Study on characteristics of spectral broadenig by thin plates

Abstract

To obtain few-cycle pulses (i.e. less than 5 fs), according to Heisenberg’s uncertainty principle [7], broadband spectrum is required. To get broadband spectrum, hollow-core fiber (HCF) is mainly used. However, HCF has disadvantages such as low-energy conversion efficiency and unstable beam profile. In 2014, Kung’s group proposed a new method [5], which is called multi-plate supercontinuum (MPSC), allowing the generation of the broadband spectrum. In the method, when an intense (around 1013 W/cm2) laser pulse penetrates through thin plates, spectral broadening takes place due to self-phase modulation, resulting in broadband spectrum. the method has advantages like having good beam profile and high energy conversion efficiency. We performed four MPSC experiments in different conditions (i.e. intedistance of plates, energy per a pulse, use of an extra stage, and GDD & TOD parameters of input laser) to investigate the characteristics of MPSC. In the first experiment, the spectrum was broadened maximally by three thin plates. Then, the spectra were recorded after slightly increase of distance between plates, to see the effect of interdistance of plates. As a result, the broadened spectrum became slightly shorten when the interdistance is increased. On the other hand, when the distance is long enough, the spectrum became rapidly narrower. These results can be explained by the simulation result [15]. According to the simulation, if a strong pulse passes some thin plates, the pulse self-focus due to Kerr effect. After focusing, the beam will diverge increasing beam size which give rise to decreasing of beam intensity. If the next plate is located far away from the previous plate, then self-phase modulation effect becomes weaker, because the pulse with lower intensity passes the plates. Therefore, in MPSC the location of plates is a critical factor determining how does the spectrum broaden. In the second experiment, the broadest spectrum was obtained until 490nm referring to the results of Wei’s group [6]. In the setup, weak plasma appeared right after some plates. Generation of these plasma is a tricky problem in the MPSC, because it is uncertain how the plasma, which could broaden spectrum or prevent spectral broadening, affects on the passing beam. Therefore generation of plasma should be prevented or minimized as far as possible to remove the uncertainty from the plasma. Introduction of vacuum or helium chamber will help to get a better spectrum prohibiting generation of plasma and allowing use of high power laser. In the third experiment, two MPSC stages were used to broaden spectrum twice. After the spectrum was maximally broadened in the first stage, the beam was focused toward the second stage by a curved mirror and penetrates the plates. As the results, although there was no further spectral broadening in the short wavelength region (around 550nm), but the spectrum slightly was broadened toward long wavelength (around 950nm) and signal intensity was increased in the short wavelength regime. These changes bring a good effect on getting more shorter pulses. As we did, an introduction of extra stages can be helpful to enhance performance of MPSC. In the fourth experiment, the spectra were recorded after changing group delay dispersion (GDD) and third order dispersion (TOD) of the input laser by dazzler unit, for investigating what does change of laser conditions affect on the spectral broadening. As the results, its bandwidth (Δλ (1%)) has been changed from 103 nm to 387 nm. This means that the spectral broadening by the plates will sensitively change against the condition of input laser. Thus, when input laser with proper parameters such as GDD & TOD is used, the spectrum can be broadened fully. Finally, the spectral broadening in MPSC will be quite affected by a variety of experimental parameters such as pulse duration, focal length, thickness of plate, material, etc. Four experiments were carried out with different experimental conditions (i.e. intedistance of plates, energy per a pulse, use of extra stage, and GDD & TOD parameters of laser). The results reveal that the experimental factors affect spectral broadening process quite well.