Efficient and high light amplification of optical resonator in organic laser is one of the critical factors for high performance organic laser. It can be achieved by using microcavity and DFB structures, which are commonly adopted methods to enhance light amplification in specific wavelength. Both of them are the more widely used structures in inorganic and organic lasers. In this work, we employed nearly 100% reflection (at 450 nm) DBR and Al to act as reflected mirror inside the microcavity device. The function of microcavity has been examined to show the ability of device in tuning laser emission wavelength and overcoming the loss of organic-metal interface. DFB structure was used to demonstrate different laser emissions with respect to different grating periods. The finding clarifies the role of the structure in enhancement of light amplification leading to lower threshold, which was half of that of amplified spontaneous emission from single layer of PFO. As designed laser mode is also an important factor to get a high performance organic laser, those laser modes of structures have been designed and estimated by simulations and consistent with the experimental results. Color tunable light source has great potential for display, lighting and bio-imaging. Current broadband light sources, however, have their own limitations in beam divergence and device size. In this work, we demonstrated a spatially variant light source with tunable color emission property by using two cascaded organic thin films, which emit blue and green ASE respectively under optical pumping. By spatially selecting the overlapping of the directional ASE from the cascaded films, we show that the color of light emission can be continuously tuned from blue, white to green.

The advances achieved using X-ray Free Electron Lasers such as the Linac Coherent Light Source (LCLS), have revolutionised the routine production of uniform solid-density plasmas. Pulses of X-rays above 1 keV and with durations shorter than 100 fs attaining intensities on target of around 10¹⁷ Wcm^-2 are now routinely created. Through simple single-photon photoionization events with atoms in ambient solid conditions, it is possible to create uniform samples that are simultaneously hot, dense, and highly ionized which may be easily modelled. This thesis describes measurements of the spectrally-resolved X-rays emitted from solid-density magnesium targets of varying sub-μm thicknesses isochorically heated by an X-ray laser. The data exhibit a thickness-independent source function, allowing the extraction of a measure of the opacity to K-shell X-rays within well-defined regimes of electron density and temperature, extremely close to Local Thermodynamic Equilibrium conditions by fitting to the simple 1D slab solution of the equation of radiative transfer. The deduced opacities at the peak of the K-α transitions of the ions are consistent with those predicted by detailed atomic-kinetics calculations. The extracted opacities transpire to be robust to a plethora of variations in X-ray drive conditions, including the shape, pulse-length, and energy content. Furthermore the approximations in using the 1D slab solution are examined in detail and found to be good. A full three-dimensional model of the plasma is advanced which includes attenuation, line-of-sight effects, full longitudinal and transverse gradients, and photon time-of-flight effects. The results from this model are found to also agree with the simpler 1D slab solution. This novel method of elucidating opacities may complement other methods based on absorption and could be important for further benchmarking of opacities in solar-interior relevant conditions.

The contributions of nonlinear absorption processes to laser-induced damage of three selected groups of transparent dielectrics were investigated. The studied materials were irradiated and non-irradiated fused silica, doped and undoped fluorozirconate glass and cubic zirconia stabilized with yttria. The laser-induced damage thresholds, prebreakdown transmission, and nonlinear absorption processes were studied for several specimens of each group. Experimental measurements were performed at wavelengths of 1064 nm and 532 nm using nanosecond and picosecond Nd:YAG laser pulses. In the irradiated fused silica and fluorozirconate glasses, we found that there is a correlation between the damage thresholds at wavelength λ and the linear absorption of the studied specimens at λ/2. In other words, the laser-induced breakdown is related to the probability of all possible two-photon transitions. The results are found to be in excellent agreement with a proposed two-photon-initiated electron avalanche breakdown model. In this model, the initial "seed" electrons for the formation of an avalanche are produced by two-photon excitations of E' centers and metallic impurity levels which are located within the bandgaps of irradiated Si02 and fluorozirconate glasses, respectively. Once the initial electrons are liberated in the conduction band, a highly absorbing plasma is formed by avalanche impact ionization. The resultant heating causes optical damage. In cubic zirconia, we present direct experimental evidence that significant energy is deposited in the samples at wavelength 532 nm prior to electron avalanche formation. The mechanism is found to be due to formation of color centers (F+ or F° centers) by the two-photon absorption process. The presence of these centers was directly shown by transmission measurements. The two-photon absorption (2PA) process was independently investigated and 2PA coefficients obtained. The accumulated effects of the induced centers on the nonlinear absorption measurements were also considered and the 2PA coefficients were measured using short pulses where this effect is negligible. At room temperature, the color centers slowly diffuse out of the irradiated region. The density of these centers was monitored as a function of time. The initial distribution of the centers was assumed to have a Gaussian profile. For this model the diffusion equation was solved exactly and the diffusion constant obtained.

The aim of this thesis is to develop both Yb and Tm fiber laser sources with all fiber cavities. Both wavelength ranges provide useful laser sources for optical pumping of helium. The goal is to develop Tm laser sources operating at 2058 nm to optically quench 3He (2058.63 nm) and 4He (2058.69 nm) singlets (21S0). We also have developed Yb laser sources at 1083 nm to optical pump the triplet states of helium and laser cool an atomic beam of helium.