Abstract
The vacuum ultraviolet (VUV) light source has been used for soft ionization in many applications. A simple method to get VUV light is by tripling the third harmonic of a Neodymium doped laser (1064 nm) in xenon gas. In order to get more fluence, higher repetition rate and conversion efficiency to enable more applications, we have investigated using a picosecond laser with a 50 kHz repetition rate as our 1064 nm light source. By using two lithium triborate (LBO) crystals, we obtained 354.7 nm UV light with >50% conversion efficiency from the fundamental. Taking this UV light and tripling it in a cell filled with a mixture of xenon and argon gases, the overall VUV power is expected to reach ~100 μW before optical loss. We tried to improve this conversion efficiency of VUV generation with a hollow waveguide. We used a quartz hollow waveguide that has a 100 μm inner diameter, 1200 μm outer diameter and is 10.5 cm long. Under the same focusing condition (32 μm beam-waist) and input average power (1 W) as in the gas mixing case, VUV output is enhanced by 1.4~2 times compared to free focusing with gas mixing. This conversion efficiency is not as high as we had expected. We attribute this low enhancement to scattering and propagation losses in the waveguide and to two-photon ionization. This means that if one wants to have a better conversion efficiency, one shall need to use a waveguide that has a larger inner diameter to reduce waveguide loss and two-photon ionization loss. We suggest that the hollow waveguide (capillary) shall be used when the laser peak power is large enough (for example 20 MW). On the other hand, if the laser peak power is small, we suggest to employ tight-focusing to generate VUV photons at an intensity just below the level where Kerr effect is induced.