Enhanced Spontaneous Emission of Monolayer MoS2 on Epitaxially Grown Titanium Nitride Epsilon-Near-Zero Thin Films.

Room-temperature photoluminescence enhancement of molybdenum disulfide (MoS2) monolayers on epitaxial titanium nitride (TiN) thin films grown by molecular-beam-epitaxy as well as magnetron-sputtered TiN films is observed by a confocal laser scanning microscope with excitation wavelengths covering the transition of TiN's macroscopic optical properties from dielectric to plasmonic. The photoluminescence enhancement increases as TiN becomes more metallic, and strong enhancement is obtained at the excitation wavelengths equal to or longer than the epsilon-near-zero (ENZ) wavelength of TiN films. A good agreement is observed between measured and calculated enhancements. The enhancement is attributed to the increased excitation field in MoS2 at TiN's ENZ wavelength and interference effects for thick spacers that separate the MoS2 flakes from TiN films in the metallic regime. This study enriches the fundamental understanding of emission properties on ENZ substrates that could be important for the development of advanced nanoscale lasers/light sources, optical/biosensors, and nano-optoelectronic devices.

Multi-pulse laser-induced bubble formation and nanoparticle aggregation using MoS2 nanoparticles.

Understanding of how particles and light interact in a liquid environment is vital for optical and biological applications. MoS2 has been shown to enhance nonlinear optical phenomena due to the presence of a direct excitonic resonance. Its use in biological applications is predicated on knowledge of how MoS2 interacts with ultrafast (< 1 ps) pulses. In this experiment, the interaction between two femtosecond pulses and MoS2 nanoparticles suspended in liquid is studied. We found that the laser pulses induce bubble formation on the surface of a nanoparticle and a nanoparticle aggregate then forms on the surface of the trapped bubble. The processes of formation of the bubble and the nanoparticle aggregation are intertwined.

Enhanced subwavelength coupling and nano-focusing with optical fiber-plasmonic hybrid probe: erratum.

In this erratum, the funding section of our paper [Optics Express, 27, 38098- 38108 (2019)] has been updated.

Optical modulation in hybrid antiresonant hollow-core fiber infiltrated with vanadium dioxide phase change nanocrystals.

We present a study of optical modulation by the effect of temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO2) nanocrystals deposited in an antiresonance hollow-core fiber (AR-HCF). We fabricate such a VO2-coated fiber by embedding alkylsilane functionalized VO2 nanocrystals into the air holes of an AR-HCF. With this fiber, we achieve an optical loss modulation of ∼60% at a temperature above ∼53∘C over an ultrabroad spectral range that encompasses the S+C+L band.

Gate-tunable optical filter based on conducting oxide metasurface heterostructure.

A gate-tunable plasmonic optical filter incorporating a subwavelength patterned metal-insulator-metal metasurface heterostructure is proposed. An additional thin transparent conducting oxide (TCO) layer is embedded in the insulator layer to form a double metal-oxide-semiconductor configuration. Heavily n-doped indium tin oxide (ITO) is employed as the TCO material, whose optical property can be electrically tuned by the formation of a thin active epsilon-near-zero layer at the ITO-oxide interfaces. Full-wave electromagnetic simulations show that amplitude modulation and shift of transmission peak are achievable with 3-5 V applied bias, depending on the application. Moreover, the modulation strength and transmission peak shift increase with a thinner ITO layer. This work is an essential step toward a realization of next-generation compact photonic/plasmonic integrated devices.

Gate-Tunable Plasmon-Induced Transparency Modulator Based on Stub-Resonator Waveguide with Epsilon-Near-Zero Materials.

We demonstrate an electrically tunable ultracompact plasmonic modulator with large modulation strength (>10 dB) and a small footprint (~1 µm in length) via plasmon-induced transparency (PIT) configuration. The modulator based on a metal-oxide-semiconductor (MOS) slot waveguide structure consists of two stubs embedded on the same side of a bus waveguide forming a coupled system. Heavily n-doped indium tin oxide (ITO) is used as the semiconductor in the MOS waveguide. A large modulation strength is realized due to the formation of the epsilon-near-zero (ENZ) layer at the ITO-oxide interface at the wavelength of the modulated signal. Numerical simulation results reveal that such a significant modulation can be achieved with a small applied voltage of ~3V. This result shows promise in developing nanoscale modulators for next generation compact photonic/plasmonic integrated circuits.

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.

Aluminum plasmonic nanoshielding in ultraviolet inactivation of bacteria.

Ultraviolet (UV) irradiation is an effective bacterial inactivation technique with broad applications in environmental disinfection. However, biomedical applications are limited due to the low selectivity, undesired inactivation of beneficial bacteria and damage of healthy tissue. New approaches are needed for the protection of biological cells from UV radiation for the development of controlled treatment and improved biosensors. Aluminum plasmonics offers attractive opportunities for the control of light-matter interactions in the UV range, which have not yet been explored in microbiology. Here, we investigate the effects of aluminum nanoparticles (Al NPs) prepared by sonication of aluminum foil on the UVC inactivation of E. coli bacteria and demonstrate a new radiation protection mechanism via plasmonic nanoshielding. We observe direct interaction of the bacterial cells with Al NPs and elucidate the nanoshielding mechanism via UV plasmonic resonance and nanotailing effects. Concentration and wavelength dependence studies reveal the role and range of control parameters for regulating the radiation dosage to achieve effective UVC protection. Our results provide a step towards developing improved radiation-based bacterial treatments.

Rapid detection of drought stress in plants using femtosecond laser-induced breakdown spectroscopy.

Drought stress disrupts the balance of macro- and micronutrients and affects the yield of agriculturally and economically significant plants. Rapid detection of stress-induced changes of relative content of elements such as sodium (Na), potassium (K), calcium (Ca) and iron (Fe) in the field may allow farmers and crop growers to counter the effects of plant stress and to increase their crop return. Unfortunately, the analytical methods currently available are time-consuming, expensive and involve elaborate sample preparation such as acid digestion which hinders routine daily monitoring of crop health on a field scale. We report application of an alternative method for rapid detection of drought stress in plants using femtosecond laser-induced breakdown spectroscopy (LIBS). We demonstrate daily monitoring of relative content of Na, K, Ca and Fe in decorative indoor (gardenia) and cultivated outdoor (wheat) plant species under various degrees of drought stress. The observed differences in spectral and temporal responses indicate different mechanisms of drought resistance. We identify spectroscopic markers of drought stress which allow for distinguishing mild environmental and severe drought stress in wheat and may be used for remote field-scale estimation of plant stress resistance and health.

Gate-Tunable Conducting Oxide Metasurfaces.

Metasurfaces composed of planar arrays of subwavelength artificial structures show promise for extraordinary light manipulation. They have yielded novel ultrathin optical components such as flat lenses, wave plates, holographic surfaces, and orbital angular momentum manipulation and detection over a broad range of the electromagnetic spectrum. However, the optical properties of metasurfaces developed to date do not allow for versatile tunability of reflected or transmitted wave amplitude and phase after their fabrication, thus limiting their use in a wide range of applications. Here, we experimentally demonstrate a gate-tunable metasurface that enables dynamic electrical control of the phase and amplitude of the plane wave reflected from the metasurface. Tunability arises from field-effect modulation of the complex refractive index of conducting oxide layers incorporated into metasurface antenna elements which are configured in reflectarray geometry. We measure a phase shift of 180° and ∼30% change in the reflectance by applying 2.5 V gate bias. Additionally, we demonstrate modulation at frequencies exceeding 10 MHz and electrical switching of ±1 order diffracted beams by electrical control over subgroups of metasurface elements, a basic requirement for electrically tunable beam-steering phased array metasurfaces. In principle, electrically gated phase and amplitude control allows for electrical addressability of individual metasurface elements and opens the path to applications in ultrathin optical components for imaging and sensing technologies, such as reconfigurable beam steering devices, dynamic holograms, tunable ultrathin lenses, nanoprojectors, and nanoscale spatial light modulators.