基于固体薄片超连续飞秒光源驱动的高次谐波产生实验

High harmonic generation experiments based on solid-state supercontinuum

本文报道了采用基于熔石英薄片超连续的少周期飞秒光源驱动高次谐波产生的实验研究.实验中通过将重复频率1kHz的飞秒钛宝石激光放大器所输出的能量0.8mJ、脉宽30fs的脉冲聚焦到7片0.1mm厚的熔融石英片中,得到了覆盖带宽大于倍频程的展宽光谱.利用啁啾镜补偿色散后,经瞬态光栅频率分辨光学开关法测得脉宽为6.3fs,对应约2.3个光学周期.利用压缩后的激光脉冲聚焦作用于惰性气体,并通过调节尖劈插入量改变脉宽,分别测得了分立以及连续的高次谐波截止区信号,结果与6.3fs的脉冲宽度相符合.

Intense few-cycle pulses are widely used in transient light synthesis, high harmonic generation（HHG） and especially in isolated attosecond pulse generation. To obtain intense few-cycle pulses, the intense supercontinuum is needed. The traditional way to generate intense supercontinuum is using rare gas filled hollow-core fibers. Since the input energy of hollow-core fiber system is limited to a level of tens of mJ, it is necessary to find new ways to achieve energy scaling. In this paper we demonstrate the efficient generation of supercontinuum by solid thin plates, compression and its application in HHG.The Ti：sapphire laser used in the present experiment emits 0.8 mJ in energy with a duration of 30 fs at 1 kHz. After passing through a 3：1 telescope, the beam has a diameter changed from 12 mm to 4 mm. Then the laser is focused by an f = 2000 mm lens into a 600 μm-diameter spot. After propagating through 7 fused silica plates placed at Brewster＇s angle（55.5°） with a thickness of 0.1 mm, the 0.7 mJ octave spanning supercontinuum is achieved, corresponding to an efficiency of 87.5%. The first three plates are placed at 31, 11, 2.5 mm in front of the beam waist, and the last four plates are placed at 2, 7, 12, 17 mm behind the beam waist respectively. With a pair of wedges and 4 pairs of chirped mirrors, the 0.68 mJ supercontinuum is compressed to a duration of 6.3 fs, which is measured by TG-FROG.The 0.5 mJ, 6.3 fs pulse is used to perform high-harmonic generation experiment. The beam diameter is 150 μm when focused by an f = 400 mm lens, with a laser intensity of 8.1×10^（14）W/cm^2. The 1 mm Ne gas jet is used to perform HHG experiment with a back pressure of 300 mbar. To block the near-infrared light, a 150 μm Zirconium foil is placed behind the gas jet. Then the XUV spectrum is detected by a spectrometer, which consists of a flat field grating and a CCD camera. For driving pulses of few-cycle regime without dispersion, the cutoff spectrum of HHG is continuous. But when the pulse is stretched by positive or negative dispersion, the cutoff spectrum turns discrete. The HHG result is that the cutoff region is continuous when the wedge is in a certain place. Then by increasing or reducing the insertion of the wedge, the cutoff spectrum becomes discrete. Our result is consistent with HHG generated by few-cycle pulses.In conclusion, we demonstrate high-harmonic generation based on supercontinuum generated by solid thin plates.The 0.7 mJ supercontinuum is achieved when 0.8 mJ pulses are injected to 7 thin fused silica plates. The supercontinuum is compressed to 0.68 mJ, 6.3 fs. The 0.5 mJ, 6.3 fs pulse is used to perform HHG experiments. The HHG result was consistent with few-cycle driving pulses. Our research indicates that solid state supercontinuum has great potential applications in HHG and isolated attosecond pulse generation.