Design and analysis of quantum dot laser (InAsP) for bio-photonic and mode-locking applications

In this thesis, an original quantum dot material (InAsP) was introduced and characterised as a prospective laser material for applications in biophotonics and monolithic mode-locking. InAsP quantum dot material was grown in conditions that are appropriate for InP QD, which is the standard device in this study. The reasons for employing this material are to shift the emission to longer wavelengths than can be achieved with InP QD laser. In principle, by using both the InP and InAsP QDs in a single structure, a very wide gain spectrum can be produced that may be advantageous for passive mode-locking. The characteristics of the InAsP QD lasers were determined and compared with the standard device (InP QD laser) in this work, such as threshold current density, laser efficiency, lasing wavelength and temperature dependency of the threshold current density for different cavity lengths (1, 2, 3 and 4mm). The results show a shifting in the emission wavelengths by 55 nm toward longer wavelengths, while maintaining useable threshold current density and laser efficiency. For example, the 2mm long InP laser has a threshold current density of 170 A.cm-2 at room temperature, whereas for the same length, the InAsP QD laser has 260 A.cm-2. Moreover, both samples delivered optical powers of at least 250 mW. Edge-photo voltage spectroscopy (E-PVS measurements) confirmed deeper dot confinements for the InAsP materials by approximately 103 meV. The modal absorption spectra show a greater degree of inhomogeneous broadening for the InAsP QD materials, which was consistent with the dot size variation shown in TEM images for the InAsP wafer. This can support mode-locking in this material by broadening the optical gain spectra, which was also observed in this material. Gain-current measurements at different temperatures; specifically, 150, 200, 250, 300, 350, and 400 K, illustrate that InAsP QD material has a wider gain band-width at all studied temperatures. The carrier distribution study shows that the InAsP QD material tends to be non-thermally populated at 150 K. And also the recombination rate of this material is faster than the InP QD materials. Both of these points can be positive in relation to mode-locked performance