The collective excitations of a double heterostructure are examined in detail. Including the important effects of the interfaces of the system the uncoupled modes are analysed, which are the long-wave optical interface phonon modes, LO phonon modes, and intra-and intersubband plasmon modes. The electron-phonon interaction is discussed and the coupling functions are calculated and discussed in graphical form. The resulting coupled modes, intra-and intersubband plasmon-phonon modes, are calculated for different approximations of the polarization function, the random-phase approximation, the plasmon-pol approximation, and the long-wave approximation. The dispersion curves of these modes are presented in graphical form for various cases. It is shown that the Landau damping of the modes strongly depends on the layer thickness and the concentration of the quasi-two-dimensional electron gas.Die kollektiven Anregungen einer Doppelheterostruktur werden ausfuhrlich untersucht. Es werden die ungekoppelten Moden, d. h. die langwellig-optischen Grenzflachen-Phononen, die LO-Phononen sowie die Intra-und Intersubband-Plasmonen unter Einbeziehung der Grenzflacheneffekte diskutiert. Im Detail werden die Elektron-Phonon-Wechselwirkung analysiert, die Kopplungsfunktionen berechnet und graphisch dargestellt. Die resultierenden gekoppelten Moden, die Intraund Intersubband-Plasmon-Phononen werden fur mehrere Naherungen der Polarisationsfunktion, der Random-Phase-Approximation, der Plasmon-Pol-Naherung und der langwelligen Naherung berechnet. Die Dispersionskurven sind fur verschiedene Falle graphisch dargestellt. Es wird gezeigt, daB die Landau-Dampfung der Moden stark von der Schichtdicke und der Konzentration des quasi-zweidimensionalen Elektronengases abhangt.

Experimental and theoretical research was carried out in order to establish the dependence of the performance of a compliant-cylinder-based fiber-optic accelerometer on the geometry and elastic properties of the transducer cylinders. The sensitivity and the natural frequency of the sensor were measured as a function of the ratio ε = (inner cylinder diameter)/(outer cylinder diameter). Two transducer materials with different elastic properties, a silicone rubber (Ecosil) and a polyetheretherketone polymer (PEEK 450G), were examined. It was found that with decreasing ε the sensitivity increases in the case of Ecosil and decreases in the case of PEEK. In both cases the natural frequency increases with decreasing ε. A simple analytical model was developed in order to explain this behavior qualitatively. The model takes into account the contributions to the effective stiffness from both the cylinder material and the fiber wrapped around the cylinder. The model is useful for the design of such types of accelerometer.

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A theory of dynamical screening in heterostructures and semiconductor quantum wells is developed which contains electron-electron interaction and electron-phonon interaction. I n this theory both the types of long-wave optical phonons, LO phonons and interface phonons, which occur in those layered structures are included. An expression is derived for the longitudinal screened interaction of subband electrons in heterostructures and quantum-wells. The obtained screened interaction potential allows the derivation of the total 1 ongitudinal dielectric function and the dispersion relation of the coupled intra-and intersubband plasmon-phonon modes. This screened interaction potential is used t o develope a dynamically screened and finite-temperature theory of the electronphonon interaction in heterostructures and quantum wells. The leading-order electron self-energy is disscnssed. An analytic expression of the polarization function of the quasi-two-dimensional electron gas is calculated within the random-phase approximation. Further approximations, t h e plasmon pole approximation and the long-wave approximation are discussed and the resulting analytic expressions of the polarization function are given. E s wird e k e Theorie der dynamischen Abschirmung in Heterostrukturen und Quanten-Wells entwickelt. Die dynamische Abschirmnng beinhaltet sowohl die Elektron-Elektron-als auch die Elektron-Phonon-Wechselwirkung, wobei in letzterer die zwei Typen von langwellig-optischen Phononen (LO Phononen und Grenzflachen-Phononen), die in solchen schichtftirmigen Strukturen auftreten, explizit beriicksichtigt werden. Das longitudinal abgeschirmte Wechelwirkungspotential der Subband-Elektronen in Heterostrmkturen und Qnanten-Wells wird berechnet. Ausgehend von diesem Potentmid wird ein Ausdruck fur die totale longitudinale dielektrische Funktion und fiir die Dispersionsrelntion der gekoppelten Tntra-und Intersubband-Plasmon-Phonon-RIoden abgeleitet. Weiterhin wird aus diesem Potential eine Theorie der dynamisch abgeschirmten Elektron-Phonon-Wechselwirkung bei endlichen Temperaturen in Heterostrukturen und Quanten-Wells abgeleitet. Die aus dieser Wechselwirkung resultierende Selbstenergie der Elektronen wird diskutiert. Es wird ein anulytischer Aosdruck fur die Polarisationsfunktion des quasi-zweidimensionalen Elektronengases in der Random-Phase Spproximation abgeleitet. Weitere Naherungen, die Plasmon-Pol-Naherung und die langwellige Niherung werden diskutiert und analytische Ausdrucke fur die Polarisationsfunktion abgeleitet.

The collective excitations of a double heterostructure in the presence of a magnetic field are examined in detail. The polarization function of the quasi-two-dimensional electron gas in the presence of a perpendicular quantizing magnetic field is calculated within various long-wave approximations and in the random-phase approximation. Using these polarization functions the dispersion curves of the coupled intra- and inter-subband magnetoplasmon-phonons are calculated and plotted in graphical form. The calculation of the dispersion relation is done for GaAs-Ga1-xAlxAs double heterostructures and include both quantum size effects and the correct spectrum of the long-wave optical phonons. In this paper the higher resonances of the quasi-two-dimensional magnetoplasma, the Bernstein modes, are also investigated. It is shown that the intra- and inter-subband magnetoplasmon-phonons are free of Landau damping for finite perpendicular quantizing magnetic fields.