1.3 μm single photon emission from InAs/GaAs quantum dots

Abstract

Single-photon emitters are crucial for the applications in quantum communication, random number generation and quantum information processing. Self-assembled InAs/GaAs quantum dots (QDs) have demonstrated to have singlephoton emission with high extraction efficiency, single-photon purity, and photon indistinguishability. Thus they are considered as the promising deterministic single-photon emitters. To extend the emission wavelength of InAs/GaAs QDs to telecom band, several methods have been developed, such as the strain engineered metamorphic quantum dots, the use of strain reducing layers and the strain-coupled bilayer of QDs. In fact, it is reported on single-photon emissions based on InAs/InP QDs with an emission wavelength of 1.55μm, but it is difficult to combine such QDs with a high-quality distributed Bragg reflector (DBR) cavity because the refractive index difference between InP and InGaAsP is too small to obtain a DBR cavity with high quality factor. Here we investigate 1.3μm single-photon emissions based on selfassembled strain-coupled bilayer of InAs QDs embedded in micropillar cavities. The studied InAs/GaAs self-assembled QDs are grown by molecular beam epitaxy on a semi-insulating (100) GaAs substrate through strain-coupled bilayer of InAs QDs, where the active QDs are formed on the seed QDs capped with an InGaAs layer, and two-layer QDs are vertically coupled with each other. In such a structure the emission wavelength of QDs can be extended to 1.3μm. The QDs with a low density of about 6×10⁸ cm^-2 are embedded inside a planar 1-^λ GaAs microcavity sandwiched between 20 and 8 pairs of Al^0.9Ga^0.1As/GaAs as the bottom and top mirror of a DBR planar cavity, respectively. Then the QD samples are etched into 3μm diameter micropillar by photolithography and dry etching. The measured quality factor of studied pillar cavity has a typical value of approximately 300. Photoluminescence (PL) spectra of QDs at a temperature of 5 K are examined by using a micro-photoluminescence setup equipped with a 300 mm monochromator and an InGaAs linear photodiode array detector. A diode laser with a continuous wave or a pulsed excitation repetition rate of 80 MHz and an excitation wavelength of 640 nm is used to excite QDs through an near-infrared objective (^NA 0.5), and the PL emission is collected by the same objective. The time-resolved PL of the QDs is obtained by a time-correlated single photon counting. The second-order correlation function is checked by a Hanbury-Brown and Twiss setup through using ID 230 infrared single-photon detectors.<br/>In summary, we find that the 1.3μm QD exciton lifetime at 5 K is measured to be approximately 1 ns, which has the same value as the 920 nm QD exciton lifetime. The second-order correlation function is measured to be 0.015, showing a good characteristic of 1.3μm single photon emission. To measure the coherence time, i.e., to perform highresolution linewidth measurements, of the QDs emitted at the wavelength of 920 and 1300 nm, we insert a Michelson interferometer in front of the spectrometer. The obtained coherence time for 1.3μm QDs is 22 ps, corresponding to a linewidth of approximately 30μeV. Whereas, the coherence time is 216 ps for 920 nm QDs, corresponding to a linewidth of approximately 3μeV. Furthermore, both emission spectral lineshapes are different. The former is of Gaussian-like type, while the latter is of Lorentzian type.