Diode pumped liquid crystal lasers

Abstract

Liquid crystal (LC) lasers have received much interest in photonics research for over twenty years. Their self-organising micro-scale structure, emission wavelengths spanning the visible spectrum and low thresholds offer many advantages over competing technologies. However, despite such benefits, LC lasers remain a lab-based curiosity. This is primarily due to the use of Q-switched lasers as the typical excitation source; the complexity, cost and size of which are orders of magnitude greater than the LC lasers they are pumping.

There has been much conjecture in the literature that a semiconductor-based pump source would offer a major step forward in the field. This thesis addresses this challenge, by demonstrating optical excitation using a laser diode (LD). Cleanroom fabrication processes and optical benchtop arrangements are presented to respectively produce and test LC lasers. A 445 nm LD was used to demonstrate the first unequivocal evidence of diode-pumped band-edge LC lasing, exhibiting a low threshold of 12.3 nJ/pulse and a narrow spectral linewidth (< 1.5 nm) at 610 nm. LC laser emission in the blue (480 nm) and green (530 nm) regions of the spectrum were also realised using the same 445 nm LD pump source.

An experimental investigation found that an increase in pump pulse length caused an increase in LC laser threshold and decrease in LC laser slope efficiency, thus verifying, for the first time, the theoretically hypothesised and anecdotally observed results of previously published work. It is only with the success of LD pumping that such an investigation was possible. LD pumping experiments also revealed the existence of an upper limit to the LC laser pulse length – irrespective of pump pulse durations that exceed this upper limit – with a value of 3.5 (± 0.1) ns for the DCM-based laser in this work. This phenomenon is attributed to the rate of intersystem crossing of the organic dye molecules.

Finally, the design, build and testing of the first diode-pumped LC laser prototype was realised, capable of producing up to 90 W at 15 kHz at wavelengths ranging from 480 nm to 610 nm. This low-cost (< £3,000) and small-footprint (20 cm x 12 cm) laser was successfully used to image dye-doped LC droplets in a biomedical imaging lab, demonstrating the application potential of diode-pumped LC lasers. The novel results presented in this thesis are anticipated to have a significant impact on the development of LC lasers, regarding pump source selection, performance optimisation and application potential.