M. Benyoucef

On-chip Si/SiOx microtube refractometer

The authors fabricate rolled up microtubes consisting of Si/SiOx on Si substrate and analyze the possibility to use them as a refractometric sensor. An aqueous sugar solution is inserted into the microtube, which leads to a change in refractive index and, as a result, to a detectable spectral shift of the whispering gallery modes. Experimental results can fit well with finite-difference time-domain simulations, which are used to determine the sensitivity of this tube refractometer. The ratio of spectral sensitivity to channel cross-sectional area of the refractometer is particularly striking and allows analysis of fluid volumes in the range of femtoliters. A comparative discussion with other existing refractometer schemes concludes this work.

Enhancing the Optical Excitation Efficiency of a Single Self-Assembled Quantum Dot with a Plasmonic Nanoantenna

We demonstrate how the controlled positioning of a plasmonic nanoparticle modifies the photoluminescence of a single epitaxial GaAs quantum dot. The antenna particle leads to an increase of the luminescence intensity by about a factor of 8. Spectrally and temporally resolved photoluminescence measurements prove an increase of the quantum dot’s excitation rate.

Light emission and wave guiding of quantum dots in a tube

We present microphotoluminescence investigations of InAs quantum dots (QDs) integrated into self-rolling InGaAs∕GaAs strained layers. The emission signal from the QDs is redshifted due to strain relaxation and increased in intensity after the strained layers are released from the substrate and rolled up into tubes. We detect waveguided light at the tube ends, which originates from the QDs at the laser excitation spot. The possibility of integrating quantum emitters into the tube walls acting as waveguides represents a major step toward the realization of flexible high quality factor optical resonators based on rolled-up nano- and microtubes.

Site-controlled growth and luminescence of InAs quantum dots using in situ Ga-assisted deoxidation of patterned substrates

Site-controlled growth of single and pairs of InAs quantum dots is demonstrated on ex situ electron-beam patterned (001) GaAs substrates using in situ Ga-assisted deoxidation prior to overgrowth. 6–8 ML of gallium deposited at a substrate temperature of 460°C in the absence of arsenic followed by a brief anneal under arsenic is used to remove the surface oxide without damaging a pattern consisting of ∼100nm wide, ∼20nm deep holes. Single dot luminescence is shown from a dilute array (10μm spacing) of such site-controlled dots, located only 8nm from the regrowth interface.

Optical properties of rolled-up tubular microcavities from shaped nanomembranes

Tubular optical microcavities have been fabricated by releasing prestressed SiO/SiO2 bilayer nanomembranes from polymer sacrificial layers, and their geometrical structure is well controlled by defining the shape of nanomembranes via photolithography. Optical measurements at room temperature demonstrate that resonant modes of microtubular cavities rolled up from circular shapes can be tuned in peak energy and relative intensity along the tube axes compared to those from square patterns. The resonant modes shift to higher energy with decreasing number of tube wall rotations and thickness, which fits well to finite-difference time-domain simulations. Polarization resolved measurements of the resonant modes indicate that their polarization axes are parallel to the tube axis, independent of the polarization of the excitation laser.Tubular optical microcavities have been fabricated by releasing prestressed SiO/SiO2 bilayer nanomembranes from polymer sacrificial layers, and their geometrical structure is well controlled by defining the shape of nanomembranes via photolithography. Optical measurements at room temperature demonstrate that resonant modes of microtubular cavities rolled up from circular shapes can be tuned in peak energy and relative intensity along the tube axes compared to those from square patterns. The resonant modes shift to higher energy with decreasing number of tube wall rotations and thickness, which fits well to finite-difference time-domain simulations. Polarization resolved measurements of the resonant modes indicate that their polarization axes are parallel to the tube axis, independent of the polarization of the excitation laser.

Ordered GaAs quantum dot arrays on GaAs(001) : Single photon emission and fine structure splitting

Ordered GaAs∕AlGaAs quantum dots (QDs) are fabricated on patterned GaAs(001) substrates and their optical properties are investigated by microphotoluminescence (PL) spectroscopy. QDs exhibit sharp excitonic lines with typical single QD emission features. Photon-correlation spectroscopy shows single photon emission for the neutral exciton transition. Polarization-dependent PL measurements reveal a sharp exciton line and a fine structure exchange splitting of about 70μeV.

Correlated photon pairs from single (In,Ga)As∕GaAs quantum dots in pillar microcavities

We demonstrate the triggered generation of photon pairs by the cascaded biexciton-exciton emission from a single (In,Ga)As∕GaAs quantum dot (QD) in a pillar microcavity. Photon cross-correlation measurements between the biexciton and exciton decay reveal highly asymmetric features under continuous wave excitation and a bunching effect under pulsed excitation due to the cascaded nature of the process. An enhancement of the QD photoluminescence (PL) intensity by a factor of 40 was found for pillar microcavities (0.6μm diameter) compared to the excitonic PL intensity in bulk semiconductors, thus reflecting the enhanced photon collection effect out of the cavity structure. The observed pillar mode structure is well understood on the basis of an extended transfer-matrix method.

Radiative emission dynamics of quantum dots in a single cavity micropillar

The light emission of self-assembled (In,Ga)As/GaAs quantum dots embedded in single GaAs-based micropillars has been studied by time-resolved photoluminescence spectroscopy. The altered spontaneous emission is found to be accompanied by a non-exponential decay of the photoluminescence where the decay rate strongly depends on the excitation intensity. A microscopic theory of the quantum dot photon emission is used to explain both, the non-exponential decay and its intensity dependence. Also the transition from spontaneous to stimulated emission is studied.

Wavelength Tunable Triggered Single-Photon Source from a Single CdTe Quantum Dot on Silicon Substrate

Triggered single-photon emission from a single CdTe quantum dot (QD) grown on Si(001) substrate is demonstrated for the first time. The emission wavelength of QDs can be tuned in a wide spectral range (more than 8 meV) using a focused laser beam. A nearly perfect single-photon emission from the exciton lines is preserved even after energy tuning. The lifetime is also measured before and after laser processing, and no appreciable change is observed.

Enhanced correlated photon pair emission from a pillar microcavity

We demonstrate the efficient generation of triggered photon pairs by placing a single quantum dot (QD) into a micropillar cavity. Photon cross-correlation measurements between biexciton and exciton decay reveal a bunching effect under pulsed excitation due to the cascaded nature of the emission. No polarization correlation between the exciton and biexciton emission is observed. Furthermore, the emission mode structure of the pillar microcavities is investigated within a theory-experiment comparison where calculations are based on an extended transfer matrix method. Efficient mode confinement perpendicular to the emission direction leads to a series of transverse modes combined with enhanced QD emission. For the photoluminescence (PL) intensity of QDs in pillar microcavities (0.6 μm diameter), an enhancement factor of 40 was found in comparison to the PL intensity of QDs in bulk semiconductors, reflecting the enhanced photon collection effect out of the cavity structure.