## 1 Introduction  
Demand for eye-safe lasers has grown over recent decades in such fields as remote sensing, range finding, optical communication. Er\({}^{3+}\) lasers generate pulses at 1540 nm and are effectively used within these application fields [1; 2; 3]. Co\({}^{2+}\):MgAl\({}_{2}\)O\({}_{4}\) (Co:MALO) can be used as a passive Q-switching to obtain laser pulses in Er:glass lasers [1]. It also allows one to reduce system size, avoid high-voltage power supplies which are necessary for electro-optic Q-switches and also decrease laser costs [4]. Designing a compact laser to achieve a higher output power usually results in increased laser radiation load on its components. Co\({}^{2+}\):MgAl\({}_{2}\)O\({}_{4}\) might be one of the components within the cavity, which also limits the peak power of generated pulses [5]. The surface laser-induced damage threshold (LIDT) of any optical substrate is always much smaller than its bulk damage. During polishing of the surface, subsurface defects (SSD) such as cracks are created, which are usually filled with remnants of polishing materials [6; 7; 8]. Moreover, the mixture of polished material and polishing slurry creates a top-most subsurface layer, which is known as a Beilby layer, or redeposition layer [6; 7]. It usually has a much higher absorption of incident laser pulse comparing to the bulk substrate material. Due to this, the potential laser damage resistance of a transparent optical component is drastically reduced. Polishing crystalline materials also induces additional deformations of the bulk crystal structure near the surface, which could also reduce laser damage resistance [9]. [PERSON] et al. identified two possible mechanisms for creating structural disorders during polishing--mechanical and thermal. Thermal gradients within a thin topmost layer create waves, which propagate and may create an amorphous layer even in the absence of mechanical deformation stresses. In case of a slow polishing of a crystalline sample, the thickness of the Beilby layer might reach 0.1-1 um [10]. [PERSON] et al. observed the amorphous subsurface layer of a single crystal yttrium aluminum garnet (YAG) having a thickness of 5 nm after fine polishing for 110 min [11]. [PERSON] et al. analyzed diamond samples with high-resolution TEM, which were prepared by a novel as well as chemomechanical polishing (CMP) technique. They found an amorphous subsurface layer with a thickness of 3.9 nm for CMP-prepared samples [12]. Various post-treatment methods have been investigated and developed, in order to remove SSD and Beilby layers. These include such techniques and their combinations as wet etching [8; 13; 14], laser conditioning [15], reactive plasma/ion etching [16; 17; 18] or physical sputtering [13; 19]. Though plasma/ion-based technologies usually have much lower etching rates than other techniques, they are very attractive for being noncontact and allowing one to execute surface treatment within the vacuum chamber just before the coating process. Several investigations have shown the successful application of plasma etching for efficient increase of the LIDT of uncoated and afterwards coated fused-silica (FS) substrates for the UV spectral range without deterioration of their surface properties [16; 17]. However, to our knowledge, only a few publications exist about plasma or ion etching of laser crystals [19; 20]. [PERSON] et al. investigated the ion-beam etching of CsLiB\({}_{6}\)O\({}_{10}\) crystals using different ion energies and etching depths. They found that etching down to a 80 nm depth while keeping the ion beam voltage at 200 V allowed them to increase the initial LIDT for the 266 nm 8 ns pulses by a factor of 1.4-1.7 up to \(\sim\)17 J/cm\({}^{2}\)[19]. This group also investigated low-energy argon plasma etching of the same crystal and found that the removal of 60 nm by plasma increased the crystal surface lifetime under \(\sim\)7 W and at a 100 Hz 266 nm laser exposure 10 times, and no damage was observed over 160 h of operation, compared to as-polished surface [20].