Si/SiO₂ dispersive optics paves the way to ultrafast mid-infrared lasers

Advanced broadband dispersive multilayer optical components have become key elements in modern laser systems [1]. Along with well-known dispersive mirrors operating effectively in the systems based on Ti:Sa lasers (∼800 nm) and near-infrared Yb:YAG lasers (∼1030 nm), the dispersive optics technology is being used in 2-μm laser systems based on Thulium- and Holmium[2]. Further development of the technology based on 2.4 μm Cr:ZnS-lasers will extend laser output to 3.2 μm [3,4]. Availability of dispersive optics covering the spectral range from 2 to 3.2 μm is demanded for this promising technology. Until now, the best achievements in this direction was a 1/3-octave dispersive mirror operating in the range 2.2-2.7 μm with the group delay dispersion of -200 fs² and a high-reflector compensating the third-order dispersion of -3000 fs³[5]. In the present work, a large progress in the development of dispersive optics for 2-4 μm region is reported. The novel 2/3-octave dispersive elements achieve reflectance exceeding 99.6% and group delay values of (-200 fs²) in the entire range from 2 to 3.2 μm. The advanced feature is a very small degradation of reflectance in the OH absorption wavelength range between 2.7 μm and 2.9 μm. Exploitation of typical thin-film materials such as Ta₂O₅/SiO₂ and Nb₂O₅/SiO₂ do not provide the refractive index ratios, which are large not enough to achieve both high reflectance and desired phase properties. Also, the use of thin-film materials typical for the visible-near-infrared ranges does not allow achieving negative group delay dispersion values required for the development of Cr:ZnS oscillator because of very high oscillations in group delay dispersion. In the present work, the thin-film materials Si and SiO₂ providing a high ratio of the refractive index values of 2.3 are used for the first time for the design and production of dispersive elements in the infrared spectral range 2-3.2 μm.