Interband Transitions Research Articles

Theoretical Study of the Exciton Binding Energy and Exciton Absorption in Different Hyperbolic-Type Quantum Wells under Applied Electric, Magnetic, and Intense Laser Fields.

In this study, we investigated the exciton binding energy and interband transition between the electron and heavy-hole for the single and double quantum wells which have different hyperbolic-type potential functions subject to electric, magnetic, and non-resonant intense laser fields. The results obtained show that the geometric shapes of the structure and the applied external fields are very effective on the electronic and optical properties. In the absence of the external fields, the exciton binding energy is a decreasing function of increasing well sizes except for the strong confinement regime. Therefore, for all applied external fields, the increase in the well widths produces a red-shift at the absorption peak positions. The magnetic field causes an increase in the exciton binding energy and provides a blue-shift of the absorption peak positions corresponding to interband transitions. The effect of the electric field is quite pronounced in the weak confinement regime, it causes localization in opposite directions of the quantum wells of the electron and hole, thereby weakening the Coulomb interaction between them, causing a decrease in exciton binding energy, and a red-shift of the peak positions corresponding to the interband transitions. Generally, an intense laser field causes a decrease in the exciton binding energy and produces a red-shift of the peak positions corresponding to interband transitions.

Angle-resolved high-order harmonics in wurtzite-type ZnO

High-order harmonics in solids provide a method of analyzing the intraband and interband dynamics of a solid driven by an ultrafast laser. This study analyzed the contributions of intraband and interband dynamics based on angle-resolved high-order harmonics. According to the simulations and experiments, we found that the angular divergences of the harmonics have an evident boundary at the bandgap when the laser is polarized along the asymmetric direction, which is primarily invoked by the interplay of an interband transition and an intraband electron movement, and the intraband and interband dynamics have different sensitivities of the spatial phase of driving laser.

CRYSTAL STRUCTURE AND ELECTRONIC PROPERTIES OF THE RHENIUM DISULFIDE

The crystal structure and electronic properties of the rhenium disulfide of a triclinic crystal system within density functional theory and pseudopotential theory are investigated. It is shown that calculated primitive cell parameters and angles within local density approximation are in good agreement with experimental data. It is established that the observed direct-gap character of rhenium disulfide is related to the interband transitions at point X. The electrons energy spectrum is characterized by a large number of the valley, the electronic structure is mainly formed by the 3p- and 5d-states of sulfur and rhenium ions, respectively, and during the transition from the valence band to the conductance band the role of 5d-states increases and the role of 3p-states decreases. The observed structure is due to the low-symmetry primitive cell and a large number of nonequivalent positions of its constituent ions.

Thermoradiative devices enabled by hyperbolic phonon polaritons at nanoscales

Theromoradiative (TR) devices, though proposed a decade ago, have seen little investigation due to their low-performance compared to other solid-state energy conversion technologies. Herein, we propose an InSb-hBN TR device with a nanoscale vacuum gap down to 10 nm that has the potential to achieve efficient waste heat recovery using solid-state technologies. By coupling the hyperbolic phonon polaritons of hBN with the interband transition of InSb, this TR device design can achieve an output power nearly 4 orders-of-magnitude higher than far-field scenarios and an upper bound efficiency up to 80% of the Carnot limit corresponding to only considering radiative loss. A detailed balance model combined with fluctuational electrodynamics for a multilayered anisotropic structure is developed to theoretically estimate the performance of TR devices from far- to near-field regime. Nonradiative losses are also investigated to elucidate their effects on performance. This work helps deepen the understanding of TR devices and provides a promising pathway to efficient low-grade (< 600 K) heat recovery.

Multi-frequency complete optical isolation based on indirect interband transition

Indirect interband photonic transition is an important approach to achieve on-chip complete optical signal isolation. We demonstrate that the operational isolation bandwidth based on this effect can be increased by utilizing non-interfering transition channels obtained from the process of transition channel engineering. Multiple optical modes can be excited through non-interfering channels and be absorbed by matched filters without interfering with each other, thus achieving multi-frequency complete optical isolation. With two non-interfering transition channels, an optical isolator design that achieves complete isolation at two frequencies is proposed as an example. The potential design of multi-frequency isolator is also discussed. All the results are verified with finite-difference time-domain simulation.

Exciton Manifolds in Highly Ambipolar Doped WS2.

The disentanglement of single and many particle properties in 2D semiconductors and their dependencies on high carrier concentration is challenging to experimentally study by pure optical means. We establish an electrolyte gated WS monolayer field-effect structure capable of shifting the Fermi level from the valence into the conduction band that is suitable to optically trace exciton binding as well as the single-particle band gap energies in the weakly doped regime. Combined spectroscopic imaging ellipsometry and photoluminescence spectroscopies spanning large n- and p-type doping with charge carrier densities up to 10 cm enable to study screening phenomena and doping dependent evolution of the rich exciton manifold whose origin is controversially discussed in literature. We show that the two most prominent emission bands in photoluminescence experiments are due to the recombination of spin-forbidden and momentum-forbidden charge neutral excitons activated by phonons. The observed interband transitions are redshifted and drastically weakened under electron or hole doping. This field-effect platform is not only suitable for studying exciton manifold but is also suitable for combined optical and transport measurements on degenerately doped atomically thin quantum materials at cryogenic temperatures.

Controlling surface plasmon polariton losses in the visible spectrum by temperature-induced interband transitions

We study theoretically the temperature dynamics at the metal-dielectric interface excited by a strong pump surface plasmon polariton pulse propagating along it. We demonstrate that the inclusion of the interband transitions leads to much stronger temperature-induced modulation of the imaginary part of the dielectric constant induced by the pump. We show that in the visible part of the spectrum a strong pump field can even lead to the decrease in the extinction coefficient for the co- or contrapropagating probe wave.

Confined high-entropy-alloy nanoparticles within graphitic shells for synergistically improved photothermal conversion

High-entropy-alloy nanoparticles (HEA-NPs) have exhibited great potential in solar steam generation, in which their photothermal conversion performances can be optimized through the interband transition (IBTs) between the composing elements. In this work, the HEA-NPs were in-situ confined in the multi-layer graphitic shells (HEA@CNPs) by the arc-discharged plasma approach to achieve a synergistic improvement in the solar steam generation performances. It can be recognized that the graphitic shells not only serve as optical absorbers but also improve the surface temperature ascribed to their low thermal conductivities. The numerical simulation reveals that the electric fields can be accumulated in the graphitic shells over the wavelength regions of 1000 to 2500 nm. As a result, the most optimized FeCoNiTiVCrMnCu@C nanocapsules demonstrated an evaporation rate of 2.66 kg m−2h−1 under one sun irradiation with an energy conversion efficiency of 98%, and the surface temperature can increase to ∼105 °C within 90 s, demonstrating an effective solar steam generation performance. The present study indicates an insight into the synergistic optical absorption behaviors between HEA-NPs and graphitic shells, and the nanocapsules could be expanded to other solar-thermal conversion applications.

Tunable large spin Nernst effect in a two-dimensional magnetic bilayer

We theoretically investigate topological spin transport of the magnon polarons in a bilayer magnet with two-dimensional square lattices. Our theory is motivated by recent reports on the van der Waals magnets which show the reversible electrical switching of the interlayer magnetic order between antiferromagnetic and ferromagnetic orders. The magnetoelastic interaction opens band gaps and allows the interband transition between different excitation states. In the layered antiferromagnet, due to the interband transition between the magnon-polaron states, the spin Berry curvature which allows the topological spin transport occurs even if the time-reversal symmetry is preserved. We find that the spin Berry curvature in the layered antiferromagnet is very large due to the small energy spacing between two magnonlike states. As a result, the spin Nernst conductivity shows a sudden increase (or decrease) at the phase transition point between ferromagnetic and antiferromagnetic phases. Our results suggest that the ubiquity of tunable topological spin transports in two-dimensional magnetic systems.

Plasmonic Properties of Al2O3 Nanoshell with a Metallic Core

Background: Al is the promising candidate for deep UV and longer wavelength range plasmonic applications. But it is difficult to have the pure aluminium nanostructure as it is easily oxidized, forming a thin layer of Al2O3. In this paper, we have evaluated the field enhancement of oxide layer on metallic shell (Al-Al2O3 and Au-Al2O3) for single and dimer core-shell configuration and showed potential of the oxide layer in SERS. Methods: The Finite Difference Time Domain (FDTD) has been used to evaluate the LSPR and field enhancement of single and dimer Al-Al2O3 and Au- Al2O3 nanostructure. Results: The results exhibit the tunable plasmon resonance on varying the inner and outer radii of the Al2O3 shell. A redshift and decrease in enhancement were observed as shell thickness increases, whereas on increasing the core size, the enhancement increases in the case of Au-Al2O3 and decreases in Al- Al2O3 due to quadrupole contribution. But on comparing the Au-Al2O3 with Al-Al2O3 for the same particle size, Al-Al2O3 shows larger enhancement because Au has to compete with its interband transition. Conclusion: By optimizing the thickness of the shell and core size, it can be concluded that an ultrathin shell of Al2O3 can give higher enhancement. With Al as a core metal, the enhancement increases as compared to Au-Al2O3. Since a single Al-Al2O3 nanoshell has shown a huge enhancement we have considered the multimer configuration of two identical nanoshells. Due to coupling between two nanoshells a huge increase in enhancement factor ~1012 was observed for Al-Al2O3 dimer nanoshell in the UV region.

Influence of Proton Ion Irradiation on the Linear–Nonlinear Optoelectronic Properties of Sb40Se20S40 Thin Films at Different Fluences for Photonic Devices

This study investigates the effects of 30 keV proton ion irradiation on the structural, morphological, and linear–nonlinear optoelectronic properties of Sb40Se20S40 thin films. The retention of an amorphous nature and vibrational bonding rearrangements caused by ion irradiation demonstrate structural tailoring. The cumulative decrease in roughness with an increase in ion dose decreases the surface energy and optical absorption loss. With the blue shifting of the absorption edges, proton irradiation increased the optical transmittance and reflectance. The variation of fluence changes the optical bandgap and Urbach energy, which are induced by local structural changes caused by defects and disorder. The refractive index decreased considerably, which supports the Moss rule. Proton irradiation reduced the interband transition and average band energy gap of the system. However, ion irradiation increased optical losses while decreasing optical conductivity and dielectric characteristics. The third-order nonlinear susceptibility and nonlinear refractive index decreased significantly as the fluence increased. Such materials with optical tuning capabilities via ion fluence are essential for cutting-edge photonic and optoelectronic applications.

Interband Transitions and Critical Points of Single‐Crystal Thoria Compared with Urania

The interband transitions of UO2are validated independently through cathode luminescence. A picture emerges consistent with density functional theory. While theory is generally consistent with experiment, it is evident from the comparison of UO2and ThO2that the choice of functional can significantly alter the bandgap and some details of the band structure, in particular at the conduction band minimum. Strictly ab initio predictions of the optical properties of the actinide compounds, based on density functional theory alone, continue to be somewhat elusive.

Low-energy interband transition in the infrared response of the correlated metal SrVO3 in the ultraclean limit

Non-Fermi-liquid behavior of correlated metals is one of the central problems in condensed matter physics. To advance the understanding of the non-Fermi-liquid phase, it is crucial to know the intrinsic electronic response of correlated metals. The authors study the optical response of an archetypal correlated metal SrVO${}_{3}$ in the ultraclean limit. The results reveal that interband transitions can give rise to a significant contribution to the low-energy electronic response, mimicking the non-Fermi-liquid behavior of the charge dynamics.

Waveguide-integrated mid-IR photodetector and all-optical modulator based on interlayer excitons absorption in a WS2/HfS2 heterostructure

Abstract Novel 2D van der Waals semiconductors facilitate the formation of heterostructures and thus support bandgap engineering for atomically thin modern photonic applications. When these heterostructures form a type II band structure, interlayer excitons (ILEs) are formed as a result of the ultrafast charge transfer between the layers. Here, we present for the first time a waveguide-coupled, mid-IR photodetector and modulator based on the ILE absorption. The device consists of a heterostructure of a single layer of tungsten disulfide (WS2) and a few layers of hafnium disulfide (HfS2) integrated to a silicon waveguide on a sapphire substrate. We measure broadband mid-IR photodetection (3.8–5.5 µm) with responsivity in the order of tens of µA/W and with no significant effect on the waveguide’s transmission. Additionally, we demonstrate waveguide-integrated, mid-IR, all-optical modulation by controlling the ILE population with the interband transition of the individual layers of the heterostructure.

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.

First-principles investigation on narrow bandgap InSb1−xBix dilute bismide alloys for highly efficient long-wavelength infrared optoelectronics

The electronic structure, optical properties and crystal structure of the dilute bismide InSb1−xBix alloys with (0 ≤ x ≤ 0.125) have been studied using the DFT FP − LAPW method within the novel TB-mBJ and WC-GGA models. The lattice constant increases quasi-linearly in InSb1−xBix for small Bi contents. Based on calculations of the formation energy, we demonstrated the thermodynamic stability of the alloys. The small content of Bi into InSb1−xBix results in a large bandgap narrowing by ∼ 264 meV up to a composition of 0.125 covering the mid- and long wavelength IR ∼ 3.4–12.5 μm spectral range. From DOS analysis, a bandgap reduction in the InSb1-xBix can be explained by both upward movement of the valence band (VB) induced by the resonance interaction of Bi-6p states at the top of VB and the downward moves of In-s and Sb/Bi-p states at the conduction band (CB). A semiconductor-semimetal transition in InSb1−xBix at × ∼ 0.137 was expected. The optical properties in terms of dielectric functions (ε1, ε2), refractive index n, extinction coefficient k, reflectivity R and the absorption coefficient α were calculated for photon energies of 0–8 eV and compared to experimental data. The sharp energy structures appear in various optical spectra associated with interband transitions from occupied In-s, Sb-p and Bi-p orbitals located at VBmax to unoccupied In-s/p, Sb-p and Bi-p states located at CBmin. Good absorbance in the ultraviolet range was observed, which is useful for solar cells design. InSb1−xBix/InSb bismide alloy is an attractive candidate for realizing highly efficient light-emitting sources and photodetectors operating in the long-wavelength infrared spectral range (λLWIR ≥ 8 μm).

Disentangling the Ultrafast Nonlinear Optical Behavior of Plasmonic Resonances Near the Interband Transition

The photoexcitation of plasmonic nanostructures with ultrashort laser pulses allows for elucidating the mechanisms underlying the ultrafast nonlinear optical response of such systems, gaining insight into the fundamental processes triggered by light absorption at the nanoscale. To date, the complex temporal and spectral features of the photoinduced response are not fully understood, especially when the photon energies are close to the interband transitions of the metallic medium. Herein, the effects of photoexcitation of plasmonic nanostructures are studied by resorting to a combinaion of broadband transient absorption spectroscopy and semiclassical nonlinear simulations of the energy relaxation processes following illumination. The proposed approach enables an in‐depth disentanglement of all the contributions to the ultrafast transient optical response of supported gold nanocrystals. From these methods, the apparent transient blue shift of the localized plasmon resonance observed in the pump–probe signals is rationalized as an interplay between different and spectrally dispersed permittivity modulations, instead of a simple negative permittivity change, as it could be concluded based on the Fröhlich condition. The results provide a comprehensive understanding of the thermo‐modulational nonlinearities of plasmonic nanostructures exhibiting resonances close to the interband transition threshold.

Optical investigation of the heavy-fermion normal state in superconducting UTe2

The recently discovered superconductor, UTe$_2$, has attracted immense scientific interest due to the experimental observations that suggest odd-parity superconductivity. It is believed that the material becomes a heavy-fermion metal at low temperatures although details of the normal state are unclear. Using Fourier transform infrared spectroscopy (FTIR), the normal state electronic structure of UTe$_2$ was investigated at zero applied magnetic field. Combining the measured reflectivity with the dc resistivity, the complex optical conductivity was obtained over a large frequency range. The frequency dependence of the real part of the optical conductivity exhibits a MIR peak around 4000 cm$^{-1}$ and a narrow Drude peak that develops below 40 K. A combination of density functional and dynamic mean field theory (DFT + DMFT) gives spectra in close correspondence to the experiment. Via this comparison we attribute the prominent MIR peak to inter-band transitions involving a narrow U 5$f$ feature that develops near the Fermi level. In this regard, our data gives spectroscopic evidence for the existence of a low energy Kondo resonance at temperatures just above the onset of superconductivity and implicates heavy electrons in the formation of the superconducting state. We find that the coherent Kondo resonance is primarily associated with a collapse of scattering and less with a transfer of spectral weight.

Photoreflectance system based on vacuum ultraviolet laser at 177.3 nm

Photoreflectance (PR) spectroscopy is a powerful and non-destructive experimental technique to explore interband transitions of semiconductors. In most PR systems, the photon energy of the pumping beam is usually chosen to be higher than the bandgap energy of the sample. To the best of our knowledge, the highest energy of pumping laser in reported PR systems is 5.08 eV (244 nm), not yet in the vacuum ultraviolet (VUV) region. In this work, we report the design and construction of a PR system pumped by VUV laser of 7.0 eV (177.3 nm). At the same time, dual-modulated technique is applied and a dual channel lock-in-amplifier is integrated into the system for efficient PR measurement. The system’s performance is verified by the PR spectroscopy measurement of well-studied semiconductors, which testifies its ability to probe critical-point energies of the electronic band in semiconductors from ultraviolet to near-infrared spectral region.

Spectroscopic imaging ellipsometry of two-dimensional TMDC heterostructures

Semiconducting two-dimensional materials and their heterostructures gained a lot of interest for applications as well as fundamental studies due to their rich optical properties. Assembly in van der Waals heterostacks can significantly alter the intrinsic optical properties as well as the wavelength-dependent absorption and emission efficiencies, making a direct comparison of, e.g., photoluminescence intensities difficult. Here, we determine the dielectric function for the prototypical MoSe2/WSe2 heterobilayer and their individual layers. Apart from a redshift of 18–44 meV of the energetically lowest interband transitions, we find that for larger energies, the dielectric function can only be described by treating the van der Waals heterobilayer as a new artificial homobilayer crystal rather than a stack of individual layers. The determined dielectric functions are applied to calculate the Michelson contrast of the individual layers and the bilayer in dependence of the oxide thickness of often used Si/SiO2 substrates. Our results highlight the need to consider the altered dielectric functions impacting the Michelson interference in the interpretation of intensities in optical measurements such as Raman scattering or photoluminescence.