Frequency extension of solid-state terahertz lasers
thesisposted on 28.03.2022, 20:29 authored by Tiago Almeida Ortega
Terahertz radiation has the potential to impact revolutionary real-world applications throughTHz spectroscopy and spectral imaging in various fields such as life-sciences, medicine, homeland security and in manufacturing. Terahertz laser sources based on intracavity stimulated polariton scattering (SPS) in magnesium oxide-doped lithium niobate crystals(MgO:LiNbO3; MgO:LN) have been extensively reported in the literature, typically producing terahertz radiation continuously tunable from 1 – 3 THz. This technology has proven to produce truly compact, reliable and cost-effective THz sources with frequency tunable output which can be interfaced with relatively simple detection mechanisms (portable spectrometers, Golay cells, and robust pyroelectric detectors). What is of particular significance is that the core technology is based on solid state laser design, which is well established, and the components are well developed. This thesis offers new insights into the design and operation of intracavity THz lasers based on stimulated polariton scattering. Contributions to the knowledge base have been made with regards to the understanding of SPS material properties, terahertz frequency coverage and terahertz output power. The SPS crystals are investigated from first principles, with their vibrational spectra (spontaneous Raman and infrared reflection) and dispersion properties being examined. Their polariton dispersion curves, refractive index and absorption coefficient are calculated, providing valuable tools to guide the design of these sources. Exploring MgO:LiNbO3, and also two other SPS-active crystals never explored in the intracavity design, potassium titanyl phosphate (KTiOPO4; KTP) and rubidium titanyl phosphate (RbTiOPO4; RTP), significant contributions to the area of SPS laser design have been made. The use of KTP and RTP in the novel intracavity surface-emitted geometry expanded the terahertz tuning range of intracavity SPS sources up to 5.98 THz. The average output power of the terahertz field was also greatly enhanced, reaching 125 μW for a modest 6 W diode pump power. This remarkable power level is the highest ever reported from an intracavity SPS laser source, and breaks the 0.1 mW THz power barrier. Moreover, this represents an increase in one order of magnitude in output power, and two orders of magnitude in diode-to-THz conversion efficiency from previously reported intracavity SPS laser systems.The beam quality parameter of the terahertz output was also measured to be excellent incomparison to previously reported linear THz SPS sources, being MH2~1.57 and MV2~1 in the horizontal and vertical dimensions respectively, indicating a substantial increase in brightness.