Optical studies of semiconductor quantum microcavities

The work presented in this thesis is concerned with the studies of radiation emitted from inorganic and organic semiconductor microcavities. For the inorganic microcavities, both single and coupled structures were investigated whilst for the organic microcavities a single microcavity was studied. In chapter 2, the PL emitted from the surface of a single microcavity is studied using nonresonant excitation. In this technique, the microcavity is excited at higher energy relative to the reflectivity stop-band. Over a wide range of detunings (∼-21 meV up to ∼+4meV), the power dependence of the PL spectra show the existence of a relaxation bottleneck for negative detuning at different temperatures (∼1.8K, ∼35K and ∼60K). To explain the collapse of the relaxation bottleneck observed at higher excitation power (P≥20W/cm2), the angular variation of the full width at half maximum of the lower polariton peak was measured. The collapse of the relaxation bottleneck was attributed to the exciton-exciton scattering observed at high excitation power. In chapter 3, resonant excitation of a particular polariton state (lower polariton branch) is employed to study stimulated scattering of polaritons. At a specific angle of excitation (∼15.5 deg), the photocreated polaritons are scattered simultaneously to populate two specific states termed as signal and idler, in a process which conserves both energy and momentum. At the aforementioned angle of excitation, the power dependence is measured in order to demonstrate the nonlinear behaviour of the intensity of the signal and idler beams as a function of power, with a marked threshold being found. The key features of stimulated scattering, the nonlinear variation of the intensity and the distinctive narrowing of the signal and idler beams above threshold, are found. The power dependence at some other angles of excitation (∼10 deg and ∼6.5 deg) are also measured. The presented results show that the stimulated scattering is weakly dependent on the angle of excitation. In chapter 4, angular dependent reflectivity and PL techniques are employed to measure the passive and active response of a coupled microcavity with three quantum wells embedded in the top cavity. The measurements are conducted at two different temperatures, namely 10K and 65K. Close to resonance, where the central polariton mode lies exactly between the upper and lower polariton modes, the central mode disappears. The disappearance of the central mode from the PL spectra near the resonance is attributed to the diminished optical field in the cavity containing the quantum well. The PL measured from the edge of the structure, which represents the excitonic distribution at high k, is used to model the surface PL spectra. In chapter 5, Raman scattering techniques are used to study the involvement of polaritons in scattering processes in organic microcavities. The organic material (J- aggregate) has a large oscillator strength, which leads to a large Rabi splitting (∼80meV) when such a material is enclosed inside a microcavity. This large splitting between the lower and upper polariton (∼80meV) allows the double resonance Raman scattering condition, where the excitation and detection are both in resonance with the polariton states, to be achieved. The involvement of the Stokes Raman line (phonon emission) in the scattering of the polariton from high to low energy, is confirmed by the angular and laser tuning measurements of the emitted PL spectra. The maximum of the Stokes Raman signal is observed when the detection angle is in resonance with the polariton energy. The anti- Stokes Raman line (phonon absorption) is also measured by exciting the microcavity close to the normal of the sample surface. Again the maximum of the anti-Stokes line is observed when the detection angle is in resonance with the polariton state. (author)