Superlattice nonlinearities for Gigahertz-Terahertz generation in harmonic multipliers

Semiconductor superlattices are strongly nonlinear media offering several technological challenges associated with the generation of high-frequency Gigahertz radiation and very effective frequency multiplication up to several Terahertz. However, charge accumulation, traps and interface defects lead to pronounced asymmetries in the nonlinear current flow, from which high harmonic generation stems. This problem requires a full non-perturbative solution of asymmetric current flow under irradiation, which we deliver in this paper within the Boltzmann-Bloch approach. We investigate the nonlinear output on both frequency and time domains and demonstrate a significant enhancement of even harmonics by tuning the interface quality. Moreover, we find that increasing arbitrarily the input power is not a solution for high nonlinear output, in contrast with materials described by conventional susceptibilities. There is a complex combination of asymmetry and power values leading to maximum high harmonic generation.Comment: 13 pages, 7 figures, Accepted for Nanophotonics (De Gruyter

Controlling the harmonic conversion efficiency in semiconductor superlattices by interface roughness design

In semiconductor superlattices, when Bragg oscillating electrons interact with an input electromagnetic field, frequency multiplication is possible. An ideal superlattice has a purely antisymmetric voltage current response and can thus produce only odd harmonics. However, real world superlattices can also have even harmonic response and that increases the range of possible output frequencies. These effects have been recently explained with a predictive model that combines an Ansatz solution for the Boltzmann Equation with a Nonequilibrium Green's Functions approach. This predictive tool, coupled with recent progress on GHz input sources, support the growing interest in developing compact room temperature devices that can operate from the GHz to the THz range. The natural question to ask is what efficiencies can be expected. This paper addresses this issue by investigating power-conversion efficiency in irradiated semiconductor superlattices. Interface imperfections are consistently included in the theory and they strongly influence the power output of both odd and even harmonics. Good agreement is obtained for predicted odd harmonic outputs with experimental data for a wide frequency range. The intrinsic conversion efficiency used is based on the estimated amplitude of the input field inside the sample and thus independent of geometrical factors that characterize different setups. The method opens the possibility of designing even harmonic output power by controlling the interface quality

High-frequency acoustoelectronic phenomena in miniband superlattices

The motion of a quantum particle in a periodic potential can generate rich dynamics in the presence of a driving field. Such systems include, but are not limited to, semiconductor superlattices which exhibit a very anisotropic band structure that results into pronounced nonlinearities and high carrier mobility. In this thesis, we investigate the semiclassical dynamics and electron transport in a spatially periodic potential driven by a propagating wave. Firstly, we examine the transport features of an electron in a single miniband superlattice driven by a high-frequency acoustic plane wave. In this system, the nonlinear electron dynamics crucially depends on the amplitude of the acoustic wave. The transport characteristics are studied by means of a non-linearised kinetic model. In particular, to provide a realistic description of the directed transport, we employ the exact path-integral solutions of the Boltzmann transport equation. The calculated electron drift velocity and the time-averaged velocity show a nonmonotonic dependence upon the amplitude of the acoustic wave with multiple pronounced extrema. We found out that the changes in the velocity-amplitude characteristics are directly associated with a series of global bifurcations due to topological rearrangements of the phase space of the system. These dramatic transformations are connected with superlattice intraminiband transitions, and accompanied by inelastic emission (absorption) of the quantum particle. The bifurcations also signify the transitions between different dynamical regimes, involving unconfined electron motion, wave-dragging and phonon-assisted Bloch oscillations. Each regime has a characteristic spectral fingerprint, which manifests itself in appearance of specific high-frequency components in the spectra of the corresponding averaging trajectory. Secondly, we consider to use the acoustically pumped superlattices for an amplification of THz electromagnetic waves, involving the mechanisms similar to the Bloch gain in electrically biased superlattices. In particular, we predict the tunable THz gain due to nonlinear oscillations which are associated with the localised motion of electrons confined by a propagating potential wave. Traditionally, one of the key issues which emerges from considering different schemes for achieving small signal gain in superlattices, is the control of electric stability. Here, it is shown that for our case of the fast miniband electrons driven by an acoustic wave, terahertz gain can occur without the electric instability. Additionally, we find that the characteristic changes in the averaged velocities are connected to the shape of gain profiles. Consequently, the analytic findings, which determine the transitions between different dynamical regimes at the bifurcations, hold up for the behaviour of amplification of high-frequency electromagnetic waves. The increase of the miniband width, results in an enhancement of the effect of phase space restructuring on the drift velocity and high-frequency gain. Finally, we analyse the case for a superlattice device utilising acoustic waves with a very slow propagation speed. Benefiting from a simple solution of the Boltzmann equation, here we clarify the role of spatial nonlinearity both in miniband electron dynamics and in amplification of an electromagnetic wave. We show that nonlinear Bloch oscillations occur at a single critical value of the wave amplitude, inducing high negative differential drift velocity. Within this model, we also explain how the amplification of a high-frequency signal can arise below the threshold for an excitation of Bloch oscillations

Structural and biomechanical alterations in rabbit thoracic aortas are associated with the progression of atherosclerosis

<p>Abstract</p> <p>Background</p> <p>Atherosclerosis is a diffuse and highly variable disease of arteries that alters the mechanical properties of the vessel wall through highly variable changes in its cellular composition and histological structure. We have analyzed the effects of acute atherosclerotic changes on the mechanical properties of the descending thoracic aorta of rabbits fed a 4% cholesterol diet.</p> <p>Methods</p> <p>Two groups of eight male New Zealand White rabbits were randomly selected and fed for 8 weeks either an atherogenic diet (4% cholesterol plus regular rabbit chow), or regular chow. Animals were sacrificed after 8 weeks, and the descending thoracic aortas were excised for pressure-diameter tests and histological evaluation to examine the relationship between aortic elastic properties and atherosclerotic lesions.</p> <p>Results</p> <p>All rabbits fed the high-cholesterol diet developed either intermediate or advanced atherosclerotic lesions, particularly American Heart Association-type III and IV, which were fatty and contained abundant lipid-filled foam cells (RAM 11-positive cells) and fewer SMCs with solid-like actin staining (HHF-35-positive cells). In contrast, rabbits fed a normal diet had no visible atherosclerotic changes. The atherosclerotic lesions correlated with a statistically significant decrease in mean vessel wall stiffness in the cholesterol-fed rabbits (51.52 ± 8.76 kPa) compared to the control animals (68.98 ± 11.98 kPa), especially in rabbits with more progressive disease.</p> <p>Conclusions</p> <p>Notably, stiffness appears to decrease with the progression of atherosclerosis after the 8-week period.</p

Σχεδίαση τελεστικών ενισχυτών διαγωγιμότητας (ΟΤΑ) χαμηλής κατανάλωσης σε δυο γενιές τεχνολογίας Bulk CMOS

This thesis submitted in partial fulfillment of the requirements for the diploma degree of electrical and computer engineering.Summarization: The continued need for accurate design methodologies mandates an ongoing research in this field. In this work, the Inversion Coefficient (IC) based methodology for low-power, low-voltage MOSFET design was explored. This methodology is based on design-oriented transistor parameter extraction, such as I0 (technology current), slope factor n, transconductance parameter KP etc. and several important performance metrics in the form of Figures-of-Merit (FoM), such as gm/ID and Av (intrinsic gain). To test the accuracy of this approach, two different operational transconductance amplifier (OTA) topologies were designed in low power mode of operation (power dissipation 24uW), a current mirror p- input, single-ended OTA and a p-input, fully differential, folded cascode (FDFC) OTA. To accentuate the prediction capability of this methodology, two process design kits (PDKs) were used; a 65nm bulk CMOS PDK and a 90nm bulk CMOS PDK. The structural design flow includes the procedure of parameter extraction for both PDKs, the mathematical analysis of each circuit, the design validation and optimization via simulation. All four designs were developed in Virtuoso ADE by Cadence and simulated using Spectre Simulation Platform. Open-Loop Gain (A0), Gain Bandwidth (GBW) , Phase Margin (PM), Slew Rate (SR), Input and Output Voltage ranges, Input referred Noise and Input DC offset were set as circuit performance criteria. Finally, comparative results between circuit topologies and technology nodes are presented and discussed.Περίληψη: Η διαρκής ανάγκη για ακριβείς μεθοδολογίες σχεδίασης απαιτεί μια συνεχή έρευνα στον τομέα αυτό. Σε αυτή την εργασία, διερευνήθηκε η μεθοδολογία που βασίζεται στον δείκτη αναστροφής (IC) για σχεδιασμό κυκλωμάτων με MOSFET, χαμηλής ισχύος και χαμηλής τάσης. Αυτή η μεθοδολογία βασίζεται στην εξαγωγή παραμέτρων των τρανσίστορ προσανατολισμένη στη σχεδίαση, όπως το I0 (ρεύματος τεχνολογίας), τον συντελεστής κλίσης n, την παράμετρο διαγωμιμότητας KP κλπ. και αρκετές σημαντικές μετρικές απόδοσης (FoM), όπως gm/ID and Av (ενδογενές κέρδος). Για να εξεταστεί η ακρίβεια αυτής της προσέγγισης, σχεδιάστηκαν δύο διαφορετικές τοπολογίες τελεστικών ενισχυτών διαγωγιμότητας (OTA) σε λειτουργία χαμηλής ισχύος (απόδοση ισχύος 24uW), έναν p-εισόδου καθρέφτη ρεύματος τελεστικό ενισχυτή διαγωγιμότητας OTA μονής εξόδου και έναν τελεστικό ενισχυτή διαφορικής εξόδου (FDFC) ΟΤΑ. Για να τονισθεί η δυνατότητα πρόβλεψης της συγκεκριμένης μεθοδολογίας, χρησιμοποιήθηκαν δύο διαφορετικά κιτ σχεδιασμού (PDKs)· ένα CMOS PDK 65nm και ένα CMOS PDK 90nm. Η διαδικασία δομικής σχεδίασης περιλαμβάνει την εξαγωγή παραμέτρων και για τα δύο PDK, τη μαθηματική ανάλυση κάθε κυκλώματος, την επαλήθευση των αποτελεσμάτων της σχεδίασης και τη βελτιστοποίηση μέσω προσομοίωσης. Και οι τέσσερις σχεδιάσεις αναπτύχθηκαν στο Virtuoso ADE από την Cadence και προσομοιώθηκαν με τη χρήση του Spectre Simulation Platform. Το κέρδος ανοικτού βρόχου (A0), το εύρος ζώνης κέρδους (GBW), το περιθώριο φάσης (PM), ο ρυθμός μετατόπισης (SR), τα εύρη τάσης εισόδου και εξόδου, καθορίστηκαν ως κριτήρια απόδοσης κυκλώματος. Τέλος, παρουσιάζονται και αναλύονται συγκριτικά αποτελέσματα μεταξύ των τοπολογιών και των διαφορετικών τεχνολογιών

Beyond the ordinary acoustoelectric effect: superluminal phenomena in the acoustic realm and phonon-mediated Bloch gain

It has been shown that coherent phonons can be used as a potent tool for controlling and enhancing optoelectronic and transport properties of nanostructured materials. Recent studies revealed that interaction of acoustic phonons and fast-moving carriers in semiconductor heterostructures can be accompanied by electron-phonon instabilities that cause ordinary and induced Cherenkov effects. However, the development of such instabilities is still poorly understood. Our study shows that other supersonic phenomena, beyond the Cherenkov instability, are possible for nonequilibrium charge transport in the miniband semiconductor superlattices (SLs) driven by an acoustic plane wave. Using semiclassical nonperturbative methods and elements of the bifurcation theory, we find the conditions for the onset of dynamical instabilities (bifurcations) which are caused by the emission of specific SL phonons by supersonic electrons, and their back action on the electrons. Notably, the underlying radiation mechanism is connected either to normal or anomalous Doppler effects in full accordance with the Ginzburg-Frank-Tamm theory. The appearance of induced Doppler effects is also discussed in relation to the formation of electron bunches propagating through the spatially periodic structure of the SL. When the amplitude of the acoustic wave exceeds a certain threshold, the dynamical instabilities developed in the system are manifested as drift velocity reversals, resonances in sound attenuation, and absolute negative mobility. We demonstrate that the discovered superluminal Doppler phenomena can be utilized for tunable broadband amplification and generation of GHz-THz electromagnetic waves, which creates a ground for the development of novel phononic devices. </p