Optical studies of three dimensional confinement in photonic and electronic systems.

In the first part of this thesis the optical properties of three dimensional opal photonic crystals are investigated. The samples were grown by sedimentation of silica spheres into a face centred cubic structure. Structural studies show that self-assembled opal photonic crystals are polycrystalline materials consisting of misoriented domains of size 50 μm to 100 μm. The angle dependent transmission technique is used to characterise the stop band of the samples. Due to the weak refractive index contrast in opal photonic crystals, only stop bands along the [111] direction are observed. The experimental transmission spectra are compared with theoretical transmission spectra calculated according to a three dimensional model based on the transfer matrix method. The experimental stop band is found to be six times broader than the calculated one, and also the experimental Bragg attenuation length is found to be five to seven times larger than the calculated one. Angle resolved diffraction and scattering techniques are used to investigate the origin of the discrepancies between experiment and theory. Analysis of the diffraction spectra indicate that the samples consist of misoriented domains of thickness 10 μm with a Gaussian distribution of 10° FWHM around the [111] direction. The scattering spectra show a strong resonant enhancement at the centre or edges (depending on the refractive index contrast) of the stop band. This observation is attributed to the multiple incoherent backward/forward reflections between the sample domains. By analysing the balance of photon flux originating from a slab of opal the shape of the experimental transmission stop band is fully explained.
To investigate the effect of an incomplete photonic structure on the emission properties of light sources located inside the photonic crystal, the samples were infiltrated with solutions of laser dyes having fluorescence bands which overlap with the photonic gap of the host crystal. The optical fluorescence spectra reveal a stop band region with a centre varying with angle according to Bragg’s law. It is found that the fluorescence stop band is much shallower and narrower than that observed in transmission spectra. The gap narrowing in the fluorescence spectra is attributed to the scattering events inside the opal taking into account the spectral dependence of the Bragg attenuation length.
High refractive index contrast opal photonic crystals are achieved by infiltration with chalcogenide glasses (AS2S3 and AsSe) by precipitation from solution. Optical imaging of the samples after infiltration shows that chalcogenide glasses tend to aggregate into submillimeter areas inside opals. Spatially resolved reflectivity spectroscopy is used to characterise the infiltrated areas. Large shifts (up to 80 nm) in the position of the stop band has been achieved from the infiltrated samples in comparison to the samples before infiltration.
In the second part of this thesis, mid-infrared (2.5 μm -25 μm) intersubband transitions in the conduction band of uncoupled and vertically coupled and also in the valence band of self-assembled In(Ga)As/GaAs quantum dots are investigated using direct absorption measurements and photocurrent spectroscopy. The investigated dots show three dimensional electronic confinement. Intersubband transitions are investigated as a function of the polarisation angle of the incident radiation, external electric field and temperature. The experimental results indicate that intersubband transitions in the conduction band of the uncoupled dots are allowed for radiation polarised in the growth direction, similar to that found for quantum wells. By strongly coupling quantum dots in the growth direction (10 Å GaAs barrier between the dots) we have achieved a reversal of the intersubband selection rule for optical transitions between conduction band states, compared with that observed for uncoupled dots. We find that for strongly coupled dots the dominant absorption occurs for light polarised perpendicular to the growth direction (s-polarised), consistent with eight-band strain dependent k.p calculations. This contrasting behaviour results from the very different composition of the basis wave functions in the conduction band states for coupled and uncoupled dots due to differences in the conduction-valence band mixing. This effect enhances the oscillator strength of transitions between the ground and excited states in the conduction band for coupled dots. The results from p-type samples show that intersubband transitions in the valence band of self-assembled quantum dots are strongly polarised perpendicular to the growth direction (s polarised). The results are attributed to the anisotropy of the hole subbands and also to the strong band mixing between the valence subbands. The results suggest the suitability of strongly coupled self-assembled quantum dots and p-type quantum dots for high efficiency normal incidence infrared photo detectors.