52 results found
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.
Breeze J, Tan K-J, Richards B, et al., 2015, Enhanced magnetic Purcell effect in room-temperature masers, NATURE COMMUNICATIONS, Vol: 6, ISSN: 2041-1723
Mihalkovic M, Richmond-Decker J, Henley CL, et al., 2014, Ab-initio tiling and atomic structure for decagonal ZnMgY quasicrystal, PHILOSOPHICAL MAGAZINE, Vol: 94, Pages: 1529-1541, ISSN: 1478-6435
Oxborrow M, Breeze JD, Alford NM, 2012, Room-temperature solid-state maser, NATURE, Vol: 488, Pages: 353-+, ISSN: 0028-0836
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
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
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
Lindström T, Burnett J, Oxborrow M, et al., 2011, Pound-locking for characterization of superconducting microresonators, Rev. Sci. Instrum., Vol: 82
We present a new application and implementation of the so-called Poundlocking technique for the interrogation of superconducting microresonators. Wediscuss how by comparing against stable frequency sources this technique can beused to characterize properties of resonators that can not be accessed usingtraditional methods. Specifically, by analyzing the noise spectra and the Allandeviation we obtain valuable information about the nature of the noise insuperconducting planar resonators. This technique also greatly improves theread-out accuracy and measurement throughput compared to conventional methods.
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
Grop S, Schäfer W, Bourgeois P-Y, et al., 2010, Unprecedented High Long Term Frequency Stability with a Macroscopic Resonator Oscillator
This article reports on the long-term frequency stabilty characterisation ofa new type of cryogenic sapphire oscillator using an autonomous pulse-tubecryocooler as its cold source. This new design enables a relative frequencystability of better than 4.5e-15 over one day of integration. This representsto our knowledge the best long-term frequency stability ever obtained with asignal source based on a macroscopic resonator.
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
Oxborrow M, Bourgeois P-Y, Kersalé Y, et al., 2010, DC-powered Fe3+:sapphire Maser and its Sensitivity to Ultraviolet Light
The zero-field Fe3+:sapphire whispering-gallery-mode maser oscillatorexhibits several alluring features: Its output is many orders of magnitudebrighter than that of an active hydrogen maser and thus far less degraded byspontaneous-emission (Schawlow-Townes) and/or receiving-amplifier noise. Itsoscillator loop is confined to a piece of mono-crystalline rock bolted into ametal can. Its quiet amplification combined with high resonator Q provide theingredients for exceptionally low phase noise. We here concentrate on noveltiesaddressing the fundamental conundrums and technical challenges that impedeprogress. (1) Roasting: The "mase-ability" of sapphire depends significantly onthe chemical conditions under which it is grown and heat-treated. We providesome fresh details and nuances here. (2) Simplification: This paper obviatesthe need for a Ka-band synthesizer: it describes how a 31.3 GHz looposcillator, operating on the preferred WG pump mode, incorporating Poundlocking, was built from low-cost components. (3) "Dark Matter": A Siegman-levelanalysis of the experimental data determines the substitutional concentrationof Fe3+ in HEMEX to be less than a part per billion prior to roasting and up toa few hundred ppb afterwards. Chemical assays, using different techniques(incl. glow discharge mass spectra spectroscopy and neutron activationanalysis) consistently indicate, however, that HEMEX contains iron atconcentrations of a few parts per million. Drawing from severalforgotten-about/under-appreciated papers, this substantial discrepancy isaddressed. (4) Excitons: Towards providing a new means of controlling theFe3+:sapph. system, a cryogenic sapphire ring was illuminated, whilst masing,with UV light at wavelengths corresponding to known electronic andcharge-transfer (thus valence-altering) transitions. Preliminary experimentsare reported.
Grop S, Bourgeois PY, Bazin N, et al., 2010, ELISA: A cryocooled 10 GHz oscillator with 10(-15) frequency stability, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 81, ISSN: 0034-6748
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
Grop S, Bourgeois PY, Bazin N, et al., 2010, Demonstration of a cryocooled 10 GHz oscillator with 10(-15) frequency stability., 24th European Frequency and Time Forum (EFTF), Publisher: IEEE
Benmessai K, Bourgeois PY, Bazin N, et al., 2009, NEW RESULTS FOR WHISPERING GALLERY MODE CRYOGENIC SAPPHIRE MASER OSCILLATORS, 7th Symposium on Frequency Standards and Metrology, Publisher: WORLD SCIENTIFIC PUBL CO PTE LTD, Pages: 398-402
Schwefel HGL, Yang L, Oxborrow M, et al., 2009, Level Crossing in Toroidal On-chip Microcavities, Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference (CLEO/QELS 2009), Publisher: IEEE, Pages: 1333-+
Grop S, Giordano V, Bourgeois PY, et al., 2009, ELISA: an Ultra-Stable Oscillator for ESA Deep Space Antennas, Joint Meeting of the 23rd European Frequency and Time Forum/IEEE International Frequency Control Symposium, Publisher: IEEE, Pages: 376-380, ISSN: 1075-6787
Carmon T, Schwefel HGL, Yang L, et al., 2008, Static envelope patterns in composite resonances generated by level crossing in optical toroidal microcavities, PHYSICAL REVIEW LETTERS, Vol: 100, ISSN: 0031-9007
Benmessai K, Bourgeois PY, Kersale Y, et al., 2007, Frequency instability measurement system of cryogenic maser oscillator, ELECTRONICS LETTERS, Vol: 43, Pages: 1436-1437, ISSN: 0013-5194
Oxborrow M, 2007, Whispering Gallery Oscillator, US 7,292,112
Oxborrow M, 2007, Traceable 2-D finite-element simulation of the whispering-gallery modes of axisymmetric electromagnetic resonators, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol: 55, Pages: 1209-1218, ISSN: 0018-9480
Marra G, Henderson D, Oxborrow M, 2007, Frequency stability and phase noise of a pair of X-band cryogenic sapphire oscillators, MEASUREMENT SCIENCE AND TECHNOLOGY, Vol: 18, Pages: 1224-1228, ISSN: 0957-0233
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