Imperial College London

ProfessorNeilAlford

Faculty of EngineeringDepartment of Materials

Professor of Materials
 
 
 
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Contact

 

+44 (0)20 7594 6724n.alford

 
 
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Assistant

 

Ms Karen Murphy +44 (0)20 7594 3330

 
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Location

 

2..05 (in RSM)Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

441 results found

Ng W, Wen Y, Attwood M, Jones DC, Oxborrow M, Alford N, Arroo Det al., 2024, Maser-in-a-Shoebox': a portable plug-and-play maser device at room-temperature and zero magnetic-field, Applied Physics Letters, Vol: 124, ISSN: 0003-6951

Masers, the microwave analogs of lasers, have seen a renaissance owing to the discovery of gain media that mase at room temperature and zero-applied magnetic field. However, despite the ease with which the devices can be demonstrated under ambient conditions, achieving the ubiquity and portability, which lasers enjoy, has to date remained challenging. We present a maser device with a miniaturized maser cavity, gain material, and laser pump source that fits within the size of a shoebox. The gain medium used is pentacene-doped para-terphenyl, and it is shown to give a strong masing signal with a peak power of −5 dBm even within a smaller form factor. The device is also shown to mase at different frequencies within a small range of 1.5 MHz away from the resonant frequency. The portability and simplicity of the device, which weighs under 5 kg, paves the way for demonstrators particularly in the areas of low-noise amplifiers, quantum sensors, cavity quantum electrodynamics, and long-range communications.

Journal article

Wen Y, Diggle PL, Alford NM, Arroo DMet al., 2023, Exploring the spin dynamics of a room-temperature diamond maser using an extended rate equation model, Journal of Applied Physics, Vol: 134, ISSN: 0021-8979

Masers—the microwave analog of lasers—are coherent microwave sources that can act as oscillators or quantum-limited amplifiers. Masers have historically required high vacuum and cryogenic temperatures to operate, but recently, masers based on diamond have been demonstrated to operate at room temperature and pressure, opening a route to new applications as ultra-low noise microwave amplifiers. For these new applications to become feasible at a mass scale, it is important to optimize diamond masers by minimizing their size and maximizing the power of signals that can be amplified. Here, we develop and numerically solve an extended rate equation model to present a detailed phenomenology of masing dynamics and determine the optimal properties required for the copper cavity, dielectric resonator, and gain medium in order to develop portable maser devices. We conclude by suggesting how the material parameters of the diamond gain media and dielectric resonators used in diamond masers can be optimized, and how rate equation models could be further developed to incorporate the effects of temperature and nitrogen concentration on spin lifetimes.

Journal article

Wang Y, Guerenneur A, Ramadan S, Huang J, Fearn S, Nabi N, Klein N, Alford NM, Petrov PKet al., 2023, Toward fabrication of devices based on graphene/oxide multilayers, ACS Applied Electronic Materials, Vol: 5, Pages: 3261-3267, ISSN: 2637-6113

Owing to its high electrical conductivity, low density, and flexibility, graphene has great potential for use as a building block in a wide range of applications from nanoelectronics to biosensing and high-frequency devices. For many device applications, it is required to deposit dielectric materials on graphene at high temperatures and in ambient oxygen. This has been proven to be highly challenging because these conditions cause significant degradation in graphene. In this work, we investigate the degradation of graphene at elevated temperatures in an oxygen atmosphere and possible protection mechanisms to enable the growth of oxide thin films on graphene at higher temperatures. We show that coating graphene with self-assembled monolayers of hexamethyldisilazane (HMDS) prior to a high-temperature deposition can significantly reduce the damage induced. Furthermore, a graphene sample treated with HMDS displayed a weaker doping effect due to weak interaction with oxygen species than bare graphene, and a much slower rate of electrical resistance degradation was exhibited during annealing. Thus, it is a promising approach that could enable the deposition of metal oxide materials on graphene at high temperatures without significant degradation in graphene quality, which is critical for a wide range of applications.

Journal article

Yao Q, Berenov AVV, Bower R, Zou B, Xiao X, Alford NMM, Oulton RFM, Petrov PKKet al., 2023, Crystalline AuNP-Decorated Strontium Niobate Thin Films: Strain-Controlled AuNP Morphologies and Optical Properties for Plasmonic Applications, ACS APPLIED NANO MATERIALS, Vol: 6, Pages: 11115-11123

Journal article

Doiron B, Li Y, Bower R, Mihai A, Dal Forno S, Fearn S, Hüttenhofer L, Cortés E, Cohen LF, Alford NM, Lischner J, Petrov P, Maier SA, Oulton RFet al., 2023, Optimizing hot electron harvesting at planar metal–semiconductor interfaces with titanium oxynitride thin films, ACS Applied Materials and Interfaces, Vol: 25, Pages: 30417-30426, ISSN: 1944-8244

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride (TiON)/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency (NFC ≈ 2.8 × 1019 m-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (NHE ≈ 1.6 × 1018 m-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.

Journal article

Wang Y, Zou B, Rente B, Alford N, Petrov PKet al., 2023, Deposition of nanocrystalline multilayer graphene using pulsed laser deposition, Crystals, Vol: 13, Pages: 1-8, ISSN: 2073-4352

The wide application of graphene in the industry requires the direct growth of graphene films on silicon substrates. In this study, we found a possible technique to meet the requirement above. Multilayer graphene thin films (MLG) were grown without a catalyst on Si/SiO2 using pulsed laser deposition (PLD). It was found that the minimum number of laser pulses required to produce fully covered (uninterrupted) samples is 500. This number of laser pulses resulted in samples that contain ~5 layers of graphene. The number of layers was not affected by the laser fluence and the sample cooling rate after the deposition. However, the increase in the laser fluence from 0.9 J/cm2 to 1.5 J/cm2 resulted in a 2.5-fold reduction in the MLG resistance. The present study reveals that the PLD method is suitable to produce nanocrystalline multilayer graphene with electrical conductivity of the same magnitude as commercial CVD graphene samples.

Journal article

Bower R, Wells MP, Johnson F, Kilmurray R, Doiron B, Calì E, Mallia G, Zou B, Mihai AP, Harrison NM, Fearn S, Oulton R, Alford NM, Cohen LF, Petrov PKet al., 2021, Tunable double epsilon-near-zero behavior in niobium oxynitride thin films, Applied Surface Science, Vol: 569, Pages: 150912-150912, ISSN: 0169-4332

Journal article

Arroo DM, Alford NM, Breeze JD, 2021, Perspective on room-temperature solid-state masers, Applied Physics Letters, Vol: 119, Pages: 1-10, ISSN: 0003-6951

The first solid-state masers to operate at room-temperature and ambient air-pressure were recently demonstrated using optically pumped spin-triplet states as the gain medium. In this Perspective, we briefly review the previous state-of-the-art in cryogenic solid-state masers and then discuss the development of the room-temperature solid-state maser: from the organic pentacene pulsed maser to the diamond nitrogen-vacancy continuous-wave maser. We characterize the operation of these masers as coherent microwave sources and ultra-low noise amplifiers before outlining how they can be adapted to act as model systems in which to explore room-temperature cavity quantum electrodynamics. After discussing challenges facing current embodiments of the room-temperature solid-state maser, we explore how they might be addressed or by-passed altogether through the development of alternative materials and masing mechanisms. Finally, we speculate on how the advent of masers that can operate in ambient conditions might lead to novel applications in metrology and quantum technologies.

Journal article

Doiron B, Gusken NA, Lauri A, Li Y, Mihai A, Matsui T, Bower R, Huettenhoffer L, Regoutz A, Forno SD, Fearn S, Petrov PK, Cortes E, Cohen LF, Alford NM, Lischner J, Petrov P, Maier SA, Oulton RFet al., 2020, Hot Carrier Optoelectronics with Titanium Nitride, Lasers and Electro-Optics Society Annual Meeting-LEOS, ISSN: 1092-8081

© 2020 OSA. Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.

Conference paper

Doiron B, Güsken NA, Lauri A, Li Y, Mihai A, Matsui T, Bower R, Huettenhoffer L, Regoutz A, Forno SD, Fearn S, Petrov PK, Cortés E, Cohen LF, Alford NM, Lischner J, Maier SA, Oulton RFet al., 2020, Hot carrier optoelectronics with titanium nitride

Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.

Conference paper

Le Goupil F, Baker A, Tonus F, Berenov A, Randall CA, Alford NMet al., 2019, Direct measurement of electrocaloric effect in lead-free (Na<sub>0.5</sub>Bi<sub>0.5</sub>)TiO<sub>3</sub>-based multilayer ceramic capacitors, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 39, Pages: 3315-3319, ISSN: 0955-2219

Journal article

Doiron B, Li Y, Mihai A, Bower R, Alford NM, Petrov PK, Maier SA, Oulton RFet al., 2019, Plasmon-enhanced electron harvesting in robust titanium nitride nanostructures, The Journal of Physical Chemistry Part C: Nanomaterials and Interfaces, Vol: 123, Pages: 18521-18527, ISSN: 1932-7447

Titanium nitride (TiN) continues to prove itself as an inexpensive, robust, and efficient alternative to gold in plasmonic applications. Notably, TiN has improved hot electron-harvesting and photocatalytic abilities compared to gold systems, which we recently attributed to the role of oxygen in TiN and its native semiconducting TiO2–x surface layer. Here, we explore the role of localized surface plasmon resonances (LSPRs) on electron harvesting across the TiN/TiO2–x interface and probe the resilience of TiN nanostructures under high-power laser illumination. To investigate this, we fabricate TiN strips, in which the lateral confinement allows for the polarization-selective excitation of the LSPR. Using ultrafast pump–probe spectroscopy, optical characterization, and Raman vibrational spectroscopy, we relate the differences and changes observed in the electron behavior to specific material properties. We observe plasmon-enhanced electron harvesting beyond what is expected resulting from the enhanced absorption of the plasmonic mode. We accredit this to the surface oxide damping the plasmon resonance, providing additional nonradiative loss channels. Subsequently, we show that low-power annealing of the surface oxide layer reduces the trap density at the interface and increases the initial harvested electron concentration. The unique properties of TiN make it important in the future development of plasmonic electron-harvesting applications.

Journal article

Pang JS, Theodorou IG, Centeno A, Petrov PK, Alford NM, Ryan MP, Xie Fet al., 2019, Tunable three-dimensional plasmonic arrays for large near-infrared fluorescence enhancement, ACS Applied Materials and Interfaces, Vol: 11, Pages: 23083-23092, ISSN: 1944-8244

Metal-enhanced fluorescence (MEF), resulting from the near-field interaction of fluorophores with metallic nanostructures, has emerged as a powerful tool for dramatically improving the performance of fluorescence-based biomedical applications. Allowing for lower autofluorescence and minimal photoinduced damage, the development of multifunctional and multiplexed MEF platforms in the near-infrared (NIR) windows is particularly desirable. Here, a low-cost fabrication method based on nanosphere lithography is applied to produce tunable three-dimensional (3D) gold (Au) nanohole–disc arrays (Au-NHDAs). The arrays consist of nanoscale glass pillars atop nanoholes in a Au thin film: the top surfaces of the pillars are Au-covered (effectively nanodiscs), and small Au nanoparticles (nanodots) are located on the sidewalls of the pillars. This 3D hole–disc (and possibly nanodot) construct is critical to the properties of the device. The versatility of our approach is illustrated through the production of uniform and highly reproducible Au-NHDAs with controlled structural properties and tunable optical features in the NIR windows. Au-NHDAs allow for a very large NIR fluorescence enhancement (more than 400 times), which is attributed to the 3D plasmonic structure of the arrays that allows strong surface plasmon polariton and localized surface plasmon resonance coupling through glass nanogaps. By considering arrays with the same resonance peak and the same nanodisc separation distance, we show that the enhancement factor varies with nanodisc diameter. Using computational electromagnetic modeling, the electric field enhancement at 790 nm was calculated to provide insights into excitation enhancement, which occurs due to an increase in the intensity of the electric field. Fluorescence lifetime measurements indicate that the total fluorescence enhancement may depend on controlling excitation enhancement and therefore the array morphology. Our findings provide important in

Journal article

Doiron B, Mota M, Wells MP, Bower R, Mihai A, Li Y, Cohen LF, Alford NM, Petrov PK, Oulton RF, Maier SAet al., 2019, Quantifying figures of merit for localized surface plasmon resonance applications: a materials survey, ACS Photonics, Vol: 6, Pages: 240-259, ISSN: 2330-4022

Using localized surface plasmon resonances (LSPR) to focus electromagnetic radiation to the nanoscale shows the promise of unprecedented capabilities in optoelectronic devices, medical treatments and nanoscale chemistry, due to a strong enhancement of light-matter interactions. As we continue to explore novel applications, we require a systematic quantitative method to compare suitability across different geometries and a growing library of materials. In this work, we propose application-specific figures of merit constructed from fundamental electronic and optical properties of each material. We compare 17 materials from four material classes (noble metals, refractory metals, transition metal nitrides, and conductive oxides) considering eight topical LSPR applications. Our figures of merit go beyond purely electromagnetic effects and account for the materials’ thermal properties, interactions with adjacent materials, and realistic illumination conditions. For each application we compare, for simplicity, an optimized spherical antenna geometry and benchmark our proposed choice against the state-of-the-art from the literature. Our propositions suggest the most suitable plasmonic materials for key technology applications and can act as a starting point for those working directly on the design, fabrication, and testing of such devices.

Journal article

Sathian J, Minassian A, Alford NM, Damzen MJet al., 2019, Enhancing performance of Ce:YAG luminescent concentrators for high power applications

High power scaling of an LED pumped luminescent concentrator (LC) can address significant excitation and illumination applications, including laser pumping and medical light therapy [1-3]. However, heat deposited into the luminescent material is critical and was already a serious issue in our prior systems [3] where forced air cooling of the Ce:YAG LC light-guide was employed but limited power and/or duration of operation. Here we report the successful demonstration of a temperature controlled Ce:YAG LC pumped with blue InGaN LEDs.

Conference paper

Sathian J, Jonathan Breeze BB, Frangeskou A, Morley G, Alford NMet al., 2019, Quantification of NV-centres in diamond for room-temperature maser

MASERs are devices that exploit stimulated emission to amplify electromagnetic waves at microwave frequencies. Their principal advantage over conventional (semiconductor-based) electronic amplifiers is lower noise. An optically-pumped, solid-state pentacence maser operating at room temperature in pulsed mode was demonstrated [1]. This was followed by the recent demonstration of a continuous maser at room temperature using colour centres (NV) in diamond [2] paving the way for a new class of masers that could find new applications in medicine, security and sensing, quantum computing/information processing.

Conference paper

Sathian J, Breeze J, Ben B, Frangeskou A, Morley G, Alford NMet al., 2019, Quantification of NV-centres in Diamond for Room-Temperature Maser, Conference on Lasers and Electro-Optics Europe / European Quantum Electronics Conference (CLEO/Europe-EQEC), Publisher: IEEE

Conference paper

Sathian J, Minassian A, Alford NM, Damzen MJet al., 2019, Enhancing Performance of Ce:YAG Luminescent Concentrators for High Power Applications, Conference on Lasers and Electro-Optics Europe / European Quantum Electronics Conference (CLEO/Europe-EQEC), Publisher: IEEE

Conference paper

Axelsson A-K, Le Goupil F, Valant M, Alford NMet al., 2018, Optimisation of SrBi2(Nb,Ta)(2)O-9 Aurivillius phase for lead-free electrocaloric cooling, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 38, Pages: 5354-5358, ISSN: 0955-2219

Journal article

Wells M, Bower R, Kilmurray B, Zou B, Mihai AP, GOBALAKRICHENANE G, Alford NM, Oulton RFM, Cohen L, Maier SA, ZAYATS A, Petrov PKet al., 2018, Temperature stability of thin film refractory plasmonic materials, Optics Express, Vol: 12, Pages: 15726-15744, ISSN: 1094-4087

Materials such as W, TiN, and SrRuO3 (SRO) have been suggested as promising alternatives to Au and Ag in plasmonic applications owing to their stability at high operational temperatures. However, investigation of the reproducibility of the optical properties after thermal cycling between room and elevated temperatures is so far lacking. Here, thin films of W, Mo, Ti, TiN, TiON, Ag, Au, SrRuO3 and SrNbO3 are investigated to assess their viability for robust refractory plasmonic applications. These results are further compared to the performance of SrMoO3 reported in literature. Films ranging in thickness from 50 to 105 nm are deposited on MgO, SrTiO3 and Si substrates by e-beam evaporation, RF magnetron sputtering and pulsed laser deposition, prior to characterisation by means of AFM, XRD, spectroscopic ellipsometry, and DC resistivity. Measurements are conducted before and after annealing in air at temperatures ranging from 300 to 1000° C for one hour, to establish the maximum cycling temperature and potential longevity at elevated temperatures for each material. It is found that SrRuO3 retains metallic behaviour after annealing at 800° C, while SrNbO3 undergoes a phase transition resulting in a loss of metallic behaviour after annealing at 400° C. Importantly, the optical properties of TiN and TiON are degraded as a result of oxidation and show a loss of metallic behaviour after annealing at 500° C, while the same is not observed in Au until annealing at 600° C. Nevertheless, both TiN and TiON may be better suited than Au or SRO for high temperature applications operating under vacuum conditions.

Journal article

Petrov PK, Wells M, Zou B, Mihai A, Bower R, Doiron B, Regoutz A, Fearn S, Maier S, Alford Net al., 2018, Multiphase strontium molybdate thin films for plasmonic local heating applications, Optical Materials Express, Vol: 8, Pages: 1806-1817, ISSN: 2159-3930

In the search for alternative plasmonic materials SrMoO3 has recently been identified as possessing a number of desirable optical properties. Owing to the requirement for many plasmonic devices to operate at elevated temperatures however, it is essential to characterize the degradation of these properties upon heating. Here, SrMoO3 thin films are annealed in air at temperatures ranging from 75 - 500{\deg} C. Characterizations by AFM, XRD, and spectroscopic ellipsometry after each anneal identify a loss of metallic behaviour after annealing at 500{\deg} C, together with the underlying mechanism. Moreover, it is shown that by annealing the films in nitrogen following deposition, an additional crystalline phase of SrMoO4 is induced at the film surface, which suppresses oxidation at elevated temperatures.

Journal article

Breeze JD, Salvadori E, Sathian J, Alford NM, Kay CWMet al., 2018, Continuous-wave room-temperature diamond maser, Nature, Vol: 555, Pages: 493-496, ISSN: 0028-0836

The maser—the microwave progenitor of the optical laser—has been confined to relative obscurity owing to its reliance on cryogenic refrigeration and high-vacuum systems. Despite this, it has found application in deep-space communications and radio astronomy owing to its unparalleled performance as a low-noise amplifier and oscillator. The recent demonstration of a room-temperature solid-state maser that utilizes polarized electron populations within the triplet states of photo-excited pentacene molecules in a p-terphenyl host1,2,3 paves the way for a new class of maser. However, p-terphenyl has poor thermal and mechanical properties, and the decay rates of the triplet sublevel of pentacene mean that only pulsed maser operation has been observed in this system. Alternative materials are therefore required to achieve continuous emission: inorganic materials that contain spin defects, such as diamond4,5,6 and silicon carbide7, have been proposed. Here we report a continuous-wave room-temperature maser oscillator using optically pumped nitrogen–vacancy defect centres in diamond. This demonstration highlights the potential of room-temperature solid-state masers for use in a new generation of microwave devices that could find application in medicine, security, sensing and quantum technologies.

Journal article

Ptashnik SV, Mikhailov AK, Yastrebov AV, Petrov PK, Liu W, Alford N, Hirsch S, Kozyrev ABet al., 2017, Ferroelectric thin film acoustic devices with electrical multiband switching ability, Scientific Reports, Vol: 7, ISSN: 2045-2322

Design principles of a new class of microwave thin film bulk acoustic resonators with multiband resonance frequency switching ability are presented. The theory of the excitation of acoustic eigenmodes in multilayer ferroelectric structures is considered, and the principle of selectivity for resonator with an arbitrary number of ferroelectric layers is formulated. A so called “criterion function” is suggested that allows to determine the conditions for effective excitation at one selected resonance mode with suppression of other modes. The proposed theoretical approach is verifiedusing thepreexisting experimental data published elsewhere. Finally, the possible application of the two ferroelectric layers structures for switchable microwave overtone resonators, binary and quadrature phase-shift keying modulators are discussed. These devices could play a pivotal role in the miniaturization of microwave front-end antenna circuits.

Journal article

Breeze JDB, Salvadori E, Sathian J, Alford N, Kay CWMet al., 2017, Room-temperature cavity quantum electrodynamics with strongly-coupled Dicke states, npj Quantum Information, Vol: 3, ISSN: 2056-6387

The strong coupling regime is essential for efficient transfer of excitations between states in different quantum systems on timescales shorter than their lifetimes. The coupling of single spins to microwave photons is very weak but can be enhanced by increasing the local density of states by reducing the magnetic mode volume of the cavity. In practice, it is difficult to achieve both small cavity mode volume and low cavity decay rate, so superconducting metals are often employed at cryogenic temperatures. For an ensembles of N spins, the spin–photon coupling can be enhanced by N−−√N through collective spin excitations known as Dicke states. For sufficiently large N the collective spin–photon coupling can exceed both the spin decoherence and cavity decay rates, making the strong-coupling regime accessible. Here we demonstrate strong coupling and cavity quantum electrodynamics in a solid-state system at room-temperature. We generate an inverted spin-ensemble with N ~ 1015 by photo-exciting pentacene molecules into spin-triplet states with spin dephasing time T∗2~3T2*~3 μs. When coupled to a 1.45 GHz TE01δ mode supported by a high Purcell factor strontium titanate dielectric cavity (Vm~0.25Vm~0.25 cm3, Q ~ 8,500), we observe Rabi oscillations in the microwave emission from collective Dicke states and a 1.8 MHz normal-mode splitting of the resultant collective spin–photon polariton. We also observe a cavity protection effect at the onset of the strong-coupling regime which decreases the polariton decay rate as the collective coupling increases.

Journal article

Wells MP, Zou B, Doiron BG, Kilmurray R, Mihai AP, Oulton RF, Gubeljak P, Ormandy K, Mallia G, Harrison N, Cohen LF, Maier S, Petrov PKet al., 2017, Tunable, Low Optical Loss Strontium Molybdate Thin Films for Plasmonic Applications, Advanced Optical Materials, Vol: 5, ISSN: 2195-1071

Strontium molybdate (SrMoO3) thin films are grown epitaxially on strontium titanate (SrTiO3), magnesium oxide (MgO), and lanthanum aluminate (LaAlO3) substrates by pulsed laser deposition and possess electrical resistivity as low as 100 µΩ cm at room temperature. SrMoO3 is shown to have optical losses, characterized by the product of the Drude broadening, ΓD, and the square of the plasma frequency, ωpu2, significantly lower than TiN, though generally higher than Au. Also, it is demonstrated that there is a zero-crossover wavelength of the real part of the dielectric permittivity, which is between 600 and 950 nm (2.05 and 1.31 eV), as measured by spectroscopic ellipsometry. Moreover, the epsilon near zero (ENZ) wavelength can be controlled by engineering the residual strain in the films, which arises from a strain dependence of the charge carrier concentration, as confirmed by density of states calculations. The relatively broad tunability of ENZ behavior observed in SrMoO3 demonstrates its potential suitability for transformation optics along with plasmonic applications in the visible to near infrared spectral range.

Journal article

Doiron B, Li Y, Mihai AP, Cohen LF, Pretrov PK, Alford NM, Oulton RF, Maier SAet al., 2017, Comparison of the ultrafast hot electron dynamics of titanium nitride and gold for plasmonic applications, SPIE Optics + Photonics Conference on Plasmonics - Design, Materials, Fabrication, Characterization, and Applications XV, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X

With similar optical properties to gold and high thermal stability, titanium nitride continues to prove itself as a promising plasmonic material for high-temperature applications in the visible and near-infrared. In this work, we use transient pump probe differential reflection measurements to compare the electron energy decay channels in titanium nitride and gold thin films. Using an extended two temperature model to incorporate the photoexcited electrons, it is possible to separate the electron-electron and electron-phonon scattering contributions immediately following the arrival of the pump pulse. This model allows for incredibly accurate determination of the internal electronic properties using only optical measurements. As the electronic properties are key in hot electron applications, we show that titanium nitide has substantially longer electron thermalization and electron-phonon scattering times. With this, we were also able to resolve electron thermal conduction in the film using purely optical measurements.

Conference paper

Braic L, Vasilantonakis N, Mihai A, Villar-Garcia I, Fearn S, Zou B, Alford N, Doiron B, Oulton R, Maier S, Zayats A, Petrov Pet al., 2017, Titanium oxynitride thin films with tuneable double epsilon-near-zero behaviour for nanophotonic applications, ACS Applied Materials and Interfaces, Vol: 9, Pages: 29857-29862, ISSN: 1944-8244

Titanium oxynitride (TiOxNy) thin films are fabricated using reactive magnetron sputtering. The mechanism of their growth formation is explained, and their optical properties are presented. The films grown when the level of residual oxygen in the background vacuum was between 5 nTorr to 20 nTorr exhibit double epsilon-near-Zero (2-ENZ) behavior with ENZ1 and ENZ2 wavelengths tunable in the 700–850 and 1100–1350 nm spectral ranges, respectively. Samples fabricated when the level of residual oxygen in the background vacuum was above 2 × 10–8 Torr exhibit nonmetallic behavior, while the layers deposited when the level of residual oxygen in the background vacuum was below 5 × 10–9 Torr show metallic behavior with a single ENZ value. The double ENZ phenomenon is related to the level of residual oxygen in the background vacuum and is attributed to the mixture of TiN and TiOxNy and TiOx phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of TiN, TiOx, and TiOxNy compounds in the films and, therefore, tune the screened plasma wavelengths. A good approximation of the ellipsometric behavior is achieved with Maxwell–Garnett theory for a composite film formed by a mixture of TiO2 and TiN phases suggesting that double ENZ TiOxNy films are formed by inclusions of TiN within a TiO2 matrix. These oxynitride compounds could be considered as new materials exhibiting double ENZ in the visible and near-IR spectral ranges. Materials with ENZ properties are advantageous for designing the enhanced nonlinear optical response, metasurfaces, and nonreciprocal behavior.

Journal article

Sathian J, Breeze J, Richards B, Alford N, Oxborrow Met al., 2017, Solid-state source of intense yellow light based on a Ce:YAG luminescent concentrator, Optics Express, Vol: 25, Pages: 13714-13727, ISSN: 1094-4087

A luminescent concentrator functioning as a bright source of yellow light is reported. It comprises a waveguide made of cerium-doped YAG crystal, in the form of a long-thin rectangular strip, surrounded by flowing air and optically pumped from both sides with blue light from arrays of high-efficiency InGaN LEDs. Phosphor-converted yellow light, generated within the strip, is guided to a glass taper that is butt-coupled to one of the strip’s end faces. Up to 20 W of optical power, centered on 575 nm with a linewidth of 76 nm, can be continuously radiated into air from the taper’s 1.67 mm × 1.67 mm square output aperture. The intensity of the outputted light is significantly greater than what any yellow (AlGaInP) LED can directly produce (either singly or arrayed), with only a modest increase in linewidth. Furthermore, the wall-plug efficiency of the source exceeds that of any yellow laser. The concept allows for further substantial increases in intensity, total output power and wall-plug efficiency through scaling-up and engineering refinements.

Journal article

Salvadori E, Breeze JD, Tan K-J, Sathian J, Richards B, Fung MW, Wolfowicz G, Oxborrow M, Alford NM, Kay CWMet al., 2017, Nanosecond time-resolved characterization of a pentacene-based room-temperature MASER, Scientific Reports, Vol: 7, ISSN: 2045-2322

The performance of a room temperature, zero-field MASER operating at 1.45 GHz has been examined. Nanosecond laser pulses, which are essentially instantaneous on the timescale of the spin dynamics, allow the visible-to-microwave conversion efficiency and temporal response of the MASER to be measured as a function of excitation energy. It is observed that the timing and amplitude of the MASER output pulse are correlated with the laser excitation energy: at higher laser energy, the microwave pulses have larger amplitude and appear after shorter delay than those recorded at lower laser energy. Seeding experiments demonstrate that the output variation may be stabilized by an external source and establish the minimum seeding power required. The dynamics of the MASER emission may be modeled by a pair of first order, non-linear differential equations, derived from the Lotka-Volterra model (Predator-Prey), where by the microwave mode of the resonator is the predator and the spin polarization in the triplet state of pentacene is the prey. Simulations allowed the Einstein coefficient of stimulated emission, the spin-lattice relaxation and the number of triplets contributing to the MASER emission to be estimated. These are essential parameters for the rational improvement of a MASER based on a spin-polarized triplet molecule.

Journal article

Pang J, Theodorou I, Centeno A, Petrov P, Alford N, Ryan M, Xie Fet al., 2016, Gold nanodisc arrays as near infrared metal-enhanced fluorescence platforms with tuneable enhancement factors, Journal of Materials Chemistry C, Vol: 5, Pages: 917-925, ISSN: 2050-7534

Metal enhanced fluorescence (MEF) is a physical effect through which the near-field interaction of fluorophores with metallic nanoparticles can lead to large fluorescence enhancement. MEF can be exploited in many fluorescence-based biomedical applications, with potentially significant improvement in detection sensitivity and contrast enhancement. Offering lower autofluorescence and minimal photoinduced damage, the development of effective and multifunctional MEF platforms in the near-infrared (NIR) region, is particularly desirable. In this work, the enhancement of NIR fluorescence caused by interaction with regular arrays of cylindrical gold (Au) nanoparticles (nanodiscs), fabricated through nanosphere lithography, is reported. Significant MEF of up to 235 times is obtained, with tuneable enhancement factors. The effect of array structure on fluorescence enhancement is investigated by semi-quantitatively de-convoluting excitation enhancement from emission enhancement, and modelling the local electric field enhancement. By considering arrays of Au nanodiscs with the same extinction maximum, it is shown that the excitation enhancement, due to increased electric field, is not significantly different for the particle sizes and separation distances considered. Rather, it is seen that the emission from the fluorophore is strongly enhanced, and is dependent on the topography, in particular particle size. The results show that the structural characteristics of Au nanodisc arrays can be manipulated to tune their enhancement factor, and hence their sensitivity.

Journal article

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