Y. X. Zheng

Broadband optical absorption by tunable Mie resonances in silicon nanocone arrays

Nanostructure arrays such as nanowire, nanopillar, and nanocone arrays have been proposed to be promising antireflection structures for photovoltaic applications due to their great light trapping ability. In this paper, the optical properties of Si nanopillar and nanocone arrays in visible and infrared region were studied by both theoretical calculations and experiments. The results show that the Mie resonance can be continuously tuned across a wide range of wavelength by varying the diameter of the nanopillars. However, Si nanopillar array with uniform diameter exhibits only discrete resonance mode, thus cant achieve a high broadband absorption. On the other hand, the Mie resonance wavelength in a Si nanocone array can vary continuously as the diameters of the cross sections increase from the apex to the base. Therefore Si nanocone arrays can strongly interact with the incident light in the broadband spectrum and the absorbance by Si nanocone arrays is higher than 95% over the wavelength from 300 to 2000u2005nm. In addition to the Mie resonance, the broadband optical absorption of Si nanocone arrays is also affected by Wood-Rayleigh anomaly effect and metal impurities introduced in the fabrication process.

Electronic and optical properties of novel carbon allotropes

Abstract The vibrational properties, electronic structures and optical properties of novel carbon allotropes, such as monolayer penta-graphene (PG), double-layer PG and T12-carbon, were studied by first-principles calculations. Results of phonon calculations demonstrate that these exotic carbon allotropes are dynamically stable. The bulk T12 phase is an indirect-gap semiconductor having a quasiparticle (QP) bandgap of ∼5.19xa0eV. When the bulk material transforms to a two-dimensional (2D) phase, the monolayer and double-layer PG become quasi-direct gap semiconductors with smaller QP bandgaps of ∼4.48xa0eV and ∼3.67xa0eV, respectively. Furthermore, the partial charge density analysis indicates that the 2D phases retain part of the electronic characteristics of the T12 phase. The linear photon energy-dependent dielectric functions and related optical properties including refractive index, extinction coefficient, absorption spectrum, reflectivity, and energy-loss spectrum were also computed and discussed. Additionally, the chemical stability of monolayer PG and the electronic and optical properties of double-side hydrogenated monolayer PG were also investigated. The results obtained from our calculations are beneficial to practical applications of these exotic carbon allotropes in optoelectronics and electronics.

High efficiency of photon-to-heat conversion with a 6-layered metal/dielectric film structure in the 250-1200 nm wavelength region

The optical properties and thermal stability of a 6-layered metal/dielectric film structure are investigated in this work. A high optical absorption average of > 98% is achieved in the broad spectral range of 250-1200 nm with experiment results, in good agreement with our simulated results. The samples have a typical layered structure of: SiO(2)(57.3 nm)/Ti(5.7 nm)/SiO(2) (67.1 nm)/Ti(11.6 nm)/SiO(2)(51.4 nm)/Cu(>100 nm), deposited on optically polished Si or K9-glass substrates by magnetron sputtering. The sample of the 6-layered metal/dielectric film structure has an AM1.5G solar absorptance of 95.5% with the features of low thermal emittance of 0.136 at 700K and good thermal stability, and will be potentially suitable for practical application in high-efficiency solar absorber devices in many fields.

The effect of interference on magneto-optics in magneto-optical layered structures

In this article the magneto-optics of magneto-optical (M-O) layered structures have been studied theoretically and experimentally. For the air/M-O/air configuration, an analytic expression between the apparent complex Faraday rotation and the eigenvalue φF=e1φF′ can be obtained, when the M-O layers are semitransparent and weakly magnetic. The interference factor e1 is a function of the optical constants and the M-O layer thickness d, and the light wavelength. In these structures, the apparent Faraday rotation consists of two parts. One oscillates as a function of the M-O layer thickness and the other is proportional to the layer thickness. The oscillation period and the amplitude are determined by the optical constants of the M-O layers. For the air/M-O/reflector configuration, the Kerr rotation φk oscillates as a function of the M-O layer thickness and approaches a constant as the thickness d→∞. If the M-O layers are semitransparent and weakly magnetic, the apparent Kerr rotation can be expressed as φk=e2φF′. For ultrathin metallic magnetic bilayered films the Kerr rotation is proportional to the M-O layer thickness and the enhancement factor is a function of the optical constants of the M-O layer and NM reflector. The magneto-optics of a Co spinel ferrite film, Co/Cu, Fe-Ni/Cu, and Co/Si structures have been studied experimentally.In this article the magneto-optics of magneto-optical (M-O) layered structures have been studied theoretically and experimentally. For the air/M-O/air configuration, an analytic expression between the apparent complex Faraday rotation and the eigenvalue φF=e1φF′ can be obtained, when the M-O layers are semitransparent and weakly magnetic. The interference factor e1 is a function of the optical constants and the M-O layer thickness d, and the light wavelength. In these structures, the apparent Faraday rotation consists of two parts. One oscillates as a function of the M-O layer thickness and the other is proportional to the layer thickness. The oscillation period and the amplitude are determined by the optical constants of the M-O layers. For the air/M-O/reflector configuration, the Kerr rotation φk oscillates as a function of the M-O layer thickness and approaches a constant as the thickness d→∞. If the M-O layers are semitransparent and weakly magnetic, the apparent Kerr rotation can be expressed as φ...

High photon-to-heat conversion efficiency in the wavelength region of 250–1200 nm based on a thermoelectric Bi 2 Te 3 film structure

In this work, 4-layered SiO2/Bi2Te3/SiO2/Cu film structures were designed and fabricated and the optical properties investigated in the wavelength region of 250–1200u2009nm for their promising applications for direct solar-thermal-electric conversion. A typical 4-layered film sample with the structure SiO2 (66.6u2009nm)/Bi2Te3 (7.0u2009nm)/SiO2 (67.0u2009nm)/Cu (>100.0u2009nm) was deposited on a Si or K9-glass substrate by magnetron sputtering. The experimental results agree well with the simulated ones showing an average optical absorption of 96.5%, except in the shorter wavelength region, 250–500u2009nm, which demonstrates the superior absorption property of the 4-layered film due to the randomly rough surface of the Cu layer resulting from the higher deposition power. The high reflectance of the film structure in the long wavelength region of 2–20u2009μm will result in a low thermal emittance, 0.064 at 600u2009K. The simpler 4-layered structure with the thermoelectric Bi2Te3 used as the absorption layer may provide a straightforward way to obtain solar-thermal-electric conversion more efficiently through future study.

Study of optical properties of metallic Aux(SiO2)1−x and Nix(SiO2)1−x films

Abstract To study the optical properties of the metal–insulator granular films, a series of Ni x (SiO 2 ) 1− x and Au x (SiO 2 ) 1− x granular films was made with a variation of x and the annealing process. The complex dielectric function was measured in the 1.5–4.8-eV photon energy range. New spectral features were found in the dielectric functions. For Au x (SiO 2 ) 1− x samples, a new peak in the e 2 spectra is seen around 2xa0eV, especially for x =0.67. The e 1 spectra change continuously from positive to negative in the low photon energy range, resulting in a true zero of e 1 associated with the plasmon occurring at 2.0–2.5xa0eV and at about 3.6xa0eV for the Au- and Ni-based films, respectively. Compared to previous data, the experimental results obtained in this work provide richer information to understand better the optical properties of the metal-granular films that are not well predicted by the theoretical model. The effect will be useful in the metallic multilayer structure design and applications.

STUDIES OF OPTICAL AND ELECTRONIC PROPERTIES IN CO/AG MULTILAYERS

We have studied the electronic structure and optical properties, in the photon energy range of 3.0–4.5 eV, of sputtered Co/Ag multilayers with a scanning ellipsometer and Kerr spectrometer. It is found for the multilayer series Co(1.2 nm)/Ag and Co/Ag with dCo/dAg=2/3 that the screened plasma frequency ωp* of the multilayers and the interband transition frequency ω0 for L3→L2′ (d band to Fermi surface near the L point) in the Ag layers shift towards lower energy with increasing Ag layer thickness or modulation wavelength. For the second series of samples, the Kerr rotation peak also shifts towards lower energy with increasing modulation wavelength. For a third series of Co/Ag(1.3 nm) multilayers, only ω0 moves towards lower energy with an increasing Co layer thickness. The variation of ω0 is not easily explained in terms of internal strains and changes of the lattice constants in the Ag layers. Further theoretical analysis is needed.

Evolution of short- and medium-range order in the melt-quenching amorphization of Ge2Sb2Te5

Phase-change memory takes advantage of the fast phase transition between amorphous and crystalline phases of phase-change materials (e.g., Ge2Sb2Te5 or GST). To date, while the “SET” process (crystallization of GST glass) has been intensively studied, studies on the “RESET” process (melt-quenching amorphization of GST) are still limited. In this work, we explored the structural changes of GST upon rapid cooling by ab initio molecular dynamics simulations and atomistic cluster alignment (ACA) analysis. Different from other methods which only focus on the nearest bonding atoms, the ACA method can study both the short- and medium-range order clusters containing atoms beyond the first-neighboring shell and enables us to explore the changes of cluster structures in a larger scale. The results reveal that low-coordinated octahedral clusters tend to become high-coordinated ones, and Ge-centered octahedral structures change to tetrahedrons whereas Sb-centered tetrahedrons transform to octahedral structures during the amorphization process. Interestingly, tetrahedrons show aggregation in liquid and supercooled liquid in contrast to 6-fold octahedrons which present notable aggregation in amorphous GST. Moreover, our study showed that wrong bonds (Ge–Ge, Sb–Sb, Ge–Sb and Te–Te bonds) can promote the formation of large rings, and irreducible rings tend to separate into smaller and larger rings as the temperature is decreased. Our findings provide useful insights into the formation process and the structure of amorphous GST, which is valuable for facilitating the application of phase change materials.

Ferromagnetic resonance studies of noble metals based sandwiches

In this paper we have prepared Fe/Cu, Ag, Au/Fe sandwiches by ion beam sputtering and studied the interlayer coupling behavior by ferromagnetic resonance technique. In these sandwiches the Fe and noble metal layers are polycrystalline in the textures of (110) and (111), respectively. It is found that the in‐plane resonance field and the linewidth oscillate as a function of nonmagnetic layer thickness in a period of about 1.0 nm for Fe/Cu, Au/Fe structures, and 1.4 nm for Fe/Ag/Fe system. It is suggested that the interlayer coupling strength between the ferromagnetic layers oscillates in the same period with the resonance field. An in‐plane anisotropy was found in Fe/Ag/Fe system.

Magnetic and magneto-optic properties in CoNb/Pd and FeSi/Cr multilayers

Abstract In this paper the magnetic and magneto-optical properties of Coue5f8Nb Pd and Feue5f8Si Cr multilayers are studied. For Coue5f8Nb/Pd(56 A) multilayers, with decreasing magnetic layer thickness the saturation magnetization M s is the same for polar Kerr rotation θ k and ellipticity ϵ k decrease. For Coue5f8Nb(15 A)/Pd Feue5f8Si(19 A)/Cr multilayers with thin non-magnetic layers M s and θ k oscillate with non-magnetic layer thickness. The oscillation period of M s is the same for these two series of samples. With non-magnetic layer thickness further increasing, M s and θ k both approach constants.