A photonic crystal nanocavity laser with ultralow threshold

Abstract

Confining photons in a small space is a difficult task. Recent nanotechnology advances now enable their confinement in a cubic wavelength volume for as long as 1 nanosecond in semiconductors when using 2D photonic crystal (PhC) structures. As shown in Figure 1, a PhC is a periodic optical structure whose periodicity is on the order of the wavelength of light. PhCs are designed to affect the motion of photons1 because they have the optical equivalent of the energy gap of conventional semiconductors. The possibility to manipulate photons in solid materials offers many attractive applications such as nanocavity lasers, tunable slow light devices, and optical integrated circuits. The PhC nanocavity laser is considered one of the best candidates to achieve ultra-low threshold lasing due to its small mode volume and high quality factor (Q, which expresses the ability of an oscillating system to keep oscillating before running out of energy). The first reported PhC laser (1999) used multiple quantumwells (QWs) as the gain material at low temperature in pulsed operation.2 Recently, continuous wave (CW) laser operation at low temperature was also reported. However, CW lasing is inherently difficult to achieve at room temperature due to fast non-radiative losses that translate into very high Q nanocavity requirements. Our group was the first to demonstrate room temperature operation of a high-Q PhC nanocavity CW laser.3 Further improvement of the cavity Q reduced the optical pumping threshold down to the microwatt level.4 3D photon confinement can be achieved by fabricating the PhC slab structure with a nanocavity (see Figure 1). The 2D triangular lattice confines photons in the plane of the slab. The excitation beam generates excitons in quantum dots (QDs) embedded into the slab layer and the excitons recombine with photon emission. Our PhC nanocavity was designed to have its resonant wavelength at the excitonic photoluminescence (PL) peak, allowing CW lasing at 1.33μm at room temperature with Figure 1. Scanning electron micrographs of a photonic crystal nanocavity structure. Top (a) and cross sectional (b) views. QD: quantum dot.