title: The quantum theory of optical bistability in nonlinear systems

authors: Steyn-Ross, Moira L.

abstract: This thesis is concerned with the investigation of the behaviour of various nonlinear systems when excited by a coherent light-field. 

Of particular interest to us is the possibility of observing optical bistability in these systems.

In Chapter One we introduce the general theoretical methods we employ to perform all investigations.

In Chapters Two and Three we develop a quantum theory of the interaction of light with a variety of different systems, i.e. Raman active media, the parametric oscillator, the two-photon absorber and a system of three-level atoms.

In each case, a master equation, containing all statistical information about the system, is derived. This enables the systematic inclusion of a damping mechanism into each model.

Discussion is limited to the steady state behaviour of the systems, and in general we assume the deterministic limit in which we ignore quantum fluctuations in the field variables. It is then possible to factorise the steady state expectation values of these variables. 

Steady state calculations indicate that each system may exhibit optical bistability in output field/intensity dependent on input field/intensity.

To determine whether a system will display optical bistability, it is necessary to perform a stability analysis. Where possible, such analytical calculations are performed. However, in certain cases the complexity of the highly nonlinear systems results in these calculations becoming extremely difficult. In such cases, conclusions relating to the stability of the system are drawn from graphical plots of its steady state behaviour.

Chapters Four to Eight are devoted to a study of the intracavity interaction of coherent radiation with semiconductors.

As explained in Chapter Four (of an introductory nature), this work was prompted by two recent experiments which indicated the existence of optical bistability in semiconductors. Although both experiments used semiconductors as the nonlinear material, the mechanism proposed to produce the observed bistability in each case was vastly different.

In Chapter Eight we discuss one of these mechanisms – interband excitation. We present a very simple theory of this effect and show that in essence it is equivalent to the theory of optical bistability in two-level systems. However, we stress that our theory is only approximate and relies on the validity of several simplifying assumptions.

Chapters Five and Six concern the other form of semiconductor optical bistability - arising from the interaction between a light field and excitons comprising the semiconductor.

In order to develop a quantum theory of this system using master equation techniques, it is necessary to transform the fermion system to a boson system. Bosonisation transformations required to effect this are developed in Chapter Five. 

In Chapter Six we present a fully quantum mechanical theory of optical bistability in excitonic systems. Two types of bistability are found: bistability in output intensity and also exciton number, dependent on input intensity.

In Chapter Seven we investigate the effects of quantum fluctuations in a low density exciton system, by considering its Fokker-Planck equation. We discuss in detail the adiabatic elimination of stochastic variables with regard to the system’s Fokker-Planck equation.

text: The quantum theory of optical bistability in nonlinear systems This thesis is concerned with the investigation of the behaviour of various nonlinear systems when excited by a coherent light-field. 

Of particular interest to us is the possibility of observing optical bistability in these systems.

In Chapter One we introduce the general theoretical methods we employ to perform all investigations.

In Chapters Two and Three we develop a quantum theory of the interaction of light with a variety of different systems, i.e. Raman active media, the parametric oscillator, the two-photon absorber and a system of three-level atoms.

In each case, a master equation, containing all statistical information about the system, is derived. This enables the systematic inclusion of a damping mechanism into each model.

Discussion is limited to the steady state behaviour of the systems, and in general we assume the deterministic limit in which we ignore quantum fluctuations in the field variables. It is then possible to factorise the steady state expectation values of these variables. 

Steady state calculations indicate that each system may exhibit optical bistability in output field/intensity dependent on input field/intensity.

To determine whether a system will display optical bistability, it is necessary to perform a stability analysis. Where possible, such analytical calculations are performed. However, in certain cases the complexity of the highly nonlinear systems results in these calculations becoming extremely difficult. In such cases, conclusions relating to the stability of the system are drawn from graphical plots of its steady state behaviour.

Chapters Four to Eight are devoted to a study of the intracavity interaction of coherent radiation with semiconductors.

As explained in Chapter Four (of an introductory nature), this work was prompted by two recent experiments which indicated the existence of optical bistability in semiconductors. Although both experiments used semiconductors as the nonlinear material, the mechanism proposed to produce the observed bistability in each case was vastly different.

In Chapter Eight we discuss one of these mechanisms – interband excitation. We present a very simple theory of this effect and show that in essence it is equivalent to the theory of optical bistability in two-level systems. However, we stress that our theory is only approximate and relies on the validity of several simplifying assumptions.

Chapters Five and Six concern the other form of semiconductor optical bistability - arising from the interaction between a light field and excitons comprising the semiconductor.

In order to develop a quantum theory of this system using master equation techniques, it is necessary to transform the fermion system to a boson system. Bosonisation transformations required to effect this are developed in Chapter Five. 

In Chapter Six we present a fully quantum mechanical theory of optical bistability in excitonic systems. Two types of bistability are found: bistability in output intensity and also exciton number, dependent on input intensity.

In Chapter Seven we investigate the effects of quantum fluctuations in a low density exciton system, by considering its Fokker-Planck equation. We discuss in detail the adiabatic elimination of stochastic variables with regard to the system’s Fokker-Planck equation.

year: 1981

title: Remote sensing of freshwater habitats for large rivers and lakes of the Waikato region using sub-pixel classification

authors: Ashraf, Muhammad Salman

subjects: remote sensing data

abstract: The overall goal of this study was to evaluate the feasibility of multi-sensor and multi-spectral remote sensing data for mapping different freshwater habitat zones of large rivers and lakes of the Waikato region. The review of satellite sensors available in NZ for aquatic mapping has identified three data options, which have been purchased for a case study area (Tongariro river delta). These data options are Landsat-5 (30m resolution), ALOS (10m and 2.5m) and QuickBird-2 (2.4m and 0.6m). There is no perfect option available as each option is a compromise between spatial and spectral resolution and cost.

text: Remote sensing of freshwater habitats for large rivers and lakes of the Waikato region using sub-pixel classification The overall goal of this study was to evaluate the feasibility of multi-sensor and multi-spectral remote sensing data for mapping different freshwater habitat zones of large rivers and lakes of the Waikato region. The review of satellite sensors available in NZ for aquatic mapping has identified three data options, which have been purchased for a case study area (Tongariro river delta). These data options are Landsat-5 (30m resolution), ALOS (10m and 2.5m) and QuickBird-2 (2.4m and 0.6m). There is no perfect option available as each option is a compromise between spatial and spectral resolution and cost.

year: 2007-09

title: Community-university collaborations: creating hybrid research and collective identities

authors: Cave, Jennifer Barbara Jane

subjects: collaborative

abstract: In this article we explore the politics of community-university collaborative research and activism. We are scholars based in universities as well as members of various 'communities'-queer, Pasifika and Māori-hence we regularly venture beyond the formal research spaces of the academy and enter into critical collaborative research with others working in 'the community'. In what follows we first outline collaborative community-based research literature that advocates hybrid research collectivities. Second, we give some context and background to our methodologies that have enabled us to re-think collaborative research. Third, examples from our research are offered to illustrate the ways in which collaborative research constructs new collective identities. Finally, we conclude by arguing that social science scholars working across university-community boundaries may expand and multiply hybrid research collectives, and thereby effect positive social change at many levels

text: Community-university collaborations: creating hybrid research and collective identities In this article we explore the politics of community-university collaborative research and activism. We are scholars based in universities as well as members of various 'communities'-queer, Pasifika and Māori-hence we regularly venture beyond the formal research spaces of the academy and enter into critical collaborative research with others working in 'the community'. In what follows we first outline collaborative community-based research literature that advocates hybrid research collectivities. Second, we give some context and background to our methodologies that have enabled us to re-think collaborative research. Third, examples from our research are offered to illustrate the ways in which collaborative research constructs new collective identities. Finally, we conclude by arguing that social science scholars working across university-community boundaries may expand and multiply hybrid research collectives, and thereby effect positive social change at many levels

year: 2012