# Few-Cycle, \(\mu\)J-Class, Deep-UV Source from Gas Media
## 3 Results and Discussion  
Figure 3 shows the measured DUV energy (a,b) and the spectra (c,d) as functions of the gas cell position, for 2 bar Ne (a,c) and 0.8 bar Ar (b,d) gas targets. Figure 3e,f shows the measured DUV energies when the supplied gas pressures are changed at fixed cell locations. In the data shown in Figure 3a,b, reflection losses from the window, prism, and  
Figure 1: **(a)** Schematic experimental setup. A gas cell for THG is housed in a small chamber, nested inside a larger main chamber. **(b)** Geometry of the laser beam near the focus. z\({}_{0}\) represents the gas cell position. A negative z\({}_{0}\) value means the gas cell is positioned before the laser focus and vice versa. The laser focus is at z\({}_{0}\) = 0 mm. Figure 2: **(a)** Spectra of the NIR laser before and after transmission through the gas cell, measured when the gas cell was positioned at the tightest focus position. The transmitted NIR spectrum was blue-shifted (higher photon energy). **(b)** Mean photon energies of the transmitted NIR laser as a function of the cell position. The amount of blue-shift was used to determine the best focus position z\({}_{0}\) = 0 mm, which is 18.25 mm in **(b)**. lens, were considered. In the case of Ne gas, the highest DUV energy was generated at the laser focus, as shown in Figure 3a,c. In contrast, the results from Ar gas in Figure 3b,d show that the DUV yield is lowest at the laser focus, while two maxima are located on either side of the laser focus. For a qualitative explanation of this cell position dependence, we numerically estimated the THG yield (Figure 4c,d) based on a zeroth-order approximation, which is proportional to the product of neutral atomic density, the cube of the electric field strength, and the \(\chi^{(3)}\) of the corresponding atom. The ADK tunneling ionization rate is used to estimate the depletion of the neutral atoms [25]. The experimental result is consistent with the numerical estimation shown in Figure 4, in which the ionization of Ne atoms is not significant (Figure 4a,c) in comparison to the Ar case (Figure 4b,d). As estimated by the numerical simulation shown in Figure 4b, a strong depletion of Ar atoms constitutes the low DUV yield at the focus. Based on this result, we fixed the cell position at z\({}_{0}\) = 16 mm in the case of Ar gas, where a maximum DUV yield was observed. Afterward, the gas pressure was changed from 0 to 2 bar. The highest DUV yield was observed at 0.8 bar. When a much higher pressure was applied, the DUV yield was reduced due to the increased plasma density and, hence, the increased nonlinearity. For Ne, with the cell located at the focus position (z\({}_{0}\) = 0 mm), a gradual increase in the DUV yield was observed by supplying higher gas pressure as shown in Figure 3e, which is in agreement with other experimental results [14]. Figure 3: DUV pulse energies and spectra are plotted by the cell position when 2 bar Ne (**a,c**), and 0.8 bar Ar (**b,d**) targets are used. (**e,f**) show the DUV pulse energies at fixed cell positions, measured by changing the gas pressure: (**e**) Ne and (**f**) Ar. Ar has a larger third-order susceptibility (\(\chi^{(3)}\) = 0.0441 (a.u.)) and a lower ionization energy (\(I_{\text{P}}\) = 15.7 eV) in comparison to Ne (\(\chi^{(3)}\) = 0.00312 (a.u. ), \(I_{\text{P}}\) = 21.5 eV) [26]. Therefore, the results from Ar show relatively high DUV yields, even in the low gas pressure (below 1 bar). As a trade-off, the neutral Ar atoms are significantly depleted at the focus as shown in Figure 4b. In general, an ionized atom has a lower susceptibility than the neutral atom [27], meaning inferior production of UV photons. Therefore, when Ar is used as a THG medium, it is not recommended to place the gas target at the laser focus. The cell position scanning is an essential task to find an optimal condition for THG. Asymmetry in the DUV yield (dashed lines in Figure 3a,b) with respect to the gas cell location is noted, which cannot be reproduced with the zeroth-order approximation shown in Figure 4c,d.