Photoluminescence from In${}_{0.5}$Ga${}_{0.5}$As/GaP quantum dots coupled to photonic crystal cavities

We demonstrate room-temperature visible-wavelength photoluminescence from In${}_{0.5}$Ga${}_{0.5}$As quantum dots embedded in a GaP membrane. Time-resolved above band photoluminescence measurements of quantum dot emission show a biexpontential decay with lifetimes of $\approx$200 ps. We fabricate photonic crystal cavities which provide enhanced outcoupling of quantum dot emission, allowing the observation of narrow lines indicative of single quantum dot emission. This materials system is compatible with monolithic integration on Si and is promising for high-efficiency detection of single quantum dot emission as well as optoelectronic devices emitting at visible wavelengths.

Figure 1

(a) QD PL from an unprocessed region of the sample as temperature is varied between 25 K and 300 K. The pump is a 405-nm CW laser diode at 700 $\mu$W. (b) Photoluminescence as a function of pump power at 10 K showing broadening at higher energies with increasing power, indicative of the presence of excited states. (c) Integrated counts of the low-energy half of the PL spectrum (to minimize the contribution of the excited states, which varies with temperature). Intensity is decreased by a factor of 4 at 300 K. (d) Semilog plot of integrated counts of the low-energy half of the PL spectrum versus inverse temperature. The fit to the exponential (solid red line) gives the activation energy $E_a=161$ meV.

Figure 2

(a) Experimental setup for time-resolved measurements. 3-ps pulses at 800 nm from a Ti:Sapphire laser are frequency doubled in a BiBO crystal. A grating is used to filter the 400-nm light, which is passed through a dichroic mirror onto the sample. The photoluminescence emitted by the quantum dots is transmitted through the dichroic, passed through a long-pass filter (LPF) to remove any residual pump, and sent to a spectrometer or streak camera (for time-resolved measurements). (b) Time-resolved streak-camera measurements showing the lifetime integrated across the low-energy half of the PL spectrum for different temperatures. Inset: The excitation pulse at 400 nm. The red line indicates the fit with a decay time of 12 ps, limited by instrument resolution. (c) Exponential fits (red solid line) of time-resolved data at 250 K, showing initial and final decay times ${\tau }_i$ and ${\tau }_f$. (d) Measured lifetimes as a function of sample temperature.

Figure 3

(a) SEM image of a photonic crystal nanocavity. (b) PL measured at 12 K with the 405-nm CW pump from an unprocessed region of thinner sample used for photonic-crystal measurements. The QD wavelength is slightly blueshifted from Fig. 1. (c) PL measurement indicating the emission of quantum dots coupled into cavity modes. The fundamental cavity mode is indicated by the black circle. (d) Lorentzian fit (red solid line) of the fundamental cavity mode at 681.7 nm with $Q=2800$. (e) FDTD-simulated electric-field intensity for the fundamental cavity mode.

Figure 4

(a) PL from a different photonic-crystal-cavity structure with the higher-order cavity mode aligned to the wavelength of the strongest QD PL measured with the 405-nm CW pump. The black circle indicates the mode of interest. Inset: The FDTD-simulated electric-field intensity for the higher-order cavity mode indicated by the circle in (a). (b) High-resolution spectra of QD PL from the cavity in (a). Measurements are taken at 10 K. The two single lines at low power are indicative of single quantum dots. (c) The change in wavelength of quantum dots (measured at 500 nW) and cavity (measured at 100 $\mu$W) as a function of temperature. Solid lines indicate quadratic fits. (d) Change in QD linewidth as a function of temperature.