Broadband Epsilon-Near-Zero Perfect Absorption in the Near-Infrared

Multi-band perfect absorber based on an elliptical cavity coupled with an elliptical metal nanorod

We proposed a triple-band narrowband device based on a metal-insulator-metal (MIM) structure in visible and near-infrared regions. The finite difference time domain (FDTD) simulated results illustrated that the absorber possessed three perfect absorption peaks under TM polarization, and the absorption efficiencies were about 99.76%, 99.99%, and 99.92% at 785 nm, 975 nm, and 1132 nm, respectively. Simulation results matched well with the results of coupled-mode theory (CMT). Analyses of the distributions of the electric field indicated the "perfect" absorption was due to localized surface plasmon polaritons resonance (LSPPR) and Fabry-Perot resonance. We developed a multi-band absorber with more ellipsoid pillars. The four band-absorbing device presented perfect absorption at 767 nm, 1046 nm, 1122 nm, and 1303 nm, and the absorption rates were 99.45%, 99.41%, 99.99%, and 99.94%, respectively. By changing the refractive index of the surrounding medium, the resonant wavelengths could be tuned linearly. The maximum sensitivity and Figure of Merit were 230 nm RIU-1 and 10.84 RIU-1, respectively. The elliptical structural design provides more tuning degrees of freedom. The absorber possessed several satisfactory performances: excellent absorption behavior, multiple bands, tunability, incident insensitivity, and simple structure. Therefore, the designed absorbing device has enormous potential in optoelectronic detection, optical switching, and imaging.

Epsilon-near-zero material-based bi-layer metamaterials for selective mid-infrared radiation

Selective mid-infrared (MIR) radiation is highly desirable in many applications. However, there are still great challenges to simultaneously achieve MIR camouflage and radiative cooling utilizing simple structure. This work theoretically and experimentally proposes a bi-layer metamaterial composed of aluminum doped zinc oxide (AZO) nanoparticles embedded in Al2O3matrix on the aluminum film. The bi-layer metamaterial exhibits high performance in MIR camouflage with radiative cooling, a low emissivity (ε3-5μm= 0.11,ε8-14μm= 0.20) in atmospheric windows and a high emissivity (ε5-8μm= 0.81) in non-atmospheric windows. The interaction of the epsilon-near-zero (ENZ) mode and localized surface plasmon resonance (LSPR) mode is responsible for the perfect emission over the wavelength range of 5-8μm. Additionally, the proposed selective MIR emitter supports large-angle incidence and has great polarization insensitivity. This demonstrates that epsilon-near-zero material-based bi-layer metamaterial is highly promising for the development of selective mid-infrared radiation.

Sorption-Induced Fiber Optic Plasmonic Gas Sensing via Small Grazing Angle of Incidence

Sensing technologies based on plasmonic nanomaterials are of interest for various chemical, biological, environmental, and medical applications. In this work, an incorporation strategy of colloidal plasmonic nanoparticles (pNPs) in microporous polymer for realizing distinct sorption-induced plasmonic sensing is reported. This approach is demonstrated by introducing tin-doped indium oxide pNPs into a polymer of intrinsic microporosity (PIM-1). The composite film (pNPs-polymer) provides distinct and tunable optical features on the fiber optic (FO) platform that can be used as a signal transducer for gas sensing (e.g., CO2 ) under atmospheric conditions. The resulting pNPs-polymer composite demonstrates high sensitivity response on FO in the evanescent field configuration, provided by the dramatic response of modes above the total-internal-reflection angle. Furthermore, by varying the pNPs content in the polymer matrix, the optical behavior of the pNPs-polymer composite film can be tuned to affect the operational wavelength by over several hundred nanometers and the sensitivity of the sensor in the near-infrared range. It is also shown that the pNPs-polymer composite film exhibits remarkable stability over a period of more than 10 months by mitigating the physical aging issue of the polymer.

Simultaneous Control of Spectral And Directional Emissivity with Gradient Epsilon-Near-Zero InAs Photonic Structures

Controlling both the spectral bandwidth and directionality of emitted thermal radiation is a fundamental challenge in contemporary photonics. Recent work has shown that materials with a spatial gradient in the frequency range of their epsilon-near-zero (ENZ) response can support broad spectrum directionality in their emissivity, enabling high total radiance to specific angles of incidence. However, this capability is limited spectrally and directionally by the availability of materials with phonon-polariton resonances over long-wave infrared wavelengths. Here, an approach is designed and experimentally demonstrated using doped III-V semiconductors that can simultaneously tailor spectral peak, bandwidth, and directionality of infrared emissivity. InAs-based gradient ENZ photonic structures that exhibit broadband directional emission with varying spectral bandwidths and directional ranges as a function of their doping concentration profile and thickness are epitaxially grown and characterized. Due to its easy-to-fabricate geometry, it is believed that this approach provides a versatile photonic platform to dynamically control broadband spectral and directional emissivity for a range of emerging applications in heat transfer and infrared sensing.

Nanoscale modeling of dynamically tunable planar optical absorbers utilizing InAs and InSb in metal-oxide-semiconductor-metal configurations

The attainment of dynamic tunability in spectrally selective optical absorption has been a longstanding objective in modern optics. Typically, Fabry-Perot resonators comprising metal and semiconductor thin films have been employed for spectrally selective light absorption. In such resonators, the resonance wavelength can be altered via structural modifications. The research has progressed further with the advent of specialized patterning of thin films and the utilization of metasurfaces. Nonetheless, achieving dynamic tuning of the absorption wavelength without altering the geometry of the thin film or without resorting to lithographic fabrication still poses a challenge. In this study, the incorporation of a metal-oxide-semiconductor (MOS) architecture into the Fabry-Perot nanocavity is shown to yield dynamic spectral tuning in a perfect narrowband light absorber within the visible range. Such spectral tuning is achieved using n-type-doped indium antimonide and n-type-doped indium arsenide as semiconductors in a MOS-type structure. These semiconductors offer significant tuning of their optical properties via electrically induced carrier accumulation. The planar structure of the absorber models presented facilitates simple thin-film fabrication. With judicious material selection and appropriate bias voltage, a spectral shift of 47 nm can be achieved within the visible range, thus producing a discernible color change.

Tailoring the Thickness-Dependent Optical Properties of Conducting Nitrides and Oxides for Epsilon-Near-Zero-Enhanced Photonic Applications

The unique properties of the emerging photonic materials, conducting nitrides and oxides, especially their tailorability, large damage thresholds, and, importantly, the so-called epsilon-near-zero (ENZ) behavior, have enabled novel photonic phenomena spanning optical circuitry, tunable metasurfaces, and nonlinear optical devices. This work explores direct control of the optical properties of polycrystalline titanium nitride (TiN) and aluminum-doped zinc oxide (AZO) by tailoring the film thickness, and their potential for ENZ-enhanced photonic applications. This study demonstrates that TiN-AZO bilayers support Ferrell-Berreman modes using the thickness-dependent ENZ resonances in the AZO films operating in the telecom wavelengths spanning from 1470 to 1750 nm. The bilayer stacks also act as strong light absorbers in the ultraviolet regime using the radiative ENZ modes and the Fabry-Perot modes in the constituent TiN films. The studied Berreman resonators exhibit optically induced reflectance modulation of 15% with picosecond response time. Together with the optical response tailorability of conducting oxides and nitrides, using the field enhancement near the tunable ENZ regime can enable a wide range of nonlinear optical phenomena, including all-optical switching, time refraction, and high-harmonic generation.

Manufacturing-Enabled Tunability of Linear and Nonlinear Epsilon-Near-Zero Properties in Indium Tin Oxide Nanofilms

Epsilon-near-zero (ENZ) materials have attracted great interest due to their exotic linear and nonlinear responses, which makes it significant to tune ENZ wavelengths for wavelength-dependent applications. However, studies to achieve tunability in a wide spectral range and link the fabrication parameters with linear and nonlinear ENZ properties have been uncovered. ENZ indium tin oxide (ITO) nanofilms are fabricated by magnetron sputtering, through which the control of ENZ properties is demonstrated. Factors in the sputtering process, such as the gas ratio and annealing, have a great impact on the ITO samples. Tunable ENZ parameters are listed to provide a beneficial database for ENZ ITO, mainly attributed to the change of carrier concentration. The influence of ENZ parameters on optical characteristics via annealing treatment is further explored. The ENZ wavelength is blue-shifted by 609 nm, and the intrinsic loss is reduced by 63.2%, while the ITO samples exhibit better linear scattering properties and stronger field intensity enhancement. Additionally, the laser testing system illustrates the change from reverse saturable absorption to saturable absorption with an absolute modulation depth of 21.9%, improved by 222.1%, and the nonlinear refractive index n₂ and nonlinear absorption coefficient β are 2.07 × 10^-16 m² W^-1 and -3.16 × 10^-10 m W^-1 for post-annealed ITO samples, respectively. The proposed sputtering protocol offers a feasible technique to control the linear and nonlinear ENZ performance, which has great potential in laser technology and nanophotonics.

Thickness-dependent loss-induced failure of an ideal ENZ-enhanced optical response in planar ultrathin transparent conducting oxide films

Ultrathin planar transparent conducting oxide (TCO) films are commonly used to enhance the optical response of epsilon-near-zero (ENZ) devices; however, our results suggest that thickness-dependent loss renders them ineffective. Here, we investigated the thickness-dependent loss of indium tin oxide (ITO) films and their effect on the ENZ-enhanced optical responses of ITO and ITO/SiO₂ multilayer stacks. The experimental and computational results show that the optical loss of ITO films increases from 0.47 to 0.70 as the thickness decreases from 235 to 52 nm, which results in a reduction of 60% and 45% in the maximum field enhancement factor of a 52-nm monolayer ITO and 4-layer ITO/SiO₂ multilayer stack, respectively. The experimental results show that the ENZ-enhanced nonlinear absorption coefficient of the 52-nm single-layer ITO film is -1.6 × 10³cm GW^-1, which is 81% lower than that of the 235-nm ITO film (-8.6 × 10³cm GW^-1), indicating that the thickness-dependent loss makes the ultrathin TCO films unable to obtain greater nonlinear responses. In addition, the increased loss reduces the cascading Berreman transmission valley intensity of the 4-layer ITO/SiO₂ multilayer stack, resulting in a 42% reduction in the ENZ-enhanced nonlinear absorption coefficient compared to the 235-nm ITO film and a faster hot electron relaxation time. Our results suggest that the thickness and loss trade-off is an intrinsic property of TCO films and that the low-loss ultrathin TCO films are the key to the robust design and fabrication of novel ENZ devices based on flat ultrathin TCO films.

All-optical AZO-based modulator topped with Si metasurfaces

All-optical communication systems are under continuous development to address different core elements of inconvenience. Here, we numerically investigate an all-optical modulator, realizing a highly efficient modulation depth of 22 dB and a low insertion loss of 0.32 dB. The tunable optical element of the proposed modulator is a layer of Al-doped Zinc Oxide (AZO), also known as an epsilon-near-zero transparent conductive oxide. Sandwiching the AZO layer between a carefully designed distributed Bragg reflector and a dielectric metasurface-i.e., composed of a two-dimensional periodic array of cubic Si-provides a guided-mode resonance at the OFF state of the modulator, preventing the incident signal reflection at λ = 1310 nm. We demonstrate the required pump fluence for switching between the ON/OFF states of the designed modulator is about a few milli-Joules per cm². The unique properties of the AZO layer, along with the engineered dielectric metasurface above it, change the reflection from 1 to 93%, helping design better experimental configurations for the next-generation all-optical communication systems.

Spinning thermal radiation from twisted two different anisotropic materials

Thermal radiation has applications in numerous fields, such as radiation cooling, thermal imaging, and thermal camouflage. Micro/nanostructures such as chiral metamaterials with polarization-dependent or symmetry-breaking properties can selectively emit circularly (spin) polarized polarization waves. In this paper, we propose and demonstrate the spinning thermal radiation from two twisted different anisotropic materials. Taking industrial polymer and biaxial hyperbolic material α-MoO₃ as an example, it is found that broadband spinning thermal radiation can be obtained from 13 µm to 18 µm. The spin thermal radiation of the proposed twisted structure originates from the combined effect of polarization conversion of circularly polarized wave and selective absorption of linearly polarized wave by the top and bottom layers of anisotropic materials, respectively. Besides, the narrowband spinning thermal radiation with 0.9 circular dichroism is achieved at wavelength of 12.39 µm and 18.93 µm for finite thickness α-MoO₃ due to the epsilon-near-zero mode, and the magnetic field distribution can confirm the phenomenon. This work achieves broadband and narrowband spin thermal radiation and significantly enhances circular dichroism, which may have applications in biological sensing and thermal detection.

Broad-Band Ultrafast All-Optical Switching Based on Enhanced Nonlinear Absorption in Corrugated Indium Tin Oxide Films

Ultrafast all-optical switches based on epsilon-near-zero (ENZ)-enhanced nonlinear refraction in transparent conducting oxides have achieved exciting results in realizing large absolute modulations. However, broad-band, polarization-independent, and wide-angle ultrafast all-optical switches have been challenging to produce, due to the inherent narrow band, polarization-dependent, and angle-dependent characteristics of the ENZ effect. To this end, we propose an ultrafast all-optical switch based on the enhanced nonlinear absorption of corrugated indium tin oxide (ITO) thin films. Taking advantage of the perfect absorption and localized field enhancement of the ENZ and localized surface plasmon resonance modes, we significantly enhanced the nonlinear absorption of the corrugated ITO film in the 1450-1650 nm telecom band. The experimental results show that the nonlinear saturable absorption coefficient of the corrugated ITO film at 1450 nm was as high as -1.5 × 10⁵ cm GW^-1, enabling all-optical switching to obtain an extinction ratio of 14.32 dB and an ultrafast switching time of 350 fs at a pump fluence of 18.51 mJ cm^-2. Furthermore, the all-optical switch achieved an extinction ratio of over 15 dB and an insertion loss of approximately 2.6 dB within the 200 nm absorption band and exhibited polarization-independent and wide-angle features. The ultrafast temporal response can be attributed to intraband transient bleaching of the corrugated ITO film. Our findings demonstrate that corrugated ENZ films can overcome the inherent narrow-band, polarization-dependent, and angle-dependent problems of natural ENZ materials without increasing the response time, making them a potential ENZ ultrafast all-optical switching material platform.

Optimization of Magnetoplasmonic ε-Near-Zero Nanostructures Using a Genetic Algorithm

Magnetoplasmonic permittivity-near-zero (ε-near-zero) nanostructures hold promise for novel highly integrated (bio)sensing devices. These platforms merge the high-resolution sensing from the magnetoplasmonic approach with the ε-near-zero-based light-to-plasmon coupling (instead of conventional gratings or bulky prism couplers), providing a way for sensing devices with higher miniaturization levels. However, the applications are mostly hindered by tedious and time-consuming numerical analyses, due to the lack of an analytical relation for the phase-matching condition. There is, therefore, a need to develop mechanisms that enable the exploitation of magnetoplasmonic ε-near-zero nanostructures' capabilities. In this work, we developed a genetic algorithm (GA) for the rapid design (in a few minutes) of magnetoplasmonic nanostructures with optimized TMOKE (transverse magneto-optical Kerr effect) signals and magnetoplasmonic sensing. Importantly, to illustrate the power and simplicity of our approach, we designed a magnetoplasmonic ε-near-zero sensing platform with a sensitivity higher than 56∘/RIU and a figure of merit in the order of 102. These last results, higher than any previous magnetoplasmonic ε-near-zero sensing approach, were obtained by the GA intelligent program in times ranging from 2 to 5 min (using a simple inexpensive dual-core CPU computer).

Effective medium optical modelling of indium tin oxide nanocrystal films

Doped semiconductor nanocrystal-based thin films are widely used for many applications, such as screens, electrochromic windows, light emitting diodes, and solar cells. Herein, we have employed spectroscopic ellipsometry to measure and model the complex dielectric response of indium tin oxide films fabricated by nanocrystal deposition and sintering. The films could be modelled as Bruggemann effective media, allowing estimation of the nanoscale interstitial porosity of the structure. The effective dielectric constants show the possibility of tuning the plasma frequency and the epsilon-near zero condition of the film.

Absorption and scattering in perfect thermal radiation absorber-emitter metasurfaces

Detailed spectral analysis of radiation absorption and scattering behaviors of metasurfaces was carried out via finite-difference time-domain (FDTD) photonic simulations. It revealed that, for typical metal-insulator-metal (MIM) nanodisc metasurfaces, absorbance and scattering cross-sections exhibit a ratio of σ_abs/σ_sca = 1 at the absorption peak spectral position. This relationship was likewise found to limit the attainable photo-thermal conversion efficiency in experimental and application contexts. By increasing the absorption due to optical materials, such as Cr metal nano-films typically used as an adhesion layer, it is possible to control the total absorption efficiency η = σ_abs/σ_sca and to make it the dominant extinction mechanism. This guided the design of MIM metasurfaces tailored for near-perfect-absorption and emission of thermal radiation. We present the fabrication as well as the numerical and experimental spectral characterisation of such optical surfaces.

Large-Area, Highly Crystalline DNA-Assembled Metasurfaces Exhibiting Widely Tunable Epsilon-Near-Zero Behavior

Metasurfaces prepared via bottom-up nanoparticle assembly enable the deliberate manipulation of light in the optical regime, resulting in media with various engineered optical responses. Here, we report a scalable method to grow highly crystalline 2D metasurfaces composed of colloidal gold nanocubes, over macroscopic areas, using DNA-mediated assembly under equilibrium conditions. Using an effective medium description, we predict that these plasmonic metasurfaces behave as dielectric media with high refractive indices that can be dynamically tuned by tuning DNA length. Furthermore, we predict that, when coupled with an underlying thin gold film, the real permittivity of these metasurfaces exhibits a crossover region between positive and negative values, known as the epsilon-near-zero (ENZ) condition, which can be tuned between 1.5 and 2.6 μm by changing DNA length. Optical characterization performed on the DNA-assembled metasurfaces reveals that the predicted optical properties agree well with the measured response. Overall, we propose an efficient method for realizing large-area plasmonic metasurfaces that enable dynamic control over optical characteristics. High-index and ENZ metasurfaces operating in the telecommunications regime could have significant implications in high-speed optical computing, optical communications, optical imaging, and other areas.

Determining femtosecond laser fluence for surface engineering of transparent conductive thin films by single shot irradiation

In recent years, there has been increasing interest in optoelectronic applications of transparent conductive oxide (TCO) thin-film-based materials and devices fabricated using patterning techniques. Meanwhile, femtosecond laser processing is a convenient method that further improves the performance of TCO-based functional devices and expands their application prospects. In this study, we proposed a simple and effective strategy to determine the fluences required for laser processing TCOs. We investigated the modification of an indium tin oxide (ITO) film induced by a femtosecond laser (45/150 fs, 800 nm) at different pulse fluences. The results reveal that the laser modification of ITO films is highly dependent on the irradiated pulse fluences. Several distinct types of final micro/nanostructures were observed and may be attributed to superficial amorphization, spallation ablation, stress-assisted delamination, boiling evaporation, and phase explosion. The final micro/nanostructures were studied in detail using optical microscopy, scanning electron microscopy, transmission electron microscopy and a surface profiler. At a lower fluence above the melting but below the ablation threshold, a laterally parabolic amorphous layer profiled with maximum thicknesses of several tens of nanometers was quantitatively attained. At a higher fluence, stress-assisted delamination and superheated liquid-induced micro-honeycomb structures emerged. Furthermore, the electron and lattice temperature evolutions were also obtained using a two-temperature model to prove the ablation mechanism and ascertain the micro/nanostructure formation principle. The predicted surface temperatures confirmed film amorphization without ablation below 0.23 J/cm². These results reveal the interaction mechanism between femtosecond laser pulse and ITO film including the competition between the free electron heating of intraband transition and the multiphoton absorption of the interband transition, which promotes the potential applications for femtosecond laser processing TCO films and other wide-band-gap semiconductors such as photodetectors, solar cells, UV-light-emitting diodes, and flat-panel displays.

Manipulation of epsilon-near-zero wavelength for the optimization of linear and nonlinear absorption by supercritical fluid

We introduce supercritical fluid (SCF) technology to epsilon-near-zero (ENZ) photonics for the first time and experimentally demonstrate the manipulation of the ENZ wavelength for the enhancement of linear and nonlinear optical absorption in ENZ indium tin oxide (ITO) nanolayer. Inspired by the SCF's applications in repairing defects, reconnecting bonds, introducing dopants, and boosting the performance of microelectronic devices, here, this technique is used to exploit the influence of the electronic properties on optical characteristics. By reducing oxygen vacancies and electron scattering in the SCF oxidation process, the ENZ wavelength is shifted by 23.25 nm, the intrinsic loss is reduced by 20%, and the saturable absorption modulation depth is enhanced by > 30%. The proposed technique offers a time-saving low-temperature technique to optimize the linear and nonlinear absorption performance of plasmonics-based ENZ nanophotonic devices.

Broadband directional control of thermal emission

Controlling the directionality of emitted far-field thermal radiation is a fundamental challenge. Photonic strategies enable angular selectivity of thermal emission over narrow bandwidths, but thermal radiation is a broadband phenomenon. The ability to constrain emitted thermal radiation to fixed narrow angular ranges over broad bandwidths is an important, but lacking, capability. We introduce gradient epsilon-near-zero (ENZ) materials that enable broad-spectrum directional control of thermal emission. We demonstrate two emitters consisting of multiple oxides that exhibit high (>0.7, >0.6) directional emissivity (60° to 75°, 70° to 85°) in the p-polarization for a range of wavelengths (10.0 to 14.3 micrometers, 7.7 to 11.5 micrometers). This broadband directional emission enables meaningful radiative heat transfer primarily in the high emissivity directions. Decoupling the conventional limitations on angular and spectral response improves performance for applications such as thermal camouflaging, solar heating, radiative cooling, and waste heat recovery.

Admittance analysis of broadband omnidirectional near-perfect absorber in epsilon-near-zero mode

In this paper, we propose a broadband omnidirectional near-perfect absorber that transforms light energy into heat. In contrast to previous research on structural metamaterials, this study focuses on light absorption in the epsilon-near-zero (ENZ) layers without any structural patterns. Chromium (Cr) thin films were applied as ENZ layers. Using the admittance method, we found the proper thicknesses of SiO₂ layers to match the incident medium and achieve perfect absorption. Also, the absorber is angular insensitive up to 60°. The temperature of the absorber increases from room temperature to 42°C, which is 4°C higher than the uncoated substrate at 38°C, after exposure to sunlight for 20 min.

Large-Area Fabrication of Complex Nanohole Arrays with Highly Tunable Plasmonic Properties

By combining nanosphere lithography with oblique angle deposition, large-area asymmetric compound Ag nanohole arrays with nanorods inside the hole were patterned on substrates. The technique enabled the production of complex nanohole arrays with controlled hole diameter, thickness, and rod structure inside the hole. The compound asymmetric Ag nanohole structures showed strong polarization-dependent optical properties, and a new extraordinary optical transmission (EOT) mode with tunable resonance wavelength at the near-IR region was observed. The transmission at the new EOT wavelength region can increase from 27% of nanohole to 69% of the compound structure, and these structures can achieve a refractive index sensitivity as high as 847 nm RIU^-1. The tunable EOT wavelength and strong polarization-dependent optical properties make the structure ideal for ultrathin optical filters, polarizers, surface-enhanced spectroscopies, etc.

Constructing Metastructures with Broadband Electromagnetic Functionality

Electromagnetic metastructures stand for the artificial structures with a characteristic size smaller than the wavelength, which may efficiently manipulate the states of light. However, their applications are often restricted by the bandwidth of the electromagnetic response of the metastructures. It is therefore essential to reassert the principles in constructing broadband electromagnetic metastructures. Herein, after summarizing the conventional approaches for achieving broadband electromagnetic functionality, some recent developments in realizing broadband electromagnetic response by dispersion compensation, nonresonant effects, and several trade-off approaches are reviewed, followed by some perspectives for the future development of broadband metamaterials. It is anticipated that broadband metastructures will have even more substantial applications in optoelectronics, energy harvesting, and information technology.

Dynamical Control of Broadband Coherent Absorption in ENZ Films

Interferometric effects between two counter-propagating beams incident on an optical system can lead to a coherent modulation of the absorption of the total electromagnetic radiation with 100% efficiency even in deeply subwavelength structures. Coherent perfect absorption (CPA) rises from a resonant solution of the scattering matrix and often requires engineered optical properties. For instance, thin film CPA benefits from complex nanostructures with suitable resonance, albeit at a loss of operational bandwidth. In this work, we theoretically and experimentally demonstrate a broadband CPA based on light-with-light modulation in epsilon-near-zero (ENZ) subwavelength films. We show that unpatterned ENZ films with different thicknesses exhibit broadband CPA with a near-unity maximum value located at the ENZ wavelength. By using Kerr optical nonlinearities, we dynamically tune the visibility and peak wavelength of the total energy modulation. Our results based on homogeneous thick ENZ media open a route towards on-chip devices that require efficient light absorption and dynamical tunability.

Ultrafast electrical switching of nanostructured metadevice with dual-frequency liquid crystal

Shortening of switching times of various soft-matter-based tunable metamaterials is one of the key challenges to improve the functionality of modern active devices. Here we show an effective strategy in the evolution of soft-matter-based tunable metamaterials that makes possible acceleration of both on and off switching processes by using a dual-frequency liquid crystal mixture. The frequency-convertible dielectric anisotropy of the dual-frequency mixture enabled us to create a fast-response in-plane switching metasurface at the nanoscale, which could be tuned by an electrical signal with different frequencies. The results clearly show that the resonance of the metamaterial can be continuously and reversibly controlled within a wavelength range of 100 nm as the applied frequency is inverted between 1 kHz and 40 kHz, with a total response time (τ = τ_ON + τ_OFF) of 1.89 ms. Furthermore, experimental characteristics of the hybrid metamaterial are in great agreement with numerical calculations, which allow us to anticipate active epsilon-near-zero behavior of the metadevice. This work indicates the future development direction of liquid-crystal-based active plasmonic systems.

Direct Observations of Surface Plasmon Polaritons in Highly Conductive Organic Thin Film

Plasmonic effect plays a significant role in many optoelectronic devices and enables various innovative applications. It has been widely studied in metallic materials, for example, Ag and Au, and later was expanded to transparent conductive oxides, etc. However, such plasmonic structures have limitations in many emerging optoelectronics including flexible optoelectronics, organic optoelectronics, and so on, due to their inorganic natures. In this manuscript, we discovered that the acid-modified highly conductive organic PEDOT:PSS film shows interesting plasmonic properties in the vis-NIR region and exhibits great potentials for activating surface plasmon polaritons. The dispersion curves of dielectric permittivity and optical constants of two modified PEDOT:PSS samples are obtained by inversion calculation of the spectroscopic ellipsometry data with the Drude-Lorentz dispersion model. The permittivity crossover wavelengths λ_C and the surface plasmon wavelengths λ_sp are found to be located squarely in the 650-900 nm range, which will enable future plasmonic device applications in the vis-NIR region. The activation of surface plasmon polaritons propagation mode of modified PEDOT:PSS is directly observed and confirmed by prism coupling experiments. In addition, further quantitative analysis revealed that our modified PEDOT:PSS samples have comparable abilities to generate, propagate, and confine surface plasmon polaritons as indium tin oxide (ITO). To the best of our knowledge, this is the first direct demonstration of an organic structure showing equivalent plasmonic properties to the inorganic ones. We believe it will open up much more possibilities for the optoelectronic devices, due to the flexibility, lightness, biological compatibility, and solution processability of the organic plasmonic materials.

Effect of oxygen stoichiometry on the structure, optical and epsilon-near-zero properties of indium tin oxide films

Transparent conductive oxide (TCO) films showing epsilon near zero (ENZ) properties have attracted great research interest due to their unique property of electrically tunable permittivity. In this work, we report the effect of oxygen stoichiometry on the structure, optical and ENZ properties of indium tin oxide (ITO) films fabricated under different oxygen partial pressures. By using spectroscopic ellipsometry (SE) with fast data acquisition capabilities, we observed modulation of the material index and ENZ wavelength under electrostatic gating. Using a two-layer model based on Thomas-Fermi screening model and the Drude model, the optical constants and Drude parameters of the ITO thin films are determined during the gating process. The maximum carrier modulation amplitude ΔN of the accumulation layer is found to vary significantly depending on the oxygen stoichiometry. Under an electric field gate bias of 2.5 MV/cm, the largest ENZ wavelength modulation up to 27.9 nm at around 1550 nm is observed in ITO thin films deposited with oxygen partial pressure of P _{O 2} =10 Pa. Our work provides insights to the optical properties of ITO during electrostatic gating process for electro-optic modulators (EOMs) applications.

Enhancement of sub-bandgap light absorption in perovskite semiconductor films via critical coupling

Light absorption in semiconductors is a fundamental problem that has broad impact on a wide range of fields. However, it is intrinsically limited by the bandgap energy of the semiconductor. Herein, we study the enhancement of sub-bandgap light absorption in inorganic-organic hybrid perovskite semiconductor films via critical coupling. This is achieved at large incidence angles by balancing radiative and nonradiative decay rates in a planar multilayer structure. We found that a very small loss in the semiconductor layer can result in substantial light absorption. This simple but general method can be used to enhance the optical and optoelectronic responses of semiconductors below the bandgap energy.

Tunable metasurface refractive index plasmonic nano-sensor utilizing an ITO thin layer in the near-infrared region

In this paper, a refractive index plasmonic nano-sensor with an array of metallic nano-rods has been designed in the near-infrared region. The tunable sensor is based on a perfect absorber, and an indium tin oxide (ITO) layer has been utilized as an active metasurface to improve sensor performance. By applying the voltage to the ITO layer, a perfect absorption has been obtained. Also, the sensitivity rises from 625 to 1125 nm/RIU (refractive index unit), and the figure of merit is 10 times higher. Moreover, the sensor application in the fields of medicine and hematology has been shown. The proposed sensor can be used to diagnose and determine anemia and malaria parasites.

Epsilon-Near-Zero Absorber by Tamm Plasmon Polariton

Two schemes of excitation of a Tamm plasmon polariton localized at the interface between a photonic crystal and a nanocomposite with near-zero effective permittivity have been investigated in the framework of the temporal coupled-mode theory. The parameters of the structure have been determined, which correspond to the critical coupling of the incident field with a Tamm plasmon polariton and, consequently, ensure the total absorption of the incident radiation by the structure. It has been established that the spectral width of the absorption line depends on the scheme of Tamm plasmon polariton excitation and the parameters of a nanocomposite film. The features of field localization at the Tamm plasmon polariton frequency for different excitation schemes have been examined. It has been demonstrated that such media can be used as narrowband absorbers based on Tamm plasmon polaritons localized at the interface between a photonic crystal and a nanocomposite with near-zero effective permittivity.

Enhanced absorption by coherent control in a photonic crystal resonator coupled with a microfiber

We demonstrate enhanced absorption in a photonic crystal resonator (PCR) coupled with an optical microfiber. Enhanced absorption is based on coherent perfect absorption (CPA) that is time-reversed lasing. The PCR is fabricated on a silicon membrane with optimized parameters obtained from a numerical simulation. In an experiment, we observed 72% of absorption for the PCR with the optimized parameters. We also verified numerically that the absorption required for CPA can be tuned by changing the distance between the PCR and the optical microfiber.

Tunable polarization-independent dual-band coherent perfect absorber based on metal-graphene nanoring structure

A dual-band polarization-independent coherent perfect absorber(CPA) based on metal-graphene nanostructure is proposed, which is composed of golden nanorings with different sizes on graphene monolayer. Based on the finite-difference time-domain (FDTD) solutions, coherent perfect absorptions of the metal-graphene CPA are achieved at frequencies of 50.54 THz and 43.60 THz, which are resulted from the excited surface plasmon resonance induced by different size nanorings. Through varying the relative phase of two incident countering-propagating beams, the absorption peaks are all-optically tuned from 98.3 % and 98.4 % to nearly 0, respectively. By changing the gate-controlled Fermi energy of the graphene layer, the resonance frequencies of the CPA are tuned simultaneously without changing the geometrical parameters. And polarization independence of the metal-graphene CPA is revealed due to the center symmetry of nanoring structure. The electrical tunability of resonance frequency and polarization independence enable the proposed CPA to be widely applied in optoelectronic and engineering technology areas for tunable active multiple-band regulation and control.

New trends in plasmonic (bio)sensing

The strong enhancement and localization of electromagnetic field in plasmonic systems have found applications in many areas, which include sensing and biosensing. In this paper, an overview will be provided of the use of plasmonic phenomena in sensors and biosensors with emphasis on two main topics. The first is related to possible ways to enhance the performance of sensors and biosensors based on surface plasmon resonance (SPR), where examples are given of functionalized magnetic nanoparticles, magnetoplasmonic effects and use of metamaterials for SPR sensing. The other topic is focused on surface-enhanced Raman scattering (SERS) for sensing, for which uniform, flexible, and reproducible SERS substrates have been produced. With such recent developments, there is the prospect of improving sensitivity and lowering the limit of detection in order to overcome the limitations inherent in ultrasensitive detection of chemical and biological analytes, especially at single molecule levels.

ITO-TiN-ITO Sandwiches for Near-Infrared Plasmonic Materials

Indium tin oxide (ITO)-based sandwich structures with the insertion of ultrathin (<10 nm) titanium nitride (TiN) are investigated as near-infrared (NIR) plasmonic materials. The structural, electrical, and optical properties reveal the improvement of the sandwich structures stemmed from TiN insertion. TiN is a well-established alternative to noble metals such as gold, elevating the electron conductivity of sandwich structures as its thickness increases. Dielectric permittivities of TiN and top ITO layers show TiN-thickness-dependent properties, which lead to moderate and tunable effective permittivities for the sandwiches. The surface plasmon polaritons (SPP) of the ITO-TiN-ITO sandwich at the telecommunication window (1480-1570 nm) are activated by prism coupling using Kretschmann configuration. Compared with pure ITO films or sandwiches with metal insertion, the reflectivity dip for sandwiches with TiN is relatively deeper and wider, indicating the enhanced coupling ability in plasmonic materials for telecommunications. The SPP spatial profile, penetration depth, and degree of confinement, as well as the quality factors, demonstrate the applicability of such sandwiches for NIR plasmonic materials in various devices.

Broadband Tunable THz Absorption with Singular Graphene Metasurfaces

By exploiting singular spatial modulations of the graphene conductivity, we design a broadband, tunable THz absorber whose efficiency approaches the theoretical upper bound for a wide absorption band with a fractional bandwidth of 185%. Strong field enhancement is exhibited by the modes of this extended structure, which is able to excite a wealth of high-order surface plasmons, enabling deeply subwavelength focusing of incident THz radiation. Previous studies have shown that the conductivity can be modulated at GHz frequencies, which might lead to the development of efficient high-speed broadband switching by an atomically thin layer.

Tunable near-infrared epsilon-near-zero and plasmonic properties of Ag-ITO co-sputtered composite films

Series of co-sputtered silver-indium tin oxide (Ag-ITO) films are systematically fabricated. By tuning the atomic ratio of silver, composite films are manifested to have different microstructures with limited silver amount (<3 at.%). Two stages for film morphology changing are proposed to describe different status and growth mechanisms. The introduction of silver improves the preferred orientations of In2O3 component significantly. Remarkably, dielectric permittivity of Ag-ITO films is highly adjustable, allowing the cross-over wavelengths λc to be changed by more than 300 nm through rapid post-annealing, and thus resulting in tunable epsilon-near-zero and plasmonic properties in the near-infrared region. Lower imaginary permittivity compared with pure metal films, as well as larger tunability in λc than pure ITO films suggest the potentiality of Ag-ITO films as substituted near-infrared plasmonic materials. Extended Maxwell-Garnett model is applied for effective medium approximation and the red-shifting of epsilon-near-zero region with the increase of silver content is well-fitted. Angle-variable prism coupling is carried out to reveal the surface plasmon polariton features of our films at optical communication wavelength. Broad dips in reflectance curves around 52-56° correspond to the SPP in Ag-ITO films.

Electrically tunable perfect light absorbers as color filters and modulators

Methods for spectrally controlling light absorption in optoelectronic devices have attracted considerable attention in recent years. It is now well known that a Fabry-Perot nanocavity comprising thin semiconductor and metal films can be used to absorb light at selected wavelengths. The absorption wavelength is controlled by tailoring the thickness of the nanocavity and also by nanostructure patterning. However, the realization of dynamically tuning the absorption wavelength without changing the structural geometry remains a great challenge in optoelectronic device development. Here it is shown how an ultrathin n-type doped indium antimonide integrated into a subwavelength-thick optical nanocavity can result in an electrically tunable perfect light absorber in the visible and near infrared range. These absorbers require simple thin-film fabrication processes and are cost effective for large-area devices without resorting to sophisticated nanopatterning techniques. In the visible range, a 40 nm spectral shift can be attained by applying a reasonable bias voltage to effect the color change. It is also shown that these electrically tunable absorbers may be used as optical modulators in the infrared. The predicted (up to) 95.3% change in reflectance, transforming the device from perfectly absorbing to highly reflective, should make this technology attractive to the telecommunication (switching) industry.

Excitation of epsilon-near-zero resonance in ultra-thin indium tin oxide shell embedded nanostructured optical fiber

We report a novel optical waveguide design of a hollow step index fiber modified with a thin layer of indium tin oxide (ITO). We show an excitation of highly confined waveguide mode in the proposed fiber near the wavelength where permittivity of ITO approaches zero. Due to the high field confinement within thin ITO shell inside the fiber, the epsilon-near-zero (ENZ) mode can be characterized by a peak in modal loss of the hybrid waveguide. Our results show that such in-fiber excitation of ENZ mode is due to the coupling of the guided core mode to the thin-film ENZ mode. We also show that the phase matching wavelength, where the coupling takes place, varies depending on the refractive index of the constituents inside the central bore of the fiber. These ENZ nanostructured optical fibers have many potential applications, for example, in ENZ nonlinear and magneto-optics, as in-fiber wavelength-dependent filters, and as subwavelength fluid channel for optical and bio-photonic sensing.

Broadband perfect infrared absorption by tuning epsilon-near-zero and epsilon-near-pole resonances of multilayer ITO nanowires

We numerically investigate the broadband perfect infrared absorption by tuning epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) resonances of multilayer indium tin oxide nanowires (ITO NWs). The monolayer ITO NWs array shows intensive absorption at ENZ and ENP wavelengths for p polarization, while only at the ENP wavelength for s polarization. Moreover, the ENP resonances are almost omnidirectional and the ENZ resonances are angularly dependent. Therefore, the absorption bandwidth is broader for p polarization than that for s polarization when polarized waves are incident obliquely. The ENZ resonances can be tuned by altering the doping concentration and volume filling factor of ITO NWs. However, the ENP resonances only can be tuned by changing the doping concentration of ITO NWs, and volume filling factor impacts little on the ENP resonances. Based on the strong absorption properties of each layer at their own ENP and ENZ resonances, the tuned absorption of the bilayer ITO NWs with the different doping concentrations can be broader and stronger. Furthermore, multilayer ITO NWs can achieve broadband perfect absorption by controlling the doping concentration, volume filling factor, and length of the NWs in each layer. This study has the potential to apply to applications requiring efficient absorption and energy conversion.

Resonant absorption of electromagnetic waves in transition anisotropic media

We study the mode conversion and resonant absorption phenomena occurring in a slab of a stratified anisotropic medium, optical axes of which are tilted with respect to the direction of inhomogeneity, using the invariant imbedding theory of wave propagation. When the tilt angle is zero, mode conversion occurs if the longitudinal component of the permittivity tensor, which is the one in the direction of inhomogeneity in the non-tilted case, varies from positive to negative values within the medium, while the transverse component plays no role. When the tilt angle is nonzero, the wave transmission and absorption show an asymmetry under the sign change of the incident angle in a range of the tilt angle, while the reflection is always symmetric. We calculate the reflectance, the transmittance and the absorptance for several configurations of the permittivity tensor and find that resonant absorption is greatly enhanced when the medium from the incident surface to the resonance region is hyperbolic than when it is elliptic. For certain configurations, the transmittance and absorptance curves display sharp peaks at some incident angles determined by the tilt angle.

Chip-scale nanophotonic switch based on a waveguide-metamaterial coupling mechanism

We demonstrate a simple and effective waveguide-metamaterial coupling mechanism to achieve switching control of the fundamental TE mode propagation in a silicon photonic stripe waveguide. The metamaterial is made of vertically stacked alternating ITO/Si layers and, theoretically, can be switched between transparent and absorptive regimes via modifying the carrier concentration of the ITO layers. In addition to its small footprint and CMOS-compatible fabrication, simulation results indicate that the optical switch features a high modulation depth (MD) of 27.8 dB, low insertion losses of 0.004 dB, and a wide operating bandwidth of 300 nm where the MD is >24.6  dB. We expect that this mechanism is a good candidate for designing high-performance and ultra-compact photonic devices in densely integrated nanophotonic circuits and computation systems.

Titanium Oxynitride Thin Films with Tunable Double Epsilon-Near-Zero Behavior for Nanophotonic Applications

Titanium oxynitride (TiO_xN_y) thin films are fabricated using reactive magnetron sputtering. The mechanism of their growth formation is explained, and their optical properties are presented. The films grown when the level of residual oxygen in the background vacuum was between 5 nTorr to 20 nTorr exhibit double epsilon-near-Zero (2-ENZ) behavior with ENZ1 and ENZ2 wavelengths tunable in the 700-850 and 1100-1350 nm spectral ranges, respectively. Samples fabricated when the level of residual oxygen in the background vacuum was above 2 × 10^-8 Torr exhibit nonmetallic behavior, while the layers deposited when the level of residual oxygen in the background vacuum was below 5 × 10^-9 Torr show metallic behavior with a single ENZ value. The double ENZ phenomenon is related to the level of residual oxygen in the background vacuum and is attributed to the mixture of TiN and TiO_xN_y and TiO_x phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of TiN, TiO_x, and TiO_xN_y compounds in the films and, therefore, tune the screened plasma wavelengths. A good approximation of the ellipsometric behavior is achieved with Maxwell-Garnett theory for a composite film formed by a mixture of TiO₂ and TiN phases suggesting that double ENZ TiO_xN_y films are formed by inclusions of TiN within a TiO₂ matrix. These oxynitride compounds could be considered as new materials exhibiting double ENZ in the visible and near-IR spectral ranges. Materials with ENZ properties are advantageous for designing the enhanced nonlinear optical response, metasurfaces, and nonreciprocal behavior.

Combining ε-Near-Zero Behavior and Stopped Light Energy Bands for Ultra-Low Reflection and Reduced Dispersion of Slow Light

We investigate media which exhibits epsilon-near-zero (ENZ) behavior while simultaneously sustaining stopped light energy bands which contain multiple points of zero group velocity (ZGV). This allows the merging of state-of-the-art phenomena that was hitherto attainable in media that demonstrated these traits separately. Specifically, we demonstrate the existence of Ferrell-Berreman (FB) modes within frequency bands bounded by points of ZGV with the goal to improve the coupling efficiency and localization of light in the media. The FB mode is formed within a double layer, thin-film stack where at subwavelength thicknesses the structure exhibits a very low reflection due to ENZ behavior. In addition, the structure is engineered to promote a flattened frequency dispersion with a negative permittivity able to induce multiple points of ZGV. For proof-of-concept, we propose an oxide-semiconductor-oxide-insulator stack and discuss the useful optical properties that arise from combining both phenomena. A transfer matrix (TM) treatment is used to derive the reflectivity profile and dispersion curves. Results show the ability to reduce reflection below 0.05% in accordance with recent experimental data while simultaneously exciting a polariton mode exhibiting both reduced group velocity and group velocity dispersion (GVD).

Improving the transmittance of an epsilon-near-zero-based wavefront shaper

Although epsilon-near-zero (ENZ) metamaterials offer many unconventional ways to play with light, the optical impedance mismatch with surroundings can limit the efficiency of future devices. We report here on the improvement of the transmittance of an ENZ wavefront shaper. In this Letter, we first address the way to enhance the transmittance of a plane wave through a layer of ENZ material, thanks to a numerical optimization approach based on the transfer matrix method. We then transpose the one-dimensional approach to a two-dimensional case where the emission of a dipole is shaped into a plane wave by an ENZ device with a design that optimizes the transmittance. As a result, we demonstrate a transmittance efficiency of 15% that is four orders of magnitude higher than previous devices proposed in the literature for wavefront shaping applications. This Letter aims to pave the way for future efficient ENZ devices by offering new strategies to optimize the transmittance through ENZ materials.

All-optical modulation in wavelength-sized epsilon-near-zero media

We investigate the interaction of two pulses (pump and probe) scattered by a nonlinear epsilon-near-zero (ENZ) slab whose thickness is comparable with the ENZ wavelength. We show that when the probe has a narrow spectrum localized around the ENZ wavelength, its transmission is dramatically affected by the intensity of the pump. Conversely, if the probe is not in the ENZ regime, its propagation is not noticeably affected by the pump. Such all-optical modulation is due to the oversensitive character of the ENZ regime, and it is so efficient that it even occurs in a wavelength thick slab.

Enhanced Nonlinear Refractive Index in ε-Near-Zero Materials

New propagation regimes for light arise from the ability to tune the dielectric permittivity to extremely low values. Here, we demonstrate a universal approach based on the low linear permittivity values attained in the ε-near-zero (ENZ) regime for enhancing the nonlinear refractive index, which enables remarkable light-induced changes of the material properties. Experiments performed on Al-doped ZnO (AZO) thin films show a sixfold increase of the Kerr nonlinear refractive index (n_{2}) at the ENZ wavelength, located in the 1300 nm region. This in turn leads to ultrafast light-induced refractive index changes of the order of unity, thus representing a new paradigm for nonlinear optics.

General Strategy for Broadband Coherent Perfect Absorption and Multi-wavelength All-optical Switching Based on Epsilon-Near-Zero Multilayer Films

We propose a general, easy-to-implement scheme for broadband coherent perfect absorption (CPA) using epsilon-near-zero (ENZ) multilayer films. Specifically, we employ indium tin oxide (ITO) as a tunable ENZ material, and theoretically investigate CPA in the near-infrared region. We first derive general CPA conditions using the scattering matrix and the admittance matching methods. Then, by combining these two methods, we extract analytic expressions for all relevant parameters for CPA. Based on this theoretical framework, we proceed to study ENZ CPA in a single layer ITO film and apply it to all-optical switching. Finally, using an ITO multilayer of different ENZ wavelengths, we implement broadband ENZ CPA structures and investigate multi-wavelength all-optical switching in the technologically important telecommunication window. In our design, the admittance matching diagram was employed to graphically extract not only the structural parameters (the film thicknesses and incident angles), but also the input beam parameters (the irradiance ratio and phase difference between two input beams). We find that the multi-wavelength all-optical switching in our broadband ENZ CPA system can be fully controlled by the phase difference between two input beams. The simple but general design principles and analyses in this work can be widely used in various thin-film devices.

Resonant absorption and amplification of circularly-polarized waves in inhomogeneous chiral media

It has been found that in the media where the dielectric permittivity ε or the magnetic permeability μ is near zero and in transition metamaterials where ε or μ changes from positive to negative values, there occur a strong absorption or amplification of the electromagnetic wave energy in the presence of an infinitesimally small damping or gain and a strong enhancement of the electromagnetic fields. We attribute these phenomena to the mode conversion of transverse electromagnetic waves into longitudinal plasma oscillations and its inverse process. In this paper, we study analogous phenomena occurring in chiral media theoretically using the invariant imbedding method. In uniform isotropic chiral media, right-circularly-polarized and left-circularly-polarized waves are the eigen-modes of propagation with different effective refractive indices n(+) and n(-), whereas in the chiral media with a nonuniform impedance variation, they are no longer the eigenmodes and are coupled to each other. We find that both in uniform chiral slabs where either n(+) or n(-) is near zero and in chiral transition metamaterials where n(+) or n(-) changes from positive to negative values, a strong absorption or amplification of circularly-polarized waves occurs in the presence of an infinitesimally small damping or gain. We present detailed calculations of the mode conversion coefficient, which measures the fraction of the electromagnetic wave energy absorbed into the medium, for various configurations of ε and μ with an emphasis on the influence of a nonuniform impedance. We propose possible applications of these phenomena to linear and nonlinear optical devices that react selectively to the helicity of the circular polarization.