Structured light beams with space-variant polarization can be efficiently generated using voltage-tunable nematic liquid-crystal (Q-plate). By appropriately selecting the input state and the retardation of the Q-plate, an optical field acquires a spatially structured polarization distribution that is capable of encoding non-trivial topological information across the beam profile. These features can be directly read out through interaction with plasmonic nano-structures, such as circular and spiral slits. Here we show that, upon illumination, polarization-dependent excitation of surface plasmons converts the hidden topology of the polarization structure into observable intensity distributions, including plasmonic vortices and characteristic interference patterns, while the tunability of the input parameters enables a rich variety of distinct topological forms.

Polarization is a fundamental property of light that carries distinct and valuable information. Consequently, its precise measurement is crucial for numerous applications, including biomedical imaging, remote sensing, and optical communication. Since polarization cannot be measured directly, it is typically inferred by converting it into intensity signals using dedicated optical elements. Conventional approaches, however, predominantly rely on bulky optical components, leading to considerably high fabrication costs and limited integration density. Here, we introduce a passive photonic integrated circuit capable of precisely determining the polarization state of visible free-space light. An silicon nitride on-chip architecture employing a compact polarization-splitting grating coupler and a set of passive interferometers encodes the polarization information into intensity signals, allowing conventional detectors to accurately reconstruct the polarization state. With increasing compactness of photonic components, however, susceptibility to fabrication tolerances as well as intrinsic design constraints increases, potentially leading to non­/ideal behaviour. To address this, we introduce a robust calibration procedure that enables precise measurements even in the presence of imperfections. The chip design, combined with the calibration procedure, offers a robust, small-footprint, and high-speed approach to polarimetry, enabling a wide range of applications.

Dielectric nanoparticles can be engineered to scatter light predominantly in the transverse direction, a phenomenon known as the transverse Kerker effect. Although complete cancelation of forward scattering from a single object is forbidden by the optical theorem, we show that a single photonic mode can nonetheless realize an ideal transverse Kerker effect. The mode remains dark under normal incidence but evolves into an accidental bound state in the continuum when the nanoparticles are arranged in metasurfaces. This enables a new route to polarization-independent quasi-bound states in the continuum whose quality factors are tunable without symmetry breaking. We experimentally demonstrate our concept in the visible, achieving the first polarization-independent bound state in the continuum without the need for Brillouin-zone folding. Furthermore, we show that our modes maintain large quality factors over a substantially broader region of momentum space than conventional bound states in the continuum. Our results establish a platform for realizing ultranarrow resonances free of the constraints for designs with standard bound states in the continuum.

Raman spectroscopy is a versatile and well-established method for the identification of materials and characterization of their properties. The underlying physical effect, i.e., inelastic scattering, is highly inefficient. Hence, various enhancement techniques have been implemented over the years, often based on enhancing the local interaction field, eventually resulting in stronger Raman signals. In the enhancement process as well as in the interaction with the Raman-active material under study, the polarization of the excitation field plays a major role. Here, we show how spatially nanostructured light can be used along with individual plasmonic nanoparticles, acting as field enhancers, to selectively address Raman resonances of a high-refractive-index material, providing an additional means for analyzing the inherent properties of materials. This technique, which we call Structured Light Nanoparticle-Enhanced Raman Spectroscopy (SL-NERS), provides the ability to explore the response of Raman modes in the subwavelength regime.

We employ the concept of quantum Fisher information to optimize the focused excitation fields in coherent scattering microscopy. Our optimization goal is to achieve the best possible localization precision for small scatterers located above a glass coverslip, while keeping the intensity of the total incoming excitation fields fixed. For small numerical aperture (NA) values, the optimal fields have linear or circular polarization, and the excitation beam can be well approximated by a Gaussian one. For larger NA values, the optimal beam acquires radial polarization. We show that the high localization precision can be attributed to high field strengths at the scatterer position, and correspondingly a large number of scattered and detected photons. Finally, we evaluate the performance of the optimized beams in interferometric scattering microscopy (i scat ), and further optimize these fields for i scat localization using the concept of Fisher information.

Efficient and precise information storage and processing using light's various degrees of freedom - intensity, phase, and polarization - have vast applications in modern photonics. The corresponding utilization necessitates the accurate measurement and decomposition of arbitrary spatial modes into their orthogonal components. In this paper, we introduce a new modal decomposition technique based on a 16-pixel reconfigurable photonic integrated circuit programmed as a spatial mode decomposer. This device uniquely identifies and quantifies the relative contributions of constituent modes in a Laguerre-Gaussian basis. The presented device not only provides the relative weights of these modes but also their relative phases, offering a novel approach based on an integrated platform for optical information processing. We further highlight a novel input interface that enables the decomposition of input beam polarization into circular polarization basis. The potential applications of this technology are vast, ranging from advanced optical communications to microscopy and beyond, marking a significant stride in the field of integrated photonics.

Phase is an intrinsic property of light,and thus a crucial parameter across numerous applications in modern optics.Various methods exist for measuring the phase of light,each presenting challenges and limitations-from the mechanical stability requirements of free-space interferometers to the computational complexity usually as-sociated with methods based on spatial light modulators.Here,we utilize a passive photonic integrated circuit to spatially probe phase and intensity distributions of free-space light beams.Phase information is encoded into intensity through a set of passive on-chip interferometers,allowing conventional detectors to retrieve the phase profile of light through single-shot intensity measurements.Furthermore,we use silicon nitride as a material platform for the waveguide architecture,facilitating multi-spectral utilization in the visible spectral range.Our approach for fast,multi-spectral,and spatially resolved measurement of intensity and phase enables a wide variety of potential applications,ranging from microscopy to free-space optical communication.

A distinguishing feature of high-index dielectric nanoparticles is their ability to support strong Mie resonances, thereby enhancing the interaction of light with matter and minimizing Ohmic losses, leading to unprecedented efficiency. An important advancement in this field is the investigation of the "transverse Kerker" effect, in which both forward and backward scattering are significantly reduced while lateral scattering is enhanced. We uncover that the realization of a perfect transverse Kerker effect is possible even in passive structures, by exploiting the physics of bound states in the continuum—electromagnetic states remaining localized in photonic structures, coexisting with outgoing waves. Such 'transverse Kerker BICs' are polarization independent, and in momentum space are pinned at the center of polarization vortices with high order topological charges.

Due to its ability to meet requirements such as e.g. telemetry, millimeter-wave transceiver technology has gained research interest for various sensor applications, including the automotive and consumer sector. This work presents a resonant metamaterial for millimeter-waves that enables telemetric position sensing. The concept is based on a resonant unit cell that can be tuned to enable position encoding. A 2D metamaterial design was developed to parametrize the resonance frequency via a geometric parameter of the structure. The tuneable range of the metamaterial was estimated using a finite element method (FEM) simulation. This allowed for a unique mapping of resonance frequency and the geometric parameter, where a linear range for the sensor effect was selected. The resonance frequency shift encodes the absolute position via the geometry parameter of the metamaterial. A linear position encoded bar was fabricated using well-known PCB manufacturing techniques for position determination. The position encoded metamaterial was successfully tested with a vector network analyser under lab conditions. This telemetric position sensor concept offers a compact and contactless read-out without mechanical interference with the moving object. The metamaterial is completely passive, resulting in low maintenance and failure issues. The overall sensor concept includes a state-of-the-art radar chip as millimeter-wave transceiver which is currently under development.

Enhancing fields is an important task in millimeter-wave applications, such as nondestructive microwave inspection, metamaterial sensing applications, or millimeter-wave imaging. This demand for compact yet high-performance devices for field enhancement operating in the millimeter-wave regime has led to innovative approaches regarding lens design. State-of-the-art lens designs in this wavelength regime tend to be bulky and operate in the far field, making them unsuitable for small form factor applications. In this context, formulating the required functionality and algorithmically looking for the desired material topology is an inversion of the standard approach. This paper presents an inverse-designed field-amplifying metalens operating in the near field of a 60GHz patch antenna. With a size of about three times the wave length, the given structure promises good performance while maintaining a smaller form factor than conventional solutions. It yields an enhancement of the power amplitude by over 7dB.

Diffractive plasmonic metasurfaces offer the possibility of controlling the flow of light in flat optical systems through the excitation of lattice plasmon modes by a careful metasurface design. Nonetheless, a remaining challenge for this type of structure is the dynamic control of its optical properties via degrees of freedom, such as the polarization states of incoming light. In this report, we explain theoretically and demonstrate experimentally the polarization control over amplitude and propagation direction of lattice plasmon modes supported by a multipolar plasmonic metasurface. These unidirectional optical waves result from the coupling between near-field effects of individual meta-atoms and far-field effects originating from the lattice modes. The device operates over a broad wavelength range, maintaining its directional behavior and enabling it to operate also as a polarization-controlled directional diffraction grating, a power splitter, or an optical router for on-chip photonics applications.

Epsilon-near-zero (ENZ) materials, i.e., materials with a vanishing real part of the permittivity, have become an increasingly desirable platform for exploring linear and nonlinear optical phenomena in nanophotonic and on-chip environments. ENZ materials inherently enhance electric fields for properly chosen interaction scenarios, host extreme nonlinear optical effects, and lead to other intriguing phenomena. To date, studies in the optical domain have mainly focused on nanoscopically thin films of ENZ materials and their interaction with light and other nanostructured materials. Here, we experimentally and numerically explore the optical response of individual nanostructures milled into an ENZ material. For the study, we employ 3D structured light beams, allowing us to fully control polarization-dependent field enhancements enabled by a tailored illumination and a vanishing permittivity. Our studies provide insight between complex near-fields and the ENZ regime while showcasing the polarization-dependent controllability they feature. Such effects can form the basis for experimental realizations of extremely localized polarization-controlled refractive index changes, which can ultimately enable ultrafast switching processes at the level of individual nanostructures.

In technology, old or new, from basic imaging through a camera lens to advanced applications such as fluorescence microscopy and optical lithography, there are countless examples that would be inconceivable without the utilization of focused light. As technology evolves, the demands on spatially confined light fields grow but so do the challenges of accurately characterizing these complex fields. This study introduces a technique to measure the full vectorial nature of light, reaching sub/wavelength spatial resolution while capturing the 3D amplitude and phase for both electric and magnetic fields. This is achieved based on a polarization-resolved far-field analysis of light scattered by a single spherical nanoparticle acting as a local probe. For experimental verification, the method is applied to tightly focused light fields under various input scenarios. Offering high resolution, precision, and flexibility, this technique shows great promise for both fundamental research and applications in technologies relying on highly localized light fields.

We present a metamaterial for orbital angular momentum generation in the millimeter wave regime that can be fabricated with low-cost fused deposition modeling (FDM) 3D printers. The metamaterial induces a spatially distributed phase shift on millimeter waves, passing through the sample, via a spatial variation of the effective permittivity. We use an analytical model to calculate the effective permittivity of the metamaterial unit cell, which consists of a square block made of a dielectric with a cylindrical air hole at its center. The analytical model is used to design a metasurface that generates a beam carrying orbital angular momentum of order $l=1$. We prove this concept in a laboratory setup using a commercially available millimeter wave chip as the source.

Structured light is a key component of many modern applications, ranging from superresolution microscopy to imaging, sensing, and quantum information processing. As the utilization of these powerful tools continues to spread, the demand for technologies that enable the spatial manipulation of fundamental properties of light, such as amplitude, phase, and polarization grows further. In this respect, technologies based on liquid-crystal cells, e.g., spatial light modulators, became very popular in the last decade. However, the rapidly advancing field of integrated photonics allows entirely new routes towards beam shaping that not only outperform liquid-crystal devices in terms of speed, but also have substantial potential with respect to robustness and conversion efficiencies. In this study, we demonstrate how a programmable integrated photonic processor can generate and control higher-order free-space structured light beams at the click of a button. Our system offers lossless and reconfigurable control of the spatial distribution of light's amplitude and phase, with switching times in the microsecond domain. The showcased on-chip generation of spatially tailored light enables an even more diverse set of methods, applications, and devices that utilize structured light by providing a pathway towards combining the strengths of programmable integrated photonics and free-space structured light.

Integrated photonic devices provide significant advantages over their conventional counterparts, such as a drastically reduced footprint as well as compatibility with other photonic or electronic circuitry. In this work, we present a high-precision optical position sensor fabricated on a silicon-on-insulator platform. The sensor relies on the principle of position-dependent directional waveguide coupling upon excitation of a monolithically integrated scatterer with a tightly focused polarization-tailored beam. We demonstrate a spatial resolution of 7.2 nm, corresponding to approximately λ/200.

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects, ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in the life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nano-particle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration.

Integrated photonics has gained growing attention in the last two decades, mainly due to its potential to increase the speed of data transfer and to shrink the footprint of integrated circuits (ICs). This lead to the rapid development of a huge variety of integrated optical components and compound devices such as on-chip light sources, optical modulators, detectors and many others [1]. The technology developed in this framework can also be of great use for the miniaturization and integration of optical sensors. Here, we present a high-precision displacement sensor based on a silicon-on-insulator platform. The core of the device is a cylindrical, all-integrated scatterer which is excited by a tightly focused vector beam. This excitation and the resulting directional coupling of light to the surrounding waveguides depends strongly on the position of the device with respect to the focused beam [2]. Recording the light coupled out of each of the waveguides with a CCD camera and employing a tailored calibration technique enables localization of the device with a resolution of $\lambda/200$ . Fig. 1 demonstrates the working principle of the device by means of two exemplary camera images. The difference among the two images in terms of the light observed at the four outcouplers is caused by a relative displacement of $\Delta x=\Delta y=160$ nm.

Polarization-preserving fibers maintain the two polarization states of an orthogonal basis. Quantum communication, however, requires sending at least two nonorthogonal states and these cannot both be preserved. We present a new scheme that allows for using polarization encoding in a fiber not only in the discrete, but also in the continuous-variable regime. For the example of a helically twisted photonic-crystal fibre, we experimentally demonstrate that using appropriate nonorthogonal modes, the polarization-preserving fiber does not fully scramble these modes over the full Poincar\'e sphere, but that the output polarization will stay on a great circle; that is, within a one-dimensional protected subspace, which can be parametrized by a single variable. This will allow for more efficient measurements of quantum excitations in nonorthogonal modes.