Large Local Density Of States Research Articles

Coupling between atoms and localized modes in a defect-free quasiperiodic photonic crystal

In this study, the evolution spectrum and emission dynamics of atoms in a defect-free quasiperiodic photonic crystal are studied. Quasiperiodic photonic crystal have advantages over periodic photonic crystal in terms of abundant and strong localized modes due to the quasiperiodic and self-approximation structures of quasiperiodic photonic crystal. The localized modes exhibit extremely large local density of states and high-quality factors in the quasiperiodic photonic crystal. The frequencies of the large local density of states of the localized modes do not depend on the polarization and the position in a particular area in the quasiperiodic photonic crystals. Due to the extremely large local density of states in the defect-free quasiperiodic photonic crystal, strong coupling between the atom and the localized mode in the quasiperiodic photonic crystal can easily occur. Correspondingly, the population of the excited state displays Rabi oscillation, and the emission spectrum of the atom splits into two peaks. Moreover, the Rabi oscillation and the evolution spectrum of the dressed atom depend on the radiation linewidth.

Manipulating the light-matter interactions in plasmonic nanocavities at 1 nm spatial resolution

The light-matter interaction between plasmonic nanocavity and exciton at the sub-diffraction limit is a central research field in nanophotonics. Here, we demonstrated the vertical distribution of the light-matter interactions at ~1 nm spatial resolution by coupling A excitons of MoS2 and gap-mode plasmonic nanocavities. Moreover, we observed the significant photoluminescence (PL) enhancement factor reaching up to 2800 times, which is attributed to the Purcell effect and large local density of states in gap-mode plasmonic nanocavities. Meanwhile, the theoretical calculations are well reproduced and support the experimental results.

Giant radiative thermal rectification using an intrinsic semiconductor film

Rectification of heat flow via a thermal diode is not only of fundamental interest, but can also enable a range of novel applications in thermal management and energy conversion. However, despite decades of extensive research, large rectification ratios of practical importance have yet to be demonstrated. Here, we theoretically achieve giant rectification ratios (3 to almost 5 orders of magnitude) by leveraging near-field radiative thermal transport between two parallel planes. Guided by a rational design approach centering on the electromagnetic local density of states (LDOS), we employ a thin film of an intrinsic semiconductor—such as silicon—as one terminal of our radiative thermal diodes, which provides the necessary nonlinearity and a substantial LDOS contrast as the temperature bias is flipped. For the other terminal, we explore two kinds of materials which either serve as a narrowband or a broadband filter, both capable of converting the large LDOS contrast into giant thermal rectification. We further consider representative multilayer configurations in order to block backside radiation and improve mechanical stability. All the designs are expected to perform well over a wide range of film thicknesses, gap sizes, and temperatures. Our work offers an opportunity for realizing thermal diodes with unprecedented rectification ratios.

Effect of B and O doping on the electronic structure and quantum capacitance of carbon nitride monolayers using first-principles calculations

The structural, electronic, and capacitance properties of B- or O-doped carbon nitride monolayers were systematically investigated using first-principles calculations. Different single-atom substitutions (i.e., B or O dopant on a Cx or Ny substitution site) were considered for this work. The substitution site plays an important role in regulating the stability and electronic structure of carbon nitride monolayers. B or O doping could make carbon nitride monolayers produce large local density of states near Fermi level contributed mainly from the hybridization of the 2p states of C, N, and the doped atom (B or O), thus significantly improving conductivity, quantum capacitance, and surface charge density of the structures. The results show that the quantum capacitances of the B-doped carbon nitride monolayers are much greater than those of the B-doped graphene monolayers. Furthermore, B-doped C3N at the C1 site, B-doped tg-C3N4 at the N2 site, and O-doped tg-C3N4 at the N1 site are strongly recommended as the electrodes in symmetrical supercapacitors, while the other doped components could also be used as cathode or anode materials in asymmetrical supercapacitors. The findings of this study suggest that doped carbon nitride structures could be considered as promising electrode materials for supercapacitors.

A DFT study on the austenitic Ni2MnGa (001) surfaces

Abstract Through the use of spin-polarized density functional theory calculations we have studied the different (001) surface terminations of austenitic Ni2MnGa. The stability analysis revealed a preference to form MnGa terminated surfaces in a large range of growth conditions, Ni-terminated surfaces are stable as well. The dominant structure, is the one engineered to be Ga-terminated, but that appears under Ga-rich conditions. STM images were simulated for all surface terminations and compared with the images obtained experimentally by Leicht et al. New J. Phys. 13 (2011) 033021. We found a perfect match between theory and experiment for MnGa and Ni terminations. From the STM images of the MnGa termination, the Mn atoms are strongly imaged, while the Ga atoms are weak. Whereas for the Ni terminated surface, there is a large local density of states due to the Ni atoms with an appreciably contribution due to the second monolayer Mn atoms. Magnetic characterization revealed surface effects on the MnGa and Ni terminated surfaces, whereas the Ga terminated experienced no change. Our results give insight about what is observed in the experimental STM images.

Cooperative interactions between nano-antennas in a high-Q cavity for unidirectional light sources

We analyse the resonant mode structure and local density of states in high-Q hybrid plasmonic-photonic resonators composed of dielectric microdisks hybridized with pairs of plasmon antennas that are systematically swept in position through the cavity mode. On the one hand, this system is a classical realization of the cooperative resonant dipole–dipole interaction through a cavity mode, as is evident through predicted and measured resonance linewidths and shifts. At the same time, our work introduces the notion of ‘phased array’ antenna physics into plasmonic-photonic resonators. We predict that one may construct large local density of states (LDOS) enhancements exceeding those given by a single antenna, which are ‘chiral’ in the sense of correlating with the unidirectional injection of fluorescence into the cavity. We report an experiment probing the resonances of silicon nitride microdisks decorated with aluminium antenna dimers. Measurements directly confirm the predicted cooperative effects of the coupled dipole antennas as a function of the antenna spacing on the hybrid mode quality factors and resonance conditions.

High-Temperature Polaritons in Ceramic Nanotube Antennas.

High-temperature thermal photonics presents unique challenges for engineers as the database of materials that can withstand extreme environments are limited. In particular, ceramics with high temperature stability that can support coupled light-matter excitations, that is, polaritons, open new avenues for engineering radiative heat transfer. Hexagonal boron nitride (hBN) is an emerging ceramic 2D material that possesses low-loss polaritons in two spectrally distinct mid-infrared frequency bands. The hyperbolic nature of these frequency bands leads to a large local density of states (LDOS). In 2D form, these polaritonic states are dark modes, bound to the material. In cylindrical form, boron nitride nanotubes (BNNTs) create subwavelength particles capable of coupling these dark modes to radiative ones. In this study, we leverage the high-frequency optical phonons present in BNNTs to create strong mid-IR thermal antenna emitters at high temperatures (938 K). Through direct measurement of thermal emission of a disordered system of BNNTs, we confirm their radiative polaritonic modes and show that the antenna behavior can be observed even in a disordered system. These are among the highest-frequency optical phonon polaritons that exist and could be used as high-temperature mid-IR thermal nanoantenna sources.

Enhanced Quantum Dot Spontaneous Emission with Multilayer Metamaterial Nanostructures

The Purcell effect of quantum dot (QD) spontaneous emission with Ag-SiO2 multilayer metamaterial nanostructures has been demonstrated in experiment and simulation. A broadband enhanced spontaneous emission rate of QDs is observed due to large local density of states in the epsilon-near-zero and hyperbolic regions of multilayer structures. Multilayer gratings are utilized to further enhance the QD spontaneous emission as the QDs located inside the grating grooves strongly interact with high-k coupled surface plasmon polariton modes. Photoluminescence decay measurements are in good agreement with both analytical treatment with a nonlocal effect and three-dimensional finite-element simulation. Detailed studies of QD position and polarization effects on emission rate enhancement for multilayer and multilayer grating nanostructures provide important insight for understanding the coupling mechanisms of emitter–multilayer interaction and the engineering of local density of states in metamaterial nanostructures. ...

Dipole plasmon resonance induced large third-order optical nonlinearity of Au triangular nanoprism in infrared region

Au triangular nanoprisms with strong dipole plasmon absorption peak at 1240 nm were prepared by wet chemical methods. Both numerical calculations and experiments were carried out to investigate the optical properties of the samples. Finite difference time domain (FDTD) and Local Density of States (LDOS) calculations demonstrate that strong electric field enhancement and large LDOS can be obtained at tip areas of the Au triangular nanoprisms. Z scan techniques were used to characterize the nonlinear absorption, nonlinear refraction, as well as one- and two-photon figures of merit (W and T, respectively) of the sample. The results show that maximum nonlinear refractive index can be obtained around the resonance absorption wavelength of 1240 nm, detuning the wavelength from the absorption peak will lead to the decrease of the nonlinear refractive index n(2), while the nonlinear absorption coefficient β doesn't change much with the wavelength. This large wavelength dependence of n(2) and small change of β enable the sample to satisfy the all-optical switching demand of W> 1 and T< 1 easily in a large wavelength range of 1200-1300 nm. These significant nonlinear properties of the sample imply that Au triangular nanoprism is a good candidate for future optical switches in infrared optical communication wavelength region.

Exact Calculation of Local Density of States in Two-Dimensional Photonic Crystals

An exact calculation method of local density of states (LDOS) in two-dimensional (2D) photonic crystals (PCs) is presented. In order to calculate the LDOS, the eigen-equation of magnetic field is first solved by the plane-wave expansion method, then the eigen-modes of electric-field are obtained. There are two different ways to solve the eigen-equantion of magnetic field and three different ways to obtain the eigen-modes of the electric-field. In comparison of the numerical results from these different ways, an exact and fast method for calculating the LDOS in PCs is found. With use of this method, we investigate the LDOS of the 2D PCs consisting of a triangular lattice of cylinders. The results show the large LDOS is favorable to reside in higher dielectric-constant medium in high frequency region, rather than in lower dielectric-constant medium.

Photoconductivity in nanocrystalline GaN and amorphous GaON

In this work we present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a-GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a-GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, we build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid our interpretation of this data set, we are able to characterize the density of states in the gap for the two materials.

Engineering infrared emission properties of silicon in the near field and the far field

We study numerically the thermal emission of highly doped silicon surfaces. We show that by modifying the doping, we can tune the frequency of emission. We also show that by taking advantage of the large local density of states due to surface-plasmon polaritons, radiative properties in the far field can be drastically modified. It is possible to realize a spatially partially coherent thermal source as well as a quasi-isotropic source. Finally, we study the radiative transfer between two doped silicon bodies. Surface-plasmon polaritons produce an enhancement of several orders of magnitude of the radiative energy transfer at nanometric distances.

NMR Studies of Silicon Clathrate Compounds

NMR spectra of 29Si and 137Ba in silicon clathrate compounds, NaxBaySi46, NaxSi136 and Ba8 T xSi46 (where T = Au, Ag, and Cu) are reported. We found that the local density of states at the Fermi level increases with rising superconducting transition temperature. The silicon Sl site was suggested to hybridize with Ba-orbital in the superconducting clathrate and has a large local density of states.

Photoexcited carrier effect on scanning tunneling microscopy of zinc-phthalocyanine and coronene molecules on Si(100)2 × 1 surfaces

Photocurrent images of zinc-phthalocyanine (ZnPc) and coronene molecules adsorbed on Si(100)2 × 1 surfaces have been obtained by scanning tunneling microscopy. The photocurrent images of molecules mainly reflect the contribution of the surface photovoltage in proportion to the local density of states (LDOS). In the photocurrent observations the ZnPc molecules are seen as bright images, indicating the large LDOS at the bias voltage. On the other hand, the images of coronene molecules show a wide variety of contrasts which depend strongly on the adsorption site against the Si dimer rows. This dissimilarity may be attributed to the difference in the tunneling probability of the photoexcited carriers, a difference which reflects the variation of the surface electronic state formed by a molecule and Si dimers.

Structural, optical, and electronic properties of magnetron-sputtered platinum oxide films

Stable platinum oxide films have been prepared through magnetron sputtering and have been analyzed on the bases of energy-sensitive microanalyses, x-ray diffraction, resistivity, and optical reflectance measurements. The complex dielectric function has been determined for various oxygen contents in the film covering the wave-number regime 50 cm−1–λ−1–50 000 cm−1. The vibrational properties are dominated through a strong band, centered at 765 cm−1, associated with a asymmetric stretching mode of the Pt—O bond. The films are amorphous, with chemical composition PtOx, where 1&amp;lt;x&amp;lt;2.1, and are considered as a homogeneous solid solution of PtO and PtO2. The materials system displays a conductor–insulator transition at x≥2, in connection with an optical band gap Eg of ∼1.2 eV in the fully oxidized state. The conduction mechanism over the whole range of compositions is thermally activated and is determined through a large density of localized states extending into the band gap. At x&amp;lt;2 the optical gap disappears, consistent with the semimetallic behavior of the materials system for this range of composition.

Perpendicular Magnetic Anisotropy of Metallic Multilayers Composed of Magnetic Layers Only: Ni/Co and Ni/Fe Multilayers

The magnetic anisotropy energies of multilayers which include only magnetic elements, Ni/Co and Ni/Fe, are calculated from first principles. Both systems show perpendicular anisotropy but the anisotropy energy is somewhat smaller than that of well-studied systems such as Pd/Co. The LDOS (local density of states) of Ni is as large as that of Co(Fe) at the Fermi level and contributes directly to perpendicular anisotropy, which means that the magnetic moments of Ni are also normal to the film plane. The anisotropy cannot be explained only in terms of the electronic structure of Co(Fe) near the Fermi level but that of Ni must also be considered. The origin of the perpendicular anisotropy is attributed to a large LDOS of |m| =2 character of Co(Fe) and Ni minority spin near the Fermi energy.

Transport in atomically engineered BiSrCaCuO multilayers

We have utilized atomic layer deposition to synthesize single crystal thin films of cuprate superconductors, as well as a variety of superlattices and multilayer heterostructures with atomically abrupt interfaces. For example, we made trilayer structures with the top and the bottom BiSrCaCuO electrodes separated by titanate barriers that were only 4 A thick and yet free of pinholes over macroscopic areas. This unique synthetic capability made it possible to study transport in thec-axis direction under controlled and systematically varied conditions. Taken together, these experiments provide a picture of thec-axis transport in 2201, 2212, and various heterostructures under study. In particular, they suggest presence of a large density of localized states near the Fermi level and thus support a two-component description of the electron system in these compounds.

Surface reconstruction and interface formation in Si and GaAs

We investigated the effects of substrate surface reconstruction upon the interface formation between In and Ge overlayers and Si(111)2 × 1, Si(111)7 × 7, and GaAs(110) substrates. Photoemission spectroscopy with synchrotron radiation was used to monitor the evolution of the interface parameters—Schottky barrier height or band discontinuities—as a function of the overlayer thickness. All interfaces involving Si(111)7 × 7 substrates exhibited a characteristic evolution pattern with saturation at 1.4–1.8 monolayers coverage. The interfaces involving Si(111)2 × 1 substrates instead exhibited faster evolution saturating below a monolayer coverage with marked differences between individual cleaved surfaces. A similarly rapid evolution was observed for the interfaces involving GaAs(110) substrates. These results indicate that the spectrum of intrinsic surface states is different for Si(111)2 × 1 and for Si(111)7 × 7. In particular for Si(111)7 × 7 a large density of localized states exists near the Fermi level.

Analysis of the thermally stimulated capacitor-discharge method for characterizing localized states in amorphous semiconductors

A detailed analysis of the method of thermally stimulated capacitor discharge (TSCD) is presented. Distinction is made between the dielectric relaxation currents and the trap-limited currents. The position, shape, and number of the depolarization peaks caused by the dielectric relaxation currents are shown to depend (i) on the conductivity of the sample as a function of temperature and electric field and (ii) on the geometry of the sample. It is pointed out that the behavior of the dielectric-relaxation-current peaks is quite similar to the trap-limited-current peaks, and an analysis of the peaks may lead to false conclusions, if done without distinguishing them from one another. We assert that no new information about the trap parameters can be obtained from TSCD, unless the structure of the polarization and depolarization curves are different. No such difference was found in the chalcogenide glass studied by us, and the observed TSCD could be explained in terms of dielectric relaxation currents only. We also point out that a low drift mobility, a short screening length, and possibly a large density of localized states near the Fermi level, make the observation of trap-limited TSCD in multicomponent chalcogenides unlikely.