Conference Agenda

Dual and multi-wavelength digital holography has demonstrated to be a relevant tool for desensitized testing of optical surfaces, large deformation of structures or surface shape profiling. With the advent of digital holography, a wide range of applications of dual/multi-wavelength holography was demonstrated, such as endoscopic imaging, calibration of mechanical structures, erosion measurements, in-line industrial inspection, melt-pool monitoring in additive laser welding manufacturing or more recently accurate profiling by coherence scanning profilometry.

However, due to the natural roughness of the inspected surface, speckle decorrelation occurs and noise is included in the data. This noise refers as the “speckle decorrelation” noise. Especially, the noise is non Gaussian, non-stationary, amplitude–dependent and may be anisotropic. In order to yield high quality data for metrology purpose, speckle decorrelation is required to be reduced. Recently deep leaning has emerged as a powerful and rapid approach for processing phase data. This paper proposes an overview of dual and multi-wavelength digital holography approaches and aims at describing the last theoretical results in the analysis of the standard deviation of decorrelation noise. The influence of noise in the measurements of the surface shape is described by an analytical approach. Numerical simulations with realistic experimental parameters are provided and discussed.

Gold standard imaging modalities in biological field are based on fluorescence signals providing high specificity and high resolution. Recently, Fluorescence Microscopy has been combined with microfluidics to develop instrumentations called Imaging Flow Cytometers, high-throughput tools that supply bright-field, darkfield and multiple-channels fluorescence images of each single cell passing in the Field Of View (FOV). Nevertheless, Fluorescence Microscopy has some drawbacks as phototoxicity, photobleaching, expensive costs for sample preparations and also the a-priori knowledge of the tags to be used. For these reasons label-free imaging methods greatly increase in the recent years as the Quantitative Phase Imaging (QPI) technologies for microscopy. One of the optical techniques to achieve QPI is Digital Holography. DH in microscopy has several advantages such as the possibility to numerically scan the focal distance, a properties that open to the integration of DH in microfluidics. Indeed DH combined with microfluidic circuits allows to image particles or cells flowing into the FOV at different depths. Here the capabilities of label-free single-cell imaging by DH are presented and their implications on next future biomedical applications discussed. Static or in-flow configurations combined with DH will be showed describing recent results and perspectives also in combination with Artificial Intelligence architectures.

Traditional methods used to realize the primary standard for the absolute optical power standard rely on expensive equipment and require well-trained personnel for maintenance and measurement activities. Silicon photonics technologies have enabled the development of predictable photodiodes with uncertainty comparable to (or perhaps better than) traditional methods. This work will report these research activities.

Terahertz time-domain spectroscopic ellipsometry (THz-TDSE) provides several advantages and opens a wide range of applications in optical metrology. We demonstrate the measurement of complete ellipsometric spectral response in terms of the Jones or Mueller matrix based on polarization switching by using a spintronic THz emitter (STE). The general method is demonstrated on THz-TDSE of anisotropic crystal of Mercury Chloride (Calomel). Applications of THz-TDSE in the field of THz optical activity and contactless measurement of surface electric properties are proposed.

An imaging Mueller matrix ellipsometer is used to measure structures in measuring fields in the micrometre range, which are too small for conventional ellipsometry. Line and grid structures are measured and evaluated with the help of numerical simulations using the finite element method to characterize the structure parameters.

We present the impact of line edge roughness (LER) on the optical critical dimension (OCD) metrology of nanostructures. The consideration of LER in OCD requires numerically expensive forward models and is therefore usually neglected. We present an analytical approach that allows estimation of the impact on the uncertainty. Systematic differences between CD measured by SEM and OCD were observed in different experiments. While SEM is basically sensitive to the local volume density, optical methods are sensitive to the permittivity of the material. We discuss an analytical upper bound on the contribution of the LER. For high index gratings, the contribution is as high as 3.7 nm for TM-polarized light and 1.2 nm for TE-polarized light, making this crucial for sub-nanometer metrology.

Gold gratings were measured by spectroscopic ellipsometry and modeled by the finite element method to investigate the capabilities of optical dimensional metrology for plasmonic diffractive structures. The gratings were prepared by electron beam lithography using parameters determined by finite element simulations for significant variations of the amplitude ratio and phase shift of the polarized reflection coefficients to achieve high sensitivity for both the measurement of the grating dimensions and the sensing capabilities. Sub-nanometer sensitivity was shown to determine the grating dimensions and the thickness of an adsorbed layer to be detected in both traditional reflection and Kretschmann-Raether (KR) configurations. The sensitivity for the refractive index of the ambient was calculated to be 10$^{-5}$ at best, which is not significantly better than the sensitivities for plane gold layers in KR configurations. However, in diffraction-based resonant setups, the high sensitivity dips can be shifted to a larger spectral range, which is highly significant in many applications. It was also revealed that 2D models assuming a perfect geometry fit the measured ellipsometry spectra only qualitatively, leaving room for model development in the future.

In the pursuit of closing the gap between nanometrology and nanofabrication, we investigate the use of advanced optical far field methods for sub-wavelength parameter reconstruction. With the goal of establishing a hybrid evaluation scheme connecting different methods and including different information channels, we performed comparison measurements on a silicon line grating sample with buried as well as not buried surface relief lines. To this end, the results of our measurement are in good agreements with each other, and the collected structure data is feasible to be used for hybrid evaluation.

Exploiting nonclassical states of light allows new imaging and sensing approaches. In particular, nonlinear interferometers enable quantum imaging with undetected light. Here, based on the effect of induced coherence, samples can be probed with light that is not detected at all. Instead, its quantum-correlated partner light is recorded and yields the information of the sample, although it never interacted with it. This enables new sensing modalities beyond classical limitations. The talk will outline the fundamental concept, recent progress, limits, and perspectives for biomedical applications of nonlinear interferometers.

We present the Hartmann sensor's capacity, which is traditionally used for wavefront sensing, to measure the coherence properties of the signal. By reinterpreting the detection theory of the conventional Hartmann sensor within the framework of quantum tomography, we unveil the ability to quantify the mutual intensity function in the form of coherence matrix analogue to the density matrix of the mixed quantum state. With this analogy, we can use the quantum-inspired algorithms to reconstruct this matrix from experimental data.

Fast and accurate asphere and freeform measurements are in high demand by the optics manufacturing industry. Interferometric methods such as tilted-wave interferometry meet these demands but require accurate surface positioning of the specimen along the optical axis, since such measurements are sensitive to such positioning errors. In this work the Cat's Eye position will be investigated in terms of accuracy and repeatability as a reference position for surface positioning in tilted-wave interferometry. For this purpose, a two-regime method for specimen alignment using different optimization criteria is investigated and its repeatability is evaluated. Accurate and reproducible positioning into the Cat's Eye position together with interferometric movement tracking will allow accurate specimen positioning along the optical axis, which will significantly reduce the surface measurement errors associated with such misalignment and improve the overall measurement uncertainty.

Shack-Hartmann wavefront sensor is applied today in broad areas of interest. Especially in optical systems quality assessment, the SHWS provides fast and accurate wavefront measurement. The sensitivity, i.e., the minimal measurable change in the wavefront, is usually omitted when it comes to a single Zernike Polynomials level. There is no ISO standard either. Comparing the specifications of SHWS among different manufacturers, one can feel confused. Here, we show the sensitivity for single Zernike polynomials up to the third radial order. In addition, we calculate the minimal wavefront RMS at the quantum limit using Fisher Information theory and compare it with the standard modal reconstruction algorithm used in SHWS.

The analysis carries two regimes: weak signal for adaptive optics and strong signal for optical metrology.

In this presentation, we propose a measurement system to observe 2-D in-plane displacement at a fast sampling rate of 5 kHz using a sinusoidal phase modulation interferometer (SPMI). The SPMI consists of a Michelson interferometer incorporating an electric-optic modulator (EOM) with a modulation frequency of 5 kHz and a high-speed camera (HSC) synchronised to a clock signal at a frequency of 60 kHz, 12 times the modulation frequency. Phase demodulation of each pixel in the camera is performed by acquiring the light intensity signal to that pixel in synchronisation with the sampling signal and performing a specific addition or subtraction of them. By applying this procedure to all pixels in the camera, the 2D in-plane displacement can be obtained. This technique has the potential to measure fast, dynamic deformation of object surfaces and dynamic wavefront aberrations due to air fluctuations.

The progression of fibrosis is frequently related to a failed healing process, and it may affect many tissues and organs causing severe consequences including post-infarct heart insufficiency, post-injury limb paralysis, cirrhosis, nephropathy, retinopathy, failure of implanted devices and even resistance to chemotherapy in solid tumours. Experimental models, both in vitro and in vivo, are widely used for studies of basic pathophysiology, and for pre-clinical testing of pharmacological therapies counteracting fibrosis. However, conventional models are not able to realistically reproduce the revascularization aspect of the inflammatory reaction that leads to the generation of fibrotic tissue. In this talk, I will present the most advanced frontier in this sector, represented by the new concept of an "organism-on-a-chip". This is a hybrid model, in which an organ-on-a-chip is implanted sub-cute in a living organism, such as a mouse or an embryonated avian egg, thus eliciting a foreign-body reaction with the formation of a fibrotic microenvironment. In an organism-on-a-chip, the fibrotic reaction can be guided in terms of extra-cellular stiffness and vascularity, using microscopic scaffolds incorporated in the implanted chip. The fibrotic microenvironment can then be imaged longitudinally, in high resolution, with the added advantage of significantly reducing animal sacrifice.

We present how to develop virtual microcylinder- or microsphere-assisted surface topography measurement instruments. As the most critical part, the interaction between light, microcylinder and measurement object is considered based on the finite element method (FEM). Results are obtained for microcylinder-assisted conventional, interference, and confocal microscopes without necessity to repeat the time-consuming FEM simulations for each sensor.

Topographical as well as microscopic imaging of nanoscale surfaces plays a pivotal role across various disciplines. Nevertheless, achieving fast, label-free, and accurate characterization of laterally expanded structures below the diffraction limit remains challenging. Recent studies highlight the use of microsphere assistance for resolution improvement. Confocal microscopy, augmented by microspheres, enables the imaging of small structures that were previously inaccessible. This is experimentally compared with microsphere-assisted microscopy (MAM) to underline the decisive role of the confocal effect.

Nearfield spectroscopy is crucial for characterizing micro- and nanostructures and it often requires hyperspectral imaging, where at each spatial point a full spectrum is recorded. Due to its combination with an atomic force microscope, nearfield hyperspectral imaging is serial in nature and results in long acquisition times and stability challenges, also restricting its industrial use. In this work, we employ a subsampling strategy combined with low-rank matrix reconstruction in a commercial nearfield spectroscopy system to significantly shorten measurement acquisition times.

Optical vortices, characterized by their helical phase topology and ability to carry orbital angular momentum, have found diverse applications in metrology. In this work, we present novel metrology systems utilizing vortex beams. The adaptation of optical vortex microscopy for rough surface measurement using fluorescent nanomarkers, and experimental setup for retardation measurement are described. Retardation measurement has been successfully applied to the calibration of spatial light modulator and can be adapted for measurement of circular dichroism. In developed methods the information on retardation or local surface height is restored from self-interference spread function of optical vortex beams carrying opposite topological charges, called Double-Helix Point Spread Function (DH PSF). The use of neural networks, enhancing measurement accuracy and enabling advanced data analysis for data processing, is described.

We present two distinct types of 'smart' surfaces designed for facilitating the quantitative exploration of dynamic processes occurring at sub-wavelength distances from interfaces, using far-field optical techniques. Based on evanescent waves in excitation and/or emission, we achieve an axial localization precision of about 10 nm. The first type of substrate incorporates nanocavities in a thin metallic film, enhancing and confining the electromagnetic field to a tiny volume. The second sample consists of a thin fluorescent film sandwiched between transparent spacer and capping layers deposited on a glass coverslip. The emission pattern from this film codes detailed information about the local fluorophore environment, namely, the refractive index, defects, reciprocal lattice, and the axial distance of the molecular emitter from the surface. An application to axial metrology in total internal reflection fluorescence and axial super-localisation microscopes is presented.

Emission patterns from molecules at interfaces encode many details about their local environment and their axial position, along the microscope’s optical axis. We introduce an advanced approach that synergizes back focal plane (BFP) imaging with innovative 'smart' surfaces make surface imaging more qualitative, more reliable, and more robust. Our method is particularly focused on accurately measuring the refractive index (RI) of transparent thin films and their imperfections close to the interfaces. Our technique utilizes a 'smart' surface, which features a uniform fluorescent thin film of about 4 nm thickness together with back-focal plane (BFP) imaging. We manage to detect bubbles or other imperfection in 100 nm thin film of polymer with RI of 1.34.

Coherent fiber bundles (CFB) are used in endoscopes for instance in biomedical diagnosis or optical inspection for industrial processes. Their working principle is based on the pixelated intensity transfer via several thousands of fiber cores, within a single monolithic structure. Due to scattering of the effective refractive index in the fiber cores, all phase information is lost. Thus, CFB endoscopes conventionally offer pixelated 2D imaging only, whereas the distal optics used for (de-) magnification enhances the endoscope diameter typically beyond 2 mm and suffers aberrations.

Measuring the optical transfer function of the CFB or more specifically the phase distortion enables correcting said distortion. Advances in CFB endoscopy are presented and discussed. This encompasses single-sided self-calibration and video rate 3D imaging without distal optics based on digital optical phase conjugation using a spatial light modulator. Deep learning is used for real-time complex light field generation as well as image deconvolution. Furthermore, a CFB with bending invariant OTF is introduced. In combination with laser-based manufacturing, this enables an ultrathin and mechanically flexible optical lens. The novel component can be integrated into standard widefield or raster scanning microscopes to enable endoscopic applications with diameters below 400 µm and 1 µm resolution.

The new polarization-holographic element of an optimal configuration is developed for the real-time complete analysis of the polarization state of light (for determining all Stokes parameters). The simultaneous measurement of the intensities in all points of images in diffracted orders using CCD camera and appropriate software allows to determine the spatial distribution of a polarization state in the images of objects, and also the dispersion of this distribution.