Recently, researchers from the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) have proposed a novel method for the generation of wavelength-switchable vacuum ultraviolet (VUV) and deep UV (DUV) sources using a nonlinear conversion process in strong-field-ionized molecules.  

The generated DUV/VUV coherent radiations exhibit unique characteristics including narrow bandwidth, switchable wavelength, and fully controllable polarization. Furthermore, they were used for the generation of DUV/VUV Raman comb. Results were published in New Journal of Physics on Feb. 08, 2021. 

Coherent VUV (100-200 nm) and DUV (200-300 nm) light sources have been widely applied in numerous studies. Until now, the generation of such light sources is still challenging and mostly relying on large facilities such as free-electron lasers. Generation of table-top sources and the development of optical-field manipulation technologies in the DUV/VUV regime is highly desirable. 

In this study, the researchers chose three electronic states of CO⁺ ions to construct a cascaded quantum system, and adopt near-infrared and violet pump pulses to realize resonant excitation among these energy states.

Taking advantage of abundant vibrational energy levels of these electronic states, DUV/VUV radiations with different wavelengths can be selectively produced through resonant four-wave mixing of the two pump lasers. The polarization states of these DUV/VUV coherent radiations can be fully controlled by changing the ellipticity of the violet pump pulse. 

Moreover, the combination of femtosecond impulsive excitation and picosecond DUV probe light can facilitate efficient Raman scattering to generate DUV/VUV Raman combs with more than 100 sidebands. Together with the advantages of their high spectral resolution and flexible polarization control, these coherent light sources will open a broad range of applications ranging from high-resolution spectroscopy and surface science to photochemistry and medicine, etc. 

This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, and the Strategic Priority Research Program of CAS, among others.

Figure 1. (a) Energy-level diagram for generation of DUV/VUV radiations through resonant excitation. (b-d) Typical spectra and beam profiles of DUV/VUV radiations. (Image by SIOM) 

Figure 2. (a) Schematic diagram for generation of Raman comb through cascaded rotational Raman scattering. (b) Raman combs recorded at different O2 gas pressures. (Image by SIOM)