Hybrid thin-film solution-processed liquid crystal lasers
Keywords: polarisation
solution processing
liquid crystals
perovskite
flexoelectricity
flexible laser
birefringent materials
laser

The ultimate goal of this thesis is to use low-cost processing techniques to create thin film lasers that are compact in form, exhibit low excitation threshold fluences, and the potential for wavelength tuning when the laser is subjected to external stimuli. This thesis explores the use of a combination of liquid crystalline materials and organic & inorganic gain media to fabricate the thin film lasers, which could potentially be important for future bio-medical sensing devices, spectroscopy applications, virtual and augmented reality displays, and environmental sensors. This thesis can be broadly divided into three parts: 1) to investigate the electro-optic properties of novel liquid crystalline materials that could be used to form the optical cavity of these thin film lasers; 2) to employ the use of novel gain materials in the form of perovskite nanocrystals to create photostable liquid crystal (LC) lasers; and 3) to develop thin film lasers with new functionality such as beam steering through mechanical deformation and polarisation control with the addition of a second LC layer.
Experiments were designed and constructed so as to study the physical parameters, such as the birefringence, elastic and dielectric constants, and the flexoelastic ratio, of a newly developed LC dimer. The dimer exhibits a large flexoelastic ratio (2.59 ~ 2.60 1/V) and high birefringence (∆n = 0.312) that enable it to be used to create fast switching photonic devices at low driving voltages, such as full 2pi phase modulators. Furthermore, the high birefringence also makes it desirable for low excitation threshold and wavelength tuneable band-edge LC devices.
The birefringence characteristics and periodic structure of the chiral nematic phase, along with its compatibility with solution processing techniques, makes them exciting candidates for thin-film laser devices. As a result, this Thesis aims to explore potential fabrication processes that could be used to disperse alternative gain materials in the form of perovskite nanocrystals (CsPbI3 and CsPbI2Br, λem = 675 +/- 2nm, size = 10 +/- 15 nm) into a chiral nematic LC host. Combining these inorganic and organic semi-conducting materials with LC host materials could open up new possibilities in terms of a route towards an electrically pumped LC laser. Therefore, a detailed study is presented on the dispersion of perovskite nanocrystals in nematic and chiral nematic LCs by employing various processing techniques. Results show that for all of the techniques considered in this thesis, nanocrystal aggregates form, which in some cases can lead to an alteration of the emission characteristics. It is believed that the alteration in the emission characteristics is potentially due to a disintegration of the perovskite structure. For several of the processes considered in this work, little to no alteration in the emission spectrum is observed, which indicates that the perovskite structure has remained intact during the dispersion process. Furthermore, thin-films of perovskite (both in the form of a bulk layer and nanocrystals) have also been employed to create a Defect-Mode LC laser structure. After optimising the material combination and fabrication process, a defect-mode structure with perovskite nanocrystals is developed which is found to exhibit multi-mode laser emission.
A novel flexible defect-mode laser is also demonstrated in this Thesis by using a photo-polymerised chiral nematic LC and a laser dye that forms a stack CLC/Barrier/ Dye doped PMMA/Barrier/CLC. This laser, which is the first demonstration of a flexible defect-mode laser, is fabricated using a film transfer technique on a transparent flexible polymer. The results show evidence of single mode emission at a wavelength of 582 nm and a low fluence excitation threshold of 12.3µJ/cm2/pulse when optically pumped with a solid-state laser. By the virtue of the mechanical flexibility, the LC laser shows a beam steering phenomenon of up to 42 degrees when the laser is held curve and scanned with the pump laser. Such lasers are of potentially useful for creating 3D imaging systems that negate the need for additional optical components.
To conclude this Thesis, a polarisation controllable band-edge LC laser device is presented that shows on-demand control of the polarisation state of the laser using an in-plane electrode configuration combined with a nematic LC layer. The optical anisotropy coupled with the orientation of the electric field results in a polarisation in a certain direction with the in-plane electrodes assembly can control the polarisation states of light that passes through the device. The device can control the polarisation state of the LC band-edge laser by generating circular and elliptical polarisation in any orientation with the limitation of the linear polarisation state. The polarisation manipulators are useful for various imaging application from biomedical to entertainment industries.