Polarisation-controlled single photon emission at high temperatures from InGaN quantum dots

Solid-state single photon sources with polarisation control operating beyond the Peltier cooling barrier of 200 K are desirable for a variety of applications in quantum technology. Using a non-polar InGaN system, we report the successful realisation of single photon emission with a g((2))(0) of 0.21, a high polarisation degree of 0.80, a fixed polarisation axis determined by the underlying crystallography, and a GHz repetition rate with a radiative lifetime of 357 ps at 220 K in semiconductor quantum dots. The temperature insensitivity of these properties, together with the simple planar epitaxial growth method and absence of complex device geometries, demonstrates that fast single photon emission with polarisation control can be achieved in solid-state quantum dots above the Peltier temperature threshold, making this system a potential candidate for future on-chip applications in integrated systems

Highly polarized electrically driven single-photon emission from a non-polar InGaN quantum dot

© 2017 Author(s). Nitride quantum dots are well suited for the deterministic generation of single photons at high temperatures. However, this material system faces the challenge of large in-built fields, decreasing the oscillator strength and possible emission rates considerably. One solution is to grow quantum dots on a non-polar plane; this gives the additional advantage of strongly polarized emission along one crystal direction. This is highly desirable for future device applications, as is electrical excitation. Here, we report on electroluminescence from non-polar InGaN quantum dots. The emission from one of these quantum dots is studied in detail and found to be highly polarized with a degree of polarization of 0.94. Single-photon emission is achieved under excitation with a constant current giving a g(2)(0) correlation value of 0.18. The quantum dot electroluminescence persists up to temperatures as high as 130 K

Defects in III-nitride microdisk cavities

Nitride microcavities offer an exceptional platform for the investigation of light-matter interactions as well as the development of devices such as high efficiency light emitting diodes (LEDs) and low-threshold nanolasers. Microdisk geometries in particular are attractive for low-threshold lasing applications due to their ability to support high finesse whispering gallery modes (WGMs) and small modal volumes. In this article we review the effect of defects on the properties of nitride microdisk cavities fabricated using photoelectrochemical (PEC) etching of an InGaN sacrificial superlattice (SSL). Threading dislocations originating from either the original GaN pseudosubstrate are shown to hinder the undercutting of microdisk cavities during the photoelectric chemical (PEC) etching process resulting in whiskers of unetched material on the underside of microdisks. The unetched whiskers provide a pathway for light to escape, reducing microdisk Q-factor if located in the region occupied by the WGMs. Additionally, dislocations can affect the spectral stability of quantum dot emitters, thus hindering their effective integration in microdisk cavities. Though dislocations are clearly undesirable, the limiting factor on nitride microdisk Q-factor is expected to be internal absorption, indicating that the further optimisation of nitride microdisk cavities must incorporate both the elimination of dislocations and careful tailoring of the active region emission wavelength and background doping levels.The original research shown in this article has been funded by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ ERC grant agreement no. 279361 (MACONS). RAO acknowledges the Royal Academy of Engineering Leverhulme Trust Senior Research Fellowship scheme.This is the author accepted manuscript. The final version is available from the Institute of Physics via https://doi.org/10.1088/1361-6641/32/3/03300

High-temperature performance of non-polar (11–20) InGaN quantum dots grown by a quasi-two-temperature method

Non-polar (11–20) a-plane InGaN quantum dots (QDs) are one of the strongest candidates to achieve on-chip applications of polarised single photon sources, which require a minimum operation temperature of ∼200 K under thermoelectrically cooled conditions. In order to further improve the material quality and optical properties of a-plane InGaN QDs, a quasi-two-temperature (Q2T) method has been developed, producing much smoother underlying InGaN quantum well than the previous modified droplet epitaxy (MDE) method. In this work, we compare the emission features of QDs grown by these two methods at temperatures up to 200 K. Both fabrications methods are shown to be able to produce QDs emitting around the thermoelectric cooling barrier. The sample fabricated by the new Q2T method demonstrates more stable operation, with an order of magnitude higher intensity at 200 K comparing to the comparable QDs grown by MDE. A detailed discussion of the possible mechanisms that result in this advantage of slower thermal quenching is presented. The use of this alternative fabrication method hence promises more reliable operation at temperatures even higher than the thermoelectric cooling threshold, and facilitates the on-going development of high temperature polarised single photon sources based on a-plane InGaN QDs.This research was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) Grants EP/M012379/1 and EP/M011682/1. T.W. is grateful for the award of the National Science Scholarship (NSS) as PhD funding by the Singapore Agency for Science, Technology and Research (A STAR). R.A.O. is grateful to the Royal Academy of Engineering and the Leverhulme Trust for a Senior Research Fellowship

Temperature-dependent fine structure splitting in InGaN quantum dots

We report the experimental observation of temperature-dependent fine structure splitting in semiconductor quantum dots using a non-polar (11-20) a-plane InGaN system, up to the on-chip Peltier cooling threshold of 200 K. At 5 K, a statistical average splitting of 443 ± 132 eV has been found based on 81 quantum dots. The degree of fine structure splitting stays relatively constant for temperatures less than 100 K, and only increases above that temperature. At 200 K, we find that the fine structure splitting ranges between 2 ~ 12 meV, which is an order of magnitude higher than that at low temperatures. Our investigations also show that phonon interactions at high temperatures might have a correlation with the degree of exchange interactions. The large fine structure splitting at 200 K makes it easier to isolate the individual components of the polarized emission spectrally, increasing the effective degree of polarization for potential on-chip applications of polarized single photon sources.This research was supported by the Engineering and Physical Sciences Research Council (EPSRC) U.K. (Grant No. EP/M012379/1 and EP/M011682/1) T.W. is grateful for the award of a National Science Scholarship (NSS) as PhD funding by the Singapore Agency for Science, Technology and Research (A*STAR). C.C.K. is grateful for the support provided by a Clarendon Scholarship and a Mary Frances and Philip Wagley Graduate Scholarship. R.A.O. is grateful to the Royal Academy of Engineering and the Leverhulme Trust for a Senior Research Fellowship

Nitride Single Photon Sources

Single photon sources are a key enabling technology for quantum communications, and in the future more advanced quantum light sources may underpin other quantum information processing paradigms such as linear optical quantum computation. In considering possible practical implementations of future quantum technologies, the nitride materials system is attractive since nitride quantum dots (QDs) achieve single photon emission at easily accessible temperatures [1], potentially enabling the implementation of quantum key distribution paradigms in contexts where cryogenic cooling is impracticable

Theoretical and experimental analysis of radiative recombination lifetimes in nonpolar InGaN/GaN quantum dots

We present here a combined experimental and theoretical analysis of the radiative recombination lifetime in a-plane (11math formula0) InGaN/GaN quantum dots. The structures have been grown by modified droplet epitaxy and time-resolved photoluminescence measurements have been performed to gain insight into the radiative lifetimes of these structures. This analysis is complemented by multi-band math formula calculations. To account for excitonic effects, the math formula theory is coupled with self-consistent Hartree calculations. Special attention is paid to the impact of the quantum dot size on the results. Our calculations show that the residual built-in fields in these nonpolar structures are compensated by the attractive Coulomb interaction, leading to the situation that the oscillator strength is almost unaffected by changes in the quantum dot size. Furthermore, our theoretical studies reveal that the radiative lifetimes are one order magnitude lower than values for c-plane systems of identical size and shape. Our theoretical findings are consistent with experimental results. Also, the calculated lifetimes are comparable in magnitude to the measured values. The majority of the measured dots produce lifetime values of 250–300 ps, highlighting the potential of these nanostructures for future high-speed single-photon emitters.This work was supported by Science Foundation Ireland (project number 13/SIRG/2210) and Engineering and Physical Sciences Research Council (EPSRC) UK (Grants EP/M012379/1 and EP/M011682/1)

Linearly polarized photoluminescence of InGaN quantum disks embedded in GaN nanorods

We have investigated the emission from InGaN/GaN quantum disks grown on the tip of GaN nanorods. The emission at 3.21 eV from the InGaN quantum disk doesn't show a Stark shift, and it is linearly polarized when excited perpendicular to the growth direction. The degree of linear polarization is about 39.3% due to the anisotropy of the nanostructures. In order to characterize a single nanostructure, the quantum disks were dispersed on a SiO2 substrate patterned with a metal reference grid. By rotating the excitation polarization angle from parallel to perpendicular relative to the nanorods, the variation of overall PL for the 3.21 eV peak was recorded and it clearly showed the degree of linear polarization (DLP) of 51.5%

Immunohistochemical Characterisation of Cell-Type Specific Expression of CK1δ in Various Tissues of Young Adult BALB/c Mice

BACKGROUND: Casein kinase 1 delta (CK1delta) phosphorylates many key proteins playing important roles in such biological processes as cell growth, differentiation, apoptosis, circadian rhythm and vesicle transport. Furthermore, deregulation of CK1delta has been linked to neurodegenerative diseases and cancer. In this study, the cell specific distribution of CK1delta in various tissues and organs of young adult BALB/c mice was analysed by immunohistochemistry. METHODOLOGY/PRINCIPAL FINDINGS: Immunohistochemical staining of CK1delta was performed using three different antibodies against CK1delta. A high expression of CK1delta was found in a variety of tissues and organ systems and in several cell types of endodermal, mesodermal and ectodermal origin. CONCLUSIONS: These results give an overview of the cell-type specific expression of CK1delta in different organs under normal conditions. Thus, they provide evidence for possible cell-type specific functions of CK1delta, where CK1delta can interact with and modulate the activity of key regulator proteins by site directed phosphorylation. Furthermore, they provide the basis for future analyses of CK1delta in these tissues

Origins of Spectral Diffusion in the Micro-Photoluminescence of Single InGaN Quantum Dots

We report on optical characterization of self-assembled InGaN quantum dots (QDs) grown on three GaN pseudo-substrates with differing threading dislocation densities. QD density is estimated via microphotoluminscence on a masked sample patterned with circular apertures, and appears to increase with dislocation density. A non-linear excitation technique is used to observe the sharp spectral lines characteristic of QD emission. Temporal variations of the wavelength of emission from single QDs are observed and attributed to spectral diffusion. The magnitude of these temporal variations is seen to increase with dislocation density, suggesting locally fluctuating electric fields due to charges captured by dislocations are responsible for the spectral diffusion in this system. © 2013 The Japan Society of Applied Physics