Quantum sensing devices such as atomic clocks enable unmatched precision in various area of metrology. Initially bulky laboratory devices, it is of great interest to miniaturize them to lower their energy consumption and deploy them in many embedded and mobile systems. To allow a dramatic cost reduction and miniaturization, CSEM developed and tested with success miniature atomic clocks based on wafer-level processes. On top of the control electronics and the atomic vapor cells, the optical design, the optical components and their assembly have been fully redesigned to be wafer-level fabricated. To achieve low aspect ratio and integrated optical management, thin glass planar waveguides have been implemented allowing to transport and beam shape the interrogating beam going from and to the atomic vapor gas cell. This proved a much simpler wafer scale assembly process, a monolithic construction less prone to single component alignment issues and provide much more compact atomic clocks

Plasmonic effects have been considered as a promising way of bringing the performances of metal oxide water splitting photoelectrodes closer to their theoretical limit. Optimizing the contribution of plasmonic effects is far more complex than material selection and needs to include, in particular, morphological aspects. In this work, we establish practical design guidelines for enhancing the performances of metal oxide photoanodes with plasmonic nanoparticles. A previously reported theoretical method modeling the contribution of optical nanostructures is used to calculate the photoelectrochemical performances (photocurrent, external quantum efficiency) and experimentally validated on fabricated devices. New insights on the contribution of plasmonic nanoparticles are found. Specifically, the performances are strongly influenced by the illumination direction, the active material morphology, and the size and position of the plasmonic scatterers. The light intensity distribution is studied to understand how plasmonic nanoparticles are modifying the light absorption in the active material. It is revealed that near-field hot-spots, exponentially decreasing with the distance to the scatterers, can beneficially impact the performances only for pure nanoparticles whereas interference effects are contributing even when the nanoparticles are covered with a protective shell (e.g., SiO2).

Metal oxide semiconductors have shown considerable potential for photoelectrochemical water-splitting. However, no ideal material has emerged which benefit from both an attractive sunlight absorption and efficient charge transport properties. In this work, we show that decorating photoanodes with high refractive index nanoparticles such as amorphous titania can result in reduced reflection losses at the electrolyte/photoanode interface, thereby increasing the performances under illumination from the electrolyte side. A proof of concept is obtained for a bismuth vanadate photoanode including a surface catalyst and a hematite photoanode. The photocurrent density and external quantum efficiency are improved by up to 10% upon nanoparticle decoration, quantitatively matching the decrease in reflectance. Simulations show that a similar enhancement happens when a thick bismuth vanadate photoanode with optimal charge transport properties is considered, thereby suggesting that this strategy can improve photoanodes suffering from high reflection losses regardless of the bare sample performance.

Wafer-scale, nanoimprint lithography-based approaches for manufacturing of high-efficiency transmission gratings at telecom wavelengths are reported. Two microns-deep, binary gratings are thereby fabricated and combined with a subwavelength, antireflective structure to achieve a cost-efficient and reliable manufacturing process. Diffraction efficiencies of 92% are experimentally achieved in the Littrow configuration. These gratings are used to compress 8 picosecond pulses with 1W of average output power at central wavelength of 1555nm to pulse duration of 378 femtoseconds

Material nanostructuring and optical phenomena on a nanoscale such as plasmonic effects and light scattering have been widely studied for improving the solar-to-hydrogen efficiency of photoelectrochemical (PEC) water-splitting electrodes. In this work, we report a method for analyzing the contributions of optical effects from nanostructures for enhancing the PEC performances. Electromagnetic simulations are performed for the precise calculation of generated power density in a semiconductor material. In addition, the transport and transfer of photogenerated charges to the electrolyte are modeled by using the conservation of minority carriers. The surface loss parameter, diffusion length, and doping density of the semiconductor material are determined by fitting the model to an incident photon to current efficiency (IPCE) curve experimentally measured on the bare reference photoelectrode. These parameters are then used to compute the IPCE spectra of the photoelectrode for which an optical enhancement strategy is used, such as nanostructuring or plasmonics. The method is validated using published experimental data. The calculated IPCE enhancement ratio originating from optical effects is in quantitative agreement with experimental observations for both periodic and random optical structures. The model can be used to study in detail the key enhancement mechanisms for the IPCE from optical nanostructures and, in particular, discriminate between optical and nonoptical (e.g., catalytic) enhancement.

Light coupling in waveguides has been extensively investigated in a variety of contexts, from photonic integrated circuits to biosensing and near-eye displays for augmented reality. Here, narrowband diffraction is reported using a Fano interference effect in hybrid nanostructures. The excitation of hybrid plasmonic and bulk waveguides allows for a selectivity of 10 nm bandwidth in the first order and strong reduction of the entire zeroth order. A Fano formalism is used to predict the maximal diffraction efficiency at critical coupling, when external mode coupling balances intrinsic losses. It is found that the first order and zeroth order are related by a Fano-like spectral profile with similar spectral widths, resonance wavelengths, and modulation depths and differ only in the asymmetry parameter. The diffraction efficiency, angle, and wavelength can be solely tuned by the thin film thickness. A semianalytical dispersion model of the hybrid system is introduced and validated experimentally. Applications are foreseen in many optical devices that require color-selective coupling or dispersive properties such as optical document security or near-eye displays. The dispersion behavior under a divergent light source can also be utilized to design inexpensive, compact, and robust spectrometers or biosensors.

Nanostructured filter arrays on image sensors are promising for miniature spectrometers and spectral imagers. In this work, we report on resonant waveguide gratings fabricated by UV nanoimprint lithography and conformal dielectric-plasmonic coatings. Optical measurements in accordance with numerical simulations report on a resonance bandwidth of 20 nm in transmission in the visible range. The impact of cladding thickness and filter lateral size on the resonance properties is investigated with the help of numerical calculations. Finally, it is shown that the proposed geometry based on conformal coatings has a very efficient blocking rate compared to other nanostructured filter approaches.

An electrically tunable filter based on a plasmonic phase retarder and liquid crystal cells is reported. The plasmonic phase retarder consists of a periodic array of deep-subwavelength metallic nanostructures. A first entrance polarizer prepares the incident light in a polarization state oriented at 45° from the nanowires orientation. A strong phase retardation between TM and TE polarizations is induced by the plasmon resonances. A polarization analyzer based on liquid crystal cells allows to project the transmitted light onto a polarization state whose orientation depends on the applied voltage. Using this approach, a range of 8V is enough to span more than 70% of the area covered by standard RGB filters in CIE color coordinates with a single filter, including yellow, orange, red, magenta, purple, blue, cyan and green as well as different tones of white. In order to ensure the applicability to large area production, UV nanoimprint lithography (UV-NIL) and thin film coatings have been used to fabricate the plasmonic phase retarder. The evaporation is performed with an angle, so that a self-shadowing effects prevents full coverage of the surface. The resulting structure consists in a periodic array of silver nanowires. Multiple interfering resonances are observed so that the nominal transmission can reach levels above 70%. The construction of the colors transmitted by the tunable filter is modeled and validated through a series of optical characterization of the individual elements.

The search for full control of amplitude and phase of the electromagnetic field from planar surfaces is of high interest for the development of highly integrated photonic systems and at optical devices. A hybrid dielectric plasmonic resonant waveguide grating which enables highly wavelength-selective first order diffraction in a multimode light guide is reported. Measurements show a narrowband peak in the first order of diffraction at resonance, while the undesired transmitted signal is strongly suppressed at other wavelengths as well as in the zeroth diffraction order. Another hybrid resonant waveguide grating is reported and shows a bandwidth of 20nm in the zeroth order of transmission. Overall, this work shows the promising use of hybrid structures for taking the best features of both plasmonic and dielectric grating resonances for designing highly integrated optical devices such as spectrometers or optical security features.

Dynamic tuning of color filters finds numerous applications including displays or image sensors. Plasmonic resonators are subwavelength nanostructures which can tailor the phase, polarization, and amplitude of the optical field, but they are limited in color vibrancy when used as filters. In this work, birefringence induced colors of plasmonic resonators and a fast switching thin liquid crystal cell are combined in a multicolored electrically tunable filter. With this mechanism, the color gamut of the plasmonic surface and the liquid crystal cell is mutually enhanced in order to generate all primary additive and subtractive colors with high saturation as well as different tones of white. A single filter is able to cover more than 70% of the color gamut of standard RGB filters by applying a voltage ranging between 2 and 6.5 V. This spectral selectivity is added in transmission without any loss in the image resolution. The presented approach is foreseen to be implemented in a variety of devices including miniature sensors or smart-phone cameras to enhance the color information, ultraflat multispectral imagers, wearable or head-worn displays, as well as high resolution display panels.

An electrically tunable filter based on a plasmonic phase retarder and liquid crystal cells is reported. The plasmonic phase retarder consists of a periodic array of deep-subwavelength metallic nanostructures. A first entrance polarizer prepares the incident light in a polarization state oriented at 45° from the nanowires orientation. A strong phase retardation between TM and TE polarizations is induced by the plasmon resonances. A polarization analyzer based on liquid crystal cells allows to project the transmitted light onto a polarization state whose orientation depends on the applied voltage. Using this approach, a range of 8V is enough to span more than 50% of the area covered by standard RGB filters in CIE color coordinates with a single filter. In order to ensure the applicability to large area production, UV nanoimprint lithography (UV-NIL) and thin film coatings have been used to fabricate the plasmonic phase retarder. The nanoimprint master consists in a periodic binary grating with a sub-wavelength period below 200nm in order to avoid diffraction effects in the visible range and maximize the angular stability. The grating master is imprinted and coated with a silver thin film and encapsulated. The evaporation is performed with an angle, so that a self-shadowing effects prevents full coverage of the surface. The resulting structure consists in a periodic array of silver nanowires of total width 50nm, with a cross section forming an inverted U-shape. This particular shape shows a high degree of tunability of the plasmon resonance position given the constraints of a sub-wavelength periodicity. Multiple interfering resonances are observed so that the nominal transmission can reach >70%. Placed between a polarizer oriented at 45° from the nanowires orientation and a liquid crystal cell, the transmission spectrum of the plasmonic phase retarder can be tuned with the applied voltage. For a low voltage, the polarization transmitted through the liquid crystal cell is oriented along the gratings lines. For higher voltage, the light transmitted through the liquid crystal cell is oriented across the grating lines and the resulting spectrum has a dip in transmission, which is the signature of a plasmon resonance. At a voltage of 8V, a full rotation of the polarization by 180° has been applied. Different colors can be obtained within this range, including orange, magenta, purple, blue, turquoise, green and yellow with the same tunable filter. Other designs have been investigated in order to obtain more saturated blue, green or red using this approach.

Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide‐mode resonant gratings, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes from UV to microwave frequencies in many different configurations. A broad range of optical effects are obtained using RWGs such as waveguide coupling, filtering, focusing, field enhancement and nonlinear effects, magneto‐optical Kerr effect, or electromagnetically induced transparency. Thanks to their high degree of optical tunability (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry: refractive index and fluorescence biosensors, solar cells and photodetectors, signal processing, polarizers and wave plates, spectrometers, active tunable filters, mirrors for lasers and optical security features. The aim of this review is to discuss the latest developments in the field including numerical modeling, manufacturing, the physics, and applications of RWGs. Scientists and engineers interested in using RWGs for their application will also find links to the standard tools and references in modeling and fabrication according to their needs.

Resonant waveguide gratings (RWGs) are thin-film structures, where coupled modes interfere with the diffracted incoming wave and produce strong angular and spectral filtering. The combination of two finite-length and impedance matched RWGs allows the creation of a passive beam steering element, which is compatible with up-scalable fabrication processes. Here, we propose a design method to create large patterns of such elements able to filter, steer, and focus the light from one point source to another. The method is based on ellipsoidal mirrors to choose a system of confocal prolate spheroids where the two focal points are the source point and observation point, respectively. It allows finding the proper orientation and position of each RWG element of the pattern, such that the phase is constructively preserved at the observation point. The design techniques presented here could be implemented in a variety of systems, where large-scale patterns are needed, such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)

Single-photon avalanche photodiode (SPAD) image sensors offer time-gated photon counting, at high binary frame rates of >100 kFPS and with no readout noise. This makes them well-suited to a range of scientific applications, including microscopy, sensing and quantum optics. However, due to the complex electronics required, the fill factor tends to be significantly lower (< 10%) than that of EMCCD and sCMOS cameras (>90%), whilst the pixel size is typically larger, impacting the sensitivity and practicalities of the SPAD devices. This paper presents the first characterisation of a cylindrical-shaped microlens array applied to a small, 8 micron, pixel SPAD imager. The enhanced fill factor, ≈50% for collimated light, is the highest reported value amongst SPAD sensors with comparable resolution and pixel pitch. We demonstrate the impact of the increased sensitivity in single-molecule localisation microscopy, obtaining a resolution of below 40nm, the best reported figure for a SPAD sensor.

This study introduces a versatile method for modifying the optical properties of plasmonic substrates by inkjet printing of commercially available, transparent inks with various refractive indices. The large-scale and cost-efficient process is demonstrated on tilted aluminum nanolamellas. They show vivid and bright colors based on a Fano resonance, which is highly susceptible to the surrounding material. Furthermore, color rendering can be made strongly angle-dependent and asymmetric, which is in contrast to the nonvariable color generation enabled by conventional inkjet printing.

A novel thin-film single-layer structure based on resonant waveguide gratings (RWGs) allows to engineer selective color filtering and steering of white light. The unit cell of the structure consists of two adjacent finite-length and cross-talking RWGs, where the former acts as in-coupler and the latter acts as out-coupler. The structure is made by only one nano-imprint lithography replication and one thin film layer deposition, making it fully compatible with up-scalable fabrication processes. We characterize a fabricated optical security element designed to work with the flash and the camera of a smartphone in off-axis light steering configuration, where the pattern is revealed only by placing the smartphone in the proper position. Widespread applications are foreseen in a variety of fields, such as multifocal or monochromatic lenses, solar cells, biosensors, security devices and seethrough optical combiners for near-eye displays.

Resonant waveguide gratings (RWGs) are subwavelength structures of great interest for biosensors, optical filters and optical security applications. We demonstrate and characterize a beam steering device, where the in-coupling and out-coupling processes make use of different RWGs that share the same ultrathin dielectric waveguide. This device enables selective color-filtering and redirection of a white light source (such as a white LED). Furthermore, this structure is compatible with up-scalable fabrication processes such as roll-to-roll replication, and is relevant for high-volume production. Because of its color selectivity and its use in low coherence illumination conditions, such a beam steering device could be implemented in a variety of optical applications such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays.

We present an angle-insensitive, miniaturized and integratable filtering system based on plasmonic substrates for multispectral imaging. Active tunability of the plasmonic filter allows color recording, estimation of unknown spectra, and determination of spectral singularities, for example, laser lines, while exploiting the full spatial resolution of a B/W conventional camera. Compared to other multispectral imaging systems, the plasmonic filtering system can be placed in front of an existing imaging system, for example, including lenses, supporting a cost-efficient fabrication and integration. Additionally, it is characterized by high angular acceptance, which we demonstrate by imaging with a field-of-view of'-50. Further, the number of nonpixelated broadband filters could be varied in situ for faster imaging or higher quality, compared to systems with a fixed number of channels.