Jonathan Breeze

Dielectric loss of oxide single crystals and polycrystalline analogues from 10 to 320 K

The key factors influencing microwave dielectric loss are examined. A comparison is made between single crystals and polycrystalline analogues. Measurements of the temperature dependence of microwave dielectric losses in various materials are reported, for temperatures between 20 and 300 K. Single crystal and polycrystalline TiO2, LaAlO3, MgO and Al2O3 are considered. The temperature dependence of dielectric losses of certain single crystals (MgO and Al2O3) are found to be in good agreement with the theory of intrinsic losses for temperatures above 100 K. At lower temperatures losses due to defects and grain boundaries dominate. The absolute value of the loss predicted by theory is considerably lower than measured values. # 2001 Published by Elsevier Science Ltd. All rights reserved.

Measurements of Permittivity, Dielectric Loss Tangent, and Resistivity of Float-Zone Silicon at Microwave Frequencies

The complex permittivity and resistivity of float-zone high-resistivity silicon were measured at microwave frequencies for temperatures from 10 up to 400 K employing dielectric-resonator and composite dielectric-resonator techniques. At temperatures below 25 K, where all free carriers are frozen out, loss-tangent values of the order of 2times10-4 were measured, suggesting the existence of hopping conductivity or surface charge carrier conductivity in this temperature range. Use of a composite dielectric-resonator technique enabled the measurement of materials having higher dielectric losses (or lower resistivities) with respect to the dielectric-resonator technique. The real part of permittivity of silicon proved to be frequency independent. Dielectric losses of high-resistivity silicon at microwave frequencies are mainly associated with conductivity and their behavior versus temperature can be satisfactory described by dc conductivity models, except at very low temperatures

On the temperature coefficient of resonant frequency in microwave dielectrics

Abstract Developing a material with a temperature coefficient of the resonant frequency (TCF) of zero is probably the most difficult aspect of research into microwave dielectric ceramics. There are a host of high-quality-factor (Q) high-relative-permittivity (ϵr) materials whose TCF is far too high to be usable, notably TiO2 (ϵr = 104; Q = 14000 at 3GHz: TCF = +427 ppm K−1). The challenge is to find ways of tuning TCF while maintaining values of ϵr and Q suitable for use in microwave components. This paper explains, in terms of octahedral tilting and ϵr dilution, the tunability of TCF or, more precisely, the temperature coefficient of ϵr in Ca and Sr titanate-based ceramics. Examples of the structural changes that occur during tuning are also shown.

Room-temperature solid-state maser

The invention of the laser has resulted in many innovations, and the device has become ubiquitous. However, the maser, which amplifies microwave radiation rather than visible light, has not had as large an impact, despite being instrumental in the laser’s birth. The maser’s relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic and free-electron masers require vacuum chambers and pumping; and solid-state masers, although they excel as low-noise amplifiers and are occasionally incorporated in ultrastable oscillators, typically require cryogenic refrigeration. Most realizations of masers also require strong magnets, magnetic shielding or both. Overcoming these various obstacles would pave the way for improvements such as more-sensitive chemical assays, more-precise determinations of biomolecular structure and function, and more-accurate medical diagnostics (including tomography) based on enhanced magnetic resonance spectrometers incorporating maser amplifiers and oscillators. Here we report the experimental demonstration of a solid-state maser operating at room temperature in pulsed mode. It works on a laboratory bench, in air, in the terrestrial magnetic field and amplifies at around 1.45 gigahertz. In contrast to the cryogenic ruby maser, in our maser the gain medium is an organic mixed molecular crystal, p-terphenyl doped with pentacene, the latter being photo-excited by yellow light. The maser’s pumping mechanism exploits spin-selective molecular intersystem crossing into pentacene’s triplet ground state. When configured as an oscillator, the solid-state maser’s measured output power of around −10 decibel milliwatts is approximately 100 million times greater than that of an atomic hydrogen maser, which oscillates at a similar frequency (about 1.42 gigahertz). By exploiting the high levels of spin polarization readily generated by intersystem crossing in photo-excited pentacene and other aromatic molecules, this new type of maser seems to be capable of amplifying with a residual noise temperature far below room temperature.

Scattering of light into silicon by spherical and hemispherical silver nanoparticles

The interaction of light with noble metal nanoparticles deposited onto the top surface of a semiconductor has been investigated using the finite-difference time-domain method. The scattering is calculated for spherical and hemispherical silver nanoparticles placed in a periodic two-dimensional array on the upper surface of a semi-infinite silicon substrate. The results show that the contact area between hemispherical particles and the silicon significantly reduces the forward scattering. The use of an oxide buffer layer to separate the particle from the semiconductor is investigated and is seen to be important if the forward scattering of light is to be enhanced.

Microwave dielectric loss in oxides: Theory and experiment

We present a model that provides a description of the microwave dielectric loss in oxides. The dielectric loss (tan δ) in single crystal and polycrystalline MgO and Al2O3 is measured over the temperature range 70–300 K. We are able to model the dielectric loss in terms of a two-phonon difference model. There are two key parameters in this model: The third derivative, φ3, of the lattice potential and the linewidth, γ, of the thermal phonons. In polycrystalline samples, rather than considering the different mechanisms of extrinsic loss, it is assumed that the main effect of extrinsic factors is a modification of the linewidth of the thermal phonons. By varying γ(T), it is shown that the model can describe the loss in both single crystals and polycrystallines materials. In single crystal and polycrystalline MgO, we use γ as a fitting parameter. In single crystal and polycrystalline Al2O3, we obtain γ(T) by Raman spectroscopy. The theory gives the right order of magnitude of the measured loss.

Ultralow loss polycrystalline alumina

Polycrystalline alumina with extremely low microwave dielectric loss is reported with properties analogous to a theoretical ensemble of randomly oriented, single crystal sapphire grains. By avoiding deleterious impurities and by careful control of microstructure, we show that grain boundaries in aluminum oxide have only a limited influence on the dielectric loss. A method of measuring the electric permittivity and loss tangent of low-loss microwave ceramic dielectrics is reported. The electrical parameters such as relative permittivity and loss tangent are extracted using the radial mode matching technique. The measured values for ultralow loss polycrystalline aluminum oxide agree well with theoretical values modelled on an ensemble of randomly oriented anisotropic single crystal sapphire grains.

Enhanced magnetic Purcell effect in room-temperature masers

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.

Enhanced quality factors in aperiodic reflector resonators

Cavity resonators that employ the high reflectivity of periodic arrays of dielectric layers exhibit enhanced quality factors compared with dielectric resonators. Their quality factor is limited by the exponential decay of the electric field penetrating the structure. We show that an aperiodic reflector array with dielectric layers thinner than a quarter-wave near the defect site and asymptotically approaching quarter-wave thickness distant from the site can exhibit very high quality factors. A spherical aperiodic reflector resonator consisting of nested alumina shells is simulated and shown to exhibit quality factors greater than 107 at 10GHz and room temperature.

Continuous-wave room-temperature diamond maser

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 host 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 diamond and silicon carbide, 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.