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All-dielectric nanoantennas, consisting of high refractive index semiconductor material, are drawing a great deal of attention in nanophotonics. Owing to their ability to manipulate efficiently the flow of light within sub-wavelength volumes, they have become the building blocks of a wide range of new photonic metamaterials and devices. The interaction of the antenna with light is largely governed by its size, geometry, and the symmetry of the multitude of optical cavity modes it supports. Already for simple antenna shapes, unraveling the full modal spectrum using conventional far-field techniques is nearly impossible due to the spatial and spectral overlap of the modes and their symmetry mismatch with incident radiation fields. This limitation can be circumvented by using localized excitation of the antenna. Here, we report on the experimental near-field probing of optical higher order cavity modes (CMs) and whispering gallery modes (WGMs) in amorphous silicon nanoantennas with simple, but fundamental, geometrical shapes of decreasing rotational symmetry: a disk, square, and triangle. Tapping into the near-field using an aperture type scanning near-field optical microscope (SNOM) opens a window on a rich variety of optical patterns resulting from the local excitation of antenna modes of different order with even and odd parity. Numerical analysis of the antenna and SNOM probe interaction shows how the near-field patterns reveal the node positions of – and allows us to distinguish between – cavity and whispering gallery modes. As such, this study contributes to a richer and deeper characterization of the structure of light in confined nanosystems, and their impact on the structuring of the light fields they generate.

Silicon spin qubits are among the most promising candidates for large scale quantum computers, due to their excellent coherence and compatibility with CMOS technology for upscaling. Advanced industrial CMOS process flows allow wafer-scale uniformity and high device yield, but off the shelf transistor processes cannot be directly transferred to qubit structures due to the different designs and operation conditions. To therefore leverage the know-how of the micro-electronics industry, we customize a 300mm wafer fabrication line for silicon MOS qubit integration. With careful optimization and engineering of the MOS gate stack, we report stable and uniform quantum dot operation at the Si/SiOx interface at milli-Kelvin temperature. We extract the charge noise in different devices and under various operation conditions, demonstrating a record-low average noise level of 0.61 ${\mu}$eV/${\sqrt{Hz}}$ at 1 Hz and even below 0.1 ${\mu}$eV/${\sqrt{Hz}}$ for some devices and operating conditions. By statistical analysis of the charge noise with different operation and device parameters, we show that the noise source can indeed be well described by a two-level fluctuator model. This reproducible low noise level, in combination with uniform operation of our quantum dots, marks CMOS manufactured MOS spin qubits as a mature and highly scalable platform for high fidelity qubits., Comment: 22 pages, 13 figures

In this work, we report the modeling and design of a high-speed Ge-based plasmonic detector coupled with a Metal-Insulator-Metal (MIM) plasmonic majority gate. The detector is designed to distinguish between multiple output levels of the integrated majority gate. Through numerical analyses we predict the proposed plasmonic detector has an intrinsic bandwidth beyond $\text{220 GHz}$ at an applied bias of only $\text{100 mV}$ . An asymmetric Metal-Semiconductor-Metal (MSM) configuration of the plasmonic detector ensures a dark current of a few nA which results in high sensitivity. The high electric field generated by the electrode asymmetry enables effective separation of the photogenerated carriers resulting in high photocurrent even at few mVs of applied bias. The low capacitance of less than $1fF$ arising from the small detector dimensions results in a high RC-limited bandwidth. Moreover, the narrow plasmonic Ge slot of the photodetector provides a short drift path and fast transit time for carriers. Unlike previously reported plasmonic detectors that use noble metals as electrodes, our proposed detector employs Al and Cu to meet CMOS compatibility requirements and thus can be a potential candidate for high-speed computational systems in industry-level applications. Additionally, the findings presented in the article will be helpful for the future realization of an integrated plasmonic system.

Biological nanopores are emerging as powerful tools for single-molecule analysis and sequencing. Here, we engineered the two-component pleurotolysin (PlyAB) toxin to assemble into 7.2 × 10.5 nm cylindrical nanopores with a low level of electrical noise in lipid bilayers, and we addressed the nanofluidic properties of the nanopore by continuum simulations. Surprisingly, proteins such as human albumin (66.5 kDa) and human transferrin (76-81 kDa) did not enter the nanopore. We found that the precise engineering of the inner surface charge of the PlyAB induced electro-osmotic vortices that allowed the electrophoretic capture of the proteins. Once inside the nanopore, two human plasma proteins could be distinguished by the characteristics of their current blockades. This fundamental understanding of the nanofluidic properties of nanopores provides a practical method to promote the capture and analysis of folded proteins by nanopores. ispartof: NANO LETTERS vol:20 issue:5 pages:3819-3827 ispartof: location:United States status: published

ispartof: 2022 INTERNATIONAL ELECTRON DEVICES MEETING, IEDM ispartof: International Electron Devices Meeting (IEDM) location:CA, San Francisco date:3 Dec - 7 Dec 2022 status: published

We explore the use of inverse design methods for the generation of optical lattices in photonic integrated circuits. A polychromatic pattern generating device that switches between optical lattices with different symmetry and periodicity, is presented.

Design of high-efficiency visible-spectrum silicon nitride grating couplers, compatible with CMOS fabrication are proposed. The design is based on amorphous-silicon overlay and distributed-Bragg-reflectors. We simulated 74% and 79% efficiency for 638 nm and 780 nm.

We present a photonic circuit, integrated with switches and phase modulators, that generates structured illumination patterns. In combination with our custom reconstruction algorithm, 100nm resolution in a structured illumination microscopy (SIM) imaging sequence is achieved.

We report on phase-change material scatterers in a half-wave plate design. A simulation study shows the cross-polarization of circularly polarized light reaches close to 20% around 800 nm in the ON state. Characterization is ongoing.

This work studies the performance of a Ge-based plasmonic detector with carrier mobility and lifetime variation. The performance is evaluated in terms of the detector's dark current, photocurrent, and detection bandwidth.

We investigate hybrid dielectric-metallic transmission color filters for cost-effective, narrow linewidth filters that are fully CMOS compatible. Through the interference of the resonance phenomena in a metallic hole array with a magnetic dipole resonance in a dielectric nanopillar, a narrow linewidth transmission window is opened. An additional metal cover on top of the pillar further suppresses transmission at longer wavelengths due to a localized surface plasmon polariton. FDTD simulations predict a transmission of over 50% and a linewidth of down to 25 nm for the green filter. The out-of-band suppression is lower compared to previously reported metal-insulator-metal filters.

Triggered by the need for arrays of individually resolvable excitation foci or trapping potentials in photonics applications, coherent lattice theory describes a unique approach to design structured interference patterns. Typically, large periodicity lattices remain unexplored due to limitations in the theoretical description. Here, we present a method for efficient computation of coherent lattices, successfully covering all periodic and quasi-periodic lattices. The previously unrelated moire theory and prime number factorization are the foundation of the proposed method. Additionally, we experimentally verify key optical coherent lattices and propose broadening their applicability towards structured light microscopy and optical trapping using photonic integrated circuits.

Supercritical angle fluorescence (SAF) is a surface sensitive detection mechanism that has been used to perform rapid, sensitive detection of biomolecules. However, the SAF biosensors that have been demonstrated so far have been either bulky and expensive or small and affordable while sacrificing on sensitivity . We present a novel interference based SAF biosensor technology that has the potential to be fast, small, inexpensive and mass manufacturable while being as sensitive as the current gold standard. This technology can therefore create biosensors in a wide range of form factors, from large, complex high multiplex devices to small handheld point of care solutions.

The data acquisition speed of point scanning microscopy techniques at sub-cellular resolution limits imaging of large samples, as sample stability and focus drift are becoming an issue. Therefore, light sheet fluorescent microscopy (LSFM) has become the method of choice for imaging cleared samples. However, this method still suffers from a trade-off between imaging depth and resolution, due to diffraction of the illuminating beam limiting the achievable field of view. After improvement of the latter with non-diffracting Bessel beams, lattice light sheet microscopy has substantially reduced the illumination point-spread-function (PSF). Here, we propose further improvement by generation of structured light sheets via phased arrays implemented as silicon nitride photonic integrated circuits (PICs). Beam generation with PICs results in much higher power efficiency than beam forming with conventional liquid crystal based spatial phase modulators, as it does not require the use of narrow blocking apertures. Moreover, this approach enables increased control over the generated field profile. Modeling of concrete PIC concepts indicates that sub-cellular resolution with mm scale imaging depths can be concomitantly achieved. Maintaining a small PSF along the axis perpendicular to the direction of light propagation is sacrificed in order to maintain it over increased imaging depth along the beam propagation axis. Rapid lateral scanning of the illumination beam inside the plane of the light sheet is then obtained by scanning of the laser wavelength within the excitation spectrum of the target fluorescent protein, allowing for a wide bidirectional field-of-view with high resolution.

An effective way to design structured coherent wave interference patterns that builds on the theory of coherent lattices, is presented. The technique combines prime number factorization in the complex plane with moiré theory to provide a robust way to design structured patterns with variable spacing of intensity maxima. In addition, the proposed theoretical framework facilitates an elegant computation of previously unexplored high-order superlattices both for the periodic and quasiperiodic case. A number of beam configurations highlighting prime examples of patterns for lattices with three-, four-, and fivefold symmetry are verified in a multibeam interference experiment. ispartof: PHYSICAL REVIEW LETTERS vol:125 issue:18 ispartof: location:United States status: published

Despite the broad success of biological nanopores as powerful instruments for the analysis of proteins and nucleic acids at the single-molecule level, a fast simulation methodology to accurately model their nanofluidic properties is currently unavailable. This limits the rational engineering of nanopore traits and makes the unambiguous interpretation of experimental results challenging. Here, we present a continuum approach that can faithfully reproduce the experimentally measured ionic conductance of the biological nanopore Cytolysin A (ClyA) over a wide range of ionic strengths and bias potentials. Our model consists of the extended Poisson-Nernst-Planck and Navier-Stokes (ePNP-NS) equations and a computationally efficient 2D-axisymmetric representation for the geometry and charge distribution of the nanopore. Importantly, the ePNP-NS equations achieve this accuracy by self-consistently considering the finite size of the ions and the influence of both the ionic strength and the nanoscopic scale of the pore on the local properties of the electrolyte. These comprise the mobility and diffusivity of the ions, and the density, viscosity and relative permittivity of the solvent. Crucially, by applying our methodology to ClyA, a biological nanopore used for single-molecule enzymology studies, we could directly quantify several nanofluidic characteristics difficult to determine experimentally. These include the ion selectivity, the ion concentration distributions, the electrostatic potential landscape, the magnitude of the electro-osmotic flow field, and the internal pressure distribution. Hence, this work provides a means to obtain fundamental new insights into the nanofluidic properties of biological nanopores and paves the way towards their rational engineering. ispartof: NANOSCALE vol:12 issue:32 pages:16775-16795 ispartof: location:England status: published

Raman spectroscopy is commonly used in analytical chemistry for identification of molecules based on the analysis of spectral lines in the Raman spectrum corresponding to molecular vibrations. For solids and gases, usually high spectral resolution is required. For many other applications, however, a high spectral resolution is less critical, while compactness and reduced system cost are increasingly important. Several studies have already shown the applicability of low resolution Raman spectroscopy (LRRS) systems to a wide variety of fields, such as in situ monitoring of process control, on-site detection of illicit drugs and explosives, on-site detection of water contamination, and safety analysis of edible oil. In this work, we illustrate how imec’s CMOS based hyperspectral (HS) filter technology can be used to build a very compact, low cost, mass-manufacturable Raman spectrometer where spectral resolution can be traded for increased sensitivity, hence better SNR and/or shorter acquisition time. Furthermore, unlike other types of Raman spectrometers, the proposed system can be tailored to the application by only targeting specific Raman bands, e.g., 400-1500cm-1 and 3000-4000cm-1, by selecting the right set of HS filters for a given wavelength. This enables further improvements in system performance, or even use for Raman imaging. The HS filter technology is currently already used in various commercially available HS cameras for imaging applications in the VIS-NIR (470-900nm), NIR (600-970nm) and SWIR range (1100-1700nm) to study the reflectance or transmission spectra of the imaged targets. Proof-of-concept Raman measurements have been successfully performed in a laboratory setup where the impact of using different HS filter selections or layouts and different cooled and uncooled cameras can be tested. Furthermore, the measurements closely match the predicted spectra obtained with our in-house developed HS-based Raman spectrometer simulation software, which can be used for design space exploration to optimize each of the system components (optical module, rejection filters, HS filters and image sensor) for the target application.

Fluorescence microscopy is an indispensable tool in biology and medicine, fuelling many breakthrough discoveries in a wide set of sub-domains. Yet, the resolution is intrinsically restricted by the diffraction limit. The last two decades have witnessed the emergence of several super-resolution fluorescence microscopy techniques that are breaking this limit (cfr. Nobel Prize in Chemistry 2014). Structured illumination microscopy (SIM) is one of such techniques that has gained much popularity and is available in commercial systems. In SIM, a biological sample is illuminated by a spatially structured light field — a sinusoidal interference pattern — which causes normally inaccessible high-resolution information to be encoded into the observed image due to the Moire effect. Typically, the illumination patterns are generated by a grating or SLM and focused onto the sample through an objective lens to excite the fluorophores. Next, the fluorescence is transmitted to the imager through the same lens. Multiple images are taken under certain illumination patterns and used to reconstruct a single super-resolved image. However, making use of free space optics, state-of-the-art SIM systems require multiple bulky and precision optical components that give these systems the drawbacks of high cost and cumbersome size. In this talk, a structured illumination microscopy system based on a photonic integrated circuit (PIC) is presented. The unique properties of photonic integrated circuits allow us to create truly innovative microscopy systems that have the potential to go well beyond the current state-of-the-art: higher resolution due to the use of high refractive index materials, easier alignment with on-chip illumination light path, large field of view, a compact form factor resulting from on-chip integration, and lower cost due to compatibility with CMOS chip fabrication.

In recent years, the protein nanopore cytolysin A (ClyA) has become a valuable tool for the detection, characterization and quantification of biomarkers, proteins and nucleic acids at the single-molecule level. Despite this extensive experimental utilization, a comprehensive computational study of ion and water transport through ClyA is currently lacking. Such a study yields a wealth of information on the electrolytic conditions inside the pore and on the scale the electrophoretic forces that drive molecular transport. To this end we have built a computationally efficient continuum model of ClyA which, together with an extended version of Poison-Nernst-Planck-Navier-Stokes (ePNP-NS) equations, faithfully reproduces its ionic conductance over a wide range of salt concentrations. These ePNP-NS equations aim to tackle the shortcomings of the traditional PNP-NS models by self-consistently taking into account the influence of both the ionic strength and the nanoscopic scale of the pore on all relevant electrolyte properties. In this study, we give both a detailed description of our ePNP-NS model and apply it to the ClyA nanopore. This enabled us to gain a deeper insight into the influence of ionic strength and applied voltage on the ionic conductance through ClyA and a plethora of quantities difficult to assess experimentally. The latter includes the cation and anion concentrations inside the pore, the shape of the electrostatic potential landscape and the magnitude of the electro-osmotic flow. Our work shows that continuum models of biological nanopores—if the appropriate corrections are applied—can make both qualitatively and quantitatively meaningful predictions that could be valuable tool to aid in both the design and interpretation of nanopore experiments.

A photonic integrated circuit chip architecture is proposed and fabricated to implement super-resolution structured illumination microscopy. The working concept is introduced with some preliminary results, followed by a pros and cons discussion. © 2020 The Author(s)

We observe a drastic increase in propagation loss at visible wavelengths in PECVD silicon nitride waveguides after exposure to ultraviolet (UV) light. Low temperature annealing or high intensity optical exposure at visible wavelengths brings the propagation loss back toward the original value before UV exposure or even lower. We postulate that these effects can be explained by the population and depopulation of defect centers in the silicon nitride. We demonstrate the importance of defect depopulation before using waveguides in any silicon nitride based visible photonic application and provide a cleaning procedure that yields reproducible, low-loss, initial waveguide conditions.

Most fluorescent immunoassays require a wash step prior to read-out due to the otherwise overwhelming signal of the large number of unbound (bulk) fluorescent molecules that dominate over the signal from the molecules of interest, usually bound to a substrate. Supercritical angle fluorescence (SAF) sensing is one of the most promising alternatives to total internal reflection fluorescence for fluorescence imaging and sensing. However, detailed experimental investigation of the influence of collection angle on the SAF surface sensitivity, i.e., signal to background ratio (SBR), is still lacking. In this Letter, we present a novel technique that allows to discriminate the emission patterns of free and bound fluorophores simultaneously by collecting both angular and spectral information. The spectrum was probed at multiple positions in the back focal plane using a multimode fiber connected to a spectrometer and the difference in intensity between two fluorophores was used to calculate the SBR. Our study clearly reveals that increasing the angle of SAF collection enhances the surface sensitivity, albeit at the cost of decreased signal intensity. Furthermore, our findings are fully supported by full-field 3D simulations.

In this paper, a computational performance analysis is presented of a wide-field time-gated fluorescence lifetime imaging microscope (FLIM) using practically realizable properties of the laser, sample, and a three-tap time-gated CMOS image sensor. The impact of these component-level properties on the accuracy and the precision of the measurement results are estimated and discussed based on Monte Carlo simulations. The correlation between the detector speed and the accuracy of the extracted fluorescence lifetime is studied, and the minimum required incident photoelectron number of each pixel is estimated for different detector speeds and different fluorescence lifetime measurements. In addition, the detection limits due to the dark current and the parasitic light sensitivity of the detector are also investigated. This work gives an overview of the required fluorescence emission condition as well as the required detector properties for a three-tap time-gated image sensor to achieve good FLIM data in biological applications.

Subwavelength optical resonators and scatterers are dramatically expanding the toolset of the optical sciences and photonics engineering. By offering the opportunity to control and shape light waves in nanoscale volumes, recent developments using high-refractive-index dielectric scatterers gave rise to efficient flat-optical components such as lenses, polarizers, phase plates, color routers, and nonlinear elements with a subwavelength thickness. In this work, we take a deeper look into the unique interaction of light with rod-shaped amorphous silicon scatterers by tapping into their resonant modes with a localized subwavelength light source-an aperture scanning near-field probe. Our experimental configuration essentially constitutes a dielectric antenna that is locally driven by the aperture probe. We show how leaky transverse electric and magnetic modes can selectively be excited and form specific near-field distribution depending on wavelength and antenna dimensions. The probe's transmittance is furthermore enhanced upon coupling to the Fabry-Perot cavity modes, revealing all-dielectric nanorods as efficient transmitter antennas for the radiation of subwavelength emitters, in addition to constituting an elementary building block for all-dielectric metasurfaces and flat optics.

High-index dielectric nanostructures support inherently strong magnetic dipole (MD) resonances at optical frequencies with minimal dissipative absorptions. They are promising candidates for MD radiation enhancement. Previous investigations, however, show that the maximum magnetic field enhancement is confined inside the nanostructure and therefore inaccessible to nearby MD emitters, limiting the achievable emission enhancement. In this paper, we design a nano-doughnut-shaped silicon disk, i.e., a disk with an open hole through its center. This way, the maximum magnetic field intensity is exposed and can be leveraged to fully enhance MD radiations. On the basis of numerical calculations, a record high enhancement factor of the radiative decay rate up to 350 has been achieved with minimal nonradiative losses. We further demonstrate the importance of spectral and spatial overlap of the MD emitter with the MD resonance in the silicon nanodisk in order to maximize the MD radiations. Our study opens new possibi...

Biological microentities, such as cells and bacteria, are often detected and identified by bulky optical setups such as fluorescent microscopes and flow cytometers. Integrated photonics provides the prospect of dramatically miniaturizing these setups, enabling highly parallelized on-chip detection. In this work we designed, fabricated, and characterized a SiN photonic circuit for fluorescence detection of microentities. A tailored diffraction grating excites fluorescent particles, whose emitted light is collected in a single-mode waveguide by a second neighboring diffraction grating. Both fluorescent polystyrene microparticles and labeled peripheral blood mononuclear cells were detected in a microfluidic channel integrated on top of the waveguide circuit. We quantified both numerically and experimentally the efficiency of this grating system and obtained a good agreement. The presented on-chip fluorescent detection structure and the obtained results will strongly contribute to the development of on-chip c...

Directional in-coupling of light into a single-mode SixNy waveguide is demonstrated using a single subwavelength asymmetrically shaped metallic nanoantenna. In simulation as well as experiments, we show that at its quadrupolar resonance the V-shaped antenna can couple near-infrared light in the waveguide with a directivity reaching 25 dB. As an ultracompact directional coupler in the near-infrared and visible spectral range, the investigated system constitutes an important building block for integrated nanophotonic circuits and sensors.

A high surface to bulk fluorescence ratio is very useful in bioimaging, sensing, sequencing, and physical chemistry characterization. We used the evanescent field of a photonic waveguide for highly localized excitation and collection of molecular fluorescence. As both near-field excitation and collection are strongly distance dependent, we were able to increase the surface to bulk fluorescence ratio significantly. We have also experimentally investigated the combined excitation and collection efficiency as a function of the position of the molecule in the near field. Finally, we formulated and experimentally verified a general condition for the waveguide–molecule interaction length for maximum optical efficiency of the device.

The ability to confine and to study single molecules has enabled important advances in natural and applied sciences. Recently, we have shown that unlabeled proteins can be confined inside the biological nanopore Cytolysin A (ClyA) and conformational changes monitored by ionic current recordings. However, trapping small proteins remains a challenge. Here, we describe a system where steric, electrostatic, electrophoretic, and electro-osmotic forces are exploited to immobilize a small protein, dihydrofolate reductase (DHFR), inside ClyA. Assisted by electrostatic simulations, we show that the dwell time of DHFR inside ClyA can be increased by orders of magnitude (from milliseconds to seconds) by manipulation of the DHFR charge distribution. Further, we describe a physical model that includes a double energy barrier and the main electrophoretic components for trapping DHFR inside the nanopore. Simultaneous fits to the voltage dependence of the dwell times allowed direct estimates of the cis and trans translocation probabilities, the mean dwell time, and the force exerted by the electro-osmotic flow on the protein (≅9 pN at -50 mV) to be retrieved. The observed binding of NADPH to the trapped DHFR molecules suggested that the engineered proteins remained folded and functional inside ClyA. Contact-free confinement of single proteins inside nanopores can be employed for the manipulation and localized delivery of individual proteins and will have further applications in single-molecule analyte sensing and enzymology studies. ispartof: ACS NANO vol:13 issue:9 pages:9980-9992 ispartof: location:United States status: published

Two-photon microscopy is a key imaging technique in biological sciences because of its superior deep tissue imaging capabilities in addition to high transverse and axial resolution. In recent years, development of low-weight miniature two-photon microscopes has been of great interest for in vivo imaging of brain activity. Limited by these mechanical constraints, most of the developed miniature two-photon microscopes utilize graded index objective lenses that usually have inferior optical characteristics compared to conventional refractive objective lenses. Dielectric metasurfaces, a recent category of diffractive optical elements with enhanced capabilities, have proven versatile in various applications ranging from lensing to holography and polarization control. Their ultrathin form factor and potentially extremely low-weight make them very attractive for applications with stringent size and weight constraints. However, despite their success in various types of microscopy and imaging applications, they have not been previously utilized for multi-photon fluorescence microscopy. The main barrier for using metasurface lenses in multi-photon microscopy arises from their large chromatic dispersion that effectively makes them single-wavelength. Here we will present a double-wavelength metasurface lens especially designed to have the same focal length at 820 and 605 nm, corresponding to the excitation and emission wavelengths of a certain fluorophore. After characterizing the poly-silicon metasurface lens at both wavelengths, we used it in a two-photon microscopy setup and demonstrated its capability to capture two-photon images qualitatively similar to images taken with a conventional objective lens. We will also discuss the effects of chromatic dispersion of the metasurface lens on its two-photon imaging performance.

We report a new type of nanosphere colloidal lithography to directly fabricate monodisperse silica (SiO2) nanorings by means of reactive ion etching of hollow SiO2 spheres. Detailed TEM, SEM, and AFM structural analysis is complemented by a model describing the geometrical transition from hollow sphere to ring during the etching process. The resulting silica nanorings can be readily redispersed in solution and subsequently serve as universal templates for the synthesis of ring-shaped core-shell nanostructures. As an example we used silica nanorings (with diameter of ∼200 nm) to create a novel plasmonic nanoparticle topology, a silica-Au core-shell nanoring, by self-assembly of Au nanoparticles (20 nm) on the ring's surface. Spectroscopic measurements and finite difference time domain simulations reveal high quality factor multipolar and antibonding surface plasmon resonances in the near-infrared. By loading different types of nanoparticles on the silica core, hybrid and multifunctional composite nanoring structures could be realized for applications such as MRI contrast enhancement, catalysis, drug delivery, plasmonic and magnetic hyperthermia, photoacoustic imaging, and biochemical sensing.

© 2009-2012 IEEE. This paper presents silicon nitride (SixNy) photonic integrated circuits (PICs) with high performance at a wavelength of 450 nm, which, therefore, is suitable for neuronal stimulation with optogenetics. These PICs consist of straight and bent waveguides, and grating couplers that are fabricated in a complementary metal-oxide-semiconductor (CMOS)-compatible plasma enhanced chemical vapor deposition SixNy platform. Their characterization shows propagation losses of 0.96 $\pm$ 0.4 dB/cm on average for straight waveguides that are 1-5 $\mu$m wide and bend insertion losses as low as 0.2 dB/90 $^\circ$ for 1 $\mu$m wide waveguides with a radius of 100 $\mu$m. Additionally, the grating coupler characterization shows that they can deliver about 10 $\mu$W of light in an area of 5 $\times$ 9 $\mu$m2 (240 mW/mm2), which is captured from an uncollimated laser diode (70 mW). Besides delivering sufficient power for optogenetic applications, the gratings have dimensions that are comparable to the size of a neuron, which would allow single cell interaction. These results demonstrate that, with this SixNy platform, high-density and large-scale implantable neural devices can be fabricated and readily integrated into existing CMOS-compatible neuro-electronic platforms. ispartof: IEEE Photonics Journal vol:8 issue:5 pages:2701211-12 status: published

Two-photon microscopy is a key imaging technique in life sciences due to its superior deep-tissue imaging capabilities. Light-weight and compact two-photon microscopes are of great interest because of their applications for in vivo deep brain imaging. Recently, dielectric metasurfaces have enabled a new category of small and lightweight optical elements, including objective lenses. Here we experimentally demonstrate two-photon microscopy using a double-wavelength metasurface lens. It is specifically designed to focus 820 and 605 nm light, corresponding to the excitation and emission wavelengths of the measured fluorophors, to the same focal distance. The captured two-photon images are qualitatively comparable to the ones taken by a conventional objective lens. Our metasurface lens can enable ultracompact two-photon microscopes with similar performance compared to current systems that are usually based on graded-index-lenses. In addition, further development of tunable metasurface lenses will enable fast axial scanning for volumetric imaging.

A large increase in propagation loss is observed when exposing PECVD silicon nitride photonic waveguides to UV light. Annealing at moderate temperatures of ∼300 °C or high intensity optical exposure at visible wavelengths brings the propagation loss back to the initial value or even lower. We postulate that the origin of this effect can be found in the population and depopulation of optically active defect states in the silicon nitride material. We experimentally demonstrate the importance of defect depopulation when using silicon nitride waveguides and present a cleaning and depopulation procedure that yields reproducible, low-loss waveguide conditions.

Supercritical angle fluorescence (SAF) is a near-field collection method that has surface sensitivities similar to or better than near-field excitation techniques like TIRF and waveguide based excitation. SAF is emitted by fluorophores that are a few hundred nanometers away from an interface, above the critical angle, into the higher index material. SAF decreases exponentially with increasing distance from the interface and is therefore more sensitive to molecules near the surface. Although a lot of research has used SAF for biosensing and microscopy, the angular dependence of SAF on both the surface and bulk fluorescence contributions hasn’t been experimentally studied. We present a method that measures the surface selectivity of SAF in the presence of bulk fluorophores. Two different fluorophores were used. One was bound to the surface and the other was suspended in the bulk. The spectrum was measured at discrete points in the back focal plane (BFP) and the contribution of the two fluorophores was extracted from it. The results of the experiment show the highest signal-to-noise ratio in the region just above the critical angle of 61.31o because of the higher signal intensity. However, for experiments where bulk exclusion is important, we observe the highest signal-to-bulk ratio at angles above 68˚ for a glass-water interface. Understanding the angular dependence on the sensitivity of a SAF biosensor enables tuning the collection angles towards specific applications and could lead to the creation of smaller, more sensitive devices.