Second-order nonlinear optical (NLO) materials are of great interest for the development of the new coherent and tunable laser applications, which role is becoming significant in many fields, including medicine, material science, civil industries, etc. [1–5]. Although technologies for the construction of such lasers are highly investigated nowadays, commercially available wavelength ranges with high efficiency are still limited [6].
Based on the transparent bands, the nonlinear optical crystals can be divided into three main categories: ultraviolet, visible, and infrared. The currently available nonlinear optical materials, such as β-BaB2O4 (β-BBO) [7], LiB3O5 (LBO) [8], LiNbO3 (LN) [9], KH2PO4 (KDP) [10], and KTiOPO4 (KTP) [11], are well applicable for ultraviolet and visible regions. Despite that metal oxide NLO crystals have been developed for the visible and UV regions, nowadays great interest is focused on the IR spectral range of 2−25 µm, the so-called fingerprint region for organic and inorganic molecules. In this region, it is possible to detect hazardous and dangerous materials such as chemical warfare agents, pollutants, biohazards, explosives, and trace gases for homeland security, environmental monitoring, and industrial process controls [2]. Therefore, the development of novel nonlinear optical crystals for the infrared range is a great challenge.
The practical applications of the infrared nonlinear optical materials require few basic conditions, i.e. broad infrared transparent region, large nonlinear optical coefficient, high laser-induced damage threshold, large bandgap, moderated birefringence, and high thermal stability [12]. Over the past decades, many chalcogenides, pnictides, and oxides have shown promising properties for the development of IR nonlinear optical materials. Among a variety of materials, the wide bandgap metal chalcogenides are the most promising for nonlinear optical applications operating in the IR region of the electromagnetic spectrum [2, 13–15].
Searching for new promising nonlinear optical materials we came to the proustite crystals Ag3AsS3, which are known quite long ago as a natural silver mineral [16]. Beginning from the 1970s, Ag3AsS3 crystals are actively investigated as promising electronics materials as they are piezoelectric, pyroelectric, as well as thermo- and photosensitive semiconductors [17]. Also, the Ag3AsS3 crystals have been demonstrated nanosecond optical parametric oscillators (OPOs) [18–20]. At low temperatures, they are semiconducting ferroelectrics, and at high temperatures, superionic conductors. In line with the aforementioned requirements to the NLO materials in the IR region, Ag3AsS3 is an acentric uniaxial crystal with large non-linear optical coefficients (d31 = 30 times d36 in KDP, d22 = 50 times d36 in KDP), large refractive indices (~3.0), a large negative birefringence (no – ne > 0.2) and high transparency over a wide range (0.6-13 µm) in the IR spectral region [17, 21, 22]. Such crystals have low hardness (2–2.5 on Moss scale)[23], therefore they are easy to process mechanically. The surfaces obtained by polishing the crystals are stable to non-aggressive atmospheric conditions [24]. Moreover, Ag3AsS3 exhibits a very low melting point (~480°C) in comparison to other state-of-the-art NLO sulfides, e.g. AgGaS2 (993°C) [25], LiGaS2 (1050°C) [26] or AgGaGeS4 (845°C) [27]. This fact leads to the much lower vapor pressure of sulfur in the sealed quartz ampoules used for the crystal growth, and thus, it can be beneficial to grow a large stoichiometric Ag3AsS3 single crystal. On the other hand, such a low melting point makes this material very plausible for commercial production due to the much lower energy consumption required for material preparation. Despite many advantages of the Ag3AsS3 crystals, the material has one downside – its low laser damage threshold. Damage thresholds have been found for four laser wavelengths spanning the range 694 nm to 10.6 µm by using Q-switched lasers [28].
Aiming to explore the potential of the Ag3AsS3 material for the fabrication of the nonlinear optical crystals, in this work we investigated the effect of the rare-earth doping by Pr, Eu, and Yb on the crystal structure and optical characteristics of this compound. Although optical characteristics of proustite have been fairly extensively investigated, no studies on the effect of rare-earth alloying on their physical properties were found. The crystal structure of Ag3AsS3 is non-centrosymmetric and contains prismatic and octahedral voids. Therefore, we were able to successfully introduce the rare-earth elements into that voids in place of silver atoms. The main effect of such doping is related to the increase of the second harmonic generation intensity, which is the main requirement of the NLO materials. In the case of the 1% Yb doping of Ag3AsS3, the increase of the SHG intensity was almost 2 times, indicating the great potential of this material for practical applications.