Advances in quasi phase-matched optical frequency converters

Abstract

Recent advances in the growth of ferroelectric crystals with low optical losses and good structural consistency in particular KTiOPO 4 and stoichiomethic Mg:LiTaO 3 allowed a vast extension of applicability of quasi-phase-matched frequency converters. In this paper we overview recent advances in structuring these materials enabling high-energy pulse generation, extremely broadband parametric gain, and some unique applications in the quasi phase-matched crystals containing submicron-periodicity 1D structures as well as in 2D-structured crystals. IntroductionThis year, 2011, marks the 50-th anniversary of the first experimental demonstration of frequency conversion of what was then the recently invented Ruby laser by employing second harmonic generation in crystal quartz [1].Only one year later, in 1962, the seminal paper by J Armstrong and co-workers [2] proposed the principle of quasi phase-matching (QPM) as a generic method to increase the efficiency of second order interactions in different classes of materials. The QPM technique provides additional degrees of freedom allowing design of spatial and temporal properties of nonlinear interactions. Over the years, the additional capabilities afforded by the QPM techniques shifted focus in the field of nonlinear materials from the synthesis of new crystals with specific phasematching and nonlinear properties towards engineering of nonlinear interactions and refining materials which can take advantage of the QPM techniques. This shift was gradually happening over the last twenty years. The result is something which can be thought of as sort of nonlinear optics engineering toolbox which could be added to the existing and developing laser engineering methods.Regardless of apparent flexibility of the QPM techniques, its success critically depended on the initial material properties. Substantial effort over the last 10 years has been devoted to tailor the properties of the most popular oxide ferroelectrics used for QPM structure fabrication, KTiOPO 4 (KTP), LiNbO 3 (LN) and LiTaO 3 (LT). All these materials had different problems related to electrical properties, induced absorption, photorefraction, etc. These problems currently have been substantially alleviated or solved in principle either by appropriate doping, carefully tailored compositions of solid solutions of isomorphic compounds and/or modification of crystal stoichiometry. In our work we primarily utilize isomorphs of KTP and stoichiometric MgO-doped LT (Mg:SLT) for engineered nonlinear structures due to the high optical damage thresholds, low coercive fields and, importantly, possibility to fabricate periodic structures on the submicrometer scale. As will be shown in the presentation such structures enable realization of novel types of devices having no counterparts in the realm of birefringence phasematched nonlinear interactions. QPM structures for high energy and broadband parametric gainHigh-energy mid-infrared sources are required for number of applications including spectroscopy [3], remote sens...