*Editorial* **Editorial for the Special Issue on "Advances in Middle Infrared Laser Crystals and Its Applications"**

**Xiaoming Duan 1,\*, Renqin Dou 2,\*, Linjun Li 3,\* and Xiaotao Yang 4,\***


In the past two decades, there has been a growing interest in middle infrared (mid-IR) laser crystals and its application to achieve mid-IR laser radiations, which has been benefited by the development of novel mid-infrared crystals and the improving quality of traditional mid-IR crystals. The recent progress in crystal growth, theoretical modelling, and generation of mid-IR laser radiations has offered a new perspective for design and growth of mid-IR crystals. In this Special Issue of *Crystals*, we gathered sixteen peerreviewed papers that shed light on recent advances in the field of mid-IR laser crystals and their applications.

Li et al. reported the lasing performance of a Ho:YVO4 crystal under laser diode (LD) pumping conditions [1]. With double-pass-pumped architecture, up to 8.7 W continuouswave (CW) output power at 2052.4 nm was achieved. In addition, good beam quality factors in horizontal and vertical directions were also obtained. Yudin et al. explored the influence of angstrom-scale roughness on the laser-induced damage threshold of ZnGeP2 crystal [2]. In the polishing process, the magnetorheological processing technology was used. Samples of the ZnGeP2 with an Angstrom level of surface roughness were achieved. The laser-induced damage threshold value at the indicated orders of magnitude of the surface roughness parameters was determined by the number of point depressions. The influence on the characteristics of multilayer Interference antireflection coatings was also investigated, based on Nb, Si, and Al oxides on the laser-induced damage threshold of ZnGeP2 crystal [3]. Under the excitation of a 2.09-μm Ho:YAG laser, the effect of the defect structure and the parameters of antireflection interference coatings based on alternating layers of Nb2O5/Al2O3 and Nb2O5/SiO2 layers on the laser-induced damage threshold of ZnGeP2 crystals were determined. Experimental results indicate that the silicon conglomerates in an interference antireflection coating is beneficial to decrease the laser-induced damage threshold of the ZnGeP2 crystal. Li et al. prepared the β-Ga2O3 saturable absorberby the optical floating zone method [4], which was used in subsequent work, the 1.08-μm Nd:GYAP laser produces 606.54-ns pulse width at repetition rate of 344.06 kHz. Experimental results present that the β-Ga2O3 crystal has great potential for the development of the 1-μm pulsed laser. Liao et al. explored the energy transfer and cross-relaxation induced 2.78 μm emission in Er3+/Tm3+: PbF2 crystal [5], which was grown by the Bridgman method. The spectroscopic properties, energy transfer mechanism, and first-principles calculations of as-grown crystals were investigated in detail. This work provides an avenue to design mid-IR laser materials with good performance. Wang et al. investigated the characteristics of plasma generated by infrared pulse laser-induced fused silica [6]. Based on the theory of fluid mechanics and gas dynamics, a two-dimensional axisymmetric gas dynamic model was established to simulate the plasma generation process

**Citation:** Duan, X.; Dou, R.; Li, L.; Yang, X. Editorial for the Special Issue on "Advances in Middle Infrared Laser Crystals and Its Applications". *Crystals* **2022**, *12*, 643. https://doi.org/10.3390/ cryst12050643

Received: 25 April 2022 Accepted: 26 April 2022 Published: 30 April 2022

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of fused silica induced by a millisecond pulse laser. The maximum expansion velocity of the laser-induced plasma was calculated. The experimental results verify the correctness of theoretical calculations. Kim et al. theoretically and numerically investigated the broadband second-harmonic properties of non-oxide mid-IR nonlinear crystals [7]. Chalcopyrite semiconductors, defect chalcopyrite and orthorhombic ternary chalcogenides have also been demonstrated. Beam propagation directions, spectral positions of resonance, effective nonlinearities, spatial walk-offs between interacting beams and spectral bandwidths were analyzed. Zheng et al. investigated the mid-IR optical property of Dy:CaF2-SrF2 crystal [8], which was fabricated by a multicrucible temperature gradient method. Sellmeier dispersion formula, absorption characteristics and fluorescence properties were demonstrated. The results indicate that the Dy:CaF2-SrF2 crystal is a promising candidate for compact mid-IR lasers. Chen et al. show the internal mechanism of APD photocurrent characteristics under ms-level pulsed infrared laser irradiation [9]. The sampling current characteristics of the external circuit and the internal mechanism of the current generation in APD were studied. Chen et al. also investigated the temperature-rise characteristics of silicon avalanche photodiodes in different external capacitance circuits irradiated by the same laser pulse [10]. Zhang et al. explored the formation laws of direction of Fano line-shape in a ring MIM plasmonic waveguide side-coupled with a rectangular resonator and nano-sensing analysis of multiple fano resonances [11]. Du et al. presented an experimental investigation of double-end pumped Tm, Ho:GdVO4 laser [12]. With operating temperature of 77 K, the output characteristics of Tm, Ho:GdVO4 laser were studied, where the continuous-wave and Q-switching performance were also demonstrated. Ni et al. investigated the mid-far properties of nonlinear crystal AGGSe [13]. The large AGGSe single crystal of 35 mm diameter and 80 mm length was obtained by the seed-aided Bridgman method. X-ray diffraction, rocking curve and transmission spectrum was used to characterize the crystalline quality. In addition, the 8 μm frequency doubling was also presented. Qian et al. reported a high energy, narrow pulse width, long-wave infrared ZGP optical parametric oscillator [14]. The maximum average output powers of 3.15 W at 8.2 μm and 11.4 W at 2.8 μm were achieved. The minimum pulse width was approximately 8.1 ns. Niu et al. reported an efficient pulsed Ho:YAP laser pumped at 1989 nm [15]. When the absorbed power was 30 W, an average power of 18.02 W with the pulse width of 104.2 ns acousto-optic (AO) Q-switched Ho:YAP laser was obtained at a repetition frequency of 10 kHz. Ma et al. experimentally investigated the plasma plume analysis of long pulse laser irradiates CFRP and GFRP composite materials [16]. The results show that GFRP is more vulnerable to breakdown than CFRP under the same conditions.

As shown in this Special Issue of *Crystals*, the study of mid-IR crystals and its applications continues to grow and expand as we, as a community, strive to acquire further understanding of the underlying potential of these crystals. The goal is to bring these and other new concepts closer to application for mid-IR crystals and beyond.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

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