arXiv subjects
Publications and source records attributed to Z. Moktadir.
A novel graphene transistor architecture is reported. The transistor has a U-shape geometry and was fabricated using a gallium focused ion beam (FIB). The channel conductance was tuned with a back gate. The Ion/Ioff ratio exceeded 10^5.
We have fabricated and tested an atom chip that operates as a matter wave interferometer. In this communication we describe the fabrication of the chip by ion-beam milling of gold evaporated onto a silicon substrate. We present data on the quality of the wires, on the current density that can be reached in the wires and on the smoothness of the magnetic traps that are formed. We demonstrate the operation of the interferometer, showing that we can coherently split and recombine a Bose-Einstein condensate with good phase stability.
A continuum one dimensional model of homoepitaxial growth under oblique incidence is investigated. We carried out numerical integration of a continuum equation incorporating a shadowing search algorithm. The interplay between the Ehrlich-Schwoebel (ES) effect and shadowing is clearly highlighted. It was found that different growth phases are separated by a well defined crossover time after which the ES and shadowing are the dominating mechanisms. Also, the model identified the existence of a transition period where deep grooves develop on the surface before the shadowing regime is fully developed. We found that growth under oblique angles accelerates the coarsening of mounds on the surface and we determined the corresponding critical exponent.
Ultra-cold atoms can be manipulated using microfabricated devices known as atom chips. These have significant potential for applications in sensing, metrology and quantum information processing. To date, the chips are loaded by transfer of atoms from an external, macroscopic magneto-optical trap (MOT) into microscopic traps on the chip. This transfer involves a series of steps, which complicate the experimental procedure and lead to atom losses. In this paper we present a design for integrating a MOT into a silicon wafer by combining a concave pyramidal mirror with a square wire loop. We describe how an array of such traps has been fabricated and we present magnetic, thermal and optical properties of the chip.
In this work, a study of epitaxial growth was carried out by means of wavelets formalism. We showed the existence of a dynamic scaling form in wavelet discriminated linear MBE equation where diffusion and noise are the dominant effects. We determined simple and exact scaling functions involving the scale of the wavelets when the system size is set to infinity. Exponents were determined for both, correlated and uncorrelated noise. The wavelet methodology was applied to a computer model simulating the linear epitaxial growth; the results showed a very good agreement with analytical formulation. We also considered epitaxial growth with the additional Ehrlich$-$Schwoebel effect. We characterized the coarsening of mounds formed on the surface during the nonlinear phase using the wavelet power spectrum. The latter have an advantage over other methods in the sense that one can track the coarsening in both frequency (or scale) space and real space simultaneously. We showed that the averaged wavelet power spectrum (also called scalegram) over all the positions on the surface profile, identified the existence of a dominant scale $a^*$, which increases with time following a power law relation of the form $a^* \sim t^n$, where $n\simeq 1/3$.
The blasting of brittle materials with sharp particles is an important fabrication technology in many industrial processes. In particular, for micro-systems, it allows the production of devices with feature sizes down to few tens of microns. An important parameter of this process is the surface roughness of post-blasted surfaces. In this work the scaling properties of Pyrex glass and silicon surfaces after bombardment with alumina particles is investigated. The targets were bombarded at normal incidence using alumina particles with two different average sizes, $29μm$ and $9μm$, respectively. This investigation indicates that the resulting surfaces have multifractal properties. Applying multifractal detrended fluctuation analysis (MFDFA) allowed us to determine the singularity spectrum of the surfaces. This spectrum did not depend on the target material or on the size of the particles. Several parameters quantifying relevant quantities were determined. We argue that for scales below $5 μm$, fracture processes are dominant while at large scales long range correlations are responsible for the multifractal behaviour.
Voltage fluctuations of Platinum wires hosted by silicon nitride beams were investigated. We considered four variants of the wires: three with an adhesion layer and one with no adhesion layer. We found that the presence of an adhesion layer changes the nature of the power spectrum which is $1/f$ for wires with no adhesion layers and $1/f^{3/2}$ for wires with an adhesion layer. We attribute the value of the exponent $α=3/2$ to the long range diffusion of hydrogen originating from adhesion layers that undergo a migration through the Pt wires.
Atom chips use current flowing in lithographically patterned wires to produce microscopic magnetic traps for atoms. The density distribution of a trapped cold atom cloud reveals disorder in the trapping potential, which results from meandering current flow in the wire. Roughness in the edges of the wire is usually the main cause of this behaviour. Here, we point out that the edges of microfabricated wires normally exhibit self-affine roughness. We investigate the consequences of this for disorder in atom traps. In particular, we consider how closely the trap can approach the wire when there is a maximum allowable strength of the disorder. We comment on the role of roughness in future atom--surface interaction experiments.
Concave pyramids are created in the (100) surface of a silicon wafer by anisotropic etching in potassium hydroxide. High quality micro-mirrors are then formed by sputtering gold onto the smooth silicon (111) faces of the pyramids. These mirrors show great promise as high quality optical devices suitable for integration into MOEMS and atom chips. We have shown that structures of this shape can be used to laser-cool and hold atoms in a magneto-optical trap.
We present a novel microfabricated optical cavity, which combines a very small mode volume with high finesse. In contrast to other micro-resonators, such as microspheres, the structure we have built gives atoms and molecules direct access to the high-intensity part of the field mode, enabling them to interact strongly with photons in the cavity for the purposes of detection and quantum-coherent manipulation. Light couples directly in and out of the resonator through an optical fibre, avoiding the need for sensitive coupling optics. This renders the cavity particularly attractive as a component of a lab-on-a-chip, and as a node in a quantum network.
We report on the integration of small-scale optical components into silicon wafers for use in atom chips. We present an on-chip fibre-optic atom detection scheme that can probe clouds with small atom numbers. The fibres can also be used to generate microscopic dipole traps. We describe our most recent results with optical microcavities and show that single-atom detection can be realised on an atom chip. The key components have been fabricated by etching directly into the atom chip silicon substrate.
Gold micro-mirrors have been formed in silicon in an inverted pyramidal shape. The pyramidal structures are created in the (100) surface of a silicon wafer by anisotropic etching in potassium hydroxide. High quality micro-mirrors are then formed by sputtering gold onto the smooth silicon (111) faces of the pyramids. These mirrors show great promise as high quality optical devices suitable for integration into MOEMS systems.