Understanding photoacoustic signal formation in the presence of transparent thin films.

Strain-induced variation of the refractive index is the main mechanism of strain detection in photoacoustic experiments. However, weak strain-optic coupling in many materials limits the application of photoacoustics as an imaging tool. A straightforward deposition of a transparent thin film as a top layer has previously been shown to provide signal enhancement due to elastic boundary effects.

Computational vibration mitigation using phase interpolation in digital holographic microscopy for overlay metrology.

Digital holographic microscopy retrieves amplitude and phase information of an image which allows us to computationally correct for imperfections in the imaging optics. However, digital holographic microscopy is an interferometric technique that is inherently sensitive to undesired phase variations between object and reference beam. These phase variations lower the fringe contrast if they are integrated over a finite exposure time which leads to a reduced amplitude of the retrieved image.

Mid-infrared optical parametric chirped-pulse amplifier at 50 W and 38 fs pumped by a high-power Yb-InnoSlab platform.

High-power Yb:InnoSlab lasers are proliferating into multiple modern application areas of laser physics ranging from plasma physics and nanolithography to driving optical parametric amplifiers for high-harmonic generation and attosecond science. Here, we present, the layout, design and first results of an optical parametric chirped-pulse amplifier system pumped by a kW-level average power Yb-InnoSlab laser. We describe the layout and concepts of the pump lasers, with particular attention to the specific design principles required for our application.

Characterizing post-compression of mJ-level ultrafast pulses via loose focusing in a gas cell.

The ability to generate high-intensity ultrashort laser pulses is a key driver for advancing the strong-field physics and its applications. Post-compression methods aim to increase the peak intensity of amplified laser pulses via spectral broadening through self-phase modulation (SPM), followed by temporal pulse compression. However, other unavoidable nonlinear self-action effects, which typically occur parallel to SPM, can lead to phase distortions and beam quality degradation.

Sub-cycle dynamics in two-color high-harmonic generation from laser-produced plasmas.

We present high-order harmonic generation (HHG) in laser-produced aluminium and tin plasmas driven by a two-color field with orthogonal polarization, leading to the generation of both odd and even harmonics. We shape the effective drive field with sub-cycle resolution by controlling the phase between the fundamental wave and its second harmonic. The shape of the drive field influences the electron trajectories of the various harmonics generated in these plasmas.

Fast spectroscopic imaging using extreme ultraviolet interferometry.

Extreme ultraviolet pulses as generated by high harmonic generation (HHG) are a powerful tool for both time-resolved spectroscopy and coherent diffractive imaging. However, the integration of spectroscopy and microscopy to harness the unique broadband spectra provided by HHG is hardly explored due to the challenge to decouple spectroscopic and microscopic information. Here, we present an interferometric approach to this problem that combines Fourier transform spectroscopy (FTS) with Fourier transform holography (FTH).

Breaking Abbe's diffraction limit with harmonic deactivation microscopy.

Nonlinear optical microscopy provides elegant means for label-free imaging of biological samples and condensed matter systems. The widespread areas of application could even be increased if resolution was improved, which the famous Abbe diffraction limit now restrains. Super-resolution techniques can break the diffraction limit but most rely on fluorescent labeling.

Evaluation of the hoof centre-of-pressure path in horses affected by chronic osteoarthritic pain.

Introduction: The Centre of Pressure (COP) is the single point summarising all forces transferred to the hoof during the stance phase of a stride. COP path (COPp) is the trajectory that COP follows from footstrike to lift-off. Aim of the present study was to characterize the COP and COPp in horses affected by osteoarthritis and chronic lameness.

PtyLab.m/py/jl: a cross-platform, open-source inverse modeling toolbox for conventional and Fourier ptychography.

Conventional (CP) and Fourier (FP) ptychography have emerged as versatile quantitative phase imaging techniques. While the main application cases for each technique are different, namely lens-less short wavelength imaging for CP and lens-based visible light imaging for FP, both methods share a common algorithmic ground. CP and FP have in part independently evolved to include experimentally robust forward models and inversion techniques.

Field-position dependent apodization in dark-field digital holographic microscopy for semiconductor metrology.

Measuring overlay between two layers of semiconductor devices is a crucial step during electronic chip fabrication. We present dark-field digital holographic microscopy that addresses various overlay metrology challenges that are encountered in the semiconductor industry. We present measurement results that show that the point-spread function of our microscope depends on the position in the field-of-view.

aPIE: an angle calibration algorithm for reflection ptychography.

Reflection ptychography is a lensfree microscopy technique particularly promising in regions of the electromagnetic spectrum where imaging optics are inefficient or not available. This is the case in tabletop extreme ultraviolet microscopy and grazing incidence small angle x ray scattering experiments. Combining such experimental configurations with ptychography requires accurate knowledge of the relative tilt between the sample and the detector in non-coplanar scattering geometries.

Aberration calibration and correction with nano-scatterers in digital holographic microscopy for semiconductor metrology.

Overlay metrology measures pattern placement between two layers in a semiconductor chip. The continuous shrinking of device dimensions drives the need to explore novel optical overlay metrology concepts that can address many of the existing metrology challenges. We present a compact dark-field digital holographic microscope that uses only a single imaging lens.

Smartphone-Based Pelvic Movement Asymmetry Measures for Clinical Decision Making in Equine Lameness Assessment.

Visual evaluation of hindlimb lameness in the horse is challenging. Objective measurements, simultaneous to visual assessment, are used increasingly to aid clinical decision making. The aim of this study was to investigate the association of pelvic movement asymmetry with lameness scores (UK scale 0-10) of one experienced veterinarian.

Monitoring of Organochlorine Pesticide and Polychlorinated Biphenyl Residues in Common Swifts () in the Region of Hannover, Lower Saxony, Germany.

The use of pesticides is associated with the decline of several avian species. In this study, we monitored the organochlorine contaminants in common swifts () in the years 2016 to 2018. These long-distance migrants breed in Europe and winter in Africa.

Ptychographic optical coherence tomography.

Ptychography is a robust computational imaging technique that can reconstruct complex light fields beyond conventional hardware limits. However, for many wide-field computational imaging techniques, including ptychography, depth sectioning remains a challenge. Here we demonstrate a high-resolution three-dimensional (3D) computational imaging approach, which combines ptychography with spectral-domain imaging, inspired by optical coherence tomography (OCT).

Diffraction-based overlay metrology using angular-multiplexed acquisition of dark-field digital holograms.

In semiconductor device manufacturing, optical overlay metrology measures pattern placement between two layers in a chip with sub-nm precision. Continuous improvements in overlay metrology are needed to keep up with shrinking device dimensions in modern chips. We present first overlay metrology results using a novel off-axis dark-field digital holographic microscopy concept that acquires multiple holograms in parallel by angular multiplexing.

High-resolution microscopy through optically opaque media using ultrafast photoacoustics.

We present a high-resolution microscope capable of imaging buried structures through optically opaque materials with micrometer transverse resolution and a nanometer-scale depth sensitivity. The ability to image through such materials is made possible by the use of laser ultrasonic techniques, where an ultrafast laser pulse launches acoustic waves inside an opaque layer and subsequent acoustic echoes from buried interfaces are detected optically by a time-delayed probe pulse. We show that the high frequency of the generated ultrasound waves enables imaging with a transverse resolution only limited by the optical detection system.

Role of scattering by surface roughness in the photoacoustic detection of hidden micro-structures.

We present an experimental study in which we compare two different pump-probe setups to generate and detect high-frequency laser-induced ultrasound for the detection of gratings buried underneath optically opaque metal layers. One system is built around a high-fluence, low-repetition-rate femtosecond laser (1 kHz) and the other around a low-fluence, high-repetition-rate femtosecond laser (5.1 MHz).

Laser-induced ultrasonics for detection of low-amplitude grating through metal layers with finite roughness.

We report on the use of laser-induced ultrasonics for the detection of gratings with amplitudes as small as 0.5 nm, buried underneath an optically opaque nickel layer. In our experiments, we use gratings fabricated on top of a nickel layer on glass, and we optically pump and probe the sample from the glass side.

Impact of coherence length on the field of view in dark-field holographic microscopy for semiconductor metrology: theoretical and experimental comparisons.

Semiconductor manufacturers continue to increase the component densities on computer chips by reducing the device dimensions to less than 10 nm. This trend requires faster, more precise, and more robust optical metrology tools that contain complex and high-precision optics with challenging imaging requirements. Here, we present dark-field digital holographic microscopy as a promising optical metrology technique that uses optics with acceptable complexity.

zPIE: an autofocusing algorithm for ptychography.

An autofocusing algorithm for ptychography is proposed. The method optimizes a sharpness metric that would be observed in a differential interference microscope and is valid for both amplitude and phase modulating specimens. We experimentally demonstrate that the algorithm, based on the extended ptychographic iterative engine (ePIE), calibrates the sample-detector distance with an accuracy within the depth of field of the ptychographic microscope.

Measuring laser beam quality, wavefronts, and lens aberrations using ptychography.

We report on an approach for quantitative characterization of laser beam quality, wavefronts, and lens aberrations using ptychography with a near-infrared supercontinuum laser. Ptychography is shown to offer a powerful alternative for both beam propagation ratio M and wavefront measurements compared with existing techniques. In addition, ptychography is used to recover the transmission function of a microlens array for aberration analysis.

Generation and characterization of focused helical x-ray beams.

The phenomenon of orbital angular momentum (OAM) affects a variety of important applications in visible optics, including optical tweezers, free-space communication, and 3D localization for fluorescence imaging. The lack of suitable wavefront shaping optics such as spatial light modulators has inhibited the ability to impart OAM on x-ray and electron radiation in a controlled way. Here, we report the experimental observation of helical soft x-ray beams generated by holographically designed diffractive optical elements.

Optical parametric chirped pulse amplifier producing ultrashort 10.5 mJ pulses at 1.55 μm.

We present an optical parametric chirped pulse amplifier (OPCPA) delivering 10.5 mJ pulses with durations down to 220 fs, at 100 Hz repetition rate, centered at 1550 nm. The system is pumped by a picosecond Nd:YAG amplifier at 1064 nm based on quasi-continuous-wave diode pumping and seeded by a femtosecond mode-locked Er fiber laser at 1550 nm.