There is an increasing demand for high performance materials many of which are incompatible with conventional manufacturing processes due to their sensitivity with heat and abrasion (e.g. carbon fibre reinforced polymer (CFRP), polymers, thin films, semiconductor wafers). Surface preparation of these advanced materials is usually essential to exploiting their enhanced properties in engineering applications. This project concerns the development of a new, low temperature material processing technique using impulsive shock waves of liquid vapour as a generic manufacturing tool, called material processing by laser induced droplet vaporisation (LIDV). LIDV uses a pulsed laser to vaporise a stream of droplets to produce a succession of impacting shock waves at the surface of the component of interest. By exploiting the precision of modern droplet generation technologies with the power and control of a laser source, well-regulated impulsive forces can be achieved at a level sufficient to remove surface contaminants and/or to modify the surface mechanical properties (roughness and hardness). The project will undertake proof-of-principle experiments to establish the benefits of LIDV as a means to clean and modify the surface of CFRP. CFRP presents a particular problem since a multi-stage surface preparation process is required as all traces of surface contaminants and chemical agents that are exhumed during production must be removed before final assembly. It is expected that the LIDV process could provide more accurate and efficient surface modification of high performance materials than at present and that it will also open the door for exciting breakthroughs in peening, thin films transfer, hardening and alloying.

In this instrument development project we will be designing, constructing and testing two laser systems producing UV light pulses with sufficient energy to mark, cut or drill various non-ferrous engineering materials, be it removing small amounts precisely or in a wholesale manner that accompanies an explosion of the target material. The key advantages of the proposed systems are that they will be efficient and offer unique properties for the emitted light that cannot be found in any other laser system in the world. The first is the colour, or wavelength, of the light that will be shorter than almost all other solid-state laser systems; next and for just one of the instruments, the energy in each pulse and their frequency of arrival will be comparable to the smaller industrial-standard excimer gas lasers, which are used for many processes in the electronics manufacturing industry but rely on toxic and corrosive gases and very high voltage discharges to generate the UV light; while the second instrument will have one thousand times less energy per pulse than the first, it will deliver the same number more pulses per second, making it very useful for rapid precision micro-processing, where speed and accuracy are a premium. For us to be able to make these novel laser systems we will exploit an old technology that has re-emerged as a potential platform architecture, cryogenic cooling. Cryogenic cooling applied to high energy laser systems with high average powers has become accepted as the credible route toward laser driven fusion reactors and extreme-peak-power laser facilities (NIF - https://lasers.llnl.gov/, DiPOLE - at STFC Rutherford Appleton Laboratory (RAL) http://www.stfc.ac.uk, HiLASE - http://www.hilase.cz), clearly evidence of the potential efficiency of the approach. Employing this method we will develop a platform technology that underpins both of the systems detailed above and will enable the unique characteristics of our proposed manufacturing laser instruments. At the end of the project we will have developed a clear route for transferring the knowledge to enable the manufacturing of these lasers and begun testing their performance for materials processing in collaboration with UK laser micro-processing industrial partners.

Until very recently the MASER could only be used in very specialist applications such as radio astronomy. The reason for this is that cryogenic cooling and to a lesser extent, high applied magnetic fields, prohibited mass production on the grounds of both complexity and cost. Despite the fact that the MASER was discovered before the LASER these issues meant that the latter, which does not need applied magnetic fields or cooling, saw widespread adoption in a huge range of applications from bar-code readers, laser discs to laser eye surgery. In 2013 Imperial and UCL were awarded an EPSRC funded research project to produce a room temperature MASER. Although we had preliminary observations that room temperature masing was possible we had not verified this in a different laboratory setting, nor did we have a clear idea of how the masing molecule interacted with light and which crystal orientations or dopant concentrations would be optimal. This collaboration was remarkably successful achieving all the objectives we set. Now, in what is another world first, the team has constructed a diamond MASER capable of continuous-wave operation at room temperature. Our previous research has concentrated solely on organic materials as the masing medium. In this proposal we will explore the potential of masing in inorganic materials at room temperature. In doing so we will obviate two key problems encountered with organics. Problem 1 - Decay rates: The primary obstacle that prevents continuous operation in organics is the relatively long lifetime of the lowest triplet sub-level, reducing the number of pentacenes available for optical pumping (bottleneck) and destroying the population inversion. Problem 2 - Heating: The organic gain medium, pentacene in p-terphenyl we first used to demonstrate a room temperature MASER cannot withstand a continuous illumination by a laser because the temperature of the terphenyl host rises above its melting point. Solution to both problems: a radical but exciting departure which will address both problems simultaneously is to explore high spin states in inorganic materials with high melting/decomposition temperature and favourable thermal conductivities (T.C.): such as diamond (M.P. 3550C; T.C. 2000 W/mK) and silicon carbide (2730C; T.C. 120 W/mK). Very recently we observed masing at room temperature in diamond exploiting NV centres. This means we can build upon a huge wealth of research in the UK and elsewhere on diamond NV centres. Again there is much research exploring defects in SiC that we can build on. We have initiated a collaboration with the group of Prof. Dr. Vladimir Dyakonov at Würzburg group who are currently exploring SiC. REF. https://arxiv.org/pdf/1709.00052.pdf Achieving this would further establish without doubt the UK as the key place to carry out fundamental research on the topic of room temperature MASERs.

Hollow core optical fibres guide light in a hollow (usually, gas-filled) core rather than in a solid glass core as in all conventional fibres. The use of a hollow core means that many of the constraints on optical fibre performance which are due to the properties of the core material are lifted (often by many orders of magnitude) and the fibres can by far outperform their more familiar conventional counterparts. There is a problem, however: how can you trap light in a hollow core? Substantial effort has been put into developing so-called photonic bandgap fibres over the last 20 years. These fibres rely on a complex cladding structure to trap light in the hollow core with low losses. They have been developed to a high degree but have been held back by some apparently insurmountable practical problems. These have especially constrained their performance at the short wavelengths which are important in many applications such as high precision laser machining and materials modification. The state-of-the-art laser systems can now deliver the necessary radiation for these applications: however, a truly flexible delivery system does not currently exist. This ability to deliver the pulsed laser light flexibly from the laser system to the point of application is a key advance required to develop practical and commercially viable applications. Over the last eighteen months, researchers in this collaboration and at a couple of other laboratories across Europe have demonstrated that a much simpler fibre design can actually be far more effective than the bandgap fibres. This is especially true at long wavelengths (in the mid-infrared) and at short wavelengths (eg 1 micron wavelength and below.) Numerical simulations now suggest that these designs can be extended to offer the possibility of their outperforming any existing optical fibres at almost any optical wavelength. This proposal is to demonstrate these fibres at a range of short wavelengths and to work with four UK-based companies to establish them as useful in manufacturing and clinical environments. This involves making fibres with several designs, verifying their performance, identifying the barriers to their use and overcoming them, and then working in the laboratories of our collaborators to establish them as useful on the factory floor and also in medical and engineering measurements. Along the way, we aim to demonstrate the lowest-loss optical fibre ever (at a longer wavelength) and to investigate whether these designs can be extended to deliver laser beams with low beam quality.

Until very recently the MASER could only be used in very specialist applications such as radio astronomy. The reason for this is that cryogenic cooling and to a lesser extent, high applied magnetic fields, prohibited mass production on the grounds of both complexity and cost. Despite the fact that the MASER was discovered before the LASER these issues meant that the latter, which does not need applied magnetic fields or cooling, saw widespread adoption in a huge range of applications from bar-code readers, laser discs to laser eye surgery. In 2013 Imperial and UCL were awarded an EPSRC funded research project to produce a room temperature MASER. Although we had preliminary observations that room temperature masing was possible we had not verified this in a different laboratory setting, nor did we have a clear idea of how the masing molecule interacted with light and which crystal orientations or dopant concentrations would be optimal. This collaboration was remarkably successful achieving all the objectives we set. Now, in what is another world first, the team has constructed a diamond MASER capable of continuous-wave operation at room temperature. Our previous research has concentrated solely on organic materials as the masing medium. In this proposal we will explore the potential of masing in inorganic materials at room temperature. In doing so we will obviate two key problems encountered with organics. Problem 1 - Decay rates: The primary obstacle that prevents continuous operation in organics is the relatively long lifetime of the lowest triplet sub-level, reducing the number of pentacenes available for optical pumping (bottleneck) and destroying the population inversion. Problem 2 - Heating: The organic gain medium, pentacene in p-terphenyl we first used to demonstrate a room temperature MASER cannot withstand a continuous illumination by a laser because the temperature of the terphenyl host rises above its melting point. Solution to both problems: a radical but exciting departure which will address both problems simultaneously is to explore high spin states in inorganic materials with high melting/decomposition temperature and favourable thermal conductivities (T.C.): such as diamond (M.P. 3550C; T.C. 2000 W/mK) and silicon carbide (2730C; T.C. 120 W/mK). Very recently we observed masing at room temperature in diamond exploiting NV centres. This means we can build upon a huge wealth of research in the UK and elsewhere on diamond NV centres. Again there is much research exploring defects in SiC that we can build on. We have initiated a collaboration with the group of Prof. Dr. Vladimir Dyakonov at Würzburg group who are currently exploring SiC. REF. https://arxiv.org/pdf/1709.00052.pdf Achieving this would further establish without doubt the UK as the key place to carry out fundamental research on the topic of room temperature MASERs.