Few-cycle light sources with broadband wavelength-tunability in the visible region are quite useful for studying excitonic relaxation and vibrational dynamics in photosynthetic systems, since many molecules have absorption bands in this spectral range. However, they have, so far, been proved difficult to achieve. 

Recently, a research team from State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, has pointed out a very practical means of producing such sources using soliton-plasma interactions in a gas-filled single-ring photonic crystal fiber (SR-PCF). 

It is found that in a He-filled SR-PCF, the central wavelength of few-cycle blueshifting soliton could be continuously tuned over hundreds of nanometers through adjusting the single pulse energy launched into the fiber, while maintaining a high energy conversion efficiency of >57%. 

Moreover, at high input pulse energies, soliton self-compression effects enhanced the plasma density in the fiber, which could modulate the phase-matching condition of ultraviolet dispersive wave generation. 

Furthermore, the effect of the core-cladding resonance on the soliton-plasma interactions was also investigated.

The results show that such effect can become profound if the core-cladding resonant wavelength is close to the pump wavelength. However, this effect could, in practice, be easily minimized through adjusting the wall thickness of the SR-PCF. 

This technique has several remarkable advantages, including compact set-up, high conversion efficiency, and broad wavelength-tuning range. Besides potential applications in pump-probe spectroscopy and strong-field physics, this work also offers useful insight for understanding the underlying physics of soliton-plasma interactions in gas-based ultrafast nonlinear optics. 

The results entitled "Wavelength-tunable few-cycle pulses in visible region generated through soliton-plasma interactions" were published in Opt. Express. 

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

The fundamental core mode of SR-PCF and its wavelength-dependent dispersion (Image by SIOM)

Temporal and spectral evolutions of the output pulses as a function of input energy (Image by SIOM)