Diffractive shear interferometry for extreme ultraviolet high-resolution lensless imaging

High-harmonic spin-shearing interferometry for spatially resolved EUV magneto-optical spectroscopy

We present a method for achieving hyperspectral magnetic imaging in the extreme ultraviolet (EUV) region based on high-harmonic generation (HHG). By interfering two mutually coherent orthogonally-polarized and laterally-sheared HHG sources, we create an EUV illumination beam with spatially-dependent ellipticity. By placing a magnetic sample in the beamline and sweeping the relative time delay between the two sources, we record a spatially resolved interferogram that is sensitive to the EUV magnetic circular dichroism of the sample. This image contains the spatially-resolved magneto-optical response of the sample at each harmonic order, and can be used to measure the magnetic properties of spatially inhomogeneous magnetic samples.

Toward Complete All-Optical Intensity Modulation of High-Harmonic Generation from Solids

Optical modulation of high-harmonics generation in solids enables the detection of material properties, such as the band structure, and promising new applications, such as super-resolution imaging in semiconductors. Various recent studies have shown optical modulation of high-harmonics generation in solids, in particular, suppression of high-harmonics generation has been observed by synchronized or delayed multipulse sequences. Here we provide an overview of the underlying mechanisms attributed to this suppression and provide a perspective on the challenges and opportunities regarding these mechanisms. All-optical control of high-harmonic generation allows for femtosecond, and in the future possibly subfemtosecond, switching, which has numerous possible applications: These range from super-resolution microscopy to nanoscale controlled chemistry and highly tunable nonlinear light sources.

Advances in laboratory-scale ptychography using high harmonic sources [Invited]

Extreme ultraviolet microscopy and wavefront sensing are key elements for next-generation ultrafast applications, such as chemically-resolved imaging, focal spot diagnostics in pump-and-probe experiments, and actinic metrology for the state-of-the-art lithography node at 13.5 nm wavelength. Ptychography offers a robust solution to the aforementioned challenges. Originally adapted by the electron and synchrotron communities, advances in the stability and brightness of high-harmonic tabletop sources have enabled the transfer of ptychography to the laboratory. This review covers the state of the art in tabletop ptychography with high harmonic generation sources. We consider hardware options such as illumination optics and detector concepts as well as algorithmic aspects in the analysis of multispectral ptychography data. Finally, we review technological application cases such as multispectral wavefront sensing, attosecond pulse characterization, and depth-resolved imaging.

Phase retrieval using bidirectional interference

In this paper we describe a phase retrieval algorithm using constraints given by diffraction patterns and phase difference obtained from bidirectional interference. Wave propagation and linear phase ramps are used to connect the recordings. At least three patterns are recorded and processed (two diffraction patterns and one interference pattern). The quality of the results can be improved when recording and processing more patterns. The method works well with non-sparse samples and short (few millimeter) recording distances. Simulations, comparisons with other methods, and experimental validations are presented.

Spatially resolved spectral phase interferometry with an isolated attosecond pulse

We demonstrate spatially resolved supercontinuum spectral phase interferometry with an isolated attosecond pulse (IAP). The measured spatial-spectral interferogram over the broadband region indicates a high degree of IAP coherence in both spatial and spectral domains. In addition, the spectral-delay interferogram shows periodic temporal oscillations over the full IAP continuous spectrum, which indicates high temporal coherence. The supercontinuum spectral phase interferometry with broadband IAP will contribute to exploring spatiotemporal dispersive electronic dynamics through phase-based spectroscopy in the future.

Phase retrieval algorithms for lensless imaging using diffractive shearing interferometry

Diffractive shearing interferometry (DSI) is a method that has recently been developed to perform lensless imaging using extreme ultraviolet radiation generated by high-harmonic generation. In this paper, we investigate the uniqueness of the DSI solution and the requirements for the support constraint size. We find that there can be multiple solutions to the DSI problem that consist of displaced copies of the actual object. These alternative solutions can be eliminated by enforcing a sufficiently tight support constraint, or by introducing additional synthetic constraints. We furthermore propose a new DSI algorithm inspired by the analogy with coherent diffractive imaging (CDI) algorithms: the original DSI algorithm is in a way analogous to the hybrid input-output algorithm as used in CDI, and we propose a new algorithm that is more analogous to the error reduction algorithm as used in CDI. We find that the newly proposed algorithm is suitable for final refinement of the reconstruction.

Extreme ultraviolet lensless imaging without object support through rotational diversity in diffractive shearing interferometry

We report on a method that allows microscopic image reconstruction from extreme-ultraviolet diffraction patterns without the need for object support constraints or other prior knowledge about the object structure. This is achieved by introducing additional diversity through rotation of an object in a rotationally asymmetric probe beam, produced by the spatial interference between two phase-coherent high-harmonic beams. With this rotational diffractive shearing interferometry method, we demonstrate robust image reconstruction of microscopic objects at wavelengths around 30 nm, using images recorded at only three to five different object rotations.

Broadband extreme ultraviolet interferometry and imaging

Using a pair of phase-locked high-harmonic generation sources, we demonstrate Fourier transform interferometry at extreme-ultraviolet (EUV) wavelengths between 17 and 55 nm. This is made possible by the adaptation of a birefringence-based ultrastable interferometer for infrared femtosecond pulses. Since we measure the interference with an EUV-sensitive CCD camera, this enables a wide range of spatially and spectrally resolved measurements at extreme ultraviolet wavelengths. We demonstrate the capabilities of this technique by performing wavelength-resolved high-resolution coherent diffractive imaging and by measuring the spatially resolved spectral absorption of a thin structured titanium film.