Search

An independently tunable dual-resonance hybrid structure incorporating graphene is proposed to achieve selective light trapping and response tuning in the visible and near-infrared band. The incident light with different wavelengths are intensely localized in two separated resonant cavities via the synergetic effect of the guided mode resonances (GMRs) and optical Tamm states (OTSs), which leads to the strong enhancement of the light-matter interaction of graphene in different parts of the structure. The dual-band perfect absorption associated with critical coupling is achieved, whereas the two resonant modes can be independently tuned by adjusting the corresponding structural parameters. Furthermore, by dynamically changing the permittivity of the resonant cavities or the chemical potential of graphene, the absorption wavelength can be continuously adjusted and the absorption efficiency can be regulated stepwise.

We propose a graphene plasmonic structure by applying two graphene layers mingled with a thin gold layer in a silicon grating. By utilizing the finite-difference time-domain (FDTD) method, we investigate the optical response of the system, and observe that the design achieves dual tunable electromagnetically induced transparency (EIT)-like effect at terahertz frequencies. The EIT-like effect arises from the destructive interference between the grapheme-layer bright modes and the gold-layer dark mode. The EIT-like phenomenon can be adjusted by the Fermi level, which is related to the applied voltage. The results show that the group delay of the present structure reaches 0.62 ps in the terahertz band, the group refractive index exceeds 1200, the maximum delay-bandwidth product is 0.972, and the EIT-like peak frequency transmittance is up to 0.89. This indicates that the device has good slow light performance. The proposed structure might enable promising applications in slow-light devices.

The applicability of resonant waveguide grating (RWG)-based structures as filters to control the spectral response in an optical communication system is investigated. A new physical model for the structure is established on the basis of the Fabry–Pérot (FP) etalon model and coupled leaky mode theory (CLMT). It is found that the flat-top spectral response of the filter is achieved by the combined effect of the guided-mode resonance of an RWG and its Fabry–Pérot resonance (FPR). The bandwidth-tunable spectral response of the filter can be varied according to the change in the eigenvalues of the RWG by changing the structural parameters such as strip width of the grating and incident angle. The flat-top and bandwidth-tunable RWG-based resonant filter is a promising application for high-performance optical communication systems.

This paper deals with the optimization of photonic structures, specifically waveguide couplers between a continuous waveguide (CWG) and a periodically segmented waveguide (PSW). For this purpose, a numerical technique called finite element method (FEM) is used in conjunction with an adapted optimization technique called Scatter Search (SS). In this work the possibilities of finding high coupling efficiencies are extended through the creation of adaptations to the Scatter Search algorithm and the enormous amount of solutions generated to be evaluated by such procedure. The results have showed that the adaptation of the technique is valid for optimization of tapers.

A mid-infrared (mid-IR) dual-band filter with ultra-high resolving power is designed and numerically investigated. It is composed of an Al2O3 layer between a gold grating and a CaF2 substrate. Each unit cell of the grating consists of two gold strips with different heights. The filter provides two high-extinction ultra-narrow stop bands. One is at a wavelength of ~3.8392 μm with a full-width at half-maximum (FWHM) of ~0.8 nm, and the other is at a wavelength of ~3.8435 μm with a FWHM of ~1.8 nm. The resolving power of the filter is >2100. The two stop bands result from the (+1) and (-1) waveguide modes, respectively. An interaction between the two resonance modes exists and affects the stop bands. We can control the interaction by adjusting the grating parameters, thus designing the FWHMs and lineshapes of the two stop bands flexibly.

We report on subwavelength reflective gratings for hyperspectral applications operating in a very large spectral band (340–1040 nm). Our study concerns a blazed-binary grating having a period of 30 μm and composed of 2D subwavelength structures with size from 120 nm to 350 nm. We demonstrate the manufacturing of the gratings on 3″ wafers by two lithography technologies (e-beam and nanoimprint) followed by classical dry etching process. Optical measurements show that the subwavelength grating approach enables a broadband efficiency, polarization behaviour and wavefront quality improvement with respect to the requirements for the next generation of spectro-imagers for Earth observation missions. An outlook towards spherical substrate based on nanoimprint lithography is also reported with the results of mixed features replication (holes and pillars in the range of 160–330 nm) on a 540 mm concave substrate which demonstrate uniformity and accuracy capabilities over 3″ surface.

In this work, the authors present their recent progress in the development of a platform for the mid-infrared wavelength range, based on suspended silicon waveguides with subwavelength metamaterial cladding. The platform has some intrinsic advantages, which make it a very promising candidate for sensing applications in the molecular fingerprint region. Specifically, it can cover the full transparency window of silicon (up to a wavelength of 8 μm), only requires one lithographic etch-step and can be designed for strong light-matter interaction. Design rules, practical aspects of the fabrication process and experimental results of a complete set of elemental building blocks operating at two very different wavelengths, 3.8 and 7.7 μm, are discussed. Propagation losses as low as 0.82 dB/cm at λ0 = 3.8 μm and 3.1 dB/cm at λ0 = 7.7 μm are attained for the interconnecting waveguides.

We present the realization of a high-efficiency, compact, and thermally tunable unidirectional launcher of surface plasmon polaritons (SPPs) in a double-slit structure filled with phase change materials (PCMs). PCMs such as VO 2 provide a feasible way for tailoring optical properties, particularly refractive index as a function of heat. By introducing a narrow hybrid dielectric layer beneath the slits, SPPs couple efficiently into waveguide mode and propagate within the slot waveguide with considerably high intensity. Our simulation results revealed up to 80% coupling efficiency into the slot waveguide with extinction ratio of 1:800 between the left and the right slots can be achieved at telecommunication wavelength for normally incident P-polarized light. By heating the coupler above the VO 2 phase transition temperature, the coupling and consequently the crosstalk approaches zero. Propagation length and other properties of the structure are characterized to confirm the feasibility of the coupler. This approach of unidirectional coupling into slot waveguides may lead to the development of integrated plasmonic circuits and on-chip applications.

In this paper, we report a new and generic method for all-dielectric resonators operating at terahertz (THz) frequencies based on polymer/ceramic composite particles. In this method, the polymer solution is added into ceramic powders (strontium titanate) and prilling technology is employed to make them become polymer/ceramic composite micro particles. The prepared micro particles are then assembled as single-layer all-dielectric resonators. By using a THz time-domain spectrometer, we find that various Mie resonances can be induced in these all-dielectric resonators. It is also demonstrated that the permittivity of the composite particles can be improved by being sintered at proper temperatures, thereby provides a new way for ceramic all-dielectric resonators in THz range. This method, free from complex process, time-consuming, expensive equipment, and clean room, provides a generic method for high-throughput and low cost THz all-dielectric resonators, which is promising for developing potential applications in wide-bandwidth THz devices.

Plasmonic filters are excellent candidates for spectral filters in the near infrared spectrum. Such filters require only a single nanoscale thickness perforated metal film that can be tailored to tune the location of the transmission maximum. Here, we demonstrate using computational methods a double coaxial aperture array (DAA) in a hexagonal geometry that can be tuned to produce plasmon resonances accompanied by strong transmission in the near-infrared spectrum and at telecommunication wavelengths with a peak wavelength insensitive to angle of incidence. The DAA consists of two concentric annular apertures and radial distance between the two apertures is varied to tune the resonant wavelength. The presented geometry finds potential applications in modulators for telecommunication, spectroscopy, solar cells, high-resolution CMOS image sensors, chemical sensors, and biosensors.

A subwavelength grating sensor based on a bimodal waveguide configuration is presented for continuous in-flow measurements of refractive index variations. An experimental bulk sensitivity of 1350nm/RIU and a limit of detection of 2x10-5RIU is obtained in a single-channel refractive index sensor.

5 pags., 4 figs., 2 tabs., Mode-division multiplexing is a promising solution to enhance the capacity of current photonic integrated circuits and thereby meet the growing demands of telecom and datacom applications. Here, we experimentally demonstrate for the first time, to the best of our knowledge, a compact and broadband mode converter and multiplexer based on a subwavelength engineered multimode interference coupler. Our device was fabricated on a standard 220-nm-thick silicon-on-insulator platform with a small footprint of 38.6 ⨯ 3.7 µm2. Experimental results for a complete back-to-back multiplexer-demultiplexer link show an insertion loss, This work has been funded in part by the Spanish Ministry of Science, Innovation and Universities (MICINN) under grants CDTI SNEO-20181232 (Alcyon Photonics S.L.), TEC2015-71127-C2-1-R (FPI scholarship BES-2016-077798), TEC2016-80718-R, IJCI-2016-30484 and RTI2018-097957-B-C33; and the Community of Madrid - FEDER funds (S2018/NMT-4326). This project has received funding from the Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant No. 734331.

Reduction in the thickness of air-suspended mirrors to the deep subwavelength scale can facilitate efficient optical and mechanical design. The thinner the mirror, the smaller is its mass, and the more excellent mechanical properties it presents. Thin mirrors are easily displaced by optical radiation pressure, and could present more intense light-mirror interactions than their thicker counterparts. Herein, we present this kind of ultraflat air-suspended mirrors that can reflect light at the telecom region, with a mirror thickness of approximately 25 nm and total thickness of 50 nm. This kind of mirrors shows many unique properties, e.g., the mirrors become increasingly transparent with diminishing film thickness at nonresonant wavelength; there is a strong evanescent field distribution outside the mirrors; the reflection wavelength is almost independent of the refractive index of the mirror materials, and nearly only depends on the lattice geometry at the deep subwavelength scale; moreover, their quality factor significantly improves, which is important for applications in laser, cavity optomechanics, and cavity quantum electrodynamics.

Wavelength-division multiplexing over optical links provides an effective solution for the bandwidth challenge of off-chip and on-chip communications. We demonstrate a compact silicon photonic four-channel optical add-drop multiplexer enabled by subwavelength-grating-based contradirectional couplers. Passbands of the device show on-chip insertion losses below 1.8 dB with wide 3 dB bandwidth of ~6.7 nm suitable for coarse wavelength-division Multiplexing in short-reach optical interconnect applications. Transmission of 10 Gb/s data stream through different channels of the multiplexer results in negligible power penalties, whereas interferometric crosstalk-induced power penalties are below 2.8 dB.

A resonant wavelength detection system based on a gradient grating period guided-mode resonance (GGP-GMR) filter and a linear charge-coupled device is demonstrated. The GGP-GMR used in this work consists of a subwavelength grating structure with a grating period that varies from 250 to 550 nm in increments of 2 nm. We successfully demonstrate that by using this system, we can monitor the shift in a sensor's dip wavelength caused by different sucrose concentrations. This technology has the potential to replace the spectrometer used in most optical biosensor systems. Given its compactness, the system can be integrated with lab-on-a-chip systems for numerous biosensing applications.

We propose, design, fabricate and characterize a subwavelength grating slot (SWGS) waveguide on silicon platform at short-wave infrared (SWIR) wavelength of 2 μm. The mode guiding mechanism, i.e. SWGS mode, is a combination of surface-enhanced supermode and Bloch mode. We also design and fabricate a low-loss strip-to-SWGS mode converter. We further demonstrate chip-scale direct modulation data transmission at 2 μm through the fabricated SWGS waveguides. Favorable operation performance is achieved in the experiment.

Tunable surface plasmon polariton launchers are highly demanded in various applications of nanophotonics for achieving controllable plasmonic signals coupling and multiplexing. Here, a tunable-focusing device with simplified structure is suggested. It consists of an array of nanofootprints. This device allows us polarization-controlled tunable plasmonic directing and focusing on the two-dimensional metallic surface. The simulation and experimental results indicate that the focal position of the excited plasmon field can be flexibly tuned between two distinct positions just by manipulating the incident polarization state. Hence, this nanofootprints structure can serve effectively as controllable plasmonic routers and demultiplexers. It also has promising application prospects in many other fields of optics.

This paper reports a compact spectrometer based on a guided-mode resonance (GMR) filter mounted on a linear charge-coupled device (CCD). The GMR is specially designed to exhibit gradient grating periods (GPPs) laterally to ensure that the GMR functions as a linear-variable bandstop filter. Each period corresponds to a resonant wavelength such that this wavelength is reflected back at its corresponding resonant period and transmitted to all other periods. Consequently, when a resonant wavelength is incident on the GGP-GMR attached to a linear CCD, the CCD pixel underneath its resonant period receives the minimum intensity, and the other pixels receive higher intensities. In terms of the wavelength range of interest, by scanning a single wavelength at a time, a transmission efficiency matrix that contains the transmission efficiency of each wavelength at each pixel can be established. An unknown incident spectrum can be reconstructed using the established transmission efficiency matrix and the intensity measured using the CCD. In this study, a GGP-GMR spectrometer less than 3 mm long that can achieve a wavelength detection range of 200 nm was demonstrated to reconstruct various incident spectra, including a single wavelength of light with a resolution of 0.5 nm, a single wavelength of light with varying intensity levels, and dual incident light sources.

Metals are commonly used in plasmonic devices because of their strong plasmonic property. However, such properties are not easily tuned. For applications such as spatial light modulators and beam steering, tunable plasmonic properties are essential, and neither metals nor other plasmonic materials possess truly tunable plasmonic properties. In this work, we show that the silver alloy silver–ytterbium (Ag-Yb) possesses tunable plasmonic properties; its plasmonic response strength can be adjusted as a function of Yb concentration. Such tunability can be explained in terms of the influence of Yb on bound charge and interaction of its dielectric with the dielectric of Ag. The change in transition characteristics progressively weakens Ag’s plasmonic properties. With a spectral ellipsometric measurement, it was shown that the Ag-Yb alloy thin film retains the properties of Ag with high transmission efficiency. The weakened surface plasmon coupling strength without dramatic change in the coupling wavelengths implies that the tunability of the Ag-Yb alloy is related to its volume ratio. The principle mechanism of the plasmonic change is theoretically explained using a model. This work points to a potential new type of tunable plasmonic material.

Good sensing performance is observed in metal-dielectric-metal (MDM) waveguides in recent research. However, Ag will be oxidized in air. The oxidization layer makes the sensing performance not as high as before. Plating an SiO2 film on the surface of the Ag waveguide not only can avoid oxidization but can keep good sensing performance through our numerical discussion as well. The decaying rate of electric field E at the interface and inside is also defined to illustrate the phenomenon mentioned above. We hope our findings will provide guidance for the integrated plasmonic devices.

The multilayer film consisting of MoO3/Ag/MoO3 (shortened as MAM) is very promising as an alternative of indium-tin oxide (ITO) to work as the transparent anode in organic solar cells (OSCs). In MAM-based thin OSCs, the absorption of light is quite poor. Here, we propose to apply a short-pitched metallic grating into the MAM-based OSC for improving the absorption in its thin active layer. Numerical calculations reveal that the obtained enhancement of integrated absorption in the active layer is more than 500% with respect to the equivalent planar control. The field distributions at the two peaks of the normalized absorption spectrum indicate that propagating surface plasmon polaritons (SPPs), as well as the horizontal metal-insulator-metal (MIM) waveguide mode, play the main role of trapping light into the active material. It is expected that the proposal could contribute to the development of the efficient ITO free thin OSCs.

The dispersion profiles of a hybrid graphene-silicon (HyGS) slot waveguide with trilayers of graphene are investigated in this paper. By tuning the bias voltage on graphene layer, the zero dispersion wavelengths (ZDW) of HyGS slot waveguide can be tailored dynamically and effectively. With small disturbed voltage (6 V), a large ZDW tuning amount (200 nm) is achieved, without the need for changing the geometric structure of the waveguide, which has been verified with numerical simulation. Moreover, the dispersion curve of HyGS slot waveguide is flat enough (group velocity dispersion slope less than 2 ps/nm2·km), in the vicinity of ZDW. This waveguide is highly beneficial to such applications as integrated optics and graphene-related active optical devices.

The paper proposes a method for calculating the phase function of subwavelength diffractive optical elements. The method is based on diffraction gratings with a varying period for generating vector beams with arbitrary-order cylindrical polarization. Formulas for the phase function of the grating are obtained with due regard for the period variation for increasing the efficiency of the polarization conversion of the incident beam. The obtained phase functions are supposed to be used for creating polarization-conversion diffractive optical elements for noise-resistant optical communication systems.

Subwavelength gratings (SWG) are periodic structures which behave as controllable homogeneous metamaterials. SWGs are extremely interesting when they are used in platforms with a limited choice of material refractive indices, enabling the design of a myriad of high-performance devices. Here we present a novel technique to gain control over the intrinsic anisotropy of the synthesized metamaterial. We show that tilting the silicon segments in a SWG structure mainly affects the in-plane (TE) modes, with little impact on the out-of-plane (TM) modes. Moreover, we present a methodology to quickly but accurately calculate the modes of a tilted periodic structure modeling the structure as a rotated uniaxial crystal which can be solved with an anisotropic mode solver. Measurements on a set of fabricated tilted SWG waveguides validate our simulation results. By using the presented technique, we design a polarization beam splitter based on a 2x2 multimode interferometer. The design is based on the optimization of the tilting angle to tone the beat length of the TE modes to be a half of the beat length of the TM modes. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech; Ministerio de Economía y Competitividad (MINECO) (IJCI-2016-30484, TEC2015-71127-C2-R, TEC2016-80718-R); Ministerio de Educación, Cultura y Deporte (MECD) (FPU16/06762); European Regional Development Fund (ERDF); Comunidad de Madrid (SINFOTON-CM S2013/MIT-2790); European Association of National Metrology Institutes (EURAMET) (H2020-MSCA-RISE-2015:SENSIBLE, JRP-i22 14IND13 Photind).

A symmetric graphene plasmon waveguide (SGPWG) is proposed here to achieve excellent subwavelength waveguiding performance of mid-infrared waves. The modal properties of the fundamental graphene plasmon mode are investigated by use of the finite element method. Due to the naturally rounded tips, the plasmon mode in SGPWG could achieve a normalized mode field area of ~10−5 (or less) and a figure of merit over 400 by tuning the key geometric structure parameters and the chemical potential of graphene. In addition, results show that the modal performance of SGPWG seems to improve over its circular counterparts. Besides the modal properties, crosstalk analysis indicates that the proposed waveguide exhibits extremely low crosstalk, even at a separation distance of 64 nm. Due to these excellent characteristics, the proposed waveguide has promising applications in ultra-compact integrated photonic components and other intriguing nanoscale devices.

The waveguiding of terahertz surface plasmons by a GaAs strip-loaded graphene waveguide is investigated based on the effective-index method and the finite element method. Modal properties of the effective mode index, modal loss, and cut-off characteristics of higher order modes are investigated. By modulating the Fermi level, the modal properties of the fundamental mode could be adjusted. The accuracy of the effective-index method is verified by a comparison between the analytical results and numerical simulations. Besides the modal properties, the crosstalk between the adjacent waveguides, which determines the device integration density, is studied. The findings show that the effective-index method is highly valid for analyzing dielectric-loaded graphene plasmon waveguides in the terahertz region and may have potential applications in subwavelength tunable integrated photonic devices.

The antireflection (AR) performance and far-field transmittance, depending on the morphology and parameters of subwavelength structures (SWSs), have been investigated by a 3-D finite-difference time-domain (FDTD) method. It is found that far-field transmission characteristics are dominated mainly by period of SWSs. If the period is increased above 220 nm, the far-field transmittance rapidly decreases for a short-wavelength region. An effective and inexpensive method for fabricating disordered SWSs on fused silica has been demonstrated in this paper. An Au nanoscale island mask was formed by lower temperatures thermal dewetted at normal atmosphere. Then, a disordered SWS was fabricated by reactive ion etching (RIE). The average diameter of the Au nanoscale island was easily controlled by the thickness of Au thin films and annealing temperature. The etched depth and shape of fused silica SWSs depend additionally on RIE duration, thus achieving efficient AR characteristics. The measured data and calculated results obtained by the FDTD method exhibit reasonably similar tendencies. The optimized single-side fused silica SWS that is tapered on the tips leads to a significantly high transmissivity value of 96.2%. In addition, it exhibits a broadband AR property at a wavelength range of 400–1100 nm.

A broadband absorber, which is composed of graphene analog of electromagnetically induced transparency (EIT) and a metal ground plane spaced by a thin SiO 2 dielectric layer, is proposed and investigated. Numerical results reveal that the working bandwidth can be dynamically tuned between broadband absorption and narrowband absorption by varying the Fermi level of graphene through controlling of the electrostatic gating. Furthermore, absorption behaviors can be tuned to the state of off when the electric polarized is rotated to another axis. It is also found that the coupling strength between the radiative element and dark element is tuned by the distance between the disconnected vertical graphene strip and horizontal graphene strip. In addition, this type of graphene-based absorber is very sensitive to the incident angles.

A wavelength-selective metasurface absorber suitable for use in multispectral microbolometer focal plane arrays in the long-wavelength infrared (LWIR) region is theoretically and experimentally investigated. We show that a thin metal metasurface, which is characterized by effective surface impedance, as an absorbing layer can be integrated with an asymmetric Fabry-Pérot cavity to construct a wavelength-selective perfect absorber. The absorbed infrared energy is mainly dissipated in a thin metal metasurface, in contrast to a multilayer metamaterial absorber. The calculated and experimental spectral responses of the metasurface absorbers show excellent wavelength-selective narrowband absorption to allow multispectral imaging in LWIR.

An all-optical-fiber orbital angular momentum (OAM) mode generator is designed based on the effective medium theory. The generator is formed by many coaxial vortex structures with the same height and gradually decreasing width following the azimuth angle, which causes the linear decrement of the effective refractive index with the azimuth angle. The different effective refractive index will cause the different optical path and the corresponding phase delay along the azimuthal angle. The OAM generation process is simulated using a finite-difference time-domain (FDTD) software package.

Currently, huge opportunities for the inclusion of new optical devices in our lives have been appearing. There are evident and irrefutable examples for nanoantenna applications. They can be used to improve already developed devices or even be used as the device. In both cases, they can be applied in diverse areas, such as medicine, environment, energy, defense, and communications. A square arrayed metallic nanoantenna composed of circular holes is studied by performing simulations using COMSOL Multiphysics. This article aims to study the influence of the nanoantenna&rsquo, s metal, silver, gold, copper and aluminum, but also the optical response dependence on the nanoantenna&rsquo, s periodicity, its thickness, the hole diameter, and the number of holes. It is evidenced that the optical response can be tuned using the structure parameters and by choosing an appropriate material. This tuning will allow developers to fulfil the specifications, since it is proven that the response peak can be deliberately shifted, amplified, or attenuated.

Plasmonic metasurfaces have attracted much attention in recent years owing to many promising prospects of applications such as polarization switching, local electric field enhancement (FE), near-perfect absorption, sensing, slow-light devices, and nanoantennas. However, many problems in these applications, like only gigahertz switching speeds of electro-optical switches, low-quality factor (Q) of plasmonic resonances, and relatively low figure of merit (FOM) of sensing, severely limit the further development of plasmonic metasurface. Besides, working as nanoantennas, it is also challenging to realize both local electric FE exceeding 100 and near-perfect absorption above 99%. Here, using finite element method and finite difference time domain methods respectively, we firstly report a novel optically tunable plasmonic metasurface based on the hybridization of in-plane near-field coupling and out-of-plane near-field coupling, which provides a good solution to these serious and urgent problems. A physical phenomenon of electromagnetically induced transparency is obtained by the destructive interference between two plasmon modes. At the same time, ultrasharp perfect absorption peaks with ultra-high Q-factor (221.43) is achieved around 1550 nm, which can lead to an ultra-high FOM (214.29) in sensing application. Particularly, by using indium-doped CdO, this metasurface is also firstly demonstrated to be a femtosecond optical reflective polarizer in near-infrared region, possessing an ultra-high polarization extinction ratio. Meanwhile, operating as nanoantennas, this metasurface achieves simultaneously strong local electric FE(|E loc|/|E 0| > 100) and a near-perfect absorption above 99.9% for the first time, which will benefit a wide range of applications including photocatalytic water splitting and surface-enhanced infrared absorption.

Achieving strong absorption of low-energy photons is one of the key issues to demonstrate quantum dot solar cells working in the intermediate band regime at practical concentration factors and operating temperatures. Guided-mode resonance effects may enable large enhancement of quantum dot intraband optical transitions. We propose quantum dot thin-film cells designed to have significant field waveguiding in the quantum dot stack region and patterned at the rear-side with a sub-wavelength diffraction grating. Remarkable increase of the optical path length at mid-infrared wavelengths is shown owing to guided-mode resonances. Design guidelines are presented for energy and strength of the second-photon absorption for III-V quantum dots, such as InAs/GaAs and GaSb/GaAs, whose intraband and intersubband transitions roughly extends over the 2 - 8 µm range. The proposed design can also be applied to quantum dot infrared detectors. Angle-selectivity is discussed in view of applications in concentrator photovoltaic systems and infrared imaging systems.

We investigate the chiroptical response of a single plasmonic nanohelix interacting with a weakly-focused circularly-polarized Gaussian beam. The optical scattering at the fundamental resonance is characterized experimentally, and the chiral behavior of the nanohelix is explained based on a multipole analysis. The angularly resolved emission of the excited nanohelix is verified experimentally and it validates the theoretical results. Further, we study the first higher-order resonance and explain the formation of chiral dipoles in both cases., Comment: 15 pages, 11 figues

International audience; When building spot array binary Fourier diffractive optical elements (DOEs) having feature sizes on the order of the wavelength, we noticed remarkable variations in the experimental diffraction efficiency compared to the simulation results. Even with the use of high-cost electron beam lithography and rigorous Fourier modal method simulations, there appear to be no publications, to the best of our knowledge, showing close agreement in diffraction efficiency between the simulation and experimental results. In this Letter, we show that the diffraction symmetry of binary Fourier DOEs can be an efficient and consistent metric for evaluating the limit of the thin-element approximation and the effects of fabrication errors.

We explain modal processes in 2-D guided-mode resonance reflectors with subwavelength periods in terms of the mode structure of quasi-equivalent 1-D grating-based reflectors. The 1-D gratings are designed via a second-order effective-medium theory. The principal features in the reflection spectra of the 2-D devices show good quantitative agreement with the corresponding 1-D grating spectra for small modulation strength. Thereby, clear connections are established with the TE and TM modal states of a reflector. For reflectors made with silicon that has a high index of refraction, there is qualitative agreement between the 2-D spectra and the concomitant 1-D modal signatures. Two-dimensional reflectors with periodic rods and holes are treated. In both cases, it is found that the spectra are dominated by contributions from a single polarization state in the 1-D grating equivalent. The results and methods provided herein enable improved understanding of the physical properties of 2-D resonant reflectors and the related 2-D modulated devices, including photonic crystal slabs. Hence, this methodology facilitates the design of 2-D reflectors in general as is straightforwardly applied to device architectures, materials, and spectral regions beyond those treated here.

The dielectric mirror is an important optical component for optoelectronic devices, passive photonic devices, and solar cells. Unfortunately, the reflection bandwidth of distributed Bragg reflectors (DBRs) and high-index contrast mirrors (HCGs) are limited by the index contrast of the material system used. Here, an aperiodic design for dielectric mirrors is proposed, and it is shown that for a fixed index contrast, the bandwidth of the reflection band can be arbitrarily widened by simply incorporating more dielectric layers. This is pronouncedly different from the fixed bandwidth of HCGs and DBRs. The physics behind the broadband reflection for the aperiodic stacking is identified as the photonic bandgap widening due to the annihilation of the quasi-guided modes in nonperiodic structures. This observation applies very well to aperiodic auto-cloned 3-D photonic crystal reflectors, to aperiodic DBRs, and even to diffuse dielectric mirrors that have recently emerged to be very promising for solar cells due to their zero plasmonic absorption nature. Experimentally, the white paint diffuse medium reflectors are applied to SiGe solar cells to confirm their high reflectance and the feasibility of enhancing solar cell efficiency.

We experimentally demonstrate apodized focusing subwavelength grating couplers for both the fundamental transverse electric (TE00) mode and the fundamental transverse magnetic (TM00) mode. A measured insertion loss of 3.2 dB with a 1-dB bandwidth of 36 nm has been obtained for the TE00 mode, and a measured insertion loss of 3.3 dB with a 1-dB bandwidth of 37 nm has been obtained for the TM00 mode. Back reflections of -24 dB and -21 dB have been obtained for the TE00 and TM00 modes, respectively.

A new type of metalens with the capability to selectively focus narrowband light is reported at different focal planes depending on polarization. Two zone planes with two different focal lengths are spatially multiplexed and encoded in the subwavelength regime with arrays of orthogonal silver nanostructures, which are designed to exhibit strong wavelength- and polarization-dependent scattering profiles. Using this principle, imaging under white-light illumination with independent focal planes is experimentally demonstrated for each polarization at the narrowband resonant wavelength. Furthermore, each focal plane can be dynamically controlled with an output polarizer. In contrast to conventional refractive and diffractive optical elements, functional metalenses allow additional control over wavelength and polarization properties of light for a wide range of applications.

We present design of plasmonic modulators using vanadium dioxide (VO2) as modulating material realized on silicon-on-insulator (SOI) wafer with only 200 nm × 140 nm modulating section within 1 μ m × 3 μm device footprint. By utilising the large refractive index contrast between the metallic and semiconductor phases of VO2, the modulator can achieve a broad working wavelength range from 1100 to 1800 nm around C-band, with a high modulation depth of 21.5 dB/μm. We also analyse effects of using seed layer of different dielectric materials for growing VO2 on modulation index by exploring the mixed combination of VO2 and different dielectric materials. Our device geometries can have potential applications in the development of next-generation miniaturised high-frequency optical modulators in silicon photonics for optical communications.

We propose a novel method for the design of binary two-level diffractive optical elements that provide an efficient diffractive function while reducing the reflectivity of a high-index interface. The structure can be fabricated with a single patterning step and is particularly useful for the mid-infrared spectral range. The concept is based on zero-order transmission gratings using subwavelength microstructures. To demonstrate the concept a Dammann grating has been realized in silicon and characterized in the mid-infrared by scanning the far-field intensity distribution.

The dataset contains 28 excel files representing data for figures created for the purposes of showing the enhancement of fluorescence in the ultraviolet. Further details of the data can be found in the Readme file provided. The data supports the article “Enhancing ultraviolet spontaneous emission with a designed quantum vacuum" to appear in Optics Express".

We show that randomization of a nanodisk array on a rectangular lattice can be used to spectrally tune the surface-enhanced Raman scattering (SERS) intensity and to control the field enhancement. The spectral selectivity of the Raman modes exhibits a nonlinear dependence on the extinction. Randomization of the rectangular pattern brings a favorable increase of SERS intensity via a reduced selectivity and extinction changes at the Stokes band. The extinction dependence for two polarizations on the spacing in the rectangular array is demonstrated. For a given polarization, the extinction strength depends primarily on the spacing between the nanodisks in the orthogonal direction. Such randomization and polarization control of SERS increases the versatility, e.g., all-optical control, of this important sensing tool.

This paper proposes guided-mode resonant GaN grating that are implemented on a GaN-on-silicon wafer structure. Numerical simulations based on rigorous coupled wave analysis (RCWA) are performed to model multiple resonances in thick GaN membranes. Back wafer etching of freestanding GaN membranes is used as a tuning mechanism to manipulate the optical performance in the visible wavelength range. Angular resolved micro reflectance measurements are conducted to characterize the fabricated GaN gratings and show the dependence of guided-mode resonances on the filling factor of the gratings, the membrane thickness, and the polarization of incident beam. The experimental results agree well with numerical simulations. This paper opens the way to fabricate guided-mode resonant GaN grating for many diverse applications.

The processing of organic substances by vacuum deposition is opening new possibilities for the properties of optical surfaces. Organic small molecules can be evaporated and deposited like optical thin films. Plasma etching, which has been successfully applied to generate nanostructures on polymer substrates, is now used for obtaining antireflective nanostructures on top of interference stacks on glass. The latest results in achieving excellent antireflective properties for a wide range of light incidence angles were accomplished through multiple etching of polymer substrates and organic layers.

The lossy nature of plasmonic wave due to absorption is shown to become an advantage for scaling-up a large area surface nanotexturing of transparent dielectrics and semiconductors by a self-organized sub-wavelength energy deposition leading to an ablation pattern—ripples—using this plasmonic nano-printing. Irreversible nanoscale modifications are delivered by surface plasmon polariton (SPP) using: (i) fast scan and (ii) cylindrical focusing of femtosecond laser pulses for a high patterning throughput. The mechanism of ripple formation on ZnS dielectric is experimentally proven to occur via surface wave at the substrate–plasma interface. The line focusing increase the ordering quality of ripples and facilitates fabrication over wafer-sized areas within a practical time span. Nanoprinting using SPP is expected to open new applications in photo-catalysis, tribology, and solar light harvesting via localized energy deposition rather scattering used in photonic and sensing applications based on re-scattering of SPP modes into far-field modes. Femtosecond laser pulses focused onto a dielectric induce ripples on its surface that are useful for photovoltaic and sensing applications. Light-driven excitation of surface charge waves—plasmons—achieves light localization on surfaces down to the deep-subwavelength nanoscale. Now, Hong-Bo Sun of Jilin University in China and co-workers from China and Australia have used surface plasmon polaritons to create permanent surface modifications whose period is smaller than the laser wavelength. They achieved this by using a cylindrical lens to create a line focus for near-infrared ultrashort laser pulses on silicon and zinc sulfide surfaces. This nanoprinting technique is well suited for texturing large areas on a wafer scale, since the patterning speed is determined simply by the repetition rate of the laser used. Potential applications include enhanced light harvesting for solar cells and improved photocatalysis.

Graphene was recently proposed as a promising alternative to support surface plasmons with superior performances in the mid-infrared range. Here, we theoretically show that high-performance and low-loss transmission of graphene plasmons can be achieved by adding a silica substrate to the graphene-covered nanowire pairs. The effect of the substrate layer on mode properties has been intensively investigated by using the finite element method. Furthermore, the results show that inserting a low index material layer between the nanowire and substrate could compensate for the loss accompanied by the substrate, thus the mode properties could be adjusted to fulfill better performance. A reasonable propagation length of 15 &mu, m and an ultra-small normalized mode area about ~10&minus, 4 could be obtained at 30 THz. The introduction of the substrate layer is crucial for practical fabrication, which provides additional freedom to tune the mode properties. The graphene-covered nanowire pairs with an extra substrate may inspire potential applications in tunable integrated nanophotonic devices.

The transmittance, reflectance and absorption of silver nanowires metamaterial embedded into a semiconductor matrix with optical gain are numerically investigated. Metamaterials may suffer from appreciable dissipative losses which are inherent for all plasmonic structures. The losses can significantly be reduced by introducing optical gain in the dielectric matrix by placing atomic or molecular impurities which are pumped by an external light source to create a population inversion. We numerically analyzed the optical properties when the semiconductor host material represents a gain medium. We calculate the transmittance, reflectance and absorption at normal incidence in the visible and near infrared ranges. We observed a peculiar behavior of their optical coefficients that can be explained by observing the field redistribution on the metamaterial.

We design and experimentally verify a topology optimized low-loss and broadband two-mode (de-)multiplexer, which is (de-)multiplexing the fundamental and the first-order transverse-electric modes in a silicon photonic wire. The device has a footprint of 2.6 µm x 4.22 µm and exhibits a loss1.2 dB in a 100 nm bandwidth measured around 1570 nm. The measured cross talk is-12 dB and the extinction ratio is14 dB in the C-band. Furthermore, we demonstrate that the design method can be expanded to include more modes, in this case including also the second order transverse-electric mode, while maintaining functionality.

8 págs.; 5 figs.; 1 tab., Self-assembly (SA) of molecular units to form regular, periodic extended structures is a powerful bottom-up technique for nanopatterning, inspired by nature. SA can be triggered in all classes of solid materials, for instance, by femtosecond laser pulses leading to the formation of laser-induced periodic surface structures (LIPSS) with a period slightly shorter than the laser wavelength. This approach, though, typically involves considerable material ablation, which leads to an unwanted increase of the surface roughness. We present a new strategy to fabricate high-precision nanograting structures in silicon, consisting of alternating amorphous and crystalline lines, with almost no material removal. The strategy can be applied to static irradiation experiments and can be extended into one and two dimensions by scanning the laser beam over the sample surface. We demonstrate that lines and areas with parallel nanofringe patterns can be written by an adequate choice of spot size, repetition rate and scan velocity, keeping a constant effective pulse number (N ) per area for a given laser wavelength. A deviation from this pulse number leads either to inhomogeneous or ablative structures. Furthermore, we demonstrate that this approach can be used with different laser systems having widely different wavelengths (1030 nm, 800 nm, 400 nm), pulse durations (370 fs, 100 fs) and repetition rates (500 kHz, 100 Hz, single pulse) and that the grating period can also be tuned by changing the angle of laser beam incidence. The grating structures can be erased by irradiation with a single nanosecond laser pulse, triggering recrystallization of the amorphous stripes. Given the large differences in electrical conductivity between the two phases, our structures could find new applications in nanoelectronics., This work has been supported by the LiNaBioFluid project of the H2020 program of the European Commission (FETOPEN- 665337) as well as by the Spanish TEC2014-52642-C2- 1-R. MG-L and JH-R acknowledge the grants respectively awarded by the Spanish Ministry of Education and the Spanish Ministry of Economy and Competiveness.