Accurate Post-Calibration Predictions for Noninvasive Glucose Measurements in People Using Confocal Raman Spectroscopy.

In diabetes prevention and care, invasiveness of glucose measurement impedes efficient therapy and hampers the identification of people at risk. Lack of calibration stability in non-invasive technology has confined the field to short-term proof of principle. Addressing this challenge, we demonstrate the first practical use of a Raman-based and portable non-invasive glucose monitoring device used for at least 15 days following calibration. In a home-based clinical study involving 160 subjects with diabetes, the largest of its kind to our knowledge, we find that the measurement accuracy is insensitive to age, sex, and skin color. A subset of subjects with type 2 diabetes highlights promising real-life results with 99.8% of measurements within A + B zones in the consensus error grid and a mean absolute relative difference of 14.3%. By overcoming the problem of calibration stability, we remove the lingering uncertainty about the practical use of non-invasive glucose monitoring, boding a new, non-invasive era in diabetes monitoring.

Critical-depth Raman spectroscopy enables home-use non-invasive glucose monitoring.

One of the most ambitious endeavors in the field of diabetes technology is non-invasive glucose sensing. In the past decades, a number of different technologies have been assessed, but none of these have found its entry into general clinical use. We report on the development of a table-top confocal Raman spectrometer that was used in the home of patients with diabetes and operated for extended periods of time unsupervised and without recalibration. The system is based on measurement of glucose levels at a 'critical depth' in the skin, specifically in the interstitial fluid located below the stratum corneum but above the underlying adipose tissue layer. The region chosen for routine glucose measurements was the base of the thumb (the thenar). In a small clinical study, 35 patients with diabetes analyzed their interstitial fluid glucose for a period of 60 days using the new critical-depth Raman (CD-Raman) method and levels were correlated to reference capillary blood glucose values using a standard finger-stick and test strip product. The calibration of the CD-Raman system was stable for > 10 days. Measurement performance for glucose levels present at, or below, a depth of ~250µm below the skin surface was comparable to that reported for currently available invasive continuous glucose monitors. In summary, using the CD-Raman technology we have demonstrated the first successful use of a non-invasive glucose monitor in the home.

Gradient metasurfaces: a review of fundamentals and applications.

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Third-order gap plasmon based metasurfaces for visible light.

Efficient control and manipulation of light using metasurfaces requires high fabrication accuracy that becomes progressively demanding when decreasing the operation wavelength. Considering gap surface plasmon (GSP) based metasurfaces, we demonstrate that the metasurfaces, which utilize the third-order GSP resonance and thereby involve relatively large nanobricks, can successfully be used for efficient polarization-controlled steering of visible light. The reflection amplitude and phase maps for a 450 nm period array of 50 nm thick nanobricks placed atop a 40 nm thick silica layer supported by an optically thick gold film are calculated for the operation wavelength of 633 nm. Exploiting the occurrence of the third-order GSP resonance for nanobricks having their lengths close to 300 nm, we design the phase-gradient metasurface, representing an array of (450 x 2250 nm2) supercells made of 5 nanobricks with different dimensions, to operate as a polarization beam splitter for linearly polarized light. The fabricated polarization beam splitter is characterized using a supercontinuum light source at the normal light incidence and found to exhibit a polarization contrast ratio of up to 40 dB near the design wavelength of 633 nm while showing better than 20 dB contrast in the range of 550 - 650 nm for both polarizations. The diffraction efficiency experimentally measured at normal incidence exceeds 10% (20% in simulations) at the design wavelength of 633 nm, with the performance for the TE polarization (electric field perpendicular to the plane of diffraction) being significantly better (experimentally > 20% and theoretically > 40%) than for the TM polarization. This difference becomes even more pronounced for the light incidence deviating from normal. Finally, we discuss possible improvements of the performance of polarization beam splitters based on third-order GSP resonance as well as other potential applications of the suggested approach.

Gap-plasmon based broadband absorbers for enhanced hot-electron and photocurrent generation.

Plasmonic hot-electron generation has recently come into focus as a new scheme for solar energy conversion. So far, however, due to the relatively narrow bandwidth of the surface plasmon resonances and the insufficient resonant light absorption, most of plasmonic photocatalysts show narrow-band spectral responsivities and small solar energy conversion efficiencies. Here we experimentally demonstrate that a three-layered nanostructure, consisting of a monolayer gold-nanoparticles and a gold film separated by a TiO2 gap layer (Au-NPs/TiO2/Au-film), is capable of near-completely absorbing light within the whole visible region. We show that the Au-NPs/TiO2/Au-film device can take advantage of such strong and broadband light absorption to enhance the generation of hot electrons and thus the photocurrent under visible irradiation. As compared to conventional plasmonic photocatalysts such as Au-NPs/TiO2 nanostructures, a 5-fold-enhanced incident photon-to-current conversion efficiency is achieved within the entire wavelength range 450-850 nm in the Au-NPs/TiO2/Au-film device. Simulations show good agreements with the experimental results, demonstrating that only the plasmon-induced losses contribute to the enhanced photocurrent generation of the Au-NPs/TiO2/Au-film device.

Random-phase metasurfaces at optical wavelengths.

Random-phase metasurfaces, in which the constituents scatter light with random phases, have the property that an incident plane wave will diffusely scatter, hereby leading to a complex far-field response that is most suitably described by statistical means. In this work, we present and exemplify the statistical description of the far-field response, particularly highlighting how the response for polarised and unpolarised light might be alike or different depending on the correlation of scattering phases for two orthogonal polarisations. By utilizing gap plasmon-based metasurfaces, consisting of an optically thick gold film overlaid by a subwavelength thin glass spacer and an array of gold nanobricks, we design and realize random-phase metasurfaces at a wavelength of 800 nm. Optical characterisation of the fabricated samples convincingly demonstrates the diffuse scattering of reflected light, with statistics obeying the theoretical predictions. We foresee the use of random-phase metasurfaces for camouflage applications and as high-quality reference structures in dark-field microscopy, while the control of the statistics for polarised and unpolarised light might find usage in security applications. Finally, by incorporating a certain correlation between scattering by neighbouring metasurface constituents new types of functionalities can be realised, such as a Lambertian reflector.

Enhanced nonresonant light transmission through subwavelength slits in metal.

We analytically describe light transmission through a single subwavelength slit in a thin perfect electric conductor screen for the incident polarization being perpendicular to the slit, and derive simple, yet accurate, expressions for the average electric field in the slit and the transmission efficiency. The analytic results are consistent with full-wave numerical calculations and demonstrate that slits of widths ∼100 nm in real metals may feature nonresonant (i.e., broadband) field enhancements of ∼100 and transmission efficiency of ∼10 at infrared or terahertz frequencies, with the associated metasurface-like array of slits becoming transparent to the incident light.

Unidirectional scattering by nanoparticles near substrates: generalized Kerker conditions.

Starting from a general description of light scattering by a nanoparticle in homogeneous surroundings and situated near a substrate, we outline the connection to multipole expansion of scattered light and derive conditions and limits on achievable half-space scattering asymmetry, including the possibility of unidirectional scattering along the propagation direction of the incident light (i.e., generalized Kerker conditions). As a way of realizing strongly asymmetric scattering, we perform a parametric study of the optical properties of disk-shaped gap-surface plasmon (GSP) resonators, consisting of a glass spacer sandwiched between two gold disks, with numerical calculations that corroborate the conditions derived from the multipole expansion. Finally, we present proof-of-principle experiments of asymmetric scattering by GSP-resonators on a glass substrate.

Analog computing using reflective plasmonic metasurfaces.

Motivated by the recent renewed interest in compact analog computing using light and metasurfaces ( Silva, A. et al. Science 2014 , 343, 160 - 163), we suggest a practical approach to its realization that involves reflective metasurfaces consisting of arrayed gold nanobricks atop a subwavelength-thin dielectric spacer and optically thick gold film, a configuration that supports gap-surface plasmon resonances. Using well-established numerical routines, we demonstrate that these metasurfaces enable independent control of the light phase and amplitude, and design differentiator and integrator metasurfaces featuring realistic system parameters. Proof-of-principle experiments are reported along with the successful realization of a high-quality poor-man's integrator metasurface operating at the wavelength of 800 nm.

Extremely confined gap surface-plasmon modes excited by electrons.

High-spatial and energy resolution electron energy-loss spectroscopy (EELS) can be used for detailed characterization of localized and propagating surface-plasmon excitations in metal nanostructures, giving insight into fundamental physical phenomena and various plasmonic effects. Here, applying EELS to ultra-sharp convex grooves in gold, we directly probe extremely confined gap surface-plasmon (GSP) modes excited by swift electrons in nanometre-wide gaps. We reveal the resonance behaviour associated with the excitation of the antisymmetric GSP mode for extremely small gap widths, down to ~5 nm. We argue that excitation of this mode, featuring very strong absorption, has a crucial role in experimental realizations of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry. The occurrence of the antisymmetric GSP mode along with the fundamental GSP mode exploited in plasmonic waveguides with extreme light confinement is a very important factor that should be taken into account in the design of nanoplasmonic circuits and devices.

Nanofocusing in circular sector-like nanoantennas.

Gold circular sector-like nanoantennas (with a radius of 500 nm and a taper angle of 60°, 90°, and 120°) on glass are investigated in a near-infrared wavelength range (900 - 2100 nm). Amplitude- and phase-resolved near-field images of circular sector-like antenna modes at telecom wavelength feature a concentric circular line of phase contrast, demonstrating resonant excitation of a standing wave of counter-propagating surface plasmons, travelling between a tip and opposite circular edge of the antenna. Transmission spectra obtained in the range 900 - 2100 nm are in good agreement with numerical simulations, revealing the main feature of this antenna configuration, viz., the resonance wavelength, in contrast to triangular antennas, does not depend on the taper angle and is determined only by the sector radius. This feature together with a robust and easily predictable frequency response makes circular sector-like nanoantennas very promising for implementing bowtie antennas and attractive for many applications.

Scaling in light scattering by sharp conical metal tips.

Using the electrostatic approximation, we analyze electromagnetic fields scattered by sharp conical metal tips, which are illuminated with light polarized along the tip axis. We establish scaling relations for the scattered field amplitude and phase, and verify the validity with numerical simulations. Analytic expressions for the wavelength at which the scattered field near the tip changes its direction and for the field decay near the tip extremity are obtained, relating these characteristics to the cone angle and metal permittivity. The results obtained have important implications for various tip-enhanced phenomena, ranging from Raman and scattering near-field imaging to photoemission spectroscopy and nano-optical trapping.

Subwavelength plasmonic color printing protected for ambient use.

We demonstrate plasmonic color printing with subwavelength resolution using circular gap-plasmon resonators (GPRs) arranged in 340 nm period arrays of square unit cells and fabricated with single-step electron-beam lithography. We develop a printing procedure resulting in correct single-pixel color reproduction, high color uniformity of colored areas, and high reproduction fidelity. Furthermore, we demonstrate that, due to inherent stability of GPRs with respect to surfactants, the fabricated color print can be protected with a transparent dielectric overlay for ambient use without destroying its coloring. Using finite-element simulations, we uncover the physical mechanisms responsible for color printing with GPR arrays and suggest the appropriate design procedure minimizing the influence of the protection layer.

Plasmonic metasurfaces for efficient phase control in reflection.

We numerically study the optical properties of metal-insulator-metal resonators and metasurfaces, emphasizing the presence of gap-surface plasmon (GSP) resonances and their connection to the optical response. In relation to birefringent metal-backed metasurfaces, we show how a combination of metal nanobrick and nanocross elements allows one to fully control the phase of reflected light for two orthogonal polarizations simultaneously. The approach is exemplified by the design of a gradient birefringent metasurface that reflects two orthogonal polarization states into +2 and -3 diffraction order, respectively, with a reflectivity up to ~ 80% and in a broad wavelength range around the design wavelength of 800 nm. Finally, we introduce the concept of metascatterers, which are wavelength-sized polarization-sensitive scatterers.

Gap plasmon-based metasurfaces for total control of reflected light.

In the quest to miniaturise photonics, it is of paramount importance to control light at the nanoscale. We reveal the main physical mechanism responsible for operation of gap plasmon-based gradient metasurfaces, comprising a periodic arrangement of metal nanobricks, and suggest that two degrees of freedom in the nanobrick geometry allow one to independently control the reflection phases of orthogonal light polarisations. We demonstrate, both theoretically and experimentally, how orthogonal linear polarisations of light at wavelengths close to 800 nm can be manipulated independently, efficiently and in a broad wavelength range by realising polarisation beam splitters and polarisation-independent beam steering, showing at the same time the robustness of metasurface designs towards fabrication tolerances. The presented approach establishes a new class of compact optical components, viz., plasmonic metasurfaces with controlled gradient birefringence, with no dielectric counterparts. It can straightforwardly be adapted to realise new optical components with hitherto inaccessible functionalities.

Broadband plasmonic half-wave plates in reflection.

We demonstrate, both numerically and experimentally, that metal-insulator-metal configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus supporting gap-surface plasmon resonances, can be designed to function as reflective broadband half-wave plates. Using gold as the metal, the constructed wave plates in the near-infrared regime show scalability, bandwidth of ~20% of the design wavelength, and theoretical reflectivity above 85%, while a reflectivity of ~50% is experimentally measured.

Efficient and broadband quarter-wave plates by gap-plasmon resonators.

We demonstrate numerically that metal-insulator-metal (MIM) configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus facilitating gap-surface plasmon resonances, can be designed to function as efficient and broadband quarter-wave plates in reflection by a proper choice of geometrical parameters. Using gold as the metal, we demonstrate quarter-wave plate behavior at λ ~/= 800 nm with an operation bandwidth of 160 nm, conversion efficiency of 82%, and angle of linear polarization fixed throughout the entire bandwidth. This work also includes a detailed analytical and numerical study of the optical properties and underlying physics of structured MIM configurations.

Broadband focusing flat mirrors based on plasmonic gradient metasurfaces.

We demonstrate that metal-insulator-metal configurations, with the top metal layer consisting of a periodic arrangement of differently sized nanobricks, can be designed to function as broadband focusing flat mirrors. Using 50-nm-high gold nanobricks arranged in a 240-nm-period lattice on the top of a 50-nm-thick layer of silicon dioxide deposited on a continuous 100-nm-thick gold film, we realize a 17.3 × 17.3 µm(2) flat mirror that efficiently reflects (experiment: 14-27%; theory: 50-78%) and focuses a linearly polarized (along the direction of nanobrick size variation) incident beam in the plane of its polarization with the focal length, which changes from ~15 to 11 µm when tuning the light wavelength from 750 to 950 nm, respectively. Our approach can easily be extended to realize the radiation focusing in two dimensions as well as other optical functionalities by suitably controlling the phase distribution of reflected light.

Localized spoof plasmons arise while texturing closed surfaces.

We demonstrate that textured closed surfaces, i.e., particles made of perfect electric conductors (PECs), are able to support localized electromagnetic resonances with properties resembling those of localized surface plasmons (LSPs) in the optical regime. Because of their similar behavior, we name these types of resonances as spoof LSPs. As a way of example, we show the existence of spoof LSPs in periodically textured PEC cylinders and the almost perfect analogy to optical plasmonics. We also present a metamaterial approach that captures the basic ingredients of their electromagnetic response.

Efficient absorption of visible radiation by gap plasmon resonators.

We demonstrate experimentally a periodic array of differently-sized and circularly-shaped gap plasmon resonators (GPRs) with the average absorption ~94% for unpolarized light in the entire visible wavelength range (400-750 nm). Finite-element simulations verify that the polarization insensitive broadband absorption originates from localized gap surface plasmons whose resonant excitations only weakly depend on the angle of incidence. Arrays of GPRs also exhibit enhanced local field intensities (~115) as revealed by scanning two-photon photoluminescence microscopy, that are spectrally correlated with the minima in corresponding linear reflection spectra.