The nonlinear responses of different materials provide useful mechanisms for optical switching, low noise amplification, and harmonic frequency generation. However, the nonlinear processes usually have an extremely weak nature and require high input power to be excited. To alleviate this severe limitation, we propose new designs of ultrathin nonlinear metasurfaces composed of patterned graphene micro-ribbons to significantly enhance third harmonic generation (THG) at far-infrared and terahertz (THz) frequencies. The incident wave is tightly confined and significantly boosted along the surface of graphene in these configurations due to the excitation of highly localized plasmons. The bandwidth of the resonant response becomes narrower due to the introduction of a metallic substrate below the graphene micro-ribbons, which leads to zero transmission and standing waves inside the intermediate dielectric spacer layer. The enhancement of the incident field, combined with the large nonlinear conductivity of graphene, can dramatically increase the THG conversion efficiency by several orders of magnitude. In addition, the resonant frequency of the metasurface can be adjusted by dynamically tuning the Fermi energy of graphene via electrical or chemical doping. As a result, the third harmonic generated wave can be optimized and tuned to be emitted at different frequencies without the need to change the nonlinear metasurface geometry. The proposed nonlinear metasurfaces provide a new way to realize compact and efficient nonlinear sources at the far-infrared and THz frequency ranges, as well as new frequency generation and wave mixing devices which are expected to be useful for nonlinear THz spectroscopy and noninvasive THz imaging applications.

In this work, industrial grade sepiolite (IG-SEP) was premodified with hydrochloric acid, and then premodified sepiolite (SEP) was impregnated with triethylenetetramine (TETA) to create a novel TETA functionalized SEP adsorbent. The effects of TETA loading and adsorption temperature on CO2 adsorption capacity, as well as the CO2 adsorption isotherm and cyclic regenerability were investigated. Results show that, when SEP was optimized at 30 wt % TETA (SEP-TETA-30%), the adsorbent attained a CO2 adsorption capacity as high as 1.93 mmol/g at 50 °C with fast adsorption kinetics and good regenerability. The Freundlich model was best able to fit the experimental adsorption isotherm, and the isosteric heat of adsorption at high CO2 coverage was 28 kJ/mol. The presence of moisture in simulated gas has a positive effect on the adsorption capacity, with less harm to adsorption stability. The adsorbent samples were further characterized by N2 adsorption/desorption, scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The increased surface area and pore volume of the SEP allow for effective immobilization of TETA within its channels. A kinetic study demonstrated that the experimental adsorption data for SEP-TETA-30% were well fitted by the pseudo-second-order model. More importantly, an index of cost efficiency was proposed to evaluate the practicability and competitiveness of amine-impregnated adsorbents. The cost efficiency of SEP-TETA-30% adsorbent was estimated as 0.19 mol CO2/$, which was higher than reported adsorbents, suggesting that the TETA functionalized SEP (SEP-TETA-30%) composite is a promising adsorbent for cyclic postcombustion CO2 capture processes.

High input intensities are usually required to efficiently excite optical nonlinear effects in ultrathin structures due to their extremely weak nature. This problem is particularly critical at low terahertz (THz) frequencies because high input power THz sources are not available. The demonstration of enhanced nonlinear effects at THz frequencies is particularly important since these nonlinear mechanisms promise to play a significant role in the development and design of new reconfigurable planar THz nonlinear devices. In this work, we present a novel class of ultrathin nonlinear hybrid planar THz devices based on graphene-covered plasmonic gratings exhibiting very large nonlinear response. The robust localization and enhancement of the electric field along the graphene monolayer, combined with the large nonlinear conductivity of graphene, can lead to boosted third harmonic generation (THG) and four-wave mixing (FWM) nonlinear processes at THz frequencies. These interesting nonlinear effects exhibit very high nonlinear conversion efficiencies and are triggered by realistic input intensities with relative low values. In addition, the THG and FWM processes can be significantly tuned by the dimensions of the proposed hybrid structures, the doping level of graphene, or the input intensity values, whereas the nonlinear radiated power remains relatively insensitive to the incident angle of the excitation source. The presented nonlinear hybrid graphene-covered plasmonic gratings have a relative simple geometry and, as a result, can be used to realize efficient third-order nonlinear THz effects with a limited fabrication complexity. Several new nonlinear THz devices are envisioned based on the proposed hybrid nonlinear structures, such as frequency generators, all-optical signal processors, and wave mixers. These devices are expected to be useful for nonlinear THz spectroscopy, noninvasive THz subwavelength imaging, and THz communication applications.

DNA self‐assembly is a powerful tool to arrange optically active components with high accuracy in a large parallel manner. A facile approach to assemble plasmonic antennas consisting of two metallic nanoparticles (40 nm) with a single colloidal quantum dot positioned at the hot spot is presented here. The design approach is based on DNA complementarity, stoichiometry, and steric hindrance principles. Since no intermediate molecules other than short DNA strands are required, the structures possess a very small gap (≈ 5 nm) which is desired to achieve high Purcell factors and plasmonic enhancement. As a proof‐of‐concept, the fluorescence emission from antennas assembled with both conventional and ultrasmooth spherical gold particles is measured. An increase in fluorescence is obtained, up to ≈30‐fold, compared to quantum dots without antenna.

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