Photocatalytic water splitting by N-TiO2 on MgO (111) with exceptional quantum efficiencies at elevated temperatures.

Nature communications 10 (2019) 4421-

Y Li, Y-K Peng, L Hu, J Zheng, D Prabhakaran, S Wu, TJ Puchtler, M Li, K-Y Wong, RA Taylor, SCE Tsang

Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In the past decades, various systems have been developed but most of them suffer from low activities, a narrow range of absorption and poor quantum efficiencies (Q.E.) due to fast recombination of charge carriers. Here we report a dramatic suppression of electron-hole pair recombination on the surface of N-doped TiO2 based nanocatalysts under enhanced concentrations of H+ and OH-, and local electric field polarization of a MgO (111) support during photolysis of water at elevated temperatures. Thus, a broad optical absorption is seen, producing O2 and H2 in a 1:2 molar ratio with a H2 evolution rate of over 11,000 μmol g-1 h-1 without any sacrificial reagents at 270 °C. An exceptional range of Q.E. from 81.8% at 437 nm to 3.2% at 1000 nm is also reported.

Unravelling the key role of surface features behind facet-dependent photocatalysis of anatase TiO2.

Chemical communications (Cambridge, England) 55 (2019) 4415-4418

Y-K Peng, B Keeling, Y Li, J Zheng, T Chen, H-L Chou, TJ Puchtler, RA Taylor, SCE Tsang

The high activity of nanocrystallites is commonly attributed to the terminal high-energy facets. However, we demonstrate that the high activity of the anatase TiO2(001) facet in photocatalytic H2 evolution is not due to its high intrinsic surface energy, but local electronic effects created by surface features on the facet.

III-V compounds as single photon emitters

Journal of Semiconductors 40 (2019)

X Wang, L Xu, Y Jiang, Z Yin, CCS Chan, C Deng, RA Taylor

© 2019 Chinese Institute of Electronics. Single-photon emitters (SPEs) are one of the key components in quantum information applications. The ideal SPEs emit a single photon or a photon-pair on demand, with high purity and distinguishability. SPEs can also be integrated in photonic circuits for scalable quantum communication and quantum computer systems. Quantum dots made from III-V compounds such as InGaAs or GaN have been found to be particularly attractive SPE sources due to their well studied optical performance and state of the art industrial flexibility in fabrication and integration. Here, we review the optical and optoelectronic properties and growth methods of general SPEs. Subsequently, a brief summary of the latest advantages in III-V compound SPEs and the research progress achieved in the past few years will be discussed. We finally describe frontier challenges and conclude with the latest SPE fabrication science and technology that can open new possibilities for quantum information applications.

Nitride Single Photon Sources


T Zhu, JC Jarman, CX Ren, F Tang, CC Kocher, TJ Puchtler, BPL Reid, T Wang, SK Patra, S Schulz, RA Taylor, RA Oliver

Light Controlled Optical Aharonov-Bohm Oscillations in a Single Quantum Ring.

Nano letters 18 (2018) 6188-6194

H Kim, S Park, R Okuyama, K Kyhm, M Eto, RA Taylor, G Nogues, LS Dang, M Potemski, K Je, J Kim, J Kyhm, J Song

We found that optical Aharonov-Bohm oscillations in a single GaAs/GaAlAs quantum ring can be controlled by excitation intensity. With a weak excitation intensity of 1.2 kW cm-2, the optical Aharonov-Bohm oscillation period of biexcitons was observed to be half that of excitons in accordance with the period expected for a two-exciton Wigner molecule. When the excitation intensity is increased by an order of magnitude (12 kW cm-2), a gradual deviation of the Wigner molecule condition occurs with decreased oscillation periods and diamagnetic coefficients for both excitons and biexcitons along with a spectral shift. These results suggest that the effective orbit radii and rim widths of electrons and holes in a single quantum ring can be modified by light intensity via photoexcited carriers, which are possibly trapped at interface defects resulting in a local electric field.

Photonic molecules defined by SU-8 photoresist strips on a photonic crystal waveguide.

Optics express 26 (2018) 32332-32345

SA Lennon, FSF Brossard, LP Nuttall, J Wu, J Griffiths, RA Taylor

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.

Temperature induced crossing in the optical bandgap of mono and bilayer MoS2 on SiO2.

Scientific reports 8 (2018) 5380-5380

Y Park, CCS Chan, RA Taylor, Y Kim, N Kim, Y Jo, SW Lee, W Yang, H Im, G Lee

Photoluminescence measurements in mono- and bilayer-MoS2 on SiO2 were undertaken to determine the thermal effect of the MoS2/SiO2 interface on the optical bandgap. The energy and intensity of the photoluminescence from monolayer MoS2 were lower and weaker than those from bilayer MoS2 at low temperatures, whilst the opposite was true at high temperatures above 200 K. Density functional theory calculations suggest that the observed optical bandgap crossover is caused by a weaker substrate coupling to the bilayer than to the monolayer.

Mitigating the photocurrent persistence of single ZnO nanowires for low noise photodetection applications.

Nanotechnology (2018)

J-P Girard, L Giraudet, S Kostcheev, B Bercu, TJ Puchtler, R Taylor, C Couteau

In this work, we investigate the optoelectronic properties of zinc oxide (ZnO) nanowires, which are good candidates for applications based on integrated optics. Single ZnO nanowire photodetectors were fabricated with ohmic contacts. By taking current transient measurements in different atmospheres (oxygen, air, vac- uum and argon), and at various temperatures, we point out the importance of surface effects on the electrical behaviour. Results confirm that oxygen chemisorption is responsible for the existence of a high photocon- ductive gain in these devices, and for the first time a two step process in the photocurrent rise transient is reported. A maximum gain of G = 7.8 × 10^7 is achieved. However, under certain conditions, the persistence of the photocurrent can last up to several hours and as such may prevent the device from operating at useful rates. From a knowledge of the photocurrent response mechanisms, we establish a method to restore the pho- todetector to its initial state, with very low dark current, by applying an appropriate gate voltage sequence. This advances the state of the art for these detectors towards commercial applications.

Room-temperature InP/InGaAs nano-ridge lasers grown on Si and emitting at telecom bands

OPTICA 5 (2018) 918-923

Y Han, WK Ng, C Ma, Q Li, S Zhu, CCS Chan, KW Ng, S Lennon, RA Taylor, KS Wong, KM Lau

Optical Aharonov-Bohm Oscillations with Disorder Effects and Wigner Molecule in a Single GaAs/AlGaAs Quantum Ring

in NanoScience and Technology, (2018) 231-254

K Kyhm, HD Kim, R Okuyama, M Eto, KC Je, RA Taylor, G Nogues, LS Dang, AAL Nicholet, M Potemski, JS Kim, JD Song

© 2018, Springer International Publishing AG, part of Springer Nature. The optical Aharonov-Bohm effect in a single quantum ring is associated with disorder effects. In the presence of structure anisotropy, localisation, internal electric field, and impurity scattering, optical Aharonov-Bohm oscillations of an electron-hole pair become modulated. Additionally, provided that a strongly correlated exciton pair is formed in a single quantum ring similar to the Wigner molecule, novel oscillations can be observed for increasing magnetic field. In this case, the biexciton emission energy changes abruptly at transition magnetic fields with a fractional oscillation period compared to that of the exciton, the so-called fractional optical Aharonov-Bohm oscillations.

Linearly polarized photoluminescence of InGaN quantum disks embedded in GaN nanorods.

Scientific reports 8 (2018) 8124-8124

Y Park, CCS Chan, L Nuttall, TJ Puchtler, RA Taylor, N Kim, Y Jo, H Im

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%.

Carrier confinement effects of InxGa1-xN/GaN multi quantum disks with GaN surface barriers grown in GaN nanorods

OPTICAL MATERIALS 78 (2018) 365-369

Y Park, CCS Chan, RA Taylor, N Kim, Y Jo, SW Lee, W Yang, H Im

CF2 -Bridged C60 Fullerene Dimers and their Optical Transitions.

Chemphyschem : a European journal of chemical physics and physical chemistry 18 (2017) 3540-3543

P Dallas, S Zhou, S Cornes, H Niwa, Y Nakanishi, Y Kino, T Puchtler, RA Taylor, GAD Briggs, H Shinohara, K Porfyrakis

Fullerene dyads bridged with perfluorinated linking groups have been synthesized through a modified arc-discharge procedure. The addition of Teflon inside an arc-discharge reactor leads to the formation of dyads, consisting of two C60 fullerenes bridged by CF2 groups. The incorporation of bridging groups containing electronegative atoms lead to different energy levels and to new features in the photoluminescence spectrum. A suppression of the singlet oxygen photosensitization indicated that the radiative decay from singlet-to-singlet state is favoured against the intersystem crossing singlet-to-triplet transition.

A Nanophotonic Structure Containing Living Photosynthetic Bacteria.

Small (Weinheim an der Bergstrasse, Germany) 13 (2017)

D Coles, LC Flatten, T Sydney, E Hounslow, SK Saikin, A Aspuru-Guzik, V Vedral, JK-H Tang, RA Taylor, JM Smith, DG Lidzey

Photosynthetic organisms rely on a series of self-assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum, a member of the green sulfur bacteria family, light is absorbed by large antenna complexes called chlorosomes to create an exciton. The exciton is transferred to a protein baseplate attached to the chlorosome, before migrating through the Fenna-Matthews-Olson complex to the reaction center. Here, it is shown that by placing living Chlorobaculum tepidum bacteria within a photonic microcavity, the strong exciton-photon coupling regime between a confined cavity mode and exciton states of the chlorosome can be accessed, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have energy distinct from that of the exciton which can be tuned by modifying the energy of the optical modes of the microcavity. It is believed that this is the first demonstration of the modification of energy levels within living biological systems using a photonic structure.

Direct generation of linearly polarized single photons with a deterministic axis in quantum dots

NANOPHOTONICS 6 (2017) 1175-1183

T Wang, TJ Puchtler, SK Patra, T Zhu, M Ali, TJ Badcock, T Ding, RA Oliver, S Schulz, RA Taylor

Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures.

Scientific reports 7 (2017) 12067-12067

T Wang, TJ Puchtler, SK Patra, T Zhu, JC Jarman, RA Oliver, S Schulz, RA Taylor

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.

Optical fabrication and characterisation of SU-8 disk photonic waveguide heterostructure cavities.

Optics express 25 (2017) 24615-24622

LP Nuttall, FSF Brossard, SA Lennon, BPL Reid, J Wu, J Griffiths, RA Taylor

In order to demonstrate cavity quantum electrodynamics using photonic crystal (PhC) cavities fabricated around self-assembled quantum dots (QDs), reliable spectral and spatial overlap between the cavity mode and the quantum dot is required. We present a method for using photoresist to optically fabricate heterostructure cavities in a PhC waveguide with a combined photolithography and micro-photoluminescence spectroscopy system. The system can identify single QDs with a spatial precision of ±25 nm, and we confirm the creation of high quality factor cavity modes deterministically placed with the same spatial precision. This method offers a promising route towards bright, on-chip single photon sources for quantum information applications.

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


CC Kocher, TJ Puchtler, JC Jarman, T Zhu, T Wang, L Nuttall, RA Oliver, RA Taylor

Organic molecule fluorescence as an experimental test-bed for quantum jumps in thermodynamics.

Proceedings. Mathematical, physical, and engineering sciences 473 (2017) 20170099-20170099

C Browne, T Farrow, OCO Dahlsten, RA Taylor, V Vlatko

We demonstrate with an experiment how molecules are a natural test bed for probing fundamental quantum thermodynamics. Single-molecule spectroscopy has undergone transformative change in the past decade with the advent of techniques permitting individual molecules to be distinguished and probed. We demonstrate that the quantum Jarzynski equality for heat is satisfied in this set-up by considering the time-resolved emission spectrum of organic molecules as arising from quantum jumps between states. This relates the heat dissipated into the environment to the free energy difference between the initial and final state. We demonstrate also how utilizing the quantum Jarzynski equality allows for the detection of energy shifts within a molecule, beyond the relative shift.

Optical polarization in mono and bilayer MoS2

CURRENT APPLIED PHYSICS 17 (2017) 1153-1157

Y Park, N Li, CCS Chan, BPL Reid, RA Taylor, H Im