Nanoantenna Arrays Research Articles

Bound State in the Continuum in Nanoantenna-Coupled Slab Waveguide Enables Low-Threshold Quantum-Dot Lasing.

Colloidal quantum dots (CQDs) are a promising gain material for solution-processed, wavelength-tunable lasers, with potential application in displays, communications, and biomedical devices. In this work, we combine a CQD film with an array of nanoantennas, made of titanium dioxide cylinders, to achieve lasing via bound states in the continuum (BICs). Here, the BICs are symmetry-protected cavity modes with giant quality factors, arising from slab waveguide modes in the planar CQD film, coupled to the periodic nanoantenna array. We engineer the thickness of the CQD film and size of the nanoantennas to achieve a BIC with good spatial and spectral overlap with the CQDs, based on a second-order transverse-electric (TE)-polarized waveguide mode. We obtain room-temperature lasing with a low threshold of approximately 11 kW/cm2 (peak intensity) under 5-ns-pulsed optical excitation. This work sheds light on the optical modes in solution-processed, distributed-feedback lasers and highlights BICs as effective, versatile, surface-emitting lasing modes.

Optical Heating Controlled With a Thermoplasmonic Metasurface

We develop a photothermal technology to control optical heating of polymer and liquid crystal films through a refractory titanium nitride (TiN) metasurface. The metasurface represents an array of identical square-shaped TiN nanoantennas on a Si substrate. Upon CW laser illumination, a TiN nanoantenna experiences anomalous Joule heating at a plasmon resonance. A temperature rise provides a unique opportunity for locally probing phase transitions. In the case of heterogeneous PMMA thin films or polymeric blends, a controlled optical heating is needed to probe the glass transition temperature (Tg) of their constituents. Here, we model a controlled thermal response originating from the TiN nanoantenna under CW laser illumination by using FDTD/FEM methods.

Design and research of dual-wavelength polarization multiplexing multifocal metalens based on superimposed nano-antenna array

In this study, based on the single-layer metasurface structure, a dual-wavelength polarization multiplexing metalens is designed at the communication wavelengths of 1310 nm and 1550 nm, respectively. Using the dual-phase modulation method, a single-wavelength polarization multiplexing metalens is proposed, which can simultaneously control left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP). Furthermore, the cross superposition method is used to combine them to achieve the dual-wavelength polarization multiplexing metalens. The results show that the system can achieve polarization multiplexing at the two wavelengths of 1310 nm and 1550 nm, which is consistent with the expected results, and the focusing efficiency reached 61%. With the increase of the numerical aperture, the focusing intensity of the left and right focal points gradually approaches, and the difference between the full width at half maximum of the two focal points also decreases accordingly. It provides a new way for the optical imaging, information detection and the realization of multifunctional ultra-surface devices.

Simulation of Solar Cells with Integration of Optical Nanoantennas.

The evolution of nanotechnology has provided a better understanding of light-matter interaction at a subwavelength scale and has led to the development of new devices that can possibly play an important role in future applications. Nanoantennas are an example of such devices, having gained interest in recent years for their application in the field of photovoltaic technology at visible and infrared wavelengths, due to their ability to capture and confine energy of free-propagating waves. This property results from a unique phenomenon called extraordinary optical transmission (EOT) where, due to resonant behavior, light passing through subwavelength apertures in a metal film can be transmitted in greater orders of magnitude than that predicted by classical theories. During this study, 2D and 3D models featuring a metallic nanoantenna array with subwavelength holes coupled to a photovoltaic cell are simulated using a Finite Element Tool. These models present with slight variations between them, such as the position of the nanoantenna within the structure, the holes’ geometry and the type of cell, in order to verify how its optical response is affected. The results demonstrate that the coupling of nanoantennas to solar cells can be advantageous and improve the capture and absorption of radiation. It is concluded that aperture nanoantennas may concentrate radiation, meaning that is possible to tune the electric field peak and adjust absorption on the main layers. This may be important because it might be possible to adjust solar cell performance to the global regions’ solar spectrum by only adjusting the nanoantenna parameters.

Coherent Multilayer Photonic Nanoantenna Array with off-Aperture Phase Adjustment

Coherent Multilayer Photonic Nanoantenna Array with off-Aperture Phase Adjustment

Enhancing Photoelectrochemical Energy Storage by Large-Area CdS-Coated Nickel Nanoantenna Arrays

The integration of thin films made up of periodic plasmonic nanostructures and semiconductors holds great potential to develop efficient technologies for photoelectrochemical solar energy conversion and storage. However, to date, only periodic nanoantenna arrays made up of Au have been explored, posing severe limitations in terms of scalability and costs. Here, we show that nickel nanopillar arrays can support complex electromagnetic resonances that strongly enhance the photoelectrochemical response of CdS thin films. By controlling the pitch size and diameter of the nanopillars, we obtain broadband light absorption from the ultraviolet (UV) to the near-infrared (NIR) wavelength range, thus achieving large photocurrent enhancements compared to a planar Ni/CdS sample and in line with those generated by previously reported Au nanostructures. The photocurrent enhancement is attributed to photonic modes in the UV and hybrid cavity-plasmonic modes in the visible and NIR ranges, which give rise to efficient energy transfer and hot carrier injection between metallic structures, the semiconductor, and the electrolyte. The developed nanopillar arrays are promising candidates for photoelectrochemical devices fully exploiting the solar spectrum and using Earth-abundant raw materials.

Influence of Plasmon Resonances and Symmetry Effects on Second Harmonic Generation in WS2-Plasmonic Hybrid Metasurfaces.

The nonlinear process of second harmonic generation (SHG) in monolayer (1L) transition metal dichalcogenides (TMD), like WS2, strongly depends on the polarization state of the excitation light. By combination of plasmonic nanostructures with 1L-WS2 by transferring it onto a plasmonic nanoantenna array, a hybrid metasurface is realized impacting the polarization dependency of its SHG. Here, we investigate how plasmonic dipole resonances affect the process of SHG in plasmonic-TMD hybrid metasurfaces by nonlinear spectroscopy. We show that the polarization dependency is affected by the lattice structure of plasmonic nanoantenna arrays as well as by the relative orientation between the 1L-WS2 and the individual plasmonic nanoantennas. In addition, such hybrid metasurfaces show SHG in polarization states, where SHG is usually forbidden for either 1L-WS2 or plasmonic nanoantennas. By comparing the SHG in these channels with the SHG generated by the hybrid metasurface components, we detect an enhancement of the SHG signal by a factor of more than 40. Meanwhile, an attenuation of the SHG signal in usually allowed polarization states is observed. Our study provides valuable insight into hybrid systems where symmetries strongly affect the SHG and enable tailored SHG in 1L-WS2 for future applications.

Direction‐Sensitive Magnetophotonic Surface Crystals

Magnetophotonic Crystals In article number 2100119 by Richard M. Rowan-Robinson, Alexandre Dmitriev, Vassilios Kapaklis, and co-workers, microscale magnetophotonic surface crystals are revealed by building arrays of magnetoplasmonic nanoantennas with nanosized rare-earth transition metal ferrimagnetic TbCo alloys, displaying perpendicular magnetic anisotropy. Magneto-optics is dramatically enhanced in such crystals via nanoantennas plasmons and their coupling into collective surface lattice modes. The result is a highly light-direction-sensitive optical surface with nanomagnetic functionality for the plasmon-assisted all-optical magnetization switching.

High Contrast, Femtosecond Light Polarization Manipulation in Epsilon-near-Zero Material Coupled to a Plasmonic Nanoantenna Array

Ultrafast manipulation of light polarization is crucial in many nonlinear optic and optoelectronic operations. However, most of the configurations are suffering from low modulation speed (gigahertz...

Direction‐Sensitive Magnetophotonic Surface Crystals

Nanometer‐thin rare‐earth–transition‐metal (RE–TM) alloys with precisely controlled compositions and out‐of‐plane magnetic anisotropy are currently in the focus for ultrafast magnetophotonic applications. However, achieving lateral nanoscale dimensions, crucial for potential device downscaling, while maintaining designable optomagnetic functionality and out‐of‐plane magnetic anisotropy is extremely challenging. Herein, nanosized Tb18Co82 ferrimagnetic alloys, having strong out‐of‐plane magnetic anisotropy, within a gold plasmonic nanoantenna array to design a micrometer‐scale magnetophotonic crystal that exhibits abrupt and narrow magneto‐optical (MO) spectral features that are both magnetic field and light incidence direction controlled are integrated. The narrow Fano‐type resonance arises through the interference of the individual nanoantenna's surface plasmons and a Rayleigh anomaly of the whole nanoantenna array, in both optical and MO spectra, which are demonstrated and explained using Maxwell theory simulations. This robust magnetophotonic crystal opens the way for conceptually new high‐resolution light incidence direction sensors, as well as for building blocks for plasmon‐assisted all‐optical magnetization switching in ferrimagnetic RE–TM alloys.

Wavelength conversion through plasmon-coupled surface states

Surface states generally degrade semiconductor device performance by raising the charge injection barrier height, introducing localized trap states, inducing surface leakage current, and altering the electric potential. We show that the giant built-in electric field created by the surface states can be harnessed to enable passive wavelength conversion without utilizing any nonlinear optical phenomena. Photo-excited surface plasmons are coupled to the surface states to generate an electron gas, which is routed to a nanoantenna array through the giant electric field created by the surface states. The induced current on the nanoantennas, which contains mixing product of different optical frequency components, generates radiation at the beat frequencies of the incident photons. We utilize the functionalities of plasmon-coupled surface states to demonstrate passive wavelength conversion of nanojoule optical pulses at a 1550 nm center wavelength to terahertz regime with efficiencies that exceed nonlinear optical methods by 4-orders of magnitude.

Plasmonic metamaterial absorbers with strong coupling effects for small pixel infrared detectors

Here we report a metal-insulator-metal (MIM) based infrared plasmonic metamaterial absorber consisting of deep subwavelength meander line nanoantennas. High absorption composed of two-hybrid modes from 11 μm to 14 μm is experimentally demonstrated with a pixel pitch of 1.47 μm corresponding to a compression ratio of 8.57. The physical mechanisms responsible for novelty spectral absorption, including the strong coupling between the plasmon resonances and the phonon vibrations, material loss from the dielectric spacer, localized surface plasmon resonance (LSPR), and Berreman mode excited by oblique incidence, have been systematically analyzed by finite-difference time-domain (FDTD) method, Fabry-Perot resonance model and two-coupled damped oscillator model. At oblique incidence, a spectral splitting related to the strong coupling between LSPR mode and Berreman mode is also observed. The distribution of local electromagnetic fields and ohmic loss are numerically investigated. Moreover, we evaluate the absorption performances with finite-sized arrays. We also show that the absorber can maintain its absorption with a 2 × 2 nanoantenna array. Such a miniaturized absorber can adapt to infrared focal plane arrays with a pixel size smaller than 5 μm, and thermal analysis is also performed. Our approach provides an effective way to minimize the antenna footprint without undermining the absorber performances, paving the way towards its integration with small pixels of infrared focal plane arrays for enhanced performances and expanded functionalities.

Near‐Infrared Active Metasurface for Dynamic Polarization Conversion

AbstractControl of optical polarization is central to harnessing the properties of electromagnetic radiation for many applications, including 3D imaging and quantum computation. However, conventional optical polarizing components are typically bulky and static, that is, fixed in their degree of polarization control. Active metasurfaces have potential for versatile polarization control in a compact form factor by temporally modulating electromagnetic amplitudes and phase between orthogonal electric field components. Here, dynamic control of the polarization state of reflected light from an active metasurface is demonstrated. The metasurface uses indium tin oxide (ITO) as an active element in an array of aluminum nanoantennas operating in the telecom range. Applying an electrical bias between the ITO layer and the back reflector modulates the carrier concentration in ITO at the gate‐dielectric/ITO interface, resulting in complex refractive index modulation in the epsilon‐near‐zero condition. This index modulation alters the degree of excitation of plasmonic modes corresponding to the orthogonal polarization components, leading to polarization modulation. By suitably biasing the metasurface, linearly polarized incident light can be converted to cross‐polarized, circularly polarized or elliptically polarized light upon reflection. Dynamic control of the reflected polarization state has potentially wide applications in dynamic wave plates, spatial light modulators, and adaptive wavefront control.

Helium ion beam lithography and liftoff

We introduce a helium ion beam lithography and liftoff process to fabricate arbitrary nanostructures. Exploiting existing high-resolution positive tone resists such as poly (methyl methacrylate) (PMMA), the process offers three significant advantages over electron beam lithography: (a) the exposing helium ion beam produces a high secondary electron yield leading to fast patterning, (b) proximity effects are negligible due to the low count of backscattered helium ions from the substrate, and (c) the process is transferrable with minimal alteration among different types of substrates (e.g. silicon, fused silica). The process can be used to pattern any material compatible with liftoff such as evaporated metals or dielectrics, and allows overlay of nanostructures precision-aligned to microstructures realised beforehand on the same substrate. The process is demonstrated for several PMMA thicknesses to liftoff different thicknesses of deposited material. Resolution trials are conducted to determine the limits of the process for each PMMA thickness. Isolated lines as narrow as 14 nm, and line-space gratings of 40 nm pitch (50% duty cycle), are produced as resolution tests by lifting off a 20 nm thick Au film. Nanostructures of aspect ratio up to ∼3:1 have been realised. Plasmonic nanoantenna arrays overlaid to microscale contacts are produced as device demonstrators, for which optical measurements are in excellent agreement with theory.

Dynamic real-time imaging of living cell traction force by piezo-phototronic light nano-antenna array.

Dynamic mapping of the cell-generated force of cardiomyocytes will help provide an intrinsic understanding of the heart. However, a real-time, dynamic, and high-resolution mapping of the force distribution across a single living cell remains a challenge. Here, we established a force mapping method based on a "light nano-antenna" array with the use of piezo-phototronic effect. A spatial resolution of 800 nm and a temporal resolution of 333 ms have been demonstrated for force mapping. The dynamic mapping of cell force of live cardiomyocytes was directly derived by locating the antennas' positions and quantifying the light intensities of the piezo-phototronic light nano-antenna array. This study presents a rapid and ultrahigh-resolution methodology for the fundamental study of cardiomyocyte behavior at the cell or subcellular level. It can provide valuable information about disease detection, drug screening, and tissue engineering for heart-related studies.

Beamforming optical antenna arrays for nano-bio sensing and actuation applications

Light is found at the basis of bidirectional interfaces between digital systems and biological systems, including those in nano-bio-sensing and nano-bio-actuation applications. Despite the potentially very small size of optical transceivers and antennas, enabled by the very small wavelength of optical electromagnetic radiation, currently, existing systems commonly rely on macro-sized and intrusive equipment, including table-top microscopes and spectroscopy systems, and fiber optical technology as opposed to wireless setups. These place several bottlenecks between the possibilities and the capabilities of these emerging fields. In this paper, a unified theory is developed to design optical nano-antenna arrays, which are able to manipulate light similarly to how antenna arrays manipulate RF radiation. First, the applications of optical nano-antenna arrays in transmission, reception and reflection are described. Then, the fundamental factors affecting the design of individual optical antennas are described and modeled. The mutual coupling between optical nano-antenna is studied to guide the development of compact antenna arrays. Different beamforming strategies are proposed, and their performance is numerically investigated by means of finite element methods.

Nanoscale refractory doped titanium nitride field emitters

Refractory materials exhibit high damage tolerance, which is attractive for the creation of nanoscale field-emission electronics and optoelectronics applications that require operation at high peak current densities and optical intensities. Recent results have demonstrated that the optical properties of titanium nitride, a refractory and CMOS-compatible plasmonic material, can be tuned by adding silicon and oxygen dopants. However, to fully leverage the potential of titanium (silicon oxy)nitride, a reliable and scalable fabrication process with few-nm precision is needed. In this work, we developed a fabrication process for producing engineered nanostructures with gaps between 10 and 15 nm, aspect ratios larger than 5 with almost 90° steep sidewalls. Using this process, we fabricated large-scale arrays of electrically-connected bow-tie nanoantennas with few-nm free-space gaps. We measured a typical variation of 4 nm in the average gap size. Using applied DC voltages and optical illumination, we tested the electronic and optoelectronic response of the devices, demonstrating sub-10 V tunneling operation across the free-space gaps, and quantum efficiency of up to 1 × 10−3 at 1.2 μm, which is comparable to a bulk silicon photodiode at the same wavelength and three orders of magnitude higher than with nearly identical gold devices. Tests demonstrated that the titanium silicon oxynitride nanostructures did not significantly degrade, exhibiting less than 5 nm of shrinking of the average gap dimensions over few-μm2 areas after 10 h of operation. Our results will be useful for developing the next generation of robust and CMOS-compatible nanoscale devices for high-speed and low-power field-emission electronics and optoelectronics applications.

Multilayered L-shaped nanoantenna arrays with an increased electric field enhancement

The performance of the composite metal/dielectric multilayered L-shaped nanoantennas for strong electric field enhancement has been investigated. The bonding and antibonding modes arising from the plasmon hybridization modes of the multilayered nanoantennas are presented. It is observed that the plasmon coupling of the proposed composite nanostructures can form three states of in-phase bonding, out-of-phase bonding and out-of-phase antibonding. The out-of-phase charge distribution indicates that dark plasmon modes can be revealed on the bonding modes of the proposed multilayered structures. The response of the proposed L-shaped nanoantennas to the attachment of a dielectric spherical nanoparticle and their molecular sensitivity are investigated. According to this study, the proposed multilayered nanostructures are good candidates for strong electric field localization in sensing and detection applications.

Bioinspired nanoantennas for opsin sensitization in optogenetic applications: a theoretical investigation

Opsins with high sensitivity are desired to reduce dependence on optical fibers and enable deep-brain optogenetic stimulation through the intact scalp and skull, while minimizing brain tissue heating and the associated biasing of neural activity. While optimized opsin engineering has produced ultrasensitive and red-shifted opsins suitable for transcranial optogenetic stimulation, further improvements in sensitivity are throttled by biological limitations. Nanostructures are capable of generating near-field intensity enhancements of over 104, but thus far nanomaterials have not been applied to amplify local light intensity for optogenetic applications. In this manuscript, we propose the use of bowtie nanoantennas for local enhancement of 470 nm light to sensitize channelrhodopsin (ChR2) to low light intensities. We begin with a comparison of the near-field intensity enhancement offered by different metals at 470 nm, before selecting aluminum as the optimal material. Next, we tune the geometric parameters of aluminum bowtie nanoantennas to maximize the intensity enhancement at 470 nm. We further optimize enhancement by constructing bowtie nanoantenna arrays inspired by patterns occurring in biology, obtaining intensity enhancements up to a factor of 5000. Monte Carlo simulations suggest that transcranial 470 nm illumination of only 50 mW mm−2 is capable of stimulating bowtie-sensitized ChR2 in the deep brain (∼5 mm) in mice, enabling minimally invasive deep-brain stimulation with opsins found in the traditional optogenetic toolbox. This computation-guided optical antenna engineering approach opens opportunities for designing multifunctional materials for enhancing the efficiency of optogenetic neuromodulation, optical neural activity imaging, and highly localized electrical microstimulation in the brain.

Ultrabroadband, compact, polarization independent and efficient metasurface-based power splitter on lithium niobate waveguides.

We propose a metasurface-based Lithium Niobate waveguide power splitter with an ultrabroadband and polarization independent performance. The design consists of an array of amorphous silicon nanoantennas that partially converts the input mode to multiple output modes creating multimode interference such that the input power is equally split and directed to two branching waveguides. FDTD simulation results show that the power splitter operates with low insertion loss (< 1dB) over a bandwidth of approximately 800 nm in the near-infrared range, far exceeding the O, E, S, C, L and U optical communication bands. The metasurface is ultracompact with a total length of 2.7 µm. The power splitter demonstrates a power imbalance of less than 0.16 dB for both fundamental TE and TM modes. Our simulations show that the device efficiency exhibits high tolerance to possible fabrication imperfections.