Characterization of angularly resolved EUV emission from 2-µm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

Abstract

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2 µm-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2 µm) pulse creates the EUV-emitting plasma. Irradiating 30-to 300 µm-diameter targets with 2 µm laser pulses, we find that the efficiency in creating EUV light around 13.5 nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2 µm drive laser intensity (0.6-1.8 × 10 11 W cm −2 ) and pulse duration (3.7-7.4 ns) are studied. It is found that the angular dependence of the emission of light within a 2% bandwidth around 13.5 nm and within the backward 2π hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2 µm drive laser. With increasing target diameter, the emission in this 2% bandwidth becomes increasingly anisotropic, with a greater fraction of light being emitted into the hemisphere of the incoming laser beam. For direct comparison, a similar set of experiments is performed with a 1 µm-wavelength drive laser. Emission spectra, recorded in a 5.5-25.5 nm wavelength range, show significant self-absorption of light around 13.5 nm in the 1 µm case, while in the 2 µm case only an opacity-related broadening of the spectral feature at 13.5 nm is observed. This work demonstrates the enhanced capabilities and performance of 2 µm-driven plasmas produced from disk targets when compared to 1 µm-driven plasmas, providing strong motivation for the use of 2 µm lasers as drive lasers in future high-power sources of EUV light.

Microdroplet-tin plasma sources of EUV radiation driven by solid-state-lasers (Topical Review)

“…Schupp et al [72,78] and Behnke et al [34] recently reported the first experimental study of 2 µm laser-driven tin plasmas. These studies, discussed in section 3.3, confirmed the simulation results with regards to the particular promise of the 2 µm driver.…”

“…In a detailed follow-up work, Schupp et al [78] reported on the EUV emission properties of tin plasmas produced by the irradiation of pre-pulse-preformed liquid tin targets by 2 µm wavelength laser pulses (cf figure 8). In a two-pulse scheme, much like in the current industrial setting, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks (cf figure 8(c)) before the main (2 µm) pulse creates the EUV-emitting plasma.…”

Production of 13.5 nm light with 5% conversion efficiency from 2 μ m laser-driven tin microdroplet plasma

“…3 Consistently, twofold higher CEs have been found using 2 lm laser light, under otherwise comparable conditions. [16][17][18] However, the experiments using focused 2 lm light did not yield CE values competitive with the CO 2 state-of-the-art, with maximum CEs being limited to approximately 3%-and no information on 2 lm laser-driven source size is currently available. It was hypothesized 17,25,26 that CE values could increase significantly if the plasma is heated homogeneously, i.e., in a manner that is constant spatially and temporally to generate a uniform (optimum) temperature plasma.…”

“…9,[11][12][13][14] Combined with the low wall-plug efficiencies of 10 lm gas lasers, this has motivated research into plasmas driven by shorter wavelength, highly efficient solid-state lasers to strongly accelerate in recent years. [15][16][17][18][19][20][21] Radiationhydrodynamic simulations have shown promise of high CE plasma sources driven by 2-5 lm lasers 13 without the need for complex target shaping. Given the maturity of 1.064 lm solid-state laser technology, and the strong progress in 1.88 lm laser development, 20,21 studies have focused on tin plasmas generated by 1-and 2 lm laser light.…”

“…A low energy 1 lm wavelength "pre-pulse" laser preshapes the tin droplet into a flat target. 3,17,28 A second, high energy 2 lm "mainpulse" laser is later fired onto the tin target, generating an EUV emitting plasma. 17,27 We control the target diameter (Ø T ) impacted by the main-pulse by varying the delay time between pre-pulse and main-pulse, allowing for larger targets at longer delays.…”