Abstract
Tunable coherent light sources operating in the vacuum ultraviolet (VUV) region in the 100–200-nm (6–12 eV) wavelength range have important spectroscopic applications in many research fields, including time-resolved angle-resolved photoemission spectroscopy. Recent advances in laser technology have enabled the upconversion of visible femtosecond lasers to the vacuum and extreme ultraviolet regions. However, the complexity of their experimental setups and the scarcity of bulk nonlinear crystals for VUV generation have hampered its widespread use. Here, we propose the use of a free-standing dielectric nanomembrane as a simple and practical method for tunable VUV generation. We demonstrate that third harmonic VUV light is generated with sufficient intensity for spectroscopic applications from commercially available SiO2 nanomembranes of submicron thicknesses under excitation with visible femtosecond laser pulses. The submicron thickness of the nanomembranes is optimal for maximizing VUV generation efficiency and prevents self-phase modulation and spectral broadening of the fundamental beam. The observed VUV photons are up to 107 photons per pulse at 157 nm with a 1-kHz repetition rate, corresponding to a conversion efficiency of 10−6. Moreover, the central VUV wavelength can be tuned in the 146–190-nm wavelength range by changing the fundamental wavelength. We also explore material and thickness dependence with experiments and calculations. The presented results suggest that dielectric nanomembranes can be used as practical nonlinear media for VUV spectroscopic applications.
Highlights
Vacuum ultraviolet (VUV) coherent light sources have been a powerful spectroscopic probe,1 allowing the observation of the electronic states of excited atoms2 and molecules.3 In the field of life sciences, electronic circular dichroism (ECD) is a powerful tool because of its sensitivity to structural conformation of biomolecules.4 ECD measurements require efficient sources of VUV light5 but have already shown promise for probing important biomolecules.6,7Another application that has recently emerged is time-resolved angle-resolved photoemission spectroscopy (ARPES).8–11 By combining ARPES with pulsed lasers in a conventional pump–probe setup, it is possible to directly explore the dynamics of nonequilibrium electronic states
We demonstrate that third harmonic generation (THG) in SiO2 nanomembranes excited at the fundamental 470-nm wavelength with commercially available femtosecond optical parametric amplifiers (OPAs) enables coherent VUV light generation at 157-nm wavelength with sufficient photon flux (∼107 photons/pulse with a 1-kHz repetition rate; 1010 photon/s) to be used as a probe beam for VUV spectroscopy, including laser ARPES
We showed that a 300-nm SiO2 nanomembrane excited by femtosecond pulses radiated from an OPA pumped by a regenerative amplifier can generate 7.9-eV (157 nm) photons at a 1-kHz repetition rate, with 1.4 × 107 photon/pulse, which corresponds to 18.1 nW and the conversion efficiency of as much as 10−6
Summary
Vacuum ultraviolet (VUV) coherent light sources have been a powerful spectroscopic probe, allowing the observation of the electronic states of excited atoms and molecules. In the field of life sciences, electronic circular dichroism (ECD) is a powerful tool because of its sensitivity to structural conformation of biomolecules. ECD measurements require efficient sources of VUV light but have already shown promise for probing important biomolecules.. In the case of femtosecond laser pulse excitation, Emax(ω) is limited by two major factors: the first is the laser damage threshold of solid-state materials; the second is self-phase modulation incurred during propagation through a bulk substrate material Such nonlinear propagation effects significantly reduce the maximum electric field amplitude of the fundamental beam and the efficiency of wavelength conversion when femtosecond pulses with energies from micro- to milli-joules are used. In the method of the previous research, only a very thin region near the surface contributes to VUV generation, the fundamental wave propagates through a bulk substrate before reaching the nonlinear medium, and the effects of nonlinear propagation there are inevitable To overcome these problems for increasing the fundamental pulse intensity, in this study, we propose and demonstrate the use of dielectric free-standing nanomembranes with a thickness of several hundred nanometers or less as a new solid-state material for VUV THG. Owing to these excellent properties and because these dielectric nanomembranes are commercially available and easy to handle, they are promising and practical nonlinear media for wavelength conversion to the VUV region for spectroscopic applications
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