Laser-induced transient opacity in helium nanodroplets probed by single-shot coherent diffraction

Abstract

Single-shot coherent diffractive imaging (CDI) with intense short-wavelength light pulses enables the structural characterization of individual nanoparticles in free flight with high spatial and temporal resolution. Conventional CDI assumes that the target object exhibits a linear scattering response and static electronic properties. Here, we extend this approach to investigate transient laser-driven modifications of the electronic structure in individual nanoparticles, imprinted in their time-resolved diffraction patterns. In the presence of a near-infrared laser pulse, we observe a pronounced reduction in the diffraction signal from helium nanodroplets when probed with ultrashort extreme ultraviolet (XUV) pulses. This effect is attributed to a light-field-induced modification of the electronic structure of the droplets, which substantially increases their XUV absorption. Our results demonstrate that single-particle diffraction can capture ultrafast light-driven electron dynamics in nanoscale systems. This paves the way for the spatiotemporal tracking of reversible changes in the electronic properties of nanoscale structures with potential applications in ultrafast X-ray optics, materials science, and all-optical signal processing.

Figures (4)

• Figure 1: a) Sketch of the experimental setup for time-resolved single-shot coherent diffraction imaging of helium nanodroplets with an intense HHG source. The inset shows the average contribution of individual harmonics to the total XUV intensity. b) The brightest size-selected diffraction image for each of four size ranges in the center of the observed size distribution as a function of XUV-NIR time delay. Delays with low statistics were omitted. The rightmost column shows the brightest size selected image for the four size ranges from an XUV-only data set of comparable size to each delay measurement (color scale see Fig. \ref{['fig:dip']}a, full-resolution data set see Data Availability).
• Figure 2: a) Exemplary diffraction pattern with indicated inner detector area, in blue up to 11° scattering angle and outer detector area in orange, from 31° up to the edge of the detector at 45°. b) Delay-dependent integrated detector signal for all hits, restricted to different areas of the diffraction detector (full detector signal as green dotted line, inner detector area as blue solid line, outer detector area as orange solid line). All curves are normalized to the respective XUV only signal. c) Delay-dependent integrated helium ion yield, normalized to its maximal value.
• Figure 3: a, b) Simulated effective refractive index of helium droplets around 20eV photon energy (absorption coefficient $\beta$, refraction coefficient $\delta$) vs. NIR field strength (bottom axis) or laser intensity (top axis). c) Line-outs of the effective refractive indices at the positions of the 13th and d) 15th harmonic are given as gray solid ($\delta$) and dashed ($\beta$) lines. In addition, the detector integrals at these energies for an exemplary droplet size of R = 708 nm are given in blue and red. The same mask as for the experimental patterns (Supplementary Fig. 7) is applied. e) Measured (blue markers) and simulated (red curve) delay-dependent detector signal as a function of the NIR-XUV delay. The effective average NIR intensities during the XUV scattering process are indicated above the curve.
• Figure 4: Delay dependent small-angle detector signal of size selected diffraction patterns for four different size ranges. Simulated curves for single sizes are given as dotted lines.