Single Quantum Dot Emission Research Articles

Silicon-Based Single Quantum Dot Emission in the Telecoms C-Band

We report the observation of single quantum dot (QD) emission in the telecoms C-band (1530–1565 nm) from an InAs QD structure grown on a Si substrate. A large red-shift of the emission is achieved by capping InAs QDs with a thin GaAsSb layer. Sharp lines, representing emission from single QDs, are observed out to wavelengths as long as 1540 nm. Comparison is made to the optical properties of a nominally identical active region structure grown on a GaAs substrate. Single QD emission from a Si-based system at 1500 nm has the potential for single photon sources compatible with current optical fibers and reduced complexity of integration with drive electronics.

Light-Induced Fluorescence Modulation of Quantum Dot-Crystal Violet Conjugates: Stochastic Off–On–Off Cycles for Multicolor Patterning and Super-Resolution

Photoswitching or modulation of quantum dots (QDs) can be promising for many fields that include display, memory, and super-resolution imaging. However, such modulations have mostly relied on photomodulations of conjugated molecules in QD vicinity, which typically require high power of high energy photons at UV. We report a visible light-induced facile modulation route for QD-dye conjugates. QD crystal violets conjugates (QD-CVs) were prepared and the crystal violet (CV) molecules on QD quenched the fluorescence efficiently. The fluorescence of QD-CVs showed a single cycle of emission burst as they go through three stages of (i) initially quenched "off" to (ii) photoactivated "on" as the result of chemical change of CVs induced by photoelectrons from QD and (iii) back to photodarkened "off" by radical-associated reactions. Multicolor on-demand photopatterning was demonstrated using QD-CV solid films. QD-CVs were introduced into cells, and excitation with visible light yielded photomodulation from "off" to "on" and "off" by nearly ten fold. Individual photoluminescence dynamics of QD-CVs was investigated using fluorescence correlation spectroscopy and single QD emission analysis, which revealed temporally stochastic photoactivations and photodarkenings. Exploiting the stochastic fluorescence burst of QD-CVs, simultaneous multicolor super-resolution localizations were demonstrated.

Single photon extraction from self-assembled quantum dots via stable fiber array coupling

We present a direct fiber output of single photons from self-assembled quantum dots (QDs) realized by a stable fiber array-QD chip coupling. The integration of distributed Bragg reflector cavity and the etching of micropillar arrays isolate QDs and enhance their normal emission. The matched periods and mismatched diameters of the pillar array and the single-mode fiber array with Gaussian-shaped light spots enable a large alignment tolerance and a stable, efficient (i.e., near-field), and chip-effective (i.e., parallel) coupling of single QD emission, as compared to the traditional “point-based” coupling via a confocal microscope, waveguide, or fiber. The single photon counting rate at the fiber end reaches 1.87 M counts per second (cps) with a time correlation g2(0) of 0.3 under a saturated excitation, and 485 K cps with a g2(0) of 0.02 under a weak excitation, demonstrating a nice “all-fiber” single-photon source.

Unidirectional Emission of a Site-Controlled Single Quantum Dot from a Pyramidal Structure

Emission control of a quantum emitter made of semiconductor materials is of significance in various optical applications. Specifically, the realization of efficient quantum emitters is important because typical semiconductor quantum dots are associated with low extraction efficiency levels due to their high refractive index contrast. Here, we report bright and unidirectional emission from a site-controlled InGaN quantum dot formed on the apex of a silver-coated GaN nanopyramidal structure. We show that the majority of the extracted light from the quantum dot is guided toward the bottom of the pyramid with high directionality. We also demonstrate that nanopyramid structures can be detached from a substrate, thus demonstrating great potential of this structure in various applications. To clarify the directional radiation, the far-field radiation pattern is measured using Fourier microscopy. This scheme will pave the way toward the realization of a bright and unidirectional quantum emitter along with easy fabrication and large-area reproducibility.

Plasmonic Effect on Exciton and Multiexciton Emission of Single Quantum Dots.

Quantum dots are nanoscale quantum emitters with high quantum yield and size-dependent emission wavelength, holding promises in many optical and electronic applications. When quantum dots are situated close to noble metal nanoparticles, their emitting behavior can be conveniently tuned because of the interaction between the excitons of the quantum dots and the plasmons of the metal nanoparticles. This interaction at the single quantum dot level gives rise to reduced or suppressed photoluminescence blinking and enhanced multiexciton emission, which is difficult to achieve in isolated quantum dots. However, the mechanism of how plasmonic structures cause the changes in the quantum dot emission remains unclear. Because of the complexity of the system, the interfaces between metal, semiconductor, and ligands must be considered, in addition to factors such as geometry, interparticle distance, and spectral overlap. The challenges in the design and fabrication of the hybrid nanostructures as well as in understanding the exciton-plasmon coupling mechanism can be overcome by a cooperative effort in synthesis, optical spectroscopy, and theoretical modeling.

Parallel Recording of Single Quantum Dot Optical Emission Using Multicore Fibers

Single indium arsenide quantum dot emission spectra have been recorded using a four-core, crosstalk-free, multicore fiber placed at the collection arm of a confocal microscope. We developed two different measurement setups depending on the relative configuration of the excitation and collection spots. In the single-matched mode, the emission from the excited area is collected by a single core in the multicore fiber, whereas the three remaining cores capture the emission from neighboring, non-excited areas. This procedure allows for the recording of the quantum dot emission from carrier diffusion between sample positions separated by more than 6 $\mu \text{m}$ . In the multiple-matched mode, the excitation spot overlaps the four-core emission area. This configuration permits the acquisition of the micro-photoluminescence spectra at different sample positions without scanning. These results show the possibilities offered by multicore fibers for the spectroscopic analysis of single semiconductor quantum dot optical emission.

In situ tuning hydrostatic pressure at low temperature using electrically driven diamond anvil cell

Traditionally, a diamond anvil cell (DAC) operated at low temperature can be pressurized by using a helium-driven piston or remote control tightening mechanism. This approach of pressurizing DAC is not convenient for operating at low temperature. Here we develop a low-temperature pressurizing technique for in situ tuning pressure in DAC at 20 K by an electrically driven method. The improved DAC pressure apparatus is composed of traditional DAC device and a piezoelectric actuator (PZT). Here the PZT used in the experiment is the PSt 150/1010/40 supplied by the Piezomechanik. Both parts are assembled together in a red copper or stainless steel cylinder. The DAC part is thermally contacted with a low temperature holder for cooling the chamber of the DAC in the experiment. The wires of the PZT connect with the voltage source through the wiring terminals of the cryostat. As the DAC apparatus cools down, two electrodes of the PZT are connected together when a voltage difference between the electrodes is generated. When the temperature of the DAC chamber arrives at the presetting value, two electrodes of the PZT are connected with the voltage source for applying voltage to the PZT. In this paper, we find that the PZT stroke shows a linear increase with increasing voltage at 300 K, whereas it is approximately linear at 80 and 6 K. The maximum strokes are 40, 26 and 15 upm at 300, 80 and 6 K respectively when the applied voltage is 120 V. The experimental results show that the PZT-driven DAC apparatus can continuously generate pressure from 0.49 to 4.41 GPa at low temperature and applied voltage of 0-290 V, where at zero voltage an initial pressure of 0.49 GPa is generated by using driven screws of the DAC device at room temperature. The pressure in the DAC chamber is determined by the red shift of ruby florescence line. The calibrated chamber temperature in DAC is determined as a function of pressure (PZT voltage) by using the intensity ration (R2/R1) of ruby R2 and R1 fluorescence lines. We find that the chamber temperature only slightly increases with increasing pressure in a range of (19 1) K. The main difference between the present device and the other tuning DAC apparatus is that the force on the DAC can be conveniently applied by using PZT voltage. This guarantees a high pressure-tuned resolution in the experiment, e. g., we tune a single InAs quantum dot (QD) emission wavelength to match the cavity mode. Such a tuning technique is found to have applications in realizing a compact tunable single photon source or completing two-photon interference of Hong-Ou-Mandel experiments between the QD and nitrogen vacancy center in diamond or atom, respectively.

Spatially uniform enhancement of single quantum dot emission using plasmonic grating decoupler.

We demonstrate a spatially uniform enhancement of individual quantum dot (QD) fluorescence emission using plasmonic grating decouplers on thin gold or silver films. Individual QDs are deposited within the grating in a controlled way to investigate the position dependency on both the radiation pattern and emission enhancement. We also describe the optimization of the grating decoupler. We achieve a fluorescence enhancement ~3 times higher than using flat plasmon film, for any QD position in the grating.

Growth of non-polar InGaN quantum dots with an underlying AlN/GaN distributed Bragg reflector by metal-organic vapour phase epitaxy

Non-polar (11–20) InGaN quantum dots (QDs) have been grown using a modified droplet epitaxy method by metal-organic vapour phase epitaxy on top of a 15-period AlN/GaN distributed Bragg reflector (DBR) on a-plane GaN pseudo-substrate prepared by epitaxial lateral overgrowth (ELOG), in which the QDs are located at the centre of a ca. 180 nm GaN layer. The AlN/GaN DBR has shown a peak reflectivity of ∼80% at a wavelength of ∼454 nm with a 49 nm wide, flat stop-band. Variations in layer thicknesses observed by cross-sectional scanning transmission electron microscopy have been identified as the main source of degradation of the DBR reflectivity. The presence of trenches due to incomplete coalescence of the ELOG template and the formation of cracks due to relaxation of tensile strain during the DBR growth may distort the DBR and further reduce the reflectivity. The DBR top surface is very smooth and does not have a detrimental effect on the subsequent growth of QDs. Enhanced single QD emission at 20 K was observed in cathodoluminescence.

Au Microdisk-Size Dependence of Quantum Dot Emission from the Hybrid Metal-Distributed Bragg Reflector Structures Employed for Single Photon Sources**Supported by the National Key Basic Research Program of China under Grant No 2013CB922304, and the National Natural Science Foundation of China under Grant Nos 11474275 and 11464034.

We investigate metallic microdisk-size dependence of quantum dot (QD) spontaneous emission rate and micro-antenna directional emission effect for the hybrid metal-distributed Bragg reflector structures based on a particular single QD emission. It is found that the measured photoluminescence (PL) intensity is very sensitive to the size of metallic disk, showing an enhancement factor of 11 when the optimal disk diameter is 2 μm and the numerical aperture of microscope objective NA=0.5. It is found that for large metal disks, the Purcell effect is dominant for enhanced PL intensity, whereas for small size disks the main contribution comes from plasmon scattering at the disk edge within the light cone collected by the microscope objective.

Direct evidence of single quantum dot emission from GaN islands formed at threading dislocations using nanoscale cathodoluminescence: A source of single photons in the ultraviolet

Intense emission from GaN islands embedded in AlN resulting from GaN/AlN quantum well growth is directly resolved by performing cathodoluminescence spectroscopy in a scanning transmission electron microscope. Line widths down to 440 μeV are measured in a wavelength region between 220 and 310 nm confirming quantum dot like electronic properties in the islands. These quantum dot states can be structurally correlated to islands of slightly enlarged thicknesses of the GaN/AlN quantum well layer preferentially formed in vicinity to dislocations. The quantum dot states exhibit single photon emission in Hanbury Brown-Twiss experiments with a clear antibunching in the second order correlation function at zero time delay.

Enhanced single photon emission from positioned InP/GaInP quantum dots coupled to a confined Tamm-plasmon mode

We report on the enhancement of the spontaneous emission in the visible red spectral range from site-controlled InP/GaInP quantum dots by resonant coupling to Tamm-plasmon modes confined beneath gold disks in a hybrid metal/semiconductor structure. The enhancement of the emission intensity is confirmed by spatially resolved micro-photoluminescence area scans and temperature dependent measurements. Single photon emission from our coupled system is verified via second order autocorrelation measurements. We observe bright single quantum dot emission of up to ∼173 000 detected photons per second at a repetition rate of the excitation source of 82 MHz, and calculate an extraction efficiency of our device as high as 7%.

Independent dynamic acousto-mechanical and electrostatic control of individual quantum dots in a LiNbO3-GaAs hybrid

We demonstrate tuning of single quantum dot emission lines by the combined action of the dynamic acoustic field of a radio frequency surface acoustic wave and a static electric field. Both tuning parameters are set all-electrically in a LiNbO3-GaAs hybrid device. The surface acoustic wave is excited directly on the strong piezoelectric LiNbO3 onto which a GaAs-based p-i-n photodiode containing a single layer of quantum dots was epitaxially transferred. We demonstrate dynamic spectral tuning with bandwidths exceeding 3 meV of single quantum dot emission lines due to deformation potential coupling. The center energy of the dynamic spectral oscillation can be independently programmed simply by setting the bias voltage applied to the diode.

Extraction efficiency enhancement of single InAs quantum dot emission through light scattering on the Au nanoparticles

Single semiconductor quantum dots (QDs) have been considered as the promising solid-state single photon sources. To obtain bright quantum sources, the key issue is to enhance extraction efficiency of the QD emission, which is challenging since QDs normally emit isotropically in a high refractive index material. In this article, we investigate the influence of Au nanoparticles on the QD photoluminescence (PL) extraction efficiency based on the techniques of optically positioned QDs and single QD emission detection. The InAs QD samples studied are grown using the molecular beam epitaxy on a (001) GaAs substrate. The sample consists of, in sequence, a 200 nm GaAs buffer layer, a 100 nm AlAs sacrificed layer, a 30 nm GaAs, a QD layer, and a 100 nm GaAs cap layer. The QD sample is mounted in a cryostat cooled down to 5 K, and excited by illumination of a 640 nm diode laser (CW or pulsed with a repetition frequency of 80 MHz). Excitation laser beam is focused to an approximately 2 μm spot on the sample using a microscope objective (NA : 0.5) which is mounted on a nanocube XYZ piezo nanopositioning stage with a scanning range of 100×100×100 μm3. The QD PL is collected using the same objective and measured using a 0.5 m focal length monochromator equipped with a silicon charge-coupled device (CCD). The PL decay measurements are performed using a silicon avalanche photodiode (APD) and a time-correlated single-photon counting (TCSPC) board.#br#In order to study the influence of different environments surrounding the QDs on the spontaneous emission rate and the extraction efficiency, the same QD emissions are measured under the conditions that: (1) A typical QD is at first chosen and optically positioned and then its emission is measured. (2) A GaAs layer containing the QDs is lifted off from the as-grown sample by an AlAs sacrificed layer and placed on the Au film with or without Au nanoparticles. (3) Optical measurements are carried out to obtain the QD emission intensity. This technique enables us to compare the same QD emission intensity for the as-grown QD sample, which is placed on the Au film or on the Au nanoparticles.#br#In summary, it is found that the measured QD emission intensity increases up to 6 times that of the original for the QD placed on the Au nanoparticles, otherwise it is only doubled for the QD placed on the Au film. The time-resolved PL measurements show that the QDs have nearly the same decay time for the QDs in different environments, implying that the QD spontaneous emission rate has not been changed. Therefore, the enhanced PL is due to the increase of extraction efficiency. The physical mechanism underlying the Au nanoparticles-induced PL enhancement is attributed to the trapped QD emission light within the sample and scattered again by Au nanoparticles and collected by the microscopy objective.

Single QD emission from arrays of QD chains obtained by patterning-free method

The selective growth of InAs quantum dot chains self-ordered at mesoscopic distances on a rippled GaAs(001) substrate is obtained by tuning the direction of the As flux at high temperature and high As/In flux ratio. Microphotoluminescence experiments demonstrate that such isolated quantum dot chains show single quantum dot emission properties.

Spectral diffusion in nitride quantum dots: Emission energy dependent linewidths broadening via giant built‐in dipole moments

AbstractWe present a study about the origin of the huge emission linewidths broadening commonly observed for wurtzite GaN/AlN quantum dots. Our analysis is based on a statistically significant number of quantum dot spectra measured by an automatized µ‐photoluminescence mapping system applying image recognition techniques. A clear decrease of the single quantum dot emission linewidths is observed with rising overall exciton emission energy. 8‐band k · p based model calculations predict a corresponding decrease of the built‐in exciton dipole moments with increasing emission energy in agreement with the measured behavior for the emission linewidths. Based on this proportionality we explain the particular susceptibility of nitride quantum dots to spectral diffusion causing the linewidth broadening via the linear quantum‐confined Stark effect. This is the first statistical analysis of emission linewidths that identifies the giant excitonic dipole moments as their origin and estimates the native defect‐induced electric field strength to ∼2 MV/m. Our observation is in contrast to less‐polar quantum dot systems as e.g. arsenides that exhibit a naturally lower vulnerability to emission linewidth broadening due to almost negligible exciton dipole moments. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sub‐critical InAs layers on metamorphic InGaAs for single quantum dot emission at telecom wavelengths

We report on the design, the growth by MBE and the optical and morphological characterization of metamorphic InAs/InGaAs quantum dots (QD) with a density low enough to allow single dot characterization without the need of complex litographic steps to isolate single QDs. InAs sub‐critical coverages were deposited on InxGa1‐xAs metamorphic buffers (MBs) and the transition from 2D growth to 3D island nucleation was monitored by reflection high energy electron diffraction (RHEED). We discuss the fundamental differences of the sub‐critical growth method compared with the Stranski‐Krastanow one, also by considering available theoretical models. AFM confirmed that the density of QDs can be controlled down to 108 cm−2 and showed that InGaAs surfaces have an effect on QD positioning uniformity. Optical properties were studied by ensemble PL at 10 K and by μ‐PL at 4 K and single dot emission was detected in the whole range from 1000 to 1315 nm for structures grown on In0.15Ga0.85As MBs, allowing to identify most of the excitonic species at the ground state. In conclusion, our results show that the sub‐critical QD growth technique can be successfully used to prepare low‐density InAs QD structures on InxGa1‐xAs (x ≥ 0.15) metamorphic buffers and that, based on advanced μPL characterization, these nanostructures can be effective single photon sources emitting at 1300 nm, with the potential of emission extension towards 1550 nm.

Manifestation of tunneling TLS dynamics of a polymer matrix in single-molecule fluorescence blinking

Blinking (stochastic intermittence) of fluorescence is a quite common phenomenon that accompanies the emission of single quantum objects-organic chromophore molecules, quantum dots, and nanocrystals. It is demonstrated that fluorescence blinking of single organic molecules embedded into a polymer matrix including the occurrence of “grey” states is due to tunneling transitions in the two-level systems (TLSs) of the matrix. The repeated registration of fluorescence excitation spectra of single molecules (SMs) is used for our analysis. The statistics of fluorescence blinking of an SM is directly related to conformational changes (tunneling transitions in TLSs) in its immediate vicinity. Individual parameters of the corresponding elementary excitation are also determined.

Cascaded emission of linearly polarized single photons from positioned InP/GaInP quantum dots

We report on the optical characterization of site-controlled InP/GaInP quantum dots (QDs). Spatially resolved low temperature cathodoluminescence proves the long-range ordering of the buried emitters, revealing a yield of ∼90% of optically active, positioned QDs and a strong suppression of emitters on interstitial positions. The emission of single QDs shows a pronounced degree of linear polarization along the [0,−1,1] crystal axis with an average degree of polarization of 94%. Photon correlation measurements of the emission from a single QD indicate the single-photon character of the exciton and biexciton emission lines as well as the cascaded nature of the photon pair.

Controlled nucleation and optical properties of InAs quantum dots grown on faceted GaAs microstructures

AbstractWe present a study of the site‐controlled nucleation of InAs quantum dots on faceted GaAs microstructures grown by migration enhanced epitaxy and demonstrate single quantum dot emission. Well defined GaAs templates with facets mostly belonging to the crystal plane families {01n }, {11n } and (001) are realized. Subsequent InAs nucleation occurs on the plane with the highest growth rate, and allows a precise positioning of optical active InAs quantum dots. The site‐controlled quantum dots reveal excitonic and biexcitonic transitions with narrow linewidths. The results are promising for the realization of single photon sources from site‐selected InAs QDs on faceted GaAs microstructures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)