# Photosensitivity of Infrared Glasses under Femtosecond Laser Direct Writing for mid-IR Applications
## 1 Introduction  
Direct laser writing consists in focusing a high-intensity laser inside a crystalline or an amorphous material in order to locally modify its physical properties [1]. Since their development about thirty years ago, femtosecond lasers have been used in particular for this laser writing technique. Thanks to ultra-short pulses, these lasers can reach intensities higher than TW/cm\({}^{2}\) at the focal point. With non-linear absorption of the material due to these high intensities, the material is permanently modified in a confined volume of a few \(\upmu\)m\({}^{3}\). This direct femtosecond laser writing (FLDW) is therefore of great interest for its numerous writing possibilities, as well as for a number of accessible materials, especially in the fabrication of complex 3D photonic components. Indeed, current advanced femtosecond laser systems offer a myriad of material interactions in glassy media: nanocrystals precipitation [2], metallic nanoparticles precipitation and shaping [3], nanogratings formation [4], or 3D refractive index changes profiling (based on nanoscale structural changes) [1]. These materials' interactions depend on both the laser parameters and the material chemical composition. The FLDW technique is especially studied for the creation of nanogratings and optical waveguides [5] but attracts the attention of researchers in many different fields such as diffractive and polarization optics but also gradient index optics [6] and geometric phase optics [7]. This technique has been extensively studied in silica in order to fabricate new integrated optical components: waveguides, lenses, birefringent optics, etc. However, these studies are mainly limited to visible and near-infrared wavelengths [1,8,9]. Today, the studies of the FLDW technique are growing in materials such as silicon performing in infrared [10]. Indeed, the development of integrated optical components in infrared has many attractive civilian (home automation, smartphones, automotive, etc.) and military applications. The materials that can be used at mid-wavelength infrared (MWIR: 3-5 \(\upmu\)m) and long-wavelength infrared (LWIR: 8-12 \(\upmu\)m), besides silicon, are non-silicate glasses such as chalcogenides [11], fluorides [12], and heavy oxide glasses [13]. In particular, chalcogenide materials have a very wide transmission band from visible to LWIR (8-14 \(\upmu\)m). Despite their growing use in the field of infrared optical imaging for their optical qualities, studies of their utilization with fs-laser writing are mainly limited to the fabrication of mid-IR waveguides for space applications [5; 12]. The use of the FLDW technique to fabricate complex optical components embedded in these infrared materials is a fairly new and developing area [5; 14]. In order to open the door to the fast prototyping of complex 3D optics (geometric phase lens, gradient index optics, etc.) for infrared applications, our study seeks to understand the modifications implemented in a set of well-chosen glasses as a function of the writing parameters (laser pulse duration, pulse energy, writing speed, etc.). We are particularly interested in the quantification of both phase and refractive index modifications, which would allow the 3D inscription of freeform objects that are more and more developed in optical imaging [15]. According to the literature, chalcogenide glasses can reach very high index contrasts: higher than \(\sim\)5 \(\times\) 10\({}^{-2}\)[16], while heavy oxides reach lower values of a few 10\({}^{-3}\)[13], but they have better thermal and mechanical stability. Besides chalcogenides, heavy oxide glasses transmit only up to MWIR, but they are chemically stable, manufacturable by conventional means (melt-quenching), and mechanically resistant. In this paper, we study the photosensitivity of one chalcogenide glass: 75(GeS\({}_{2}\))-15(In\({}_{2}\)S\({}_{3}\))-10(CsCl), also named GeS\({}_{2}\).