Tunable nonlinear coherent perfect absorption with epsilon-near-zero plasmonic waveguides

Propagation characteristics of single and multilayer Ga:ZnO in the epsilon near zero region

We numerically investigated the propagation characteristics of Ga:ZnO (GZO) thin films embedded in a ZnWO₄ background in the epsilon near zero (ENZ) region. We found that, for GZO layer thickness ranging between 2 - 100 nm (∼ 1/600 - 1/12 of ENZ wavelength), such structure supports a novel non-radiating mode with its real part of effective index lower than surrounding refractive index or even less than 1. Such a mode has its dispersion curve lying to the left of the light line in the background region. However, the calculated electromagnetic fields display non-radiating nature contrary to the Berreman mode, because the transverse component of the wave vector is complex, ensuring a decaying field. Furthermore, while the considered structure supports confined and highly lossy TM modes in the ENZ region, no TE mode is supported. Subsequently, we studied the propagation characteristics of a multilayer structure constituting an array of GZO layers in the ZnWO₄ matrix considering the modal field's excitation using the end-fire coupling. Such a multilayer structure is analyzed using high-precision rigorous coupled-wave analysis and shows strong polarization selective and resonant absorption/emission, the spectral location and bandwidth of which can be tuned by judiciously selecting the thickness of the GZO layer and other geometrical parameters.

Ultra-broadband coherent perfect absorption via elements with linear phase response

Increasing interest in perfect absorption of metasurface has initiated a discussion on the implementation of ultra-broadband coherent perfect absorption (CPA). Here, we present a mirror symmetric coherent absorption metasurface (CAMS) with polarization independence based on resistive thin films and annular metal patterns to force the fulfillment of ultra-broadband CPA in terahertz (THz) regime, controlling the interplay between electromagnetic waves and matter. By incorporating internal and external ring-shaped films with attached phase-delay lines, the desired phase response can be obtained, laying the foundation for implementing ultra-broadband coherent absorption. Simultaneously, by building a metal-medium composite structure superseding the dielectric substrate, additional promotion of the coherent absorptivity over the operation frequencies is realized. Manipulating the phase difference of two back-propagation coherent beams, the coherent absorptivity at 8.34-25.07 THz can be tailored successively from over 95.7% to as low as 38.1%. Moreover, with the incident angle up to 70° for the transverse electric wave, the coherent absorptivity is still over 74.8% from 8.34 THz to 25.07 THz. And for the transverse magnetic wave, at 6.67-24.2 THz, above 81.3% coherent absorptivity is visible with the incident angle increased from 0° to 60°. Our finding provides an interesting approach to designing ultra-broadband coherent absorption devices and may serve applications in THz modulators, all-optical switches, and signal processors.

Plasmon mode manipulation based on multi-layer hyperbolic metamaterials

Metamaterial with hyperbolic dispersion properties can effectively manipulate plasmonic resonances. Here, we designed a hyperbolic metamaterial (HMM) substrate with a near-zero dielectric constant in the near-infrared region to manipulate the plasmon resonance of the nano-antenna (NA). For NA arrays, tuning the equivalent permittivity of HMM substrate by modifying the thickness of Au/diamond, the wavelength range of plasmon resonance can be manipulated. When the size of the NA changes within a certain range, the spectral position of the plasmon resonance will be fixed in a narrow band close to the epsilon-near-zero (ENZ) wavelength and produce a phenomenon similar to "pinning effect." In addition, since the volume plasmon polaritons (VPP) mode is excited, it will couple with the localized surface plasmon (LSP) mode to generate a spectrum splitting. Therefore, the plasmon resonance is significantly affected and can be precisely controlled by designing the HMM substrate.

Optomechanical Anti-Lasing with Infinite Group Delay at a Phase Singularity

Singularities which symbolize abrupt changes and exhibit extraordinary behavior are of a broad interest. We experimentally study optomechanically induced singularities in a compound system consisting of a three-dimensional aluminum superconducting cavity and a metalized high-coherence silicon nitride membrane resonator. Mechanically induced coherent perfect absorption and anti-lasing occur simultaneously under a critical optomechanical coupling strength. Meanwhile, the phase around the cavity resonance undergoes an abrupt π-phase transition, which further flips the phase slope in the frequency dependence. The observed infinite discontinuity in the phase slope defines a singularity, at which the group velocity is dramatically changed. Around the singularity, an abrupt transition from an infinite group advance to delay is demonstrated by measuring a Gaussian-shaped waveform propagating. Our experiment may broaden the scope of realizing extremely long group delays by taking advantage of singularities.

Ultrathin multi-band coherent perfect absorber in graphene with high-contrast gratings

High-contrast gratings (HCGs) can be designed as a resonator with high-quality factor and surface-normal emission, which are excellent characters for designing optical devices. In this work, we combine HCGs with plasmonic graphene structure to achieve an ultrathin five-band coherent perfect absorber (CPA). The presented CPA can achieve multi- and narrow-band absorption with high intensity under a relatively large incident angle. The good agreement between theoretical analysis and numerical simulated results demonstrates that our proposed HCGs-based structure is feasible to realize CPA. Besides, by dynamically adjusting the Fermi energy of graphene, we realize the active tunability of resonance frequency and absorption intensity simultaneously. Benefitting from the combination of HCGs and the one-atom thickness of graphene, the proposed device possesses an extremely thin feature. Our work proposes a novel method to manipulate coherent perfect absorption and is helpful to design tunable multi-band and ultrathin absorbers.

Coupled-mode theory for plasmonic resonators integrated with silicon waveguides towards mid-infrared spectroscopic sensing

Advances in mid-IR lasers, detectors, and nanofabrication technology have enabled new device architectures to implement on-chip sensing applications. In particular, direct integration of plasmonic resonators with a dielectric waveguide can generate an ultra-compact device architecture for biochemical sensing via surface-enhanced infrared absorption (SEIRA) spectroscopy. A theoretical investigation of such a hybrid architecture is imperative for its optimization. In this work, we investigate the coupling mechanism between a plasmonic resonator array and a waveguide using temporal coupled-mode theory and numerical simulation. The results conclude that the waveguide transmission extinction ratio reaches maxima when the resonator-waveguide coupling rate is maximal. Moreover, after introducing a model analyte in the form of an oscillator coupled with the plasmonics-waveguide system, the transmission curve with analyte absorption can be fitted successfully. We conclude that the extracted sensing signal can be maximized when analyte absorption frequency is the same as the transmission minima, which is different from the plasmonic resonance frequency. This conclusion is in contrast to the dielectric resonator scenario and provides an important guideline for design optimization and sensitivity improvement of future devices.

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.

Resonance energy transfer and quantum entanglement mediated by epsilon-near-zero and other plasmonic waveguide systems

The resonance energy transfer and entanglement between two-level quantum emitters are typically limited to sub-wavelength distances due to the inherently short-range nature of the dipole-dipole interactions. Moreover, the entanglement of quantum systems is hard to preserve for a long time period due to decoherence and dephasing mainly caused by radiative and nonradiative losses. In this work, we outperform the aforementioned limitations by presenting efficient long-range inter-emitter entanglement and large enhancement of resonance energy transfer between two optical qubits mediated by epsilon-near-zero (ENZ) and other plasmonic waveguide types, such as V-shaped grooves and cylindrical nanorods. More importantly, we explicitly demonstrate that the ENZ waveguide resonant energy transfer and entanglement performance drastically outperforms the other waveguide systems. Only the excited ENZ mode has an infinite phase velocity combined with a strong and homogeneous electric field distribution, which leads to a giant energy transfer and efficient entanglement independent of the emitters' separation distances and nanoscale positions in the ENZ nanowaveguide, an advantageous feature that can potentially accommodate multi-qubit entanglement. Moreover, the transient entanglement can be further improved and become almost independent of the detrimental decoherence effect when an optically active (gain) medium is embedded inside the ENZ waveguide. We also present that efficient steady-state entanglement can be achieved by using a coherent external pumping scheme. Finally, we report a practical way to detect the steady-state entanglement by computing the second-order correlation function. The presented findings stress the importance of plasmonic ENZ waveguides in the design of the envisioned on-chip quantum communication and information processing plasmonic nanodevices.

Bandwidth bounds of coherent perfect absorber in resonant metasurfaces

Coherent perfect absorber (CPA), a resonator with critical losses that can perfectly absorb all incident light, has been observed at various frequency regimes (from microwave to visible light). Besides the functional frequency, the bandwidth is also an important parameter in characterizing the performance of CPA. Here, we explore the bandwidth of CPA in a kind of weakly-coupled-resonance metasurfaces with 4κ²-γs2<0, where κ is the near-field coupling between the radiative and non-radiative resonant modes, and γ_sis the scattering loss rate of the radiative resonant mode. Based on the coupled mode theory, we analytically derive the upper and lower bounds of the bandwidth, and show that they are determined by the dissipation loss rates of the composed modes. To narrow the bandwidth, it is better to increase the radiative loss rate when designing a weakly coupled resonator. We also show that CPA is associated with a robust phase singularity with a winding number of ± 1. The conclusions are numerically verified in a designed resonant metasurface and could perform as a guideline for designing CPA in various resonant systems.

Induced reflection in Tamm plasmon systems

We present an induced reflection response analogue to electromagnetically induced transparency (EIT) in a novel Tamm plasmon system, consisting of a thin metal film and a Bragg grating with a defect layer. The results show that an induced narrow peak can be generated in the original broad reflection dip, which is attributed to the coupling and interference between the Tamm plasmon and defect modes in the grating structure. It is found that the EIT-like induced reflection is strongly dependent on the thickness of defect layer, grating period number between the metal and defect layers, thickness of Bragg grating layer, refractive index of defect layer, and thickness of metal film. Additionally, the induced reflection can be dynamically tuned by adjusting the angle of incident light. The numerical simulations agree extremely well with theoretical calculations. The coupling strength between the Tamm plasmon and defect modes is determined by the above parameters. These results will provide a new avenue for light field control and devices in multilayer photonic systems.

Coherent perfect absorber and laser for nonlinear waves in optical waveguide arrays

A localized non-Hermitian potential can operate as a coherent perfect absorber or as a laser for nonlinear waves. The effect is illustrated for an array of optical waveguides, with the central waveguide being either active or absorbing. The arrays situated to the left and to the right of the center can have different characteristics. The result is generalized to setups with the central waveguide carrying additional nonlinear dissipation or gain and to the two-dimensional arrays with embedded one-dimensional absorbing or lasing subarrays.

Coherent perfect absorption of nonlinear matter waves

Coherent perfect absorption is the complete extinction of incoming radiation by a complex potential in a physical system supporting wave propagation. The concept was proven for linear waves in a variety of systems including light interacting with absorbing scatterers, plasmonic metasurfaces, and graphene films, as well as sound waves. We extend the paradigm to coherent perfect absorption of nonlinear waves and experimentally demonstrate it for matter waves in an atomic Bose-Einstein condensate. Coherent absorption of nonlinear matter waves is achieved easier than its linear analogs because the strength of two-body interactions offers additional freedom for control. Implementation of the coherent perfect absorber of Bose-Einstein condensates paves the way toward broad exploitation of the phenomenon in nonlinear optics, exciton-polariton condensates, acoustics, and other areas of nonlinear physics. It also opens perspectives for designing atom lasers.