Ultraviolet Frequency Region Research Articles

First principles study on optical and thermal properties of BaTiS3

Electronic, optical and thermal properties of hexagonal perovskite structure of BaTiS3, are studied, by means of the density functional theory (DFT) implemented on CASTEP, Material Studio, computer program. The data is optimized with generalized gradient approximation (GGA) and exchange correlation function called local density approximation (LDA), within the Broyden-Goldfarb-Shanno (BFGS) scheme of minimization and Koelling-Harmon relativistic treatment. Structure of BaTiS3, which we studied, crystallizes in the space group P63/mmc. Our results for structural, electronic and thermal properties of BaTiS3 are reported for the first time in comparison with experimental and other theoretical results wherever these are available. The electronic band structure reveals p-type semiconducting behavior and major contribution occurs from S-3p orbitals in Valence band and the conduction band is basically constructed from Ti-3d states. Reflectivity of BaTiS3 is also founded to be higher within visible and in ultraviolet region up to 25 eV, which indicates the potentiality of this material to be utilized as a jacket to elude solar heating. Coefficient of absorption spectra of BaTiS3 reveals the conducting nature of this material. The negative values of ε1(ω) is additionally observed,which is obvious indication of Drude like behavior seen in metals. Index of refraction is relatively higher within the infrared section and steadily lowered within the visible and ultraviolet frequency region. Analysis for conductivity also favors the photoconductivity with small photon energy. Thermo dynamical properties reported for the first time with Debye temperature of 1000 K.

Free electrons excited SPASER.

Surface plasmon amplification by stimulated emission of radiation (SPASER) is discovered and used for realizing lasers at nanometer scale. The conventional gain media that are applied in SPASER are solid materials, such as organic dye or semiconductor, which limits the frequency range of SPASER. The free electrons could be considered as a kind of gain medium for emitting radiation. Here, we investigate theoretically the SPASER, which is excited by free electrons. We also demonstrate the tunable, deep-ultraviolet, and ultracompact laser numerically by having free electrons interact with surface plasmon polariton mode supported on metal surface. The output power density could reach about 30 W/μm2 and the wavelength in deep ultraviolet could be widely tuned by varying the electron energy. This work offers a way of realizing integrated free electron laser in the ultraviolet frequency region.

Enantiomer-Selective Molecular Sensing Using Racemic Nanoplasmonic Arrays.

Building blocks of life show well-defined chiral symmetry which has a direct influence on their properties and role in Nature. Chiral molecules are typically characterized by optical techniques such as circular dichroism (CD) where they exhibit signatures in the ultraviolet frequency region. Plasmonic nanostructures have the potential to enhance the sensitivity of chiral detection and translate the molecular chirality to the visible spectral range. Despite recent progress, to date, it remains unclear which properties plasmonic sensors should exhibit to maximize this effect and apply it to reliable enantiomer discrimination. Here, we bring further insight into this complex problem and present a chiral plasmonic sensor composed of a racemic mixture of gammadions with no intrinsic CD, but high optical chirality and electric field enhancements in the near-fields. Owing to its unique set of properties, this configuration enables us to directly differentiate phenylalanine enantiomers in the visible frequency range.

Thin film (6,5) semiconducting single-walled carbon nanotube metamaterial absorber for photovoltaic applications

A wide-band (6,5) single-walled carbon nanotube metamaterial absorber design with near unity absorption in the visible and ultraviolet frequency regions for solar cell applications is proposed. The frequency response of the proposed design provides wide-band with a maximum of 99.2% absorption. The proposed design is also simulated with (5,4), (6,4), (7,5), (9,4), and (10,3) chiralities, and results are compared to show that the proposed design works best with (6,5) carbon nanotube (CNT) but also good for other chiral CNTs in the visible and ultraviolet frequency region. The geometric structure was carefully analyzed for its contribution to the absorption behavior. The absorber design is highly flexible and capable of keeping the wide-band with high absorption. Due to the excellent symmetric characteristics of the proposed design, which provides polarization independency under normal incidence (transverse electromagnetic mode), the proposed metamaterial absorber is a good candidate for the solar cell application, where absorbance can be kept high with respect to the polarization angle.

Polarization independent triple-band (5,4) semiconducting carbon nanotube metamaterial absorber design for visible and ultraviolet regions

Various metamaterial absorber designs operating in the microwave, infrared, visible, and ultraviolet frequency regions have been proposed in the literature. However, only a few studies have been done on the metamaterials that absorb in both visible and ultraviolet solar spectra. A triple-band polarization-insensitive metamaterial absorber structure with semiconducting single-walled carbon nanotube as the dielectric layer is proposed to efficiently absorb the incident electromagnetic radiations in visible and ultraviolet frequency regions. A unit cell of this design comprises three basic components in the form of metal–semiconductor–metal layers. The metallic part of the structure is aluminum, and the (5,4) single-walled carbon nanotube is used as the semiconducting material. The electromagnetic response of the proposed design is numerically simulated in the visible and ultraviolet regions with the maximum absorption rates of 99.75% at 479.4 THz, 99.94% at 766.9 THz, and 97.33% at 938.8 THz with corresponding skin depths of 13.0, 12.8, and 12.9 nm, respectively. Thus, solar cells based on this metamaterial absorber can offer nearly perfect absorption in the suggested frequency regions. The simple configuration of the design provides flexibility to control geometric parameters to be used in the solar cell and possesses the capability to be rescaled for other solar spectrum.

Coherent and tunable radiation with power enhancement from surface plasmon polaritons**Project supported by the National Basic Research Program of China (Grant No. 2014CB339801), the National Natural Science Foundation of China (Grant Nos. 61231005, 11305030, and 612111076), and the National High Technology Research and Development Program of China (Grant No. 2011AA010204).

Surface plasmon polaritons excited by an electron beam can be transformed into coherent and tunable light radiation waves with power enhancement in the simple structure of a metal film with a dielectric medium loading. In this paper, the process of the radiation transformation of this radiation, and the dependencies of the radiation characteristics on the parameters of the structure and the electron beam are studied in detail. The radiation power enhancement is greatly influenced by the beam energy and the film thickness in the infrared to ultraviolet frequency region. Up to 122 times radiation power enhancement and 6.5% radiation frequency tuning band can be obtained by optimizing the beam energy and the parameters of the film.

Deep-ultraviolet Raman scattering studies of monolayer graphene thin films

We present joint experimental and theoretical investigations of the deep-ultraviolet Raman scattering spectra of monolayer graphene thin films. We found that upon a 266nm laser excitation, while the G mode remains pronounced, the G′ mode is not observed. The G′ mode exhibits distinctive linewidth broadening towards the ultraviolet frequency region. The peak intensity ratio of the G′ and G modes increases dramatically excited at 355, 532, and 785nm wavelengths. All of the experimental findings are in good agreement with the theory calculated by the tight-binding method. We theoretically show that the integrated intensity of the G′ mode is inversely proportional to the incident photon energy, whereas that of the G mode increases as the fourth power of the incident photon energy. These results extend our understanding of the double resonance Raman scattering process of graphene away from the Dirac cones and provide the foundation for future technologically important developments of graphene in the deep-ultraviolet frequency range.

Optical properties of nitrogen-doped graphene thin films probed by spectroscopic ellipsometry

Nitrogen-doped graphene thin films were prepared by either chemical vapor deposition (CVD) or electrochemical exfoliation (ECE). Their optical properties were determined in the spectral region of 0.73–6.42eV and at temperatures between 200 and 350K by spectroscopic ellipsometry. The parameters of the dispersive structures were derived by numerical fitting of the experimental data to the stacked layer model. The optical absorption spectrum of the CVD-grown thin films is characterized by an asymmetric Fano resonance in the ultraviolet frequency region. In contrast, the line shape of the ECE-grown thin films displays less asymmetric. The excitonic resonance of the nitrogen-doped thin films is overall blue shifted by ~0.2–0.3eV compared with that of undoped analog. We interpret these results in terms of the exothermic nature of triazine molecule adsorption due to binding to graphene's surface via electron rich nitrogen.

Optical conductivity of multilayer graphene

The optical conductivity of graphene monolayer and multilayers are theoretically studied taking into account of the full dispersion, yielding results that are valid up to the ultraviolet region. Compared to the optical conductivity of monolayer graphene, extra peak structures appear in both the infrared and ultraviolet frequency regions for the multilayer case, which is understood as a combined effect of van Hove singularities in the energy band structures and optical transition selection rules. The number of ultraviolet peaks is equal to the number of layers. The number of infrared peaks in an even-layered graphene is equal to one half of the layer number. In odd-layered graphene, due to the existence of a monolayer-like subband, the number of infrared peaks is equal to that in the corresponding even-layered graphene with one layer less.

Band gaps of two-dimensional photonic crystal structure using conjugated polymer (3-octylthiophenes)

The possibility of using conjugated polymer (3-octylthiophenes, P3OT) as two-dimensional (2D) photonic band gap crystals was investigated. The different aspects were examined of the absolute photonic band gap (PBG) formation for 2D photonic crystals (PCs) consisting of P3OT pillars in air. The formation of PBG was exhibited and confirmed by a calculation of transfer matrix method (TMM). It was found that P3OT triangular structure could be good candidate for absolute inhibition of reflection in ultraviolet frequency region for given orientation.

Ab initiorates of multiphoton ionization of the isoelectronic species H,He+, andLi2+in the vuv and xuv frequency regions

Ab initio theoretical investigation of two- and three-photon ionization of the isoelectronic species H, He${}^{+}$, and Li${}^{2+}$ in the vacuum ultraviolet and x-ray ultraviolet frequency regions, is carried out using the fully nonperturbative Floquet-coupled-channel method. We present the results for the total ionization rates in the wavelength region between approximately 260 to 10 nm, and in the intensity domain between about ${10}^{10}$ and ${10}^{17}$ W/cm${}^{2}$, depending on the member of the isoelectronic species investigated. Comparison with the perturbation theoretical results at moderate intensities is made, a corresponding scaling law is tested, and the finite values of the rates of ionization at the resonance frequencies are obtained. The dependence of the rates on the intensity of the field shows the presence of the so-called ``stability windows,'' for all three species investigated here. The role of polarization on the ionization rates shows a general dominance of the linear polarization over the circular polarization in the intensity and frequency domains investigated, for all the three species concerned.

Superconductivity-type fluctuations in the electronic spectra of charge-transfer organic solids

The interaction between an excited electron and a large-radius exciton (Wannier-Mott exciton) has been studied in the visible and ultra-violet frequency regions for a two-level system of a molecular solid. Using a self-consistent approach, it is shown that at low temperatures and when certain conditions prevail, a bound-state may exist arising from the electron-exciton pairing. The excitation spectrum is found to be of the superconductivity type and the electron- exciton quasiparticle migrates through the crystal with definite energy and wave vector. The gap function due to the electron-exciton pairing is calculated at zero temperature and then the expression for the transition temperature is established. A formula for the ground-state energy is derived corresponding to the electron-exciton pairing and a discussion of the parameters that appear in the theory is presented.

Point defects in reduced strontium titanate

Strontium titanate single crystals have been carefully reduced at temperatures ranging from 1200 to 1400°C in oxygen partial pressures of 10 −7−10 −12 atm and quenched to room temperature where optical absorption coefficients, Hall coefficients, mass densities and lattice parameters were measured. (Optical absorption coefficients and Hall coefficients were also measured at 77 K). The predominant defects at room temperature were found to be doubly ionized oxygen vacancies and conduction band electrons. No evidence was found for singly ionized oxygen vacancies as has been suggested by some workers. Optical absorption in the visible and ultraviolet frequency region was found to be dependent upon the concentration of free carriers. For photon energies of 0.5 eV to about 1 eV, a free carrier absorption effect occurs which may be due to intraband transitions. Three peaks occur at photon energies of 1.66, 2.44, and 2.95 eV and may be due to phonon assisted interband transitions among the titanium d-like conduction bands.