Search

We present a novel device capable of enhanced and broadband collection of pairs of photons emitted by a single semiconductor quantum dot.

The desire to push beyond ‘Rayleigh’s curse’ has resulted in new techniques for super resolution imaging by deconstructing scattered light from point sources into several spatial modes, as coupling to higher order modes is exquisitely sensitive to lateral displacement. Here we implement such an approach for high numerical aperture objectives and demonstrate that for gold nanoparticles, their intrinsic asymmetry results in coupling to higher order modes without lateral displacement. This situation not only applies to practical nanoparticles but is applicable to any dipole emitter due to the asymmetry of the emission. However, with full polarization analysis we suggest that one may be able to apply such spatial mode demultiplexing techniques.

The polarisation properties of a quantum well spin – vertical cavity surface emitting laser (spin – VCSEL), both without injection and with variable polarisation optical injection, are investigated experimentally and compared with the spin flip model (SFM). Without injection, we demonstrate two distinct types of VCSEL-pump response depending on the signs of the linewidth enhancement factor, birefringence and dichroism: firstly where the pump and VCSEL have the same sign of the ellipticity, and secondly where the VCSEL ellipticity, accompanied by the linear polarisation, switches sign. We show that by controlling the injected power, ellipticity or linear angle, near circular polarisation can be obtained. These responses bothgive insight into the electro-optical injected spin-VCSEL system, and have practical implications for the use of spin VCSELs in unique applications exploiting the ellipticity degree of freedom.

Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarisation (C points), with elliptical polarisations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor $\sim 30$), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots., 6 pages, 4 figures, plus supplementary

The generation of photon pairs in single quantum dots is based on a process that is, in its nature, deterministic. However, an efficient extraction of these photon pairs from a high-index semiconductor host material requires engineering of the photonic environment. We report on a micropillar-based device featuring an extraction efficiency of 69.4(10)$\%$ that is achieved by harnessing a broadband operation suitable for extraction of photon pairs emitted from a single quantum dot. Opposing the approaches that rely solely on Purcell enhancement to realize the enhancement of the extraction efficiency, our solution exploits a suppression of the emission into the modes other than the cavity mode. Our technological implementation requires modest fabrication effort enabling higher device yields that can be scaled up to meet the growing needs of quantum technologies. Furthermore, the design of the device can be further optimized to allow for an extraction efficiency of 85$\%$.

So-called ‘iridoplasts’ found in certain species of the genus Begonia are alike to common chloroplasts found in most plants, but with a modified, periodic internal structure reminiscent of a 1D photonic crystal. Modelling indicates that this structure gives the Begonias their iridescent blue leaves, while also enhancing the absorption of photosynthetically active radiation. We will present an overview of the nano-optical theory underlying the model of iridoplasts, contrasting the design goals and constraints of biological and artificial systems. We use a simplified optical model based on Lorentz oscillators to answer the question of if the measured structure parameters of the iridoplasts are photonically optimised or if they reflect other biological constraints. Our results show that optimised photonic absorbers will not necessarily have high reflectance, raising the possibility that photonic structures for light harvesting in nature are more common than previously believed.

By employing Tamm optical states—states localized between a thin metal microdisk and a semiconductor distributed Bragg reflector—we demonstrate near 1300 nm (O-band), narrow wavelength (20 nm FWHM), highly spatially localized photodetection. By varying the size of the microdisk, we show spectral tuning (7 nm) of the peak optical response. Furthermore, by reducing the symmetry of the microstructures, we lift the degeneracy of the polarization, producing a polarization sensitivity in our detector. These confined Tamm states are useful not only for sensing but also for photoconductive switch applications such as optical microwave switches and terahertz generation.

Spontaneous emission from excitonic transitions in InAs/GaAs quantum dots embedded in photonic crystal waveguides at 5K into non-guided and guided modes is determined by direct hyperspectral imaging. This enables measurement of the absolute coupling efficiency into the guided modes, the beta-factor, directly, without assumptions on decay rates used previously. Notably, we found beta-factors above 90% over a wide spectral range of 40meV in the fast light regime, reaching a maximum of (99 $\pm$ 1)%. We measure the directional emission of the circularly polarized transitions in a magnetic field into counter-propagating guided modes, to deduce the mode circularity at the quantum dot sites. We find that points of high directionality, up to 97%, correlate with a reduced beta-factor, consistent with their positions away from the mode field antinode. By comparison with calibrated finite-difference time-domain simulations, we use the emission energy, mode circularity and beta-factor to estimate the quantum dot position inside the photonic crystal waveguide unit cell.

We investigate the effect of nuclear spins on the phase shift and polarisation rotation of photons scattered off a quantum dot-cavity system. We show that as the phase shift depends strongly on the resonance energy of an electronic transition in the quantum dot, it can provide a sensitive probe of the quantum state of nuclear spins that broaden this transition energy. By including the electron-nuclear spin coupling at a Hamiltonian level within an extended input-output formalism, we show how a photon scattering event acts as a nuclear spin measurement, which when rapidly applied leads to an inhibition of the nuclear spin dynamics via the quantum Zeno effect, and a corresponding stabilisation of the optical resonance. We show how such an effect manifests in the intensity autocorrelation $g^{(2)}(\tau)$ of scattered photons, whose long-time bunching behaviour changes from quadratic decay for low photon scattering rates (weak laser intensities), to ever slower exponential decay for increasing laser intensities as optical measurements impede the nuclear spin evolution., Comment: 8 pages, 3 figures

We experimentally demonstrate gold microdisc structures that produce confined Tamm plasmons (CTPs), interface modes between a metal layer and a distributed Bragg reflector, resonant around 1.3 {\mu}m. Quantum dots grown within the structures show an order of magnitude increase in the photoluminescence emitted at room temperature. Varying the disc diameter, we show spectral tuning of the resonance and measure the dispersion relation as evidence of mode confinement. The simplicity of fabrication and tuneability of these structures make CTPs an ideal platform for making scalable telecom devices based on quantum dots., Comment: 8 pages, 4 figures

We determine the propagation loss of GaAs photonic crystal waveguides by spectral imaging of the spontaneous emission from the embedded InAs/GaAs quantum dots. The results are compared with the loss obtained by imaging the near field of the out-of-plane radiation of the waveguide mode propagating within the light cone. From the corresponding far field, we furthermore measure the mode wavevector, from which we determine the waveguide dispersion. Additionally, we show that spectral imaging allows to determine the relative efficiencies of the couplers. Using the same experiment, and detailed photonic simulations, we have determined the beta factor and the directionality of the emission of the QDs, finding beta factors up to 99% and high directionalities.

This work was funded by the Future Emerging Technologies (FET)-Open FP7-284743 [project Spin Photon Angular Momentum Transfer for Quantum Enabled Technologies (SPANGL4Q)] and the German Ministry of Education and research (BMBF) and Engineering and Physical Sciences Research Council (EPSRC) (EP/M024156/1, EP/N003381/1 and EP/M024458/1). J.J.H. was supported by the Bristol Quantum Engineering Centre for Doctoral Training, EPSRC grant EP/L015730/1. We acknowledge the GW4 network for funding of A.Y. Solid-state quantum emitters have long been recognised as the ideal platform to realize integrated quantum photonic technologies. We demonstrate that a self-assembled negatively charged quantum dot (QD) in a low Q-factor photonic micropillar is a suitable design for deterministic polarisation switching and spin-photon entanglement. We show this by measuring a shift in phase of an input single photon of at least 2π/3. As we explain in the text, this is strong experimental proof that input photons can interact with the emitter deterministically. A deterministic photon-emitter interaction is a viable and scalable means to achieve several vital functionalities such as single photon switches and entanglement gates. Our experimentally determined value is limited by mode mismatch between the input laser and the cavity, QD spectral fluctuations and spin relaxation. When on-resonance we estimate that up to ∼80% of the collected photons couple into the cavity mode and have interacted with the QD and undergone a phase shift of π. Postprint

It is shown that cavities formed between a multilayer quarter-wave Bragg reflector and a metal mirror which support Tamm plasmons can be modelled by using a hard-mirror approximation including appropriate penetration depths into the mirrors. Results from this model are in excellent agreement with those found by numerical methods. In addition Tamm modes that are laterally confined by the presence of a metallic disc deposited on the Bragg reflector can be described by the effective index model that is commonly used for vertical-cavity surface-emitting lasers (VCSELs). This enables the lateral modes confined by a circular disc to be found from conventional weakly-guiding waveguide theory similar to that used for optical fibres. The resonant wavelengths of these linearly-polarised (LP) guided modes are calculated as functions of disc diameter and other parameters.

Using continuous-wave optical pumping of a spin-VCSEL at room temperature, we find high spin amplification of the pump close to threshold within the communications wavelength window, here at 1300 nm. This facilitates a strong switch from left to right circularly polarised light emission, which has potential applications in polarisation encoding for data communications. We use a simple spin flip model to fit the experimental results and discuss the VCSEL parameters that affect this amplification. publishedVersion

Iridescence in shade-dwelling plants has previously been described in only a few plant groups, and even fewer where the structural colour is produced by intracellular structures. In contrast with other Selaginella species, this work reports the first example in the genus of structural colour originating from modified chloroplasts. Characterization of these structures determines that they form one-dimensional photonic multilayers. The Selaginella bizonoplasts present an analogous structure to recently reported Begonia iridoplasts; however, unlike Begonia species that produce iridoplasts, this Selaginella species was not previously described as iridescent. This therefore raises the possibility of widespread but unobserved and uncharacterized photonic structures in plants.

A photosynthetic algal intracellular organelle containing a living opal responds dynamically to environmental illumination., Natural photonic crystals are responsible for strong reflectance at selective wavelengths in different natural systems. We demonstrate that intracellular opal-like photonic crystals formed from lipids within photosynthetic cells produce vivid structural color in the alga Cystoseira tamariscifolia. The reflectance of the opaline vesicles is dynamically responsive to environmental illumination. The structural color is present in low light–adapted samples, whereas higher light levels produce a slow disappearance of the structural color such that it eventually vanishes completely. Once returned to low-light conditions, the color re-emerges. Our results suggest that these complex intracellular natural photonic crystals are responsive to environmental conditions, changing their packing structure reversibly, and have the potential to manipulate light for roles beyond visual signaling.

The authors present here designs for tuneable confined Tamm plasmons (CTPs) resonant at 1.3 μm, consisting of an AlAs/GaAs distributed Bragg reflector and gold disc. Using numerical methods they explored the effect of disc diameter on the CTP resonance and position of a dipole source (modelling a quantum dot) on emission through the disc. They found decreasing disc diameter resulted in a blue-shifted fundamental mode and that a dipole positioned at the centre of the disc emitted with an angular distribution that collected 90% of the transmitted power within a numerical aperture of 0.7. They also explore the Purcell enhancement under the CTP as a function of dipole position.

Semiconductor quantum dots are often proposed as an ideal means to achieve non-linear interactions and photon switching in semiconductor integrated circuits. We examine here the use of the electron spin degree of freedom as a means to achieve a controllable quantum switching of single photons and trains of photons. We examine some types of spin-based quantum switches, and the design principles upon which a nanophotonic quantum switch that uses narrow band photons should be based.

There has recently been great interest in chiral waveguides, where the polarisation of the forwards and backwards propagating light fields are oppositely circular at certain positions in space (C-points) [1]. This results in new phenomena, such as the one way emission of light from spin-transitions in quantum dots (QDs) or atoms (simulation, fig. b). The emitted photon's direction is determined by the spin state of the emitter.

Quantum dots (QDs) can be incorporated into solid state photonic devices such as cavities or waveguides that enhance the light-matter interaction. A near unit efficiency light-matter interaction is essential for deterministic, scalable quantum information devices [1]. In this limit, a single photon input into the device will undergo a large rotation of the polarization of the light field due to the strong interaction with the QD. In the past preliminary results have indicated that a low quality-factor (Q∼290) pillar microcavity possesses a high s-factor and that the instantaneous interactions should be deterministic and with high fidelity [2].

We report on the coherent optical response from an ensemble of (In,Ga)As quantum dots (QDs) embedded in a planar Tamm-plasmon microcavity with a quality factor of approx. 100. Significant enhancement of the light-matter interaction is demonstrated under selective laser excitation of those quantum dots which are in resonance with the cavity mode. The enhancement is manifested through Rabi oscillations of the photon echo, demonstrating coherent control of excitons with picosecond pulses at intensity levels more than an order of magnitude smaller as compared with bare quantum dots. The decay of the photon echo transients is weakly changed by the resonator indicating a small decrease of the coherence time $T_2$ which we attribute to the interaction with the electron plasma in the metal layer located close (40nm) to the QD layer. Simultaneously we see a reduction of the population lifetime $T_1$, inferred from the stimulated photon echo, due to an enhancement of the spontaneous emission by a factor of 2, which is attributed to the Purcell effect, while non-radiative processes are negligible as confirmed from time-resolved photoluminescence., 7 pages, 5 figures

We demonstrate a deterministic shift in phase of an input single photon by a negatively charged quantum dot in a low Q-factor, high output efficiency micropillar cavity, with values up to 2π/3.

Joint or simultaneous measurements of non-commuting quantum observables are possible at the cost of increased unsharpness or measurement uncertainty. Many different criteria exist for defining what an "optimal" joint measurement is, with corresponding different tradeoff relations for the measurements. Understanding the limitations of such measurements is of fundamental interest and relevant for quantum technology. Here, we experimentally test a tradeoff relation for the sharpness of qubit measurements, a relation which refers directly to the form of the measurement operators, rather than to errors in estimates. We perform the first optical implementation of the simplest possible optimal joint measurement, requiring less quantum resources than have previously often been employed. Using a heralded single-photon source, we demonstrate quantum-limited performance of the scheme on single quanta., 7 pages, 6 figures

By performing a full analysis of the projected local density of states (LDOS) in a photonic crystal waveguide, we show that phase plays a crucial role in the symmetry of the light-matter interaction. By considering a quantum dot (QD) spin coupled to a photonic crystal waveguide (PCW) mode, we demonstrate that the light-matter interaction can be asymmetric, leading to unidirectional emission and a deterministic entangled photon source. Further we show that understanding the phase associated with both the LDOS and the QD spin is essential for a range of devices that can be realized with a QD in a PCW. We also show how suppression of quantum interference prevents dipole induced reflection in the waveguide, and highlight a fundamental breakdown of the semiclassical dipole approximation for describing light-matter interactions in these spin dependent systems.

We present slow-light photonic crystal waveguide designs that provide a $\times$8.6 improvement of the local density of optical states at a fully chiral point over previous designs., 4 pages, 3 figures

Enhanced light harvesting is an area of interest for optimizing both natural photosynthesis and artificial solar energy capture1,2. Iridescence has been shown to exist widely and in diverse forms in plants and other photosynthetic organisms and symbioses3,4, but there has yet to be any direct link demonstrated between iridescence and photosynthesis. Here we show that epidermal chloroplasts, also known as iridoplasts, in shade-dwelling species of Begonia5, notable for their brilliant blue iridescence, have a photonic crystal structure formed from a periodic arrangement of the light-absorbing thylakoid tissue itself. This structure enhances photosynthesis in two ways: by increasing light capture at the predominantly green wavelengths available in shade conditions, and by directly enhancing quantum yield by 5-10% under low-light conditions. These findings together imply that the iridoplast is a highly modified chloroplast structure adapted to make best use of the extremely low-light conditions in the tropical forest understorey in which it is found5,6. A phylogenetically diverse range of shade-dwelling plant species has been found to produce similarly structured chloroplasts7-9, suggesting that the ability to produce chloroplasts whose membranes are organized as a multilayer with photonic properties may be widespread. In fact, given the well-established diversity and plasticity of chloroplasts10,11, our results imply that photonic effects may be important even in plants that do not show any obvious signs of iridescence to the naked eye but where a highly ordered chloroplast structure may present a clear blue reflectance at the microscale. Chloroplasts are generally thought of as purely photochemical; we suggest that one should also think of them as a photonic structure with a complex interplay between control of light propagation, light capture and photochemistry.

Photonic crystal waveguides can support polarisation singularities, which are predicted to be useful in future quantum information applications. For example, C-points possess local chirality, allowing spin-photon entanglement when a quantum dot is placed at its location. Photonic crystal waveguides also support slow-light modes, and we have studied the enhancement of this coupling in the slow-light regime. We have also studied the practicalities of using such C-points in real world waveguides with disorder.

Quantum dots (QDs) are semiconductor nanostructures in which a three dimensional potential trap produces an electronic quantum confinement, thus mimicking the behaviour of single atomic dipole-like transitions. However unlike atoms, QDs can be incorporated into solid state photonic devices such as cavities or waveguides that enhance the light-matter interaction. A near unit efficiency light-matter interaction is essential for deterministic, scalable quantum information (QI) devices. In this limit, a single photon input into the device will undergo a large rotation of the polarization of the light field due to the strong interaction with the QD. In this paper we measure a macroscopic ($\sim6^o$) phase shift of light as a result of the interaction with a negatively charged QD coupled to a low quality-factor (Q$\sim290$) pillar microcavity. This unexpectedly large rotation angle demonstrates this simple low Q-factor design would enable near deterministic light-matter interactions., Comment: 6 pages, 3 figures

We report the first experimental observation of an excitonic optical Tamm state supported at the interface between a periodic multilayer dielectric structure and an organic dye-doped polymer layer. The existence of such states is enabledby the metal-like optical properties of the excitonic layer based on aggregated dye molecules. Experimentally determined dispersion curves, together with simulated data, including field profiles, allow us to identify the nature of these new modes. Our results demonstrate the potential of organic excitonic materials as a powerful means to control light at the nanoscale, off ering the prospect of a new alternative photonic building block for nanophotonics designs based on molecular materials.

We present the simulation, fabrication, and optical characterization of low-index polymeric rod-connected diamond (RCD) structures. Such complex three-dimensional photonic crystal structures are created via direct laser writing by two-photon polymerization. To our knowledge, this is the first measurement at near-infrared wavelengths, showing partial photonic bandgaps for this structure. We characterize structures in transmission and reflection using angular resolved Fourier image spectroscopy to visualize the band structure. Comparison of the numerical simulations of such structures with the experimentally measured data show good agreement for both P- and S-polarizations.

The first solid state structures for cavity quantum electrodynamic studies were realized analog to established designs from the field of trapped atom or ions: Single emitters (e.g. semiconductor quantum dots) were place in optical resonators with small mode volumes and a large quality factors. These devices have allowed the demonstration of effects that were already observed for atom/ion systems, like the strong coupling of a single emitter to an optical cavity mode [1,2]. However, a solid state platform offers much more control over the optical density of states than the open resonators used in quantum optics experiments with atoms or ions. Modern nano-fabrication tools can be used to pattern the dielectric environment of the emitters with nanometer precision, opening completely new possibility to tailor their photonic environment. This has led to the development of alternative structures, e.g. for single photon sources. Cavity based single photon sources rely on the Purcell effect to direct the spontaneous emission into a defined optical mode. However, it is also possible to collect a large fraction of the emitted photons in a single mode when the emission into other modes is suppressed. This can be achieved by embedding the quantum dots in photonic crystal waveguides [3] or nanowires [4]. A big advantage of these structures is their broadband operation that is not limited by the bandwidth of a cavity resonance.

Semiconductor and plasmonic nanostructures have attracted considerable attention during the last decade. Semiconductors are the basis of today's information technology because of the possibility of tailoring electrical and optical properties on a detailed level. An elementary optical excitation in semiconductors is the electron-hole pair (exciton) with large oscillator strength which potentially enables ultrafast information processing. However, for quantum information applications excitons have been scarcely considered because of their limited optical coherence time due to complex many body interactions and their short radiative lifetime (below 1 ns) being the downside of the large oscillator strength. In nanostructures such as quantum dots (QDs) the optical decoherence is weak but still limited by radiative decay. Therefore approaches to involve the long-lasting coherence of resident electron spins have been pursued recently. The key excitation in plasmonics is the surface plasmon polariton (SPP), which is a coupled oscillation of the electromagnetic field and electron plasma in a metal. SPPs allow electromagnetic energy to be concentrated in nanoscale volumes at the interface between a metal and a dielectric, leading to an enhancement of linear and nonlinear optical effects. Current state of the art in nanotechnology allows fabrication of sophisticated metallic structures thus making possible light engineering at the nanoscale. Plasmonic structures may be shaped to manipulate the polarization of the light into novel “orbital” angular momentum states. Therefore, placing semiconductor nanostructures in the vicinity of plasmonic structures may establish transfer of angular momentum to a QD spin as well as efficient coherent control of electronic states. In order to achieve these goals detailed knowledge of near-field distribution of the electromagnetic fields and their propagation, optical response of the system under excitation with optical pulses which is governed by dynamics of electronic states in each of the constituents (semiconductor and plasmonic nanostructures) and combined hybrid structures are required.

The effects of short range disorder on the polarization characteristics of light in W1 photonic crystal waveguides were simulated. It was found that points of local circular polarization (C-points) were robust in both quantity and location.

We report the measurement of macroscopic phase shifts of several degrees for reflected incident light resonant with a bright negatively charged quantum dot (QD) in a micropillar structure of Q-factor less than 200.

The generation of electron spin coherence has been studied in n-modulation-doped (In,Ga)As/GaAs self-assembled quantum dots (QDs) which contain on average a single electron per dot. The doping has been confirmed by pump–probe Faraday rotation experiments in a magnetic field parallel to the heterostructure growth direction. For studying spin coherence, the magnetic field was rotated by 90° to the Voigt geometry, and the precession of the electron spin about the field was monitored. The coherence is generated by resonant excitation of the QDs with circularly polarized laser pulses, creating a coherent superposition of an electron, and a trion state. The efficiency of the generation can be controlled by the pulse intensity, being most efficient for (2n+1)π pulses.

A key to ultra-long electron spin memory in quantum dots (QDs) at zero magnetic field is polarization of the nuclei, such that the electron spin is stabilized along the nuclear magnetic field. We demonstrate that spin-polarized electrons in n-doped QDs align the nuclear field via hyperfine interaction. A feedback onto the electrons occurs, leading to stabilization of polarization, for which spin memory is retained over 0.2 seconds.

Spin coherence has been studied in self-assembled (In,Ga)As/GaAs quantum dots which contain on average a single electron per dot due to n-type modulation doping. A magnetic field was applied in the Voigt geometry, in order to study the precession of the electron spins which are oriented normal to the field. Here we address the mechanism by which such a normal orientation can be achieved by resonant excitation with circularly polarized laser pulses. Further, we discuss, how long this coherence is maintained within an ensemble of quantum dots. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have performed single dot photoluminescence and time-resolved ensemble photoluminescence measurements on InAs quantum dots embedded in a lateral in-plane p–i–n or n–i–n device, respectively, which makes the application of lateral electric fields, i.e. field direction perpendicular to the growth direction, feasible. Time-resolved measurements show an increase in the radiative lifetime of up to 30% with increasing field. We attribute this to the reduced overlap between the electron and hole wave functions. Single dot spectroscopy revealed a small red-shift of the emission energies of maximum 0.5 meV. This shift can be explained by the quantum confined Stark effect taking into account that the red-shift due to the band-tilting is partly compensated by a decrease in exciton binding energy.