Large Optical Nonlinearity of Dielectric Nanocavity-Assisted Mie Resonances Strongly Coupled to an Epsilon-near-Zero Mode

Field Enhancement and Nonlocal Effects in Epsilon-Near-Zero Photonic Gap Antennas

In recent years, the large electric field enhancement and tight spatial confinement supported by the so-called epsilon near-zero (ENZ) mode has attracted significant attention for the realization of efficient nonlinear optical devices. Here, we experimentally demonstrate ENZ photonic gap antennas (PGAs), which consist of a dielectric pillar within which a thin slab of indium tin oxide (ITO) material is embedded. In ENZ PGAs, hybrid dielectric-ENZ modes emerge from strong coupling between the dielectric antenna modes and the ENZ bulk plasmon resonance. These hybrid modes efficiently couple to free space and allow for large enhancements of the incident electric field over nearly an octave bandwidth, without the stringent lateral nanofabrication requirements of conventional plasmonic or dielectric nanoantennas. To understand the modal features, we probe the linear response of single ENZ PGAs with dark field scattering and interpret the results in terms of a simple coupled oscillator framework. Third harmonic generation (THG) is used to probe the ITO local fields and large enhancements are observed in the THG efficiency over a broad spectral range. Surprisingly, sharp peaks emerge on top of the nonlinear response, which were not predicted by full wave calculations. These peaks are attributed to the ENZ material's nonlocal response, which once included using a hydrodynamic model for the ITO permittivity improves the agreement of our calculations for both the linear and nonlinear response. This proof of concept demonstrates the potential of ENZ PGAs, which we have previously shown can support electric field enhancements of up to 100-200×, and the importance of including nonlocal effects when describing the response of thin ENZ layers. Importantly, inclusion of the ITO nonlocality leads to increases in the predicted field enhancement, as compared to the local calculation.

Dynamically logical modulation for THz wave within a dual gate-controlled 2DEG metasurface

High-speed logic modulation of terahertz (THz) waves is crucial for future communications, yet a technology gap persists. Here, we report a dual gate-controlled two-dimensional electronic gas logic modulation metasurface that enables symmetric and asymmetric electron distribution states through independent control of the two electron transport channels. The transition between these two states leads to various response modes in the metasurface and notably increases the diversity of the spectrum transformation, resulting in a multivalued relationship in which each output corresponds to more than one input signal, thus establishing the logical modulation. Our results demonstrate the common logical functions of AND, OR, XOR, XNOR, NOR, and NAND at different frequencies with a modulation speed faster than 250 picoseconds. This work offers a unique avenue for the high-speed, free-space logical operation of THz waves and increases the security of secure communication.

Strong Coupling of Resonant Metasurfaces with Epsilon-Near-Zero Guided Modes

We study, both theoretically and experimentally, strong interaction between a quasi-bound state in the continuum (QBIC) supported by a resonant metasurface with an epsilon-near-zero (ENZ) guided mode excited in an ultrathin ITO layer. We observe and quantify the strong coupling regime of the QBIC-ENZ interaction in the hybrid metasurface manifested through the mode splitting over 200 meV. We also measure experimentally the resonant nonlinear response enhanced near the ENZ frequency and observe the effective nonlinear refractive index up to ∼4 × 10-13 m2/W in the ITO-integrated dielectric nanoresonators, which provides a promising platform for low-power nonlinear photonic devices.

All-optical ultrafast polarization switching with nonlinear plasmonic metasurfaces

Optical switching has important applications in optical information processing, optical computing, and optical communications. The long-term pursuit of optical switch is to achieve short switching time and large modulation depth. Among various mechanisms, all-optical switching based on Kerr effect represents a promising solution. However, it is usually difficult to compromise both switching time and modulation depth of a Kerr-type optical switch. To circumvent this constraint, symmetry selective polarization switching via second-harmonic generation (SHG) in nonlinear crystals has been attracting scientists' attention. Here, we demonstrate SHG-based all-optical ultrafast polarization switching by using geometric phase controlled nonlinear plasmonic metasurfaces. A switching time of hundreds of femtoseconds and a modulation depth of 97% were experimentally demonstrated. The function of dual-channel all-optical switching was also demonstrated on a metasurface, which consists of spatially variant meta-atoms. The nonlinear metasurface proposed here represents an important platform for developing all-optical ultrafast switches and would benefit the area of optical information processing.

Unlocking Efficient Ultrafast Bound-Electron Optical Nonlinearities via Mirror Induced Quasi Bound States in the Continuum

The operation of photonic devices often relies on modulation of their refractive index. While the sub-bandgap index change through bound-electron optical nonlinearity offers a faster response than utilizing free carriers with an overbandgap pump, optical switching often suffers from inefficiency. Here, we use a recently observed metasurface based on mirror-induced optical bound states in the continuum, to enable superior modulation characteristics. We achieve a pulsewidth-limited switching time of 100 fs, reflectance change of 22%, remarkably low energy consumption of 255 μJ/cm2, and an enhancement of modulation contrast by a factor of 440 compared to unpatterned silicon. Additionally, the narrow photonic resonance facilitates the detection of the dispersive nondegenerate two-photon nonlinearity, allowing tunable pump and probe excitation. These findings are explained by a two-band theoretical model for the dispersive nonlinear index. The demonstrated efficient and rapid switching holds immense potential for applications, including quantum photonics, sensing, and metrology.

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.

Colloidal self-assembly based ultrathin metasurface for perfect absorption across the entire visible spectrum

Perfect absorption over the entire visible spectrum can create a dark background for acquiring images with high contrast and improved resolution, which is crucial for various applications such as medical imaging, biological detection, and industrial non-destructive testing. The broadband absorption is desired to be achieved in an ultrathin structure for low noise as well as high integration. Here, we experimentally demonstrate a metasurface broadband perfect absorber with an ultrathin thickness of 148 nm and a large area of ∼10 cm2. Such a metasurface, with more than 97% absorption in the wavelength range from 400 to 800 nm, is composed of chromium nanodisk hexagonal array deposited on a chromium substrate with a silica spacer. A self-assembly based colloidal lithography nanofabrication method is developed for the scalable fabrication of the proposed nanostructure. We attribute the broadband absorption to the spectrally overlapped Fabry-Perot resonance, surface plasmon polariton, and localized surface plasmon resonances. Our results offer a novel approach to wafer-scale and low-cost manufacturing of absorption-based devices for applications such as high-contrast imaging and optical modulation.

Strongly Coupled Plasmon Polaritons in Gold and Epsilon-Near-Zero Bifilms

Epsilon-near-zero (ENZ) polaritons in a thin transparent conducting-oxide film exhibit a significant electric field enhancement and localization within the film at frequencies close to their plasma frequency, but do not propagate. Meanwhile, plasmon polariton modes in thin metallic films can propagate for several microns, but are more loosely confined in the metal. Here, we propose a strongly coupled bilayered structure of a thin gold film on a thin indium tin oxide (ITO) film that supports hybrid polariton modes. We experimentally characterize the dispersion of these modes and show that they have propagation lengths of 4-8 μm while retaining mode confinement greater than that of the polariton in gold films by nearly an order of magnitude. We study the tunability of this coupling strength by varying the thickness of the ITO film and show that ultrastrong coupling is possible at certain thicknesses. The unusual linear and nonlinear optical properties of ITO at ENZ frequencies make these bifilms useful for the active tuning of strong coupling, ultrafast switching, and enhanced nonlinear interactions at near-infrared frequencies.

Origin of an Anticrossing between a Leaky Photonic Mode and an Epsilon-Near-Zero Point of Silver

Strong light-matter coupling hybridizes light and matter to form states known as polaritons, which give rise to a characteristic anticrossing signature in dispersion plots. Here, we identify conditions under which an anticrossing can occur in the absence of strong coupling. We study planar silver/dielectric structures and find that, around the epsilon-near-zero point in silver, the impedance matching between the silver and dielectric layers gives rise to an anticrossing. Our work shows that care must be taken to ensure that anticrossing arising from impedance matching is not misattributed to strong coupling.