical loss and control the laser's far-field radiation profile. I will show an interesting analogy of this nanolaser design to the famous cQED example of a point dipole source in front of a mirror: original radiation characteristics of the PhC nanocavity - such as Q factors and far-field
radiation patterns - can be controlled by changing the distance between the metal and the cavity. Based on this principle, we designed the most practical form of PhC nanolasers, made of InP/InGaAsP material systems aimed at the telecommunication wavelength of 1.3 um. 3-D
numerical simulations have shown that, assuming the use of gold as the bottom cladding, one can achieve linearly-polarized unidirectional vertical emission with radiation efficiency of over 70%. Most recent experimental results will be followed with a focus on various abricationrelated issues. Finally, I will discuss an important implication of the proposed device in the field of plasmonics and try to define the concept of ‘plasmonicity’ that can account for the fraction of total energy of the mode that goes into surface plasmons.