DETERMINACION DE LA POBLACION CANINA Y FELINA ESTIMADA CON

This thesis has described the design, implementation and characterisation of novel intracavity terahertz optical parametric oscillators that make the parametric technique an attractive option for investigating the “THz Gap”. Indeed the original system outlined in Chapter 3 has been protected by several patents, which have been licensed to the company MSquared Lasers of Glasgow who have commercialised the system, and are now selling units internationally.

The advantage of the intracavity cavity design in reducing the pump energy requirements for the THz OPO has been proven, explicitly so in Chapter 3 where the laser pulse energy that was required to reach threshold was an order of magnitude lower than a similar extracavity THz SRO. The temporal profile of the laser pulse in this system shows the near optimal condition of the cavity dumping of the laser field into the downconverted waves at the peak of the pulse. This condition allows for maximal extraction of the population inversion stored in the laser crystal (when running several times above laser threshold) providing efficient operation. This condition is not guaranteed however, and it seems somewhat fortuitous that this was systematically observed in various OPOs when running at twice OPO threshold, as it is dependent on both the laser build up time as well as that of the idler. Modelling of the laser and idler pulse dynamics should be undertaken to find the dependence of this characteristic on the idler cavity properties (with respect to cavity length and mirror reflectivities) for a given laser design.

The pulse amplitude jitter of THz wave was found to be significant and correlation with the idler pulse energy was poorer than expected. This can be worked around by implementing reference detectors, but it is still an undesirable attribute. The proposal made in Chapter 3 that the THz pulse energy is being unevenly split between the two opposite generation directions (corresponding to the two opposite passes of the pump and idler waves) needs to be investigated, perhaps by the use of a thin (in the plane of the phasematching process) MgO:LiNbO3 crystal such that the THz pulses can be

reflectivity mirrors to increase the number of round trips taken while maintaining a constant idler wave build up time.

The terahertz beam profile was shown to be Gaussian like and of good quality in the vertical component of the beam in particular exhibiting an M2 factor of 2.3. The Horizontal beam quality was not as good, having an M2factor of 6.7, but this could be improved by moving to a single prism output coupling method rather than an array that distorts the terahertz wave front. The benefits of this generally good beam quality are in the increased propagation lengths and smaller focal spot sizes achievable. These features are advantageous for imaging systems and aid the beam handling and collection of the terahertz wave over longer distances, and assist in making the intracavity cavity THz OPO an attractive source.

The disappointing attribute of generating THz wave parametrically in MgO:LiNbO3

crystals is the low energy of THz pulse that is extracted. In this research maximum pulse energies of 10 to 20nJ were regularly obtained, which is a significant improvement over extracavity pumped systems in the literature and capable of saturating the bolometer used for measurements, giving dynamic ranges in excess of 10,000:1. However it is not as easily detected using cheap and robust pyroelectric detectors (dynamic ranges of ~100:1 were obtained with various pyroelectric elements). The disappointment arises from the fact that, internally to the crystal, pulses of several microjoules are being generated, and it is material absorption that limits the output power. There are a number of ways this absorption may be reduced: the implementation of a quasi-phasematching scheme that generates a faster exiting THz wave (although this has been found in this research to be non trivial), the optimum pump beam profile should be investigated to find if an advantage can be gained this way (for instance the implementation of a tall, thin profile that can edge up to the crystal interface more closely – although this would reduce the THz walkoff length), or perhaps most the promising route is to look at implementing new nonlinear materials such as orientation-patterned gallium arsenide (OP GaAs) – which has already been demonstrated in optical rectification and difference frequency generation techniques.

The techniques used for narrowing the THz wave output by restricting the pump and idler linewidths proved successful and what are estimated to be transform limited linewidths were obtained with injection seeding. This came at the cost of greatly

increased system complexity compared to utilising intracavity etalons. A thorough analysis of the optimal etalon choices should be made to attempt to approach the transform limit by this method, and to enable the system (if possible) to be tuned continuously with this narrow linewidth across the whole 1 to 3THz tuning range. Once this has been accomplished the construction of a THz spectrometer would then allow a critical comparison of this method with that of THz-TDS and FTIR techniques.

The implementation of quasi phasematching techniques in this research ran in to several difficulties. Of most importance was the realisation that poled crystals designed for the orthogonal generation of THz radiation, to enable its rapid exit and therefore minimum absorption, could have a second solution due to the bidirectionality of the grating vector. In the crystals modelled and implemented this second solution was found to be the solution to reach threshold first, and was not propagating at an extractible angle. The investigation of quasi phasematching schemes is continuing in the group, with the aims of designing crystals that will allow collinear propagation of pump and idler waves (to increase the nonlinear gain region and also allow cascaded downconversion processes to be phasematched) whilst producing extractable THz radiation, and also to extend the tuning range of the non- collinear phasematching scheme to higher and lower frequencies. The investigation of the interplay between laser and idler wave build up times would provide insights into how the collinear pump and idler (where the laser cavity is also the idler cavity) system might be successfully realised. Future systems will be able to avoid the reduced intracavity beam waists of the devices reported herein, and so alleviate the crystal damage problems encountered, by taking advantage of advances in crystal poling techniques that now allow 3mm thick PPLN to be produced. A deeper understanding of the refractive index of MgO:LiNbO3 in the terahertz range would be

of great benefit to this work, by improving the accuracy of the phasematching calculations.

It was realised early in the research presented herein that the advantages in having a room temperature, robust and simple source technology are somewhat obviated if a cryogenically cooled detection system was required. To this end the development of

electro-optic effect in a material such as ZnTe, which would then be observed via the change in the polarisation state of a probe laser. This technique could theoretically resolve the temporal profiles of the terahertz pulses. An additional detector technology has proven utile – the Golay cell. Golay cells are not as robust or inexpensive as pyroelectric elements, but the one under test (a Tydex GC1-P) sits between the pyroelectric detectors and the composite silicon bolometer, typically providing a dynamic range in excess of 1000:1.

Overall the research presented in this thesis has broken new ground in the parametric generation of THz waves, yielding 5 patents and 4 research papers (as well as several conference presentations). The work has had impact in the field in that a system was developed which, through licensing and knowledge transfer activity with MSquared lasers, is now enabling more research groups to delve into the THz Gap.