Beam drift and partial probe coherence effects in EUV reflective-mode coherent diffractive imaging.

While the industrial implementation of extreme ultraviolet lithography for upcoming technology nodes is becoming ever more realistic, a number of challenges have yet to be overcome. Among them is the need for actinic mask inspection. We report on reflective-mode lensless imaging of a patterned multi-layer mask sample at extreme ultraviolet wavelength that provides a finely structured defect map of the sample under test. Here, we present the imaging results obtained using ptychography in reflection mode at 6° angle of incidence from the surface normal and 13.5 nm wavelength. Moreover, an extended version of the difference map algorithm is employed that substantially enhances the reconstruction quality by taking into account both long and short-term variations of the incident illumination.

Time-resolved photoelectron imaging with a femtosecond vacuum-ultraviolet light source: Dynamics in the A∼/B∼- and F∼-bands of SO2.

Time-resolved photoelectron imaging is demonstrated using the third harmonic of a 400-nm femtosecond laser pulse as the ionization source. The resulting 133-nm pulses are combined with 266-nm pulses to study the excited-state dynamics in the A∼/B∼- and F∼-band regions of SO2. The photoelectron signal from the molecules excited to the A∼/B∼-band does not decay for at least several picoseconds, reflecting the population of bound states. The temporal variation of the photoelectron angular distribution (PAD) reflects the creation of a rotational wave packet in the excited state. In contrast, the photoelectron signal from molecules excited to the F∼-band decays with a time constant of 80 fs. This time constant is attributed to the motion of the excited-state wave packet out of the ionization window. The observed time-dependent PADs are consistent with the F∼ band corresponding to a Rydberg state of dominant s character. These results establish low-order harmonic generation as a promising tool for time-resolved photoelectron imaging of the excited-state dynamics of molecules, simultaneously giving access to low-lying electronic states, as well as Rydberg states, and avoiding the ionization of unexcited molecules.

In situ frequency gating and beam splitting of vacuum- and extreme-ultraviolet pulses.

Monochromatization of high-harmonic sources has opened fascinating perspectives regarding time-resolved photoemission from all phases of matter. Such studies have invariably involved the use of spectral filters or spectrally dispersive optical components that are inherently lossy and technically complex. Here we present a new technique for the spectral selection of near-threshold harmonics and their spatial separation from the driving beams without any optical elements. We discover the existence of a narrow phase-matching gate resulting from the combination of the non-collinear generation geometry in an extended medium, atomic resonances and absorption. Our technique offers a filter contrast of up to 104 for the selected harmonics against the adjacent ones and offers multiple temporally synchronized beamlets in a single unified scheme. We demonstrate the selective generation of 133, 80 or 56 nm femtosecond pulses from a 400-nm driver, which is specific to the target gas. These results open new pathways towards phase-sensitive multi-pulse spectroscopy in the vacuum- and extreme-ultraviolet, and frequency-selective output coupling from enhancement cavities.