This thesis describes a number of studies on organic semiconductors focused around using them as gain media for lasers. The photophysical properties of organic semiconductors are studied using a wide range of experimental techniques, allowing the evaluation of new materials and novel excitation schemes for use in organic semiconductor lasers. Polyfluorene is a well-established conjugated polymer laser gain medium and in this thesis its excellent lasing properties are combined with its two photon absorption properties to demonstrate a tunable two-photon pumped solid-state laser based on a commercially available organic semiconductor. A family of bisfluorene dendrimers was studied using a number of photophysical techniques to evaluate their potential as laser materials. Distributed feedback lasers based on one of the dendrimers are demonstrated with lasing thresholds comparable to polyfluorene. The same materials were found to have enhanced two-photon absorption properties in comparison to polyfluorene, leading to the fabrication of tunable two-photon pumped dendrimer lasers. A member of a novel family of star-shaped oligofluorene truxenes was evaluated as a laser gain material and the distributed feedback lasers made from them show some of the lowest lasing thresholds reported for organic semiconductors, partly as a consequence of exceptionally low waveguide losses in comparison to other single-material thin films. Finally, an organic laser dye is blended with a conjugated polymer, where the dye molecules harvest the excitation light of a GaN laser diode and transfer its energy to the polymer molecules. This is the first time such a scheme is used in an organic laser and in combination with a novel surface-emitting distributed Bragg reflector resonator allows the demonstration of a diode-pumped organic laser, a significant step towards simplifying organic lasers.

Nano-emitters are the new generation of laser devices. A photonic-crystal cavity, which highly confines light in small volumes, in combination with quantum-dots can enhance the efficiency and lower the threshold of this device. The practical realisation of a reliable, electrically pumped photonic-crystal laser at room-temperature is, however, challenging. In this project, a design for such a laser was established. Its properties are split up into electrical, optical and thermal tasks that are individually investigated via various device simulations. The resulting device performance showed that with our design the quantum-dots can be pumped in order to provide gain and to overcome the loss of the system. Threshold currents can be as low as 10’s of μA and Q-factors in the range of 1000’s. Gallium arsenide wafers were grown according to our specifications and their diode behaviour confirmed. Photonic-crystal cavities were fabricated through a newly developed process based on a TiOₓ hard-mask. Beside membraned cavities, also cavities on oxidised AlGaAs were fabricated with help to a unique hard-mask removal method. The cavities were measured with a self-made micro-photoluminescence setup with the highest Q-factor of 4000 for the membrane cavity and a remarkable 2200 for the oxide cavity. The fabrication steps, regarding the electrically pumped photonic-crystal laser, were developed and it was shown that this device can be fabricated. During this project, a novel type of gentle confinement cavity was developed, based on the adaption of the dispersion curve (DA cavity) of a photonic-crystal waveguide. Q-factors of as high as 600.000 were measured for these cavities made in Silicon.

<p>Over the past few years various successful miniaturization attempts of External Cavity Semiconductor Lasers (ECSL) were published. They built upon the rich literature of ECSL configurations that were extensively analyzed and improved upon since the 1960s. This was merged with the microfabrication techniques of 3D structures based on MEMS technology. The main drive for miniaturizing such tunable lasers in the recent past was the huge potential for such devices in all optical networks specifically as signal sources that enable Wavelength Division Multiplexing (WDM).<br />This thesis compares the different configurations chosen to build tunable lasers using MOEMS technology. Our criteria of comparison include wavelength tuning range, side mode suppression, tuning speed and device dimensions. Designs based on the simple ECSL with a movable external mirror suffered from the tradeoff between tuning range and Side Mode Suppression SMS. To overcome this limitation most designs adopted grating based tuning using the Littrow or Littman/Metcalf configurations. These configurations allow for much better tuning results but don’t lend themselves easily to miniaturization. The grating based devices were bulky and quite complicated to realize.<br />We propose the adoption of the Zhu/Cassidy double external cavity configuration. It retains the simplicity of the single external mirror configuration along with the tuning range and the SMS of including multiple tuning elements. In its original form this configuration suffered from mode hopping within the tuning range. Thorough simulation, design and experimental evidence is presented in this work to show that by extending the configuration to allow full control over both optical tuning elements this drawback can be eliminated.<br />Our proposed design would reduce the form factor to < 300μm x 200μm x 200μm. The voltage required to tune through all the modes is < 40V and the resonant frequency of the mirror is in the 10s of MHz order of magnitude. When coupled with a multimode laser of a sufficiently broad lasing profile this setup should enable a tuning range > 72nm with a SMS >20dB.</p> / Master of Applied Science (MASc)