S. Michaelis de Vasconcellos

Bright solid-state sources of indistinguishable single photons

Bright sources of indistinguishable single photons are strongly needed for the scalability of quantum information processing. Semiconductor quantum dots are promising systems to build such sources. Several works demonstrated emission of indistinguishable photons while others proposed various approaches to efficiently collect them. Here we combine both properties and report on the fabrication of ultrabright sources of indistinguishable single photons, thanks to deterministic positioning of single quantum dots in well-designed pillar cavities. Brightness as high as 0.79±0.08 collected photon per pulse is demonstrated. The indistinguishability of the photons is investigated as a function of the source brightness and the excitation conditions. We show that a two-laser excitation scheme allows reducing the fluctuations of the quantum dot electrostatic environment under high pumping conditions. With this method, we obtain 82±10% indistinguishability for a brightness as large as 0.65±0.06 collected photon per pulse.

Controlling spontaneous emission with plasmonic optical patch antennas.

We experimentally demonstrate the control of the spontaneous emission rate and the radiation pattern of colloidal quantum dots deterministically positioned in a plasmonic patch antenna. The antenna consists of a thin gold microdisk separated from a planar gold layer by a few tens of nanometers thick dielectric layer. The emitters are shown to radiate through the entire patch antenna in a highly directional and vertical radiation pattern. Strong acceleration of spontaneous emission is observed, depending on the antenna geometry. Considering the double dipole structure of the emitters, this corresponds to a Purcell factor up to 80 for dipoles perpendicular to the disk.

Single photon source using confined Tamm plasmon modes

We evaluate the potential of confined Tamm plasmon structures for single photon extraction. A single quantum dot is inserted in a structure consisting of a gold microdisk deposited on a distributed Bragg reflector. The quantum dot exciton line experiences acceleration of spontaneous emission while the biexciton line experiences inhibition. This property leads to non-linearities in the emission dynamics and the photon statistics as a function of the excitation rate. We show that the device operates as a single photon source with a measured brightness of 0.20 ± 0.02 collected photons per pulse. By modeling the extraction efficiency, we show that efficiencies around 60% can be reached in optimized structures.

An intentionally positioned (In,Ga)As quantum dot in a micron sized light emitting diode

We have integrated individual (In,Ga)As quantum dots (QDs) using site-controlled molecular beam epitaxial growth into the intrinsic region of a p-i-n junction diode. This is achieved using an in situ combination of focused ion beam prepatterning, annealing, and overgrowth, resulting in arrays of individually electrically addressable (In,Ga)As QDs with full control on the lateral position. Using microelectroluminescence spectroscopy we demonstrate that these QDs have the same optical quality as optically pumped Stranski–Krastanov QDs with random nucleation located in proximity to a doped interface. The results suggest that this technique is scalable and highly interesting for different applications in quantum devices.

Spatial, spectral, and polarization properties of coupled micropillar cavities

We report on an experimental and numerical study of the spatial and spectral properties of the optical modes in coupled pillar microcavities. Highly efficient photon blockade or bright sources of entangled photon pairs can be implemented by coupling a single quantum emitter to coupled cavities. Parameters for optimal coupling with a single quantum emitter are identified. Polarization properties, which are critical for both applications, are finally discussed. We show that an extremely small polarization splitting is obtained for the first modes in a wide range of parameters.

Single photon emission based on coherent state preparation

The authors report here on deterministic single photon emission after coherent optical state preparation in the p-shell of a single InGaAs∕GaAs quantum dot. In the approach, they use p-shell Rabi flopping followed by relaxation to the s-shell ground state with subsequent spontaneous single photon emission. Pulsed photon correlation experiments show complete suppression of the correlation peak at zero time delay and hence demonstrate clean single photon emission.

Coherent optoelectronics with single quantum dots

The optical properties of semiconductor quantum dots are in many respects similar to those of atoms. Since quantum dots can be defined by state-of-the-art semiconductor technologies, they exhibit long-term stability and allow for well-controlled and efficient interactions with both optical and electrical fields. Resonant ps excitation of single quantum dot photodiodes leads to new classes of coherent optoelectronic functions and devices, which exhibit precise state preparation, phase-sensitive optical manipulations and the control of quantum states by electrical fields.

Exciton spectroscopy on single CdSe/ZnSe quantum dot photodiodes

We have investigated the properties of neutral and charged excitons in single CdSe/ZnSe QD photodiodes by @m-photoluminescence spectroscopy. By applying a bias voltage, we have been able to control the number of electrons in a single QD by shifting the energy levels of the QD with respect to the Fermi level in the back contact. Also the quantum-confined Stark effect was observed as a function of the applied electric field.

Micro-Raman imaging and micro-photoluminescence measurements of strain in ZnMgSe/ZnSe microdiscs

Semiconductor microdiscs are promising for applications in photonics and quantum-information processing, such as efficient solid-state-based single-photon emitters. Strain in the multilayer structure of those devices has an important influence on their optical properties. We present measurements of the strain distribution in ZnMgSe/ZnSe microdiscs by means of micro-photoluminescence and micro-Raman imaging. Photoluminescence measurements of microdiscs reveal substantially broadened emission lines with a shift to lower energy at the undercut part of microdiscs, indicating local relaxation in this area. The distribution of the strain in the microdiscs is obtained from an imaging micro-Raman analysis, revealing that the freestanding part of the microdiscs is free of defects.

High purcell effect and directional emission for semi-conductor nanocrystals deterministically positionned in a plasmonic patch antenna

Plasmonic nano-antennas provide broadband spontaneous emission control by confining light on highly sub-wavelength volumes. This property assures efficient coupling and spectral matching of spectrally broad emitters like defects in diamonds or colloidal quantum dots with nanostructures. Precise positioning of emitters inside the nanostructures has been a limitation for efficient coupling and remains a key parameter to control for efficient light nanostructures interaction. In this paper, we propose to optimise the interaction of nanoemittors with antennas by assuring spatial and spectral matching. Although spontaneous emission acceleration ensuring large coupling to the mode has been evidenced in antennas, this property has not been combined with high emission directionnality.Here, we control both the spontaneous emission rate and radiation pattern of nanocrystals in a plasmonic antenna. We use a patch antenna, as proposed in [1], consisting in a thin gold microdisk 30 nm above a thick gold layer (fig. 1a), with a emitter positioned in the dielectric spacer. The small 30 nm separation between the disk and the gold film provide a large confinement of the electromagnetic field. The emitters are clusters of CdSe/CdS colloidal nanocrystals. A deterministic positioning of clusters inside each antenna with a precision of 25nm is provided by an optical in situ lithography technique as demonstrated in [2] for quantum dots in micropillars. The emitters are shown to radiate through the entire patch antenna in a highly directional mode. The theory predicts an acceleration of dipole emission up to 70 for a dipole perpendicular to the gold layers, and much lower for a dipole parallel (fig. 2a). For an antenna with a gold disk diameter ranging from 1.4 to 2.2. m, emission is directive in accordance with the theory (fig 1b-1c). The average cluster lifetime is reduced by a factor ranging from 5 to 15 (fig. 2b) which correspond, taking into account the 2D dipole transition of the emitters, their orientation and distribution of lifetime inside the cluster, to accelerations consistent with an acceleration of 70 for vertical dipoles. These measurements, in good agreement with our theoretical calculations, evidence high directionality and acceleration of spontaneous emission. Our work demonstrates the potential of plasmonic patch antennas to fabricate efficient single photon sources.