Microwave absorption and emission from magnetized afterglow plasmas

The microwave scattering properties of an axially magnetized afterglow plasma column in an S-band waveguide have been investigated experimentally. The column axis is perpendicular to the electric field and the direction of wave propagation in the H_(10)-mode waveguide. Strong absorption is found in the range of upper hybrid frequencies, ω_c ≤ ω ≤ [ω^2_c + ω^2_p(r,t)]^(1/2) where ω_c is the electron cyclotron frequency and ω_p is the locally and temporally varying electron plasma frequency. With the high absorption the noise emission approaches the blackbody limit. A microwave radiometer has been used to measure the noise power and with a comparison and null-technique the electron temperature. As emission and absorption are largely confined to a resonant layer, spatially resolved temperature data are obtained. Time resolution is obtained by gating the radiometer. The peak electron density is derived from the emission or absorption onset at the maximum upper hybrid frequency and confirmed by independent measurements. With this diagnostic technique the electron density and temperature decay has been studied under a variety of experimental conditions. Ambipolar diffusion and collisional cooling essentially account for the plasma decay, but impurities and metastable ions play an important role. The diagnostic method is successfully applied in a microwave heating experiment. The existence of absorbing resonant layers is shown by a peak in the radial temperature profile where the local upper hybrid frequency equals the heating frequency. The knowledge of the plasma parameters is important in the study of hot plasma effects. Buchsbaum-Hasegawa modes are investigated in a wide range of magnetic fields (.5 < ω_c/ω < .985).

Phase variations and phase shift in a TE011 mode microwave cavity used for in a miniature Rb fountain frequency standard

It is the first time in China that the phase variations and phase shift of microwave cavity in a miniature Rb fountain frequency standard are studied, considering the effect of imperfect metallic walls. Wall losses in the microwave cavity lead to small traveling wave components that deliver power from the cavity feed to the walls of cavity. The small traveling wave components produce a microradian distribution of phase throughout the cavity ity, and therefore distributed cavity phase shifts need to be considered. The microwave cavity is a TE011 circular cylinder copper cavity, with round cut-hole of end plates (14mm in diameter) for access for the atomic flux and two small apertures in the center of the side wall for coupling in microwave power. After attenuation alpha is calculated, field variations in cavity are solved. The field variations of the cavity are given. At the same time, the influences of loaded quality factor QL and diameter/height (2a/d) of the microwave cavity on the phase variations and phase shift are considered. According to the phase variation and phase shift of microwave cavity we select the parameters of cavity, diameter 2a = 69.2mm, height d = 34.6mm, QL = 5000, which will result in an uncertainty delta(Delta f / f0 ) < 4.7 x 10(-17) and meets the requirement for the miniature Rb fountain frequency standard with accuracy 10(-15).

Phase shift caused by microwave field based on light storage

We have theoretically investigated the phase shift of a probe field for a four-level atomic system interacting successively with two fields tuned near an EIT resonance of an atom, a microwave field, and a coupling field. It has been found that the phase of retrieved signal has been shifted due to the cross-phase modulation when the stored spin wave was disturbed by a microwave. Because of the low relaxation rates of the ground hyperfine state, our proposed technique can impart a large phase rotation onto the probe field with low absorption of retrieved field and very low intensity of the microwave field.

Ultranarrow absorptive spectral line induced by microwave field

This paper investigates the absorptive spectral lines of four-level atomic system driven by a coupling, probe and microwave fields. Due to the perturbation of the microwave field, the original electromagnetically induced transparency is changed to electromagnetically induced absorption and the absorptive spectral line can be very narrow. This ultranarrow spectral line has potential applications to the microwave atomic frequency standard and the measurement of very weak magnetic field.

Measurement of the Polarization of the Cosmic Microwave Background with the BICEP2 and Keck Array Telescopes

Precision polarimetry of the cosmic microwave background (CMB) has become a mainstay of observational cosmology. The ΛCDM model predicts a polarization of the CMB at the level of a few μK, with a characteristic E-mode pattern. On small angular scales, a B-mode pattern arises from the gravitational lensing of E-mode power by the large scale structure of the universe. Inflationary gravitational waves (IGW) may be a source of B-mode power on large angular scales, and their relative contribution to primordial fluctuations is parameterized by a tensor-to-scalar ratio r. BICEP2 and Keck Array are a pair of CMB polarimeters at the South Pole designed and built for optimal sensitivity to the primordial B-mode peak around multipole l ~ 100. The BICEP2/Keck Array program intends to achieve a sensitivity to r ≥ 0.02. Auxiliary science goals include the study of gravitational lensing of E-mode into B-mode signal at medium angular scales and a high precision survey of Galactic polarization. These goals require low noise and tight control of systematics. We describe the design and calibration of the instrument. We also describe the analysis of the first three years of science data. BICEP2 observes a significant B-mode signal at 150 GHz in excess of the level predicted by the lensed-ΛCDM model, and Keck Array confirms the excess signal at > 5σ. We combine the maps from the two experiments to produce 150 GHz Q and U maps which have a depth of 57 nK deg (3.4 μK arcmin) over an effective area of 400 deg² for an equivalent survey weight of 248000 μK². We also show preliminary Keck Array 95 GHz maps. A joint analysis with the Planck collaboration reveals that much of BICEP2/Keck Array's observed 150 GHz signal at low l is more likely a Galactic dust foreground than a measurement of r. Marginalizing over dust and r, lensing B-modes are detected at 7.0σ significance.

Microwave dynamics of quasiparticles and critical fields in superconducting films

The microwave response of the superconducting state in equilibrium and non-equilibrium configurations was examined experimentally and analytically. Thin film superconductors were mostly studied in order to explore spatial effects. The response parameter measured was the surface impedance.

For small microwave intensity the surface impedance at 10 GHz was measured for a variety of samples (mostly Sn) over a wide range of sample thickness and temperature. A detailed analysis based on the BCS theory was developed for calculating the surface impedance for general thickness and other experimental parameters. Experiment and theory agreed with each other to within the experimental accuracy. Thus it was established that the samples, thin films as well as bulk, were well characterised at low microwave powers (near equilibrium).

Thin films were perturbed by a small dc supercurrent and the effect on the superconducting order parameter and the quasiparticle response determined by measuring changes in the surface resistance (still at low microwave intensity and independent of it) due to the induced current. The use of fully superconducting resonators enabled the measurement of very small changes in the surface resistance (< 10^-9 Ω/sq.). These experiments yield information regarding the dynamics of the order parameter and quasiparticle systems. For all the films studied the results could be described at temperatures near T_c by the thermodynamic depression of the order parameter due to the static current leading to a quadratic increase of the surface resistance with current.

For the thinnest films the low temperature results were surprising in that the surface resistance decreased with increasing current. An explanation is proposed according to which this decrease occurs due to an additional high frequency quasiparticle current caused by the combined presence of both static and high frequency fields. For frequencies larger than the inverse of the quasiparticle relaxation time this additional current is out of phase (by π) with the microwave electric field and is observed as a decrease of surface resistance. Calculations agree quantitatively with experimental results. This is the first observation and explanation of this non-equilibrium quasiparticle effect.

For thicker films of Sn, the low temperature surface resistance was found to increase with applied static current. It is proposed that due to the spatial non-uniformity of the induced current distribution across the thicker films, the above purely temporal analysis of the local quasiparticle response needs to be generalised to include space and time non-equilibrium effects.

The nonlinear interaction of microwaves arid superconducting films was also examined in a third set of experiments. The surface impedance of thin films was measured as a function of the incident microwave magnetic field. The experiments exploit the ability to measure the absorbed microwave power and applied microwave magnetic field absolutely. It was found that the applied surface microwave field could not be raised above a certain threshold level at which the absorption increased abruptly. This critical field level represents a dynamic critical field and was found to be associated with the penetration of the app1ied field into the film at values well below the thermodynamic critical field for the configuration of a field applied to one side of the film. The penetration occurs despite the thermal stability of the film which was unequivocally demonstrated by experiment. A new mechanism for such penetration via the formation of a vortex-antivortex pair is proposed. The experimental results for the thinnest films agreed with the calculated values of this pair generation field. The observations of increased transmission at the critical field level and suppression of the process by a metallic ground plane further support the proposed model.

Microwave absorption properties of permalloy nanodots in the vortex and quasi-uniform magnetization states

When the in-plane bias magnetic field acting on a flat circular magnetic dot is smaller than the saturation field, there are two stable competing magnetization configurations of the dot: the vortex and the quasi-uniform (C-state). We measured microwave absorption properties in an array of non-interacting permalloy dots in the frequency range 1-8 GHz when the in-plane bias magnetic field was varied in the region of the dot magnetization state bi-stability. We found that the microwave absorption properties in the vortex and quasi-uniform stable states are substantially different, so that switching between these states in a fixed bias field can be used for the development of reconfigurable microwave magnetic materials.

Fluorescence properties and laser demonstrations of Nd-doped high silica glasses prepared by sintering nanoporous glass

Porous glass with high-SiO2 content was impregnated with Nd ions, and subsequently sintered at 1100 degrees C into a compact non-porous glass in air or reducing atmosphere. Sintering in a reducing atmosphere produced an intense violet-blue fluorescence at 394 nm. However, the sintering atmospheres almost did not affect the fluorescence properties in the infrared range. A good performance Nd3+-doped silica microchip laser operating at 1064 nm was demonstrated. The Nd-doped sintering glasses with high-SiO2 content are potential host materials for high power solid-state lasers and new transparent fluorescence materials. (c) 2007 Elsevier B.V. All rights reserved.

Blue green emission from a Cu+-Doped transparent material prepared by sintering porous glass

A colorless transparent, blue green emission material was fabricated by sintering porous glass impregnated with copper ions. The emission spectral profile obtained from Cu+ -doped high silica glass (HSG) by 267-mn monochromatic light excitation matches that obtained by pumping with an 800-nm femtosecond laser, indicating that the emissions in both cases come from an identical origin. The upconversion emission excited by 800-nm femtosecond laser is considered to be a three-photon excitation process. A tentative scheme of upconverted emission from Cu+ -doped HSG was also proposed. The glass materials presented herein are expected to find application in lamps, high density optical storage, and three-dimensional color displays.

Microwave Resonant and Zero-Field Absorption Study of Doped Magnetite Prepared by a Co-Precipitation Method

Fe3O4 and ZnxFe3-xO4 pure and doped magnetite magnetic nanoparticles (NPs) were prepared in aqueous solution (Series A) or in a water-ethyl alcohol mixture (Series B) by the co-precipitation method. Only one ferromagnetic resonance line was observed in all cases under consideration indicating that the materials are magnetically uniform. The shortfall in the resonance fields from 3.27 kOe (for the frequency of 9.5 GHz) expected for spheres can be understood taking into account the dipolar forces, magnetoelasticity, or magnetocrystalline anisotropy. All samples show non-zero low field absorption. For Series A samples the grain size decreases with an increase of the Zn content. In this case zero field absorption does not correlate with the changes of the grain size. For Series B samples the grain size and zero field absorption behavior correlate with each other. The highest zero-field absorption corresponded to 0.2 zinc concentration in both A and B series. High zero-field absorption of Fe3O4 ferrite magnetic NPs can be interesting for biomedical applications.

Preparation and spectroscopic properties of Er3+-doped high silica glass fabricated by sintering nanoporous glass

A compact nonporous high silica (SiO2 % > 96%) glass containing 3400 ppm Er3+ ions, which was about ten times higher than that in Er-doped silica fiber amplifier (EDSFA), was synthesized by sintering porous glass immersed into erbium nitrate solution. The 1532 nm fluorescence has a FWHM (Full Width at Half Maximum) of 45 nm wider than that of EDSFA and possesses the glass with potential application in broadband fiber amplifiers. The Judd-Ofelt theoretical analysis reflects that Er3+ ions are located in a higher covalent environment which are comparable to those of aluminosilicate glass. (c) 2005 Elsevier B.V. All rights reserved.

Spectroscopic properties of Nd3+-doped high silica glass prepared by sintering porous glass

A new kind of Nd3+, -doped high silica glass (SiO2 > 96% (mass fraction)) was obtained by sintering porous glass impregnated with Nd3+, ions. The absorption and luminescence properties of high silica glass doped with different Nd3+, concentrations were studied. The intensity parameters Omega(t) (t = 2, 4, 6), spontaneous emission probability, fluorescence lifetime, radiative quantum efficiency, fluorescence branching ratio, and stimulated emission cross section were calculated using the Judd-Ofelt theory. The optimal Nd3+ concentration in high silica glass was 0.27% (mole fraction) because of its high quantum efficiency and emission intensity. By comparing the spectroscopic parameters with other Nd3+ doped oxide glasses and commercial silicate glasses, the Nd3+-doped high silica glasses are likely to be a promising material used for high power and high repetition rate lasers.

High quantum efficiency and high concentration erbium-doped silica glasses fabricated by sintering nanoporous glasses

A new method was used to prepare erbium-doped high silica (SiO2% > 96%) glasses by sintering nanoporous glasses. The concentration of erbium ions in high silica glasses can be considerably more than that in silica glasses prepared by using conventional methods. The fluorescence of 1532 nm has an FWHM (Full Wave at Half Maximum) of 50 nm, wider than 35 nm of EDSFA (erbium-doped silica fiber amplifer), and hence the glass possesses potential application in broadband fiber amplifiers. The Judd-Ofelt theoretical analysis reflects that the quantum efficiency of this erbium-doped glass is about 0.78, although the erbium concentration in this glass (6 x 103) is about twenty times higher than that in silica glass. These excellent characteristics of Er-doped high silica glass will be conducive to its usage in optical amplifiers and microchip lasers.

Rare-earth-doped silica microchip laser fabricated by sintering nanoporous glass

We report a new method for fabricating rare-earth-doped silica glasses for laser materials obtained by sintering nanoporous silica glasses impregnated with rare-earth-doped ions. The fabricated materials have no residual pores and show good optical and mechanical properties. Good performance from a Nd3+-doped silica microchip laser operating at 1.064 mum is successfully demonstrated, suggesting that the fabricated silica glasses have potential for use as active materials for high-power solid-state lasers. (C) 2005 Optical Society of America.