Naser Qureshi

Imaging and probing electronic properties of self-assembled InAs quantum dots by atomic force microscopy with conductive tip

Atomic force microscopy with a conductive probe has been used to study both the topography and the electronic properties of 10-nm-scale self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy on n-type GaAs. The current flowing through the conductive probe normal to the sample surface is measured for imaging local conductance, while the deflection of cantilever is optically detected for disclosing geometrical structure. The conductance on InAs QDs is found to be much larger than that on the wetting layer, allowing imaging of QDs through measurements of local current. We attribute this change in conductance to the local modification of surface band bending associated with surface states on InAs QD surface. Mechanisms of electron transport through QDs are discussed based on current–voltage characteristics measured on QDs of various sizes.

Cavity enhancement of the magneto-optic Kerr effect for optical studies of magnetic nanostructures

We present a study of cavity enhancement of the magneto-optic Kerr effect using dielectric multilayers in order to facilitate optical studies of individual single-domain nanomagnets. We develop a transfer matrix theory to analyze Kerr rotation from an arbitrary number of possibly lossy dielectric layers. The combination of one lossless and one thin metallic layer is found to be most favorable for studying individual nanomagnets, providing the best tradeoff between signal enhancement and spatial resolution. Accounting for the microscopic surface structure, we find good agreement between theory and experiment. Using this technique, we demonstrate Kerr enhancements by a factor of more than 16.

Null Ronchi-Hartmann test for a lens

A method to design Ronchi-Hartmann screens for improved alignment in the testing of fast plano-convex spherical lenses is presented. We design null screens that produce aligned straight fringes for observed patterns. The designs of these null screens are based on knowledge of the caustic by refraction. A qualitative test for a lens is presented.

Hot spin-wave resonators and scatterers

A method used to achieve efficient optical control of a spin wave system in yttrium iron garnet is described. Using a focused laser beam, spin wave resonators and reflectors are induced by controlled local thermal demagnetization of a thin film. We report on the formation of an optically induced potential well for magnetostatic surface spin waves (MSSWs) leading to the formation of a high-Q MSSW resonator, and a high potential barrier that efficiently reflects magnetostatic backward volume waves.

Emergent ultra–long-range interactions between active particles in hybrid active–inactive systems

Significance Particle–particle interactions determine the state of a system. Control over the range and magnitude of such interactions is critical for science and technology. Here, we show that active particles experience an emergent ultra–long-range attractive interaction in the presence of a passive medium. The range and magnitude of this interaction are controlled by the elasticity of the medium and the activity of the particles. For the conditions studied here, we have found the range to be as large as 20 particle diameters, which is much larger than the typical interaction range between colloids. This interaction may open up new routes of control between active objects in passive environments and help us to understand the emergent interactions in nonequilibrium (biological) systems. Particle–particle interactions determine the state of a system. Control over the range of such interactions as well as their magnitude has been an active area of research for decades due to the fundamental challenges it poses in science and technology. Very recently, effective interactions between active particles have gathered much attention as they can lead to out-of-equilibrium cooperative states such as flocking. Inspired by nature, where active living cells coexist with lifeless objects and structures, here we study the effective interactions that appear in systems composed of active and passive mixtures of colloids. Our systems are 2D colloidal monolayers composed primarily of passive (inactive) colloids, and a very small fraction of active (spinning) ferromagnetic colloids. We find an emergent ultra–long-range attractive interaction induced by the activity of the spinning particles and mediated by the elasticity of the passive medium. Interestingly, the appearance of such interaction depends on the spinning protocol and has a minimum actuation timescale below which no attraction is observed. Overall, these results clearly show that, in the presence of elastic components, active particles can interact across very long distances without any chemical modification of the environment. Such a mechanism might potentially be important for some biological systems and can be harnessed for newer developments in synthetic active soft materials.

Synthesis and processing of pseudo noise signals by spin precession in Y3 Fe5O12 films

A simple method for synthesis of phase shift keying (PSK) signals in the microwave frequency range is presented. It is shown that the signal coding and processing can be efficiently realized by spin excitations in thin ferrite films. PSK signals are constructed through control of magnetization precession in a magnetic material by a pulsed magnetic field, and their compression is performed by a spin-wave based correlator, eliminating the need for semiconductor circuitry.

Mapping of spin wave propagation in a one-dimensional magnonic crystal

The formation and evolution of spin wave band gaps in the transmission spectrum of a magnonic crystal have been studied. A time and space resolved magneto inductive probing system has been used to map the spin wave propagation and evolution in a geometrically structured yttrium iron garnet film. Experiments have been carried out using (1) a chemically etched magnonic crystal supporting the propagation of magnetostatic surface spin waves, (2) a short microwave pulsed excitation of the spin waves, and (3) direct spin wave detection using a movable magneto inductive probe connected to a synchronized fast oscilloscope. The results show that the periodic structure not only modifies the spectra of the transmitted spin waves but also influences the distribution of the spin wave energy inside the magnonic crystal as a function of the position and the transmitted frequency. These results comprise an experimental confirmation of Bloch′s theorem in a spin wave system and demonstrate good agreement with theoretical o...

An active resonator based on magnetic films for near field microwave microscopy

An active resonator perturbation method is introduced as a sensitive and versatile way to probe material properties in near field microwave microscopy. An active ring microwave oscillator based on magnetostatic excitations in a yittrium iron garnet thin film has been developed with a coaxial near field probe connected directly to the resonator ring. The probe tunes the resonator’s emission frequency, and the high Q-factor of the magnetostatic oscillations allows for a very sensitive spatially resolved probe of surface impedance and material properties with a much larger dynamic range than conventional resonant probes for microwave microscopy.

Analysis of near field microwave and conventional optical images

In this work we present near field microwave images of microelectronic circuits and their interpretation to complement the conventional optical analysis. We show a highly simplified design of a resonant probe with dynamically tunable capacitive coupling and with high sensitivity. Images were obtained by measuring the microwave reflection coefficient operating a 7 GHz. This design represents a simplified and highly effective approach to implementing near field microwave microscopy.

Near-field optical magnetometry and magnetic imaging of nanomagnets

We present an all-optical approach to detecting magnetization reversal events in submicron ferromagnetic structures that is non-perturbative and compatible with ultrafast optical techniques. We demonstrate experimentally that structures much smaller than the wavelength of light can be probed using both near-field and far-field laser techniques combined with a cavity Kerr enhancement technique and two different polarimetry methods. Controlled magnetization reversal events are detected in nickel magnets approaching the 100nm scale. This leads to a promising way to measure sub-picosecond dynamics of nanomagnets for fast device applications.