Techniques for MIR rare earth doped waveguide lasers in chalcogen based glasses.

Abstract

Over the last 20 to 30 years, chalcogen based devices have received significant attention due to their promising properties and potential photonics applications in the MIR wavelength region. Emission from rare earth doped materials has therefore been an active field of research, though achieving longer wavelength emission from planar waveguide devices has proven challenging. Several issues have emerged such as dopant clustering, photoinduced absorption, and concentration dependent lifetime etc, all of which were explored in this work. This work explores improved deposition techniques, film post processing and several host materials with better rare earth ion solubility plus film consolidation as mechanisms to attempt to overcome issues faced in the past. One of the key goals of this thesis was to investigate and mitigate the effects of concentration dependent lifetime decrease in rare earth doped chalcogen films to enable high gain and high power on chip amplifiers to be built. A significant barrier to this in reactively sputtered films has been achieving good control over the film stoichiometry. Deep studies of issues in reactive sputtering and custom enhancement of the sputtering system were therefore performed and means to fabricate consistent and good quality undoped and Er doped Tellurite films found. Given the previously promising performance of Er doped As2S3 sputter deposited material, work was conducted to determine if Rapid Thermal Annealing could overcome the known limitations of these films. RTA was systematically tested and the morphological and spectroscopic properties of fabricated and annealed films characterised. Post RTA treated samples were green light irradiated to investigate PL performance for comparison with prior results. Unfortunately RTA did not prove able to resolve the known issues with water ingress to the films. The use of appropriate host glasses with high rare earth ion solubility and low photo-induced absorption was previously identified as an alternative route to high performance MIR devices. GeGaS glasses are anticipated to be good materials for this purpose. The thermal evaporation of Ge28.5 Ga6.2 S6 5.3 bulk glass was studied including the flash evaporation regime films fabricated at less than 1100C suffered significant stoichiometry variation. Even for flash evaporation where better stoichiometry was achieved there was genuine concern that the film does not have uniform composition through its depth. It was previously identified that the doping of thermally evaporated chalcogenide films needs to be performed with a rare earth doped compound rather than metal to eliminate clustering from the evaporation source. Thus, work was conducted on evaporating Erbium Sulphide, which was previously very problematic due to overheating of the deposited films as at much above 70C wafer temperature. Depositions were therefore performed at a series of temperatures from 1800C down to 1550C. Whilst successful with the deposition of Er2S3 , the work identified many challenges with the evaporation process some of which are poorly understood. Gallium Lanthanum Sulphide (GLS) has long been considered as one of the most promising candidates for chalcogenide RE doped devices. RF sputtering was used to fabricate undoped and doped films at varying Er concentrations and PL properties of as deposited and annealed films were systematically investigated. It was also found that fabricated films can easily become highly contaminated by oxygen (seemingly present as both oxides and hydroxyl groups ), which was confirmed by XPS and WDS. A potential source of high oxygen concentration in Er doped GLS films is explored and mitigation strategies discussed. Fabricated films with oxygen content identical to the target material were ultimately fabricated and characterised with attendant care. Show more