Optical Materials Group
Projects
- Nano-crystals in Semiconductor Doped Glasses
- Laser Selective Excitation Studies of Nd3+ Doped Fluoride Crystals
- Waveguiding in Single Crystal Fibres
Nano-crystals in Semiconductor Doped Glasses
Supervisors: Ivan Ruddock and Thomas Han
Semiconductor nano-crystals in transparent media, such as CdS, CdSe doped glasses, have recently received much attention due to their promising applications in non-linear optics (NLO) and as optical switches. These nanometer-sized crystals exhibit intermediate behaviour between a bulk crystal and an isolated molecule with unique semi-properties, characteristic of neither the molecular nor solid state limits. Nano-crystals have optical spectra that can be tuned in wavelength simply by varying the crystallite size. As the sizes of the nanocrystallites decrease, controlled by the preparation method, the band gap shifts to higher energies due to the quantum size effect. In three-dimensions, they are analogous to the quantum well semiconductor heterostructure that exhibits a one-dimensional quantum size effect. With the bulk NLO response depending on the number of polarisable electrons per unit volume, a high-density collection of quantum nano-clusters is required, both for achieving a basic understanding of the quantum-structure phenomenon and for device development. Absorption and luminescence spectroscopy enables the shift to be determined while four-wave-mixing techniques provides information on the resonance enhancement effect. The project will focus on the preparation and properties of nano-crystals embedded in glass and an investigation of their potential applications in non-linear optics and optoelectronics.
Laser Selective Excitation Studies of Nd3+ Doped Fluoride Crystals
Supervisor: T. P. J. Han
Luminescence rare-earth elements are of scientific and technological importance in designing, synthesizing and utilizing photonic materials. The crystal structure of the fluorides is one of the most important factors influencing the luminescent properties. Fluorides of the type MNF4 (M=Mg, Zn, Fe; N=Ca, Sr, Ba) belongs to space group Cmc21 . This means that these compound lack a centre of symmetry and is one of the rather rare fluorides with a polar structure. This material is a very suitable host for rare-earth ion impurities and has promising properties for solid-state laser applications. Within this context, identification of the defect site(s) occupied by the impurity cation in the crystal is important. This project is to investigate the optical properties of a subset of these rare-earth doped compounds. Laser Selective Excitation and Crystal-Field modelling are used to characterise the defect centres.
Waveguiding in Single Crystal Fibres
Supervisors: Ivan Ruddock and Thomas Han
Combining the linear and nonlinear optical properties
of crystalline materials with the wave-guiding property of fibres is an
important goal in optoelectronics research. The motivation is to produce
crystalline materials in a fibre geometry with various optical and nonlinear
properties. Due to the limit on smallest diameter of fibre that can be
grown and the lack of suitable cladding materials, the project will focus
instead on solving the problem of mode control by the creation of a graded
index profile. The Laser Heated Miniature Pedestal Growth method will
be used to grow fibres of important materials such as oxides and borates.
Following optimisation of the growth process, most of the effort will
investigate two approaches to modifying the refractive index - (i) the
in-diffusion of dopants, and (ii) photorefractive writing. To demonstrate
the applicability of the resulting graded index fibres, experiments such
as harmonic generation and parametric operation will be performed.
