Graphene-based metamaterials have been theoretically demonstrated as an enabler for applications as perfect absorbers, photodetectors, light emitters, modulators, and tunable spintronic devices. However, challenges associated with conventional film deposition techniques have made the multilayered metamaterial difficult to fabricate, which have severely limited experimental validations. Herein, the experimental demonstration of the phototunable graphene-based multilayered metamaterials on diverse substrates by a transfer-free, solution-phase deposition method is presented. The optical properties of the metamaterials are tuned dynamically by controllable laser-mediated conversion from graphene oxide layers into graphene counterparts, which exhibit different degrees of conversion, which would offer huge potential for devices design and fabrication. The converted graphene layers present comparable (within 10%) optical conductivity to their chemical vapor deposited analogues. Moreover, laser patterning leads to functional photonic devices such as ultrathin flat lenses embedded in the lab-on-chip device, which maintains consistency and exhibits subwavelength focusing resolution in aqueous environments without any noticeable degradation compared with the original lens. This graphene-based metamaterial provides a new experimental platform for broad applications in on-chip integrated photonic, biomedical, and microfluidic devices.

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An ideal solar-thermal absorber requires efficient selective absorption with a tunable bandwidth, excellent thermal conductivity and stability, and a simple structure for effective solar thermal energy conversion. Despite various solar absorbers having been demonstrated, these conditions are challenging to achieve simultaneously using conventional materials and structures. Here, we propose and demonstrate three-dimensional structured graphene metamaterial (SGM) that takes advantages of wavelength selectivity from metallic trenchlike structures and broadband dispersionless nature and excellent thermal conductivity from the ultrathin graphene metamaterial film. The SGM absorbers exhibit superior solar selective and omnidirectional absorption, flexible tunability of wavelength selective absorption, excellent photothermal performance, and high thermal stability. Impressive solar-to-thermal conversion efficiency of 90.1% and solar-to-vapor efficiency of 96.2% have been achieved. These superior properties of the SGM absorber suggest it has a great potential for practical applications of solar thermal energy harvesting and manipulation.

High-quality continuous (GO) thin films are prepared by a self-assembly method. Z-scan measurements during the laser-induced reduction process unveil in situ nonlinear responses in the GO film. Third-order nonlinear responses of the GO film can be tuned dynamically by varying the laser input fluence. GO thin films with tunable nonlinear responses and versatile patterning opportunities by using direct laser writing may serve as promising solid-state materials for novel nonlinear functional devices.

Recently plasmonic effects have gained tremendous interest in solar cell research because they are deemed to be able to dramatically boost the efficiency of thin-film solar cells. However, despite of the intensive efforts, the desired broadband enhancement, which is critical for real device performance improvement, has yet been achieved with simple fabrication and integration methods appreciated by the solar industry. We propose in this paper a novel idea of using nucleated silver nanoparticles to effectively scatter light in a broadband wavelength range to realize pronounced absorption enhancement in the silicon absorbing layer. Since it does not require critical patterning, experimentally these tailored nanoparticles were achieved by the simple, low-cost and upscalable wet chemical synthesis method and integrated before the back contact layer of the amorphous silicon thin-film solar cells. The solar cells incorporated with 200 nm nucleated silver nanoparticles at 10% coverage density clearly demonstrate a broadband absorption enhancement and significant superior performance including a 14.3% enhancement in the short-circuit photocurrent density and a 23% enhancement in the energy conversion efficiency, compared with the randomly textured reference cells without nanoparticles. Among the measured plasmonic solar cells the highest efficiency achieved was 8.1%. The significant enhancement is mainly attributed to the broadband light scattering arising from the integration of the tailored nucleated silver nanoparticles.

COMMUNICATIONTo demonstrate effi cient radiative coolers, selective IR emitters have been extensively studied. [1][2][3][4][5][6][7][8][9][10][11] In particular, composite materials, [ 2,9 ] white pigmented paints, [ 5,8 ] SiO fi lms, [ 6,7,10 ] and polymeric materials [ 3,11 ] are demonstrated to possess IR emission within the atmospheric transparency window. However, almost all of these materials either lack near-unity emission or broadband emission within the entire 8-13 μm window. [ 1,2,[5][6][7][8][9][10] In addition, signifi cant IR absorption outside the transparency window, where the atmosphere is highly emissive, also restricts the materials to cool down well below the ambient temperature. [2][3][4][5]11 ] On the other hand, artifi cial metallic nanostructures, such as, plasmonic nanostructures, [12][13][14][15][16][17][18] and metallic photonic crystals [19][20][21][22] possess highly selective IR optical absorptions. However, in most cases, their absorption spectra are diffi cult to optimize for wide-band absorption. Metamaterials, on the contrary, can provide both selective and broadband IR absorption. [23][24][25][26][27] Recently, multilayer metal-dielectric anisotropic metamaterials have been demonstrated to possess intriguing optical properties. [ 25,[28][29][30] By employing dispersive properties and anisotropy, ultra-broadband, spectrally selective and polarization sensitive absorption was achieved in the visible to microwave frequencies. Here, for the fi rst time, we propose the use of anisotropic metamaterials toward the application of highly effi cient radiative cooling. To achieve an ideal thermal emitter, we design and demonstrate a microstructure consisted of an array of symmetrically shaped conical metamaterial (CMM) pillars leading to a near unity absorption of unpolarized light. By selectively matching the thermal emission (absorption) to the entire 8-13 μm atmospheric transparency window, the CMM structure can possess a practical radiative cooling power of 116.6 W m −2 .Our design concept of an elementary metal-dielectric CMM pillar consists of alternating layers of aluminum and germanium as depicted in Figure 1 a. Each of the metal and dielectric layers maintains the circular symmetry along the vertical axis and the diameters of the layers decrease gradually from bottom to top which gives the structure a conical shape. The thickness of the aluminum layer is 30 nm and the thickness of the germanium layer is 110 nm. The top and bottom diameters of the CMM pillars are defi ned t and b . The substrate of the CMM structure is set to 150 nm thick aluminum which is optically thick enough to diminish any IR transmission through the substrate. The dispersive permittivity of aluminum is defi ned from reference [ 31 ] and the permittivity of germanium is 16. The periodicity of the CMM pillars, p is set to 1.3 times b , providing a suffi cient gap between the adjacent CMM pillars to avoid any proximity effects. [ 29 ] The aspect ratio, s , of t and b is optimized to 0.6 and seven periods of metal...

Nanometric flat lenses with three-dimensional subwavelength focusing are indispensable in miniaturized optical systems. However, they are fundamentally challenging to achieve because of the difficulties in accurately controlling the optical wavefront by a film with nanometric thickness. Based on the unique and giant refractive index and absorption modulations of the sprayable graphene oxide thin film during its laser reduction process, we demonstrate a graphene oxide ultrathin (∼200 nm) flat lens that shows far-field three-dimensional subwavelength focusing (λ3/5) with an absolute focusing efficiency of >32% for a broad wavelength range from 400 to 1,500 nm. Our flexible graphene oxide lenses are mechanically robust and maintain excellent focusing properties under high stress. The simple and scalable fabrication approach enables wide potential applications in on-chip nanophotonics. The wavefront shaping concept opens up new avenues for easily accessible, highly precise and efficient optical beam manipulations with a flexible and integratable planar graphene oxide ultrathin film.

The 2H-to-1T' phase transition in transition metal dichalcogenides (TMDs) has been exploited to phase-engineer TMDs for applications in which the metallicity of the 1T' phase is beneficial. However, phase-engineered 1T'-TMDs are metastable; thus, stabilization of the 1T' phase remains an important challenge to overcome before its properties can be exploited. Herein, we performed a systematic study of the 2H-to-1T' phase evolution by lithiation in ultrahigh vacuum. We discovered that by hydrogenating the intercalated Li to form lithium hydride (LiH), unprecedented long-term (>3 months) air stability of the 1T' phase can be achieved. Most importantly, this passivation method has wide applicability for other alkali metals and TMDs. Density functional theory calculations reveal that LiH is a good electron donor and stabilizes the 1T' phase against 2H conversion, aided by the formation of a greatly enhanced interlayer dipole-dipole interaction. Nonlinear optical studies reveal that air-stable 1T'-TMDs exhibit much stronger optical Kerr nonlinearity and higher optical transparency than the 2H phase, which is promising for nonlinear photonic applications.