Trajectory-based interpretation of laser light diffraction by a sharp edge

In the diffraction pattern produced by a half-plane sharp edge when it obstructs the passage of a laser beam, two characteristic regions are noticeable. There is a central region, where it can be noticed the diffraction of laser light in the region of geometric shadow, while intensity oscillations are observed in the non-obstructed area. On both sides of the edge, there are also very long light traces along the normal to the edge of the obstacle. The theoretical explanation to this phenomenon is based on the Fresnel-Kirchhoff diffraction theory applied to the Gaussian beam propagation behind the obstacle. Here we have supplemented this explanation by considering electromagnetic flow lines, which provide a more complete interpretation of the phenomenon in terms of electric and magnetic fields and flux lines, and that can be related, at the same time, with average photon paths.Comment: 13 pages, 5 figure

Fine Structure in Energy Spectra of Ultrasmall Au Nanoparticles

We have studied tunneling into individual Au nanoparticles of estimated diameters 2-5 nm, at dilution refrigerator temperatures. The I-V curves indicate resonant tunneling via discrete energy levels of the particle. Unlike previously studied normal metal particles of Au and Al, in these samples we find that the lowest energy tunneling resonances are split into clusters of 2-10 subresonances. Such effects appear to be increasingly important in smaller grains, as might be expected from the larger characteristic energies.Comment: 1 pdf fil

On Wheeler's delayed-choice Gedankenexperiment and its laboratory realization

Here, we present an analysis and interpretation of the experiment performed by Jacques et al. (2007 Science 315, 966), which represents a realization of Wheeler's delayed-choice Gedankenexperiment. Our analysis is based on the evolution of the photon state, since the photon enters into the Mach-Zehnder interferometer with a removable beam-splitter until it exits. Given the same incident photon state onto the output beam-splitter, BS_output, the photon's state at the exit will be very different depending on whether BS_output is on or off. Hence, the statistics of photon counts collected by the two detectors, positioned along orthogonal directions at the exit of the interferometer, is also going to be very different in either case. Therefore, it is not that the choice of inserting (on) or removing (off) a beam-splitter leads to a delayed influence on the photon behavior before arriving at the beam-splitter, but that such a choice influences the photon state at and after BS_output, i.e., after it has exited from the Mach-Zehnder interferometer. The random on/off choice at BS_output has no delayed effect on the photon to behave as a wave or a corpuscle at the entrance and inside the interferometer, but influences the subsequent evolution of the photon state incident onto BS_output.Comment: 7 pages, 4 figure

Saturation of Spin-Polarized Current in Nanometer Scale Aluminum Grains

We describe measurements of spin-polarized tunnelling via discrete energy levels of single Aluminum grains. In high resistance samples ($\sim G\Omega$), the spin-polarized tunnelling current rapidly saturates as a function of the bias voltage. This indicates that spin-polarized current is carried only via the ground state and the few lowest in energy excited states of the grain. At the saturation voltage, the spin-relaxation rate $T_1^{-1}$ of the highest excited states is comparable to the electron tunnelling rate: $T_1^{-1}\approx 1.5\cdot 10^6 s^{-1}$ and $10^7s^{-1}$ in two samples. The ratio of $T_1^{-1}$ to the electron-phonon relaxation rate is in agreement with the Elliot-Yafet scaling, an evidence that spin-relaxation in Al grains is governed by the spin-orbit interaction.Comment: 5 pages, 4 figure

Transport in Graphene Tunnel Junctions

We present a technique to fabricate tunnel junctions between graphene and Al and Cu, with a Si back gate, as well as a simple theory of tunneling between a metal and graphene. We map the differential conductance of our junctions versus probe and back gate voltage, and observe fluctuations in the conductance that are directly related to the graphene density of states. The conventional strong-suppression of the conductance at the graphene Dirac point can not be clearly demonstrated, but a more robust signature of the Dirac point is found: the inflection in the conductance map caused by the electrostatic gating of graphene by the tunnel probe. We present numerical simulations of our conductance maps, confirming the measurement results. In addition, Al causes strong n-doping of graphene, Cu causes a moderate p-doping, and in high resistance junctions, phonon resonances are observed, as in STM studies.Comment: 22 pages, 5 figure

Modelling Electron Spin Accumulation in a Metallic Nanoparticle

A model describing spin-polarized current via discrete energy levels of a metallic nanoparticle, which has strongly asymmetric tunnel contacts to two ferromagnetic leads, is presented. In absence of spin-relaxation, the model leads to a spin-accumulation in the nanoparticle, a difference ($\Delta\mu$) between the chemical potentials of spin-up and spin-down electrons, proportional to the current and the Julliere's tunnel magnetoresistance. Taking into account an energy dependent spin-relaxation rate $\Omega (\omega)$, $\Delta\mu$ as a function of bias voltage ($V$) exhibits a crossover from linear to a much weaker dependence, when $|e|\Omega (\Delta\mu)$ equals the spin-polarized current through the nanoparticle. Assuming that the spin-relaxation takes place via electron-phonon emission and Elliot-Yafet mechanism, the model leads to a crossover from linear to $V^{1/5}$ dependence. The crossover explains recent measurements of the saturation of the spin-polarized current with $V$ in Aluminum nanoparticles, and leads to the spin-relaxation rate of $\approx 1.6 MHz$ in an Aluminum nanoparticle of diameter $6nm$, for a transition with an energy difference of one level spacing.Comment: 37 pages, 7 figure

Electronic Properties of Clean Au-Graphene Contacts

The effects of Au grains on graphene conduction and doping are investigated in this report. To obtain a clean Au-graphene contact, Au grains are deposited over graphene at elevated temperature and in high vacuum, before any chemical processing. The bulk and the effective contact resistance versus gate voltage demonstrate that Au grains cause p-doping in graphene. The Fermi level shift is in agreement with first principles calculations, but the equilibrium separation we find between the graphene and the top-most Au layer is larger than predicted. Nonequilibrium electron transport displays giant-phonon thresholds observed in graphene tunnel junctions, demonstrating the tunneling nature of the contact, even though there are no dielectrics involved.Comment: 11 pages, 4 figure

Properties of the quantum state arising after the L-photon state has passed trough a linear quantum amplifier

We consider the system of N two-level atoms, of which N0 atoms are unexcited and N1 are excited. This system of N two-level atoms, which forms a linear quantum amplifier, interacts with a single-mode electromagnetic field. The problem of amplification of the L-photon states using such an amplifier is studied. The evolution of the electromagnetic field density matrix is described by the master equation for the field under amplification. The dynamics of this process is such that it can be described as the transformation of the scale of the phase space. The exact solution of the master equation is expressed using the transformed Husimi function of the L-quantum state of the harmonic oscillator. The properties of this function are studied and using it the average photon number and its fluctuations in the amplified state are found. © 2021, Editura Academiei Romane. All rights reserved