Coherent Fourier Scatterometry (CFS) is a powerful optical metrology technique for the precise characterisation of nanostructures. Conventional CFS systems rely on piezo-based scanning stages for raster scanning, which limits throughput due to slow scanning speeds. In this work, we present a high-speed CFS system incorporating a galvanometric (galvo) mirror for beam scanning. This approach significantly enhances scanning speed while maintaining measurement accuracy. Although galvo mirrors are widely used in optical systems, their implementation in CFS has unique challenges such as off-axis beam aberrations and angle-dependent beam shifts at the split detector. These issues are analysed and mitigated through optical design, alignment and system calibration. Additionally, we derive the minimum detector bandwidth required to capture high-frequency signals generated by the fast scanning. The effectiveness of the system is demonstrated through the calibration of pits with various diameters that are etched onto a silicon wafer. Results show a substantial improvement in scanning speed as compared with piezo-based systems without compromising measurement precision, making this approach highly suitable for high-throughput metrology applications.

We present a digital micro-mirror device (DMD)-enabled scan head for coherent Fourier scatterometry (CFS) that performs lateral scanning without macroscopic moving parts, while maintaining a diffraction-limited probe. A binary Fresnel zone plate (FZP) is displayed and translated on the DMD to steer a single focused spot across the sample, providing an electronically programmable alternative to scanning using piezo-based translation devices. To the best of our knowledge, this is the first published CFS implementation in which a DMD is the primary lateral scanning element. Furthermore, the DMD programmability is used to compensate for the scan-position-dependent aberrations using an iterative optimisation algorithm. Across a 400 × 200 DMD-pixel scan area, the peak-intensity coefficient of variation improves from 39.4% (uncorrected) to 16.1% (after correction) and to 4.47% with additional power normalisation, demonstrating substantially improved probe uniformity. Finally, we demonstrate particle detection on a Si wafer with 1 μm polystyrene latex particles, achieving an signal-to-noise ratio of 16.04 ± 1.11dB. The results establish DMD-FZP scanning with integrated aberration correction as a compact, fast, and scalable CFS architecture, with a clear pathway to higher throughput via multi-spot parallelisation.

Recovering both amplitude and phase information from a system is a fundamental goal of optical imaging. At the same time, it is crucial to operate at low photon doses to avoid altering the sample, particularly in biological applications. Quantum imaging provides a powerful route to extract more information per photon than classical techniques, which are ultimately limited by shot-noise. However, the trade-off between quantum noise reduction and spatial resolution has long been regarded as a major obstacle to the application of quantum techniques to small cellular and sub-cellular structures, where they could offer the greatest benefits. Here, we overcome this limitation by demonstrating sub-shot-noise quantitative phase imaging of biological cells based on the transport-of-intensity equation, enabling high-fidelity, label-free imaging of key cellular and sub-cellular features. We achieve high-resolution phase imaging limited only by the numerical aperture, while simultaneously obtaining a resolution-independent quantum advantage. Unlike other quantum imaging approaches, our method operates in a quasi-single-shot, wide-field configuration, retrieves both phase and amplitude information, and does not rely on interferometric measurements, making it intrinsically fast and stable. These results pave the way for the immediate application of sub-shot-noise imaging in biological microscopy.

Coherent Fourier Scatterometry (CFS) enables low-power, high- resolution, non-destructive metrology for nanoscale structures. Recent advancements have extended its applications to improving the measurement of critical dimensions, such as steep-sidewall angles of fabricated nanostructures and the detection and shape determination of defects for semiconductor and power electronics applications. Innovations like beam scanning, multi-beam setups, and synthetic optical holography enhance its speed and sensitivity, making CFS increasingly viable for industrial in-line inspection.

Coherent Fourier scatterometry (CFS) is a very sensitive optical metrology technique that has been applied for detection and characterisation of nanostructures. It is a scanning-based technique where the samplie is illuminated with a focused light spot. However, in practical CFS systems, residual optical aberrations can distort the focused spot and degrade the signal-to-noise ratio during measurements. Here, we present a systematic study of the influence of low-order aberrations: defocus, spherical, astigmatism, oblique astigmatism, and coma on the differential split-detector CFS signal. Controlled amounts of each aberration, described by Zernike polynomials, were introduced into the Fourier plane via a spatial light modulator. Two-dimensional differential scattering maps were recorded on a reference sample of 425 nm diameter, 150 nm deep pits etched in silicon, and the peak-to-peak differential signal was quantified as a function of peak-valley (PV) wavefront error. We find that defocus has the strongest impact, halving the signal at just 0.27λ PV, followed by spherical (0.32λ) and coma (0.40λ), whereas astigmatism and oblique astigmatism require larger wavefront errors (> 0.6λ) to produce comparable signal loss. These results define quantitative aberration tolerances for CFS systems. The insights gained here can guide the design and optimisation of different CFS implementations for in-line process control and nanostructure metrology.

We exploit quantum correlations to enhance quantitative phase retrieval of an object in a non-interferometric setting, only measuring the propagated intensity pattern after interaction with the object.

Quantum entanglement and squeezing have significantly improved phase estimation and imaging in interferometric settings beyond the classical limits. However, for a wide class of non-interferometric phase imaging/retrieval methods vastly used in the classical domain, e.g., ptychography and diffractive imaging, a demonstration of quantum advantage is still missing. Here, we fill this gap by exploiting entanglement to enhance imaging of a pure phase object in a non-interferometric setting, only measuring the phase effect on the free-propagating field. This method, based on the so-called “transport of intensity equation", is quantitative since it provides the absolute value of the phase without prior knowledge of the object and operates in wide-field mode, so it does not need time-consuming raster scanning. Moreover, it does not require spatial and temporal coherence of the incident light. Besides a general improvement of the image quality at a fixed number of photons irradiated through the object, resulting in better discrimination of small details, we demonstrate a clear reduction of the uncertainty in the quantitative phase estimation. Although we provide an experimental demonstration of a specific scheme in the visible spectrum, this research also paves the way for applications at different wavelengths, e.g., X-ray imaging, where reducing the photon dose is of utmost importance.

In recent years, a lot of works have been published that use parameter retrieval using orbital angular momentum (OAM) beams. Most make use of the OAM of different Laguerre-Gauss modes. However, those specific optical beams are paraxial beams and this limits the regime in which they can be used. In this paper, we report on the first results on retrieving the geometric parameters of a diffraction grating by analysing the corresponding complex-valued (i.e. amplitude and phase) Helmholtz Natural Modes (HNM) spectra containing both the azimuthal (i.e. n) and radial (i.e. m) indices. HNMs are a set of orthogonal, non-paraxial beams with finite energy carrying OAM. We use the coherent Fourier scatterometry (CFS) setup to calculate the field scattered from the diffraction grating. The amplitude and phase contributions of each HNM are then obtained by numerically calculating the overlap integral of the scattered field with the different modes. We show results on the sensitivity of the HNMs to several grating parameters.