Three-Dimensional Mid-Infrared Photonics Recent Progress in Ultrafast Laser Writing of Waveguides A. Ródenas 1* , R. R. Thomson 1 , G. Martin 2 , P. Kern 2 , and A. K. Kar 1 1 School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh EH14 4AS, Scotland, United Kingdom 2 UJF-Grenoble 1/ CNRS-INSU, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), UMR 5274, France * a.rodenas@hw.ac.uk Abstract— We present here our recent progress in the three- dimensional (3D) direct laser writing (DLW) of step-index core waveguides inside diverse technologically relevant dielectric substrates, with specific emphasis on the demonstration of DLW mid-infrared waveguiding in the whole transparency range of these materials. IR photonics; lithium niobate; YAG; chalcogenide; laser writing; laser microfabrication; three-dimensional photonics I. INTRODUCTION The mid-infrared (MIR) range of the electromagnetic spectrum, between 3 µm to 25 µm (100-12 THz), is a key region for a large number of photonic applications such as: sensing in the medical, industrial and environmental fields, high resolution molecular spectroscopy, remote thermal imaging in modern satellites, space science, free-space communication, automotive and aerospace industries, and ground or spaceborne astronomy. MIR light suffers from less Rayleigh scattering than its near-IR or Visible counterparts, and it is also the region where the Earth’s atmosphere exhibits important transmission windows for either Earth or space observation: the so-called L (3-4 µm), M (4.6-5 µm) and N (8- 12 µm) bands. The MIR is the spectral region which covers the fingerprints of most molecular energy transitions, enabling a vast amount of applications to be developed such as for instance the direct detection of greenhouse gases like CO 2 and CH 4 , or of pollutants such as HCN and CF 4 . Within astronomy, the MIR is also an exciting observing window, for example to distinguish the heat signature of an Earth-like planet from its host star, to reveal the chemical make-up of remote planets in the search for essential bio-markers such as H 2 O, CO 2 or O 3 , or to study warm and distant objects, like galaxies, newly forming stars, growing black-holes, comets, asteroids or stellar dust. Regardless these important potential applications, MIR photonics are still in its maiden infancy. IO technologies are now extremely well developed for the visible and near-infrared range; but the situation is quite different for the MIR spectral domain. Despite a number of two-dimensional (2D) MIR lithographic fabrication approaches have been recently demonstrated, these are all based on multiple-step surface processing and/or make use of thin-films for vertical confinement, therefore constraining the optical designs to strict in-plane propagations. In this presentation we review our recent progress on the DLW of waveguides (Wgs) for near future 3D MIR photonic technologies. The Wgs are fabricated by an ultrafast ultrashort- pulse laser-writing process which enables the incorporation of the optical circuits into any other instrument device or substrate, as far as the material optical absorption band edge is below the laser wavelength, and without requiring any lithography or clean room facility [1]. With the 3D DLW technique the as-fabricated Wgs typically have average propagation losses in the 0.5 dB/cm range, the Wg cores can be sized and tailored in index contrast to match the specific numerical aperture or modality of connecting fibers, and more importantly, they can be spatially positioned at will inside the sample, therefore giving free access to new refractive-index topologies which were formerly impossible with planar fabrication techniques, and allowing to create photonic devices with unique optical properties which open up the range of imaginable designs, such as photonic lanterns [2] or complex evanescent coupling schemes with before unforeseen properties [3]. Figure 1. 3D 3-Telescope Beam Combiner for the N-Band. This design was embedded 350 µm deep in the substrate and consists of three input/output combining 3D Y-junctions with zero cross-talk crossovers. All scales are in mm’s. [4] Yet, whereas many of these design possibilities have increasingly been tested and reported since the field of Wg laser writing started in 1996, this has been only done for the visible and near-IR range, and there would be no report of guiding above ~2 µm wavelengths until 2011, when the first Wgs capable of guiding in the whole transparency range of materials such as chalcogenide sulphide glass (≤ 11 µm) were reported [4]. Here we present recent results on the fabrication and preliminary optical characterization of MIR step-index