Xizhe Liu

Structural color change in longhorn beetles Tmesisternus isabellae

The elytra of longhorn beetles Tmesisternus isabellae show iridescent golden coloration which stems from long and flat scales imbricated densely on the elytral surface. The scales are able to change coloration from golden in the dry state to red in the wet state with water absorption. Structural characterizations revealed that the iridescent coloration of scales originates from a multilayer in the scale interior. Measurements on both water contact angle and chemical composition indicated that scales are hydrophilic. The change in scale coloration to red in the wet state is due to both the swelling of the multilayer period and water infiltration. The unraveled structural color change and its strategy may not only help us get insight into the biological functionality of structural coloration but also inspire the designs of artificial photonic devices.

Pressure Controlled Self-Assembly of High Quality Three-Dimensional Colloidal Photonic Crystals

A concise pressure controlled isothermal heating vertical deposition (PCIHVD) method is developed, which provides an optimal growing condition with better stability and reproducibility for fabricating photonic crystals (PCs) without the limitation of colloidal sphere materials and sizes. High quality PCs are fabricated with PCIHVD from polystyrene spheres with diameters ranging from 200 nm to 1 mu m. The deep photonic band gap and steep photonic band edge of the samples are most favorable for realizing ultrafast optical devices, photonic chips, and communications. This method makes a meaningful advance in the quality and diversity of PCs and greatly promotes their wide applications.

Room temperature fabrication of porous ZnO photoelectrodes for flexible dye-sensitized solar cells

We fabricated ZnO photoelectrodes at room temperature by doctor-blading ZnO gel; the adequate interparticle connection and the effective ammonia activation process improved the flexible DSCs efficiency to 3.8% (under 100 mW cm(-2)).

Synthesis of large-area germanium cone-arrays for application in electron field emission

High-vacuum electron-beam evaporation was used to synthesize germanium (Ge) cone-arrays on N+-type (100) Si. The surface morphology of Ti nanocrystals’ catalyst and Ge cone-arrays was investigated by scanning electron microscopy and atomic force microscopy. The lattice constant of the Ge cones determined from x-ray diffraction was nearly identical to that of bulk Ge. The mechanism of Ge cone-arrays’ formation was investigated. The electron field emission properties of the as-prepared Ge cone-arrays were studied based on current–voltage measurements and the Fowler–Nordheim equation.

RAMAN SCATTERING CHARACTERIZATION OF THE CRYSTALLINE QUALITIES OF ZNSE FILMS GROWN ON S-PASSIVATED GAAS(100) SUBSTRATES

A comparative study of the Raman spectra of ZnSe films grown by molecular beam epitaxy on GaAs(100) substrates passivated by NH4)2)Sx and S2Cl2 solutions is presented. Based on the analysis of the line shape of the first‐order longitudinal‐optical phonon of ZnSe with spatial correlation model of Raman scattering, it is shown that the ZnSe films grown on the GaAs substrates passivated by S2Cl2 solutions have longer coherence lengths, which indicate that their crystalline qualities are better than those passivated by NH4)2Sx solutions. In addition, the barrier heights of ZnSe/GaAs interfaces for different S passivations have been obtained from the ratios of the intensity of the coupled longitudinal‐optical phonon‐plasmon mode to that of the longitudinal‐optical mode of GaAs Raman peak. The results show that the ZnSe/GaAs samples passivated by S2Cl2 solutions have lower density of interfacial states.

Study of strain in partially relaxed Ge epilayers on Si(100) substrate

Ge epilayers of different thicknesses are grown by molecular-beam epitaxy with Sb as a surfactant on Si(100) substrates. X-ray diffraction illustrates that these Ge thin films are partially strained, and the strains decrease gradually with increasing epilayer thickness. Raman spectra reveal a downward shift of the Ge–Ge mode peak as the epilayer thickness increases. In the regions of high strain, the relationship between the Raman shift of this mode and the strain in the partially relaxed samples is considerably different from the linear relationship reported before, which is mainly attributed to the spatial confinement effect of phonons in a nanocrystal.

Observation of the focusing of liquid surface waves

The transmission properties of liquid surface waves through a biconvex array of rigid cylinders, arranged in a two-dimensional periodic lattice, are studied experimentally and numerically. Both experimental and numerical results show that this biconvex array of rigid cylinders can act as a lens, which can focus plane liquid surface waves in a wide range of frequencies.

Spontaneous emission modulation of colloidal quantum dots via efficient coupling with hybrid plasmonic photonic crystal

The spontaneous emission of colloidal CdSe/ZnS quantum dots (CQDs) modified by the hybrid plasmonic-photonic crystal is reported in this paper. By using a spin coater, the spatial overlap between CQDs and the surface resonance modes in this quasi-2D crystal slab is achieved. In this case, the coupling efficiency of them is enhanced greatly and most excited CQDs radiate through the surface modes. Consequently, despite the low refractive index contrast of our hybrid structure, the directionality of spontaneous emission, increased radiative probability and narrowed full width at half maximum of emission peak are all clearly observed by our home-made microscopic angle-resolved spectroscopy and time-resolved photoluminescence system. Our results manifest that the quasi-2D hybrid plasmonic-photonic crystal is an ideal candidate to tailor the radiative properties of CdSe/ZnS CQDs, which might be significant for the applications of light emitting devices.

Symmetry breaking induced excitations of dark plasmonic modes in multilayer graphene ribbons

Multilayer graphene can support multiple plasmon bands. If structured into graphene ribbons, they can support multiple localized plasmonic modes with interesting optical properties. However, not all such plasmonic modes can be excited directly due to the constrains of the structural symmetry. We show by numerical simulations that by breaking the symmetry all plasmonic modes can be excited. We discuss the general principles and properties of two-layer graphene ribbons and then extend to multilayer graphene ribbons. In multilayer graphene ribbons with different ribbon widths, a tunable broadband absorption can be attained due to the excitations of all plasmonic modes. Our results suggest that these symmetry-broken multilayer graphene ribbons could offer more degrees of freedom in designing photonic devices.

Multilayer manipulated diffraction in flower beetles Torynorrhina flammea: intraspecific structural colouration variation

We report that the intraspecific structural colouration variation of the beetle Torynorrhina flammea is a result of diffraction shifting manipulated by a multilayer sub-structure contained in a three-dimensional (3D) photonic architecture. With a perpendicularly 2D quasiperiodic diffraction grating inserted into the multilayer, the 3D photonic structure gives rise to anticrossing bandgaps of diffraction from the coupling of grating and multilayer bands. The angular dispersion of diffraction induced by the multilayer band shift behaves normally, in contrast to the ultranegative behaviour controlled by the quasiperiodic grating. In addition, the diffraction wavelength is more sensitive to the multilayer periodicity than the diffraction grating constant, which explains the smart biological selection of T. flammea in its intraspecific colouration variation from red to green to blue. The elucidated mechanism could be advantageous for the potential exploration of novel dispersive optical elements.