Giant ultrafast optical nonlinearities of annealed Sb2Te3layers

International audienceThe optimization of thin Sb 2 Te 3 films in order to obtain giant ultrafast optical nonlinearities is reported. The ultrafast nonlinearities of the thin film layers are studied by the Z-scan technique. Giant saturable absorption is obtained, which is the highest ever reported, by means of the Z-scan technique. There is a growing interest in materials exhibiting signicant ultrafast nonlinear optical (NLO) properties. 1 In particular, the saturable absorption is currently intensively being investigated because it can be directly applied to the mode-locking of laser systems. 2 Many different materials, for example, carbon nano-tubes, 3 graphene 4 and perovskites 5,6 have been reported in the past to exhibit a signicant saturable absorption. Moreover, nowadays, intense research is being carried out in the eld of nano-patterning materials in order to offer them unique optical/nonlinear optical properties. 7 This has a strong impact in many applications, for example, optical ltering, 8,9 diffractive element fabrication 10 and optical data storage, 11 to name a few. The direct laser writing technique, 12 using fs laser pulses, is one of the most promising and cost-efficient techniques used in order to pattern the matter with custom shapes. However, the resolutions are usually limited by diffraction. The latter can be surpassed by nonlinear phenomena, but the resolutions obtained are very highly dependent on the material used for patterning. 13,14 An improved way to surpass the diffraction limit is to use a super-resolution mask exhibiting a signicant saturable absorption. 15 A family of 2D materials, called topological insulators, exhibiting a "graphene-like" energy band gap structure, is currently very intensively being investigated, as these materials exhibit very high optical nonlinearities. 16,17 Among them, Sb 2 Te 3 is currently among the best candidates for the mode-locking of ultrafast laser systems. 18,19 Despite the high interest of this material for photonic applications, its NLO parameters at the ultrafast regime, are rather unknown. There is currently only one study reporting on the nonlinear absorption coefficient (b) of Sb 2 Te 3 using ultrafast pulses. 20 An optimization of its NLO response is very complex because it highly depends on its crystallization state. Research on this topic is of high interest because it will further enhance the applicability of Sb 2 Te 3 in the eld of photonics. Our group is aiming at using thin Sb 2 Te 3 layers as super-resolution masks in the ultrafast regime. Towards this objective , a thorough optimization of the crystallization state of Sb 2 Te 3 thin lms was initiated in order to obtain an optimal saturable absorption. In this work deposition, post treatment and NLO studies are presented. The crystallization was done by oven and laser annealing. In both cases, unprecedented nonlinear absorption parameters have been found, which are compared with state-of-the-art materials. The Sb 2 Te 3 thin lms were deposited using the electron beam deposition technique. The thickness of the lms has been determined to be 24 nm, by locally removing the thin layer and performing atomic force microscopy measurements (Fig. S1 †). A 10 nm thick silica layer was deposited over Sb 2 Te 3 , which allowed us to perform annealing in air without the oxidation of the layers. The substrate and the thin silica layer have been found to exhibit no optical nonlinearities under the experimental conditions used in this work. Aer the deposition, annealing was performed in two different ways. Firstly, the thin lms were annealed by using an oven at 300 C for 24 hours, following a procedure previously reported by our group. 21 A second alternative way to crystallize the thin lms has been employed in this work. In this case, laser annealing was performed by using a pump-probe setup (Fig. S2 and description in the ESI †). 22 During laser annealing the temperature on the sample was in situ measured by using a thermal camera throughout crystallization. Moreover, the relative reectance, which is well-known to be strongly dependent upon the crystallization state of the Sb 2 Te 3 material, 23 wa