Linear Optical Absorption Spectra Research Articles

Magnetic field-engineered optical nonlinearity in germanene nanotubes

Abstract The linear and nonlinear optical properties of germanene nanotubes (GeNTs) under the influence of magnetic fields are investigated through theoretical analysis. By employing a tight-binding model beyond the Dirac cone approximation, the electronic band structure, density of states, dipole matrix elements, and third-order nonlinear optical susceptibilities are calculated. The application of a magnetic field induces a Zeeman splitting of the energy levels, breaking the band degeneracy. Consequently, the dipole matrix elements exhibit a magnetic field dependence, altering the intensity and number of allowed transitions. In the infrared and ultraviolet energy regions, the linear optical absorption spectrum displays the splitting of peaks into dual neighboring peaks, with the splitting distance linearly dependent on the magnetic field strength. The third-order nonlinear optical susceptibilities, χTHG(3)3ω and χIDIR(3)ω, reveal the emergence of multiple resonance peaks in different energy regions due to two-photon and three-photon absorption processes. The magnetic field significantly influences the number, position, and height of these optical peaks, enabling effective control over the nonlinear optical response. By investigating the scaled energy (E/E_g) dependence, a universal behavior is observed for the three- and two photon resonance peaks, where their positions remain constant, but their peak intensities are significantly enhanced under the influence of the magnetic field. These findings demonstrate the potential of GeNTs as tunable nonlinear optical materials, with the magnetic field serving as an effective parameter for engineering their optical properties for various applications in optoelectronics, photonics, and related fields.

Spectroscopic Identification of the Charge Transfer State in Thiophene/Fullerene Heterojunctions: Electroabsorption Spectroscopy from GW/BSE Calculations.

Creation of charge transfer (CT) states in bulk heterojunction systems such as C60/polymer blends is an important intermediate step in the creation of carriers in organic photovoltaic systems. CT states generally have small oscillator strengths in linear optical absorption spectroscopy owing to limited spatial overlap of electron and hole wave functions in the CT excited state. Electroabsorption spectroscopy (EA) exploits changes in wave function character of CT states in response to static electric fields to enhance detection of CT states via nonlinear optical absorption spectroscopies. A 4 × 4 model Hamiltonian is used to derive splittings of even and odd Frenkel (FR) excited states and changes in wave function character of CT excited states in an external electric field. These are used to explain why FR and CT states yield EA lineshapes which are first and second derivatives of the linear optical absorption spectrum. The model is applied to ammonia-borane molecules and pairs of molecules with large and small B-N separations and CT or FR excited states. EA spectra are obtained from differences in linear optical absorption spectra in the presence or absence of a static electric field and from perturbative sum over states (SOS) configuration interaction singles χ(2) and χ(3) nonlinear susceptibility calculations. Good agreement is found between finite field (FF) and SOS methods at field strengths similar to those used in EA experiments. EA spectra of three C60/oligothiophene complexes are calculated using the SOS method combined with GW/BSE methods. For these C60/oligothiophene complexes, we find several CT states in a narrow energy range in which charge transfer from the thiophene HOMO level to several closely spaced C60 acceptor levels yields an EA signal around 10% of the signal from oligothiophene.

Benchmarking Gaussian Basis Sets in Quantum-Chemical Calculations of Photoabsorption Spectra of Light Atomic Clusters.

The choice of Gaussian basis functions for computing the ground-state properties of molecules and clusters, employing wave function-based electron-correlated approaches, is a well-studied subject. However, the same cannot be said when it comes to the excited-state properties of such systems, in general, and optical properties, in particular. The aim of the present study is to understand how the choice of basis functions affects the calculations of linear optical absorption in clusters, qualitatively and quantitatively. For this purpose, we have calculated linear optical absorption spectra of several small charged and neutral clusters, namely, Li2, Li3, Li4, B2 +, B3 +, Be2 +, and Be3 +, using a variety of Gaussian basis sets. The calculations were performed within the frozen-core approximation, and a rigorous account of electron correlation effects in the valence sector was taken by employing various levels of configuration interaction (CI) approach both for the ground and excited states. Our results on the peak locations in the absorption spectra of Li3 and Li4 are in very good agreement with the experiments. Our general recommendation is that for excited-state calculations, it is very important to utilize those basis sets which contain augmented functions. Relatively smaller aug-cc-pVDZ basis sets also yield high-quality results for photoabsorption spectra and are recommended for such calculations if the computational resources are limited.

Calculated linear and nonlinear optical absorption spectra of phosphine-ligated gold clusters.

Although prediction of optical excitations of ligated gold clusters by time-dependent density functional theory (TDDFT) is relatively well-established, limitations still exist, for example in the choice of the exchange-correlation functional. In aiming to improve on the accuracy of the calculated linear absorption, we report a theoretical study on phosphine-ligated gold clusters, specifically Au9(PR3)83+ and Au8(PR3)72+ characterized by highly resolved UV/Vis spectra, using mass-selective electronic absorption photofragmentation spectroscopy (A. Cirri, H. M. Hernández and C. J. Johnson, J. Phys. Chem. A, 2020, 124, 1467-1479, and references therein). The optical absorption spectra of the Au9(PR3)83+ and Au8(PR3)72+ clusters were calculated using TDDFT and the many-body GW (G-Green's function, and W-screened Coulomb interaction)-BSE (Bethe Salpeter Equation) method, and compared to the experimental measurements. The evGW-BSE results demonstrated fair agreement with the experimental data, comparable to the TDDFT results, but with less dependence on the reference exchange-correlation functional. Experimentally observed ligand-effects in these materials were reproduced in our calculations as well. Finally, to assess the utility of the materials for nonlinear optical absorption, a theoretical evaluation of two-photon absorption cross-sections is included.

Microscopic theory for the incoherent resonant and coherent off-resonant optical response of tellurium

An $\mathit{ab}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathit{initio}$-based fully microscopic approach is applied to study the nonlinear optical response of bulk tellurium. The structural and electronic properties are calculated from first principles using the shLDA-1/2 method within density functional theory. The resulting band structure and dipole matrix elements serve as input for the quantum mechanical evaluation of the anisotropic linear optical absorption spectra, yielding results in excellent agreement with published experimental data. Assuming quasiequilibrium carrier distributions in the conduction and valence bands, absorption/gain and spontaneous emission spectra are computed from the semiconductor Bloch and luminescence equations. For ultrafast intense off-resonant excitation, the generation of high harmonics is evaluated and the emission spectra are calculated for samples of different thicknesses.

Plasmon-induced photoluminescence and Raman enhancement in Pr:CaF2 crystal by embedded silver nanoparticles

Photoluminescence (PL) enhancement of rare earth ions is in high demand due to their crucial applications in optoelectronics devices and biomedical frequency up-convertors for endoscopy, which could be realized by a plasmonic field enhancement effect of metallic nanoparticles (NPs). In this work, both the PL emission and Raman scattering enhancement of Pr3+ ions in CaF2 crystal host have been observed for the first time by embedding silver NPs. By Ag+ ion implantation, Ag NPs (around 5 nm) have been encapsulated into the Pr:CaF2 matrix. Analysis of the difference between linear optical absorption and PL excitation spectra within the plasmon band, the mechanism of observed PL enhancement has been attributed to the local field enhancement induced by the plasmonic NPs. The classical energy transfer process nanoparticle-emitter was not present. The detailed analysis of the PL enhancement induced by unannealed and annealed Ag NPs is compared under the measurement of PL decay, showing the dominant mechanism of the increased absorption and radiative decay rate, respectively. This work shows an efficient method to enhance PL emission of rare earth ions in the crystal based on the embedded metallic NPs, which could be used to optimize the photonic sensors and light emitting devices.

Correction to "Systematic First-Principles Configuration-Interaction Calculations of Linear Optical Absorption Spectra in Silicon Hydrides: Si2H2n (n = 1-3)".

We have performed first-principles electron-correlated calculations employing large basis sets to optimize the geometries and to compute linear optical absorption spectra of various low-lying conformers of silicon hydrides: Si2H2n, n = 1, 2, 3. The geometry optimization for various isomers was carried out at the coupled-cluster singles-doubles-perturbative-triples [CCSD(T)] level of theory, while their excited states and absorption spectra were computed using a large-scale multireference singles-doubles configuration-interaction approach, which includes electron-correlation effects at a sophisticated level. Our calculated spectra are the first ones for Si2H2 and Si2H4 conformers, while for Si2H6, we obtain excellent agreement with the experimental measurements, suggesting that our computational approach is reliable. Our calculated absorption spectra exhibit a strong structure-property relationship, suggesting the possibility of identifying various conformers based on their optical absorption fingerprints. Furthermore, we have also performed geometry optimization for the selected optically excited states, providing insights into their character.

Systematic First-Principles Configuration-Interaction Calculations of Linear Optical Absorption Spectra in Silicon Hydrides: Si2H2n (n = 1-3).

We have performed first-principles electron-correlated calculations employing large basis sets to optimize the geometries and to compute linear optical absorption spectra of various low-lying conformers of silicon hydrides: Si2H2n, n = 1, 2, 3. The geometry optimization for various isomers was carried out at the coupled-cluster singles-doubles-perturbative-triples [CCSD(T)] level of theory, while their excited states and absorption spectra were computed using a large-scale multireference singles-doubles configuration-interaction approach, which includes electron-correlation effects at a sophisticated level. Our calculated spectra are the first ones for Si2H2 and Si2H4 conformers, while for Si2H6, we obtain excellent agreement with the experimental measurements, suggesting that our computational approach is reliable. Our calculated absorption spectra exhibit a strong structure-property relationship, suggesting the possibility of identifying various conformers based on their optical absorption fingerprints. Furthermore, we have also performed geometry optimization for the selected optically excited states, providing insights into their character.

Excited States and Optical Properties of Hydrogen-Passivated Rectangular Graphenes: A Computational Study

In this paper, we perform large-scale electron-correlated calculations of optoelectronic properties of rectangular graphene-like polycyclic aromatic hydrocarbon molecules. Theoretical methodology employed in this work is based upon Pariser-Parr-Pople (PPP) π-electron model Hamiltonian, which includes long-range electron-electron interactions. Electron-correlation effects were incorporated using multi-reference singles-doubles configurationinteraction (MRSDCI) method, and the ground and excited state wave functions thus obtained were employed to calculate the linear optical absorption spectra of these molecules, within the electric-dipole approximation. As far as the ground state wave functions of these molecules are concerned, we find that with the increasing size, they develop a strong diradical open-shell character. Our results on optical absorption spectra are in very good agreement with the available experimental results, outlining the importance of electron-correlation effects in accurate description of the excited states. In addition to the optical gap, spin gap of each molecule was also computed using the same methodology. Calculated spin gaps exhibit a decreasing trend with the increasing sizes of the molecules, suggesting that the infinite graphene has a vanishing spin gap.

Plasmonic Ag nanoparticles embedded in lithium tantalate crystal for ultrafast laser generation

We report the Ag nanoparticles (NPs) embedded in LiTaO3 (AgNP:LT) by direct Ag+ ion implantation. Transmission electron microscope imaging indicates that the embedded Ag NPs have an average diameter of 3.65 nm. The linear optical absorption spectrum of AgNP:LT peaking at 477 nm is observed owing to the typical effect of localized surface plasmon resonance. Z-scan investigation shows ultrafast saturable absorption of AgNP:LT at the near infrared 1 μm wavelength, which enables AgNP:LT to be a new saturable absorber (SA) for the generation of 1 μm Q-switched mode-locked pulsed laser with pulse duration of 35 ps and repetition rate of 8.74 GHz. This work not only opens a new way to tailor the nonlinearity of LiTaO3 by embedding Ag+ NPs, but also develops AgNP:LT as a new SA for ultrafast laser generation.

First principles study of structural and optical properties of [formula omitted] isomers

In this work we undertake a comprehensive numerical study of the ground state structures and optical absorption spectra of isomers of B12 cluster. Geometry optimization was performed at the coupled-cluster-singles-doubles (CCSD) level of theory, employing cc-pVDZ extended basis sets. Once the geometry of a given isomer was optimized, its ground state energy was calculated more accurately at the coupled-cluster-singles-doubles along with perturbative treatment of triples (CCSD(T)) level of theory, employing larger cc-pVTZ basis sets. Thus, our computed values of binding energies of various isomers are expected to be quite accurate. Our geometry optimization reveals eleven distinct isomers, along with their point group, and electronic ground state symmetries. We also performed vibrational frequency analysis on the three lowest energy isomers, and found them to be stable. Therefore, we computed the linear optical absorption spectra of these isomers of B12, employing large-scale multi-reference singles-doubles configuration-interaction (MRSDCI) approach, and found a strong structure-property relationship. This implies that the spectral fingerprints of the geometries can be utilized for optical detection, and characterization, of various isomers of B12. We also explored the stability of the isomer with the structure of a perfect icosahedron, with Ih symmetry. In bulk boron icosahedron is the basic structural unit, but, our vibrational frequency analysis reveals that it is unstable in the isolated form. We speculate that this instability could be due to Jahn-Teller distortion because five-fold degenerate HOMO orbitals in Ih structure are unfilled.

Tunable edge bands and optical properties in black phosphorus nanoribbons under electric field**Project supported by the National Natural Science Foundation of China (Grant No. 10947004) and the Jiangsu Government Scholarship for Overseas Studies.

For several types of the applied electric field configuration on the normal-zigzag black phosphorus nanoribbon (nZZ-BPNR) we investigate the band structure and the linear optical absorption spectrum, especially for the edge states and the corresponding low-energy absorption peaks. The obtained results show that the applied electric field can not only open another band gap at k = 0.5 point, but also can change completely the spacial probabilities of edge states in the two edge bands. The strength of electric field can tune the two band gaps at the Γ point and 0.5 point. Further, one remarkable feature is that the forbidden transition E12 and E21 are allowed. The lowest-excited-energy linear absorption peak E11 originates from the transition between two edge bands at the Γ point. Finally, in comparison with the lowest-excited-energy peaks among various configurations, the second type of electric field configuration can move this peak blue-shift larger than otherconfigurations.

Enhanced nonlinear optical susceptibilities in phosphorene nanoribbons: Ab initio study

Using Density Functional Theory (DFT) method we compute linear optical absorption spectra and nonlinear optical susceptibilities of hydrogen passivated armchair and zigzag Phosphorous Nanoribbons (aPNR and zPNR) as well as \alpha-phase phosphorous monolayer. We observe that: (a) Crystallographic direction has a strong effect on the band edge absorption which causes optical anisotropy as well as a red shift of absorption spectra by increasing the nanoribbon width. (b) The absorption values are in the order of $10^{5} cm^{-1}$ which are similar to the experimentally measured values. (c) There is two orders of magnitude enhancement of the 2nd order nonlinear optical susceptibility, $\chi^{(2)}$, in nanoribbons which emanates from breaking the centro-symmetric structure of a monolayer phosphorene by hydrogen surface terminations. (d) Chief among our results is that the 3rd order susceptibility, $\chi^{(3)}$, for phosphorene monolayer and nanoribbons are about $~10^{-13}$ esu ($~10^{-21} \frac{m^{2}}{V^{2}}$) which are in close agreement with experimentally reported values as well as a recently calculated value based on semi-analytic method. This strongly supports reliability of our method in calculating nonlinear optical susceptibilities of phosphorene and in general other nanostructures. Enhanced 2nd order optical nonlinearity in phosphorene promises better second harmonic and frequency difference (THz) generation for photonics applications.

Nonlinear Absorption Response Correlated to Embedded Ag Nanoparticles in BGO Single Crystal: From Two-Photon to Three-Photon Absorption

We report on the embedded silver (Ag) nanoparticles fabricated by Ag+ ion implantation into the Bi4Ge3O12 (BGO) crystal. Localized surface plasmon resonance (LSPR) phenomenon has been observed by linear optical absorption spectrum, which is accordance with the expectation based on Mie theory calculation. Further proofs are given by SRIM, TEM and SAED analysis, which explain the slight difference between experiment and calculation. Based on the z-scan system, it is found that the nonlinear optical response is converted from two-photon absorption to three-photon absorption under the 515 nm femtosecond pulse excitation within the LSPR band. The nonlinear absorption coefficient is measured to be ~3.1 × 10−9 cm/W (two-photon absorption coefficient) and ~8.9 × 10−14cm3/W2 (three-photon absorption coefficient) for pure BGO crystal and the sample embedded with Ag nanoparticles (Ag:BGO), respectively. Finally, we have proposed a model to explain the asymmetric nonlinear transmittance, which is in good agreement with the experimental results.

Confined surface plasmon of fundamental wave and second harmonic waves in graphene nanoribbon arrays.

The confined surface plasmon of fundamental wave and second harmonic wave (SHW) are investigated in graphene grating structure. The linear-optical absorption spectra with various fermi energy and carrier mobility are investigated with the finite difference time domain (FDTD) simulations and coupled mode theory (CMT). Based on the CMT, a theoretical model for the graphene grating is established to study the spectrum features of fundamental wave. The lifetimes of linear-optical resonant modes in theoretical model are investigated through the theoretical fitting of exact values in simulation, which are tunable with both the fermi energy and carrier mobility. We also have investigated the second-order nonlinearity of graphene grating by introducing the second-order nonlinear source. The proposed configuration and method are useful for research of the absorption, local field enhancement factor, lifetime of light, and nonlinear optical processes in highly integrated graphene photoelectric devices.

First principles electron-correlated calculations of optical absorption in magnesium clusters

In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of various isomers of magnesium clusters Mg$_{n}$ (n=2--5), corresponding to valence transitions. Geometry optimization of several low-lying isomers of each cluster was carried out using coupled-cluster singles doubles (CCSD) approach, and these geometries were subsequently employed to perform ground and excited state calculations using either the full-CI (FCI) or the multi-reference singles-doubles configuration interaction (MRSDCI) approach, within the frozen-core approximation. Our calculated photoabsorption spectrum of magnesium dimer (Mg$_{2}$) isomer is in excellent agreement with the experiments both for peak positions, and intensities. Owing to the sufficiently inclusive electron-correlation effects, these results can serve as benchmarks against which future experiments, as well as calculations performed using other theoretical approaches, can be tested.

Mixed-metal cluster chemistry. 39. Syntheses and X-ray structures of Mo3Ir3(μ4-η2-CO)(μ3-CO)(CO)10(η5-C5H5)3 and Mo3RhIr3(μ-CO)4(CO)7(η5-C5H5)3(η5-C5Me5)

Thermolysis of tetrahedral Mo2Ir2(μ-CO)3(CO)7(η5-C5H5)2 in refluxing toluene affords small amounts of Mo3Ir3(μ4-η2-CO)(μ3-CO)(CO)10(η5-C5H5)3 (1). Reaction of 1 with Rh(CO)2(η5-C5Me5) in refluxing dichloromethane gives Mo3RhIr3(μ-CO)4(CO)7(η5-C5H5)3(η5-C5Me5) (2) in moderate yield. X-ray diffraction studies show the metal cores adopt edge-bridged trigonal bipyramidal and bicapped trigonal bipyramidal structures, respectively. Cluster 2 is electron precise with 96 CVE, in contrast to the only precedent group 6-group 9 mixed-metal cluster with an identical core geometry, the 94 CVE W3Ir4(μ-H)(CO)12(η5-C5H5)3 (3). DFT studies have been employed to rationalize the contrasting behavior of 2 and 3. Time-dependent DFT has been employed to assign the key transitions in the linear optical absorption spectrum of 2.

Optical absorption spectra of boron clusters Bn (n = 2–5) for application in nano scintillator – a time dependent density functional theory study

Boron nano-clusters of various shapes and sizes have potential applications as scintillating detector and hydrogen storage material. Using time dependent density functional theory (TDDFT) as implemented in CASIDA we have studied the linear optical absorption spectra for boron clusters Bn (n = 2–5) and compared with previously reported results using Hatree-Fock (H-F) based method where the spectrum is limited to 8 eV due to exclusion of excitation into very high energy unoccupied orbital. The optical spectra fall in the visible and near UV region and are very much dependent on the shape of the isomer. We have obtained additional peaks for B2 linear, B3 triangular, B4 rhombus and square shaped isomers beyond 8 eV which were missing in the previous H-F based study and has significance as they fall below the ionization potential. We correlate the optical spectrum with the shape of the Kohn-Sham orbitals and HUMO-LUMO gap and assess comparative stability of various Bn (n = 2–5) clusters in terms of HUMO-LUMO gap, bond-length and relative energy. TDDFT computed optical spectroscopy correlated with Kohn-Sham orbitals and HUMO-LUMO gap and its comparison with H-F based method may give significant knowledge regarding geometry and optical properties of Bn (n = 2–5) clusters enabling to distingush between various isomers of Bn clusters.

Organometallic Complexes for Non-Linear Optics. 59. Syntheses and Optical Properties of Some Octupolar (N-Heterocyclic Carbene)gold Complexes

The syntheses of octupolar alkynes 1,3,5-{4-(4-HC≡CC6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (4) and 1,3,5-{4-(4-HC≡CC6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (6), diphenylamino-substituted 1,3,5-(4-Ph2NC6H4-1-C≡C)3C6H3 (7), 1,3,5-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C)3C6H3 (8), 1,3,5-{4-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (9), and 1,3,5-{4-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (10), and (N-heterocyclic carbene)gold-appended 1,3,5-{[(NHC-iPr)Au]C≡C}3C6H3 (11), 1,3,5-{[(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C}3C6H3 (12), 1,3,5-{4-([(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (13), and 1,3,5-{4-([(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (14) [NHC-iPr = κC-cyclo-CN(2,6-C6H3iPr2)CH=CHN(2,6-C6H3iPr2)] are reported. The low-energy bands in the linear optical absorption spectra of all three sets of compounds are red-shifted and increase in intensity upon π-delocalizable ‘arm’ lengthening. The diphenylamino- and (NHC-iPr)gold-terminated compounds do not exhibit measurable second-harmonic generation as assessed by hyper-Rayleigh scattering at 1064 nm using nanosecond pulses. Computational studies have been employed to rationalize the optical properties of the new compounds. Calculations on 7–10 reveal that the lowest-energy transitions with large oscillator strengths are predominantly [Ph2NC6H4] (π) → [arms + core] (π*) in character, whereas calculations on 11–14 suggest that the low-energy transitions relate to the transfer of electron density from the Au-alkynyl core group to the terminal NHC groups.

Nonlinear absorption, optical limiting behavior and structural study of a new chalcone derivative-1-(3, 4-dimethylphenyl)-3-[4(methylsulfanyl) phenyl] prop-2-en-1-one

A new third order nonlinear optical (NLO) organic material-1-(3, 4-dimethylphenyl)-3-[4(methylsulfanyl) phenyl] prop-2-en-1-one (4DPMS) belonging to chalcone family has been crystallized in acetone solution. The 4DPMS crystals are characterized by CHNS analysis, FTIR, UV–visible spectral and thermal techniques. The single crystal X-ray diffraction study reveals that 4DPMS crystallizes in monoclinic system with P21/n space group. The linear optical absorption spectrum revealed that the 4DPMS crystals are transparent in the entire visible region. Thermogravimetric data shows absence of phase transition before melting point and from differential scanning calorimetry analysis the melting point of the crystal is found to be 106°C. Third order nonlinear absorption and optical limiting experiment on 4DPMS was carried out using open aperture Z-scan technique with Nd: YAG laser operating at 532nm. It was found that the calculated values of excited state absorption cross section for 4DPMS molecules is much greater than the ground state absorption cross section. A decrease in effective nonlinear absorption coefficient was observed with increase in the input irradiance of laser. The observed optical limiting property in 4DPMS is attributed to reverse saturable absorption.