A. S. Vengurlekar

Quantum functional devices: resonant-tunneling transistors, circuits with reduced complexity, and multiple valued logic

Recent advances in the area of quantum functional devices are discussed. After a discussion of the functional device concept, resonant-tunneling bipolar transistors (RTBTs) with a double barrier in the base region are described. Design considerations for RTBTs with ballistic injection and the first observation of minority-electron ballistic RT are presented. RTBTs using thermionic injection and exhibiting a high peak-to-valley ratio at room temperature in the transfer characteristics are also described. Multiple-state RTBTs and their DC and microwave performance are then discussed. Circuit applications of RTBTs also are discussed. It is shown that RTBTs allow the implementation of many analog and digital circuit functions with a greatly reduced number of transistors and show considerable promise for multiple-valued logic. Experimental results on frequency multipliers and parity bit generators are presented. Analog-to-digital converters are memory circuits are also discussed. Two novel superlattice-base transistors are reported. Negative transconductance is achieved by suppression of injection into minibands. Gated quantum-well RT transistors are also discussed. >

Logarithmic detrapping response for holes injected into SiO2 and the influence of thermal activation and electric fields

Relaxation of trapped holes that are introduced into silicon dioxide from silicon by the avalanche injection method is studied under various conditions of thermal activation and external electric fields. It is found that the flat band voltage recovery in time follows a universal behavior in that the response at high temperatures is a time scaled extension of the response at low temperatures. Similar universality exists in the detrapping response at different external bias fields. The recovery characteristics show a logarithmic time dependence in the time regime studied (up to 6000 s). We find that the recovery is thermally activated with the activation energy varying from 0.5 eV for a field of 2 MV/cm to 1.0 eV for a field of −1 MV/cm. There is little discharge in 3000 s at room temperature for negative fields beyond −4 MV/cm. The results suggest that the recovery is due to tunneling of electrons in the silicon conduction band into the oxide either to compensate or to remove the charge of trapped holes.

Photoluminescence study of exciton-optical phonon scattering in bulk GaAs and GaAs quantum wells

We obtain the temperature dependence of the homogeneous linewidth of excitons in GaAs quantum wells (QWs) and bulk GaAs using photoluminescence measurements. The results indicate that exciton scattering rates with optical phonons are larger in bulk GaAs than in QWs.

Plasmonic crystals for ultrafast nanophotonics: Optical switching of surface plasmon polaritons

We demonstrate that the dispersion of surface plasmon polaritons in a periodically perforated gold film can be efficiently manipulated by femtosecond laser pulses in spectral regions far from the intrinsic gold resonances. Using a time- and frequency-resolved pump-probe technique we observe shifting of the plasmon polariton resonances with response times from 200 to 800 fs depending on the probe photon energy, by which we obtain comprehensive insight into the electron dynamics in gold. We show that Wood anomalies in the optical spectra provide pronounced resonances indifferential transmissionand reflection with magnitudes upto 3%for moderate pump fluences of 0.5 mJ/cm 2 .

Surface plasmon enhanced photon drag in metal films

We report a significantly enhanced photon drag effect (PDE) in a Au thin film at the surface plasmon resonance when the film is mounted in the Kretschmann-Raether geometry. The PDE is observed in the form of an electrical direct current induced by optical excitation. We discuss a model to describe how excitation of surface plasmons may give rise to a current in the Au thin film.

Fabry–Perot plasmonic structures for nanophotonics

It is demonstrated that the presence of the metal on the walls of dielectric grating slits opens new possibilities for tailoring optical properties of metal–dielectric plasmonic gratings. In particular, a new kind of metal-thickness-sensitive resonances appear due to the excitation of the Fabry–Perot plasmonic modes in the horizontal cavity formed inside the slits by vertical metalized walls. It makes the considered plasmonic structures of great interest for applications where the concentration of the electromagnetic energy is vital. Moreover, transmission peaks related to the Fabry–Perot modes inside the slits etched in the dielectric part exhibit significant enhancement and blueshift as the thickness of the metal on the slit walls increases.

Dynamics of the pump-probe reflectivity spectra in GaAs and GaN

The pump-probe reflectivity (PPR) technique is a quick way to characterize the short carrier lifetime in materials which may be potentially good terahertz (THz) emitters or detectors. Here, we study the PPR signal in semiconductors theoretically in the frequency domain (at various energies above and below the band gap) as a function of pump-probe delay. We consider two conditions of carrier relaxation. In one, the carriers are assumed to form a hot, thermalized energy distribution during excitation itself and then to cool via phonon emission, as is expected in the case of high density excitation in GaAs. In the other case, the carriers essentially remain in a nonequilibrium, nonthermal state even as they relax. This can happen when the carrier-longitudinal optical phonon interaction is stronger than carrier–carrier scattering, as is likely in GaN even at moderately high densities. In addition, effects of carrier trapping and recombination determining the carrier lifetime are included. The calculation take...

Femtosecond pulse distortion at surface plasmon resonances in a plasmonic crystal: Effect of surface plasmon lifetime

Measurements of temporal profile of a femtosecond laser pulse propagating through a one-dimensional metallic plasmonic crystal show that the pulses are severely distorted when they excite surface plasmon (SP) resonances. Significant differences in the nature of the distortion are found for the SP states at the two edges of the SP band gap, reflecting sharp contrasts in their lifetime.

Miniband conduction of minority electrons and negative transconductance by quantum reflection in a superlattice transistor

Transport of electrons tunnel injected into a superlattice (SL) is studied. The SL is placed in the base of an n‐p‐n bipolar transistor. By varying the emitter‐base forward bias (VEB), the energy of electrons injected into the base is varied. From the emitter and collector current measurements in the common‐base configuration, it is found that the electron transmission to the collector is strongly dependent on the injection energy, the currents showing a sharp peak, and the associated negative transconductance. The measurements show excellent agreement with the calculated values of VEB at the onset of miniband conduction and at the suppression of injection into the SL due to enhanced quantum reflection by the SL minigap. The transfer characteristics also reveal a low‐current gain regime of electron transport below the onset of miniband conduction, implying conduction mediated by subminiband gap states.

Optical properties of metallo-dielectric deep trench gratings: role of surface plasmons and Wood-Rayleigh anomaly.

We present optical measurements performed on a gold film deposited on deep subwavelength grooves etched in a quartz substrate at visible wavelengths where the Wood-Rayleigh (WR) anomaly and surface plasmon polaritons (SPs) are well separated. The measurements show that transmission of TM-polarized light through the slit array vanishes when SPs are excited at the air-metal interface, whereas the WR anomaly appears as small peaks.