Liquid Crystal Lasers

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 Liquid crystal lasers have been a fascinating area of photonics research for over twenty years. As the name implies, liquid crystals, similar to those in LCD TVs, play a crucial role.  The liquid crystal molecules form a laser cavity and act as an optical resonator, allowing only specific wavelengths (colours) to escape, forming a laser beam. The wavelength is determined by an organic dye integrated with the liquid crystals, which absorbs light and re-emits it at longer wavelengths. The laser's energy comes from another laser, known as the pump laser. 

 Liquid crystal lasers offer several advantages over competing technologies. Firstly, they can produce any wavelength from the ultraviolet through the visible spectrum to the near-infrared within a compact package of less than       1 cm. Achieving this range with other technologies often requires bulky and expensive systems like supercontinuum lasers. Liquid crystal lasers achieve the desired wavelength simply by optimising the liquid crystal structure and selecting an appropriate dye. Additionally, their small size, low cost and scalable fabrication process, offer potential for large-scale production, in contrast to the current method of hand-made small batches. 

 Since the first demonstration of liquid crystal lasers in the 1990s, the pump laser required has typically been a Q-switched laser, capable of delivering the short, intense pulses needed for operation. However, these lasers are much larger and more expensive than the liquid crystal laser itself, negating its low-cost and compact advantages. Before 2020, various research groups attempted to use smaller, cheaper pump sources but faced challenges due to limited energy or fabrication issues. When I began my PhD in 2018, this problem was a recurring theme in the literature. Although it wasn't the primary focus of my research, I felt it was worth exploring to potentially make progress in the field. 

I spent many hours, days and weeks in a cleanroom  (this is a controlled space in which many measures are taken to minimise contamination from dirt, dust etc.), honing the liquid crystal laser fabrication process that had been optimised by a previous PhD student in the group. Eventually, I was able to make cells that required as little pump energy as possible to produce a liquid crystal laser beam (this energy - the minimum amount of energy required to produce a laser beam - is known as the laser threshold). This was not only an indication of a well-made cell, but was also a desirable specification as it meant that it was more likely that a smaller (and typically lower-energy) pump source would work. 

  The most obvious candidate for an alternative pump source was a laser diode, as these are significantly smaller and cheaper than Q-switched lasers (albeit less powerful). Furthermore, the fact the most powerful laser diode emits 

440 nm (blue) light, meant that, if successful, this single pump source could produce all longer visible wavelengths . After several months of optimising the experimental set-up, back-of-the-envelope calculations, and fabricating different cell geometries, I successfully demonstrated, for the first time, a liquid crystal laser beam using a laser diode pump source. This work was published in the journal "Optics and Laser Technology" in 2021,  showcasing not only the ability to generate a liquid crystal laser beam from a pump source of comparable size and cost, but also laying the foundation for much of the subsequent research in my PhD 

In addition to the journal publications and conference proceedings generated from this work, I was awarded the University of Edinburgh Principal's Innovation Award, which was accompanied by a cash price for further development. I used this to design and build the world's first diode pumped liquid crystal laser system. This laser is capable of visible wavelength selectivity simply through swapping in different liquid crystal laser cells, and can generate average powers of 100s of uW at up to 20 kHz. Its modular design also enables simple integration with Q-switched pump sources, if higher peak powers are required.

To demonstrate the capabilities of the laser, I took it to the Institute for Genetics and Cancer in Edinburgh to see if I could image fluorescent samples in a microscopy lab. This image shows fluorescing  dye-doped liquid crystal droplets under 585 nm liquid crystal laser illumination and was a successful proof of principle to highlight the potential of this compact laser system in medical imaging. 

 There is still much work to be done in this field, with exciting opportunities for improving the repetition rate, pulse-to-pulse stability, fabrication process, and wavelength tuning (to name a few). While dye-based lasers have fundamental limitations, their ability to generate such a broad range of wavelengths in a compact and low-cost system is strong motivation for further research and application development.