Ultrashort femtosecond laser pulses are proved to be useful tools to better understand the basic physical and chemical properties underlying in the ultrafast optical responses. In particular, a tunable laser source across the wide spectral range is strongly demanded for investigating ultrafast dynamics in various materials.  

Recently, a research team from State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, employed a gas-filled single-ring photonic crystal fiber (SR-PCF) to generate tunable ultrashort laser pulses via soliton-plasma interactions.  

The experimental set-up includes two stages: the pulse-compression stage and soliton-dynamics stage.

At first, ~45-fs pulses from a commercial Ti:Sapphire laser system centered at 800 nm were compressed to ~16 fs through a l-m-length hollow-core fiber (HCF) filled with 0.2-bar Ar. In the soliton-dynamics stage, the compressed pulses were focused into a 17.2-cm-length SR-PCF filled with 1.3-bar Ar or 10-bar He.  

What was amazing was that, when Ar gas was filled into the SR-PCF, some interference fringes on the blueshifting soliton were observed at high pulse-energy levels due to plasma-induced pulse fission.

Apart from that, in a He-filled SR-PCF, a sharp narrow-band spectral peak was achieved at the first resonant spectral region of the SR-PCF, which resulted from phase-matched nonlinear processes.  

To the best of our knowledge, it was the first time to investigate the influence of the core-cladding resonance on the blueshifting soliton. These two experimental observations are confirmed by numerical simulations.  

Furthermore, through properly adjusting input pulse energy, researchers found that the blueshifting soliton can obtain a high conversion efficiency (～84%) and its wavelength can be tuned over hundreds of nanometers (~240 nm).  

The experiment paves the way to generate broadband wavelength-tunable light sources in the visible spectral region, which may have plenty of potential applications in ultrafast pump-probe spectroscopy.  

The corresponding results, entitled "Continuously wavelength-tunable blueshifting soliton generated in gas-filled photonic crystal fibers", were published in Opt. Letters.

This work was supported by the International S&T Cooperation Program of China, the National Natural Science Foundation of China (NSFC), the Program of Shanghai Academic/Technology Research Leader, the Strategic Priority Research Program of the Chinese Academy of Sciences, the Major Project Science and Technology Commission of Shanghai Municipality (STCSM).

Spectral evolutions of the out put pulses as a function of input energy (Image by SIOM)