Jeffrey F. Webb

Free Vibration Analysis of Single-Walled Carbon Nanotubes Based on the Continuum Finite Element Method

This paper presents a continuum finite element mechanics approach to model the vibration behaviours of single-walled carbon nanotubes (SWCNTs) of varying lengths, aspect ratios, chiralities, boundary conditions, axial loads and with initial strain applied. The results are in good agreement with the open literature and show that resonance-based carbon nanotubes sensors have the potential to meet the high level performance requirements inherent of many sensor based applications such as mass detectors, biomedical sensors, monitoring for metal deposition and chemical reactions amongst others. Currently, the sensitivity of many electromechanical transducers used for these applications have reached their respective theoretical limit. The merit of carbon nanotubes is that, due to their miniature dimensional structures, the sensitivity of these sensor based applications is vastly improved.

Theory of Reduction of Residual Amplitude Modulation in Mid-Infrared Wavelength Modulation Spectroscopy by Injection Locking of Quantum Cascade Lasers

A simplified rate equation model is used to investigate suppression of residual amplitude modulation in injection-locked quantum cascade lasers with the master laser modulated by its drive current. Quasi-static and dynamic expressions for intensity modulation suppression and frequency modulation reproduction by the slave laser are presented. The suppression peaks at a specific value of the injection ratio for a given detuning and linewidth enhancement factor. The frequency modulation reproduction is less than 100% and decreases rapidly at frequencies higher than 300 MHz. The intensity modulation suppression remains constant during this frequency range. The effects of injection ratio, detuning, coupling efficiency and linewidth enhancement factor are discussed.

A Study on Maximizing the Energy Density of a System by Choosing a Suitable Flywheel

Different types of flywheel energy storage systems have been studied in this work. A flywheel energy storage system can be thought of as a mechanical battery that stores energy kinetic energy. The objective is to maximize the energy density of the system. However, the parameters that we change to increase the energy stored in the system might cause the system to collapse if we exceed the maximum allowable stress (the yield strength of the flywheel’s rotor material), hence, a stress analysis study for flywheel rotors is essential in order for us to find out if a rotor is capable of withstanding the given conditions in order to avoid failure. The challenge is to reduce the stress on the rotor in order for us to be able to have a greater angular speed. The results show that having a very large or very low number of spokes will cause failure to occur faster. A large number of spokes caused the failure to occur on the spokes, whereas a small number of spokes caused failure to occur on the rim of the rotor.

Harmonic and Intermodulation Distortion in Direct Intensity Modulated Quantum Cascade Lasers

In this paper, nonlinear distortion in a quantum cascade laser arising from direct intensity modulation is studied. Harmonic and intermodulation distortion expressions for small sinusoidal modulation signals are derived using a perturbation method. The expressions are validated by making a comparison with a large signal simulation of a quantum cascade laser equivalent circuit. The effects of bias current, optical modulation depth, and modulation frequency on harmonic and intermodulation distortion are examined.

Turn-on analysis of quantum cascade lasers

An analytical analysis on turn-on delay and rise time for quantum cascade laser upon switching on is presented here. Numerical solution for the rate equations is presented and comparison between analytical and numerical solution shows good agreement. Effect of varying injected current value on turn-on delay and rise time is observed. Variation of bias current from zero up to threshold value is used.

Approximate analytical expressions for pulse modulation response of quantum cascade lasers

Abstract Approximate analytical expressions for turn-on delay, rise time and fall time for pulse modulation of the drive current of quantum cascade lasers are presented. These time expressions are obtained using piece-wise analysis of rate equations. From the analytical expressions, the effects of laser parameters and off and on currents are discussed. A numerical analysis shows reasonable agreement with analytical results. It also confirms the predictions from analytical results. The pulse response is compared with that of near-infrared interband lasers.

Harmonic, Intermodulation and Cross-Modulation Distortion in Directly Modulated Quantum Cascade Lasers

Using a simplified rate equation model, expressions for harmonic, intermodulation and cross-modulation distortion for a directly modulated quantum cascade laser can be derived. This paper shows how such derivations can be done and discusses some implications for quantum cascade lasers. It is important to understand such distortion, especially for applcations in communication systems.

A Landau theory of ferroelectric nanoribbons

Most of the work involving the Landau-Devonshire theory of ferroelectrics has been either for bulk materials or thin films. By extending such calculations to two dimensions it is possible to consider a ferroelectric strip. If the thickness and width of the strip are small compared to the surface are of the corresponding surfaces it is expected that the spontaneous polarisation near the surfaces will be different from that at points further inside the strip. This paper will show how Landau-Devonshire theory can be applied to such a strip. This is of particular interest now with increasing focus on nanoscale dimensions, where these size effects are expected to be more pronounced. Thus the work will concern relevant strips whose thickness and length are on the nanoscale but whose length is much greater, and from now on will mostly be referred to as nanoribbons. After showing how spontaneous polarisation profiles can be calculated, an outline will be given of how Maxwells equations together with Landau-Khalatnikov dynamical equations can be brought in to study the interaction of the nanoribbon with incident electromagnetic waves. The work here forms a fundamental basis for numerical analysis of ferroelectric strips and will be useful in future computer aided design implementations which require modelling of ferroelectric elements.

A Study of Residual Amplitude Modulation Suppression in Injection Locked Quantum Cascade Lasers Based on a Simplified Rate Equation Model

Using results that come out of a simplified rate equation model, the suppression of residual amplitude modulation in injection locked quantum cascade lasers with the master laser modulated by its drive current is investigated. Quasi-static and dynamic expressions for intensity modulation are used. The suppression peaks at a specific value of the injection ratio for a given detuning and linewidth enhancement factor. The intensity modulation suppression remains constant over a range of frequencies. The effects of injection ratio, detuning, coupling efficiency and linewidth enhancement factor are considered.

Nanoscale Ferroelectric Structures: Landau Theory for the Polarization Profile in a Ferroelectric Nano-Box and its Response to an Incident Electromagnetic Field

The Landau-Devonshire theory of ferroelectrics to date has concentrated mainly on bulk materials, thin film geometries, spherical geometries or other, essentially one-dimensional situations. By extending such calculations to three dimensions it is possible to consider a ferroelectric in the form of a box in which the influence of all surfaces of the box are included in the theory. This influence is expected to be important as the size of the box decreases and the surface to volume ratio increases. With interest in nanoscale materials burgeoning, it is of interest to study such size effects and how they influence the distribution in a ferroelectric box. This paper will show how Landau-Devonshire theory can be applied to such a box. After showing how spontaneous polarization profiles can be calculated, an outline of how Maxwells equations together with Landau-Khalatnikov dynamical equations can be brought in to study the interaction of the nano-box with incident electromagnetic waves.