Self-Tuned Quantum Dot Gain in Photonic Crystal Lasers

We demonstrate that very few (2–4) quantum dots as a gain medium are sufficient to realize a photonic-crystal laser based on a high-quality nanocavity. Photon correlation measurements show a transition from a thermal to a coherent light state proving that lasing action occurs at ultralow thresholds. Observation of lasing is unexpected since the cavity mode is in general not resonant with the discrete quantum dot states and emission at those frequencies is suppressed. In this situation, the quasicontinuous quantum dot states become crucial since they provide an energy-transfer channel into the lasing mode, effectively leading to a self-tuned resonance for the gain medium.

Figure 1

(a) Scanning electron micrograph around the defect region of a PCL laser with a lattice constant of 260 nm. (b) Plot of the electric field intensity of the lasing mode as calculated from 3D finite-difference time-domain simulations [23]. (c) Normalized lasing mode spectra for 9 different PCL devices taken at a pump power of $5\mu \mathrm{W}$, $T=4.5\mathrm{K}$. (d) Measured cavity mode intensity as a function of polarizer angle showing a polarization ratio of $25:1$.

Figure 2

(a) Lasing mode intensity (solid dots) and SE background (open dots) as a function of excitation pump power for a cavity emitting at 960 nm. Red line: linear fit. (b) Magnification of threshold region. (c) Photon correlation function $g^2(\tau =0)$ of the cavity mode as a function of normalized pump power $P/P_{\mathrm{thr}}$. Absorbed pump power at threshold, $P_{\mathrm{thr}}$, for this device emitting at 940 nm is 25 nW. $T=4.5\mathrm{K}$.

Figure 3

Cavity mode linewidth (a) and intensity (b) as a function of excitation pump power (solid dots). The $x$ axis displays optical power sent into the cavity. The corresponding absorbed pump powers are 3.6% of these values. Solid lines: Rate equation model solutions for various values of $\beta$. Red line: Best fit corresponding to $\beta =0.85\pm 0.03$. Dotted blue lines indicate the measured behavior for nonlasing devices.

Figure 4

(a) Single QD spectra recorded at pump powers below (1.5 nW), around (150 nW) and above ($1.5\mu \mathrm{W}$) the lasing threshold of the PCLs. (b) 150 nW spectrum on a log scale illustrating the broad background. (c) Schematic illustration of the possible in-plane transitions of a single QD adjacent to a WL (inset) and the corresponding density of states (DOS). (d) PCL spectrum taken at 200 nW (right $y$ axis) and measured SE lifetimes as a function of energy using a 0.1 meV broad spectral window (left $y$ axis). Sharp lines right next to the cavity mode have been tentatively labeled by QD1-QD4.