Lasing from a Large-Area 2D Material Enabled by a Dual-Resonance Metasurface.

Semiconducting transition metal dichalcogenides (TMDs) have gained significant attention as a gain medium for nanolasers, owing to their unique ability to be easily placed and stacked on virtually any substrate. However, the atomically thin nature of the active material in existing TMD lasers and the limited size due to mechanical exfoliation presents a challenge, as their limited output power makes it difficult to distinguish between true laser operation and other "laser-like" phenomena. Here, we present room temperature lasing from a large-area tungsten disulfide (WS2) monolayer, grown by a wafer-scale chemical vapor deposition (CVD) technique. The monolayer is placed on a dual-resonance dielectric metasurface with a rectangular lattice designed to enhance both absorption and emission, resulting in an ultralow threshold operation (threshold well below 1 W/cm2). We provide a thorough study of the laser performance, paying special attention to directionality, output power, and spatial coherence. Notably, our lasers demonstrated a coherence length of over 30 µm, which is several times greater than what has been reported for 2D material lasers so far. Our realization of a single-mode laser from a CVD-grown monolayer presents exciting opportunities for integration and the development of real-world applications.

A Fully Metaoptical Zoom Lens with a Wide Range.

Metalenses are typically designed for a fixed focal length, restricting their functionality to static scenarios. Various methods have been introduced to achieve the zoom function in metalenses. These methods, however, have a very limited zoom range, or they require additional lenses to achieve direct imaging. Here, we demonstrate a zoom metalens based on axial movement that performs both the imaging and the zoom function. The key innovation is the use of a polynomial phase profile that mimics an aspheric lens, which allows an extended depth of focus, enabling a large zoom range. Experimental results show that this focal length variation, combined with the extended depth of focus, translates into an impressive zoom range of 11.9× while maintaining good imaging quality. We see applications for such a zoom metalens in surveillance cameras of drones or microrobots to reduce their weight and volume, thus enabling more flexible application scenarios.

Phase noise matching in resonant metasurfaces for intrinsic sensing stability.

Interferometry offers a precise means of interrogating resonances in dielectric and plasmonic metasurfaces, surpassing spectrometer-imposed resolution limits. However, interferometry implementations often face complexity or instability issues due to heightened sensitivity. Here, we address the necessity for noise compensation and tolerance by harnessing the inherent capabilities of photonic resonances. Our proposed solution, termed "resonant phase noise matching," employs optical referencing to align the phases of equally sensitive, orthogonal components of the same mode. This effectively mitigates drift and noise, facilitating the detection of subtle phase changes induced by a target analyte through spatially selective surface functionalization. Validation of this strategy using Fano resonances in a 2D photonic crystal slab showcases noteworthy phase stability (σ<10-4π). With demonstrated label-free detection of low-molecular-weight proteins at clinically relevant concentrations, resonant phase noise matching presents itself as a potentially valuable strategy for advancing scalable, high-performance sensing technology beyond traditional laboratory settings.

RGB Achromatic Metalens Doublet for Digital Imaging.

Chromatic aberration is a major challenge faced by metalenses. Current methods to achieve broadband achromatic operation in metalenses usually suffer from limited size, numerical aperture, and working bandwidth due to the finite group delay of meta-atoms, thus restricting the range of practical applications. Multiwavelength achromatic metalenses can overcome those limitations, making it possible to realize larger numerical aperture (NA) and sizes simultaneously. However, they usually require three layers, which increases their fabrication complexity, and have only been demonstrated in small sizes, with low numerical aperture and modest efficiencies. Here, we demonstrate a 1 mm diameter red-green-blue achromatic metalens doublet with a designed NA of 0.8 and successfully apply the metalens in a digital imaging system. This work shows the potential of the doublet metasurfaces, extending their applications to digital imaging systems such as digital projectors, virtual reality glasses, high resolution microscopies, etc.

Beyond Q: The Importance of the Resonance Amplitude for Photonic Sensors.

Resonant photonic sensors are enjoying much attention based on the worldwide drive toward personalized healthcare diagnostics and the need to better monitor the environment. Recent developments exploiting novel concepts such as metasurfaces, bound states in the continuum, and topological sensing have added to the interest in this topic. The drive toward increasingly higher quality (Q)-factors, combined with the requirement for low costs, makes it critical to understand the impact of realistic limitations such as losses on photonic sensors. Traditionally, it is assumed that the reduction in the Q-factor sufficiently accounts for the presence of loss. Here, we highlight that this assumption is overly simplistic, and we show that losses have a stronger impact on the resonance amplitude than on the Q-factor. We note that the effect of the resonance amplitude has been largely ignored in the literature, and there is no physical model clearly describing the relationship between the limit of detection (LOD), Q-factor, and resonance amplitude. We have, therefore, developed a novel, ab initio analytical model, where we derive the complete figure of merit for resonant photonic sensors and determine their LOD. In addition to highlighting the importance of the optical losses and the resonance amplitude, we show that, counter-intuitively, optimization of the LOD is not achieved by maximization of the Q-factor but by counterbalancing the Q-factor and amplitude. We validate the model experimentally, put it into context, and show that it is essential for applying novel sensing concepts in realistic scenarios.

Perturbation approach to improve the angular tolerance of high-Q resonances in metasurfaces.

The interest in high quality factor (high-Q) resonances in metasurfaces has been rekindled with the rise of the bound states in the continuum (BIC) paradigm, which describes resonances with apparently limitlessly high quality-factors (Q-factors). The application of BICs in realistic systems requires the consideration of the angular tolerance of resonances, however, which is an issue that has not yet been addressed. Here, we develop an ab-initio model, based on temporal coupled mode theory, to describe the angular tolerance of distributed resonances in metasurfaces that support both BICs and guided mode resonances (GMRs). We then discuss the idea of a metasurface with a perturbed unit cell, similar to a supercell, as an alternative approach for achieving high-Q resonances and we use the model to compare the two. We find that, while sharing the high-Q advantage of BIC resonances, perturbed structures feature higher angular tolerance due to band planarization. This observation suggests that such structures offer a route toward high-Q resonances that are more suitable for applications.

Dielectric nanohole array metasurface for high-resolution near-field sensing and imaging.

Dielectric metasurfaces support resonances that are widely explored both for far-field wavefront shaping and for near-field sensing and imaging. Their design explores the interplay between localised and extended resonances, with a typical trade-off between Q-factor and light localisation; high Q-factors are desirable for refractive index sensing while localisation is desirable for imaging resolution. Here, we show that a dielectric metasurface consisting of a nanohole array in amorphous silicon provides a favourable trade-off between these requirements. We have designed and realised the metasurface to support two optical modes both with sharp Fano resonances that exhibit relatively high Q-factors and strong spatial confinement, thereby concurrently optimizing the device for both imaging and biochemical sensing. For the sensing application, we demonstrate a limit of detection (LOD) as low as 1 pg/ml for Immunoglobulin G (IgG); for resonant imaging, we demonstrate a spatial resolution below 1 µm and clearly resolve individual E. coli bacteria. The combined low LOD and high spatial resolution opens new opportunities for extending cellular studies into the realm of microbiology, e.g. for studying antimicrobial susceptibility.

Broadband c-Si metasurfaces with polarization control at visible wavelengths: applications to 3D stereoscopic holography.

Visual arts and entertainment related industries are continuously looking at promising innovative technologies to improve users' experience with state-of-the-art visualization platforms. This requires further developments on pixel resolution and device miniaturization which can be achieved, for instance, with high contrast materials, such as crystalline silicon (c-Si). Here, a new broadband stereoscopic hologram metasurface is introduced, where independent phase control is achieved for two orthogonal polarizations in the visible spectrum. The holograms are fabricated with a birefringent metasurface consisting of elliptical c-Si nanoposts on Sapphire substrate. Two holograms are combined on the same metasurface (one for each polarization) where each is encoded with four phase levels. The theoretical bandwidth is 110 nm with a signal to noise ratio (SNR) >15 dB. The stereoscopic view is obtained with a pair of cross-polarized filters in front of the observers' eyes. The measured transmission and diffraction efficiencies are about 70% and 15%, respectively, at 532 nm (the design wavelength). The metasurfaces are also investigated at 444.9 nm and 635 nm to experimentally assess their bandwidth performance. The stereoscopic effect is surprisingly good at 444.9 nm (and less so at 635 nm) with transmission and diffraction efficiencies around 70% and 18%, respectively.

Highly efficient holograms based on c-Si metasurfaces in the visible range.

This paper reports on the first hologram in transmission mode based on a c-Si metasurface in the visible range. The hologram shows high fidelity and high efficiency, with measured transmission and diffraction efficiencies of ~65% and ~40%, respectively. Although originally designed to achieve full phase control in the range [0-2π] at 532 nm, these holograms have also performed well at 444.9 nm and 635 nm. The high tolerance to both fabrication and wavelength variations demonstrate that holograms based on c-Si metasurfaces are quite attractive for diffractive optics applications, and particularly for full-color holograms.

Paths to light trapping in thin film GaAs solar cells.

It is now well established that light trapping is an essential element of thin film solar cell design. Numerous light trapping geometries have already been applied to thin film cells, especially to silicon-based devices. Less attention has been paid to light trapping in GaAs thin film cells, mainly because light trapping is considered less attractive due to the material's direct bandgap and the fact that GaAs suffers from strong surface recombination, which particularly affects etched nanostructures. Here, we study light trapping structures that are implemented in a high-bandgap material on the back of the GaAs active layer, thereby not perturbing the integrity of the GaAs active layer. We study photonic crystal and quasi-random nanostructures both by simulation and by experiment and find that the photonic crystal structures are superior because they exhibit fewer but stronger resonances that are better matched to the narrow wavelength range where GaAs benefits from light trapping. In fact, we show that a 1500 nm thick cell with photonic crystals achieves the same short circuit current as an unpatterned 4000 nm thick cell. These findings are significant because they afford a sizeable reduction in active layer thickness, and therefore a reduction in expensive epitaxial growth time and cost, yet without compromising performance.

High speed e-beam writing for large area photonic nanostructures - a choice of parameters.

Photonic nanostructures are used for many optical systems and applications. However, some high-end applications require the use of electron-beam lithography (EBL) to generate such nanostructures. An important technological bottleneck is the exposure time of the EBL systems, which can exceed 24 hours per 1 cm(2). Here, we have developed a method based on a target function to systematically increase the writing speed of EBL. As an example, we use as the target function the fidelity of the Fourier Transform spectra of nanostructures that are designed for thin film light trapping applications, and optimize the full parameter space of the lithography process. Finally, we are able to reduce the exposure time by a factor of 5.5 without loss of photonic performance. We show that the performances of the fastest written structures are identical to the original ones within experimental error. As the target function can be varied according to different purposes, the method is also applicable to guided mode resonant grating and many other areas. These findings contribute to the advancement of EBL and point towards making the technology more attractive for commercial applications.

Insights into directional scattering: from coupled dipoles to asymmetric dimer nanoantennas.

Strong and directionally specific forward scattering from optical nanoantennas is of utmost importance for various applications in the broader context of photovoltaics and integrated light sources. Here, we outline a simple yet powerful design principle to perceive a nanoantenna that provides directional scattering into a higher index substrate based on the interference of multiple electric dipoles. A structural implementation of the electric dipole distribution is possible using plasmonic nanoparticles with a fairly simple geometry, i.e. two coupled rectangular nanoparticles, forming a dimer, on top of a substrate. The key to achieve directionality is to choose a sufficiently large size for the nanoparticles. This promotes the excitation of vertical electric dipole moments due to the bi-anisotropy of the nanoantenna. In turn, asymmetric scattering is obtained by ensuring the appropriate phase relation between the vertical electric dipole moments. The scattering strength and angular spread for an optimized nanoantenna can be shown to be broadband and robust against changes in the incidence angle. The scattering directionality is maintained even for an array configuration of the dimer. It only requires the preferred scattering direction of the isolated nanoantenna not to be prohibited by interference.

Spatial resolution effect of light coupling structures.

The coupling of light between free space and thin film semiconductors is an essential requirement of modern optoelectronic technology. For monochromatic and single mode devices, high performance grating couplers have been developed that are well understood. For broadband and multimode devices, however, more complex structures, here referred to as "coupling surfaces", are required, which are often difficult to realise technologically. We identify general design rules based on the Fourier properties of the coupling surface and show how they can be used to determine the spatial resolution required for the coupler's fabrication. To our knowledge, this question has not been previously addressed, but it is important for the understanding of diffractive nanostructures and their technological realisation. We exemplify our insights with solar cells and UV photodetectors, where high-performance nanostructures that can be realised cost-effectively are essential.

Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells.

The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC71BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10-50 nm wide and 200-400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.

Dual gratings for enhanced light trapping in thin-film solar cells by a layer-transfer technique.

Thin film solar cells benefit significantly from the enhanced light trapping offered by photonic nanostructures. The thin film is typically patterned on one side only due to technological constraints. The ability to independently pattern both sides of the thin film increases the degrees of freedom available to the designer, as different functions can be combined, such as the reduction of surface reflection and the excitation of quasiguided modes for enhanced light absorption. Here, we demonstrate a technique based on simple layer transfer that allows us to independently pattern both sides of the thin film leading to enhanced light trapping. We used a 400 nm thin film of amorphous hydrogenated silicon and two simple 2D gratings for this proof-of-principle demonstration. Since the technique imposes no restrictions on the design parameters, any type of structure can be made.

Deterministic quasi-random nanostructures for photon control.

Controlling the flux of photons is crucial in many areas of science and technology. Artificial materials with nano-scale modulation of the refractive index, such as photonic crystals, are able to exercise such control and have opened exciting new possibilities for light manipulation. An interesting alternative to such periodic structures is the class of materials known as quasi-crystals, which offer unique advantages such as richer Fourier spectra. Here we introduce a novel approach for designing such richer Fourier spectra, by using a periodic structure that allows us to control its Fourier components almost at will. Our approach is based on binary gratings, which makes the structures easy to replicate and to tailor towards specific applications. As an example, we show how these structures can be employed to achieve highly efficient broad-band light trapping in thin films that approach the theoretical (Lambertian) limit, a problem of crucial importance for photovoltaics.

Experimental high numerical aperture focusing with high contrast gratings.

We demonstrate high aperture (up to NA~0.64) three-dimensional focusing in free space based on wavefront-engineered diffraction gratings. The grating lens' optical response is tailored by spatially varying the grating ridge and groove width in two dimensions to achieve focal lengths of order 100 µm that are crucial for micro-optical applications. The phase profile of the lens includes multiple 2π phase jumps and was obtained by applying an algorithm for finding the optimal path for both phase and amplitude. Experimental measurements reveal a lateral spot size of 5 µm that is close to the size of a corresponding Airy disk.

Theoretical analysis of supercontinuum generation in a highly birefringent D-shaped microstructured optical fiber.

This paper carries out a rigorous analysis of supercontinuum generation in an improved highly asymmetric microstructured fiber (MF) design. This geometry, defined simply as D-MF, has the advantage of being produced with a regular stacking and drawing technology. We have obtained birefringence values on the order of 4.87x10(-3) at the adopted pump wavelength and a significantly smaller effective area when compared to a whole MF, which makes this fiber quite attractive for SCG. Therefore, this D-MF design is a promising alternative for SCG since it provides new degrees of freedom to control field confinement, birefringence, and dispersion characteristics of MFs.