Observation of Refractive Index Line Shape in Ultrafast XUV Transient Absorption Spectroscopy

Ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy measures the time- and frequency-dependent light losses after light–matter interactions. In the linear region, the matter response to an XUV light field is usually determined by the complex refractive index n ˜ . The absorption signal is directly related to the imaginary part of n ˜ , namely, the absorption index. The real part of n ˜ refers to the real refractive index, which describes the chromatic dispersion of an optical material. However, the real refractive index information is usually not available in conventional absorption experiments. Here, we investigate the refractive index line shape in ultrafast XUV transient absorption spectroscopy by using a scheme that the XUV pulse traverses the target gas jet off-center. The jet has a density gradient in the direction perpendicular to the gas injection direction, which induces deflection on the XUV radiation. Our experimental and theoretical results show that the shape of the frequency-dependent XUV deflection spectra reproduces the refractive index line profile. A typical dispersive refractive index line shape is measured for a single-peak absorption; an additional shoulder structure appears for a doublet absorption. Moreover, the refractive index line shape is controlled by introducing a later-arrived near-infrared pulse to modify the phase of the XUV free induction decay, resulting in different XUV deflection spectra. The results promote our understanding of matter-induced absorption and deflection in ultrafast XUV spectroscopy.