Intraband Transitions Research Articles

First-principle study of two-photon absorption in Fe3O4.

Utilizing the length gauge and the electron density operator, we calculate the two-photon absorption (2PA) coefficient of Fe3O4 based on a full ab initio band structure. The pure two-photon interband transitions and those modulated by intraband transitions have been separated explicitly to determine their impact on the 2PA spectrum. A single main absorption peak instead of two will present in the spectrum by including the hybridization of inter- and intra-band transitions, which can be identified easily in future experimental studies. In addition, we find that in almost the whole 2PA-active spectral range, circularly polarized light can be absorbed with a much lower threshold power than the linearly polarized one. But the linearly polarized light is more easily to be manipulated. One can control the magnitude and resonance frequency of the 2PA coefficient by simply adjusting the polarization angle of the incident light. Our findings will deepen the understanding of the 2PA phenomenon in Fe3O4 nanoparticles.

Ab initio calculations on magnetic and optical characteristics of pure ZnO and Mn(II)-doped ZnO monolayers with and without neutral charged vacancies defects

At high Curie temperatures, diluted magnetic semiconductors exhibit induced ferromagnetism and spin-dependent interactions, making them useful in magneto-electronic devices. In spite of the fact that ferromagnetism and optical emission dynamics are characterized by intricate microstructural and compositional features, the origins of these phenomena are not yet completely understood. Here, we used first principles calculations to study how vacancy defects affect the electrical, optical, and magnetic properties of pure ZnO and Mn@ZnO monolayers. In the pristine monolayer, the Zn vacancy produces magnetism with a total magnetic moment of 2 μB, but the oxygen vacancy does not. The magnetic semiconducting characteristic of the Mn substitution at the Zn site is shown by a total magnetization of 5 μB. The magnetic ground state of the ZnO monolayer doped with Mn is significantly influenced by the presence of native defects. More specifically, Zn vacancies are essential for stabilizing the FM states due to the bound magnetic polarons (BMPs) formation, but O vacancies have no influence on the magnetic ground state of the Mn(II)@ZnO monolayer. Based on the optical study, we discovered that Zn and O vacancies in the ZnO ML produce optical absorption peaks at 1.92 eV and 2.90 eV, respectively. The substitution of Mn(II) at Zn site not only increases the band gap of the ZnO monolayer but also produces d-d transition bands at lower energy side of fundamental energy gap peak. The Zn vacancies in the Mn(II)@ZnO ML produce an infrared absorption band around 1.13 eV, which could be due to the formation of the BMP, and enhance the optical absorption efficiency. The ZnO ML’s energy gap and Mn ion’s d-d transition bands exhibit a blueshift in AFM systems and a redshift in the FM systems. In far configuration, the Mn(II)@ZnO monolayer show no shift in the fundamental bandgap and intra-band transition of the Mn(II) spins due to the Mn(II) ion’s paramagnetic nature.

Two-photon absorption in quantum dots with Hellmann potential

Abstract We study the nonlinear optical absorption properties of a spherical quantum dot (SQD) with Hellmann potential, focusing on the two-photon absorption (TPA) process, using GaAs/AlGaAs material as an example. The radial Schrödinger equation is solved using the Nikiforov-Uvarov method, while the two-photon absorption coefficient is determined through second-order perturbation theory concerning the electron-photon interaction. Our study shows that the intraband transition has a smaller energy transition than the interband transition, leading to TPA spectra for the intraband transition that is restricted within a smaller energy range and exhibits a higher peak value than those for the interband transition. The peak corresponding to the orbital quantum number of electrons in SQD ℓ = 2 consistently appears to the left of the peak corresponding to ℓ = 1 in both intraband and interband transition cases. Additionally, the dependence of the absorption peak position on the order of transition, n, differs between intra- and inter-band transitions. We also observe blue shift behavior in the TPA spectra as all three parameters, r 0, V 0e , and η, increase. Our investigation has the potential to enable the design of novel photonic devices, ultra-fast optical switches, and highly efficient solar cells through the optimization of quantum dot material properties.

Fast carrier separation induced by the metal-like O-doped MoS2/CoS cocatalyst for achieving photocatalytic and photothermal hydrogen production

Full solar-spectrum-driven hydrogen (H2) production from water-splitting is highly desirable but remains a great challenge. A heterojunction composite photocatalyst with a full-spectrum absorption range of up to 1020 nm was constructed by introducing 2D/2D metallic oxygen-doped MoS2/CoS (O-MC) nanosheets onto 1D Zn0.1Cd0.9S nanorods (ZCS NR) for photocatalytic and photothermal H2 production. Consequently, the optimal ZMC1.5 heterojunction exhibited a significantly improved H2 production activity of 95.5 mmol g−1 h−1 under 420 nm monowavelength irradiation, which is 13.84 times higher than that of pristine ZCS NR. Even under 1020 nm monowavelength irradiation, the catalyst also shows an expressive H2 generation rate of 0.202 mmol g−1 h−1. Moreover, it gives an apparent quantum yield (AQY) of 39.5% at 420 nm. Femtosecond transient absorption spectroscopy (Fs-TAS) results reveal the multiple intraband and interband excited-charge transitions in the partially occupied band of O-MC and interfacial electron transfer between O-MC and ZCS NR result in the enhancement H2 evolution rate. This study highlights the extended light absorption and special partially occupied energy bands of metallic cocatalysts, which hold great potential for the design and synthesis of efficient full solar-spectrum (UV–vis-NIR light) responsive photocatalysts.

Transparent Conducting TiO2 Thin Film Induced by Electric‐Field Controlled Hydrogen Ion Intercalation

AbstractRealizing transformation from transparent insulating to transparent conducting, is a pursuing goal in designing and fabricating novel optoelectronic materials and devices. Here, a pronounced insulating to metal transition in anatase TiO2 thin films is achieved through ionic liquid gating, and interestingly the material maintains an invariable high optical transparency. It is revealed that the emergent metallic state can be attributed to the electron doping associated with the hydrogen ion intercalation. Importantly, the hydrogenation leads to the almost rigid shift of the Fermi energy and therefore maintains nicely the transparency at the visible light region. This result is in strong contrast with the case of oxygen vacancy doping, in which the optical bandgap is suppressed due to improved orbitals hybridization and intraband transition. Moreover, through synergistic ion‐electron doping, the selective control of the gating area and pattern is realized in the micrometer or even nanometer scale with extremely distinct physical properties, which can be employed to fabricate novel optical and electronic devices. The result greatly deepens the understanding of the underlying physics and formation mechanism of transparent conducting oxide (TCO) materials. It is envisioned that this work would provide a new pathway to design other potential optoelectronic materials with novel functionalities.

Near-Field Radiative Heat Transfer between Layered β-GeSe Slabs: First-Principles Approach.

The group-IV monochalcogenide monolayers, GeSe, are interesting and novel two-dimensional (2D) semiconductor materials due to their highly anisotropic physical properties. Monolayers of the different GeSe polymorphs have already had their physical properties and potential applications extensively investigated. However, few-layer homostructures, which can also be approximated as 2D systems in many cases, have not received the same attention. For this reason, in this work, we investigate the optical properties of a free-standing few-layer β-GeSe system and use this information to investigate their performance in the near-field radiative heat transfer (NFRHT). The required optical conductivity of the few-layer 2D material is calculated by using density functional theory (DFT), including spin-orbit coupling. The band structure is investigated for up to five layers, and the effective electron masses are calculated correspondingly. Using this information, both the intraband transitions due to the presence of free electrons introduced by doping and the interband transitions are considered. The contribution of the ionic vibrations is also included in calculating the optical properties because of its relevance to NFRHT through the resulting active optical phonons. With all these contributions included, more realistic predictions of the NFRHT between the layered 2D β-GeSe materials can be obtained. It is found that the heat transfer attainable with the layered system is similar to that of a single layer of β-GeSe we have obtained previously.

Influence of external magnetic field on intraband transitions in lens-shaped quantum dot

Intraband linear and nonlinear optical absorption in a strongly oblate lens-shaped Ge/Si quantum dot in the presence of an axial magnetic field was theoretically studied. Quantum transitions are considered in the heavy hole subband, when the scalar effective mass approximation is correct. The linear and nonlinear absorption coefficients, refractive index changes and the second harmonic generation coefficient were determined. The influence of the effects of temperature, size quantization and magnetic field on the behavior of the above parameters was revealed.

Structural, band profiles and optical reflectivity studies of ALi2B (A = Cu, Ag; B = Ge, Sn, Pb) compounds

Abstract The subject of given project is to highlight the basic elastic, electronic and optical reflectivity of ALi2B (A = Cu, Ag; B = Ge, Sn, Pb). For the calculation of these properties, we used the full potential linearized augmented plane wave (FP-LAPW) procedures carry through Wien2k package. Specifically, the Perdew, Burke and Ernzerhof’s generalized gradient approximation (PBE-GGA) and Wu and Cohen generalized gradient approximation (WC-GGA) have been used. The obtained results fit well with existing experimental data. Different elastic parameters, such as constant of elasticity, elastic moduli, Poisson’s ratio, anisotropy factor and Cauchy pressure, are calculated for the first time for the compounds. The elastic properties clearly summarized the compound’s elastically stability and brittleness in both zinc blend phase. The band structure results for ALi2B shows that the compounds are metallic having overlapping bands across the electron chemical potential. The valance band highest energy state is composed from combination of Cu-d and Sn-p state, disclosed through these compound’s total (DOS) plots, whereas the conduction band primarily constitution is from Li-p, Sn-s Cu-s and Sn-p states. The intraband transitions play vital rule in the description of the optical reflectivity of the ALi2B compounds.

Electromagnetically induced transparency in a quantum ring: Interband transition

In this paper, we study the electromagnetically induced transparency (EIT) of a quantum ring by considering both interband and intraband transitions, under the influence of external electric and magnetic fields, as well as Rashba spin–orbit interaction (RSOI). Four band k⋅p model along with effective mass one band approximation were respectively employed to compute valence and conduction bands energy eigenvalues. We investigate the effects of applied electric and magnetic fields, as well as RSOI and the ring dimensions, on EIT. Our results reveal that the external magnetic field and the ring dimensions have a great influence on the EIT phenomenon. Ultimately, our numerical results will be used to develop devices with controllable nonlinear optical absorption and refraction.

Analytical formulation of spatiotemporal modulated graphene-based waveguides using Floquet-Bloch theory

In this work, an analytical model to study graphene-based spatiotemporal modulated structures is developed and verified through comparison with full wave numerical simulations. Graphene is an ideal material for realizing spatiotemporal modulated structures at high frequencies of THz and optics. In this analysis, the electromagnetic response of studied structures is expressed in terms of weighted Floquet-Bloch modes supported by the structure, while graphene is modeled by a spatiotemporal modulated surface current that imposes certain boundary conditions on the modes. The developed analytical technique is a comprehensive tool and can be used for accurate modeling of different kinds of spatiotemporal devices including lossy, guided, and leaky wave structures. To demonstrate the accuracy of the model, two plasmonic waveguides with space and time modulated graphene conductivity are analyzed and their interband and intraband transition between modes are thoroughly investigated. Using the developed analytical model, spatiotemporal modulation phenomena such as mode conversion, wave amplification and nonreciprocal response are explored and discussed for the studied structures.

DFT assessments of optical and thermoelectric characteristics of (III/V)-doped elements into graphene sheets

First-principles simulations are conducted to explore the structural stability, electronic properties, and optical responses of pristine and boron- or nitrogen-doped monolayers graphene. The computed electronic density of states revealed that the substitutional doping of boron impurity atoms on monolayer graphene (MLG) shifts the Dirac point upward, although the substitution of nitrogen impurity atoms in graphene pushes the Dirac point downward the Fermi level. This could exhibit that upon the doping of MLG with boron or nitrogen, respectively, p-type or n-type semimetal is acquired. The overall optical spectral properties of the substituted graphene with boron or nitrogen atoms are simulated and compared with the optical spectra results of pure graphene. The optical features of pristine and doped MLG are determined by taking the interband and intra-band transitions into account ranging from the far-infrared to the ultraviolet regime of the electromagnetic radiation. A remarkable red shift in the optical spectra of the doped MLG towards the visible regime of radiation is established. An enhanced reflectivity illustrated that concentration-dependent optical properties of boron and nitrogen-doped MLG happen at lower electromagnetic radiation regimes. In addition, we explored the thermoelectric behaviors of the pristine/doped graphene monolayers with [Formula: see text] supercells. We found a significant improvement in the electrical conductivity of graphene when doped with boron or nitrogen impurities. However, an increase in the electrical conductivity has textured a decrease in the Seebeck coefficients. Improvement in the electrical conductivity is attributed to an interesting effect on the graphene monolayers’ power factor (PF). These findings indicate a positive impact of the dopants on the thermoelectric properties of graphene monolayers and reveal that they are potential materials for thermoelectric applications and nanodevices.

The conductivity effect of the top coating on optical properties of thin Cu(Ag)-layered structures

This study examines the optical properties of thin Cu (Ag)-layered structures covered with protective layers based on graphene, titanium (TiO2), or aluminium (Al2O3) oxides. The objective is to investigate the impact of these coatings on the optical behaviors of underlying metallic layers, specifically in the spectral range of excitation of surface plasmon resonances. Combining the methods of spectroreflectometry and spectro-ellipsometry was used to analyze the optical characteristics of the hybrid metal-oxide-graphene films. The study shows that graphene, due to its exceptional electrical conductivity and unique optoelectronic properties, significantly modifies the optical behavior of investigated structures. It includes notable changes in refractive and absorption indices, and optical conductivity indicating potential for enhancing light-matter interactions in plasmonic-graphene layered structures with the aim to apply as biosensor. It is important that addition of TiO2 and Al2O3 layers has also strong effects on the optical properties, which are relevant to their respective applications in the fields of optoelectronics and microelectronics. Employing the effective medium approximation and the Tauc–Lorentz model promotes deeper understanding the interplay between interband and intraband electronic transitions at the nanoscale level. It was revealed that the layer thickness of constituted materials and their individual dielectric functions together with addition of a graphene monolayer commit the significance for altering the optical properties of hybrid layered structures. The obtained results are important for the fields of plasmonics and nanotechnology, providing insights for designing sensors and devices with improved optical characteristics.

Refractory Plasmonics of Reactively Sputtered Hafnium Nitride Nanoparticles: Pushing Limits

AbstractHigh‐temperature plasmonics deals with optically active nanostructures that can withstand high temperatures. A conventional approach relying on standalone noble metal nanoparticles fails to deliver refractory plasmonic nanomaterials, and an alternative route envisions metal nitrides. The main challenge remains the development of advanced synthesis techniques and the insight into thermal stability under real‐life application conditions. Here, hafnium nitride nanoparticles (HfN NPs) can be produced by gas aggregation using reactive magnetron sputtering, a technique with a small environmental footprint are shown. As‐deposited NPs are of 10 nm mean size and consist of stoichiometric, crystalline fcc HfN. They are characterized by optical absorption below 500 nm caused by interband transitions and in the red/near‐infrared (NIR) region due to intraband transitions and localized surface plasmon resonance (LSPR). The optical response can be engineered by tuning the NP composition as predicted by finite‐difference time‐domain (FDTD) calculations. Going beyond the state‐of‐the‐art, the HfN NP thermal stability is focued under ultrahigh vacuum (UHV) and in air. During UHV annealing to 850 °C, the NPs retain their morphology, chemical and optical properties, which makes them attractive in space mission and other applications. During air annealing to 800 °C, HfN NPs remain stable until 250 °C, which sets a limit for air‐mediated use.

High-Order Harmonic Generation in Solids: The Role of Intraband Transitions in Extreme Nonlinear Optics.

High-order harmonic generation (HHG) in gases is frequently used nowadays to produce attosecond pulses and coherent radiation in the visible-to-soft X-ray spectral range. HHG in solids is a natural extension of the idea of HHG in gases, and its first observation about ten years ago opened the door to investigations on attosecond electron dynamics in solids and the development of solid-state attosecond light sources. The common process in both types of HHG is nonlinear photocarrier generation, and thus, transitions between different bands (interband transitions) are always important for HHG. As well, in the case of solids, the transitions within a band (intraband transitions) also need to be considered, because efficient carrier acceleration is possible due to them. This Perspective focuses on experimental findings that show how intraband transitions can be controlled because such an understanding will be essential in the development of unique optoelectronics that can operate at petahertz frequencies.

Strain-tuned optical properties of bilayer silicon at midinfrared wavelengths

Optical properties of two-dimensional bilayer silicon have been explored at midinfrared wavelengths using density functional theory. In this work, progressive atomic structural deformation and the resultant variations in the optical properties of the bilayer silicon films were investigated under external in-plane strain. A phase transformation of the atomic structure has been observed at an applied in-plane tensile strain of 5.17%, at which the atomic lattice is changed from a low buckled to a buckle-free honeycomb structure. Evaluations of the optical properties were carried out by taking into account the inter- and intraband transitions. An abrupt change in the optical refraction index was observed at the phase transition. In addition, the buckle-free honeycomb structure presents a strain-resistive absorption edge pinned at 1.14 μm wavelength. Exceeding a strain threshold of 12.26% results in the development of both direct- and indirect-energy bandgap openings. The direct bandgap induced interband optical transitions, resulting in absorption peaks at midinfrared wavelengths and a drastic increase in the refraction index. Moreover, by adjusting the strain, the optical absorptions can be tuned in a wide range of wavelength at midinfrared from 1.5 to 11.5 μm.

Spin dynamics of room temperature van der Waals (vdW) ferromagnets and their usage in microwave devices

Quasi-two-dimensional van der Waals (vdW) materials exhibiting room-temperature (RT) long-range ferromagnetic nature have emerged as a significant research field to explore fundamental condensed matter physics due to their intriguing physical properties. These vdW materials enable a futuristic platform for implementing novel spintronics devices. Here, we examined the spin dynamics of polycrystalline Fe5GeTe2 and Fe4.8Co0.2GeTe2 vdW materials using ferromagnetic resonance (FMR) spectroscopy. Vibrating Sample Magnetometer (VSM) study reveals that both materials have a soft ferromagnetic character at room temperature. From room temperature FMR measurements, the effective magnetization of Fe5GeTe2 and Fe4.8Co0.2GeTe2 derived ∼0.54 ± 0.056 and 0.50 ± 0.017 kOe, respectively. These results are consistent with reported VSM data. Fe5GeTe2 and Fe4.8Co0.2GeTe2 exhibit broad FMR linewidths of 0.697 ± 0.036 and 0.748 ± 0.056 kOe, respectively, which can be due to inhomogeneous line broadening. Besides its intrinsic contribution to linewidth, it is also affected by extrinsic Gilbert damping (αext). The value of αext is influenced by conflicting intra-band and inter-band electronic transitions, according to Modified Kambersky's theory. Furthermore, the effective Gilbert damping constant (α) obtained is 0.0513 ± 0.0046 for Fe5GeTe2 and 0.0526 ± 0.0031 for Fe4.8Co0.2GeTe2 at RT. Additionally, we developed microwave signal processing devices using these materials and evaluated their functionality both as a microwave band-reject filter and an adjustable phase shifter. The stop-band response was studied across the 5 to 25 GHz frequency range under an applied magnetic field as high as 7 kOe. For these flip-chip-based devices, attenuation is −5 dB/cm for the Fe5GeTe2-based filter and −3.2 dB/cm on sample Fe4.8Co0.2GeTe2 at 6.95 and 5.37 kOe, respectively. The same micro-strip filter was used as a tunable phase shifter in the off-resonance region. The optimal differential phase shift studied for Fe5GeTe2 and Fe4.8Co0.2GeTe2-based phase shifters in the high-frequency region (22 GHz for Fe5GeTe2 and 18 GHz for Fe4.8Co0.2GeTe2) is 23°/cm and 14°/cm, respectively, at high magnetic fields. These versatile devices find integration across a wide spectrum of applications, such as phased-array antennas, radar systems, and wireless communication systems, offering their benefits to diverse fields.

Enhancement of Spontaneous Photon Emission in Inverse Photoemission Transitions in Semiconductor Quantum Dots.

Inverse photoemission (IPE) is a radiative electron capture process where an electron is transiently captured in the conduction band (CB) followed by intraband de-excitation and spontaneous photon emission. IPE in quantum dots (QDs) bypasses optical selection rules for populating the CB and provides insights into the capacity for electron capture in the CB, the propensity for spontaneous photon emission, intraband transition energies where both initial and final states are in the CB, and the generation of photons with frequencies lower than the bandgap. Here, we demonstrate using time-dependent perturbation theory that judicious application of electric fields can significantly enhance the IPE transition in QDs. For a series of CdSe, CdS, PbSe, and PbS QDs, we present evidence of field-induced enhancement of IPE intensities (188% for Cd54Se54), field-dependent control of emitted photon frequencies (Δω = 0.73 eV for Cd54Se54), and enhancement of light-matter interaction using directed Stark fields (103% for Cd54Se54).

Intraband Exciton Transitions in Photosynthetic Complexes Revealed by Novel Five-Wave-Mixing Spectroscopy.

We calculate the χ(4) optical response of an oriented photosystem II reaction center of purple bacteria described by the Frenkel exciton model using nonlinear exciton equations (NEE). This approach treats each chromophore as an anharmonic oscillator and provides an intuitive quasiparticle picture of nonlinear spectroscopic signals of interacting excitons. It provides a computationally powerful description of nonlinear spectroscopic signals that avoids complete diagonalization of the total Hamiltonian. Expressions for the second- and the fourth-order nonlinear signals are derived. The NEE have been successfully employed in the past to describe even-order-wave-mixing. Here, we extend them to aggregates with broken inversion symmetries. Even-order susceptibilities require the introduction of permanent dipoles, which allow to directly probe low-frequency intraband transitions of excitons.

Exchange-correlation effects on the structural, electronic, and optical properties of Sr2RuO4 using DFT calculations

We perform a first principle study on the electronic, structural, and optical properties of Sr2RuO4 by various functionals: LSDA, GGA, GGA-mBJ, PBE-D3, optB88-vdW, vdW-DF2 and HSE06 using the plane-wave method of density functional theory (DFT) in the presence of the spin-orbit coupling (SOC) and Coulomb interaction parameter, U. The dynamical stability and the thermal stability are calculated via phonon dispersions and ab initio molecular dynamics simulations, respectively. The band structure calculation and the orbital-resolved density of states of LSDA and the schemes of GGA reveal different ground state. For GGA-mBJ and the hybrid functional, there is an indirect gap in the band structure. In the case of vdW functionals, our findings are in agreement with GGA. We compute a variety of optical responses and compare them with existing experimental studies. The results of the dielectric function suggest that GGA-mBJ (LSDA and GGA) may better account for the experimental findings in the presence (absence) of SOC at the infrared spectral region (infrared and visible spectral regions). We observe that the optical responses are strongly influenced by SOC and intraband transitions. Among the optical spectra, we focus more on the electron energy loss spectrum, which depends on the excitations, the exchange-correlation effects, and the multiorbital nature of Sr2RuO4.

Optical Transitions of a Single Nodal Ring in SrAs_{3}: Radially and Axially Resolved Characterization.

SrAs_{3} is a unique nodal-line semimetal that contains only a single nodal ring in the Brillouin zone, uninterrupted by any trivial bands near the Fermi energy. We performed axis-resolved optical reflection measurements on SrAs_{3} and observed that the optical conductivity exhibits flat absorption up to 129meV in both the radial and axial directions, confirming the robustness of the universal power-law behavior of the nodal ring. The axis-resolved optical conductivity, in combination with theoretical calculations, further reveals fundamental properties beyond the flat absorption, including the overlap energy of the topological bands, the spin-orbit coupling gap along the nodal ring, and the geometric properties of the nodal ring such as the average ring radius, ring ellipticity, and velocity anisotropy. In addition, our temperature-dependent measurements revealed a spectral weight transfer between intraband and interband transitions, indicating a possible violation of the optical sum rule within the measured energy range.