image: (a): Generation and characterization of stable high energy SC light in air at kHz repetition rate. (b): Filament jitter measurement. Real color images of laser filament in air with high voltage off (c) and on (d). The corresponding far-field forward beam patterns on a white screen in real color: (e) high voltage off and (f) high voltage on.
Credit: by Yaoxiang Liu, Fukang Yin, Tie-Jun Wang, Yuxin Leng, Ruxin Li, Zhizhan Xu, and See Leang Chin
Supercontinuum (SC) white light (the spectrum stretching from the near ultraviolet to the infrared wavelengths) has advanced ultrafast laser spectroscopy in condensed matter science, biology, physics and chemistry. Compared with the frequently used photonic crystal fibers and bulk materials, femtosecond laser filamentation in gases is damage-immune for SC generation.
However, the milliseconds time scale of thermal diffusion in an air filament leads to air density reduction at the arrival of the next laser pulse for a kHz repetition laser. The thermal self-action effect results in significant beam pointing and intensity jitters of the laser filament, which leads to a challenge for applications using kHz filament and its SC light source.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Tie-Jun Wang from State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, China and co-workers have demonstrated a simple method to improve both the beam pointing and intensity stabilities of the air filament induced SC light. This was accomplished by simply applying an external DC electric field on the filament’s plasma channel. With the external electric field, plasma recombination is significantly suppressed resulting in less thermal deposition in the filament zone together with the overwhelmed filament thermal jitter by generating ionic wind from the electrode.
The beam pointing jitters of the 1 kHz air filament induced SC light were reported to suppress by more than 2-fold. The signal to noise ratio of the SC light was significantly improved as well. The scientists successfully generate a stable high-intensity and high-repetition supercontinuum white light source in air.
This is of great significance for the application of supercontinuum white light and is also very important and useful for filament-based other secondary sources, such as, third harmonic generation, THz, air lasing, and filament-based imaging and micromachining of condensed materials.