Intraband Transitions Research Articles

RPA Plasmons in Graphene Nanoribbons: Influence of a VO2 Substrate.

We study the effect of the phase-change material VO on plasmons in metallic arm-chair graphene nanoribbons (AGNRs) within the random-phase approximation (RPA) for intra- and inter-band transitions. We assess the influence of temperature as a knob for the transition from the insulating to the metallic phase of VO on localized and propagating plasmon modes. We show that AGNRs support localized and propagating plasmon modes and contrast them in the presence and absence of VO for intra-band (SB) transitions while neglecting the influence of a substrate-induced band gap. The presence of this gap results in propagating plasmon modes in two-band (TB) transitions. In addition, there is a critical band gap below and above which propagating modes have a linear negative or positive velocity. Increasing the band gap shifts the propagating and localized modes to higher frequencies. In addition, we show how the normalized Fermi velocity increases plasmon modes frequency.

Discovery of New Plasmonic Metals via High‐Throughput Machine Learning

AbstractThe field of plasmonics aims to manipulate and control light through nanoscale structuring and choice of materials. Finding materials with low‐loss response to an applied optical field while exhibiting collective oscillations due to intraband transitions is an outstanding challenge. This is viewed as a materials selection problem that bridges the gap between the large number of candidate materials and the high computational cost to accurately compute their individual optical properties. To address this, online databases that compile computational data for numerous properties of tens to hundreds of thousands of materials are combined with first‐principles simulations and the Drude model. By means of density functional theory (DFT), a training set of geometry‐dependent plasmonic quality factors for ≈1000 materials is computed and subsequently random‐forest regressors are trained on these data. Descriptors are limited to symmetry, quantities obtained using the chemical formula, and the Mendeleev database, which allows to rapidly screen 7445 candidates on Materials Project. Using DFT to compute quality factors for the 233 most promising materials, AlCu3, ZnCu, and ZnGa3 are identified as excellent potential new plasmonic metals. This finding is substantiated by analyzing their electronic structure and interband optical properties in detail.

Ultrafast transient absorption spectroscopic studies on the Impact of growth time on size, stability, and optical characteristics of colloidal gold nanoparticles

Metallic gold (Au) nanoparticles (NPs) with an average size of less than 20 nm exhibit surface plasmon resonance (SPR), which makes them intriguing candidates for optical and optoelectronic applications. In this work, Turkevich approach is used to synthesize different samples of Au NPs by tuning the growth times during synthesis. Transmission electron micrographs depict that the Au NPs are highly crystalline, monodisperse, and spherical, with an average size of ~13–30 nm. Moreover, a shift in the ultraviolet-visible (UV–visible) absorption spectrum is observed from 525 nm to 542 nm. The change in the SPR band corroborates with the growth time-dependent size enhancement. Ultrafast transient absorption spectroscopy (UTAS) has been employed to characterize the optical properties as well as the stability of NPs, and it is worth noticing that as the growth time increases, the intensity of the ground-state bleach and the intraband transitions reduces and increases at 400 fs and 5 ps, respectively, demonstrating the size dependence of the hot-electron distribution (e--e- scattering). These heated electrons subsequently relax into the lattice through two different forms of scattering, namely electron-phonon (e--ph) and phonon-phonon (ph-ph) scatterings. The stability of the Au NPs has been evaluated by kinetic parameters, with a longer average decay time (τavg) showing enhanced stability of NPs with longer growth durations. Transient absorption spectroscopy reveals the size-dependent dynamics of interfacial charge/energy transfer between the Au NPs and the lattice.

Impurity effect on the spectral parameters of an electron in a quantum dot–quantum ring semiconductor nanostructure

In the model of effective masses and rectangular potentials, the effect of a hydrogen-like donor impurity on the energy spectrum, electron wave functions and oscillator strengths of intraband quantum transitions in a quantum dot–quantum ring semiconductor nanostructure (GaAs/AlxGa1-xAs) is studied theoretically. The stationary Schrödinger equation for an electron interacting with an impurity cannot be solved analytically exactly. For its approximate solution, the unknown wave functions are sought in the form of an expansion over a complete set of cylindrically symmetric wave functions in the nanostructure without impurity, and the electron energy is found by solving the corresponding secular equation. The dependences of the energy spectrum, the binding energy of the electron with the impurity and the oscillator strengths of the quantum intraband transitions on the geometric parameters of the nanostructure are analyzed.

Photoluminescence of metallic single-walled carbon nanotubes: Role of interband and intraband transitions

The photoluminescence (PL) properties of metallic single-walled carbon nanotubes were investigated. Steady-state measurements showed interband excitonic PL in the first hyperbolic dispersion region. The decay kinetics of this interband excitonic PL was observed by femtosecond time-resolved measurements, which also showed the occurrence of intraband PL in the linear dispersion region. The decay time constant of interband excitonic PL is approximately 40 fs, while that of intraband PL is slower at the lower photon energy. This study demonstrates the role of interband and intraband transitions in the PL of a one-dimensional metal.

Computational insights into electronic, magnetic and optical properties of Mn(II)-doped ZnTe with and without vacancy defects

First principles calculations have been performed to investigate the optoelectronic and magnetic properties of Mn(II); ZnTe DMS with and without native defects using first principle calculations. Our results find that anti-ferromagnetic coupling dominates in the pure Mn(II)-doped ZnTe system due to super-exchange (SE) mechanism. The impact of p-type and n-type defects on magnetic coupling in Mn(II)-doped ZnTe were discussed and we found that p-type defect such as Zn vacancy defects play important role in stabilization of FM state due to the formation of bound magnetic polaron (BMP) while n-type defect such as Te vacancy defect does not affect the ground state of Mn(II); ZnTe. The magnetic coupling of Mn(II) spins in the absence and presence of Zn vacancy were explained on the basis of phenomenological band structure model. The optical absorption spectra of pure ZnTe and Mn(II); ZnTe with and without vacancies have been investigated and we found that single Mn(II)-doping in the lattice of ZnTe generates an absorption band at energy around 2.02 eV, which is assigned to intra-band d-d transitions of Mn(II) dopant. The band at energy around 0.44 eV in the absorption spectrum of Mn(II)-doped ZnTe with Zn-vacancy may be due to the acceptor states produced by Zn vacancy at Fermi level and band around 1.6 eV in the spectrum of Te vacancy defect system are related to the donor states produced by Te vacancy defect. Moreover, the correlation of magnetic coupling with intra-band d-d transition bands and fundamental bandgaps of ZnTe were also discussed and it was found that intra-band transition peak of Mn(II) ions and optical band gap of ZnTe are blue-shift in AFM coupled Mn(II) ions and are red-shift in FM coupled ions system. Our present findings highlight the worthwhile half-metallic properties of Mn(II)-doped ZnTe, which can be obtained via Zn vacancy defect engineering; this can find broad applications for the fabrication of optoelectronic and spintronic devices.

Diffusion photocurrent influenced by intraband excitation in an intermediate band solar cell with type-I band alignment

We elucidate a photocarrier collection mechanism in intermediate band solar cells (IBSCs) with InAs-quantum dots (QDs)-in-an-Al0.3Ga0.7As/GaAs-quantum well structures. When the Al0.3Ga0.7As barrier is excited, the device electrical output can be varied by additional infrared light for the electron intraband optical transition in QDs. The photocurrent in IBSC with a single QDs-in-a-well structure shows a monotonic increase with the intraband-excitation density. Conversely, IBSC with a multilayered QDs-in-a-well structure exhibits a photocurrent reduction when electrons in QDs are optically pumped out. The simultaneously measured photoluminescence spectra proved that the polarity of QD states changes depending on the intraband-excitation density. We discuss the drift and diffusion current components and point out that the hole diffusion current is significantly influenced by carriers inside the confinement structure. Under strong intraband excitations, we consider an increased hole diffusion current occurs by blocking hole-capture in the quantum structures. This causes unexpected photocurrent reduction in the multilayered device.

Intraband Transitions of Nanocrystals Transforming from Lead Selenide to Self-doped Silver Selenide Quantum Dots by Cation Exchange.

In search of heavy metal-free mid-IR active colloidal materials, self-doped silver selenide colloidal quantum dots (CQDs) can be an alternative offering tunable mid-IR wavelength with a narrow bandwidth. One of the challenges in the study of the intraband transition is developing a method to widen the intraband transition energy range as well as reducing the toxicity of the materials. Here, we present AgxSe (x > 2) CQDs exhibiting an intraband transition up to 0.39 eV, produced by the cation exchange (CE) method from PbSe CQDs. The major electronic transition efficiently changes from the SWIR band gap of PbSe CQDs to the mid-IR intraband transition of the AgxSe CQDs by the CE. The intraband exciton is verified by examining the absorption and emission of the CE AgxSe CQDs as well as their applications on electrochemical mid-IR luminescence and mid-IR intraband photodetectors.

Even-order harmonics in the nitrogen vacancy center in diamond from intertwined intraband and interband transitions

Different from odd-order harmonics, even-order harmonics are a result of broken inversion symmetry, where the second harmonic generation has long been used in the study of surface magnetism. Their underlying mechanisms are always intriguing. Here we reveal an unreported mechanism of even harmonics from the diamond with a nitrogen-vacancy (NV) center from the viewpoint of group theory. Two different types of interband transition channels are identified. One is the direct transitions between a valence band and a conduction band, called type-I interband transition. Type II is defined as the indirect interband transitions between two conduction bands or two valence bands. Once type-II transitions are involved only through the inversion symmetry breaking, even-order harmonics occur in the NV center in diamond. The physical origin of this new type of interband transitions is rooted in group theory, where the inversion symmetry forbids type-II transitions between two valence or conduction bands with the same irreducible representation. By contrast, the broken inversion symmetry in the NV center lifts this restriction, giving rise to even-order harmonics. Our study represents a paradigm shift in high harmonic generation.

Size-controlled quantum dots reveal the impact of intraband transitions on high-order harmonic generation in solids

Since the discovery of high-order harmonic generation (HHG) in solids, much effort has been devoted to understanding its generation mechanism and both interband and intraband transitions are known to be essential. However, intraband transitions are affected by the electronic structure of a solid, and how they contribute to nonlinear carrier generation and HHG remains an open question. Here, we use mid-infrared laser pulses to study HHG in CdSe and CdS quantum dots (QDs), where quantum confinement can be used to control the intraband transitions. We find that both the HHG intensity per excited volume and the generated carrier density increase when the average QD size is increased from about 2 nm to 3 nm. We show that the reduction of the subband gap energy in larger QDs enhances intraband transitions, and this in turn increases the rate of photocarrier injection by coupling with interband transitions, resulting in enhanced HHG.

Flat-band plasmons in twisted bilayer transition metal dichalcogenides

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in the hole-doped twisted bilayer ${\mathrm{MoS}}_{2}$ (tb-${\mathrm{MoS}}_{2}$) by employing a full tight-binding model and the random phase approximation. When considering lattice relaxations in tb-${\mathrm{MoS}}_{2}$, the flat band is not separated from remote valence bands, which makes the contribution of interband transitions in transforming the plasmon dispersion and energy significantly different. In particular, low-damped and quasiflat plasmons emerge if we only consider intraband transitions in the doped flat band, whereas a $\sqrt{q}$ plasmon dispersion emerges if we also take into account interband transitions between the flat band and remote bands. Furthermore, the plasmon energies are tunable with twist angles and doping levels. However, in a rigid sample that suffers no lattice relaxations, lower-energy quasiflat plasmons and higher-energy interband plasmons can coexist. For rigid tb-${\mathrm{MoS}}_{2}$ with a high doping level, strongly enhanced interband transitions quench the quasiflat plasmons. Based on the lattice relaxation and doping effects, we conclude that two conditions, the isolated flat band and a proper hole-doping level, are essential for observing the low-damped and quasiflat plasmon mode in twisted bilayer transition metal dichalcogenides. We hope that our study on flat-band plasmons can be instructive for studying the possibility of plasmon-mediated superconductivity in twisted bilayer transition metal dichalcogenides in the future.

Near-zero-index ultra-fast pulse characterization

Transparent conducting oxides exhibit giant optical nonlinearities in the near-infrared window where their linear index approaches zero. Despite the magnitude and speed of these nonlinearities, a “killer” optical application for these compounds has yet to be found. Because of the absorptive nature of the typically used intraband transitions, out-of-plane configurations with short optical paths should be considered. In this direction, we propose an alternative frequency-resolved optical gating scheme for the characterization of ultra-fast optical pulses that exploits near-zero-index aluminium zinc oxide thin films. Besides the technological advantages in terms of manufacturability and cost, our system outperforms commercial modules in key metrics, such as operational bandwidth, sensitivity, and robustness. The performance enhancement comes with the additional benefit of simultaneous self-phase-matched second and third harmonic generation. Because of the fundamental importance of novel methodologies to characterise ultra-fast events, our solution could be of fundamental use for numerous research labs and industries.

An ambiguity of laser‐induced degradation in LiFePO4 and advantages of single‐particle approach to Raman spectroscopy

AbstractLaser‐induced degradation is a common issue for the Raman spectroscopy study of lithium battery materials. In this work, the combination of single‐particle Raman measurements with scanning electron microscope imaging helped us reveal the variation of the decomposition pathways and products. For the first time, we observed possible laser‐induced melting of the LiFePO4 particles and an unidentified decomposition product, an alternative to previously reported α‐Fe2O3 and γ‐Li3Fe2(PO4)3. These results and analysis of the time series of Raman measurements raise doubts about laser‐induced heating. We suppose non‐thermal amorphization and melting processes due to electronic system excitation by intraband transitions with the following covalent bond destabilization. The role of defects in laser‐induced decomposition is discussed. We hope that this article could help develop a non‐destructive tool for quality control and the study of the electrochemical or thermal degradation of LiFePO4.

Emergent plasmonic excitations in Mexican-hat and bell-shaped bands of hybridized Dirac electrons in graphene/topological insulator heterostructures

Stacking a graphene monolayer onto a three-dimensional topological insulator results in a van der Waals heterostructure that can serve as a versatile platform for rich emergent exotic physics. In particular, the Dirac electrons of the graphene and that of the two-dimensional surface states of the topological insulator hybridize effectively, leading to the emergence of two distinct electronic bands around the Fermi level, with respective Mexican-hat and bell shapes. In this paper, we explore the emergent collective plasmonic excitations in the representative heterostructure of graphene/${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$, combining first-principles calculations and low-energy effective Hamiltonian descriptions. We discover two different plasmon modes of the system, and our band-resolved analysis allows us to attribute the two modes to the intraband transitions within the Mexican-hat and bell-shaped bands, respectively. We also analyze the relative strengths and damping rates of the plasmon modes upon variations of the bias potential across the heterojunction, enabling potential experimental detections of those collective excitations. These findings provide alternate avenues for regulating the electronic states and plasmonic excitations in the hybridized Dirac electron systems for potential device applications.

Plasmon-Induced Hot Carriers from Interband and Intraband Transitions in Large Noble Metal Nanoparticles

Hot electrons generated from the decay of localized surface plasmons in metallic nanostructures have the potential to transform photocatalysis, photodetection and other optoelectronic applications. However, the understanding of hot-carrier generation in realistic nanostructures, in particular the relative importance of interband and intraband transitions, remains incomplete. Here we report theoretical predictions of hot-carrier generation rates in spherical nanoparticles of the noble metals silver, gold and copper with diameters up to 30 nanometers obtained from a novel atomistic linear-scaling approach. As the nanoparticle size increases the relative importance of interband transitions from d-bands to sp-bands relative to surface-enabled sp-band to sp-band transitions increases. We find that the hot-hole generation rate is characterized by a peak at the onset of the d-bands, while the position of the corresponding peak in the hot-electron distribution can be controlled through the illumination frequency. In contrast, intraband transitions give rise to hot electrons, but relatively cold holes. Importantly, increasing the dielectric constant of the environment removes hot carriers generated from interband transitions, while increasing the number of hot carriers from intraband transitions. The insights resulting from our work enable the design of nanoparticles for specific hot-carrier applications through their material composition, size and dielectric environment.

Optimization of etching and sonication time to prepare monolayer Ti3C2Tx MXene flakes: A structural, vibrational, and optical spectroscopy study

Two-dimensional (2D) Ti3C2Tx MXene and its heterostructure exhibit promising performances in several applications such as energy storage, optoelectronics, electrocatalysis, water purification, biomedical, etc. Our investigation shows that an etching duration of 24 h is sufficient to remove Al layers from Ti3AlC2 and to obtain a well-separated accordion-like structure. Different lateral size monolayer MXene flakes ranging from 2 μm to 35 nm are obtained by tuning the sonication time from 2 to 8 h. The monolayer nature of the flakes is confirmed by both Atomic Force Microscope (AFM) and Transmission Electron Microscope (TEM). Raman vibrational modes and X-ray photoelectron spectroscopy (XPS) show that the flakes are containing –O, –F, and –OH functional groups. A unique type of absorption spectra is observed that comes from both interband and intraband transition. An in-depth structural, vibrational, and optical property study gives a clear idea to use these flakes for heterostructure formation with other materials for various applications.

Synergizing Inter and Intraband Transitions in Defective Tungsten Oxide for Efficient Photocatalytic Alcohol Dehydration to Alkenes.

Photocatalysis under mild conditions is an intriguing avenue for organic chemical manufacturing to confront the serious fossil energy crisis. Herein, we report a direct light-driven alkene production through alcohol dehydration, using nonstoichiometric tungsten oxide of W18O49 nanowires with abundant lattice defects as a photocatalyst. A representative ethylene (C2H4) production rate of 275.5 mmol gcat–1 h–1 is achieved from ethanol (C2H5OH) dehydration, together with excellent selectivity up to 99.9%. The universality of our approach is further demonstrated with other alcohol dehydration. Combining ultrafast transient absorption spectroscopy with in situ X-ray photoelectron spectroscopy, we underline that the inter- and intraband transitions synergistically contribute to such excellent activity. In particular, the intraband transition excites the electrons in defect bands into an energetically “hot” state, largely alleviating the charge recombination. As a result, the C–OH bond of chemisorbed C2H5OH molecules can be effectively dissociated to furnish the formation of C=C bonds. Our work offers a fresh insight into sustainable alkene production with renewable energy input under mild conditions.

Magnetic field dependent two-photon absorption properties in monolayer MoS2

We report our theoretical investigations on the magnetic field dependence of two-photon absorption (TPA) properties in ${\mathrm{MoS}}_{2}$. Based on the Landau levels obtained by the effective massive Dirac model, we developed TPA theories associated with both interband transitions and intraband transitions for transitional metal dichalcogenides (TMDCs) with a magnetic field under the second-order perturbation theory with respect to the electron-photon interaction. Our results reveal that the TPA coefficient is effectively enhanced by five orders of magnitude under the application of an external magnetic field, which results in the quantized horizontal Landau levels with the spin-orbit coupling (SOC) and valley Zeeman effects. The TPA peaks are located in the terahertz regime for intraband transitions and in the visible regime for interband transitions. The amplitude of TPA contributed by intraband transitions is much larger than that of interband transitions owing to the resonant transitions. Oscillatory behavior appeared in the interband transition-related TPA spectra. The separation of TPA for the transitions in the spin-up and spin-down states caused by the spin splitting of Landau levels resulting from the strong SOC is illustrated. We also analyzed the anisotropy of TPA associated with the direction of the incident light. Our results are of much importance for the application of TMDCs in optical devices.

Managing photoinduced electron transfer in AgInS2-CdS heterostructures.

Ternary semiconductors such as AgInS2, with their interesting photocatalytic properties, can serve as building blocks to design light harvesting assemblies. The intraband transitions created by the metal ions extend the absorption well beyond the bandgap transition. The interfacial electron transfer of AgInS2 with surface bound ethyl viologen under bandgap and sub-bandgap irradiation as probed by steady state photolysis and transient absorption spectroscopy offers new insights into the participation of conduction band and trapped electrons. Capping AgInS2 with CdS shifts emission maximum to the blue and increases the emission yield as the surface defects are remediated. CdS capping also promotes charge separation as evident from the efficiency of electron transfer to ethyl viologen, which increased from 14% to 29%. The transient absorption measurements that elucidate the kinetic aspects of electron transfer processes in AgInS2 and CdS capped AgInS2 are presented. The improved performance of CdS capped AgInS2 offers new opportunities to employ them as photocatalysts.

Terahertz displacive excitation of a coherent Raman-active phonon in V2O3

Nonlinear processes involving frequency-mixing of light fields set the basis for ultrafast coherent spectroscopy of collective modes in solids. In certain semimetals and semiconductors, generation of coherent phonon modes can occur by a displacive force on the lattice at the difference-frequency mixing of a laser pulse excitation on the electronic system. Here, as a low-frequency counterpart of this process, we demonstrate that coherent phonon excitations can be induced by the sum-frequency components of an intense terahertz light field, coupled to intraband electronic transitions. This nonlinear process leads to charge-coupled coherent dynamics of Raman-active phonon modes in the strongly correlated metal V2O3. Our results show an alternative up-conversion pathway for the optical control of Raman-active modes in solids mediated by terahertz-driven electronic excitation.