Ballistic Single-Electron Quputer

We propose a new solid state implementation of a quantum computer (quputer) using ballistic single electrons as flying qubits in 1D nanowires. We use a single electron pump (SEP) to prepare the initial state and a single electron transistor (SET) to measure the final state. Single qubit gates are implemented using quantum dots as phase shifters and electron waveguide couplers as beam splitters. A Coulomb coupler acts as a 2-qubit gate, using a mutual phase modulation effect. Since the electron phase coherence length in GaAs/AlGaAs heterostructures is of the order of 30$\mu$m, several gates (tens) can be implemented before the system decoheres.

Level-oscillation-induced pump effect in a quantum dot with asymmetric constrictions

We have investigated the pump effect induced by the level oscillation in a quantum dot with asymmetric constrictions. The curve of pumped current versus the frequency of level oscillation undulates at zero temperature. The oscillation of the pumped current can be smeared by increasing the temperature and the coupling strength between the quantum dot and the leads. Either the temperature increase or the coupling strength enhancement can lead to a positive or negative effect on the pumped current, depending on the parameters of the quantum dot system. A larger level-oscillation magnitude results in a larger pumped current, especially in the low-frequency case. An analytical expression of the pumped current is obtained in the regime far from adiabatic. A convenient physical picture based on our analytic result is proposed, with which we can explain all the features of the pumped current curves.

Electron and proton transfer in NADH:ubiquinone oxidoreductase (Complex I) from Escherichia coli

Cells of every living organism on our planet − bacterium, plant or animal − are organized in such a way that despite differences in structure and function they utilize the same metabolic energy represented by electrochemical proton gradient across a membrane. This gradient of protons is generated by the series of membrane bound multisubunit proteins, Complex I, II, III and IV, organized in so-called respiratory or electron transport chain. In the eukaryotic cell it locates in the inner mitochondrial membrane while in the bacterial cell it locates in the cytoplasmic membrane. The function of the respiratory chain is to accept electrons from NADH and ubiquinol and transfer them to oxygen resulting in the formation of water. The free energy released upon these redox reactions is converted by respiratory enzymes into an electrochemical proton gradient, which is used for synthesis of ATP as well as for many other energy dependent processes. This thesis is focused on studies of the first member of the respiratory chain − NADH:ubiquinone oxidoreductase or Complex I. This enzyme has a boot-shape structure with hydrophilic and hydrophobic domains, the former of which has all redox groups of the protein, the flavin and eight to nine iron-sulfur clusters. Complex I serves as a proton pump coupling transfer of two electrons from NADH to ubiquinone to the translocation of four protons across the membrane. So far the mechanism of energy transduction by Complex I is unknown. In the present study we applied a set of different methods to study the electron and proton transfer reactions in Complex I from Escherichia coli. The main achievement was the experiment that showed that the electron transfer through the hydrophilic domain of Complex I is unlikely to be coupled to proton transfer directly or to conformational changes in the protein. In this work for the first time properties of all redox centers of Complex I were characterized in the intact purified bacterial enzyme. We also probed the role of several conserved amino acid residues in the electron transfer of Complex I. Finally, we found that highly conserved amino acid residues in several membrane subunits form a common pattern with a very prominent feature – the presence of a few lysines within the membrane. Based on the experimental data, we suggested a tentative principle which may govern the redox-coupled proton pumping in Complex I.

Proton translocation coupled to electron transfer reactions in terminal oxidases

Terminal oxidases are the final proteins of the respiratory chain in eukaryotes and some bacteria. They catalyze most of the biological oxygen consumption on Earth done by aerobic organisms. During the catalytic reaction terminal oxidases reduce dioxygen to water and use the energy released in this process to maintain the electrochemical proton gradient by functioning as a redox-driven proton pump. This membrane gradient of protons is extremely important for cells as it is used for many cellular processes, such as transportation of substrates and ATP synthesis. Even though the structures of several terminal oxidases are known, they are not sufficient in themselves to explain the molecular mechanism of proton pumping. In this work we have applied a complex approach using a variety of different techniques to address the properties and the mechanism of proton translocation by the terminal oxidases. The combination of direct measurements of pH changes during catalytic turnover, time-resolved potentiometric electrometry and optical spectroscopy, made it possible to obtain valuable information about various aspects of oxidase functioning. We compared oxygen binding properties of terminal oxidases from the distinct heme-copper (CcO) and cytochrome bd families and found that cytochrome bd has a high affinity for oxygen, which is 3 orders of magnitude higher than that of CcO. Interestingly, the difference between CcO and cytochrome bd is not only in higher affinity of the latter to oxygen, but also in the way that each of these enzymes traps oxygen during catalysis. CcO traps oxygen kinetically - the molecule of bound dioxygen is rapidly reduced before it can dissociate. Alternatively, cytochrome bd employs an alternative mechanism of oxygen trapping - part of the redox energy is invested into tight oxygen binding, and the price paid for this is the lack of proton pumping. A single cycle of oxygen reduction to water is characterized by translocation of four protons across the membrane. Our results make it possible to assign the pumping steps to discrete transitions of the catalytic cycle and indicate that during in vivo turnover of the oxidase these four protons are transferred, one at a time, during the P→F, F→OH, Oh→Eh, and Eh→R transitions. At the same time, each individual proton translocation step in the catalytic cycle is not just a single reaction catalyzed by CcO, but rather a complicated sequence of interdependent electron and proton transfers. We assume that each single proton translocation cycle of CcO is assured by internal proton transfer from the conserved Glu-278 to an as yet unidentified pump site above the hemes. Delivery of a proton to the pump site serves as a driving reaction that forces the proton translocation cycle to continue.

ZnO film based surface acoustic wave micro-pump

In this study, a micro-pump unit based on surface acoustic wave (SAW) on piezoelectric ZnO film is designed and fabricated as a micro-fluidic device. It employs a mechanical wave, which is generated electrically using an aluminum interdigital transducer (IDT), and propagates on the surface of the ZnO film. The ZnO film was used in this study because it has a high electromechanical coefficient and an excellent bonding with various substrate materials, in particular silicon. The sputtering parameters for ZnO film deposition have been optimized, and the ZnO films with different thickness from 1 micron to 5.5 microns were prepared. The film properties have been characterized using different methods, such as scanning electron microscopy, X-ray diffraction and atomic force microscopy. Aluminum IDT with a finger width and spacing of 8 microns was patterned on the ZnO film using a lift-off process. The frequency generated was measured using a network analyzer, and it varies from 130 MHz to 180 MHz as a function of film thickness. A signal generator was used to generate the frequency with a power amplifier to amplify the signal, which was then applied to aluminum IDT to generate the surface acoustic wave. If a liquid droplet exists on the surface carrying the acoustic wave, the energy and the momentum of the SAW will be coupled into the fluid, causing the liquid to vibrate and move on film surface. The strength of this movement is determined by the applied voltage and frequency. The volume of the liquid drop loaded on the SAW device in this study is of several hundreds of nanoliters. The movement of the liquid inside the droplet and also on the ZnO film surface can be demonstrated. The performance of ZnO SAW device was characterized as a function of film thickness. © 2007 IOP Publishing Ltd.

Luminescence properties of electron-hole-droplets in pure and doped germanium

This work contains 4 topics dealing with the properties of the luminescence from Ge.

The temperature, pump-power and time dependences of the photoluminescence spectra of Li-, As-, Ga-, and Sb-doped Ge crystals were studied. For impurity concentrations less than about 10¹⁵cm^-3, emissions due to electron-hole droplets can clearly be identified. For impurity concentrations on the order of 10¹⁶cm^-3, the broad lines in the spectra, which have previously been attributed to the emission from the electron-hole-droplet, were found to possess pump-power and time dependent line shape. These properties show that these broad lines cannot be due to emission of electron-hole-droplets alone. We interpret these lines to be due to a combination of emissions from (1) electron-hole- droplets, (2) broadened multiexciton complexes, (3) broadened bound-exciton, and (4) plasma of electrons and holes. The properties of the electron-hole-droplet in As-doped Ge were shown to agree with theoretical predictions.

The time dependences of the luminescence intensities of the electron-hole-droplet in pure and doped Ge were investigated at 2 and 4.2°K. The decay of the electron-hole-droplet in pure Ge at 4.2°K was found to be pump-power dependent and too slow to be explained by the widely accepted model due to Pokrovskii and Hensel et al. Detailed study of the decay of the electron-hole-droplets in doped Ge were carried out for the first time, and we find no evidence of evaporation of excitons by electron-hole-droplets at 4.2°K. This doped Ge result is unexplained by the model of Pokrovskii and Hensel et al. It is shown that a model based on a cloud of electron-hole-droplets generated in the crystal and incorporating (1) exciton flow among electron-hole-droplets in the cloud and (2) exciton diffusion away from the cloud is capable of explaining the observed results.

It is shown that impurities, introduced during device fabrication, can lead to the previously reported differences of the spectra of laser-excited high-purity Ge and electrically excited Ge double injection devices. By properly choosing the device geometry so as to minimize this Li contamination, it is shown that the Li concentration in double injection devices may be reduced to less than about 10¹⁵cm^-3 and electrically excited luminescence spectra similar to the photoluminescence spectra of pure Ge may be produced. This proves conclusively that electron-hole-droplets may be created in double injection devices by electrical excitation.

The ratio of the LA- to TO-phonon-assisted luminescence intensities of the electron-hole-droplet is demonstrated to be equal to the high temperature limit of the same ratio of the exciton for Ge. This result gives one confidence to determine similar ratios for the electron-hole-droplet from the corresponding exciton ratio in semiconductors in which the ratio for the electron-hole-droplet cannot be determined (e.g., Si and GaP). Knowing the value of this ratio for the electron-hole-droplet, one can obtain accurate values of many parameters of the electron-hole-droplet in these semiconductors spectroscopically.

Ultrafast non-thermal electron dynamics in single layer graphene

We study the ultrafast dynamics of non-thermal electron relaxation in graphene upon impulsive excitation. The 10-fs resolution two color pump-probe allows us to unveil the non-equilibrium electron gas decay at early times. © 2012 OSA.

Ultrafast non-thermal electron dynamics in single layer graphene

We study the ultrafast dynamics of non-thermal electron relaxation in graphene upon impulsive excitation. The 10-fs resolution two color pump-probe allows us to unveil the non-equilibrium electron gas decay at early times. © Owned by the authors, published by EDP Sciences, 2013.

Ultrafast non-thermal electron dynamics in single layer graphene

We study the ultrafast dynamics of non-thermal electron relaxation in graphene upon impulsive excitation. The 10-fs resolution two color pump-probe allows us to unveil the nonequilibrium electron gas decay at early times. © OSA 2012.

Ultrafast carrier dynamics in undoped and p-doped InAs/GaAs quantum dots characterized by pump-probe reflection measurements

We investigate the dependence of the differential reflection on the structure parameters of quantum dot (QD) heterostructures in pump-probe reflection measurements by both numerical simulations based on the finite-difference time-domain technique and theoretical calculations based on the theory of dielectric films. It is revealed that the value and sign of the differential reflection strongly depend on the thickness of the cap layer and the QD layer. In addition, a comparison between the carrier dynamics in undoped and p-doped InAs/GaAs QDs is carried out by pump-probe reflection measurements. The carrier capture time from the GaAs barrier into the InAs wetting layer and that from the InAs wetting layer into the InAs QDs are extracted by appropriately fitting differential reflection spectra. Moreover, the dependence of the carrier dynamics on the injected carrier density is identified. A detailed analysis of the carrier dynamics in the undoped and p-doped QDs based on the differential reflection spectra is presented, and its difference with that derived from the time-resolved photoluminescence is discussed. (C) 2008 American Institute of Physics.

A novel DC-DC charge pump circuit for passive RFID transponder

This paper presents a low-voltage, high performance charge pump circuit suitable for implementation in standard CMOS technologies. The proposed charge pump has been used as a part of the power supply section of fully integrated passive radio frequency identification(RFID) transponder IC, which has been implemented in a 0.35-um CMOS technology with embedded EEPROM offered by Chartered Semiconductor. The proposed DC/DC charge pump can generate stable output for RFID applications with low power dissipation and high pumping efficiency. The analytical model of the voltage multiplier, the comparison with other charge pumps, the simulation results, and the chip testing results are presented.

Ultrafast Kerr rotations and zero-field dephasing time of electron spins in InAs/GaAs quantum disks

Time-resolved Kerr rotation (TRKR) measurements based on pump-probe arrangement were carried out at 5 K on the monolayer fluctuation induced InAs/GaAs quantum disks grown on GaAs substrate without external magnetic field. The lineshape of TRKR signals shows an unusual dependence on the excitation wavelength, especially antisymmetric step-shaped structures appearing when the excitation wavelength was resonantly scanned over the heavy- and light-hole subbands. Moreover, these step structures possess an almost identical decay time of similar to 40 Ps which is believed to be the characteristic spin dephasing time of electrons in the extremely narrow InAs/GaAs quantum disks.

Ultrafast Probing of Collective Electron Dynamics Driven by Dielectronic Repulsion

We use the time-dependent R-matrix approach to investigate an ultrashort pump-probe scheme to observe collective electron dynamics in C(+). The ionization probability of a coherent superposition of the 2s2p(2) (2)D and (2)S states shows rapid modulation due to collective dynamics of the two equivalent 2p electrons, with the modulation frequency linked to the dielectronic repulsion. The best insight into this collective dynamics is achieved by a transformation from LS symmetry to the uncoupled basis. Such dynamics may be important in high-harmonic generation using open-shell atoms and ions.