Publications
62 results found
Ng W, Wen Y, Attwood M, et 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.
Wu H, Li T, Yin Z-Q, et al., 2023, Toward Simultaneous Coherent Radiation in the Visible and Microwave Bands with Doped Molecular Crystals, ADVANCED FUNCTIONAL MATERIALS, Vol: 33, ISSN: 1616-301X
Ng W, Xu X, Attwood M, et al., 2023, Move Aside Pentacene: Diazapentacene-Doped <i>para</i>-Terphenyl, a Zero-Field Room-Temperature Maser with Strong Coupling for Cavity Quantum Electrodynamics, ADVANCED MATERIALS, Vol: 35, ISSN: 0935-9648
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Attwood M, Xu X, Newns M, et al., 2023, N-Heteroacenes as an Organic Gain Medium for Room-Temperature Masers, CHEMISTRY OF MATERIALS, Vol: 35, Pages: 4498-4509, ISSN: 0897-4756
Wu H, Yang S, Oxborrow M, et al., 2022, Enhanced quantum sensing with room-temperature solid-state masers., Science Advances, Vol: 8, Pages: 1-9, ISSN: 2375-2548
Quantum sensing with solid-state electron spin systems finds broad applications in diverse areas ranging from material and biomedical sciences to fundamental physics. Exploiting collective behavior of noninteracting spins holds the promise of pushing the detection limit to even lower levels, while to date, those levels are scarcely reached because of the broadened linewidth and inefficient readout of solid-state spin ensembles. Here, we experimentally demonstrate that such drawbacks can be overcome by a reborn maser technology at room temperature in the solid state. Owing to maser action, we observe a fourfold reduction in the electron paramagnetic resonance linewidth of an inhomogeneously broadened molecular spin ensemble, which is narrower than the same measured from single spins at cryogenic temperatures. The maser-based readout applied to near zero-field magnetometry showcases the measurement signal-to-noise ratio of 133 for single shots. This technique would be an important addition to the toolbox for boosting the sensitivity of solid-state ensemble spin sensors.
Xu X, Newns M, Oxborrow M, 2022, Simulating the magnetic fields generated by piezoelectric devices using FEM software: Beyond the quasistatic approximation, Journal of Applied Physics, Vol: 132, ISSN: 0021-8979
A method for simulating coupled electromagnetic and mechanical vibrations on arbitrarily shaped piezoelectric structures is presented. This method is based on weak forms and can be implemented in any finite-element-method software, allowing editable access to their definitions. No quasi-static approximation is imposed, meaning that magnetic fields generated by displacement currents within piezoelectric materials are captured, enabling the flow of electromagnetic energy inside and around structures containing such material to be accurately simulated. The method is particularly relevant to the design of piezoelectric antennas, resonators, and waveguides exploiting either bulk or surface-acoustic waves. The accuracy and capabilities of the method are demonstrated by simulating, in COMSOL Multiphysics, (i) a Rayleigh mode on the surface of Z-cut lithium niobate crystal and (ii) a torsional mode of a cylinder of lead zirconium titanate (PZT-5H) ceramic functioning as a micro-antenna.
Ng W, Wu H, Oxborrow M, 2021, Quasi-continuous cooling of a microwave mode on a benchtop using hyperpolarized NV- diamond, Applied Physics Letters, Vol: 119, Pages: 1-5, ISSN: 0003-6951
We demonstrate the cooling of a microwave mode at 2872 MHz through its interaction with optically spin-polarized NV− centers in diamond at zero applied magnetic field, removing thermal photons from the mode. By photo-exciting (pumping) a brilliant-cut red diamond jewel with a continuous-wave 532-nm laser, outputting 2 W, the microwave mode is cooled down to a noise temperature of 188 K. This noise temperature can be preserved continuously for as long as the diamond is optically excited and kept cool. The latter requirement restricted operation out to 10 ms in our preliminary setup. The mode-cooling performance of NV− diamond is directly compared against that of pentacene-doped para-terphenyl, where we find that the former affords the advantages of cooling immediately upon light excitation (whereas pentacene-doped para-terphenyl undesirably mases before it begins cooling) and being able to cool continuously at substantially lower optical pump power.
Attwood M, Kim DK, Hadden JHL, et al., 2021, Asymmetric N-heteroacene tetracene analogues as potential n-type semiconductors, Journal of Materials Chemistry C, Vol: 9, Pages: 17073-17083, ISSN: 2050-7526
In the search for high performance n-type organic semiconductors (OSCs) a simple strategy might be substitution of aromatic CH groups for nitrogen heteroatoms. Here, we report the synthesis and characterisation of two novel N-heteroacene compounds, namely, 1,5,12-triazatetracene (TrAT1) and 2,5,12-triazatetracene (TrAT2). Their potential as n-type materials is evaluated against 5,12-diazatetracene (DAT) by UV/vis and EPR spectroscopy, cyclic voltammetry, DFT, single crystal X-ray diffraction and thin film characterisation. Increasing the number of N-heteroatoms was found to stabilise the HOMO and LUMO leading to electron affinities for TrAT1 and TrAT2 of ca. −4 eV. Both compounds were found to exhibit columns of co-facial π-stacked molecules. For TrAT1, molecules are also linked by hydrogen bonding, while the crystal structure of TrAT2 was found to be inherently disordered. Thin films of DAT, TrAT1 and TrAT2 were grown by organic molecular beam deposition (OMBD) and found to form discontinuous films, where TrAT1 exhibited a preferential orientation.
Wu H, Mirkhanov S, Ng W, et al., 2021, Bench-top cooling of a microwave mode using an optically pumped spin refrigerator, Physical Review Letters, Vol: 127, Pages: 1-6, ISSN: 0031-9007
We experimentally demonstrate the temporary removal of thermal photons from a microwavemode at 1.45 GHz through its interaction with the spin-polarized triplet states of photo-excitedpentacene molecules doped within a p-terphenyl crystal at room temperature. The crystal functions electromagnetically as a narrow-band cryogenic load, removing photons from the otherwiseroom-temperature mode via stimulated absorption. The noise temperature of the microwave modedropped to 50+18−32 K (as directly inferred by noise-power measurements) while the metal walls of thecavity enclosing the mode remained at room temperature. Simulations based on the same system’sbehavior as a maser (which could be characterized more accurately) indicate the possibility of themode’s temperature sinking to ∼10 K (corresponding to ∼140 microwave photons).These observations, when combined with engineering improvements to deepen the cooling, identify the systemas a narrow-band yet extremely convenient platform —free of cryogenics, vacuum chambers andstrong magnets— for realizing low-noise detectors, quantum memory and quantum-enhanced machines (such as heat engines) based on strong spin-photon coupling and entanglement at microwavefrequencies.
Ng W, Zhang S, Wu H, et al., 2021, Exploring the triplet spin dynamics of the charge-transfer co-crystal phenazine/1,2,4,5-tetracyanobenzene for potential use in organic maser gain media, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 125, Pages: 14718-14728, ISSN: 1932-7447
Charge-transfer co-crystals have garnered interest for use in organic electronics and photovoltaics, where their ease of growth by solution/vapor deposition and flexibility in choice of constituent molecules allows tailoring of their band gaps and triplet spin dynamics. Here, we report on the triplet spin dynamics of one such charge-transfer co-crystal phenazine/1,2,4,5-tetracyanobenzene (PNZ/TCNB), as investigated using algorithm-assisted continuous-wave electron paramagnetic resonance and transient powder electron paramagnetic resonance (EPR) at X-band, as well as transient EPR at zero field (ZF-trEPR). The zero-field splitting parameters are |D| = 2118 ± 2 MHz and |E| = 297 ± 1 MHz, with initial triplet sublevel populations Px/Py/Pz being 0.764:0.127:0.109. The triplet spin dynamics were found to have decay times in the sub-microsecond regime. Furthermore, with the aid of a Q-boosted dielectric cavity, PNZ/TCNB can output a maser burst at 2412 MHz at room temperature and zero field under 445 nm pulsed excitation. This demonstrates that it is feasible for PNZ/TCNB to mase if a cavity with a sufficiently high-quality factor is provided and opens up avenues toward investigating other charge-transfer co-crystals with similar triplet spin dynamics as potential maser gain media at room temperature and zero field.
Wu H, Xie X, Ng W, et al., 2020, Room-temperature quasi-continuous-wave pentacene maser pumped by an invasive Ce:YAG luminescent concentrator, Physical Review Applied, Vol: 14, Pages: 064017 – 1-064017 – 10, ISSN: 2331-7019
We present in this work a quasi-continuous-wave (cw) pentacene maser operating at 1.45 GHz in the Earth’s magnetic field at room temperature with a duration of approximately 4 ms and an output power of up to −25 dBm. The maser is optically pumped by a cerium-doped yttrium-aluminum-garnet (YAG) (Ce:YAG) luminescent concentrator (LC), the wedge-shaped output of which is embedded inside a 0.1% pentacene-doped para-terphenyl (Pc:Ptp) crystal. The pumped crystal is located inside a ring of strontium titanate (STO) that supports a TE01δ mode of high magnetic Purcell factor. Combined with simulations, our results indicate that cw operation of pentacene masers at room temperature is perfectly feasible so long as excessive heating of the crystal is avoided.
Keyser AKV, Burnett JJ, Kubatkin SE, et al., 2020, Pulsed electron spin resonance of an organic microcrystal by dispersive readout, JOURNAL OF MAGNETIC RESONANCE, Vol: 321, ISSN: 1090-7807
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- Citations: 3
Wu H, Mirkhanov S, Ng W, et al., 2020, Invasive optical pumping for room-temperature masers, time-resolved EPR, triplet-DNP, and quantum engines exploiting strong coupling, Optics Express, Vol: 28, Pages: 29691-29702, ISSN: 1094-4087
We explore an approach for optically pumping a body of optically dense magnetic material. This challenge arises in time-resolved electron paramagnetic resonance (TREPR), triplet-based dynamic nuclear polarisation (DNP), and cavity QED. Crystals of pentacene-doped p-terphenyl were grown around variously shaped ends of optical waveguides, through which pump light could be injected deeply into the crystal. When incorporated into a maser as the gain medium, we found that, compared to conventional side-pumping, 11 times less pump beam intensity was needed to reach the masing threshold and 54 times more pulse energy could be absorbed by the gain medium without damage, resulting in a record peak output power of -5 dBm.
Dejean V, Konowalczyk M, Gravell J, et al., 2020, Detection of magnetic field effects by confocal microscopy, Chemical Science, Vol: 11, Pages: 7772-7781, ISSN: 2041-6520
Certain pairs of paramagnetic species generated under conservation of total spin angular momentum are known to undergo magnetosensitive processes. Two prominent examples of systems exhibiting these so-called magnetic field effects (MFEs) are photogenerated radical pairs created from either singlet or triplet molecular precursors, and pairs of triplet states generated by singlet fission. Here, we showcase confocal microscopy as a powerful technique for the investigation of such phenomena. We first characterise the instrument by studying the field-sensitive chemistry of two systems in solution: radical pairs formed in a cryptochrome protein and the flavin mononucleotide/hen egg-white lysozyme model system. We then extend these studies to single crystals. Firstly, we report temporally and spatially resolved MFEs in flavin-doped lysozyme single crystals. Anisotropic magnetic field effects are then reported in tetracene single crystals. Finally, we discuss the future applications of confocal microscopy for the study of magnetosensitive processes with a particular focus on the cryptochrome-based chemical compass believed to lie at the heart of animal magnetoreception.
Wu H, Ng W, Mirkhanov S, et al., 2019, Unraveling the room-temperature spin dynamics of photoexcited pentacene in its lowest triplet state at zero field, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 123, Pages: 24275-24279, ISSN: 1932-7447
Photoexcited pentacene, upon arriving via intersystem crossing into its lowest triplet state, has been extensively studied due to the large and relatively long-lived spin polarization that it exhibits. However, the spin dynamics of these triplets has not hitherto been accurately determined, with glaring inconsistencies between published values. Using zero-field transient electron paramagnetic resonance (ZF-trEPR), we here report the determination of a complete set of depopulation and spin–lattice relaxation rates for the lowest triplet state of pentacene doped at 0.1% into a p-terphenyl host crystal at room temperature in zero applied magnetic field. The rates of spin–lattice relaxation between the triplet’s sublevels are found to be highly anisotropic (i.e., transition-specific) and not negligible compared to the rates of depopulation from the same three sublevels back to pentacene’s ground state. The spin dynamics, as well as the ZF-trEPR technique reported here, can aid the rational, quantitative engineering of applications such as room-temperature masers and triplet dynamic nuclear polarization.
Sathian J, Oxborrow M, 2017, A Light Source
There is provided a light source arranged to output light at a first wavelength. The light source comprises a luminescent concentrator having a slab-shaped geometry. The luminescent concentrator comprises: an input port arranged to receive light and define a first area; an output port arranged to transmit light and define a second area which is smaller than the first area; and surfaces arranged to direct light inside the luminescent concentrator to the output port. The luminescent concentrator further comprises lumophores arranged to receive light at a second wavelength and emit light at the first wavelength; and a pump light supply coupled to the input port and arranged to illuminate the input port with light at the second wavelength.
Sathian J, Breeze J, Richards B, et 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.
Salvadori E, Breeze JD, Tan K-J, et 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.
Bogatko SA, Haynes PD, Sathian J, et al., 2016, Molecular design of a room-temperature maser, The Journal of Physical Chemistry C, Vol: 120, Pages: 8251-8260, ISSN: 1932-7447
Breeze J, Tan K-J, Richards B, et al., 2015, Enhanced magnetic Purcell effect in room-temperature masers, Nature Communications, Vol: 6, Pages: 1-6, ISSN: 2041-1723
Recently, the world’s first room-temperature maser was demonstrated. The maser consisted of a sapphire ring housing a crystal of pentacene-doped p-terphenyl, pumped by a pulsed rhodamine-dye laser. Stimulated emission of microwaves was aided by the high quality factor and small magnetic mode volume of the maser cavity yet the peak optical pumping power was 1.4 kW. Here we report dramatic miniaturization and 2 orders of magnitude reduction in optical pumping power for a room-temperature maser by coupling a strontium titanate resonator with the spin-polarized population inversion provided by triplet states in an optically excited pentacene-doped p-terphenyl crystal. We observe maser emission in a thimble-sized resonator using a xenon flash lamp as an optical pump source with peak optical power of 70 W. This is a significant step towards the goal of continuous maser operation.
Mihalkovic M, Richmond-Decker J, Henley CL, et al., 2014, <i>Ab-initio</i> tiling and atomic structure for decagonal ZnMgY quasicrystal, PHILOSOPHICAL MAGAZINE, Vol: 94, Pages: 1529-1541, ISSN: 1478-6435
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- Citations: 11
Oxborrow M, Breeze JD, Alford NM, 2012, Room-temperature solid-state maser, NATURE, Vol: 488, Pages: 353-+, ISSN: 0028-0836
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- Citations: 78
Lindstroem T, Burnett J, Oxborrow M, et al., 2011, Pound-locking for characterization of superconducting microresonators, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 82, ISSN: 0034-6748
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- Citations: 28
Breeze J, Oxborrow M, Alford NM, 2011, Better than Bragg: Optimizing the quality factor of resonators with aperiodic dielectric reflectors, APPLIED PHYSICS LETTERS, Vol: 99, ISSN: 0003-6951
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- Citations: 4
Grop S, Schaefer W, Bourgeois P-Y, et al., 2011, Unprecedented Long-Term Frequency Stability With a Microwave Resonator Oscillator, IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL, Vol: 58, Pages: 1694-1697, ISSN: 0885-3010
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- Citations: 13
Jones BD, Oxborrow M, Astratov VN, et al., 2011, Splitting and lasing of whispering gallery modes in quantum dot micropillars, Conference on Lasers and Electro-Optics (CLEO), Publisher: IEEE, ISSN: 2160-9020
Jones BD, Oxborrow M, Astratov VN, et al., 2010, Splitting and lasing of whispering gallery modes in quantum dot micropillars., Optics Express, Vol: 18, Pages: 22578-22592
We have studied the whispering gallery mode (WGM) resonances of GaAs/AlGaAs microcavity pillars containing InAs quantum dots. High quality factor WGMs are observed from a wide range of pillars with diameters from 1.2 to 50 μm. Multimode lasing with sub-milliwatt thresholds and high beta-factors approaching unity is observed under optical pumping in a 4 μm diameter pillar. Mode splitting is observed in WGMs from pillars with diameters of 5 μm, 20 μm and 50 μm.We develop a model in which the mode splitting in the larger pillars is caused by resonant scattering from the quantum dots themselves. The model explains why splittings are observed in all of the larger pillars and that the splitting decreases with increasing wavelength. Numerical simulations by COMSOL confirm that the model is plausible. This mechanism of splitting should be general for all circular resonant structures containing quantum dots such as microdisks, rings, toroids, and microspheres.
Avlund M, Ellegaard C, Oxborrow M, et al., 2010, Observation of Periodic Orbits on Curved Two-Dimensional Geometries, PHYSICAL REVIEW LETTERS, Vol: 104, ISSN: 0031-9007
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- Citations: 8
Grop S, Bourgeois PY, Bazin N, et al., 2010, ELISA: A cryocooled 10 GHz oscillator with 10<SUP>-15</SUP> frequency stability, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 81, ISSN: 0034-6748
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- Citations: 44
Grop S, Bourgeois PY, Bazin N, et al., 2010, High Stability Cryocooled 10 GHz Oscillator For The European Space Agency, 2010 IEEE International Frequency Control Symposium, Publisher: IEEE, Pages: 665-669, ISSN: 1075-6787
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