Conference Agenda

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For the purpose of Automatic Differentiation 3D imaging, a method which is simultaneously reliable and computationally performant is highly desired.

Most methods to solve Maxwell’s equations are either poorly performant, or numerically unstable. One method which has broken this dichotomy is the modified Born series (MBS) by Vellekoop et al.. This method formulates a simple iteration which stably converges without sacrificing per-iteration performance. However, in the case of highly-scattering materials, the iteration must be altered to ensure convergence, generally degrading the convergence rate.

After studying the modified Born series, we derived a new variant iteration, making use of a Cayley transform to guarantee stability regardless of scattering strength. Beside this improved stability being desireable in and of itself, this property can be leveraged to potentially outperform the modified Born series, in cases of scattering from strongly-scattering media.

Extreme ultraviolet (EUV) or soft X-ray scatterometry is a powerful technique enabling contactless non-destructive structural characterization of patterned nano-stacks with down to sub-nanometer precision1. The technique requires coherent EUV / soft X-ray radiation and is thus typically limited to a synchrotron or free-electron laser facilities. With the continual improvement of table-top high-harmonic-generation (HHG) sources over the last few decades, this approach has become feasible in a lab-scale environment. Here we present EUV scatteromertric results obtained using the coherent 92 eV HHG source in imec’s AttoLab on grating-type samples highly relevant for semiconductor industry. Roughness characterization was performed via direct detection of scattered light utilizing a synthetic aperture and high-dynamic-range detection coupled with noise suppression techniques. Structural parameters (CD, pitch, height, angles, layer and interlayer diffusion thicknesses) were obtained by fitting the sample model to match simulated diffraction patterns to the experimental ones. The latter were acquired through a wide angular scanning (ranging 0 to 65° from grazing). The simulated patterns were calculated by propagating light (using JCMsuite software) through a model of the structured sample. The results demonstrate sub-nanometer precision and are in good agreement with AFM and TEM measurements.

Accurate surface measurements are crucial for industrial applications. Multiwavelength digital holography (MDH) is a cutting-edge technology that extends the range of holographic measurement to the mesoscopic scale while maintaining sub-micron axial accuracy. MDH achieves this by digitally "beating" precisely measured phasers at two wavelengths to create phase maps at longer wavelength scales. However, laser mode instability and wavelength uncertainty can cause errors in the scaling factor. This leads to misestimations and unwanted phase jumps in the heightmap. Therefore, MDH typically requires highly stable lasers.

Our algorithm addresses this problem by detecting wavelength shifts during post-processing. It uses the residuals in the spatial frequency of the phasers that arises due to the misestimation of wavelength to compensate for the scaling factors and synthetic phase maps. This ensures an accurate, phase-jump-free heightmap. We find this technique essential for making MDH viable with environmental wavelength shifts and more affordable laser sources while maintaining accuracy and precision.

Microscopy with table-top high-harmonic generation (HHG) sources enable high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. However, accurate characterization of dispersive samples in terms of composition and thickness remains challenging due to the limitations of lens-based optics in this spectral range. Here, we performed spectrally resolved lensless imaging using multiple high harmonics. The diffractive shearing interferometry reconstruction serves as a foundational step for element-sensitive metrology, while ptychographic reconstruction enabled the retrieval of high-precision spectral imaging and quantitative thickness mapping. Our non-destructive method offers a powerful tool to extract both the material composition and layer thicknesses of complex nanostructured samples.

We demonstrate centimeter-scale dual-wavelength digital holography with expanding wavefront illumination that overcomes pixel-size resolution limitations thereby achieving a diffraction-limited spatial resolution of 3.91 micrometer compared to pixel-size limited resolution of 6.9 micrometer. The proposed holographic scheme provides an efficient, high-speed, high-resolution 3D optical inspection tool for industrial metrology.

This work presents a data-driven approach for reconstructing the longitudinal

component of tightly focused optical fields using only experimentally

accessible polarimetric intensity images. A custom-designed deep neural network

is trained on simulated polarimetric mappings generated from aberrated

wavefronts through a high-NA objective. The model successfully reconstructs

the complex amplitude of the longitudinal field with high fidelity, offering a

practical and instant method for indirect measurement of longitudinal components

in tightly focused beams.

A valuable method in biological imaging is Oblique Plane Microscopy (OPM), in which a single objective lens is used to launch a tilted (oblique) light sheet into the sample and to capture the fluorescence emission light. The captured light can be focused remotely onto a glass-immersion objective, tilted to match the oblique light sheet. Combined with this bespoke objective, OPM enables the use of a high-NA, short working distance objective with high magnification, maximising resolution and fluorescence capture efficiency.

Accurate modelling of the OPM’s 3D Point Spread Function (PSF) is needed for performance and resolution analysis, image deconvolution, and combinations of OPM with Structured Illumination Microscopy (SIM). The high NA necessitates taking into account all effects of polarisation and all directions of propagation, the remote focusing construction, and the non-orthogonal optical axes of the different lenses. We have developed a fully exact vectorial model of the 3D PSF for OPM. Key elements are the vignetting and deformation of the different non-overlapping pupil planes in the optical train, and correct handling of the remote focusing. It appears that a modification of standard Fourier optics methods using non-Cartesian coordinate frames provides an efficient route to compute the 3D PSF.

Icosahedral colloidal supraparticles are of interest because of their strong spherical symmetry. A polarization-complete tomographic imaging technique is developed to study the scattering properties of icosahedral supraparticles. The results reveal that our particles exhibit angle-dependent scattering behavior and inner structures.

In this talk, we discuss the effect of plasmonic resonances on the Fisher information in the far field. We consider a metallic nanowire embedded in a silicon substrate, illuminated by a dark-field focused spot, and we investigate how its position can be estimated from the scattered far-field intensities. The Fisher information is computed for both lateral and longitudinal displacements of the nanowire, and the dependence on the illumination frequency is analyzed. We compute the complex resonance frequencies of the nanowires and show that frequencies near the real part of the plasmonic resonance frequency enhance the Fisher information. However, at the resonance frequency itself, the Fisher information drops sharply, leading to an Information Dark State in which the position of the nanowire becomes nearly undetectable. This effect is analyzed and illustrated for both gold and silver nanowires.

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We present a fully quantitative full-field optical coherence tomography realized in transmission. This method allows, for the first time, a direct and fast retrieval of the complex amplitude of light that propagated through the sample of interest. We show that with an appropriate spectral range of the swept-source laser, this method separates weakly-scattered photons from the multiply-scattered ones, thus allowing measurement of thick and scattering samples, like tissue slices or organoids. With a relatively easy optical setup and straightforward numerical processing, this method is superior to classical quantitative phase imaging methods like digital holographic microscopy.

Interferometers for displacement measurement with picometer resolution are required. The main cause of degradation of interferometer resolution is air fluctuation. To investigate the effect of air fluctuation on displacement resolution, we have developed a sinusoidal phase modulation interferometer capable of measuring 1D displacement with 10 picometer resolution and 2D in-plane displacement (air fluctuation). The interferometer is of Michelson type with different polarizations in the reference and measurement arms. By exchanging the phase-modulated electro-optical modulator and photodetector of this interferometer, 1D and 2D in-plane displacements can be measured, respectively.

We propose a maximum-likelihood estimation (MLE) framework tailored to event-driven detectors, such as Timepix3, to perform computational image reconstruction from event trigger data. Unlike frame-based acquisitions, which aggregate intensities over an exposure period, event-driven cameras asynchronously record time-of-arrival data whenever a pixel exceeds a photon-count threshold. Our approach models the Poissonian probability of photon arrivals and incorporates both triggered (event) and untriggered (non-event) pixels into a unified log-likelihood function that remains well-defined across low-dose and saturation regimes. This naturally addresses challenges such as imaging under scarce photon conditions and pixel saturation in a consistent manner. Using gradient-based optimization, this MLE framework can be applied to several computational imaging techniques such as ptychography or coherent diffraction imaging.

Iterative reconstruction algorithms are commonly employed in in-line digital holography to retrieve phase information from one or more intensity images. The methods rely on iterative propagation of the complex wavefront between the measurement and reconstruction planes. To address the ill-posed nature of the reconstruction problem, constraints are applied in each plane. Despite yielding quantitative phase information, these approaches ignore aberrations introduced by the optical system. We propose a straightforward extension to these iterative algorithms to account for these aberrations, modelled here as a complex point spread function (PSF). Because the PSF can vary across the field of view, two scenarios have been investigated: one accounting for PSF variation and one ignoring it. With equal computational cost for convolution, and hence reconstruction time, the proposed method improves the accuracy of aberrated holograms reconstructions.

The demand for 100% inspection is increasing due to rising quality requirements in manufacturing. Multiwavelength Digital Holography (MDH) has emerged as a promising sensing technology capable of achieving both high throughput and high resolution. However, despite its potential, MDH has not yet been widely adopted in commercial metrology applications.

One of the key challenges in MDH system design is minimizing noise in the laser-illuminated optical imaging system. Due to the high coherence of laser light, digital holography systems are particularly sensitive to particle contamination and internal reflections within optical components.

We address this challenge by introducing a phase randomizing mirror (PRM) that controls coherent length by dynamically modulating the spatial profile of the illumination. Compared to conventional mechanical modulation methods, the PRM operates at significantly higher frequencies, enabling shorter exposure times. This results in a system that is not only high-throughput but also more resilient to environmental vibrations.

In this presentation, we will discuss the impact of the PRM on our newly developed phase-shifting digital holography system and demonstrate its application in real-world inspection scenarios.

Zernike polynomials are widely used to approximate optical aberrations and represent them in a compact, yet sufficiently accurate way. They are essential in precision optical manufacturing to characterize surface figure errors and wavefront aberrations of optical systems measured by interferometers or wavefront sensors. Zernike polynomials also describe freeform surfaces in optical design and are used in adaptive optics to correct for dynamic aberrations such as atmospheric turbulence.

The aberration coefficients of the polynomials are typically presented in tables or simple bar graphs that can be difficult to interpret at first glance. This work presents intuitive visualizations of Zernike aberrations that emphasize the magnitude and orientation of the dominant terms. Depending on the polynomial degree of the approximation, we use bubble plots or heat maps to represent low-order and mid-spatial frequency terms, respectively. In addition, we propose an alternative definition for the angular orientation of the paired polynomials with azimuthal orders m ≠ 0.

These graphical tools provide immediate visual feedback, benefiting tasks such as aberration compensator adjustment, real-time wavefront monitoring, and optical alignment. They can also improve the clarity of acceptance reports and measurement certificates.

We present a calibration method that correlates in-line measurement results from a digital holographic microscope (DHM) integrated into a two-photon polymerisation system (TPP) with offline results from a high-resolution laser scanning microscope (LSM). Our findings indicate that a single calibration step for a specific polymer is sufficient to reliably monitor the fabrication of various structures. In future, this approach will make it possible to adapt the production process to the manufactured structure geometry in real time.

Conventional interferometers provide uniform phase sensitivity across the measurement range. However, for applications involving small phase changes, amplifying the response within a specific phase range—at the expense of reduced sensitivity elsewhere—can be advantageous. A key example is the Gires-Tournois etalon, used in gravitational wave interferometers. In this work, we introduce an ultrasensitive phase measurement system based on time-holographic recording with a conventional Mach-Zehnder interferometer, operated near the intensity minimum at the dark output port. Phase fluctuations in the milliradian range around this point are converted into rad-sized dark output phases, with amplification factors exceeding 1,000. The adjustable imbalance between the interferometer arms controls this magnification, which is revealed by heterodyning the output with a frequency-shifted beam. Phase is digitally retrieved from the time-hologram using Fourier processing, with noise subtraction for correction. The system achieves phase sensitivities better than λ/3,000, enabling sub-nanometer precision for dimensional measurements. This versatile platform provides powerful tools for ultrasensitive phase measurements in a wide range of scientific and technological applications.

In this work we investigate the efficiency and accuracy of different perturbation methods, for the important case of a 1D periodic grating. These methods are potentially important in the study of the influence on optical performance of mid-spatial frequency errors on the surfaces of optical components.

The scope is limited to reflective optical systems. The perturbation methods are based on the Rayleigh hypothesis, boundary integral equations and volume integral equations. The Padé approximant is used to improve the convergence of the perturbation series. Results for a specific grating are included in a to be published paper and will be shown during the presentation.

Ptychography is a powerful computational imaging technique that

reconstructs both the complex object function and the illumination probe from

overlapping diffraction patterns. While it provides high-resolution, aberrationcorrected

imaging, its reliance on stepwise mechanical scanning limits acquisition

speed. In this work, we propose a fly-scan ptychographic approach that

enables continuous sample translation along arbitrary trajectories, significantly

reducing measurement time. To account for motion-induced decoherence, we

incorporate an object mode decomposition model combined with automatic differentiation

for accurate trajectory correction. This method enables diffractionlimited

reconstructions without the need for high-speed tracking, allowing fast

and precise measurements using standard ptychographic setups.

Light revolutionized our vision of technology. Carrying five internal degrees of freedom, it offers unlimited capabilities in light-matter interactions. The recent advances in coherent light sources enable remarkable progress in harnessing degrees of freedom and boost light-induced applications. The further increasing demand for controlling the light states catalyzes technological innovations in laser-based systems. In this presentation, I will focus on our recent advances in developing laser systems capable of delivering high power optical vortices with exceptionally high modal purity. By employing the coherent beam combining technique in the filled-aperture configuration, we show the power scaling of short pulsed high-dimensional optical vortices up to 100 W and modal purity in the range of 92-97%.

We report on a MidIr source generating bursts of intense pulses with

tunable repetition rates in the GHz range. The wavelength is also adjustable

between 2.88 to 3.03 μm. The system is based on an electro-optic frequency

comb emitting at 1030 nm out of which bursts of ns duration are carved and

amplified to several W leading to significant pulse peak power. This Yb-based

fiber system is then used to pump an optical parametric amplifier seeded by a

tunable CW Er-doped fiber laser. Due to the second-order non-linear process

properties, the temporal and spectral structure of the pump are transferred to the

idler wave leading to a transient optical frequency comb in the MidIr spectral

range.

We report on the development of the first red diode-pumped fs regenerative alexandrite amplifier. For seeding the amplifier, we will utilize our previously developed SESAM modelocked red diode-pumped alexandrite oscillator, which generates sub-100-fs pulse durations with nJ pulse energy. We will then amplify these pulses to achieve a pulse energy of 10 μJ. We achieved a continuous-wave (CW) output power of up to 2.8 W with 8 W of pump power, and watt-level output power with up to 4% output coupling using additional amplifier components inside the cavity. This demonstrates that there is sufficient net gain for regenerative amplification.

Passively modelocked solid-state lasers can exhibit two types of instabilities with very different origins. Near threshold, pulses are prone to the Q-switching instability, where pulse energy shows strong periodic modulation over successive roundtrips. This behaviour disappears as the pump power increases, giving way to the fundamental modelocked state—characterized by a single stable pulse circulating in the cavity. At higher pump levels, this state can become unstable again, leading to the generation of multiple equidistant pulses per roundtrip, forming harmonic modelocking states with 2, 3, …, n pulses. These instabilities critically affect laser performance, especially in systems using slow saturable absorbers, where accurate modelling becomes particularly challenging. Despite their practical relevance, analytical expressions for the boundaries of these instability regimes are scarce. In this work, we derive such expressions from a recently proposed generalization of the Haus master equation, providing a compact framework to describe the onset of both Q-switching (QML) and harmonic modelocking (HML) in passively modelocked solid-state lasers. These results contribute to a deeper understanding of the dynamics involved and offer valuable guidance for experimental design and optimization.

We report the post-compression of 180 fs pulses with 610 μJ pulse

energy to 44 fs using an LG0,3 beam in a compact bulk multi-pass cell. As a

result, the peak power of an Ytterbium laser system is boosted from 2.5 GW to

9.1 GW and the topological charge of the beam is shown to be conserved after

compression.

Field-resolved detection at near-petahertz frequencies provides exceptional sensitivity, broad bandwidth, and high dynamic range, enabling attosecond temporal and sub-diffraction spatial resolution. In a technique known as Femtosecond Fieldoscopy, ultrashort laser pulses impulsively excite molecular vibrations in resonance with the near-petahertz carrier frequency. This initiates vibrational coherence at the trailing edge of the pulse, which decays exponentially based on the molecular dephasing time.

The transmitted electric field encodes information about the excitation pulse, the sample’s picosecond-scale response, and a prolonged signal from atmospheric gases lasting hundreds of nanoseconds. By detecting this response in the time domain and applying Fourier analysis, the technique yields spectroscopic data with outstanding sensitivity and dynamic range. This performance is achieved by temporally gating the molecular response away from the excitation pulse.

Enabled by recent advancements in ytterbium laser systems, Femtosecond Fieldoscopy has successfully resolved overtone, Raman, and combination bands in liquid samples. Moreover, the method has been advanced for real-time sampling and extended to non-perturbative, label-free imaging.

This talk presents an overview of these recent developments as demonstrated by my group, highlighting the broad potential of this emerging spectroscopic tool.

We combine super-resolution and label-free microscopy by using a donut-shaped pump beam to confine harmonic generation to a sub-diffraction region. This Harmonic Deactivation Microscopy (HADES) can enable (sub-)fs temporal and at least 100-nm spatial resolution.

We present the comprehensive characterization of a series of harmonic fields generated in a ZnO crystal by a few-cycle MIR driving pulse that spans the visible to mid-infrared (MIR) spectral region. The characterization is conducted using the recently developed Plasma-Induced Frequency Resolved Optical Switching (PI-FROSt) technique. We demonstrate the ability of this method to accurately characterize the MIR driving field (λ = 3.2 μm), as well as both odd and even harmonics up to the fifth order. The total spectral bandwidth extends over an exceptionally wide range of 2.6 octaves. All assessments validate the high precision of the field reconstructions and confirm the suitability of the PI-FROSt method for the metrology of over-octave-spanning waveforms. The results offer valuable insights into the fundamental mechanisms governing harmonic generation and emphasize the crucial influence of propagation and cascading.

A plethora of 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 multi-pulse sequences. These works illustrate that high-harmonic generation can effectively be used to study the microscopic electron dynamics in solids on the femtosecond timescale. Moreover, the all-optical switching demonstrated has numerous potential applications: These range from super-resolution microscopy to nanoscale-controlled chemistry, and highly tunable nonlinear light sources. Here we demonstrate the use of elliptically polarized light to spatially shape and confine high-harmonic generation from solids. This technique of spatial polarization gating provides a step towards a universal high-harmonic-based all-optical super-resolution imaging technique in solids. We demonstrate the common ellipticity response of high-harmonic generation in solids and show that we can reproduce these results with simulations based on the semiconductor Bloch equations. Proof-of-principle measurements show that with spatial polarization gating we can obtain sharper and smaller emission features than with conventional high-harmonic imaging, enabling higher-resolution imaging. This opens the door to resolving ultra-fast phenomena such as the insulator-to-metal phase transition in strongly correlated materials not only in time but also in space.

Plasma mirrors driven at kHz-repetition-rate with waveform-controlled relativistic-intensity near-single-cycle laser pulses produce extreme ultraviolet spectral continua supporting isolated attosecond pulses with diffraction limited beam quality. A newly developed liquid leaf target yields unprecedented stability and duration of operation. This lifts a major obstacle to exploiting plasma mirrors as attosecond pulse sources.

We present studies on boosting high-harmonic generation (HHG) efficiency from solids in the extreme-ultraviolet (XUV) region by two-color non-collinear wave mixing in silica. Our results show that the conversion efficiency of two-color wave mixing exceeds that of single-color HHG by a factor of at least ten. We analyze the underlying mechanisms, revealing that the synergy between Floquet-Bloch dressing and inter- and intraband dynamics under laser irradiation enables efficient carrier excitation, enhancing harmonic yield. Integrating wave mixing into solid-state HHG could help establish compact, coherent XUV sources for applications where traditional gas-HHG is impractical.

Shaping of the spectrum of frequency comb sources is highly beneficial for application that require controllable optical source, such as ranging or communications. However, efficient in-situ reconfiguration of the spectrum is a challenge. We demonstrate spectral shaping of frequency combs in fast-gain active devices by engineering an Aharonov-Bohm phase in a synthetic frequency lattice. By modulating a fast gain circular laser at its resonance and twice this frequency with a relative phase, we created a triangular ladder in the modal space, where the phase added with staggered phase flux. This broke the time-reversal symmetry in the system, allowing for non-trivial gauge fields that manipulate the light. This enable the control over the frequency lattice dynamics, enabling full bandwidth tuning of a strong central lobe in a Quantum Cascade Laser comb. This paves the wave to new efficient and simple reconfigurable optical frequency comb sources.

Light beams exhibit two intrinsic quantized degrees of freedom: spin angular momentum (SAM) and orbital angular momentum (OAM) whose manipulation enables precise control over the topological characteristics of electromagnetic fields. Recently, structured fields formed from a non-separable combination of SAM and OAM have attracted significant interest. These fields correspond to eigenstates of the generalized angular momentum (GAM), an operator merging SAM and OAM components which can yield remarkable fractional eigenvalues. The conservation of GAM, observed through harmonic generation, suggests its potential significance as a fundamental quantum number. In this work, we extend the analysis by examining its conservation laws through second-harmonic generation within an underdense, isotropic, and inhomogeneous plasma; a process governed by dipole-forbidden interaction which implies spin-orbit coupling. Our findings indicate that the symmetry and topological properties of the field are disrupted, leading to conservation of the GAM charge only on average. This symmetry breaking provides an easily detectable signature of the driving field’s topology