Epsilon-near-zero Mode Research Articles

Numerical study of a tapered fiber magnetic field sensor based on the ENZ mode

This study introduces what we believe to be a novel magnetic field sensor that utilizes a tapered optical fiber coated with indium tin oxide (ITO) and titanium dioxide (TiO2), operating on an epsilon-near-zero (ENZ) mode. The evanescent field around the tapered optical fiber can excite the ENZ mode of the ITO film, and the TiO2 layer can ensure the phase matching between the fiber core mode and the ENZ mode. The sensor, immersed in magnetic fluid, leverages the unique properties of ENZ materials to achieve a high sensitivity and resolution in the near-infrared (NIR) region. Through a simulation, the sensor demonstrated a maximum sensitivity of 6900 nm/RIU and a magnetic field sensitivity of 370 pm/Oe. The findings suggest that ENZ mode-based optic sensing presents a promising alternative to traditional plasmonic sensing methods, offering potential applications in various fields requiring precise magnetic field measurements.

Fano resonance based on ENZ mode in multilayer film and its optical switch application

This paper proposes a multilayer film structure consisting of an Au film, a Dysprosium-doped cadmium oxide (CdO:Dy) layer, and a SiO2 dielectric layer. The epsilon-near-zero (ENZ) mode is indirectly excited by the surface plasmon polariton (SPP) mode, and Fano resonance can be excited by the coupling of the two modes. The higher the wavelength-sensitivity of the real part of the dielectric constant of the ENZ material at the ENZ wavelength, the narrower the bandwidth of Fano resonance in the reflection spectrum. The indium tin oxide (ITO) is used to replace CdO:Dy as a switching material, and the ENZ wavelength is dynamically controlled by bias voltage. The ON and OFF states of the reflection spectrum can be switched at the wavelength of 1410 nm, achieving an insertion loss of 0.56 dB, an extinction ratio of 13.44 dB, and a figure of merit of 23.48 at this wavelength. This multilayer film structure has potential applications in the field of optical switches.

Strong coupling of an epsilon-near-zero mode to a chiral plasmon.

The reconfigurable chiroptical effect is highly desirable for spin photonics, chiral spectroscopy, and photocatalysis due to its merits for dynamic and broadband applications. The coupling of an epsilon-near-zero (ENZ) mode to a chiral plasmon is expected to enable active and effective manipulation of the chiroptical effect but remains unexplored. Here we, for the first time to our knowledge, propose and demonstrate the strong coupling of an ENZ mode to a chiral plasmon by using a hybrid system composed of two identical vertically placed gold nanorods and an in-between ENZ film. An analytical three-oscillator model combined with numerical simulations is established to study the coupling mechanism, which predicts a Rabi splitting up to 240 meV with an ENZ film thickness of 60 nm in circular dichroism.

Exploring localized ENZ resonances and their role in superscattering, wideband invisibility, and tunable scattering

While the role and manifestations of the localized surface plasmon resonances (LSPRs) in anomalous scattering, like superscattering and invisibility, are quite well explored, the existence, appearance, and possible contribution of localized epsilon-near-zero (ENZ) resonances still invoke careful exploration. In this paper, that is done along with a comparison of the resonances of two types in the case of thin-wall cylinders made of lossy and loss-compensated dispersive materials. It is shown that the localized ENZ resonances exist and appear very close to the zero-permittivity regime, i.e., at near-zero but yet negative permittivity that is similar to the ENZ modes in thin planar films. Near- and far-field characteristics of the superscattering modes are investigated. The results indicate that the scattering regimes arising due to LSPRs and localized ENZ resonances are distinguishable in terms of the basic field features inside and around the scatterer and differ in their contribution to the resulting scattering mechanism, e.g., in terms of the occupied frequency and permittivity ranges as well as the sensitivity to the wall thickness variations. When the losses are either weak or tend to zero due to the doping with gain enabling impurities, the sharp peaks of the scattering cross-section that are yielded by the resonances can be said to be embedded into the otherwise wide invisibility range. In the case of lossy material, a wide and continuous invisibility range is shown to appear not only due to a small total volume of the scatterer in the nonresonant regime, but also because high-Q superscattering modes are suppressed by the losses. For numerical demonstration, indium antimonide, a natural lossy material, and a hypothetical, properly doped material with the same real part of the permittivity but lower or zero losses are considered. In the latter case, variations of permittivity with a control parameter can be adjusted in such a way that transitions from one superscattering mode to another can be achieved. In turn, transition from the strong-scattering to the invisibility regime is possible even for the original lossy material. The basic properties of the studied superscattering modes may be replicable in artificial structures comprising natural low-loss materials.

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.

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.

High-contrast optical bistability using a subwavelength epsilon-near-zero material.

Optical bistability opens up a promising avenue toward various optical nonlinear functions analogous to their electrical counterparts, such as switches, logic gates, and memory. Free-space bistable devices have unique advantages in large-scale integration. However, most proposed free-space schemes for optical bistability have limitations in one or more aspects of low contrast ratio, compromised compatibility, slow switching speed, and bulk size. Epsilon-near-zero (ENZ) materials have recently shown an ultrafast and giant optical nonlinearity within a subwavelength scale, potentially overcoming these obstacles. Using large-mobility indium-doped cadmium oxide (CdO) as the ENZ material, we numerically demonstrate two efficient schemes for high-contrast optical bistability within a deep subwavelength size based on the ENZ mode and the Berreman mode. The ENZ wavelength can be optically tuned with a typical time scale of sub-picoseconds, giving rise to a switchable bistability between the near-zero state and the high-reflection state. Our work contributes to the advances on compact and ultrafast all-optical signal processing.

Magnetically Modulated Hybridization of Epsilon-Near-Zero Mode in Multilayered Heterostructure

Manipulation of optical Tamm states (OTSs) near an interface between polar thin film and one-dimensional (1D) photonic crystals (PhCs) provides a framework for confinement of surface phonon polaritons (SPhPs). Here, we offer a lithography-free avenue to create structure for the realization of strong coupling between Tamm phonon polaritons (TPhPs) and epsilon-near-zero (ENZ) modes. Infrared reflectivity also reveals that the optical response of the hetero-stack can be combined with dual discrete absorption events, each associated with an individual ENZ mode. In the presence of external magnetic field, an alteration of the hybridization will be met, which enable an efficient way for the control of light-matter interaction at nanoscale in the mid-infrared (MIR) range.

In-Plane and Out-of-Plane Investigation of Resonant Tunneling Polaritons in Metal-Dielectric-Metal Cavities.

Polaritons can be generated by tuning the optical transitions of a light emitter to the resonances of a photonic cavity. We show that a dye-doped cavity generates resonant tunneling polaritons with Epsilon-Near-Zero (ENZ) effective permittivity. We studied the polariton spectral dispersion in dye-doped metal-dielectric-metal (MDM) cavities as a function of the in-plane (k||) and out-of-plane (k⊥) components of the incident wavevector. The dependence on k|| was investigated through ellipsometry, revealing the ENZ modes. The k⊥ dependence was measured by varying the cavity thickness under normal incidence using a Surface Force Apparatus (SFA). Both methods revealed a large Rabi splitting well exceeding 100 meV. The SFA-based investigation highlighted the collective nature of strong coupling by producing a splitting proportional to the square root of the involved photons. This study demonstrates the possibility of generating ENZ polaritons and introduces the SFA as a powerful tool for the characterization of strong light-matter interactions.

Excitation of Epsilon‐Near‐Zero Mode in Optical Fiber

AbstractExperimental excitation of a highly confined epsilon‐near‐zero (ENZ) mode in a side‐polished optical fiber coated with a deep subwavelength thick layer of aluminum‐doped zinc oxide (AZO) is reported. The uniform AZO layer on the fiber is fabricated by atomic layer deposition technique and optimized to exhibit close‐to‐zero permittivity at the near‐infrared wavelength. Highly polarization‐ and wavelength‐dependent transmission with strong resonance strength up to 25 dB is observed in a 30‐nm ENZ‐coated fiber that is 17 mm long. Different from the excitation of the ENZ mode in a planar conducting oxide thin film, the hybrid ENZ mode can be excited via direct phase matching between the fundamental mode of the fiber and the ENZ mode supported by the AZO thin film. The hybrid ENZ mode in the fiber exhibits a relatively long propagation/light–matter interaction length which is a few orders of magnitude longer than those on the planar ENZ substrates. It is further shown that the hybrid resonance in the ENZ fiber can be actively tuned through the refractive index of surrounding medium and the large ENZ's nonlinearity. These ENZ‐optical fibers serve as emerging in‐fiber optical devices, such as advanced in‐fiber ultrafast optical switches/modulators, mode‐locked fiber lasers, and in‐fiber optical gas/biomolecule sensors.

Weyl semimetal mediated epsilon-near-zero hybrid polaritons and the induced nonreciprocal radiation.

Polaritonic excitation and management in ultra-thin polar crystals has recently received significant attention and holds new promise for epsilon-near-zero (ENZ) modes. However, manipulation of the ENZ mode via anisotropic magneto-optic (MO) material remains elusive. Herein, we provide an effective strategy for constructing an ENZ polar thin film with dependence on Weyl semimetals (WSM). The thermal radiation of the proposed device is explored with electromagnetic (EM) simulations that utilize the anisotropic rigorous coupled-wave analysis (aRCWA) method. Strong coupling of the ENZ mode to WSM polaritons has been demonstrated, and the structural parameters hold tolerance on the order of hundreds of nanometers, which is highly favorable for low-cost fabrication and high-performance application. By changing both the azimuthal angle (ϕ) and angle of incidence (θ), the nonreciprocity (η) can be effectively influenced. The distribution of η is symmetrical with ϕ = 180°, η = 0 when ϕ = 90° and ϕ = 270°. The mechanism of this proposal is owing to the hybrid polaritons supported by the polar thin film and nonreciprocal radiation of WSM, which is validated by examining the amplitude distribution of the magnetic field. The nonreciprocal emitter described herein allows simultaneous control of spectral distribution and polarization of radiation, which will facilitate the active design and application of mid-infrared (MIR) thermal emitters.

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 1750nm. 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.

Engineering the Dispersion of Surface Plasmon Polariton/Epsilon‐Near‐Zero Modes through Modal Separation and Optical Confinement

Doped cadmium oxide (CdO) films support tunable, low‐loss surface plasmon polaritons (SPPs) and epsilon‐near‐zero (ENZ) modes throughout the mid‐infrared. Prior work demonstrated that polaritonic strong coupling could be realized in bilayer films of CdO supporting adjacent SPP and ENZ modes. The hybridized SPP‐ENZ dispersion displays a prominent anti‐crossing with an amplitude dependent on the spectral and spatial overlap of the modes. Here, we leverage the low carrier diffusivity, in concert with the broad tunability of the plasma frequency in CdO to control the SPP‐ENZ dispersion within monolithic, multilayer CdO structures featuring dielectric, unintentionally doped CdO coatings or spacer layers. In the case of the former, the coating induces further compression of the polaritonic fields, altering the dielectric environment. In contrast, when incorporated as a spacer layer, the spatial overlap of the constituent polaritonic near‐fields is reduced. In all cases, the anti‐crossing behavior of the hybrid SPP‐ENZ modes is maintained, thus inheriting the extreme light confinement of the ENZ mode and extended propagation length of the SPPs. Through simultaneous control over the SPP‐ENZ coupling strength and group velocities, significant tunability over the SPP‐ENZ dispersion is demonstrated, enabling applications such as in‐plane waveguiding, tailored thermal emission, and tunable near‐field heat transfer.

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.

Polarization-Independent, tunable, broadband perfect absorber based on semi-sphere patterned Epsilon-Near-Zero films

Epsilon-near-zero (ENZ) optical materials, whose real part of the dielectric constant crosses zero, have been demonstrated to enhance linear and nonlinear optical responses in the ENZ region. Here, a new broadband light absorption design concept based on the coupling of ENZ and localized surface plasmon resonance (LSPR) modes is proposed. It is verified by a single-layer semi-sphere patterned indium tin oxide (ITO) film based on large-area and low-cost self-assembly technology. Simulation and experimental results show that the 91 nm thick ITO film on the two-dimensional microsphere array can simultaneously excite the ENZ and LSPR modes without complex coupling structures, which enable >98% light absorption in the range of 1435–1680 nm. The perfect absorber based on a corrugated ITO film has typical polarization-independent, large-angular-spectrum, adjustable-absorption-bandwidth, and field-enhanced resonance absorption characteristics. These findings provide a new design solution for low-cost broadband perfect absorbers based on ENZ materials, which have wide prospects for application in near-infrared sensors, photoelectric detection, and ENZ photonics.

Realizing PIT-like transparency via the coupling of plasmonic dipole and ENZ modes.

Plasmon induced transparency (PIT), known as the coupling of plasmon modes in metamaterials, has attracted intensive research interests in photonic applications. In this work, a PIT-like transparency is realized via the strong coupling of plasmonic dipole and epsilon-near-zero (ENZ) mode. Two types of metasurfaces, namely the gold nanoantenna and dolmen-like metasurface, are designed with an integrated ENZ material aluminum doped zinc oxide (AZO) film. Simulations with the finite element method (FEM) demonstrate that single and double transparent windows are achieved respectively. The adjustments of the peak position and transmittance of transparent windows via the structure parameters and the AZO film thickness are further investigated. This work provides an alternative coupling scheme of realizing PIT-like transparency with simple metasurface design, and offers great potential for future metamaterial applications.

Broad-band spatial light modulation with dual epsilon-near-zero modes

Epsilon-near-zero (ENZ) modes have attracted extensive interests due to its ultrasmall mode volume resulting in extremely strong light-matter interaction (LMI) for active optoelectronic devices. The ENZ modes can be electrically toggled between on and off states with a classic metal-insulator-semiconductor (MIS) configuration and therefore allow access to electro-absorption (E-A) modulation. Relying on the quantum confinement of charge-carriers in the doped semiconductor, the fundamental limitation of achieving high modulation efficiency with MIS junction is that only a nanometer-thin ENZ confinement layer can contribute to the strength of E-A. Further, for the ENZ based spatial light modulation, the requirement of resonant coupling inevitably leads to small absolute modulation depth and limited spectral bandwidth as restricted by the properties of the plasmonic or high-Q resonance systems. In this paper, we proposed and demonstrated a dual-ENZ mode scheme for spatial light modulation with a TCOs/dielectric/silicon nanotrench configuration for the first time. Such a SIS junction can build up two distinct ENZ layers arising from the induced charge-carriers of opposite polarities adjacent to both faces of the dielectric layer. The non-resonant and low-loss deep nanotrench framework allows the free space light to be modulated efficiently <italic>via</italic> interaction of dual ENZ modes in an elongated manner. Our theoretical and experimental studies reveal that the dual ENZ mode scheme in the SIS configuration leverages the large modulation depth, extended spectral bandwidth together with high speed switching, thus holding great promise for achieving electrically addressed spatial light modulation in near- to mid-infrared regions.

Length-Irrelevant Dual-Polarized Antenna Based on Antiphase Epsilon-Near-Zero Mode

In this communication, a novel dual-polarized (DP) epsilon-near-zero (ENZ) antenna is proposed with its operating frequencies independent of lengths for both polarizations. To obtain a DP implementation of ENZ antenna, an antiphase ENZ (AP-ENZ) mode is used based on a slot-fed substrate-integrated waveguide (SIW) antenna at its cutoff frequency, in which the electric field presents a sign-function-like distribution, presenting a fixed operating frequency independent on length. The proposed DP length-irrelevant antenna design is composed of a cross-shaped SIW operating on two orthogonal AP-ENZ modes, which are excited by a cavity-backed cross-shaped slot with good port isolation. The SIW lengths for both polarizations can be tuned independently without disturbing the operation frequency with controllable radiation patterns. To be specific, we study two cases of different lengths where sidelobe elimination and high gain radiation are realized, respectively. To verify this design, two antenna prototypes Ant. 1 and Ant. 2 are fabricated and tested, exhibiting agreements between simulation and experiment. The proposed Ant. 1 presents no sidelobes at 2.42 GHz while Ant. 2 shows a high gain of 10.56 dBi from 2.45 to 2.55 GHz. The proposed design method enables antennas with a variety of radiation patterns to be designed at a fixed frequency and both symmetric and asymmetric patterns for DP antennas. It also inspires the design of a flexible antenna with unchanged operating frequency when folded in both dimensions.

Tailoring Infrared Absorption and Thermal Emission with Ultrathin Film Interferences in Epsilon‐Near‐Zero Media

Engineering modal dispersions of ultrathin, planar structures enables significant control over infrared perfect absorption (PA) and thermal emission characteristics. Herein, the optical response of ultrathin, low loss, epsilon‐near‐zero (ENZ) films on reflecting surfaces is simulated to investigate the wavelength and angular ranges over which they absorb and emit radiation most efficiently and identify the design parameters that tailor the ENZ mode dispersion of the system. A generic interference model is shown to elucidate the underlying physics of these ultrathin film absorption resonances, occurring well below the conventional quarter‐wavelength thickness limit. While the absorption is spectrally limited to wavelengths where the ENZ film is optically a dielectric with refractive index below unity, the angular spread is delimited by the material dispersion. Further, these resonant interferences are understood to arise from universal wave phenomena, realizable in simple planar structures having appropriate refractive index contrast, not restricted to ENZ materials. The results show that appropriate choice of material, film thickness and loss allows tailoring the modal dispersions, which enables precise directional control and wide tunability in spectral range (width ≈0.1–1.0 μm) of PA and thermal emission, paving the way towards efficient ENZ‐based infrared optical and thermal coatings.

Temporal dynamics of strongly coupled epsilon near-zero plasmonic systems

Subwavelength epsilon-near-zero (ENZ) films with decorated nanoantennae are an emerging platform for coupling radiation into the ENZ mode: recent experiments have shown how strong coupling between the two systems underpins significant nonlinear effects. Field enhancement and the temporal dynamic of the radiation in ENZ films strongly coupled with plasmonic antennae play a substantial role in the light–matter interaction, and the latter has not yet been fully disclosed. We numerically investigate the pulse shaping of radiation impinging on two representative strongly coupled systems: a transparent conductive oxide with ENZ in the NIR band (ITO) and a polar material with ENZ in the MIR (SiC). In both cases, we demonstrate that the temporal dynamics and field enhancement are particularly pronounced within the strong coupling region, and we highlight the different polarization responses of the ITO-based case from the SiC one. The latter, owing to lower losses, shows extreme reshaping of resonant radiation close to the ENZ spectral region and is, therefore, a promising candidate for slow-light-enhanced effects.