Kevin M. Connolly

Quantum transport in open mesoscopic cavities

Abstract In this review we describe the results of magneto-transport studies in open quantum dots, in which electronic motion is expected to be predominantly ballistic in nature. The devices themselves are realized in different semiconductor materials, using quite distinct fabrication techniques. Electron interference is an important process in determining the electrical properties of the devices at low temperatures and is manifested through the observation of periodic magneto-conductance fluctuations. These are found to result from selective excitation of discrete cavity eigenstates by incoming electrons, which are directed into a collimated beam by the input point contact. Under conditions of such restricted injection, quantum mechanical simulations reveal highly characteristic wavefunction scarring, associated with the remnants of a few classical orbits. The scarring is built up by interference between electrons, confined within the cavities over very long time scales, suggesting the underlying orbits are highly stable in nature. This characteristic is also confirmed by the results of experiment, which reveal the discrete components dominating the interference to be insensitive to changes in lead opening or temperature. The fluctuations decay with increasing temperature, although they can nonetheless still be resolved at a few degrees kelvin. This characteristic is confirmed by independent studies of devices, fabricated using very different techniques, further demonstrating the universal nature of the behavior we discuss here. These results therefore demonstrate that the correct description of electron interference in open quantum cavities, is one in which only a few discrete orbits are excited by the collimating action of the input lead, giving rise to striking wavefunction scarring with measurable magneto-transport results.

Carrier Transport in Nanodevices

Future VLSI scaling realization of gate lengths is expected to 70 nm and below. While we do not know all the underlying physics, we are beginning to understand some limiting factors, which include quantum transport, in these structures. The discrete nature of impurities, the fact that devices have critical lengths comparable to their coherence lengths, and size quantization will all be important in these structures. These phenomena will lead to pockets of charge, which will appear as coupled quantum dots in the device transport. We review some of the physics of these dots.

Quantum Transport in Single and Multiple Quantum Dots

Ballistic quantum dots have been used in a wide variety of studies ranging from single-electron charging to chaotic systems. However, in open, ballistic quantum dots, the behavior is significantly different. Here, we discuss (1) the observation of regular, periodic fluctuations arising from the existence of stable orbits, (2) the regular and chaotic behavior of coupled dots, and (3) the theory of such dots. The regular orbit properties of these dots are their most stable, generic property, and are clearly reflected in the magnetoresistance. These give rise to periodic fluctuations, which are the result of a very few, periodic orbits within the dot that give rise to scarred wave functions and harmonically related frequencies in the Fourier spectrum. The orbits arise from the role of regular trajectories in the oscillatory density of states and the crucial collimation effects of the quantum point contacts.

Coupling Maxwell's equation time-domain solution with Monte-Carlo technique to simulate ultrafast optically controlled switches

An accurate model for studying transients in semiconductor devices and optical switches is presented. This approach couples a direct solution of Maxwells equations with an ensemble Monte-Carlo model. This model is capable of simulating very-high-frequency devices and switches on the subpicosecond scale. Optical interaction with semiconductor devices can also be modeled.<<ETX>>

Physical modeling of ultrafast electrical waveform generation and characterization

Ultrafast lasers have been used along with an electro-optic sampling technique to generate and characterize electrical waveforms with subpicosecond rise-times. We study these transients by coupling a three dimensional solution of Maxwells equations in the time domain with a bipolar ensemble Monte- Carlo model. The physical parameters of interest can be accurately calculated and related to the measured data.

Dynamic simulation of a photoconductive switching experiment

Improved modelling of photoconductive switching experiments can be obtained by embedding a bipolar ensemble Monte Carlo model of the photoconductive gap into a time domain solution of Maxwells equations. In addition to making fewer assumptions than previous models, this technique allows for the simulation of a probe pulse used in pump and probe experiments. Thus it is possible to accurately simulate the entire experiment and plot the quantity actually observed in the experiments-the phase shift of the probe beam.

New Modeling Approach for High Frequency Devices: Application to Ultrafast Optically Controlled Switches

In many ultrafast semiconductor devices, the effects of generated electromagnetic waves cannot be neglected. These devices can be accurately simulated by combining a direct finite difference time domain solution of Maxwells equations with a Bipolar Monte-Carlo model of semiconductor carrier transport. This approach is utilized to investigate the response of an electro-optic switch with a subpicosecond risetime. Discussion and results showing the effects of wave propagation will also be presented.

Photoconductive Switch Simulation with Absorbing Boundary Conditions

Ultrafast lasers have been used along with electro-optic sampling techniques to characterize photoconductive switching devices and create transient waveforms with sub-picosecond rise times. We study these fast transients by coupling a three dimensional solution of Maxwell’s equations in the time domain with a bipolar Ensemble Monte Carlo model. Physical parameters of interest can be accurately computed and directly related to the measured data. We find that a proper inclusion of absorbing boundary conditions and details of the electro-optic material are crucial to transient velocity overshoot calculations.