Interband Transitions Research Articles

Electron dynamics in α -quartz induced by two-color 10-femtosecond laser pulses

The time dependent density functional theory (TDDFT) is used to study the femtosecond laser induced excitation dynamics of electrons in bulk $\ensuremath{\alpha}$-quartz, and evaluate the laser energy deposition into this material. In order to properly distinguish the contributions of ionization (electron transitions from the valence band to the conduction band) and laser heating (electron transitions in the conduction band), two 10-femtosecond laser pulses exhibiting different wavelengths are used. Short wavelengths are expected to enhance the ionization rate, whereas longer wavelengths should be more suitable for excitation in the conduction band, thus providing a possible control of the whole electron dynamics. The influence of the pulse-to-pulse delay and intensities is studied. A significant enhancement of the interaction efficiency, in terms of excited electron density and their energy density, is observed for zero pulse-to-pulse delay. It is attributed to the opening of new ionization pathways involving various combinations of both photon energies ensuring the energy conservation, i.e., the sum of photon energies bridges the bandgap. This analysis is supported by a semi-analytical quantum model in the multiphoton absorption regime. The role and strength of direct interband transitions for the electron dynamics in the conduction band are highlighted. The associated laser energy deposition into the material is shown to be as efficient as collisional processes.

Epitaxial indium antimonide for multiband photodetection from IR to millimeter/terahertz wave

Conventional photodetection converts light into electrical signals only in a single electromagnetic waveband. Multiband detection technology is highly desirable because it can handle multispectral information discrimination, identification, and processing. Current epitaxial solid-state multiband detection technologies are mainly within the IR wave range. Here, we report epitaxial indium antimonide on gallium arsenide for IR and millimeter/terahertz wave multiband photodetection. The photoresponse originates from interband transition in optoelectrical semiconductors for IR wave, and surface plasmon polaritons induced nonequilibrium electrons for a millimeter/terahertz wave. The detector shows a strong response for an IR wave with a cutoff wavelength of 6.85 μm and a blackbody detectivity of 1.8 × 10 9 Jones at room temperature. For a millimeter/terahertz wave, the detector demonstrates broadband detection from 0.032 THz (9.4 mm) to 0.330 THz (0.9 mm); that is, from K a to the W and G bands, with a noise equivalent power of 1.0 × 10 − 13 W Hz − 1 / 2 at 0.270 THz (1.1 mm) at room temperature. The detection performance is an order of magnitude better while decreasing the temperature to 170 K, the thermoelectric cooling level. Such detectors, capable of large scale and low cost, are promising for advanced uncooled multiband detection and imaging systems.

Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates

Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates

Spectral Behaviour of the low concentrations of Coumarin 334 with Broadband Cavity Enhanced Absorption Spectroscopy

The current research is a spectroscopic study of Coumarin 334 dissolved in methanol. The range of concentrations of the prepared stock solution was (3.39x10-9 to 2.03x10-8) M. Some optical characteristics of this dye were investigated such as absorbance and transmission spectra, absorption coefficient, refractive and extinction coefficients, oscillation and dispersion energies, and energy band gap. The absorbance spectra were recorded at 452 nm using Broad Band Cavity Enhanced Absorption Spectroscopy (BBCEAS) which depends on increasing the path length of the traveling light from the source to the detector. The minimum absorbance amount was 0.07 with a low concentration of 3.39x10-9 M. As a result, the other optical properties were calculated on the basis of the lowest values of absorbance. The energy band gap, which is important to detect the electronic band structure of the material, was determined; it was found to be equal to ~2.55 eV. The low values of the concentration made less collision between the molecules in the materials and the incident light. This led to a reduction in the background noise and in the percentages of losses. Furthermore, the dispersion and single oscillator energies, which help to calculate the average strength of the inter-band optical transitions and to prepare the quantifiable information about the band structure of the material, were calculated to reduce with the increasing concentrations. The refractive and extinction coefficients were determined because they are considered important factors for the optical materials and found to increase with the increasing concentrations. As a result, the study of the optical behavior of Coumarin 334 highlighted the promising materials for photonics applications at very low concentrations. All these properties are considered the main factors to determine the usefulness of the materials in advanced applications and to develop the performance of the devices which depend on the optical characteristics.

Deep ultra-violet plasmonics: exploiting momentum-resolved electron energy loss spectroscopy to probe germanium.

Germanium is typically used for solid-state electronics, fiber-optics, and infrared applications, due to its semiconducting behavior at optical and infrared wavelengths. In contrast, here we show that the germanium displays metallic nature and supports propagating surface plasmons in the deep ultraviolet (DUV) wavelengths, that is typically not possible to achieve with conventional plasmonic metals such as gold, silver, and aluminum. We measure the photonic band spectrum and distinguish the plasmonic excitation modes: bulk plasmons, surface plasmons, and Cherenkov radiation using a momentum-resolved electron energy loss spectroscopy. The observed spectrum is validated through the macroscopic electrodynamic electron energy loss theory and first-principles density functional theory calculations. In the DUV regime, intraband transitions of valence electrons dominate over the interband transitions, resulting in the observed highly dispersive surface plasmons. We further employ these surface plasmons in germanium to design a DUV radiation source based on the Smith-Purcell effect. Our work opens a new frontier of DUV plasmonics to enable the development of DUV devices such as metasurfaces, detectors, and light sources based on plasmonic germanium thin films.

Effect of Iodine-doping toward ZnO critical optical constants and their applications to latent fingerprints detection

The present work focus on the influence of Iodine (I) doping on the improvement of properties of ZnO such as structural, optoelectronic, optical, dielectric and transport. I-doped ZnO materials were synthesized via an easy sol gel method assisted by microwave post-heated treatment. I doping level was 0, 0.025, 0.050, 0.075, and 0.100 at% with respect to Zn content. The deducted physico-chemical properties were based on XRD, Raman spectroscopy, EDS, XPS, SEM imaging, photoluminescence (PL) and UV-Visible diffuse reflectance (UV-Vis DRS) spectroscopy. The prepared nanomaterials were applied to latent fingerprints detection. The determined optical constants were based on diffuse reflectance data and consequently the refractive index and extinction coefficient. Tauc’s equation was used to evaluate both band gaps and inter-band transitions as impacted by doping. The dielectric properties, optical and electrical conductivities dependence on both frequencies and I-doping level were studied. Dissipation loss factors including surface and bulk contributions were evaluated. The dynamics of transport with the consideration of the four components dielectric relaxation time (τ), optical resistivity (ρopt), optical mobility (μopt), and optical charge carriers (Nopt), were discussed in terms of photons energy assessment and materials composition. The optical covalency and optical electronegativity modification were considered. IxZnO nanomaterials were applied successfully to fingerprint detection on various substrates such as glass surfaces, coins, and plastic sheets. Both I0.025ZnO and I0.1ZnO materials were identified as a potential candidate for forensic application.

The band structure calculation of tensile strained GaNAsBi/GaAs quantum well heterostructure

The optical properties of interband and intersubband transitions in the GaNAsBi/GaAs quantum well (QW) heterostructure have been systematically studied by solving the Schrödinger equation. Theoretical optimization of physical parameters of the GaNAsBi tensile strained layer has pointed out the importance of the polarization mode in the change of the optical absorption magnitude. Indeed, the N and Bi compositions and the quantum well width variations are thoroughly discussed. For specific well parameters, the intersubband absorption spectrum of QW operating at 9.5μm as well as its magnitude dependence on the applied electric field and doping concentration are simulated. Based on the calculated absorption spectra, we can expect that the photodetection spectra of the future designed system can be tuned by controlling these relevant parameters.

The free path and generation rate of fast-moving electron interacting with dielectric media

In the framework of macroscopic continuous medium approach, we have studied interaction between a fast-moving charged particle and dielectric or semiconducting media with low energy electrically active excitations. The excitations contribute to frequency dispersion of the media dielectric permittivity. Two types of processes induced by a moving charged particle have been considered: electron-hole generation under interband transitions and excitation of polar optical phonons. For both processes, we calculated and analyzed the time- and space-dependent electric potential generated by the charged particle, polarization of the media, energy losses of the particle and other important constituents of the interaction patterns. Obtained results can contribute to deeper understanding of the charged particle beams interaction with a semiconducting medium, as well as may be useful for versatile applications of charged beams.

Optical constants of single-crystalline Ni(100) from 77 to 770 K from ellipsometry measurements

Ellipsometry measurements were taken on single-crystalline Ni(100) at various temperatures between 77 and 770 K. DC conductivity and resistivity are extracted from the model optical constants and their temperature dependence is discussed. The authors find only qualitative agreement in the general trend of the resistivity measured by ellipsometry and electrical measurements. The temperature dependence of the main absorption peak at 4.8 eV indicates that the interband transitions are scattered by magnons with an effective energy of about 53 meV. The width of the main absorption peak reduces by 0.38 eV as the temperature rises, which is interpreted as the ferromagnetic exchange energy at the L-point. The small absorption peak at 1.5 eV is prominent only in the ferromagnetic phase and almost disappears in the paramagnetic phase. This peculiarity is explained by assigning the peak to K3↓→K2↓ transitions, which accounts for the decrease of the magnitude of the peak and its constant energy.

Explanation of the Opposing Shifts in the Absorption Edge and the Optical Resonance in CuFeS2 Nanoparticles

Size-dependent change of the electronic band structure is one of the key features of nanoparticles in the quantum confinement region. CuFeS2 nanoparticles have a strong absorption feature in the visible region that has, controversially, been described as neither an excitonic transition nor a free carrier plasmon oscillation. Instead, the absorption feature in CuFeS2 nanoparticles has been attributed to quasi-static optical resonances from inter-band transitions between the valence band (VB), intermediate band (IB), and conduction band (CB). As such, we hypothesized that the feature should be subject to quantum confinement effects through modification of the electronic bands. In this paper, we show experimentally that the optical resonance absorption peak red-shifts and the optical band gap blue-shifts as the particle size decreases. Through density functional theory (DFT) and the tight binding (TB) modeling, we elucidate the size dependence of the band structure, especially focusing on the change in the IB. Using a Lorentzian oscillator optical model to simulate the absorption spectrum with inputs from the DFT-calculated band structure and band shifts from the TB model, we find that the size-dependent shifts of the optical resonance peak position in CuFeS2 are due to a tri-band quantum confinement effect that results in both the VB to IB and IB to CB gap expansion that accompanies a decrease in particle size. We also find that the transitions between the IB and CB play only a minor role in the optical spectrum. Moreover, the linear optical Lorentzian model predicts that the optical resonance peak is tunable across the visible range by partially filling the IB, lowering the CB, or expanding the IB.

Photoinduced interlayer dynamics in Td-MoTe2: A broadband pump-probe study

We report on time-resolved broadband transient reflectivity (tr-bb-TR) measurements performed on a bulk single crystal of Td-MoTe2 as a function of the incident pump fluence (F). Tr-bb-TR data unveil photoinduced electronic changes progressing on the sub-picosecond timescale as well as the dynamics of the coherent low-frequency 1A1 interlayer shear phonon. Our results indicate a gradual evolution of both the TR and the 1A1 Fourier intensity spectra as a function of F, ruling out the threshold-like change that has been associated with the ultrafast photoinduced Td → 1T ′ phase transition. We also observe a large redshift of the 1A1 Fourier spectral features, which implies that large renormalization effects are taking place on the interband transitions that are dielectrically susceptible to the 1A1 interlayer shear phonon displacement.

The correlation of quadrupole transition rates of deformed nuclei by non-parametric approach

In this study, we tried to investigate the possible correlation of inter and intra-band quadrupole transition probabilities of rotational bands. To this aim, all the available experimental values of quadrupole transition rates between different levels of the ground band together quadrupole transitions of gamma and beta bands, 2γ+→0g+ and 2β+→0g+, of even-even prolate nuclei in the 150<A<250 mass region are analyzed. We extended the kernel density estimation approach as a non-parametric estimation formalism for this analysis and compared its predictions with the results of the commonly used parametric technique with emphasis on Porter-Thomas distribution. The results show that the statistical correlation is dominant for such inter-band transitions between different levels of the ground band but for intra-band transitions which originated from the beta band, deviation from correlated behavior is obvious. Also, the comparison of the statistical behavior of 2+→0+ transitions of different rotational bands, suggests the most GOE-like behavior for the quadrupole transitions between the levels of the ground band.

Tracking Ultrafast Change of Multiterahertz Broadband Response Functions in a Photoexcited Dirac Semimetal Cd3As2 Thin Film.

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd3As2 by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 μm), which covers the crossover between the inter- and intraband response. We resolve dynamics of the photoexcited nonthermal electrons, which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics.

Photoluminescence of Cis-Polyacetylene Semiconductor Material

Photoluminescence (PL) is one of the key experimental characterizations of optoelectronic materials, including conjugated polymers (CPs). In this study, a simplified model of an undoped cis-polyacetylene (cis-PA) oligomer was selected and used to explain the mechanism of photoluminescence (PL) of the CPs. Using a combination of the ab initio electronic structure and a time-dependent density matrix methodology, the photo-induced time-dependent excited state dynamics were computed. We explored the phonon-induced relaxation of the photoexcited state for a single oligomer of cis-PA. Here, the dissipative Redfield equation of the motion was used to compute the dissipative excited state dynamics of electronic degrees of freedom. This equation used the nonadiabatic couplings as parameters. The computed excited state dynamics showed that the relaxation rate of the electron is faster than the relaxation rate of the hole. The dissipative excited-state dynamics were combined with radiative recombination channels to predict the PL spectrum. The simulated results showed that the absorption and emission spectra both have a similar transition. The main result is that the computed PL spectrum demonstrates two mechanisms of light emission originating from (i) the inter-band transitions, corresponding to the same range of transition energies as the absorption spectrum and (ii) intra-band transitions not available in the absorption spectra. However, the dissipative Redfield equation of the motion was used to compute the electronic degrees of freedom of the nonadiabatic couplings, which helped to process the time propagation of the excited dynamic state. This excited dynamic state shows that the relaxation rate of the electron is faster than the relaxation rate of the hole, which can be used for improving organic semiconductor materials for photovoltaic and LED applications.

Proton-neutron symplectic model description of 20Ne

A microscopic description of the low-lying positive-parity rotational bands in 20Ne is given within the framework of the symplectic-based proton-neutron shell-model approach provided by the proton-neutron symplectic model (PNSM). For this purpose, a model Hamiltonian is adopted. This includes an algebraic interaction lying in the enveloping algebra of the dynamical group of the PNSM, which introduces both horizontal and vertical mixings of different irreducible representations within the irreducible collective space of 20Ne. A good overall description is obtained for the excitation energies of the ground and first two excited β bands, including the ground state intraband quadrupole collectivity and the known interband transition probabilities between the low-lying collective states, without utilizing an effective charge.

High-entropy-alloy nanoparticles with 21 ultra-mixed elements for efficient photothermal conversion.

Multi-metallic nanoparticles have been proven to be an efficient photothermal conversion material, for which the optical absorption can be broadened through the interband transitions (IBTs), but it remains a challenge due to the strong immiscibility among the repelling combinations. Here, assisted by an extremely high evaporation temperature, ultra-fast cooling and vapor-pressure strategy, the arc-discharged plasma method was employed to synthesize ultra-mixed multi-metallic nanoparticles composed of 21 elements (FeCoNiCrYTiVCuAlNbMoTaWZnCdPbBiAgInMnSn), in which the strongly repelling combinations were uniformly distributed. Due to the reinforced lattice distortion effect and excellent IBTs, the nanoparticles can realize an average absorption of >92% in the entire solar spectrum (250 to 2500 nm). In particular, the 21-element nanoparticles achieve a considerably high solar steam efficiency of nearly 99% under one solar irradiation, with a water evaporation rate of 2.42 kg m–2 h–1, demonstrating a highly efficient photothermal conversion performance. The present approach creates a new strategy for uniformly mixing multi-metallic elements for exploring their unknown properties and various applications.

Bismuth microsphere for photo-assisted nitrate removal: Experimental and theoretical investigations

• The nitrate was effectively and selectively removed by bismuth microspheres. • The photo activity of bismuth was attributed to electron transfer. • The effects of adsorption on the orbitals of nitrogen-compounds were analyzed. • Theoretical calculation reveals the process of electron transfer. Nitrate is a common pollutant in both natural and engineered water environments; nitrate removal has thus been a long-standing challenge. In this study, inert bismuth microspheres were developed for the efficient removal of nitrate from water under ultraviolet irradiation. Based on the experimental conditions of a 25 mg dosage of bismuth microspheres and a reaction time of 60 min, the nitrate could be removed with an efficiency as high as 98.2%, with a total nitrogen (TN) removal of 93.7% and negligible accumulation of nitrite. This indicated the high activity and selectivity of the bismuth microspheres for nitrate removal. Combined with the properties of bismuth and the morphology of the samples, the optical properties of bismuth microspheres were mainly due to the inter-band transition, and the electron transfer in the inter-band transition resulted in the reduction of nitrate. The detailed electron transfer mechanism was elucidated by density functional theory calculations, confirming that both direct and indirect electron transfer occurred between bismuth and nitrate. In conclusion, this study provides a proof-of-concept demonstration of the use of bismuth metal for more effective nitrate removal in water treatment.

Synergistic Effect of Au Interband Transition on Graphene Oxide/ZnO Heterostructure: Experimental Analysis with FDTD Simulation.

We furnish a comprehensive study on light-induced carrier generation due to the synergistic contribution of Au interband transition and graphene oxide (GO)/ZnO heterostructure. Plasmonic gold nanoparticles (Au_nps) are incorporated as a substructure sandwiched between GO and ZnO, assisting in additional photo-induced charge carrier generation. GO is prepared by a single-step plasma-enhanced chemical vapor deposition process. The GO/ZnO heterostructure having an active working area of 0.25 cm2 is created to unleash the pyroelectric property of ZnO, and subsequently, Au_np is introduced at the interface of GO/ZnO. Here, the interband transition of Au_np and its capability for charge carrier generation combined with the excitonic charge carrier generation of the highly crystalline non-centrosymmetric hexagonal wurtzite ZnO enhances the photoresponse. Furthermore, the interaction of Au_np with ZnO and its spatial electric field intensity distribution is demonstrated by finite difference time domain simulation which indicate toward an efficient carrier generation at the interface of Au_np and ZnO. The fabricated heterostructure has an active working wavelength in the UV-A region with the highest responsivity at 375 nm of the electromagnetic spectrum. The ultrafast response time (∼29 μs) of the device is due to the pyro-phototronic effect of the GO/ZnO heterostructure enhanced by the interband transition of Au. In the comparative study of the Au_np-enriched GO/ZnO heterostructure device with a GO/ZnO device, the former shows better performance. Both the devices work in the self-powered mode as well as the photoconductive mode, but with a higher on–off current ratio in the photoconductive mode. Hence, this work helps in properly understanding photo-induced charge generation in a Au interband transition enriched GO/ZnO heterostructure.

Quantitative evaluation of LAL productivity of colloidal nanomaterials: Which laser pulse width is more productive, ergonomic, and economic?

Near-IR (wavelength ≈ 1 μm) laser ablation of bulk, chemically-inert gold in water was compared for different laser pulse width in broad the range of 300 fs–100 ns, comparing a number of key ablation characteristics: mass loss, single-shot crater volume and extinction coefficient of the generated colloidal solutions taken in the spectral ranges of interband transitions and localized plasmon resonance. Comparing to related air-based ablation results, at the given fluences laser ablation in the liquid resulted in the maximum ablation yield per unit energy and maximum NP yield per pulse and per unit energy for the picosecond lasers, occurring at subcritical peak pulse powers for laser self-focusing. The self-focusing effect was demonstrated to yield in incomplete, effectively weaker focusing in the water filaments of ultrashort laser pulses with supercritical peak powers, comparing to linear (geometrical) focusing at sub-critical peak powers. At the other, nanosecond-pulse extreme the high ablation yield per pulse, but low ablation yield per unit energy and low NP yield per pulse and per unit energy were related to strong ablation plasma screening, providing mass removal according to the well-established scaling relationships for plasma. Illustrative comparison of the ablation and nanoparticle generation efficiency versus the broad fs–ns laser pulse width range was enabled in terms of productivity, economicity, and ergonomicity, using the proposed universal quantitative criteria.

Nonlinear antidamping spin-orbit torque originating from intraband transport on the warped surface of a topological insulator

Motivated by recent experiments observing a large antidamping spin-orbit torque (SOT) on the surface of a three-dimensional topological insulator, we investigate the origin of the current-induced SOT beyond linear-response theory. We find that a strong antidamping SOT arises from intraband transitions in non-linear response, and does not require interband transitions as is the case in linear transport mechanisms. The joint effect of warping and an in-plane magnetization generates a non-linear antidamping SOT which can exceed the intrinsic one by several orders of magnitude, depending on warping parameter and the position of Fermi energy, and exhibits a complex dependence on the azimuthal angle of the magnetization. This nonlinear SOT provides an alternative explanation of the observed giant SOT in recent experiments.