Intersubband transitions in n-doped multi-quantum-well semiconductor heterostructures make it possible to engineer one of the largest known nonlinear optical responses in condensed matter systems--but this nonlinear response is limited to light with electric field polarized normal to the semiconductor layers. In a different context, plasmonic metasurfaces (thin conductor-dielectric composite materials) have been proposed as a way of strongly enhancing light-matter interaction and realizing ultrathin planarized devices with exotic wave properties. Here we propose and experimentally realize metasurfaces with a record-high nonlinear response based on the coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered electronic intersubband transitions in semiconductor heterostructures. We show that it is possible to engineer almost any element of the nonlinear susceptibility tensor of these structures, and we experimentally verify this concept by realizing a 400-nm-thick metasurface with nonlinear susceptibility of greater than 5 × 10(4) picometres per volt for second harmonic generation at a wavelength of about 8 micrometres under normal incidence. This susceptibility is many orders of magnitude larger than any second-order nonlinear response in optical metasurfaces measured so far. The proposed structures can act as ultrathin highly nonlinear optical elements that enable efficient frequency mixing with relaxed phase-matching conditions, ideal for realizing broadband frequency up- and down-conversions, phase conjugation and all-optical control and tunability over a surface.

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A novel class of visible-light-activated TiO 2 photocatalysts were prepared by direct hydrolysis of tetrabutyl titanate through iodine-doping. When calcination temperature is at 673 K, these nanoparticles (mean diameter of ∼5 nm) show stronger absorption in the 400-550 nm range with a red shift in the band gap transition and significantly higher photocatalytic activity than pure TiO 2 prepared by the same procedure and Degussa P-25 titania nanoparticles in aqueous phenol solution under visible light irradiation (λ > 400 nm). Furthermore, I-doped TiO 2 (673 K) still showed pronounced photocatalytic activity under UV and visible light irradiation.

Low-dimensional metal halides have recently attracted extensive attention owing to their unique structure and photoelectric properties.H erein, we report the colloidal synthesis of all-inorganic low-dimensional cesium copper halide nanocrystals (NCs) by adopting ah ot-injection approach.U sing the same reactants and ligands,b ut different reaction temperatures,b oth 1D CsCu 2 I 3 nanorods and 0D Cs 3 Cu 2 I 5 NCs can be prepared. Density functional theory indicates that the reduced dimensionality in 1D CsCu 2 I 3 compared to 0D Cs 3 Cu 2 I 5 makes the excitons more localized, which accounts for the strong emission of 0D Cs 3 Cu 2 I 5 NCs. Subsequent optical characterization reveals that the highly luminescent, strongly Stokes-shifted broadband emission of 0D Cs 3 Cu 2 I 5 NCs arises from the self-trapped excitons.O ur findings not only present am ethod to control the synthesis of low-dimensional cesium copper halide nanocrystals but also highlight the potential of 0D Cs 3 Cu 2 I 5 NCs in optoelectronics. Scheme 1. Colloidal synthesis of cesium copper halide nanocrystals.Supportinginformation and the ORCID identification number for one of the authors of this article can be found under: https://doi.

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.

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We report a simple technique that allows obtaining mid-infrared absorption spectra with nanoscale spatial resolution under low-power illumination from tunable quantum cascade lasers. Light absorption is detected by measuring associated sample thermal expansion with an atomic force microscope. To detect minute thermal expansion we tune the repetition frequency of laser pulses in resonance with the mechanical frequency of the atomic force microscope cantilever. Spatial resolution of better than 50 nm is experimentally demonstrated.

spatial profi le. Gradient metasurfaces provide a much richer control of the wavefront in both local amplitude and phase of the emerging transmitted and refl ected beams. [4][5][6][7] Such structures enable fl at optical components for beam focusing, polarization control and phase correction, to name a few examples. [4][5][6][7][8][9] The next challenge and exciting perspective for this technology consists in enabling real-time reconfi gurability of the metasurface platform with a fast response time, which may produce fl at optical components for rapid wavefront modulation, phase tuning and beam steering. [ 7 ] A number of methods to tune the spectral response of plasmonic nanoresonators have been reported in the recent past, based on thermal, mechanical, optical, and electrical control, as summarized in the literature. [ 10 ] Electrical tuning techniques are of particular interest, since they open a route to on-chip integration of metasurfaces with electronics, potentially enabling GHz-level switching speeds. The most common electrical tuning mechanisms reported so far are based on phasechange media, [11][12][13] the use of liquid crystals, [ 14,15 ] and carrier concentration control on a semiconductor substrate [ 16,17 ] or graphene. [18][19][20][21][22][23] Approaches based on phase-change media and liquid crystals rely on intrinsically slow physical processes and cannot produce metasurfaces with nanosecond switching time. Carrier-concentration control produces metasurfaces with much faster switching speeds with the best results in terms of tuning range and switching speeds achieved with hybrid metal−graphene structures, demonstrating spectral tuning with switching speeds up to 30 MHz, [ 23 ] limited by the RC (circuit resistance × circuit capacitance) time constant of the biasing circuit.It has recently been suggested and experimentally demonstrated by our team [ 24,25 ] as well as by Brener's group in Sandia Labs [26][27][28] that metasurfaces made of plasmonic nanoresonators polaritonically coupled [29][30][31][32] to intersubband transitions in multi-quantum-well (MQW) semiconductor heterostructures engineered for large quantum confi ned Stark effect [33][34][35] may display voltage-tunable optical response. Here we demonstrate that these polaritonic metasurfaces may provide one of the fastest electrical switching of optical response demonstrated in metasurfaces to date. Our structures utilize well-established InGaAs/AlInAs MQW semiconductor technology and demonstrate comparable absorption modulation speed compared to hybrid metal−graphene structures while using lower bias Electrically tunable mid-infrared metasurfaces with nanosecond response time and broad tuning range are reported. Electrical tuning is achieved by employing strong polaritonic coupling of electromagnetic modes in metallic nanoresonators with voltage-tunable inter-subband transitions in semiconductor heterostructures, tailored for a giant quantum-confi ned Stark effect. Experimentally, a 220-nm-thick multi-quantum-well semiconductor layer...