The atomic and molecular ultrafast dynamics research team of the Innovation Academy for Precision Measurement Science and Technology (APM) has recently made significant progress in the research and development of vacuum ultraviolet/extreme ultraviolet optical frequency combs (VUV/XUV optical frequency combs). Based on the independently-built VUV/XUV optical frequency comb platform in the early stage, the team members innovatively designed a new type of coupling output device, successfully increasing the coupling output efficiency of the XUV optical frequency comb by 2 times. By introducing the optical field vortex characteristics during the generation process of the VUV/XUV optical frequency combs, they achieved the output of ultraviolet optical frequency combs with Laguerre-Gaussian transverse modes.
As a new type of narrow-linewidth, mesa-type coherent light source, vacuum ultraviolet/extreme ultraviolet (VUV/XUV) optical frequency combs hold significant application value in the field of precision measurement physics and have drawn extensive attention from scientific researchers. Conducting precise spectroscopic measurements on few-electron atomic systems in the vacuum ultraviolet (VUV) band can enable the exploration of the applicable boundaries of quantum electrodynamics (QED) theory, the precise determination of fundamental physical constants, etc. Applying vacuum ultraviolet (VUV) optical frequency comb technology to the precise measurement of thorium-229 nuclear transition spectra will significantly promote the establishment of a new type of optical clock-the nuclear optical clock. The Atomic and Molecular Ultrafast Dynamics Team of APM initiated the independent research and development of vacuum ultraviolet/extreme ultraviolet (VUV/XUV) optical frequency combs in 2016. In 2020, they took the lead in successfully developing the first domestic VUV/XUV optical frequency comb system with performance parameters reaching the international advanced level (Chin. Phys. Lett., 2020). In 2024, they successfully achieved atomic and molecular precision spectroscopy measurements based on VUV optical frequency combs (Opt. Lett., 2024).
To further address the issue of low coupling-output efficiency in the existing vacuum ultraviolet/extreme ultraviolet (VUV/XUV) optical frequency comb system, team members recently innovatively designed a new type of micro-nano grating. This grating exhibits mirror-like characteristics with high reflectivity at the operating wavelength of the driving light (~1040 nm) and shows grating characteristics in the extreme ultraviolet (XUV) band. Thus, it can be used to achieve efficient coupling-output of XUV light by taking advantage of its diffraction effect. Team members optimized the design of grating structural parameters (such as grating period, groove depth, and duty cycle) through optimization, enabling the grating to exhibit relatively high coupling - output efficiency in the extreme ultraviolet (XUV) band (~50 nm). The team members collaborated with the Laser Thin Film and Application Team from Tongji University to successfully fabricate the micro-nano grating with specific parameters. By utilizing the laboratory's extreme ultraviolet (XUV) light source, team members successfully achieved a two-fold increase in the coupling-output efficiency (within the range of 40-60 nm) and verified the superiority of this grating in terms of XUV light coupling-output performance.
To address the shortcomings of the existing vacuum ultraviolet/extreme ultraviolet (VUV/XUV) optical frequency comb, such as a single polarization mode in the output and difficulty in controlling the spatial phase, team members innovatively introduced vortex characteristics into the optical frequency comb system, modulating the driving light source into a Laguerre-Gaussian vortex beam. By utilizing femtosecond resonant cavity enhancement technology, they amplified the Laguerre-Gaussian light field to the kilowatt level for the first time. Further combining the intracavity harmonic generation process, they successfully realized a new type of ultraviolet optical frequency comb in the Laguerre-Gaussian mode, laying a solid foundation for the all-round regulation of the polarization and spatial phase of the VUV optical frequency comb in the next step.
Schematic diagram of the working scheme for the new-type UV/VUV optical frequency comb
The related research results were recently published respectively in Optics Express under the title "Out-coupling grating for XUV frequency combs in the short wavelength region" and in Optics Letters under the title "Cavity-enhanced harmonic generation of an optical frequency comb with Laguerre-Gaussian mode". Ph.D. student XIAO Zhengrong from APM is the first author of the aforementioned achievements, while associate researcher HUA Linqiang and researcher LIU Xiaojun are the co-corresponding authors.
This research work has received strong and long-term support from departments such as the National Natural Science Foundation of China (NSFC) and the Chinese Academy of Sciences.
Link to the article:
https://doi.org/10.1364/OE.559622
https://doi.org/10.1364/OL.569141