Doppler effect in the oscillator radiation process in the medium

The purpose of this paper is to investigate the radiation process of the charged particle passing through an external periodic field in a dispersive medium. In the optical range of spectrum we will consider two cases: first, the source has not eigenfrequency, and second, the source has eigenfrequency. In the first case, when the Cherenkov radiation occurs, the non-zero eigenfrequency produces a paradox for Doppler effect. It is shown that the absence of the eigenfrequency solves the paradox known in the literature. The question whether the process is normal (i.e. hard photons are being radiated under the small angles) or anomalous depends on the law of the medium dispersion. When the source has an eigenfrequency the Doppler effects can be either normal or anomalous. In the X-ray range of the oscillator radiation spectrum we have two photons radiated under the same angle- soft and hard. In this case the radiation obeys to so-called complicated Doppler effect, i.e. in the soft photon region we have anomalous Doppler effect and in the hard photon region we have normal Doppler effect.Comment: 6 pages, no figure

On the Possibility of Medium-Energy Compact X-ray Free-Electron Laser

The problem of X-ray Free-Electron Laser operating on self-amplified spontaneous emission in irregular microundulator is considered. The case when the spectrum width of spontaneous radiation is conditioned by the spatial distribution of sources creating the undulating field is considered. In this case gain function of the stimulated radiation is dozens of times higher than that of the conventional undulators. We propose a model of irregular microundulator, which can be used to construct a drastically cheap and compact X-ray free-electron laser operating on medium energy electron bunch.Comment: 6 pages, 5 figures, revtex4, accepted by Armenian Journal of Physic

Positron Bunch Radiation in the System of Tightly-Packed Nanotubes

The problem of channeling radiation of positron bunch in the system of packed nanotubes was investigated in the present work. Used the model of harmonic potential which is justified since on the one hand the number of positrons in the region near the axis of nanotube is small, and on the other hand their contribution to the formation of the total radiation is also small. The problem is solved in the dipole approximation. The radiation at first harmonic occurs at zero angle too. At zero angle are radiated both extremely hard and extremely soft photons due to the medium polarization. The frequency-angular distribution of number of emitted photons was received. The distribution does not depend on the azimuthal angle, since the task has cylindrical symmetry. Radiation at the zero angle is fully circularly polarized. For formation of radiation there is an energy threshold: lower threshold is due to the polarization of medium, the upper threshold depends on the oscillation amplitude of channelling positrons. When the bunch energy coincides with the upper threshold then in radiation contribute all channeled positrons. Each positron in average radiates one photon. Thus is formed intensive, quasi-monochromatic and circularly olarized X-ray photon beam which may have important practical application

Tunable thin film bulk acoustic wave resonators with improved Q-factor

The tunable solidly mounted Ba0.25Sr0.75TiO3 (BSTO) thin film bulk acoustic wave resonators (TFBARs) with improved Q-factor are fabricated and characterized. The BSTO films are grown by magnetron sputtering at temperature 600 degrees C and extremely low sputter gas pressure 2 mTorr using on-axis configuration. The measured TFBARs Q-factor is more than 250 and mechanical Q-factor is more than 350 at 5 GHz resonance frequency. The improvement in the Q-factor is associated with reduction in the BSTO film grain misorientation. The latter is responsible for generation of shear waves leaking through the Bragg reflector and corresponding acoustic loss. (C) 2010 American Institute of Physics

Ferroelectric film bulk acoustic wave resonators for liquid viscosity sensing

A concept of accurate liquid viscosity sensing, using bulk acoustic wave (BAW) resonators, is proposed. The proposed BAW resonators use thin ferroelectric films with the dc field induced piezoelectric effect allowing for generation of pure longitudinal acoustic waves in the thickness excitation mode. This makes it possible to utilize exclusively shear liquid particle displacement at the resonator side walls and, therefore, accurate viscosity evaluation. The BAW resonators with the dc field induced piezoelectric effect in 0.67BiFeO(3)-0.33BaTiO(3) ferroelectric films are fabricated and their liquid viscosity sensing properties are characterized. The resonator response is analyzed using simple model of a harmonic oscillator damped by a viscous force. It is shown that the resonator Q-factor is inversely proportional to the square root of the viscosity-density product. The viscosity measurement resolution is estimated to be as high as 0.005 mPa.s, which is 0.5% of the water viscosity

Composite Ferroelectric FBARs That Are Switchable Between the First and Second Harmonics: Experimental Demonstration

Digital switching between the first and second harmonics, in a composite thin-film bulk acoustic wave resonator (FBAR), is demonstrated experimentally. The FBAR consists of two 180-nm-thick paraelectric-phase Ba0.25Sr0.75TiO3 films separated by a 50-nm-thick SrRuO3 conducting layer. The resonant frequency of this composite resonator (with Pt bottom and Al top electrodes) is switched from 3.6 GHz to 7.6 GHz, where the polarity of the 5 V dc bias is reversed on one of the ferroelectric films. The frequency switching ratio (f(2)/f(1) >= 2) depends on the thickness of the electrodes. Some adjustment of f(2)/f(1) is possible by changing the applied dc bias

Correlations between microstructure and Q-factor of tunable thin film bulk acoustic wave resonators

Correlations between microstructure and Q-factor of tunable solidly mounted Ba(0.25)Sr(0.75)TiO(3) (BSTO) thin film bulk acoustic wave resonators are studied using analysis of test structures prepared at different growth temperatures of the BSTO films varying in the range 450-650 degrees C. The observed changes in the Q-factor with growth temperature are correlated with related changes in microstructure, including the grain size, texture misalignment, interfacial amorphous layer, surface roughness, and deterioration of the Bragg reflector layers. The correlations are established through analysis of corresponding extrinsic acoustic loss mechanisms, including Rayleigh scattering at localized defects, acoustic attenuation by amorphous layer, generation of the shear waves leaking into the substrate, waves scattering by surface roughness, and resonance broadening by local thickness variations. It is shown that the waves scattering by surface roughness at the BSTO film interfaces is the main loss mechanism limiting the Q-factor of the BSTO thin film bulk acoustic wave resonators. (C) 2011 American Institute of Physics. [doi:10.1063/1.3626939

Adhesion layer-bottom electrode interaction during BaxSr1−xTiO3 growth as a limiting factor for device performance

Changes in bottom electrode morphology and adhesion layer composition upon deposition of Ba_xSr_1-xTiO₃ (BSTO) at elevated temperatures have been found, which have a negative impact on acoustic wave resonator device performance. The difference between nominal and actual adhesion layer composition are explained by grain boundary diffusion of Ti or W and their oxidation by in-diffusing oxygen, which leads to an increased interface roughness between the Pt bottom electrode and the BSTO. It is shown, that room-temperature deposited TiO₂ diffusion barriers fail to protect against Ti oxidation and diffusion. Also W adhesion layers are prone to this phenomenon, which limits their ability to act as high temperature resistant adhesion layers for bottom electrodes for ferroelectric thin films

Dielectric model of point charge defects in insulating paraelectric perovskites

Some point defects (i.e., oxygen vacancies) create deep trapping levels in the bandgap of the paraelectric phase ferroelectric crystals. Under applied DC field the traps release electrons via the Poole-Frenkel mechanism and become charged. The electric field of a point charge polarizes the crystal locally reducing its permittivity. In this paper a simple theory is proposed for calculating the DC field dependent apparent (measureable) permittivity of a paraelectric crystal with point charge defects. It is shown that the apparent permittivity of a paraelectric crystal may be sufficiently lower as compared with the defectless crystal. This reduction is in addition to the possible reduction of the apparent permittivity associated with the interfacial "dead" layers and strain