Coherent Tabletop EUV Ptychography of Nanopatterns.

Coherent diffraction imaging (CDI) or lensless X-ray microscopy has become of great interest for high spatial resolution imaging of, e.g., nanostructures and biological specimens. There is no optics required in between an object and a detector, because the object can be fully recovered from its far-field diffraction pattern with an iterative phase retrieval algorithm. Hence, in principle, a sub-wavelength spatial resolution could be achieved in a high-numerical aperture configuration. With the advances of ultrafast laser technology, high photon flux tabletop Extreme Ultraviolet (EUV) sources based on the high-order harmonic generation (HHG) have become available to small-scale laboratories. In this study, we report on a newly established high photon flux and highly monochromatic 30 nm HHG beamline. Furthermore, we applied ptychography, a scanning CDI version, to probe a nearly periodic nanopattern with the tabletop EUV source. A wide-field view of about 15 × 15 µm was probed with a 2.5 µm-diameter illumination beam at 30 nm. From a set of hundreds of far-field diffraction patterns recorded for different adjacent positions of the object, both the object and the illumination beams were successfully reconstructed with the extended ptychographical iterative engine. By investigating the phase retrieval transfer function, a diffraction-limited resolution of reconstruction of about 32 nm is obtained.

Direct compression of 170-fs 50-cycle pulses down to 1.5 cycles with 70% transmission.

We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.

Supercontinuum generation in ZBLAN glass photonic crystal fiber with six nanobore cores.

Photonic crystal fibers (PCFs) made from ZBLAN glass are of great interest for generating broadband supercontinua extending into the ultraviolet and mid-infrared regions. Precise sub-micrometer structuring makes it possible to adjust the modal dispersion over a wide range, making the generation of new frequencies more efficient. Here we report a novel ZBLAN PCF with six cores, each containing a central nanobore of a diameter ∼330 nm. Each nanobore core supports several guided modes, and the presence of the nanobore significantly modifies the dispersion, strongly influencing the dynamics and the extent of supercontinuum generation. Spectral broadening is observed when a single core is pumped in the fundamental and first higher order core modes with 200 fs long pulses at a wavelength of 1042 nm. Frequency-resolved optical gating is used to characterize the output pulses when pumping in the lowest order mode. The results are verified by numerical simulations.