Enhanced and Polarization Dependent Coupling for Photoaligned Liquid Crystalline Conjugated Polymer Microcavities

ACS Photonics American Chemical Society (ACS) (2020) acsphotonics.9b01596

RA Taylor, F Le Roux, DDC Bradley

Non-polar nitride single-photon sources

Journal of Optics IOP Publishing 22 (2020) 073001-073001

T Wang, RA Oliver, RA Taylor

Purcell enhancement of a deterministically coupled quantum dot in an SU-8 laser patterned photonic crystal heterostructure

Applied Physics Letters AIP Publishing 117 (2020) 043103-043103

H Shao, G Ying, S Lennon, F Brossard, J Griffiths, L Nuttall, V Osokin, E Clarke, H He, R Taylor

Enhanced photoluminescence quantum yield of MAPbBr3 nanocrystals by passivation using graphene

Nano Research Springer 13 (2020) 932-938

Y Park, A Jana, CW Myung, T Yoon, G Lee, C Kocher, G Ying, V Osokin, R Taylor, KS Kim

Diminishing surface defect states in perovskite nanocrystals is a highly challenging subject for enhancing optoelectronic device performance. We synthesized organic/inorganic lead-halide perovskite MAPbBr3 (MA = methylammonium) clusters comprising nanocrystals with diameters ranging between 20–30 nm and characterized an enhanced photoluminescence (PL) quantum yield (as much as ~ 7 times) by encapsulating the MAPbBr3 with graphene (Gr). The optical properties of MAPbBr3 and Gr/MAPbBr3 were investigated by temperature-dependent micro-PL and time-resolved PL measurements. Density functional theory calculations show that the surface defect states in MAPbBr3 are removed and the optical band gap is reduced by a 0.15 eV by encapsulation with graphene due to partial restoration of lattice distortions.

Coarse and fine-tuning of lasing transverse electromagnetic modes in coupled all-inorganic perovskite quantum dots

Nano Research Springer Science and Business Media LLC (2020)

Y Park, G Ying, A Jana, V Osokin, CC Kocher, T Farrow, RA Taylor, KS Kim

<jats:title>Abstract</jats:title> <jats:p>Inorganic perovskite lasers are of particular interest, with much recent work focusing on Fabry-Pérot cavity-forming nanowires. We demonstrate the direct observation of lasing from transverse electromagnetic (TEM) modes with a long coherence time ∼ 9.5 ps in coupled CsPbBr<jats:sub>3</jats:sub> quantum dots, which dispense with an external cavity resonator and show how the wavelength of the modes can be controlled via two independent tuning-mechanisms. Controlling the pump power allowed us to fine-tune the TEM mode structure to the emission wavelength, thus providing a degree of control over the properties of the lasing signal. The temperature-tuning provided an additional degree of control over the wavelength of the lasing peak, importantly, maintained a constant full width at half maximum (FWHM) over the entire tuning range without mode-hopping.</jats:p>

Transmissivity and reectivity of a transverse-electric polarized wave incident on a microcavity containing strongly coupled excitons with in-plane uniaxially oriented transition dipole moments

physica status solidi (b) Wiley 257 (2020) 2000235

R Taylor, F Le Roux, D Bradley

This work examines the reflectivity and transmissivity of a transverse‐electric (TE) polarized wave incident on a microcavity containing strongly coupled excitons with in‐plane uniaxially oriented transition dipole moments, and a different interpretation to a previous report is presented. The propagation of the electric field inside the cavity is discussed, and a distinction is made between two different physical cases: the first, previously observed, and the second, which enables the interpretation of measurements carried out on a microcavity containing an oriented layer of liquid‐crystalline poly(9,9‐dioctylfluorene). In all cases, the reflected and transmitted electric fields derive from photons leaking parallel and perpendicular to the transition dipole moment orientation.

2D photocatalysts with tuneable supports for enhanced photocatalytic water splitting

Materials Today Elsevier (2020)

Y Li, S Wu, J Zheng, Y-K Peng, D Prabhakaran, R Taylor, S Tsang

Sustainable hydrogen production is attracting increasing attention and visible-light-driven water splitting is considered as one of the most promising approaches for hydrogen evolution and solar energy storage. Different materials have been screened at mild conditions in recent decades and 2-dimensional (2D) layered materials are considered good candidates for the photocatalytic water splitting reaction. 2D single layer MoS2 has shown its potential in various catalytic systems, and has also been used in photocatalytic water splitting reaction recently. However, current studies of MoS2 monolayers give low intrinsic activity, preventing it from practical applications. This is attributed to the rapid recombination of the photo-excited charge carriers at room temperature, resulting in poor quantum efficiency (QE). Herein, a state-of-the-art strategy to prolong the exciton lifetimes is reported, which is achieved by combining the 2D MoS2 nanosheets with solid state polar-faceted supports. The charge separation process can be facilitated by the strong local polarisation introduced by the polar-faceted supports, and tuned by changing the supports with different surface polarities. Polar oxide surface is the exposure of oxygen-terminated high energetic facet, which is known to give a net dipole moment perpendicular to its surface. Such variation in the surface properties of the support to the above metal would lead to a difference in metal-support interaction(s). The resulting composite structures show great enhancement toward the visible-light-driven photocatalytic water splitting reaction, giving hydrogen and oxygen evolution in a stoichiometric 2:1 ratio at elevated temperatures from pure water. Photocatalytic performances are improved by the prolonged exciton lifetimes and exceptional hydrogen evolution activity of 2977 μmol g−1 h−1 with impressive QEs are obtained over Ru-doped MoS2 nanosheets on polar ceria support, which is among the best of the reported results of similar catalytic systems to date. More excitingly, the linear relationship between the exciton lifetimes and strength of the local polarisation is also observed, indicating that the rational design of photocatalysts can be simply achieved via engineering their local polarisation by incorporation of polar-faceted materials.

Faraday-cage-assisted etching of suspended gallium nitride nanostructures

AIP Advances AIP Publishing 10 (2020) 055319

GP Gough, A Sobiesierski, R Taylor, S Shabbir, S Thomas, DM Beggs, AJ Bennett

We have developed an inductively coupled plasma etching technique using a Faraday cage to create suspended gallium-nitride devices in a single step. The angle of the Faraday cage, gas mix, and chamber condition define the angle of the etch and the cross-sectional profile, which can feature undercut angles of up to 45°. We fabricate singly- and doubly-clamped cantilevers of a triangular cross section and show that they can support single optical modes in the telecom C-band.

Near-strain-free GaN/AlGaN narrow line width UV light emission with very stable wavelength on excitation power by using superlattices

ACS Applied Electronic Materials American Chemical Society 2 (2020) 571-579

M Li, F Chen, C Kocher, H Zhang, S Li, F Huang, J Zhang, RA Taylor

<p>Because of the strong strain in nitrides, superlattice layers have been used to release the strain in the QW and reduce the quantum confined Stark effect. However, few reports discuss comprehensively the strain relaxation behavior and optical performance of a GaN/AlGaN single quantum well (QW) with inserted GaN/AlGaN superlattices (SLs). In this work, we examined a group of graded Al content GaN/Al<sub><em>x</em></sub>Ga<sub>1&ndash;<em>x</em></sub>N SL layers under the GaN/Al<sub>0.3</sub>Ga<sub>0.7</sub>N single QW grown on&nbsp;<em>c</em>-plane sapphire. Both the excitation power and temperature dependence of the time-integrated micro-photoluminescence (&mu;-PL) and time-resolved &mu;-PL were measured. The samples exhibited very narrow UV emission and had almost unchanged emission wavelength and stable line width behavior with excitation power as well as &ldquo;S-shape&rdquo; and weak &ldquo;W-shape&rdquo; characteristics with temperature due to the localization. The temperature-dependent PL lifetime was measured from 5 to 300 K, and the relatively fast recombination lifetime of the two samples was examined. Micro-Raman spectroscopy was also conducted to probe the strain state. All the results showed that adopting SLs around the QW structure produced a much more stable and desirable performance, which can be attributed to an effective relaxation of the strain in the QW.</p>

Optical shaping of the polarization anisotropy in a laterally coupled quantum dot dimer

Light: Science and Applications Springer Nature 9 (2020) 100

H Kim, K Kyhm, RA Taylor, JS Kim, JD Song, S Park

We find that the emission from laterally coupled quantum dots is strongly polarized along the coupled direction [1 1&#xAF; 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 1&#xAF; 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 by changing the orientation of the polarized excitation. An energy difference is also observed between the polarized emission spectra parallel and perpendicular to the coupled direction, and it increases by up to three times by changing the excitation polarization orientation from [110] to [1 1&#xAF; 0]. These results suggest that the dipole-dipole interaction across the two separate quantum dots is mediated and that the anisotropic wavefunctions of the excitons and biexcitons are shaped by the excitation polarization.

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

Nature Communications Springer Nature 10 (2019) 4421

Y Li, Y-K Peng, L Hu, J Zheng, D Prabhakaran, S Wu, TJ Puchtler, M Li, K-Y Wong, R 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 Royal Society of Chemistry 55 (2019) 4415-4418

Y-K Peng, B Keeling, Y Li, J Zheng, T Chen, H-L Chou, TJ Puchtler, R Taylor, S 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 IOP Publishing 40 (2019) 071906

X Wang, L Xu, Z Yin, C Chan, C Deng, R Taylor

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 American Chemical Society 18 (2018) 6188-6194

H Kim, S Park, R Okuyama, K Kyhm, M Eto, R 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 Optical Society of America 26 (2018) 32332-32345

SA Lennon, FSF Brossard, LP Nuttall, J Wu, J Griffiths, R 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.

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 silicon emitting at telecom-bands

Optica Optical Society of America 5 (2018) 918-923

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

Semiconductor nano-lasers grown on silicon and emitting at the telecom bands are advantageous ultra-compact coherent light sources for potential Si-based photonic integrated circuit applications. However, realizing room-temperature lasing inside nano-cavities at telecom bands is challenging and has only been demonstrated up to the E band. Here, we report on InP/InGaAs nano-ridge lasers with emission wavelengths ranging from the O, E, and S bands to the C band operating at room temperature with ultra-low lasing thresholds. Using a cycled growth procedure, ridge InGaAs quantum wells inside InP nano-ridges grown on patterned (001) Si substrates are designed as active gain materials. Room-temperature lasing at the telecom bands is achieved by transferring the InP/InGaAs nano-ridges onto a SiO2∕Si substrate for optical excitation. We also show that the operation wavelength of InP/InGaAs nano-lasers can be adjusted by altering the excitation power density and the length of the nano-ridges formed in a single growth run. These results indicate the excellent optical properties of the InP/InGaAs nano-ridges grown on (001) Si substrates and pave the way towards telecom InP/InGaAs nano-laser arrays on CMOS standard Si or silicon-on-insulator substrates.

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