41 results found
Abdurakhimov LV, Khan S, Panjwani NA, et al., 2019, Magnon-photon coupling in the noncollinear magnetic insulator Cu <inf>2</inf> OSeO <inf>3</inf>, Physical Review B, Vol: 99, ISSN: 2469-9950
© 2019 American Physical Society. UK. Anticrossing behavior between magnons in the noncollinear chiral magnet Cu2OSeO3 and a two-mode X-band microwave resonator was studied in the temperature range 5-100 K. In the field-induced ferrimagnetic phase, we observed a strong-coupling regime between magnons and two microwave cavity modes with a cooperativity reaching 3600. In the conical phase, cavity modes are dispersively coupled to a fundamental helimagnon mode, and we demonstrate that the magnetic phase diagram of Cu2OSeO3 can be reconstructed from the measurements of the cavity resonance frequency. In the helical phase, a hybridized state of a higher-order helimagnon mode and a cavity mode - a helimagnon polariton - was found. Our results reveal a class of magnetic systems where strong coupling of microwave photons to nontrivial spin textures can be observed.
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.
Breeze JDB, Salvadori E, Sathian J, et 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.
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
Donchev E, Pang JS, Gammon PM, et al., 2014, The rectenna device: From theory to practice (a review), MRS Energy & Sustainability, Vol: 1, ISSN: 2329-2229
<jats:title>ABSTRACT</jats:title><jats:p><jats:bold><jats:italic>This review article provides the state-of-art research and developments of the rectenna device and its two main components – the antenna and the rectifier. Furthermore, the history, efficiency trends, and socioeconomic impact of its research are also featured.</jats:italic></jats:bold></jats:p><jats:p>The rectenna (RECTifying antENNA), which was first demonstrated by William C. Brown in 1964 as a receiver for microwave power transmission, is now increasingly researched as a means of harvesting solar radiation. Tapping into the growing photovoltaic market, the attraction of the rectenna concept is the potential for devices that, in theory, are not limited in efficiency by the Shockley–Queisser limit. In this review, the history and operation of this 40-year old device concept are explored in the context of power transmission and the ever increasing interest in its potential applications at terahertz frequencies, through the infrared and visible spectra. Recent modeling approaches that have predicted controversially high efficiency values at these frequencies are critically examined. It is proposed that to unlock any of the promised potential in the solar rectenna concept, there is a need for each constituent part to be improved beyond the current best performance, with the existing nanometer scale antennas, the rectification and the impedance matching solutions all falling short of the necessary efficiencies at terahertz frequencies. Advances in the fabrication, characterization, and understanding of the antenna and the rectifier are reviewed, and common solar rectenna design approaches are summarized. Finally, the socioeconomic impact of success in this field is discussed and future work is proposed.</jats:p>
Oxborrow M, Breeze JD, Alford NM, 2012, Room-temperature solid-state maser, NATURE, Vol: 488, Pages: 353-+, ISSN: 0028-0836
Lei DY, Kéna-Cohen S, Zou B, et al., 2012, Spectroscopic ellipsometry as an optical probe of strain evolution in ferroelectric thin films, Opt. Express, Vol: 20, Pages: 4419-4427
Heteroepitaxial strain in ferroelectric thin films is known to have a significant impact on both their low and high frequency dielectric properties. In this paper, we use ex-situ spectroscopic ellipsometry to study the strain evolution with film thickness, and strain relaxation in ferroelectric Ba0.5Sr0.5TiO3 epitaxial films grown on single crystal substrates. For films grown on MgO substrates, a critical thickness for strain relaxation is observed. In addition, studies of Ba0.5Sr0.5TiO3 films grown on different single crystal substrates reveal that the strain relaxation rate can be inferred from changes in the optical properties. Using this information, we show that the optical constants of Ba0.5Sr0.5TiO3 can be readily tuned via strain engineering.
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
Centeno A, Xie F, Breeze J, et al., 2011, Calculations of scattering and absorption efficiencies of nobel metal nanoparticles, Applied Electromagnetics Conference (AEMC), 2011 IEEE, Pages: 1-4
Shimada T, Ichikawa K, Minemura T, et al., 2010, Intrinsic Microwave Dielectric Loss of Lanthanum Aluminate, IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL, Vol: 57, Pages: 2243-2249, ISSN: 0885-3010
Centeno A, Breeze J, Ahmed B, et al., 2010, Scattering of light into silicon by spherical and hemispherical silver nanoparticles, OPTICS LETTERS, Vol: 35, Pages: 76-78, ISSN: 0146-9592
Shimada T, Ichikawa K, Minemura T, et al., 2010, Temperature and frequency dependence of dielectric loss of Ba(Mg1/3Ta2/3)O-3 microwave ceramics, 11th Electroceramics Conference 2008, Publisher: ELSEVIER SCI LTD, Pages: 331-334, ISSN: 0955-2219
Breeze JD, Perkins JM, McComb DW, et al., 2009, Do Grain Boundaries Affect Microwave Dielectric Loss in Oxides?, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 92, Pages: 671-674, ISSN: 0002-7820
Breeze J, Krupka J, Centeno A, et al., 2009, Temperature-stable and high Q-factor TiO2 Bragg reflector resonator, APPLIED PHYSICS LETTERS, Vol: 94, ISSN: 0003-6951
Shimada T, Ichikawa K, Minemura T, et al., 2009, Intrinsic Microwave Dielectric Loss of Lanthanum Aluminate, Joint Meeting of 12th International Meeting on Ferroelectricity/18th IEEE International Symposium on Applications of Ferroelectrics (IMF-ISAF-2009), Publisher: IEEE, Pages: 250-+, ISSN: 1099-4734
Centeno A, Breeze JD, Alford NMN, et al., 2008, Measurement of high loss dielectric materials using a resonant technique
This paper will report on a technique that has been developed for the measurement of the dielectric loss of materials which have a relatively high loss tangent. The method used employs a composite resonator that consists of the material under test being placed in the centre of a low loss ceramic ring resonator. The important feature of this configuration is that the electric energy filling factor in the sample is significantly lowered. This has the desirable effect of reducing the contribution to the resonator Q-factor of the sample material which has a high loss compared to the ceramic ring resonator. This enables the measurement of dielectric loss tangents which are significantly larger than those that can be measured using a conventional technique, such as making a cylindrical dielectric resonator of the sample material. In this technique the choice of resonant mode is very important. The electrical energy filling factor must low enough to ensure a measurable Q factor but at the same time high enough to measure a detectable perturbation of the resonance. It is also important to know the electrical filling factor of the ring resonator, which is made of a well characterised ceramic material, and the Q-factor contribution of any conducting surfaces in the resonant structure such as cavity walls. The choice of resonant mode and the effects of the other components in the resonator are obtained by using Finite Difference Time Domain (FDTD) and Mode Matching modelling tools. Measurement of the loss tangent of high resistivity Silicon over temperature has already been carried out with this technique by employing a BZT ring resonator resonating at around 5GHz in the TE01? mode. The results are very promising with loss tangents being measured that are an order of magnitude higher than could be measurable using a more conventional resonant technique. The current focus of this work is the measurement of liquids at microwave and millimetre wavelengths using Alumina resonators
Krupka J, Breeze J, Alford NMN, et al., 2007, Measurements of permittivity and dielectric loss tangent of high resistivity float zone silicon at microwave frequencies
Real part of permittivity and the dielectric loss tangent of float zone high resistivity Silicon were measured at microwave frequencies at temperatures from 10 K up to 380 K employing dielectric resonator technique. The real part of permittivity proved to be frequency independent and the decrease in dielectric loss tangent versus frequency proved to be not entirely proportional to the inverse of frequency. At temperatures below 25 K where all free carriers are frozen-out loss tangents values the order of 10-4 were measured.
Breeze J, Krupka J, Alford NM, 2007, Enhanced quality factors in aperiodic reflector resonators, APPLIED PHYSICS LETTERS, Vol: 91, ISSN: 0003-6951
Centeno A, Breeze JD, Krupkaf J, et al., 2006, Evaluating the properties of dielectric materials for microwave integrated circuits, Pages: 21-26
It is important to be able to accurately evaluate the electrical properties of dielectric materials to enable the accurate design of passive microwave integrated circuit components. This paper reports on research that has been undertaken in this area at London South Bank University. Three measurement techniques are reported. The first is a novel technique for measuring dielectric materials with a large tan δ using a composite resonator. The second is the measurement of the permittivity of ferroelectric thin films using a planar capacitor. The third is the use of an evanescent microwave probe to find the electrical properties at the surface of a sample.
Krupka J, Breeze J, Centeno A, et al., 2006, Measurements of permittivity, dieletric loss tangent, and resistivity of float-zone silicon at microwave frequencies, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol: 54, Pages: 3995-4001, ISSN: 0018-9480
Dunne LJ, Axelsson AK, Alford NM, et al., 2006, Quasi-classical fluctuation-dissipation description of dielectric loss in oxides with implications for quantum information processing, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Vol: 106, Pages: 986-993, ISSN: 0020-7608
Pullar RC, Breeze JD, Alford NM, 2005, Characterization and microwave dielectric properties of M2+Nb2O6 ceramics, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 88, Pages: 2466-2471, ISSN: 0002-7820
Aupi X, Breeze J, Ljepojevic N, et al., 2004, Microwave dielectric loss in oxides: Theory and experiment (vol 95, pg 2639, 2004), JOURNAL OF APPLIED PHYSICS, Vol: 96, Pages: 6943-6943, ISSN: 0021-8979
Alford NM, Breeze J, Penn SJ, 2004, Temperature stabilisation of dielectric resonator, US6803132 B1
A temperature stabilised ceramic material which has low dielectric loss and high thermal conductivity is a layered structure with a first layer formed of titania doped alumina and the second layer is a thin layer of titanium dioxide, with the layers attached by solid state diffusion.
Aupi X, Breeze J, Ljepojevic N, et al., 2004, Microwave dielectric loss in oxides: Theory and experiment, JOURNAL OF APPLIED PHYSICS, Vol: 95, Pages: 2639-2645, ISSN: 0021-8979
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