Weak coupling interactions of silicon photonic crystals with lead sulphide nanocrystals at room temperature

We demonstrate weak coupling interactions of silicon photonic crystals with PbS nanocrystals at room temperature. Coupling is verified through cold-cavity integrated waveguide measurements, with polarization extinction of 1.7 and emission enhancements that match simulations. ©2007 Optical Society of America OCIS Codes: (270.5580) Quantum Electrodynamics; (999.9999) Photonic Crystals. 1. Introduction: For silicon photonic crystals (PhC), a viable approach for studying cavity quantum electrodynamics is to integrate colloidal quantum dots (QD) as a post-processing step (1). Here, PbS QD (2,3) embedded in a PMMA matrix (4) is suggested for weak coupling interactions with localized cavity field modes of silicon 2D photonic crystal (PhC) nanocavities. Spin-coating is preferred as the deposition technique since it allows for the achievement of a thin layer of QD as well as surface uniformity. The surface changes due to PMMA deposition are characterized using atomic force microscope (AFM) measurements. Weak coupling interactions of QD in the cavity environment lead to spontaneous emission enhancement and inhibition (5) through the well-known Purcell effect, depending on spectral and spatial alignment of the QD and the cavity field mode. This enhancement, along with the cavity field mode-directionality, allows for the decoration of cavity modes using ensemble QD. 2. Design and theory: L3 silicon nanocavities are used with an additional center hole for localized PbS QD interaction (Fig 1a, b). The PhC lattice parameter a is 420 nm; the radius of air holes r is 0.319a with lateral detuning of air holes near the cavity (6). The center hole radius is either 0.286a or 0.308a. The silicon PhC are designed on a SiO2 substrate. The devices incorporate integrated waveguides that allow for lensed optical fiber characterization using a tunable laser, and are designed to have resonances near 1550 nm, within both the tunable laser and PbS QD inhomogeneous bandwidths. The devices are fabricated either using electron-beam lithography at Columbia University or using deep UV lithography at IME Singapore (Fig 1c). The PhC are characterized using 3D FDTD analysis and exhibit strongly confined modes at the center hole for enhanced CQED interactions (Fig 1b), with Q on the order of 10 3 , and a mode volume of 0.07 µm 3 . The change in cavity Q following the spin-coating of PMMA is monitored using lensed-fiber waveguide measurements. Ensemble enhancements for weakly coupled QD are computed using the computed cavity mode field profile and the experimentally observed Q. The optimal Purcell factor for the designed cavities is estimated at 75. Due to the emission preference of the cavity field mode, an estimated 11% of the cavity field mode is collected by the optics (objective lens with N. A. of 0.85), compared to 8% of the QD emission collected from the PMMA thin-film. The QD emission efficiency is also modified due to an inhibition induced by the PhC lattice and the resulting spectral bandgap. The QD area-density is approximately 1000 µm-2. Based on these parameters, an overall enhancement - averaged over all enhanced and inhibited nanocrystals - of 1.14 is calculated. Exciton linewidth evolution (7) and QD surface proximity effects are not quantitatively included in this theory, and will be further considered.