S. J. Allen

Absorption of terahertz radiation by plasmon modes in a grid-gated double-quantum-well field-effect transistor

The terahertz absorption spectrum of plasmon modes in a grid-gated double-quantum-well (DQW) field-effect transistor structure is analyzed theoretically and numerically using a first principles electromagnetic approach and is shown to faithfully reproduce important physical features of recent experimental observations. We find that the essential character of the response—multiple resonances corresponding to spatial harmonics of standing plasmons under the metal grating—is caused by the static spatial modulation of electron density in the channel. Higher order plasmon modes become more optically active as the depth of the electron density modulation in the DQW tends towards unity. The maximum absorbance, at plasma resonance, is shown to be 1/2. Furthermore, the strongest absorption also occurs when the standing plasmon resonance coincides with the fundamental dipole mode of the ungated portion of the channel.

Dispersion and spin wave “tunneling” in nanostructured magnetostatic spin waveguides

Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion. The measured attenuation lengths, of the order of 3u2002μm, are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires. Spin waves effectively “tunnel” through air gaps, produced by focused ion beam etching, as large as 1.5u2002μm.

NONEQUILIBRIUM AC JOSEPHSON EFFECT IN MESOSCOPIC NB-INAS-NB JUNCTIONS

Microwave irradiation of Nb-InAs-Nb junctions reveals frequency-doubled Josephson currents which persist to high temperatures, in the absence of a critical current. A nonequilibrium dynamical model, based on time-dependent Andreev bound states, successfully accounts for the resulting half-integer Shapiro step and an enhancement in the conductance near zero bias.

Spin wave modes in ferromagnetic tubes

Resonances are observed in the transmission between two coplanar waveguides coupled by ferromagnetic Co90Ta5Zr5 tubes that wrap around their shorted ends. The resonances are assigned to the magnetostatic surface waves that counter propagate along the tube perimeter. We use a model based on an infinite ferromagnetic tube, with elliptical cross section of roughly the same dimensions as the studied structure. Additional theoretical analysis of the fundamental precession mode observed in experiment is carried out. Periodic boundary conditions dictated by the tube perimeter and applied to magnetostatic surface waves quantitatively account for the experimentally observed bandwidth of excited modes, despite the contorted tubular shape. The tubular topology appears to be more important than the shape details.

Micro-structured ferromagnetic tubes for spin wave excitation

Micron scale ferromagnetic tubes placed on the ends of ferromagnetic CoTaZr spin waveguides are explored in order to enhance the excitation of backward volume magnetostatic spin waves. The tubes produce a closed magnetic circuit about the signal line of the coplanar waveguide and are, at the same time, magnetically contiguous with the spin waveguide. This results in a ten-fold increase in spin wave amplitude. However, the tube geometry distorts the magnetic field near the spin waveguide, and relatively high biasing magnetic fields are required to establish well-defined spin waves. Only the lowest (uniform) spin wave mode is excited.

Magnetostatic Spin-Wave Modes in Ferromagnetic Tube

Magnetostatic spin-wave modes were excited and detected in micron size ferromagnetic Co90Ta5 Zr5 rectangular tubes that wrapped around the shorted ends of coplanar waveguides. The observed modes can be assigned to surface spin waves of an infinite ferromagnetic film but with periodic boundary conditions and quantized wave vector imposed by the finite circumference of the tube.

Low‐temperature carrier distributions in wide quantum wells of different shapes from capacitance‐voltage measurements

The carrier distributions in modulation‐doped wide graded quantum wells that a measurement of the capacitance C between a surface gate and an ohmic contact to the carriers as a function of the applied bias V would yield are calculated. These capacitance‐voltage (C‐V) distributions are found to agree inexactly, but closely, with the calculated true carrier distributions. Density modulation features, induced by superlattices or by abrupt changes in the curvature of band‐gap grading, are strikingly reproduced. Electron distributions extracted from actual measurements on a wide parabolic well and on a parabolic well with superimposed superlattice are in good agreement with theory. For the case of the parabolic well, the occupancy of a finite number of subbands is manifested as structure in the C‐V distributions. This technique is relevant to the measurement of carrier distributions in any wide carrier system with more than one electric subband occupied.

Submm-wave monolithic RTD oscillator arrays to 650 GHz

Monolithic slot antenna coupled Schottky-collector resonant tunnel diode (SRTD) oscillator arrays have been fabricated and tested. Earlier we have reported oscillators with oscillation frequencies to 200 GHz. Here we report recent results which indicate oscillation frequencies to 650 GHz for a 64-element monolithic array. To our knowledge this is the highest oscillation frequency for a monolithic oscillator. Additionally, both the 28 /spl mu/W output power of the 310 GHz array and the associated 440 W/cm/sup 2/ power density represent records for RTDs operating in this frequency range.

ABSENCE OF SHAPIRO-LIKE STEPS IN CERTAIN MESOSCOPIC S-N-S JUNCTIONS

In DC transport through mesoscopic S-N-S junctions, it is known that the Josephson coupling decreases exponentially with increasing temperature, but the phase dependence of the conductance persists to much higher temperatures and decreases only as 1/T. It is pointed out here that, despite the fact that such a phase-dependent conductance does bring about an AC current for a pure DC voltage, it cannot, by itself, lead to the formation of Shapiro steps.

Inelastic light scattering from terahertz standing waves in a slab waveguide

Second order, χ(2) nonlinear mixing of optical and terahertz (THz) radiation is enhanced by phase matching by the spatial distribution of THz standing waves in a slab waveguide. The interference pattern due to selectively excited waveguide modes is thus detected and modeled along a crystal slab waveguide.