Ultrashort laser pulses featuring remarkable spectral tunability are highly demanded for investigating basic physical and chemical properties in the ultrafast dynamic processes, which are based on time-resolved spectroscopy (also called ultrafast spectroscopy). For example, ultrashort pulses in the visible region permit time-resolved capture of the excitionic relaxation and vibrational dynamics of molecules, since many molecules have absorption bands within this spectral range.

Recently, a research team from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, reported the highly-tunable visible ultrashort pulses generation in a short length of He-filled single-ring photonic crystal fiber (SR-PCF). The corresponding results were published in Opt. Express and Opt. Lett., respectively.

Using He-filled SR-PCF, researchers demonstrated that the spectral bandwidths of blueshifting solitons could be manipulated through adjusting the input pulse energy, gas pressure and core diameter of the SR-PCF, while the central wavelengths of these solitons could be continuously tuned over 200 nm.

Particularly in a large-core SR-PCF (24.6-μm core diameter), the bandwidths of blueshifting solitons could be effectively broadened to near 100 nm, pointing out the possibility of generating few-cycle, wavelength-tunable visible pulses.

It was amazing that, in a small-core SR-PCF (17-μm core diameter), they observed the plasma-driven blueshifting solitons showed little residual light near the pump wavelength in a certain pulse-energy region, leading to a high-efficiency frequency up-conversion process.

In contrast, at high pulse-energy levels, they found that the quality of the frequency up-shifting process was impaired due to the existence of a dynamical loss channel induced by the coupling of the soliton to linear modes near the pump wavelength.

In addition, by adjusting input pulse energy, the central wavelength of blueshifting solitons could be continuously tuned over 300 nm due to ionization-induced adiabatic soliton compression.

These experimental results, which have been confirmed by numerical simulations, could not only offer a deep insight into ionization-induced soliton-plasma dynamics in gas-filled hollow-core photonic crystal fibers, but also make it easy to develop highly-tunable ultrafast light sources at visible wavelengths, which may have many applications in ultrafast spectroscopy.

The work was supported by the International S&T Cooperation Program of China, the National Natural Science Foundation of China, the Program of Shanghai Academic/Technology Research Leader, ect.

Measured normalized spectral intensities and filtered beam profiles (Image by SIOM) 

Measured spectral evolutions and highly-tunable soliton wavelength (Image by SIOM)