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We present the design, modeling, and optimization of high-performance plasmonic electro-optic modulators leveraging voltage-gated carrier density in indium tin oxide (ITO) where the gated carrier density is modeled using both the Classical Drift-Diffusion (CDD) and Schr\"odinger-Poisson Coupling (SPC) methods. The latter ensures a more detailed and precise description of carrier distributions under various gate voltages which gains particular significance when applied to an epsilon-near-zero (ENZ) medium such as ITO. Combining the nanoscale confinement and field enhancement enabled by surface plasmon polaritons with the ENZ effect in ITO, modulator designs integrated with silicon waveguides and optimized for operation at {\lambda}0 = 1550 nm achieve a 3-dB bandwidth of 210 GHz, an insertion loss of 3 dB, and an extinction ratio of 5 dB for an overall length of < 4 {\mu}m as predicted by the SPC model. Our results illustrate trade-offs between high-speed modulator operation and low insertion loss, vs. extinction ratio, and the need for precise modelling of carrier distributions in ENZ materials., Comment: This manuscript has been submitted to Optics Express and is currently under review. Manuscript ID: 544216

Color centers in hexagonal boron nitride (hBN) advantageously combine excellent photophysical properties with a potential for integration in highly compact devices. Progress towards scalable integration necessitates a high quantum efficiency and an efficient photon collection. In this context, we compare the optical characteristics of individual hBN color centers generated by electron irradiation, in two different electromagnetic environments. We keep track of well-identified emitters that we characterize before and after dry transfer of exfoliated crystals. This comparison provides information about their quantum efficiency - which we find close to unity - as well as their vertical position in the crystal with nanometric precision, which we find away from the flake surfaces. Our work suggests hybrid dielectric-metal planar structures as an efficient tool for characterizing quantum emitters in addition to improving the count rate, and can be generalized to other emitters in 2D materials or in planar photonic structures.

Wafer-level testing is an important step for process and quality control of electronic chips in integrated circuit (IC) manufacturing which occurs before packaging. The process of wafer probing in its conventional contacting schemes, becomes more complicated as ICs move to smaller technology nodes and more compact designs, greatly increasing testing costs. Non-contact optical wafer probing can overcome physical probing complications, reducing costs, and increasing throughput and reliability. In this paper, a CMOS compatible, broadband (22 GHz), small footprint (5 {\mu}m dia.) plasmonic electro-optic modulator of low insertion loss (4 dB) and wide optical working bandwidth (100 nm) is proposed and demonstrated as a potential solution for wafer-level optical testing. The device modulates in reflection an incident optical carrier emerging from an optical fiber in a non-contact arrangement, to work as a data output channel from the wafer. A modulation depth of over 2% is achieved which should be sufficient to meet the requirements of wafer-level testing. The device can be placed anywhere on wafer., Comment: 14 pages, 6 figures, this paper is submitted to a peer review journal and is under review

We report the fabrication and characterization of a plasmonic metasurface comprising electrically contacted sub-wavelength gold dipole nanoantennas, conformally coated by a thin hafnia film, an indium tin oxide layer and a backside mirror, forming metal-oxide-semiconductor (MOS) capacitors, for use as an electrically-tunable reflectarray or metasurface. By voltage biasing the nanoantennas through metallic connectors and leveraging the carrier refraction effect in the MOS capacitors, our measurements demonstrate phase control in reflection over a range of about 30 degrees, with a constant magnitude of reflection coefficient of 0.5, and the absence of secondary lobes. Comprehensive electromagnetic and quantum carrier models of the structure are developed and are in excellent agreement with the measurements. The metasurface holds promise for use as an optical phased array., Comment: 17 pages, 6 figures

Surface plasmon polaritons (SPPs) are optical waves that propagate along a metal surface. They exhibit properties such as sub-wavelength localization and field enhancement which make them attractive for surface sensing, as commonly encountered in surface plasmon biosensors - the most widespread of all optical biosensors. Electrochemistry also occurs on metal surfaces, and electrochemical approaches are widely used to implement biosensors - electrochemical biosensors are the most prevalent biosensors in use. Given that metal surfaces are inherent to both techniques, it is natural to combine them into a single platform. The motivation may be (i) to realise a multimodal biosensor (electrochemical, optical), (ii) to use SPPs to probe electrochemical activity or the electrochemical double layer, thereby revealing additional or complementary information on the redox reactions occurring thereon, or (iii) to use SPPs to affect (pump) electrochemical reactions with non-equilibrium energetic (hot) electrons and holes created in working electrodes by SPP absorption, potentially leading to novel redox reaction pathways (plasmonic electrocatalysis). We introduce in a tutorial-like fashion basic concepts related to SPPs on planar structures and to electrochemistry, then we review non-exhaustively but representatively literature on the integration of these techniques., Comment: 53 pages, 22 figures

Surface plasmon polaritons (SPPs) propagating along a waveguide working electrode are sensitive to changes in local refractive index, which follow changes in the concentration of reduced and oxidised species near the working electrode. The real-time response of the output optical power from a waveguide working electrode is proportional to the time convolution of the electrochemical current density, precluding the need to compute the latter a posteriori via numerical integration. The theoretical optical response of a waveguide working electrode is derived, and validated experimentally via chronoamperometry and cyclic voltammetry measurements under low power SPP excitation, for various concentrations of potassium ferricyanide in potassium nitrate electrolyte at various scan rates. Increasing the SPP power induces a regime where the SPPs no longer act solely as a probe of electrochemical activity, but also as a pump creating energetic electrons and holes via absorption in the working electrode. In this regime the transfer of energetic carriers (electrons and holes) to the redox species dominates the electrochemical current density, which becomes significantly enhanced relative to equilibrium (low SPP power) conditions. In this regime the output optical power remains proportional to the time convolution of the current density, even with the latter significantly enhanced by the transfer of energetic carriers., Comment: 21 pages, 9 figures

Plasmonic catalysis holds significant promise for opening new reaction pathways inaccessible thermally, or for improving the efficiency of chemical processes. However, challenges persist in distinguishing photothermal effects from effects due to energetic electrons and/or holes excited in the metal comprising a plasmonic structure. Here we use a Au stripe waveguide along which surface plasmon polaritons (SPPs) propagate, operating simultaneously as a working electrode in a 3-electrode electrochemical cell. Cyclic voltammograms were obtained under SPP excitation as a function of incident optical power and wavelength ~ 1350 nm, enabling processes involving energetic holes to be separated from processes involving energetic electrons by investigating oxidation and reduction reactions separately. Redox current densities increase by 10x under SPP excitation. The oxidation, reduction and equilibrium potentials drop by as much as 2x and split beyond a clear threshold with SPP power in correlation with the photon energy. The temperature of the working electrode under SPP excitation is monitored in situ and independent control experiments isolate thermal effects. Chronoamperometry measurements with on-off modulated SPPs at 600 Hz, yield a diffusion-limited rapid current response modulated at the same frequency, ruling out thermally-enhanced mass transport. Our observations are attributed to the opening of optically controlled non-equilibrium redox channels associated with energetic carrier transfer to the redox species., Comment: 21 Pages, 7 Figures

Optical phased arrays are of strong interest for beam steering in telecom and LIDAR applications. A phased array ideally requires that the field produced by each element in the array (a pixel) is fully controllable in phase and amplitude (ideally constant). This is needed to realize a phase gradient along a direction in the array, and thus beam steering in that direction. In practice, grating lobes appear if the pixel size is not sub-wavelength, which is an issue for many optical technologies. Furthermore, the phase performance of an optical pixel may not span the required $2\pi$ phase range, or may not produce a constant amplitude over its phase range. These limitations result in imperfections in the phase gradient, which in turn introduce undesirable secondary lobes. We discuss the effects of non-ideal pixels on beam formation, in a general and technology-agnostic manner. By examining the strength of secondary lobes with respect to the main lobe, we quantify beam steering quality, and make recommendations on the pixel performance required for beam steering within prescribed specifications. By applying appropriate compensation strategies, we show that it is possible to realize high-quality beam steering even when the pixel performance is non-ideal, with intensity of the secondary lobes be two orders of magnitude smaller than the main lobe., Comment: 19 pages, 10 figures, 3 movies and 1 Python code

Biosensors exploiting long-range surface plasmon polariton (LRSPP) waveguides comprised of Au stripes embedded in Cytop with integrated and encapsulated microfluidic channels are fabricated and demonstrated. A fabrication approach was devised where the lower cladding and recessed Au stripes are fabricated on a Si substrate, and the upper cladding and microfluidic channels are fabricated on a glass substrate, followed by wafer bonding to assemble the wafers into complete structures. The bond is centered over the full length of the optical path, yet no evidence of optical scattering or excess loss due to the bond could be observed, and no evidence of a bonding interface could be discerned from high-magnification cross-sectional images. We also demonstrate wafer-scale fabrication of in-plane microfluidic inlets and outlets, along with a fixture for fluidic edge coupling that provides sealed interfaces to external fluidic tubing and components. In-plane microfluidic interfaces are automatically defined along chip facets upon wafer dicing, precluding the need to drill through holes in lids. The performance of the chips was assessed by measuring the attenuation of LRSPPs on fully cladded reference waveguides, on waveguides passing through microfluidic channels, and by measuring the response of sensors to changes in refractive index produced by injecting various sensing solutions. Our fabrication approach based on wafer bonding and in-plane fluidic interfacing is compelling for low-cost high-volume manufacturing.

Controlling the phase and amplitude of light emitted by the elements (i.e., pixels) of an optical phased array is of paramount importance to realizing dynamic beam steering for LIDAR applications. In this paper, we propose a plasmonic pixel composed of a metallic nanoantenna covered by a thin oxide layer, and a conductive oxide, e.g., ITO, for use in a reflectarray metasurface. By considering voltage biasing of the nanoantenna via metallic connectors, and exploiting the carrier refraction effect in the metal-oxide-semiconductor capacitor in the accumulation and depletion regions, our simulations predict control of the reflection coefficient phase over a range $>330^{\circ}$ with a nearly constant magnitude. We discuss the physical mechanism underlying the optical response, the effect of the connectors, and propose strategies to maximize the magnitude of the reflection coefficient and to achieve dual-band operation. The suitability of our plasmonic pixel design for beam steering in LIDAR is demonstrated via 3D-FDTD simulations.

Picosecond laser pulses have been used as a surface colouring technique for noble metals, where the colours result from plasmonic resonances in the metallic nanoparticles created and redeposited on the surface by ablation and deposition processes. This technology provides two datasets which we use to train artificial neural networks, data from the experiment itself (laser parameters vs. colours) and data from the corresponding numerical simulations (geometric parameters vs. colours). We apply deep learning to predict the colour in both cases. We also propose a method for the solution of the inverse problem -- wherein the geometric parameters and the laser parameters are predicted from colour -- using an iterative multivariable inverse design method.

We develop a concept for a waveguide that exploits spatial control of nonlinear surface-polaritonic waves. Our scheme includes an optical cavity with four-level $\text{N}$-type atoms in a lossless dielectric placed above a negative-index metamaterial layer. We propose exciting a polaritonic Akhmediev breather at a certain position of the interface between the atomic medium and the metamaterial by modifying laser-field intensities and detunings. Furthermore, we propose generating position-dependent polaritonic frequency combs by engineering widths of the electromagnetically induced transparency window commensurate with the surface-polaritonic modulation instability. Therefore, this waveguide acts as a high-speed polaritonic modulator and position-dependent frequency-comb generator, which can be applied to compact photonic chips.

We have studied the chemical and electronic properties of LaAlO3/SrVO3 ultrathin films by combining hard x-ray photoemission spectroscopy and transport measurements. We compare single SrVO3 (SVO) ultrathin films and SrVO3 buried below a polar LaAlO3 (LAO) thin layer, both epitaxially grown on SrTiO3. While ultrathin films (4 unit cells) of SVO do show insulating behavior over the entire temperature range, the LAO/SVO interface has a resistivity minimum at 250 K. When increasing the SVO layer thickness, the minimum is observed to shift to higher temperatures, but the resistivity stays always smaller than that of comparable SVO single films. Hard x-ray photoemission spectroscopy reveals a surface or interface related V5+ component in the V 2p spectra for SVO films and LAO/SVO heterostructures, respectively, attributed to a strongly oxidized component. This chemical reconstruction is weaker in LAO/SVO heterostructures compared to single SVO films. We show that this dead layer in SVO ultrathin films has to be considered when the film thickness reaches the few unit-cells limit and propose solutions on how to prevent this detrimental effect., Comment: 15 pages, 4 figures + SI 2 pages, 1 figure

Emergent phenomena at interfaces between oxides and metals can appear due to charge transfer and mass transport that modify the bulk properties. By coating the metallic oxide LaNiO$_3$ by aluminium, we fabricated a junction exhibiting a diode-like behaviour. At the equilibrium, the interface is insulating. The metallic behaviour can be recovered by applying a voltage drop across the junction in one polarity only. The electrical properties in direct and reverse bias are investigated. The observed electro-resistive effect rises up to $10^5$ \% and can be interpreted in terms of (i) a spontaneous redox reaction occurring at the interface and (ii) its reversal induced by charge injection in direct bias.

Hexagonal boron nitride (hBN) has recently gained a strong interest as a strategic component in engineering van der Waals heterostructures built with two dimensional crystals such as graphene. This work reports micro-Raman measurements on hBN flakes made of a few atomic layers, prepared by mechanical exfoliation. The temperature dependence of the Raman scattering in hBN is investigated first such as to define appropriate measurements conditions suitable for thin layers avoiding undesirable heating induced effects. We further focus on the low frequency Raman mode corresponding to the rigid shearing oscillation between adjacent layers, found to be equal to 52.5 cm-1 in bulk hBN. For hBN sheets with thicknesses below typically 4 nm, the frequency of this mode presents discrete values, which are found to decrease down to 46.0(5) cm-1 for a three-layer hBN, in good agreement with the linear-chain model. This makes Raman spectroscopy a relevant tool to quantitatively determine the number of layers in ultra thin hBN sheets, below 8L.

We have investigated the effects of low-energy ion beam irradiations using argon clusters on the chemical and electronic properties of LaAlO3/SrTiO3 (LAO/STO) heterointerfaces by combining X-ray photoelectron spectroscopy (XPS) and electrical transport measurements. Due to its unique features, we show that a short-time cluster ion irradiation of the LAO surface induces indirect modifications in the chemical properties of the buried STO substrate, with (1) a lowering of Ti atoms oxidation states (from Ti4+ to Ti3+ and Ti2+) correlated to the formation of oxygen vacancies at the LAO surface and (2) the creation of new surface states for Sr atoms. Contrary to what is observed by using higher energy ion beam techniques, this leads to an increase of the electrical conductivity at the LAO/STO interface. Our XPS data clearly reveal the existence of dynamical processes on the titanium and strontium atoms, which compete with the effect of the cluster ion beam irradiation. These relaxation effects are in part attributed to the diffusion of the ion-induced oxygen vacancies in the entire heterostructure, since an increase of the interfacial metallicity is also evidenced far from the irradiated area. These results demonstrate that a local perturbation of the LAO surface can induce new properties at the interface and in the entire heterostructure. This study highlights the possibility of tuning the electronic properties of LAO/STO interfaces by surface engineering, confirming experimentally the intimate connection between LAO surface chemistry and electronic properties of LAO/STO interfaces., Comment: 23 pages, 5 figures

Plasmonic nanoantennas have been recently proposed to boost nonlinear optical processes. In a metal dipole nanoantenna with a dielectric nanoparticle placed in the gap, the linear field enhancement can be exploited to enhance third harmonic emission. Since both metals and dielectrics exhibit nonlinearity, the nonlinear far-field contains contributions from each, and the impossibility of measuring these contributions separately has led to seemingly contradictory interpretations about the origin of the nonlinear emission. We determine that the origin of the third harmonic from metal-dielectric dipole nanoantennas depends on nanoantenna design, and in particular, the width. We find that the emission from gold dominates in thin threadlike nanoantennas, whereas the emission from the gap material dominates in wider nanoantennas. We also find that monopole nanoantennas perform better than dipoles having the same width, and due to their simplicity should be preferred in many applications.

We report on a chiral nanostructure, which we term a "butterfly nanoantenna," that, when used in a metasurface, allows the direct conversion of a linearly polarized beam into a nonlinear optical far-field of arbitrary complexity. The butterfly nanoantenna exhibits field enhancement in its gap for every incident linear polarization, which can be exploited to drive nonlinear optical emitters within the gap, for the structuring of light within a frequency range not accessible by linear plasmonics. As the polarization, phase and amplitude of the field in the gap are highly controlled, nonlinear emitters within the gap behave as an idealized Huygens source. A general framework is thereby proposed wherein the butterfly nanoantennas can be arranged on a surface to produce a highly structured far-field nonlinear optical beam with high purity. A third harmonic Laguerre-Gauss beam carrying an optical orbital angular momentum of 41 is demonstrated as an example, through large-scale simulations on a high-performance computing platform of the full plasmonic metasurface with an area large enough to contain up to 3600 nanoantennas.

We report on the creation of angle-independent colors on silver using closely time-spaced laser bursts. The use of burst mode, compared to traditional non-burst is shown to increase the Chroma (color saturation) by ~50% and to broaden the lightness range by up to ~60%. Scanning electron microscope analysis of the surfaces created using burst mode, reveal the creation of 3 distinct sets of laser induced periodic surface structures (LIPSS): low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL) and large laser-induced periodic surface structures (LLIPSS) that are 10 times the laser wavelength and parallel to the laser polarization. Nanoparticles are responsible for each plasmonic color and their distributions are observed to be similar for both burst and non-burst modes, indicating that the underlying structures (i.e. LIPSSs) are responsible for the increased Chroma and Lightness. Two-temperature model simulations of silver irradiated by laser bursts show significant increase in the electron-phonon coupling coefficient which is crucial for the creation of well-defined ripple structures. Finite difference time domain (FDTD) simulations of the colored surfaces show the increase in Chroma to be attributable to the HSFL arising in burst mode., Comment: 8 pages, 7 figures

The use of metal nanostructures for colourization has attracted a great deal of interest with the recent developments in plasmonics. However, the current top-down colourization methods based on plasmonic concepts are tedious and time consuming, and thus unviable for large-scale industrial applications. Here we show a bottom-up approach where, upon picosecond laser exposure, a full colour palette independent of viewing angle can be created on noble metals. We show that colours are related to a single laser processing parameter, the total accumulated fluence, which makes this process suitable for high throughput industrial applications. Statistical image analyses of the laser irradiated surfaces reveal various distributions of nanoparticle sizes which control colour. Quantitative comparisons between experiments and large-scale finite-difference time-domain computations, demonstrate that colours are produced by selective absorption phenomena in heterogeneous nanoclusters. Plasmonic cluster resonances are thus found to play the key role in colour formation., Comment: 9 pages, 5 figures

We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO$_3$. By means of angle-resolved photoemission (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range were the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated to the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas (2DEG) at the surface of insulating $d^0$ transition metal oxides., Comment: Manuscript + Supplemental Material, 12 pages, 9 figures

Cathodoluminescence (CL) experiments at low temperature have been undertaken on various bulk and exfoliated hexagonal boron nitride (hBN) samples. Different bulk crystals grown from different synthesis methods have been studied. All of them present the same so-called S series in the 5.6--6 eV range, proving its intrinsic character. Luminescence spectra of flakes containing 100 down to 6 layers have been recorded. Strong modifications in the same UV range are observed and discussed within the general framework of 2D exciton properties in lamellar crystals.

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices meets now a huge research focus, and it becomes particularly important to evaluate the role played by crystalline defects in them. In this work, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First the link between the presence of structural defects and the recombination intensity of bound-excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replica of exciton recombinations. Second the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations indicating extremely compact excitons in h-BN.