Effect Of Interband Transitions Research Articles

Optical properties of liquid pure copper by density functional theory

The optical properties of pure liquid copper were investigated using density functional theory with the Quantum ESPRESSO package. The effects of structural changes were investigated by comparing the electron density of states and imaginary part of the dielectric function between the crystalline and liquid states with densities near the melting point. The results indicated that the effect of interband transitions remains in the structural changes near the melting point.

Plasmons in the van der Waals charge-density-wave material 2H-TaSe2

Plasmons in two-dimensional (2D) materials beyond graphene have recently gained much attention. However, the experimental investigation is limited due to the lack of suitable materials. Here, we experimentally demonstrate localized plasmons in a correlated 2D charge-density-wave (CDW) material: 2H-TaSe2. The plasmon resonance can cover a broad spectral range from the terahertz (40 μm) to the telecom (1.55 μm) region, which is further tunable by changing thickness and dielectric environments. The plasmon dispersion flattens at large wave vectors, resulted from the universal screening effect of interband transitions. More interestingly, anomalous temperature dependence of plasmon resonances associated with CDW excitations is observed. In the CDW phase, the plasmon peak close to the CDW excitation frequency becomes wider and asymmetric, mimicking two coupled oscillators. Our study not only reveals the universal role of the intrinsic screening on 2D plasmons, but also opens an avenue for tunable plasmons in 2D correlated materials.

Plasma frequency in doped highly mismatched alloys

Highly mismatched alloys (HMAs) have band structures strongly modified due to the introduction of the alloying element. We consider HMAs where the isolated state of the alloying element is near the host conduction band, which causes the conduction band to split into two bands. We determine the bulk plasma frequency when the lower-energy band is partially occupied, as by doping, using a semi-analytical method based on a disorder-averaged Green's function. We include the nontrivial effects of interband transitions to the higher-energy band, which limit the plasma frequency to be less than an effective band gap. We show that the distribution of states in the split bands causes plasmons in HMAs to behave differently than plasmons in standard metals and semiconductors. The effective mass of the lower split band $m^*$ changes with alloy fraction, and we find that the plasmon frequency with small carrier concentration $n$ scales with $\sqrt{n}/m^*$ rather than the $\sqrt{n/m^*}$ that is expected in standard materials. We suggest experiments to observe these phenomena. Considering the typical range of material parameters in this group of alloys and taking a realistic example, we suggest that HMAs can serve as highly tunable low-frequency plasmonic materials.

Investigation of the optical behavior of indium oxide thin films with the aid of spectroscopic ellipsometry technique

Indium oxide thin films were deposited at different substrate temperatures onto silicon substrates using pulsed laser deposition technique. An optical model consisting of Tauc-Lorentz and Gaussian oscillators along with a surface roughness layer on top was used to fit the spectroscopic ellipsometry data in the range of photon energy 1.3–5 eV. Effect of interband transitions occurring in the range 3.67–4.82 eV was ensured by a combination of four Gaussian oscillators. Energy of these transitions as well as the band gap retrieved from ellipsometry analysis increased with the increment in substrate temperature. Band gap obtained from diffuse reflectivity measurements using Kubelka-Munk method depicted an exactly similar type of variation. Further, band gaps retrieved from Ellipsometry (3.51–3.93 eV) were more close to the bulk indium oxide band gap value (3.6 eV) than that obtained from Kubelka-Munk method (2.63–3.06 eV). Atomic force microscopy measurements provided further support to our analysis by producing surface roughness values that were closely related to that obtained from modeling. Band gap variation with the substrate temperature was correlated to incremental grain growth in the films as evident from atomic force microscopy, field emission scanning electron microscope and X-ray analysis.

Magnetic circular dichroism of thiolate-protected plasmonic gold nanoparticles: separating the effects of interband transitions and surface magnetoplasmon resonance

Magneto-optical activity is demonstrated in thiolate-protected Au nanoparticles with magnetic circular dichroism (MCD) spectroscopy. The samples examined are decanethiolate-protected Au nanoparticles with the mean diameters ranging from 2.0 to 4.7 nm. The nanoparticles larger than 2.4 nm in diameter exhibit a derivative-like MCD signal, indicating the presence of two circular modes of surface magnetoplasmon, but the spectral shape is so asymmetric that its identification is rather difficult. This is due to the contribution of interband transitions occurring at around the localized surface plasmon resonance (LSPR) frequency. We then develop an efficient method to phenomenologically separate the effects of magnetoplasmonic intraband (= Drude) and interband transitions in the measured MCD spectra using an approximation that the optical response of the Au nanoparticle with a critical size (∼2.0 nm) for the disappearance of LSPR, which is also experimentally obtainable, is substantially dominated by the interband transitions. The consistency of the method is ensured for tiopronin-protected Au nanoparticles, and a very small bisignate magnetoplasmonic response hidden in the total MCD spectrum can be extracted. The practical advantage of the proposed method is that we can intuitively and effectively evaluate the characteristic features of the surface magnetoplasmon of thiolate-protected Au nanoparticles without performing complicated Mie or quasielectrostatic calculations.

Plasmon Enhanced Internal Quantum Efficiency of CdSe/ZnS Quantum Dots

The effects of surface plasmon oscillations on the radiative recombination rate and internal quantum efficiency of silver coated CdSe/ZnS quantum dots imbedded in quartz matrix is studied theoretically and numerically. The analysis is carried out by introducing the local field enhancement factor for the description of the dielectric function of the quantum dots together with the modified Drude model for the metal-coat that takes into account the effects of interband transitions and the size dependent damping parameter. It is found out that the internal quantum efficiency of photoluminescence emission of the metalcoated CdSe/ZnS quantum dots is enhanced by about 5-folds compared with the quantum dots without metal-coat. Moreover, increasing the metal fraction of the coated CdSe/ZnS quantum dots from 0.80 to 0.92 results in a significant increase of the intensity that is accompanied by a blue-shift of the quantum efficiency curves. The results obtained can be utilized in the design and fabrication of optoelectronic devices that require high intensity of photoluminescence emission.

Scattering of surface plasmons on graphene by a discontinuity in surface conductivity

The scattering of graphene surface plasmons from an arbitrary, one-dimensional discontinuity in graphene surface conductivity is treated analytically by an exact solution of the quasi-static integral equation for surface current density in the spectral domain. It is found that the reflection and transmission coefficients are not governed by the Fresnel formulas obtained by means of the effective medium approach. Furthermore, the reflection coefficient generally exhibits an anomalous reflection phase, which has so far only been reported for the particular case of reflection from abrupt edges. This anomalous phase becomes frequency-independent in the regime where the effect of inter-band transitions on graphene conductivity is negligible. The results are in excellent agreement with full-wave electromagnetic simulations, and can serve as a basis for the analysis of inhomogeneous graphene layers with a piecewise-constant conductivity profile.

Optimum Surface Plasmon Excitation and Propagation on Conductive Two-Dimensional Materials and Thin Films

The surface conductivity of two-dimensional (2-D) materials and thin conductive films is considered for surface plasmon (SP) excitation and propagation. It is shown that an ideal surface conductivity exists to maximize the SP field at a given position, based on a tradeoff relating to propagation loss and near-field excitation amplitude associated with the local density of photonic states. Dispersionless and Drude dispersion models are considered, as well as the effect of interband transitions. Simple formulas are presented to obtain a maximal SP field at a given distance from a canonical source. Examples are shown for graphene and thin metal films, and a discussion of the competition between propagation loss and SP excitation is provided.

Plasmon Line Width in Liquid Metals

With the recent development of synchrotron radiation sources and the instrumentations, inelastic X-ray scattering (IXS) has become an important tool for observing plasmons in materials. One of the significant advantages of IXS is that it can be applied to liquid samples, which is considerably difficult for electron energy loss spectroscopy. Thus far, the IXS study of plasmons in liquid samples has been performed for several systems such as liquid lithium, liquid aluminum, or lithium ammonia solutions. However, the properties of plasmons in the liquid phase are less clearly understood than those in the crystalline phase. One of such properties is the plasmon line width. For the solid state, the finite line width observed in experiments can be evaluated using the formulations derived in previous theoretical studies, in which the effect of interband transitions, phonon-assisted intraband (or interband) transitions, and the excitations of two electron–hole (e–h) pairs are considered. These effects should also play an important role in liquid, because a finite line width is also observed in liquid samples. The effects of phonon and two e–h pairs are readily extended to liquid systems, since these effects can be calculated without using information on the ionic structure. On the other hand, the effect of interband transitions is difficult to extend to the liquid state, because it strongly depends on the ionic structure. In this work, we show the derivation of a formula for evaluating the plasmon line width at the zero momentum transfer, q 1⁄4 0, in metallic liquid. The formula describes how the ionic structure influences the plasmon line width for metallic liquid. The application of the formula to liquid alkali metals is also shown. The plasmon line width at q 1⁄4 0, E1=2ð0Þ, is written as E1=2ð0Þ 1⁄4 h !p Im ð!pÞ; ð1Þ

Shape-Templated Growth of Au@Cu Nanoparticles

We report the formation of copper nanoparticles with various morphologies and low polydispersity, using Au nanoparticles as templates. This seeded growth strategy is based on the reduction of Cu2+ with hydrazine in water at low temperature. Additionally, the use of poly(acrylic acid) as capping agent allows synthesis under aerobic conditions. The dimensions of the resulting Au@Cu nanoparticles can be readily tuned through either the dimensions of the Au cores or the Cu/Au molar ratio. Although Au and Cu show a significant lattice mismatch, epitaxial growth of Cu onto single crystal Au nanorods was confirmed through high-resolution electron microscopy and electron diffraction analysis. The effects of core morphology on the optical properties of the core–shell nanoparticles were analyzed by vis-NIR spectroscopy and were found to agree with simulations based on the boundary element method. This work contributes to understand the strong effect of interband transitions on the optical response of Au@Cu and to confirm the importance of tuning the localized surface plasmon resonance away from the interband transitions.

Simulation of the absorption spectra of nanometallic Al particles with core–shell structure: size-dependent interband transitions

Nanoaluminum combined with an oxidizing polymer binder is representative of a new class of nanotechnology energetic materials termed “structural energetic materials” that can be laser initiated by near-infrared heating of the Al particles. The visible and near-IR absorption spectra of Al nanoparticles passivated by the native oxide Al2O3, embedded in nitrocellulose (NC) binder, are simulated numerically using a model for the metallic dielectric function that incorporates the effects of interband transitions. The effects of oxide thickness, nanoparticle size and size distribution, and particle shape on the absorption characteristics are investigated. The nanoparticle spectra evidence an absorption peak and valley in the 550–1,100 nm range that redshift with decreasing nanoparticle size. Calculations indicate that this peak-valley structure results from interband transitions, and the unusual redshift cannot be explained without using an interband transition onset frequency that varies with nanoparticle size.

Strong oscillations detected by picosecond ultrasonics in silicon: Evidence for an electronic-structure effect

We present the results of picosecond ultrasonic experiments performed at different laser wavelengths in different samples all grown on silicon (100) substrates. In the blue range we report on strong oscillations in the reflectivity curves. We present experimental and numerical results that identify the origin of these oscillations in a strong acousto-optic interaction in the silicon substrate. Then we propose to explain the high amplitude of these oscillations by a relation with direct interband transitions in silicon which fall in the same range. This explanation is supported by two other experimental evidences: First, sudden changes in the phase of the detected signal are observed as the laser wavelength is tuned around the interband transitions. Second, we report on a shift of this phase change as the substrate is heated. This study shows that the effect of interband transitions on picosecond ultrasonic studies we had first demonstrated in thin metallic films can be extended to semiconductors.

Collective modes and Raman scattering in one dimensional electron systems

In this paper, we review recent development in the theory of resonant inelastic light (Raman) scattering in one-dimensional electron systems. The particular systems we have in mind are electron doped GaAs based semiconductor quantum wire nanostructures, although the theory can be easily modified to apply to other one-dimensional systems. We compare the traditional conduction-band-based non-resonant theories with the full resonant theories including the effects of interband transitions. We find that resonance is essential in explaining the experimental data in which the single particle excitations have finite spectral weights comparable to the collective charge density excitations. Using several different theoretical models (Fermi liquid model, Luttinger liquid model, and Hubbard model) and reasonable approximations, we further demonstrate that the ubiquitously observed strong single particle excitations in the experimental Raman spectra cannot be explained by the spinless multi-spinon excitations in the Luttinger liquid description. The observability of distinct Luttinger liquid features in the Raman scattering spectroscopy is critically discussed.

Strong effect of interband transitions in the picosecond ultrasonics response of metallic thin films

We report on the strong laser wavelength effects in the shape of acoustical pulses generated and detected in thin metallic films using ultrashort optical pulses. The experiments have been performed on aluminum and copper. In both cases, we have observed a sign change of the acoustical part in the detected signals in the vicinity of a well-known interband transition. We identify the origin of the phenomenon in the detection mechanism. These results let us demonstrate the role picosecond ultrasonics may play in the study of the electronic structure of thin films.

Investigation of quantum effects on the Bloch electron velocity around closely spaced bands at high electric fields

In conventional Monte Carlo simulations of electron and hole transport in semiconductors, the possibility of electric-field induced interband transitions between scattering events is not taken into account. In this article we study the effects of interband transitions on the electron (or hole) velocity and on the distance traveled by the charge carrier. The resulting quantum velocity, defined as the expectation value of the velocity operator for a state which is a superposition of Bloch states, contains an average group velocity and a quantum interference term. We calculate these terms for two cases, namely, for a simple parameterized two-band k⋅p model and for hole motion in wurtzite GaN, and compare with the one-band group velocity used in the conventional Monte Carlo method. The results show that interband transitions around closely spaced energy bands, which may arise in crystals with relatively large unit cells, like wurtzite GaN and hexagonal and rhombohedral SiC polytypes, may have a major effect on the particle velocity and particle trajectory, and therefore constitute a major influence on, for instance, electrical breakdown.

Absolute intensity measurements of the optical second-harmonic response of metals from 0.9 to 2.5 eV

The absolute intensity of the optical second-harmonic response and its spectral (ωfund≈0.9–2.5 eV) dependence has been measured for Ag(111), polycrystalline Ag, 4-Aminothiophenol/Ag (4-ATP/Ag) and decanethiol/Ag (DT/Ag) surfaces in contact with a liquid electrolyte. Preliminary spectra are also reported for polycrystalline Au and Cu(111) samples. For second-harmonic energies below the plasmon resonance, the magnitude of the nonlinear optical response of clean Ag samples increases as electrode potentials are made more positive. This trend reverses itself for energies above the plasmon resonance. The adsorbate-covered surfaces show a weak or nonexistent potential dependence. A unique feature is found in the 4-ATP/Ag spectra which could possibly be due to a surface charge-transfer state. The Ag results are discussed in the context of a free-electron response from which the spectral and potential dependence of the complex microscopic parameter, a(ω), are extracted. The features in the Au and Cu(111) spectra are not adequately described by this free-electron model and must be related to the effects of interband transitions on the nonlinear optical response.

C-Axis Transport in a Normal-State Bilayer Cuprate and Relation to Pseudogap

We study the effect of interband transitions on the normal-state optical conductivity, dc resistivity, and thermal conductivity along the c-axis, for a plane-chain bilayer cuprate coupled by a perpendicular hopping matrix element (t⊥). When t⊥ is small, the c-axis dc resistivity shows a characteristic upturn as the temperature is lowered, and the c-axis optical conductivity develops a pseudogap at low frequencies. As t⊥ is increased, intraband transitions start to dominate and a more conventional response is obtained. Similar pseudogap behavior is predicted in the thermal conductivity for which strong depression at low temperature is found. Analytical results for a simple plane-plane bilayer are also given, including the frequency sum rule of the optical conductivity.

Effect of interband transitions on c-axis penetration depth of layered superconductors

The electromagnetic response of a system with two planes per unit cell involves, in addition to the usual intraband contribution, an added interband term. These transitions affect the temperature dependence and the magnitude of the zero-temperature c-axis penetration depth (${\ensuremath{\lambda}}_{\mathrm{c}}$). When the interplane hopping ${\mathrm{t}}_{\mathrm{\ensuremath{\perp}}}$ is sufficiently small, the interband transitions dominate the low-temperature behavior of \ensuremath{\lambda}(T) which then does not reflect the linear temperature dependence of the intraband term and in comparison becomes quite flat even for a d-wave gap. It is in this regime that the pseudogap was found in our previous normal-state calculations of the c-axis conductivity, and the effects are connected.

Interband contribution to the long-wavelength damping of the surface plasmon.

The effect of interband transitions on the long-wavelength damping of the surface plasmon for simple metals is evaluated with use of first-order degenerate perturbation theory. The calculated values are compared with recent experimental results for alkali metals.

Electron-hole recombination in narrow-band-gap Hg1-xCdxTe and stimulated emission of LO phonons.

We investigate the dielectric properties of narrow-band-gap ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Te under the resonant condition of the band gap ${\mathit{E}}_{\mathit{g}}$ matching the longitudinal-optic (LO) phonon energy \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathrm{L}}$. The effect of interband transitions, which are now also induced by LO phonons, is to renormalize the mixed LO-phonon--plasmon mode downward in energy by \ensuremath{\sim}20%. We evaluate the gain in the mixed mode under conditions of population inversion and find that the stimulated emission of the LO-phonon--plasmon mixed mode from electron-hole recombination requires a pumping level of about 1--10 kW/${\mathrm{cm}}^{2}$. The feasibility of a stimulated-LO-phonon-emission laser is considered.