Red-Green-Blue 2D tuneable liquid crystal laser devices

In this paper, we review our recent experimental work on coherent and blue phase liquid crystal lasers.We will present results on thin-film photonic band edge lasing devices using dye-doped low molar mass liquid crystals in self-organised chiral nematic and blue phases. We show that high Q-factor lasers can be achieved in these materials and demonstrate that a single mode output with a very narrow line width can be readily achievable in well-aligned mono-domain samples. Further, we have found that the performance of the laser, i.e. the slope efficiency and the excitation threshold, are dependent upon the physical parameters of the low molar mass chiral nematic liquid crystals. Specifically, slope efficiencies greater than 60% could be achieved depending upon the materials used and the device geometry employed. We will discuss the important parameters of the liquid crystal host/dye guest materials and device configuration that are needed to achieve such high slope efficiencies. Further we demonstrate how the wavelength of the laser can be tuned using an in-plane electric field in a direction perpendicular to the helix axis via a flexoelectric mechanism as well as thermally using thermochromic effects. We will then briefly outline data on room temperature blue phase lasers and further show how liquid crystal/lenslet arrays have been used to demonstrate 2D laser emission of any desired wavelength. Finally, we present preliminary data on LED/incoherent pumping of RG liquid crystal lasers leading to a continuous wave output. © 2009 SPIE.

High efficiency liquid crystal lasers giving "white light" emission from arrays and flexible substrates

The self-organization of the helical structure of chiral nematic liquid crystals combined with their sensitivity to electric fields makes them particularly interesting for low-threshold, wavelength tunable laser devices. We have studied these organic lasers in detail, ranging from the influence specific macroscopic properties, such as birefringence and order parameter, have on the output characteristics, to practical systems in the form of two-dimensional arrays, double-pass geometries and paintable lasers. Furthermore, even though chiral nematics are responsive to electric fields there is no facile means by which the helix periodicity can be adjusted, thereby allowing laser wavelength tuning, without adversely affecting the optical quality of the resonator. Therefore, in addition to studying the liquid crystal lasers, we have focused on finding a novel method with which to alter the periodicity of a chiral nematic using electric fields without inducing defects and degrading the optical quality factor of the resonator. This paper presents an overview of our research, describing (i) the correlation between laser output and material properties,(ii) the importance of the gain medium,(iii) multicolor laser arrays, and (iv) high slope efficiency (>60%) silicon back-plane devices. Overall we conclude that these materials have great potential for use in versatile organic laser systems.

High-efficiency InGaN/GaN quantum well structures on large area silicon substrates

The growth techniques which have enabled the realization of InGaN-based multi-quantum-well (MQW) structures with high internal quantum efficiencies (IQE) on 150mm (6-in.) silicon substrates are reviewed. InGaN/GaN MQWs are deposited onto GaN templates on large-area (111) silicon substrates, using AlGaN strain-mediating interlayers to inhibit thermal-induced cracking and wafer-bowing, and using a SiN x interlayer to reduce threading dislocation densities in the active region of the MQW structure. MQWs with high IQE approaching 60% have been demonstrated. Atomic resolution electron microscopy and EELS analysis have been used to study the nature of the important interface between the Si(111) substrate and the AlN nucleation layer. We demonstrate an amorphous SiN x interlayer at the interface about 2nm wide, which does not, however, prevent good epitaxy of the AlN on the Si(111) substrate. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Progress in ultra-high modulation efficiency tapered lasers for free-space communications

Simultaneous high power (2W), high modulation speed (1Gb/s) and high modulation efficiency (14 W/A) operation of a two-electrode tapered laser is reported. © 2011 IEEE.

Ultrafast switching liquid crystals for electro-optic transmissive and reflective displays and microscopic lasers

We report on novel liquid crystals with extremely large flexoelectric coefficients in a range of ultra-fast photonic modes, namely 1) the uniform lying helix, that leads to in-plain switching, birefringence phase devices with 100 μs switching times at low fields, i.e.2-5 V/μm, and analogue or grey scale capability, 2) the uniform standing helix, using planar surface alignment and in-plane fields, with sub ms response times and optical contrasts in excess of 5000:1 with a perfect optically isotropic or black "off state", 3) the wide temperature range blue phase that leads to field controlled reflective color, 4) chiral nematic optical reflectors electric field tunable over a wide wavelength range and 5) high slope efficiency, wide wavelength range tunable narrow linewidth microscopic liquid crystal lasers. © 2011 Materials Research Society.