## 3 DUV Laser at 193 nm  
The reported typical results of the solid-state DUV laser at 193 nm in the past decades are listed in Table 2.  
Two methods are usually applied to generate the solid-state DUV laser at the wavelength shorter than 200 nm. One is by SHG directly from a longer wavelength; the other one is by stages of SFG. [PERSON] et al. and [PERSON] et al., respectively, summarized the nonlinear optical materials for DUV generation from 5 th to 7 th harmonics of Nd:YAG laser at 1064 nm [35; 36] by SHG and SFG including CLBO, KBBF, LBO, BBO, K\({}_{2}\)Al\({}_{2}\)B\({}_{2}\)O\({}_{7}\) (KABO) etc. Recently, coherent radiation in the vacuum ultraviolet (VUV) region was generated with a wavelength as short as 121 nm in strontium tetra borate (SBO) by \"random quasi-phase matching\" (RQPM), which was the shortest wavelength generated by second-order nonlinear optical process in a solid-state material [37]. However, this SBO crystal does not fit for the high-power DUV laser generation due to the low efficiency of around \(10^{-5}\). Table 2 summarizes the typical results of the solid-state DUV laser at 193 nm by use of the different nonlinear crystals. Among the above crystals, the KBBF crystal is currently known as the only one, enabling high power and short wavelength directly by SHG [28; 29; 30; 36; 38; 39]. For the application of seeding the hybrid ArF excimer laser, the DUV laser should be at the wavelength of 193 nm. In 2008, [PERSON] reported the result of 2.23 mW at 193 nm average power output directly by SHG in KBBF [28]. Later, [PERSON] et al. reported a 1.05 W 193-nm laser also by SHG in KBBF with the pulse duration of several hundred of picoseconds, of which the fundamental laser was from a Ti:Sapphire amplifier [29]. This is also the highest average power of 193-nm laser by SHG until now. In 2012, [PERSON] et al. presented a 0.2 W 193-nm laser by SHG in KBBF with nanoseconds pulse which is suitable for seeding the ArF laser [30]. In addition, CW DUV laser at 193 nm was reported by [PERSON] et al. with the output power of 15 mW in KBBF by SHG with which the fundamental laser was from an external cavity laser [27]. The wavelength conversion of SHG to 193 nm by using KBBF was successfully demonstrated by the aforementioned groups. However, the KBBF should be packed in a prism coupling device (PCD) consisting of CaF\({}_{2}\) or SiO\({}_{2}\) prisms to achieve the phase-matching condition [14; 29]. Moreover, KBBF is significantly difficult to be grown into single crystal because of the weak interaction of alkali metal and fluorine cations [35; 36], which limit its potential to be grown into large dimension and to be applied in generating a higher power 193-nm laser for seeding the hybrid ArF laser. Hence, the SFG process in other crystals is the practical method for high-power 193-nm laser generation. The cut-off \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Wavelength (nm)** & **Method** & **Crystal** & **Power/Energy** & **Repetition Rate** & **Reference** \\ \hline
193 & SHG & KBBF & 15 mW & CW & [27] \\
193 & SHG & KBBF & 2.23 mW & 8 kHz (-ns) & [28] \\
193 & SHG & KBBF & 1.05 W & 1 kHz (-ps) & [29] \\
193 & SHG & KBBF & 0.2 W & 6 kHz (-ns) & [30] \\
193.4 & SFG (234.1 nm + 1110 nm) & CLBO & 120 mW & CW & [18] \\
193 & SFG (221 nm + 1547 nm) & CLBO & 140 mW & 200 kHz (-ns) & [31] \\
193.5 & SFG (221 nm + 1553 nm) & CLBO & 310 mW & 6 kHz (-ns) & [12] \\
193 & SFG (226 nm + 1342 nm) & CLBO & 230 mW & 6 kHz (-ns) & [32] \\
193 & SFG (221 nm + 1553 nm) & CLBO & 1.02 W & 10 kHz (-ns) & [20] \\
193 & SFG (235.8 nm + 1064 nm) & KABO & 0.2 W & 10 kHz (-ns) & [33] \\
193.4 & SFG (266 nm + 708.6 nm) & BBO & 2.5 mW & 5 kHz (-ns) & [19] \\
193 & SFG (258 nm + 774 nm) & BBO & 2 μJ & 20 Hz (-fs) & [34] \\
193 & SFG (213 nm + 2100 nm) & LBO & 770 μJ & 60 Hz (-ns) & [21] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Typical results of the solid-state DUV laser at 193 nm (CW: continuous-wave; KABO: K\({}_{2}\)Al\({}_{2}\)B\({}_{2}\)O\({}_{7}\)).