Robert Taylor | University of Oxford (original) (raw)

Papers by Robert Taylor

Optics Express

We present experimental and numerical investigations of photonic molecules obtained from laser pa... more We present experimental and numerical investigations of photonic molecules obtained from laser patterned SU-8 photoresist strips on photonic crystal waveguides. Properties of cavities defined by a single strip are investigated and we show that two adjacent strips on a waveguide form a pair of optically coupled cavities. Simulation results and micro-photoluminescence mapping measurements demonstrate that the coupling strength is tunable by controlling the separation between the strips. Confocal mapping with decoupled collection and excitation points is used to explicitly show coupling between two cavities of a photonic molecule. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Advanced Optical Materials

Light: Science & Applications

We find that the emission from laterally coupled quantum dots is strongly polarized along the cou... more We find that the emission from laterally coupled quantum dots is strongly polarized along the coupled direction [1$$\bar 1$$ 1 ¯ 0], and its polarization anisotropy can be shaped by changing the orientation of the polarized excitation. When the nonresonant excitation is linearly polarized perpendicular to the coupled direction [110], excitons (X1 and X2) and local biexcitons (X1X1 and X2X2) from the two separate quantum dots (QD1 and QD2) show emission anisotropy with a small degree of polarization (10%). On the other hand, when the excitation polarization is parallel to the coupled direction [1$$\bar 1$$ 1 ¯ 0], the polarization anisotropy of excitons, local biexcitons, and coupled biexcitons (X1X2) is enhanced with a degree of polarization of 74%. We also observed a consistent anisotropy in the time-resolved photoluminescence. The decay rate of the polarized photoluminescence intensity along the coupled direction is relatively high, but the anisotropic decay rate can be modified b...

Applied Physics Letters

Enhancement of single photon source emission through cavity quantum electrodynamics is key to the... more Enhancement of single photon source emission through cavity quantum electrodynamics is key to the realization of applicable emitters in many quantum optics technologies. In this work, we present a flexible and convenient cavity fabrication process that writes a SU-8 microstrip onto a photonic crystal waveguide deterministically, in which InGaAs/GaAs quantum dots are present as emitters. The strip cavity is laser patterned at the location of a quantum dot with a chosen emission wavelength. Micro-photoluminescence studies are undertaken, which demonstrate an enhanced emission intensity by a factor of 2.1 with weak coupling to a single quantum dot, and time-resolved photoluminescence further shows a Purcell enhancement factor of 2.16. The fabrication process is, thus, verified as a reliable recipe to introduce deterministic cavity coupling to a chosen quantum dot.

Optics Express, Nov 21, 2018

Nano Research

published as an ASAP article. Please note that technical editing may introduce minor changes to t... more published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these "Just Accepted" manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.

Scientific Reports

We report the successful realisation of intrinsic optical polarisation control by growth, in soli... more We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k•p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions. The optical polarisation control of a quantum dot (QD) system is an essential requirement for a number of optoelectronic and quantum information applications, such as QD liquid crystal displays 1 , optical quantum computing 2 , and quantum key distribution 3-5. These 0D nanostructures possess a delta function-like density of states similar to atoms, whilst being thousands of times larger. Since their size and shape can be manipulated in the solid-state, QDs offer an easier path towards integration and development on semiconductor platforms than other comparable systems, such as atoms 6 , molecules 7 , carbon nanotubes 8 , and material defects embedded in nitride wafer substrates 9 , diamond 10 , silicon carbide 11 , and 2D nanostructures 12,13. However, in most QD systems, polarised light output can only be achieved using an external polarisation filter to define a desired polarisation state. Such configurations can be cumbersome and will incur a loss of at least 50%. To mitigate these undesired losses, direct polarisation control from the light source would be desirable. More importantly, for realistic scalable on-chip applications, one challenge that a QD system will inevitably face is the operation at temperatures high enough to reach the regime of on-chip electronic temperature regulation by thermoelectric cooling. Commercial Peltier coolers can maintain a stable temperature difference of ~100 K, allowing devices to work at ~190 K. As such, it is important not only to demonstrate QD properties, such as polarisation control, under cryogenic conditions, but also to investigate their performance and behaviour at thermoelectrically cooled temperatures. In this respect, nitride materials possess large band offsets and exciton binding energies 14 , and are suitable for operation at such high temperatures. The polarisation of light emitted by a QD is governed by the underlying crystal structure and the QD geometry. In general, the dipole moment is preferentially enhanced parallel to the direction of anisotropy of the QD geometry in a simplified picture. This effect is also found to be stronger in nitrides than in systems based on other materials, such as CdSe or InAs 15-17 , owing to particularities of the III-N valence band structure 18,19. As

Excited exciton and biexciton localised states in a single quantum ring

Applied Physics Letters, 2013

We have shown that photolithography can be used to create alignment markers on a semiconductor su... more We have shown that photolithography can be used to create alignment markers on a semiconductor substrate at cryogenic temperatures. The epoxy resist SU-8 can be exposed effectively by two-photon absorption at a temperature of 4 K. By this means a spectroscopy apparatus can be used to find the positions of randomly distributed structures at low temperatures, such as InGaAs/ GaAs quantum dots, and mark their positions. We present a systematic study of the optical exposure parameters for cryogenic two-photon laser photolithography with SU-8.

The surprising recent observation of highly emissive triplet-states in lead halide perovskites ac... more The surprising recent observation of highly emissive triplet-states in lead halide perovskites accounts for their orders-ofmagnitude brighter optical signals and high quantum efficiencies compared to other semiconductors. This makes them attractive for future optoelectronic applications, especially in bright low-threshold nanolasers. While nonresonantly pumped lasing from allinorganic lead-halide perovskites is now well-established as an attractive pathway to scalable low-power laser sources for nanooptoelectronics, here we showcase a resonant optical pumping scheme on a fast triplet-state in CsPbBr 3 nanocrystals. The scheme allows us to realize a polarized triplet-laser source that dramatically enhances the coherent signal by 1 order of magnitude while suppressing noncoherent contributions. The result is a source with highly attractive technological characteristics, including a bright and polarized signal and a high stimulated-to-spontaneous emission signal contrast that can be filtered to enhance spectral purity. The emission is generated by pumping selectively on a weakly confined excitonic state with a Bohr radius ∼10 nm in the nanocrystals. The exciton fine-structure is revealed by the energy-splitting resulting from confinement in nanocrystals with tetragonal symmetry. We use a linear polarizer to resolve 2-fold nondegenerate sublevels in the triplet exciton and use photoluminescence excitation spectroscopy to determine the energy of the state before pumping it resonantly.

Interplay between many body effects and Coulomb screening in the optical bandgap of atomically thin MoS2

Nanoscale, Jan 23, 2017

Due to its unique layer-number dependent electronic band structure and strong excitonic features,... more Due to its unique layer-number dependent electronic band structure and strong excitonic features, atomically thin MoS2 is an ideal 2D system where intriguing photoexcited-carrier-induced phenomena can be detected in excitonic luminescence. We perform micro-photoluminescence (PL) measurements and observe that the PL peak redshifts nonlinearly in mono- and bi-layer MoS2 as the excitation power is increased. The excited carrier-induced optical bandgap shrinkage is found to be proportional to n(4/3), where n is the optically-induced free carrier density. The large exponent value of 4/3 is explicitly distinguished from a typical value of 1/3 in various semiconductor quantum well systems. The peculiar n(4/3) dependent optical bandgap redshift may be due to the interplay between bandgap renormalization and reduced exciton binding energy.

Long Stokes shifts and vibronic couplings in perfluorinated polyanilines

Chem. Commun., 2017

We report the effect of surfactant addition on the optical properties of perfluorinated polyanili... more We report the effect of surfactant addition on the optical properties of perfluorinated polyanilines synthesized through liquid–liquid interfaces.

Electrically tunable organic-inorganic hybrid polaritons with monolayer WS2

Nature communications, Jan 17, 2017

Exciton-polaritons are quasiparticles consisting of a linear superposition of photonic and excito... more Exciton-polaritons are quasiparticles consisting of a linear superposition of photonic and excitonic states, offering potential for nonlinear optical devices. The excitonic component of the polariton provides a finite Coulomb scattering cross section, such that the different types of exciton found in organic materials (Frenkel) and inorganic materials (Wannier-Mott) produce polaritons with different interparticle interaction strength. A hybrid polariton state with distinct excitons provides a potential technological route towards in situ control of nonlinear behaviour. Here we demonstrate a device in which hybrid polaritons are displayed at ambient temperatures, the excitonic component of which is part Frenkel and part Wannier-Mott, and in which the dominant exciton type can be switched with an applied voltage. The device consists of an open microcavity containing both organic dye and a monolayer of the transition metal dichalcogenide WS2. Our findings offer a perspective for electr...

Quasi-one-dimensional density of states in a single quantum ring

Scientific reports, Jan 5, 2017

Generally confinement size is considered to determine the dimensionality of nanostructures. While... more Generally confinement size is considered to determine the dimensionality of nanostructures. While the exciton Bohr radius is used as a criterion to define either weak or strong confinement in optical experiments, the binding energy of confined excitons is difficult to measure experimentally. One alternative is to use the temperature dependence of the radiative recombination time, which has been employed previously in quantum wells and quantum wires. A one-dimensional loop structure is often assumed to model quantum rings, but this approximation ceases to be valid when the rim width becomes comparable to the ring radius. We have evaluated the density of states in a single quantum ring by measuring the temperature dependence of the radiative recombination of excitons, where the photoluminescence decay time as a function of temperature was calibrated by using the low temperature integrated intensity and linewidth. We conclude that the quasi-continuous finely-spaced levels arising from ...

Quantum dot-like excitonic behavior in individual single walled-carbon nanotubes

Scientific reports, Nov 16, 2016

Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects ... more Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects for applications such as optoelectronic and quantum information devices. Yet, their optical performance is hindered by low fluorescent yield. Highly mobile excitons interacting with quenching sites are attributed to be one of the main non-radiative decay mechanisms that shortens the exciton lifetime. In this paper we report on time-integrated photoluminescence measurements on individual polymer wrapped semiconducting carbon nanotubes. An ultra narrow linewidth we observed demonstrates intrinsic exciton dynamics. Furthermore, we identify a state filling effect in individual carbon nanotubes at cryogenic temperatures as previously observed in quantum dots. We propose that each of the CNTs is segmented into a chain of zero-dimensional states confined by a varying local potential along the CNT, determined by local environmental factors such as the amount of polymer wrapping. Spectral diffusi...

Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires

Nano letters, Dec 14, 2016

We demonstrate single-photon emission from self-assembled m-plane InGaN quantum dots (QDs) embedd... more We demonstrate single-photon emission from self-assembled m-plane InGaN quantum dots (QDs) embedded on the side-walls of GaN nanowires. A combination of electron microscopy, cathodoluminescence, time-resolved microphotoluminescence (μPL), and photon autocorrelation experiments give a thorough evaluation of the QD structural and optical properties. The QD exhibits antibunched emission up to 100 K, with a measured autocorrelation function of g((2))(0) = 0.28(0.03) at 5 K. Studies on a statistically significant number of QDs show that these m-plane QDs exhibit very fast radiative lifetimes (260 ± 55 ps) suggesting smaller internal fields than any of the previously reported c-plane and a-plane QDs. Moreover, the observed single photons are almost completely linearly polarized aligned perpendicular to the crystallographic c-axis with a degree of linear polarization of 0.84 ± 0.12. Such InGaN QDs incorporated in a nanowire system meet many of the requirements for implementation into quant...

Exciton Dipole–Dipole Interaction in a Single Coupled-Quantum-Dot Structure via Polarized Excitation

Nano Letters, 2016

LASERS, OPTICS, AND OPTOELECTRONICS 111101 Strongly coupled single quantum dot in a photonic crystal waveguide cavity (3 pages)

Time-integrated and time-resolved photoluminescence studies of InGaN quantum dots

physica status solidi (c), 2004