Epsilon-near-zero Materials Research Articles

Enhancement of second-order vortex harmonics in epsilon-near-zero materials

When a spatially-inhomogeneous vortex laser field irradiating an epsilon-near-zero (ENZ) material is theoretically investigated, second-order vortex harmonics occur due to the symmetry breaking from the field spatial-inhomogeneity. Their signal intensity is sensitive to three factors: the ENZ field enhancing ratio, the ENZ material thickness, and the field inhomogeneity degree. Their competition supports a high conversion efficiency of around 10−6 for second-order harmonics from the incident fundamental laser field under an optimum thickness of ENZ materials.

Vertical Thermal Emission from Optical Antennas on an Epsilon‐Near–Zero Substrate

AbstractThis work presents a novel approach to achieve directional and normal thermal emission from epsilon‐near–zero (ENZ) materials. ENZ materials exhibit near–zero permittivity at the ENZ point, resulting in some unique properties compared to conventional optical materials including infinite wavelength, constant phase distribution, and decoupling of spatial and temporal fields inside the ENZ material. These properties are used to engineer the far‐field thermal emission from optical antennas fabricated on ENZ film in the mid‐infrared. By coupling the antenna resonance mode with the Berreman mode of the ENZ material, highly directional and normal emission is demonstrated. This approach could have significant implications for thermal management, energy conversion, and sensing applications.

Tunable ENZ properties in organic material PEDOT:PSS treated with different solutions

Epsilon-near-zero (ENZ) materials have drawn significant attention due to their novel properties near ENZ wavelengths. One such material, PEDOT:PSS, is a conducting polymer whose performances can be easily modified using convenient solution treatment methods. In this paper, pristine, ethylene glycol (EG) -treated, and concentrated sulfuric acid -treated PEDOT:PSS films were found to achieve ENZ wavelengths at 1657 nm, 1450 nm, and 1162 nm, respectively. Spectrum numerical fitting, Hall measurement, structural characterizations and differential analysis were performed to investigate the effects of the solution modification method on the ENZ performances of PEDOT:PSS films. Furthermore, the imaginary part of permittivity was decomposed through the Drude-Lorentz dispersion model to explain dynamics of polarons and bipolarons in PEDOT:PSS, the bipolarons in acid-treated film was found to play an important role on ENZ properties. The quality factors QSPP and QLSPR were used to evaluate the performance of different ENZ materials for device applications. Our findings pave the way for further research on ENZ photonics of organic materials.

Tunable bandpass filter based on epsilon-near-zero metamaterials using liquid crystals

ABSTRACT In the article, a design method of tunable bandpass filter based on epsilon-near-zero (ENZ) metamaterials using liquid crystals (LCs) is proposed. Thanks to the strong and directionally consistent electric field distribution caused by energy tunnelling effect of ENZ metamaterials, the ENZ unit cell with filled LC in the narrow tunnel can theoretically achieve the bandpass filtering characteristic with a large tuning rate. On this basis, a third-order LC-based tunable bandpass filter is constructed by three series unit structures, which achieves obviously tunable bandpass filtering characteristic with different external biased voltages. Finally, one prototype is designed, fabricated and measured, respectively. The measurement results show that the proposed bandpass filter based on ENZ materials with LCs can achieve effective tuning of the operating frequency with different external biased voltages, which verifies the effectiveness of the proposed design method. In addition, the impact of shape variation of the proposed ENZ-based tunable filter on electromagnetic characteristics is also further discussed and analysed in experiments, which is of great significance for achieving RF miniaturisation and high integration.

Enhanced optical nonlinearity of epsilon-near-zero metasurface by quasi-bound state in the continuum

Bound states in the continuum (BICs) provide a powerful way to enhance the nonlinear properties of materials, epsilon-near-zero (ENZ) materials are considered as promising candidates with strong nonlinearities. However, the realization of BIC based on ENZ materials in the near-infrared (NIR) is very challenging due to the large loss in the NIR. Here, a high-quality quasi-BIC based on the ENZ metasurface is proposed for the first time, which is composed of patterned ENZ films embedded in a dielectric-metal hybrid structure, and realizes destructive interference between the Berreman mode and photonic mode to form the Friedrich-Wintergen BIC (FW-BIC). The electric field is strongly confined in the ENZ film, resulting in considerable field enhancement, and the nonlinear refractive index coefficient is 1.63 × 10−12 m2/W, which is three orders of magnitude larger than that of the ITO film. The instantaneous response time is 600 fs and extremely high modulation speed up to the THz level. Benefiting from the perfect absorption and narrow linewidth of quasi-BIC and the change in refractive index of the metasurface induced by Kerr nonlinearity, the absolute modulation is from near-zero to 92% with an extinction ratio of 23.2 dB. It provides a promising platform for the development of integrated ultrafast high-speed photonics.

Entangled dark state mediated by a dielectric cavity within epsilon-near-zero materials.

Two emitters can be entangled by manipulating them through optical fields within a photonic cavity. However, maintaining entanglement for a long time is challenging due to the decoherence of the entangled qubits, primarily caused by cavity loss and atomic decay. Here, we found the entangled dark state between two emitters mediated by a dielectric cavity within epsilon-near-zero (ENZ) materials, ensuring entanglement maintenance over an extended period. To obtain the entangled dark state, we derived an effective model with degenerate mode modulation. In the dielectric cavities within ENZ materials, the decay rate of emitters can be regarded as 0, which is the key to achieving the entangled dark state. Meanwhile, the dark state immune to cavity loss exists when two emitters are in symmetric positions in the dielectric cavity. Additionally, by adjusting the emitters to specific asymmetric positions, it is possible to achieve transient entanglement with higher concurrence. By overcoming the decoherence of the entangled qubits, this study demonstrates stable, long-term entanglement with ENZ materials, holding significant importance for applications such as nanodevice design for quantum communication and quantum information processing.

Time-Dependent Ultrafast Quadratic Nonlinearity in an Epsilon-Near-Zero Platform.

Ultrafast nonlinearity, which results in modulation of the linear optical response, is a basis for the development of time-varying media, in particular those operating in the epsilon-near-zero (ENZ) regime. Here, we demonstrate that the intraband excitation of hot electrons in the ENZ film results in a second-harmonic resonance shift of ∼10 THz (40 nm) and second-harmonic generation (SHG) intensity changes of >100% with only minor (<1%) changes in linear transmission. The modulation is 10-fold enhanced by a plasmonic metasurface coupled to a film, allowing for ultrafast modulation of circularly polarized SHG. The effect is described by the plasma frequency renormalization in the ENZ material and the modification of the electron damping, with a possible influence of the hot-electron dynamics on the quadratic susceptibility. The results elucidate the nature of the second-order nonlinearity in ENZ materials and pave the way to the rational engineering of active nonlinear metamaterials and metasurfaces for time-varying applications.

Hollow core optical fiber enabled by epsilon-near-zero material

Abstract Hollow core optical fibers of numerous guiding mechanisms have been studied in the past decades for their advantages on guiding light in air core. This work demonstrates a new hollow core optical fiber based on a different guiding mechanism, which confines light with a cladding made of epsilon-near-zero (ENZ) material through total internal reflection. We show that the addition of a layer of ENZ material coating (e.g. indium tin oxide layer) significantly reduces the loss of the waveguide compared to the structure without the ENZ layer. We also show that the propagation loss of the ENZ hollow core fiber can be further improved by integrating ENZ materials with lower loss. This study presents a novel type of hollow core fiber, and can find advanced in-fiber photonic applications such as laser surgery/spectroscopy, novel gas-filled/discharge laser, in-fiber molecular/gas sensing, and low-latency optical fiber communication.

Ultra-broadband directional thermal emission

Abstract Directional control of thermal emission over its broad wavelength range is a fundamental challenge. Gradient epsilon-near-zero (ENZ) material supporting Berreman mode has been proposed as a promising approach. However, the bandwidth is still inherently limited due to the availability of ENZ materials covering a broad bandwidth and additional undesired omnidirectional modes in multilayer stacking with increased thickness. Here, we show that broadband directional thermal emission can be realized beyond the previously considered epsilon-near-zero and Berreman mode region. We then establish a universal approach based on effective medium theory to realizing ultra-broadband directional thermal emitter. We numerically demonstrate strong (emissivity &gt;0.8) directional (80 ± 5°) thermal emission covering the entire thermal emission wavelength range (5–30 μm) by using only two materials. This approach offers a new capability for manipulating thermal emission with potential applications in high-efficiency information encryption, energy collection and utilization, thermal camouflaging, and infrared detection.

Clarifying the origin of second-harmonic generation from an epsilon-near-zero flim-coupled plasmonic nanoparticle-on-mirror system by size-dependence properties

We conduct a comprehensive numerical investigation on the size-dependence properties of second harmonic generation (SHG) from an epsilon-near-zero (ENZ) film-coupled plasmonic nanoparticle on-mirror (NPoM) system. The distinct size dependence of gold and indium-tin-oxide (ITO) leads to the existence of a critical point where the SHGs from these two materials are balanced. This study offers valuable guidance in the design of plasmonic systems containing ENZ materials for enhancing SHG.

Interactions of Fundamental Mie Modes with Thin Epsilon-near-Zero Substrates.

Extensive research has focused on Mie modes in dielectric nanoresonators, enabling the creation of thin optical devices surpassing their bulk counterparts. This study investigates the interactions between two fundamental Mie modes, electric and magnetic dipoles, and the epsilon-near-zero (ENZ) mode. Analytical, simulation, and experimental analyses reveal that the presence of the ENZ substrate significantly modifies these modes despite a large size mismatch. Electric and magnetic dipole modes, both with ∼12 THz line widths, exhibit 21 and 26 THz anticrossings, respectively, when coupled to the ENZ mode, indicating strong coupling. We also demonstrate that this strongly coupled system yields notably large subpicosecond nonlinear responses. Our results establish a solid foundation for designing functional, nonlinear, dynamic dielectric metasurfaces with ENZ materials.

Epsilon-near-zero gratings for polarization selectivity.

Zero-index materials have emerged as a topic of significant scientific interest in recent years. In this Letter, we investigate the electromagnetic properties of epsilon-near-zero (ENZ) gratings composed of materials with near-zero effective permittivity. Our study reveals that ENZ gratings exhibit a unique polarization selectivity that is opposite to that observed in perfect conductor gratings. Furthermore, we demonstrate that hybrid gratings combining perfect conductors and ENZ materials can block omnidirectional electromagnetic waves of any polarization. In addition, we propose a practical design of the ENZ and hybrid gratings based on dielectric ENZ MMs, exhibiting excellent polarization selectivity and blocking effect. Our research presents a promising approach for the flexible manipulation of polarizations using ENZ gratings.

Addressing the Impact of Surface Roughness on Epsilon-Near-Zero Silicon Carbide Substrates.

Epsilon-near-zero (ENZ) media have been very actively investigated due to their unconventional wave phenomena and strengthened nonlinear response. However, the technological impact of ENZ media will be determined by the quality of realistic ENZ materials, including material loss and surface roughness. Here, we provide a comprehensive experimental study of the impact of surface roughness on ENZ substrates. Using silicon carbide (SiC) substrates with artificially induced roughness, we analyze samples whose roughness ranges from a few to hundreds of nanometer size scales. It is concluded that ENZ substrates with roughness in the few nanometer scale are negatively affected by coupling to longitudinal phonons and strong ENZ fields normal to the surface. On the other hand, when the roughness is in the hundreds of nanometers scale, the ENZ band is found to be more robust than dielectric and surface phonon polariton (SPhP) bands.

Effect of Epsilon-Near-Zero Modes on the Casimir Interaction between Ultrathin Films.

Vacuum fluctuation-induced interactions between macroscopic metallic objects result in an attractive force between them, a phenomenon known as the Casimir effect. This force is the result of both plasmonic and photonic modes. For very thin films, field penetration through the films will modify the allowed modes. Here, we theoretically investigate the Casimir interaction between ultrathin films from the perspective of force distribution over real frequencies for the first time. Pronounced repulsive contributions to the force are found due to the highly confined and nearly dispersion-free epsilon-near-zero (ENZ) modes that only exist in ultrathin films. These contributions persistently occur around the ENZ frequency of the film irrespective of the interfilm separation. We further associate the ENZ modes with a striking thickness dependence of a proposed figure of merit (FOM) for conductive thin films, suggesting that the motion of objects induced by Casimir interactions is boosted for deeply nanoscale sizes. Our results shed light on the correlation between special electromagnetic modes and the vacuum fluctuation-induced force as well as the resulting mechanical properties of ultrathin ENZ materials, which may create new opportunities for engineering the motion of ultrasmall objects in nanomechanical systems.

Enhanced spin Hall effect of light in the PT-symmetric trilayer structure containing epsilon-near-zero materials

We systematically study the spin Hall effect of light (SHEL) in the parity-time (PT)-symmetric trilayer structure containing epsilon-near-zero (ENZ) materials, and design a high sensitivity refractive index sensor with an adjustable sensing range. It is revealed that the SHEL shift in the PT-symmetric trilayer structure is clearly enhanced, which is two orders of magnitude larger than that in the conventional sandwich structure containing ENZ materials. The enhancement of the SHEL shift is attributed to the fact that the change of reflection coefficient induced by the quasi-bound states in the continuum (quasi-BIC) in the former structure is smoother than that induced by the bound states in the continuum in the latter structure. It is further found that when the refractive index of the interlayer dielectric in the PT-symmetric structure is fixed, the SHEL shift is significantly enhanced near the quasi-BIC resonance angle determined by the gain-loss coefficient. Meanwhile, the SHEL shift enhanced by excitation of quasi-BIC is very sensitive to the gain-loss coefficient and the refractive index of the interlayer dielectric. Finally, we design a high sensitivity refractive index sensor with an adjustable sensing range based on the quasi-BIC-enhanced SHEL shift. These studies provide a pathway to enhance the SHEL and may open avenues for the application of optical sensors.

Optically tunable bianisotropy in a sphere made from an epsilon-near-zero material.

Bianisotropic media can be used to engineer absorbance, scattering, polarization, and dispersion of electromagnetic waves. However, the demonstration of a tunable light-induced bianisotropy at optical frequencies is still lacking. Here, we propose an experimentally feasible concept for a light-induced tunable bianisotropic response in a homogeneous sphere made of an epsilon-near-zero (ENZ) material. By exploiting the large linear absorption and the large possible intensity-dependent changes in the permittivity of ENZ materials, the direction-dependent scattering and absorption cross sections could be obtained. Our findings pave the way for further studies and applications in the optical regime requiring full dynamic control of the bianisotropic behavior.

Flexible and biocompatible poly (vinyl alcohol)/multi-walled carbon nanotubes hydrogels with epsilon-near-zero properties

• Flexible PVA/MWCNTs hydrogels were fabricated by solution method, and cytotoxicity experiments proved that the process was environmentally friendly and hydrogels were biocompatible. • Epsilon-near-zero properties were realized at radio frequency from PVA/MWCNTs hydrogels, originated from the resonance of the induced electric dipole. • After the frequency achieving epsilon-near-zero, negative permittivity which was different from Drude model but related to interband transition, was observed. Epsilon-near-zero (ENZ) material has been a research hotspot in recent years due to unique physical properties such as inverse Doppler effect and negative refractive index, showing great potentials in the fields of flexible electronics, wearable devices, sensors, etc. The ENZ materials are mostly reported at visible, infrared and terahertz wavelengths, while the report about ENZ materials at radio frequency is rare. In this work, flexible and biocompatible poly (vinyl alcohol)/multi-walled carbon nanotubes (PVA/MWCNTs) hydrogels, which were successfully fabricated by an environmentally friendly method, were used as ENZ materials at radio frequency for the first time. Cytotoxicity experiments proved that the experimental process and products are green, environmentally friendly and biocompatible. The microstructure, crystalline structure, chemical composition and dielectric properties were investigated. Two different water states, which were free water molecules and bound water molecules, coexisted in these PVA/MWCNTs hydrogels and accounted for about 37.5 wt% and 15.9 wt% respectively by analyzing the thermogravimetric analysis curves. When the MWCNTs content reached 12 wt% and 15 wt%, the continuous conductive MWCNTs network was formed in the hydrogel, and ENZ phenomenon was observed at about 760 kHz and 580 kHz respectively, which was attributed to the interband transition. Considering the flexibility and non-toxicity of PVA/MWCNTs hydrogels, the ENZ properties of this structure at the radio frequency can be well used in wearable invisibility cloak, flexible electronics, skin sensors and other wearable devices.

Nonlinear epsilon-near-zero material: Explanation with time-varying thermal energy

<p indent="0mm">In recent years, transparent conducting oxides with epsilon-near-zero (ENZ) response and their associated applications in nonlinear optics have drawn wide attention of the research community. It is commonly thought that the physical origin of the giant optical nonlinearity in ENZ materials is the large change of effective mass of free electrons within the conduction band of the transparent conducting oxide material. In a recent study, by introducing the concept of time-varying thermal energy, the transient nonlinear optical response of ENZ material is more thoroughly explained. The theory is further corroborated by experimental studies.

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.

Fluorescence inhibition near spherical ENZ nanoparticles: competition between radiative and non-radiative processes

In this work, we aim to study numerically the emission decay rate of a hybrid system combining a quantum emitter (QE) and an epsilon-near-zero (ENZ) spherical nanoparticle (SNP). Inspired by the peculiar behavior of ENZ materials and their high potential in developing unusual abilities in controlling the emission properties of QE. More specifically the control of fluorescence inhibition, or the amplification of the lifetime of the excited state. This can naturally find applications in quantum information storage for optical quantum memories based on light-atom interaction which naturally benefit from storage time control. We demonstrate that the key process in limiting fluorescence inhibition is the competition between inhibition of fluorescence from the radiative processes and energy dissipation due to the non-radiative channels. Furthermore, we illustrate that this balance can be shifted to optimize inhibition as function of the QE position. The optimization happens via SNP size control, material composition, and λENZ of the SNP. This detailed study introduces and paves the way for new research directions on the manipulation and optimization of QE properties in the vicinity of ENZ materials.