applied sciences Review High-Power, Solid-State, Deep Ultraviolet Laser Generation Hongwen Xuan 1,*, Hironori Igarashi 2, Shinji Ito 1, Chen Qu 2, Zhigang Zhao 1 and Yohei Kobayashi 1 1 The Institute for Solid State Physics, the University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan; [email protected] (S.I.); [email protected] (Z.Z.); [email protected] (Y.K.) 2 GIGAPHOTON INC., 400 Yokokurashinden, Oyama, Tochigi 323-8558, Japan; [email protected] (H.I.); [email protected] (C.Q.) * Correspondence: [email protected]; Tel.: +81-47-136-3356 Received: 8 November 2017; Accepted: 30 January 2018; Published: 3 February 2018 Abstract: At present, deep ultraviolet (DUV) lasers at the wavelength of fourth harmonics of 1 µm (266 nm/258 nm) and at the wavelength of 193 nm are widely utilized in science and industry. We review the generation of these DUV lasers by nonlinear frequency conversion processes using solid-state/fiber lasers as the fundamental frequency. A DUV laser at 258 nm by fourth harmonics generation (FHG) could achieve an average power of 10 W with a beam quality of M2 < 1.5. Moreover, 1 W of average power at 193 nm was obtained by sum-frequency generation (SFG). A new concept of 193-nm DUV laser generation by use of the diamond Raman laser is also introduced. A proof-of-principle experiment of the diamond Raman laser is reported with the conversion efficiency of 23% from the pump to the second Stokes wavelength, which implies the potential to generate a higher power 193 nm DUV laser in the future. Keywords: deep ultraviolet laser; frequency conversion; Raman laser 1. Introduction Deep ultraviolet (DUV) lasers are currently widely employed in various applications. For instance, a DUV laser at 260 nm has been applied as an “external seed” of a free-electron laser (FEL) with output wavelengths as short as 4.3 nm, which would make it possible to do the scientific research beyond the carbon K-edge [1]. In the industrial applications, laser machining of wide bandgap materials would be also beneficial from the DUV lasers owing to their characteristics of high photon energy [2]. In the past decades, with the fast development of the solid-state lasers and the nonlinear optical crystals, high-power DUV lasers at 266 nm/258 nm have been studied by many research groups fruitfully [3–17]. These DUV lasers are usually generated by fourth-harmonics generation (FHG) of the near-infrared (NIR) laser at 1 µm as shown in Figure1. Until now, the highest average output power of these DUV lasers was 40 W at the wavelength of 266 nm, of which the fundamental light was a high-power Nd:Y3Al5O12(Nd:YAG) laser at 1064 nm and the nonlinear optical crystal was a CsLiB6O10 (CLBO) [8]. In the scientific and industrial applications, the beam quality of the DUV laser is also an important parameter. Particularly, the beam quality of the DUV laser dramatically affects the laser machining results. The beam quality of the DUV laser is determined by that of the fundamental laser and the crystal for the frequency conversion. Recently, a high-power DUV laser (>10 W) at 258 nm was reported with a beam quality of M2 < 1.5, which was generated by FHG of a Yb:YAG laser at 1030 nm using the CLBO crystal [13]. This laser would lead to a better laser machining result compared to the previous DUV lasers with larger M2. Appl. Sci. 2018, 8, 233; doi:10.3390/app8020233 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, x 2 of 13 Appl.Yb:YAG Sci. 2018 laser, 8, 233at 1030 nm using the CLBO crystal [13]. This laser would lead to a better 2laser of 13 machining result compared to the previous DUV lasers with larger M2. Figure 1. Schematic setup of the solid-state DUV laser by FHG of 1-μm laser (DUV: deep ultra-violet; Figure 1. Schematic setup of the solid-state DUV laser by FHG of 1-µm laser (DUV: deep SHG: second-harmonics generation; FHG: fourth-harmonics generation; YAG:Y3Al5O12; BBO: β - ultra-violet; SHG: second-harmonics generation; FHG: fourth-harmonics generation; YAG:Y3Al5O12; BaB2O4; LBO: LiB3O5; CLBO: CsLiB6O10; KBBF: KBe2BO3F2). BBO: β-BaB2O4; LBO: LiB3O5; CLBO: CsLiB6O10; KBBF: KBe2BO3F2). In terms of the solid-state DUV laser at 193 nm, the laser machining as well as other applications suchIn as terms mask-inspection of the solid-state or lithography DUV laser require at 193 a nm, high-power the laser machiningDUV laser asoutput well [4,18,19]. as other applications The highest suchoutput as mask-inspectionpower of the continuous-wave or lithography require(CW) 193-nm a high-power laser was DUV 120 laser mW output by stages [4,18 of,19 sum-frequency]. The highest outputgeneration power (SFG) of the between continuous-wave DUV lasers (CW) (wavelengt 193-nmh laserlonger was than 120 193 mW nm) by stagesand corresponding of sum-frequency NIR generationlasers [4]. 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AArF high-power excimer laser, solid-state and it DUVwould seeding also result laser in with suppressing high coherence the amplified is beneficial spontaneous to the final emission output of(ASE) the hybridof the amplifier. ArF excimer laser, and it would also result in suppressing the amplified spontaneous emission (ASE)SFG of the is the amplifier. most popular method to generate the DUV laser at 193 nm. 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