Changes In Absorption Coefficient Research Articles

Switching the Nonlinear Optical Absorption of Titanium Carbide MXene by Modulation of the Surface Terminations.

Surface terminations of two-dimensional materials should have a strong influence on the nonlinear optical (NLO) properties, but the relationship between surface terminations and NLO properties has not yet been reported. In this work, switching the NLO properties of MXenes (Ti3C2Tx) via "surface terminations modulation" is explored. The surface terminations of Ti3C2Tx are modulated by electrochemical treatment, resulting in different states (viz., Ti3C2Tx(pristine), Ti3C2Tx(═O rich), and Ti3C2Tx(-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (βeff) change with the surface terminations. Ti3C2Tx(═O rich) shows a relatively large saturable absorption (SA) with laser excitation at 515 nm (βeff = -1020 ± 136.2 cm GW-1), while reverse saturable absorption (RSA) is found in Ti3C2Tx(pristine) and Ti3C2Tx(-OH rich). The RSA of Ti3C2Tx(pristine) and Ti3C2Tx(-OH rich) is attributed to excited-state absorption, while the SA of Ti3C2Tx(═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the βeff of Ti3C2Tx(-OH rich) is 113 ± 3.2 cm GW-1, 1.68 times that of Ti3C2Tx(pristine); the RSA is caused by photon-induced absorption. Our results reveal a correlation between surface terminations and NLO properties, highlighting the potential of MXenes in photoelectronics.

Theoretical and experimental research of femtosecond laser processing fused silica

The ablation threshold, depth and crater shape of fused silica for femtosecond laser processing are investigated theoretically and experimentally. Based on tracking the spatiotemporal distribution of the free electron density, free electron temperature, and laser intensity, the electron dynamics as well as the transient optical and thermophysical properties of femtosecond laser irradiated fused silica are quantitatively determined. The numerical model is validated by comparing the calculated threshold fluence, depth and crater shape of ablation with the experimentally measured ones at a wavelength of 800 nm. The free electron relaxation time at different laser fluences and pulse durations throughout the photoionization process and impact ionization process are probed. In the present work, the findings are as follows. 1) The electron relaxation time significantly affects the material optical properties and femtosecond laser energy absorption. The optical properties change dramatically. The fused silica becomes opaque for the case of laser irradiation with fluence higher than the ablation threshold. Moreover, the transition from electron-phonon collision to electron-ion collision accompanies with the femtosecond laser ablation of fused silica. 2) By using the proposed model, the experimentally observed saturation of ablation depth at high laser fluence is elucidated by the significant change of optical reflectivity and absorption coefficient. Both the results of theoretical simulation and experimental observation indicate that laser fluence has a strong influence on the shape of the ablation crater. The ablation volume increases sharply with the increase of laser fluence for femtosecond laser irradiation, compared with that for picosecond laser irradiation. 3) With the increase of femtosecond laser fluence, the ablation depth removal efficiency and ablation efficiency are both saturated, followed by slight decrements. The peak of ablation depth removal efficiency peak occurs at the femtosecond laser fluence close to 1.4 times of the ablation threshold. While the accuracy is slightly low due to the higher sensitivity of the ablation characteristics (ablation crater depth and ablation volume) to the shorter femtosecond laser pulse. For the femtosecond laser fluence higher than 3.5 times of the ablation threshold, good repeatability over a very wide fluence range can achieve accurate processing results, because a more consistent flat-bottom ablation profile tends to appear. However, the heat-affected zone leads the processing quality to degrade, compared with the scenario of femtosecond laser fluence close to the ablation threshold.

The Total Absorption Coefficient and Refractive Index Change of a Spherical Quantum Dot in the Confinement Radial Potential

The Total Absorption Coefficient and Refractive Index Change of a Spherical Quantum Dot in the Confinement Radial Potential

Effects of electron-phonon interaction and pressure on optical properties of wedge-shaped quantum dots

The energy levels and lattice vibrational modes in quantum dots (QDs) become discrete due to 3D quantum confinement. Therefore, the influence of electron-phonon (e-p) interaction on physical properties of QDs is an important subject to study. In this work, the effects of e-p interaction and pressure in a GaAs wedge-shaped (WS) quantum dot are considered. In first step, using the dielectric continuum model, the Hamiltonian of longitudinal optical (LO) phonons and the e-p interaction Hamiltonian are analytically derived. In the next step, we have studied the e-p effect on energy eigenvalues, magnetic susceptibility and optical properties of the WS quantum dot. We have also investigated the pressure effect on optical properties of the system in the presence of the e-p interaction. It is found that the polaronic energy shift reduces when the QD size increases. The magnetic susceptibility enhances with raising the QD size. The e-p effect on magnetic susceptibility is strong for smaller QD sizes. The absorption coefficient shows two peaks in the presence of the e-p interaction. The refractive index change enhances and moves to higher energies under the e-p interaction. We found that the pressure and e-p interaction, simultaneously, have important effects on absorption coefficient and refractive index changes.

Complex optical properties of polymeric composite materials mixed with quartz powder and investigated by THz time-domain spectroscopy

With the rapid development of terahertz (THz) technology comes the need to further explore the prospects for various applications of THz systems. Due to the strong need, components and equipment involving the exploration are indispensable. In order to find the most suitable material for THz technology, we selected three common materials for different 3D printing techniques—polyamide (PA), polylactic acid (PLA), and light-curable resin. After mixing each material with a quartz powder of a different weight percentage, we observed the change in absorption coefficients and refractive indices of the mixtures by THz time-domain spectroscopy (THz-TDS). The higher the ratio of a quartz powder to a mixture was, the smaller the absorption coefficient of the mixture would be. The optimum rate of change in the absorption coefficient was attained when the weight percentage of a quartz powder in a mixture was 50 wt%. At 1 THz of the measurement of THz-TDS, the average reduction in the absorption coefficients of the three different materials mixed respectively with a 50 wt% quartz powder was 39.17%. Besides reduced absorption coefficients, the mixtures’ refractive indices also changed as the weight percentage of a quartz powder in the mixtures varied. The PLA-based sample mixed with a 50 wt% quartz powder had the highest increase in the refractive index. Mixing quartz powders with materials, therefore, is an effective method to increase refractive indices and decrease absorption coefficients. The method can be applied in 3D printing techniques in the future to enhance the efficiency of THz components manufactured with 3D printing techniques.

Optical Properties of Gaas/Alxga1-xas Superlattice Under E-Field for Quantum Cascade Laser Application

Optical properties of GaAs/AlxGa1-xAs superlattice are studied dependent on quantum well thickness of gain region and doping density of injector layers underperformed electric field. Conduction band alignment of the superlattice is obtained by using effective mass approximation. 1d-Schrodinger formula is solved by using FDM. Intersubband transition energies, linear (nonlinear and total) absorption coefficients and linear (nonlinear and total) refractive index changes are plotted under applied electric field intensity. Intersubband transition energy of electron from second excited state to first excited state shows 147 meV. It is found that -45 kV/cm electric field intensity and 5 nm layer thickness of last quantum well of the gain region are the best values for studied structure. After that, linear absorption coefficient is investigated dependent on carrier number in the injector region under electric field. It is found that carrier number over 5 𝑥 1016 𝑐𝑚−2 can causes huge internal absorption of the radiative emission obtained in gain region due to increase in linear absorption coefficient by factor 10. As a conclusion, total absorption coefficient and total refractive index change are calculated for optimized parameters.

First-principles study on the electronic and optical properties of strain-tuned mixed-halide double perovskites Cs2TiI[formula omitted]Br[formula omitted

The properties of Cs2TiI6−xBrx using density functional theory were investigated. With the increase of Br doping, the structural stability is improved. Then the pressure is introduced to change the energy gap of Cs2TiI6−xBrx. Meanwhile, the optical absorption coefficient changes obviously with both pressure and Br doping. The major absorption region narrows and the peak value becomes higher with the increasing of pressure. While the peak value starts with a small decrease and then a rise with Br doping. Normalized the absorption rate and compared with the wavelength in sunlight, we get the optical coincidence index (OCI). These results demonstrate that under the influence of doping, both Cs2TiI2Br4 and Cs2TiI3Br3 are suitable materials for solar cells. When the pressure increases, the stability of Cs2TiI6−xBrx is enhanced, while the optical absorption is attenuated. Based on suitable absorption coefficient and strong OCI, Cs2TiI2Br4 and Cs2TiIBr5 are ideal light harvesting materials.

Phonon-assisted pressure-dependent opto-electronic properties in a CdS/CdSe/CdS asymmetric quantum well

ABSTRACT Pressure-associated phonon-induced opto-electronic properties are brought out in a CdS/CdSe/CdS double quantum well. The effects of pressure on the interband associated with the changes of refractive index and optical absorption coefficients are investigated in the asymmetric well by altering its right well size and the barrier size. Aldrich and Bajaj polaronic effect is employed in the Hamiltonian. The effects of pressure on the optical properties due to the interband transition in the double quantum well are carried out. The single band effective mass, envelope function approximation and density matrix are used as theoretical tools in order to obtain these properties. The pressure produces a reduction in a radiative life time. The enhancement in the magnitude of absorption coefficients occurs due to the applications of hydrostatic pressure. The results bring out the fact that the optical susceptibilities suffer a blue shift and its magnitudes are determined by the matrix elements.

Efficient Electronic Modulation of g-C 3 N 4 Photocatalyst by Implanting Atomically Dispersed Ag 1 -N 3 for Extremely High Hydrogen Evolution Rates

Efficient Electronic Modulation of g-C ₃ N ₄ Photocatalyst by Implanting Atomically Dispersed Ag ₁ -N ₃ for Extremely High Hydrogen Evolution Rates

Gradual Carrier Filling Effect in "Green" InGaN/GaN Quantum Dots: Femtosecond Carrier Kinetics with Sequential Two-Photon Absorption.

Quantum dots (QDs) allow for a significant amount of strain relaxation, which is helpful in GaN systems where a large lattice mismatch needs to be accommodated. InGaN QDs with a large indium composition are intensively investigated for light emitters requiring longer wavelengths. These are especially important for developing high-efficiency white light sources. Understanding the carrier dynamics in this large lattice-mismatched system is essential to improving the radiative efficiency while circumventing high defect density. This work investigates femtosecond carrier and photon dynamics in self-organized In0.27Ga0.73N/GaN QDs grown by molecular beam epitaxy using transient differential absorption spectroscopy, which measures the differential absorption coefficient (Δα) with and without an optical pump. Due to 3D quantum confinement and the small effective mass of InGaN, the low density of states in the conduction band is easily filled with electrons. In contrast, the GaN barrier region is replete with a high density of electrons due to a large effective mass. This contrast in carrier density creates a unique phenomenon in the dynamics, showing a change in the differential absorption coefficient (Δα) sign from negative to positive with time. The ultrafast microscopic processes indicate that right after the optical pump and first photon absorption, the valence (conduction) band states are depleted (replete) of electrons. This ground-state bleaching process makes Δα negative, and the probe beam is not absorbed. The electrons are then gradually transferred from the GaN barrier into InGaN QDs, which absorb the second photon from the probe beam (excited-state absorption), making Δα positive. The presence of excited-state carriers with a long lifetime is indicative of the enhanced availability of carriers for radiative recombination. This effect also promotes stimulated emission and amplified spontaneous emission, which can be used to develop lasers and superluminescent LEDs, respectively. Measurements with multiple pump powers and temperatures further confirm that the efficacy of InGaN QDs is enhanced by this effective mass contrast and 3D reservoir of carriers from the GaN barrier. This effect can be used to improve the internal quantum efficiency of GaN-based light emitters.

Just noticeable difference of sound pressure level of speech in open-plan office

In this study, the general sound environment characteristics of open-plan office (OPO) were investigated, and just noticeable difference (JND) of sound pressure level of speech at a distance of 4 m (Lp,A,S,4m) suggested in ISO 3382-3 was suggested. First, in order to understand the sound environment characteristics of OPO, one minute sound sources recorded in 8 offices were collected and physical and psychological acoustic characteristics were analyzed. A total of 30 office workers were subject to subjective evaluation on 8 sound sources, and they were asked to respond to questionnaires related to annoyance, work satisfaction, and speech privacy. Next, to investigate the JND, two computer simulation models identical to those of the actual OPO were implemented, and sound sources each having six different Lp,A,S,4m values were generated through the change of the sound absorption coefficient of the interior finish. The JND of Lp,A,S,4m was presented by performing paired comparison for the same subjects. It is expected that the JND of Lp,A,S,4m proposed in this study can be used for the sound environment rating of OPO.

Ultra-wide frequency tuning range for optical properties in coaxial quantum well driven by intense laser field

A new approach for controlling intersubband transitions and optical properties of coaxial cylindrical quantum well is proposed. Considering the correct dressing effect for the confinement potential, the new ultra-wide tuning mechanism is based on the variation of quasibound states with the intensity of long-wavelength laser field. By means of the finite difference method, the results provide a new explanation for the anticrossing of energy levels and exhibit great potential for ultra-wide frequency tunability (ranging from terahertz to the mid-infrared region) of the optical absorption coefficient and refractive index changes by modulating the laser parameters and barrier thickness.

Suppression of lowering in conversion efficiency under temperature variation for multijunction laser power converters

Conventional multijunction photovoltaic cells used for optical wireless power transmission, often referred to as laser power converters, currently involve a serious problem. The conversion efficiency lowers under temperature variation more notably than that of single-junction cells, because of changing absorption coefficient and resultant current mismatching among the subcells. Light-trapping using an anti-reflection coating on the front face and a diffuse reflector on the rear face solves this problem, because intensity distribution of the incident light inside the cell remains almost uniform even though the absorption coefficient changes. An angular-selective filter on the front face further improves the efficiency.

DFT Study on Regulating the Electronic Structure and CO2 Reduction Reaction in BiOBr/Sulphur-Doped G-C3N4 S-Scheme Heterojunctions

Photocatalytic CO2 reduction is a promising method to mitigate the greenhouse effect and energy shortage problem. Development of effective photocatalysts is vital in achieving high photocatalytic activity. Herein, the S-scheme heterojunctions composed by BiOBr and g-C3N4 with or without S doping are thoroughly investigated for CO2 reduction by density functional theory (DFT) calculation. Work function and charge density difference demonstrate the existence of a built-in electric field in the system, which contributes to the separation of photogenerated electron-hole pairs. Enhanced strength of a built-in electric field is revealed by analysis of Bader charge and electric field intensity. The results indicate that S doping can tailor the electronic structures and thus improve the photocatalytic activity. According to the change in absorption coefficient, system doping can also endow the heterojunction with increased visible light absorption. The in-depth investigation indicates that the superior CO2 reduction activity is ascribed to low rate-determining energy. And both of the heterojunctions are inclined to generate CH3OH rather than CH4. Furthermore, S doping can further reduce the energy from 1.23 to 0.44 eV, indicating S doping is predicted to be an efficient photocatalyst for reducing CO2 into CH3OH. Therefore, this paper provides a theoretical basis for designing appropriate catalysts through element doping and heterojunction construction.

Enhanced two-photon absorption of sandwich-like coordination complexes based on squaraine and metallomacrocycle derivatives

Covalent or noncovalent multi-chromophore structures have expanded the discovery of appreciable nonlinear optical (NLO) materials. We present here a report of how drastic changes in two-photon absorption (2PA) coefficients can enhance from slight changes of electronic delocalization through coordination interaction. Experimental results illustrate that 2PA is the dominant phenomenon contributing to the NLO properties of the complexes recorded in the range of 750–1550 nm by open aperture Z-scan method. The values of degenerate 2PA cross sections (δ2PA) can be enhanced obviously by coordination interactions assisted by steric effect and intermolecular charge transfer, not showing a simply sum of the nonlinear parameters of the starting components. Importantly, the complexes feature high δ2PA values up to 12,700 GM at 1400 nm in the second near-infrared region. Both complexes with large 2PA coefficients and multiple excitation wavelengths may offer a novel avenue for creating useful nonlinear absorbing materials.

Nonlinear optical properties in AlGaAs/GaAs symmetric coupled quantum wells

Based on the density matrix approach and iterative method, we calculated the analytic forms of linear and third-order nonlinear optical absorption coefficients and refractive index changes for symmetric coupled quantum wells (SCQWs) considering a two-level system, and gave the corresponding numerical results. The calculated results show that the optical properties of SCQWs are significantly better than those of asymmetrically coupled quantum wells, and the optical absorption coefficient and refractive index change are affected by the structure parameters of the SCQWs and the incident optical intensity. Their physical mechanisms are elucidated.

Magneto‐Electric Directional Anisotropy in Polar Soft Ferromagnets of Two‐Dimensional Organic–Inorganic Hybrid Perovskites

AbstractTwo‐dimensional organic–inorganic hybrid perovskites (2D‐OIHPs) are attracting interest due to their structural tunability and rich functional characteristics, such as ferroelectricity and ferromagnetism. Here, we report the chiral‐polar ferromagnetic 2D‐OIHP copper chlorides with discernable electric polarization in the inorganic layers. In these systems, the magneto‐electric (ME) correlation has been clearly observed by measuring a magneto‐electric directional anisotropy (MEA), in which an optical absorption coefficient changes with reversal of the light propagating direction. We have found that the MEA can be induced by a low magnetic field of about 50 mT, reflecting soft magnetic nature. The present results suggest a new paradigm for designing functional ME multiferroics, which effectively couples magnetic and electric properties.

Magneto-Electric Directional Anisotropy in Polar Soft Ferromagnets of Two-Dimensional Organic-Inorganic Hybrid Perovskites.

Two-dimensional organic-inorganic hybrid perovskites (2D-OIHPs) are attracting interest due to their structural tunability and rich functional characteristics, such as ferroelectricity and ferromagnetism. Here, we report the chiral-polar ferromagnetic 2D-OIHP copper chlorides with discernable electric polarization in the inorganic layers. In these systems, the magneto-electric (ME) correlation has been clearly observed by measuring a magneto-electric directional anisotropy (MEA), in which an optical absorption coefficient changes with reversal of the light propagating direction. We have found that the MEA can be induced by a low magnetic field of about 50 mT, reflecting soft magnetic nature. The present results suggest a new paradigm for designing functional ME multiferroics, which effectively couples magnetic and electric properties.

Reconstituting C-Centered Hexagold(I) Clusters with N-Heterocyclic Carbene Ligands

Abstract Ubiquitous intramolecular aurophilic interactions in gold clusters have usually been assigned as the main determinants of their molecular geometry and various properties. By utilizing N-heterocyclic carbene (NHC) ligands instead of phosphine ligands which have been exclusively used, we have developed ligand-specific photophysical properties of C-centered hexagold(I) (C@Au6) clusters. This account describes the recent development of carbene-protected C@Au6 clusters. First of all, a strong intramolecular C–H⋯Au interaction was found as a key structural feature of these clusters. These are also present in the newly emerged NHC-gold nanoclusters, but are negligible in the phosphine counterparts. Interestingly, the use of NHC ligands also produces clusters with a nearly identical metal kernel, but their luminescence changes significantly. In addition, a simple extension of the π-conjugated system of NHCs from imidazolylidene to benzimidazolylidene is expected to produce significant changes in both absorption coefficient and emission wavelength. These results show the promising potential of carbene ligands in character modulation of metal clusters, and may help in the rational design of carbene ligands for constructing functional metal clusters.

Linear and nonlinear optical properties in spherical quantum dots: Inversely quadratic Hellmann potential

The aims of the reported work are to provide new insights into the quantum dot optical properties confined in an inverse of a quadratic Hellmann potential. The Schrödinger equation is solved using the Nikiforov-Uvarov (NU) method, in order to obtain the analytical expressions of the eigenenergies and the eigenfunctions. The linear together with the third-order nonlinear changes in absorption coefficients and refractive index are investigated using the density matrix formalism. The absorption coefficients and the refractive index changes are strong related to the structure parameters and the optical intensity.