Optical studies of laser transmission at 10.6μm

In the development of optical components, a thorough knowledge of the behaviour and performance of materials intended for use is required. This project arose from some early measurements on the laser initiation of the semiconductor-to-metal transition in vanadium dioxide thin film coatings. There are several parameters that can be measured to assess the optical behaviour of a substance, among which transmission, reflectivity and the refractive index are included. The work in this thesis develops and uses a number of techniques for assessing the high energy laser transmission of several materials at a wavelength of 1O.6μm under different experimental conditions. Spectrophotometer facilities have also been used to examine low-energy transmission between 2.5 and 26μm. The first test system was developed for investigations requiring high total incident energies. This was achieved using focused pulsed radiation from a TEA CO2 laser and the development work on this system involved pulse energy, spatial pulse profile and temporal pulse profile measurements. Attenuation was provided by a combination of calcium fluoride discs with additional polyethylene tetra phthalate sheets of varying thicknesses for fine control. Testing showed them to behave linearly over the required incident energy ranges. Pyroelectric detectors enabled both total energy and temporal observations to be made. In the case of the temporal observations, both the transmitted and reflected pulses from the specimen were observed and compared to the incident temporal profile. Temporal studies of this kind were carried out on thin film vanadium dioxide coatings on germanium substrates, matching plain germanium substrates, and indium antimonide (InSb) and cadmium mercury telluride (CMT) wafers. Incident energy against transmitted energy characteristics were also obtained for these specimens as well as for an AR coated mounted germanium window. The initiation of the semiconductor-to-metal transition (SMT) in vanadium dioxide was achieved and appeared to be power rather than energy-dependent. This was confirmed by both the time-resolved studies and transmitted energy measurement techniques. The germanium specimens behaved linearly over the total incident energy range of 0-600mJ used for testing, whilst optical limiting was observed in the InSb and CMT wafers. Damage thresholds for all specimens except the mounted germanium window were also obtained. The optical nature of the SMT in vanadium dioxide was examined further using multi shot post-sample profiling techniques. This showed the occurrence of diffraction by the laser-induced metallic state, which appeared to be acting as an optical stop. An experimental model using a substrate disc with a metal stop attached was successfully developed to examine this conjecture further. The second test system developed was based on a 6W continuous wave CO2 waveguide laser. Fixed position pyroelectric detectors were used to give transmission readings of the chopped beam through a range of low incident energies. Alternatively, the system could be operated as a scanning spectrophotometer to produce a spatial transmission profile across the diameter of a sample. As one of the problems associated with using coherent radiation is the formation of interference fringes from light reflected from the front and rear surfaces of a sample, this technique is particularly useful for illustrating fringing caused by etalon effects or optically uneven sample surfaces. Results obtained this way have been successfully compared to theoretical computer models of fringe structures. It was found to be necessary to AR coat some of the samples to simplify the measurement techniques developed and this system was used to measure the effectiveness of the AR coatings. Finally, by combining the pulsed and continuous wave lasers into a probe beam system, it was possible to observe the recovery of the vanadium dioxide coating from its laser-induced metallic state to the semiconductor state normally maintained at room temperature.