Ralu Divan

Folded waveguide traveling-wave tube sources for terahertz radiation

Microfabricated folded waveguide traveling-wave tubes (TWTs) are potential compact sources of wide-band, high-power terahertz radiation. We present feasibility studies of an oscillator concept using an amplifier with delayed feedback. Simulations of a 560-GHz oscillator and experimental evaluation of the concept at 50 GHz are presented. Additionally, results from various fabrication methods that are under investigation, such as X-ray lithography, electroforming, and molding (LIGA), UV LIGA, and deep reactive ion etching are presented. Observations and measurements are reported on the generation of stable single-frequency oscillation states. On varying the feedback level, the oscillation changes from a stable single-frequency state at the threshold to multifrequency spectra in the overdriven state. Simulation and experimental results on amplifier characterization and dynamics of the regenerative TWT oscillator include spectral evolution and phase stability of the generated frequencies. The results of the experiment are in good agreement with the simulations.

Magnetic Vortex Core Dynamics in Cylindrical Ferromagnetic Dots

We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-sized circular permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10 ns time scale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio of the dots are in agreement with theoretical calculations presented for the same geometry.

Surface and size effects on the electrical properties of Cu nanowires

Copper nanowires were patterned with e-beam lithography and fabricated with a copper film deposited by e-beam evaporation. Various electrical properties of these nanowires (including resistivity, temperature coefficient of resistance, and failure current density) were characterized. It was experimentally found that surface and size have apparent effects on the electrical properties. Smaller values for the temperature coefficient of resistance and higher failure current density were found for Cu nanowires with decreasing wire width. The experimental finding of width dependent failure current density also agrees with finding for theoretical heat transfer of the nanowire and substrate system as calculated with the finite element method.

Determination of the Pt spin diffusion length by spin-pumping and spin Hall effect

The spin diffusion length of Pt at room temperature and at 8 K is experimentally determined via spin pumping and spin Hall effect in permalloy/Pt bilayers. Voltages generated during excitation of ferromagnetic resonance from the inverse spin Hall effect and anisotropic magnetoresistance effect were investigated with a broadband approach. Varying the Pt layer thickness gives rise to an evolution of the voltage line shape due to the superposition of the above two effects. By studying the ratio of the two voltage components with the Pt layer thickness, the spin diffusion length of Pt can be directly extracted. We obtain a spin diffusion length of ∼1.2 nm at room temperature and ∼1.6 nm at 8 K.

Ferromagnetic microdisks as carriers for biomedical applications

We report the fabrication process, magnetic behavior, as well as the surface modification of ferromagnetic microdisks suspended in aqueous solution. They posses unique properties such as high magnetization of saturation, zero remanence due to spin vortex formation, intrinsic spin resonance at low frequencies, and the capability of delivering various biomolecules at once. Furthermore, because of their anisotropic shape, our magnetic particles rotate under alternating magnetic fields of small amplitude. This can be used to promote the idea of advanced therapies, which include combined drug delivery and magnetomechanical cell destruction when targeting tumor cells. The approach enables us to fabricate suitable magnetic carriers with excellent size tolerances, and then release them from the wafer into solution, ready for surface modification and therapeutic use. The particles have a magnetic core and are covered with few nanometers of gold on each side to provide stability at ambient conditions as well as bio...

An in situ investigation of electromigration in Cu nanowires.

Electromigration in copper (Cu) nanowires deposited by electron beam evaporation has been investigated using both resistance measurement and the in situ scanning electron microscopy technique. During electromigration, voids formed at the cathode end while hillocks (or extrusions) grew close to the anode end. The failure lifetimes were measured for various applied current densities and the mean temperature in the wire was estimated. Electromigration activation energies of 1.06 eV and 0.94 eV were found for the wire widths of 90 nm and 141 nm, respectively. These results suggest that the mass transport of Cu during electromigration mainly occurs along the wire surfaces. Further investigations of the Auger electron spectrum show that both Cu atoms and the surface contaminants of carbon and oxygen migrate from cathode to anode under the electrical stressing.

Direct-write e-beam patterning of stimuli-responsive hydrogel nanostructures

The need for stimuli-responsive components in microfluidic systems has led to the development of hydrogel-based patterned microstructures. The most commonly practiced means for fabricating micropatterned hydrogels is based on in situ photopolymerization using 365nm UV light in a liquid medium. This approach has been found to be very successful for patterning hydrogel-based features with tens or hundreds of microns resolution, but its main drawback lies in having to contain the liquid prepolymer mixture within the device for irradiation. We instead propose an alternate approach that uses direct-write electron-beam radiation to cross-link a dry, spin-coated thin film of linear polymer. After exposure, the linear polymer is dissolved in water leaving behind the cross-linked regions. When immersed in water, the cross-linked regions assume the properties of hydrogel and undergo naturally thermoreversible swelling and shrinking. This direct-writing approach can be used to fabricate hydrogel-based nanostructures...

Roughning and smoothing dynamics during KOH silicon etching

Abstract We studied the influence of surface preparation prior to KOH etching and of surfactants added to the etchant over the etching rates and roughness of the Si (111) and Si (100) surfaces. The investigated etchants were 25% KOH at 70°C, and 25% KOH with small amounts of anionic, cationic and non-ionic surfactants. The surface preparation refers to the use of the following solutions for the native oxide removal: HF:H2O 1:10 (followed by DI water rinsing and drying), HF:C2H5OH 1:10 (dried without any further rinsing), and 10% HCl in HF:H2O 1:1 (also dried without rinsing). The evaluations were made by mechanical profilometry and AFM. No difference between the samples dipped in HF–water and HF–alcohol could be observed. The etching rate of the samples dipped in HCl-containing solutions were greater, while their roughness was diminished. We analyzed the influence of surfactants on the roughness and the anisotropy. The etch rates increased when using cationic and anionic surfactants and decreased with non-ionic ones. The anisotropy is modified by surfactants. Tentative explanations for the roughning mechanisms are proposed.

Fabrication of high density, high aspect-ratio polyimide nanofilters.

A novel fabrication process produces high porosity polymer nanofilters with smooth, uniform, and straight pores with high aspect ratios. The process utilizes electron beam lithography and energetic neutral atom beam lithography and epitaxy techniques. The method has the potential to produce a new generation of high-precision, very-high-porosity, biocompatible filters with pore sizes down to 100nm.

Quantum dots by ultraviolet and x-ray lithography

Highly luminescent semiconductor quantum dots have been synthesized in porous materials with ultraviolet and x-ray lithography. For this, the pore-filling solvent of silica hydrogels is exchanged with an aqueous solution of a group II metal ion together with a chalcogenide precursor such as 2-mercaptoethanol, thioacetamide or selenourea. The chalcogenide precursor is photodissociated in the exposed regions, yielding metal chalcogenide nanoparticles. Patterns are obtained by using masks appropriate to the type of radiation employed. The mean size of the quantum dots is controlled by adding capping agents such as citrate or thioglycerol to the precursor solution, and the quantum yield of the composites can be increased to up to about 30% by photoactivation. Our technique is water-based, uses readily available reagents, and highly luminescent patterned composites are obtained in a few simple processing steps. Polydispersity, however, is high (around 50%), preventing large-scale usage of the technique for the time being. Future developments that aim at a reduction of the polydispersity are presented.