Nonlinear Optical Response Research Articles

Deciphering the impact of superalkali metals [M3O (M = Li, Na, K)] doping on sulflower for enhanced NLO responses with efficient energetic offsets: A density functional theory perspective

With the motive to enhance nonlinear optical (NLO) materials, the geometries, stabilities, and NLO responses of superalkali metals [M3O (M = Li, Na, K)] doped sulflower (Octathio[8]circulene) is explored using DFT simulations and computational characterization. Binding energies from −6.07 to −29.27 kcalmol-1 demonstrate the thermal stability of M3O@sf isomers. The E(H-L) gap was reduced to 84.84 % upon M3O@sf doping. Superalkali metal doping of sulflower substantially improves the first hyperpolarizability (βo) value of complexes. A comparative study illustrates that the hyperpolarizability of the superalkalis has a monotonic relationship with the nature of metal atoms. Among all the studied isomers, K3O@sf-III has the highest βo value of 3.2×107 au. The significant charge transfer (determined by NBO), decrease in excitation energy values, increase in dipole and polarizability values justify that there has been a significant increase in the βo values. This work introduces an opportunity to develop thermally stable NLO materials which can potentially be used for high-performance NLO applications.

Universal Enhancement Effect of Nonlinear Optical Response from Band Hybridization

AbstractBulk photovoltaic effect, i.e. shift current, is a nonlinear second‐order optical response that can rectify an alternating current (AC) electric field into a direct current (DC). Depending on the wavelength of the incident light, shift current finds applications in various fields, including solar energy conversion and radiation detection. Its promising application in energy conversion and information processing has inspired investigations to uncover the relationship between shift current and electronic structures of materials. Despite numerous efforts dedicated to designing principles for strong bulk photovoltaic effect materials, the only widely accepted crucial parameter is the joint density of states (JDOS). In this study, employing effective model analysis and first‐principles calculations, an enhancement effect of bulk photovoltaic effect is found to arise from band hybridization that is typically along with anti‐crossing‐like electronic band structures, similar to the Berry curvature effects in intrinsic anomalous Hall conductivity. While this mechanism does not offer a comprehensive understanding of the relationship between electronic structure and the magnitude of bulk photovoltaic effect, it represents practical progress in the design of materials with strong bulk photovoltaic effect.

Wide-Bandgap 2D SnS2-Based All-Optical Photonic Devices with Strong Nonlinear Optical Response due to Spatial Self-Phase Modulation

Wide-Bandgap 2D SnS₂-Based All-Optical Photonic Devices with Strong Nonlinear Optical Response due to Spatial Self-Phase Modulation

Synthesis of α-Bi/SrTiO3 heterostructure through the Eu-induced reduction of Bi2O2Se/SrTiO3

Thin Bi films, especially noncentrosymmetric α-phase Bi films (α-Bi), have attracted considerable attention in recent years due to their intriguing physical properties such as ferroelectricity, nonlinear optical response, and coherent spin transport. However, the current experimental preparation of α-Bi films still presents substantial challenges, resulting in only isolated α-Bi islands being achieved. In this study, α-Bi/SrTiO3 (α-Bi/STO) was synthesized from a Bi2O2Se/STO (BOS/STO) heterostructure by depositing a Eu layer and reducing a BOS film. The so-formed α-Bi/STO interface features metallic conductivity with electron mobility of 7.7 × 104 cm2/V s and ultralow resistivity of 1 nΩ cm at 2 K, as well as high residual resistance ratios R300 K/R2 K up to 2353. Furthermore, we observed a complex weak antilocalization–weak localization–weak antilocalization crossover with increasing magnetic field at temperatures below 10 K. Our work presents the synthesis of α-Bi films, which could potentially serve as a valuable platform for experimental validation of diverse theoretical predictions associated with α-Bi heterostructures. Furthermore, this special synthesis method provides valuable insights into the preparation of other metastable films.

Exploring the impact of end-capped moieties on nonlinear optical response in d-[formula omitted]-a configurated based organoboron derivatives: A DFT/TD-DFT study

A new series of the organic derivatives (BDPTPD1-BDPTPD8) with push–pull architectures (D-π-A) was developed using various efficient acceptors into reference molecule (BDPTPR). To obtain nonlinear optical insights, BDPTPR and its derived compounds were investigated at M06/6-311G(d,p) level. All derivatives (BDPTPD1-BDPTPD8) have a lesser band gap (2.190–1.152 eV) and bathochromic shift (λmax = 713.086–722.350 nm) than that of reference molecule (Egap = 2.270 eV and λmax = 699.212 nm). Global reactivity calculations described that BDPTPD5 displayed smaller values of hardness (1.036 eV) with a larger value of softness (0.482 eV) relative to all studied chromophores. Similarly, it exhibited the highest values of βtot and γtot, as 1.783 × 10-27 and 2.400 × 10-32esu, respectively, among all investigated compounds. This framework elucidates that structural modification involving diverse end-capped acceptor units performs a crucial role in achieving desirable NLO substances for advanced optoelectronic technology.

Enhanced Optical Limiting of Gold Nanoparticles/Porous Carbon Nanocomposites.

With the wide application of laser weapons, the requirements of laser protection technology are becoming more and more strict. Therefore, it is important to find ideal optical limiting (OL) materials to protect human eyes and detectors. In this work, the nonlinear optical responses of gold nanoparticles/porous carbon (Au NPs/PC) nanocomposites prepared by the reduction method were studied using the nanosecond Z-scan technique. Compared with porous carbon, the Au NPs/PC nanocomposites show a lower damage threshold, a bigger optical limiting index and a wider absorption spectrum. The interaction between gold nanoparticles and porous carbon enhances the nonlinear scattering effect of suspended bubbles. These results indicate that Au NPs composites have potential applications in the protection of human eyes and detectors.

Nonlinear optical signatures of topological Dirac fermion.

Dirac fermion in topological materials exhibits intriguing nonlinear optical responses. However, their direct correlation with the linearly dispersed band remains elusive experimentally. Here, we take topological semimetal ZrSiS as a paradigm, unveiling three unique nonlinear optical signatures of Dirac fermion. These signatures include strong quadrupolar response, quantum interference effect, and exponential divergent four-wave mixing (FWM), all of which are described by the prominent third-order nonlinear optical susceptibility. Resonantly enhanced by linear bands, quadrupolar second harmonic generation in centrosymmetric bulk overwhelms the electric-dipole contribution at the surface with inherent inversion symmetry breaking. Furthermore, owing to the interference between multiple resonant transition pathways within linear bands, difference-frequency FWM is several orders of magnitude stronger than sum-frequency FWM and third harmonic generation. The difference-frequency FWM further displays an inverse-square divergence toward degenerate excitation, whose scaling law perfectly matches with the long-sought behavior of Dirac fermion. These signatures lay the solid foundation toward the practical applications of topological materials in nonlinear optoelectronics and photonics.

Simultaneous Circular Dichroism and Wavefront Manipulation with Generalized Pancharatnam-Berry Phase Metasurfaces.

Simultaneous circular dichroism and wavefront manipulation have gained considerable attention in various applications, such as chiroptical spectroscopy, chiral imaging, sorting and detection of enantiomers, and quantum optics, which can improve the miniaturization and integration of the optical system. Typically, structures with n-fold rotational symmetry (n ≥ 3) are used to improve circular dichroism, as they induce stronger interactions between the electric and magnetic fields. However, manipulating the wavefront with these structures remains challenging because they are commonly considered isotropic and lack a geometric phase response in linear optics. Here, we propose and experimentally demonstrate an approach to achieve simultaneous circular dichroism (with a maximum value of ∼0.62) and wavefront manipulation using a plasmonic metasurface made up of C3 Archimedes spiral nanostructures. The circular dichroism arises from the magnetic dipole-dipole resonance and strong interactions between adjacent meta-atoms. As a proof of concept, two metadevices are fabricated and characterized in the near-infrared regime. This configuration possesses the potential for future applications in photodetection, chiroptical spectroscopy, and the customization of linear and nonlinear optical responses.

DFT-assisted design of curcumin-based donor-π-bridge-acceptor molecular structures: Advancing nonlinear optical materials and sustainable organic solar cells

This study explores the theoretical design of novel curcumin-based structures with enhanced nonlinear optical (NLO) characteristics. By systematically substituting hydroxyl (OH) functionalities in bisdemethoxycurcumin (BdMC) with various donor and acceptor functionalities at the edges of the π-bridge backbone, we constructed 12 different derivatives. Density-functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed at the B3LYP/6-311++G(d,p) and CAM-B3LYP/6-311++G(d,p) levels to analyze the optoelectronic properties. The introduction of donors and acceptors significantly improved NLO characteristics. Notably, BdMC-1 (N(C2H5)2-π-bridge-NO2) exhibited a remarkable enhancement in NLO responses, with the average polarizability (<α>) and the first hyperpolarizability (βtot) increasing from 465.3 to 660.9 (a.u.) and 14254.4 to 143487.6 (a.u.), respectively. UV–Vis spectroscopy showed BdMC-1 as the most red-shifted compound, with a maximum wavelength (λmax) of 431.1 nm, the lowest optical bandgap of 2.88 eV, and an ultrahigh light harvesting efficiency (LHE) of 97.4%. The designed compounds exhibited narrower HOMO-LUMO electronic bandgaps (as low as 2.08 eV) compared to BdMC (3.32 eV), contributing to the improved NLO responses. Natural bond orbital (NBO) analysis confirmed the formation of a charge separation state, facilitating effective electron migration from donors to acceptors through the π-bridge. Additionally, using BdMC-based materials as acceptors in organic solar cells (OSCs) yielded open-circuit voltages (Voc) from 1.62 to 2.23 V, fill factors (FF) above 90%, and low exciton binding energies (Ee−b) in the range of 0.08 to 0.80 eV.

Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor.

Graphene is a unique platform for tunable opto-electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light-matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all-optical fashion, but each approach involves a trade-off between speed and modulation depth. Here, lattice temperature, electron doping, and all-optical tuning of third-harmonic generation are combined in a hexagonal boron nitride-encapsulated graphene opto-electronic device and demonstrate up to 85% modulation depth along with gate-tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out-of-equilibrium chemical potential. The work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors, and frequency converters.

Plasma etching promoting the saturable absorption of BiOI

Materials exhibiting exceptional saturable absorption performance and etching compatibility are significant in the context of advanced optical devices. Our study demonstrates that plasma etching can effectively promote the saturable absorption of BiOI nanosheets because of defect formation. Remarkably, the BiOI nanosheet subjected to the optimized H2/Ar-plasma treatment manifests a substantial nonlinear absorption coefficient (βeff) of (−5.5 ± 0.7) × 103 cm GW−1 under laser excitation at 800 nm, while the pristine BiOI sample exhibits no discernible nonlinear optical response. The βeff of plasma-treated BiOI samples is (−4.2 ± 0.9) × 103 cm GW−1 at 515 nm, surpassing that of the untreated counterpart by more than fourfold. A comprehensive structural and spectroscopic analysis indicates that oxygen vacancies serve to increase the density of in-gap states, thereby narrowing the bandgap of the BiOI. This reduced bandgap effectively facilitates the Pauli-blocking effect and, in turn, augments the saturable absorption properties of the samples. In contrast to oxygen vacancies, iodine vacancies make a negligible contribution to the performance enhancement of BiOI. Our result would provide a promising way to improve the nonlinear optical response of materials.

Quantum chemical framework for designing high-performance ladder-shape NLO molecules and study of implicit vs explicit solvent effects on their NLO properties

Researchers in materials science are greatly fascinated by helicene chemistry due to its excellent optical and structural properties. In this work, we introduced symmetrical substitution of donor and acceptor groups on the [7]helicene ([7]HC-1 to [7]HC-5). DFT analysis is used to explore nonlinear optical (NLO) response for designed compounds. For [7]HC-5, the linear isotropic (αiso) and anisotropic polarizability (αaniso) are 31.59 × 10−24 esu and 59.86 × 10−24 esu, respectively. [7]HC-5 has the highest average third-order NLO polarizability (<γ>) of 47.24 × 10−36 esu which is approximately six times greater than that of para-nitroaniline, a well-known prototype NLO molecule. The effect of implicit and explicit solvents on <γ> and αiso of three selected compounds ([7]HC-1, [7]HC-3, and [7]HC-5) is also evaluated. As compared to explicit and gas phase methods, PCM model predicts an overestimation by an increment of ∼ 4-5 times for selected compounds. TD-DFT calculations show a higher transition dipole moment (4.24) and a lower transition energy (3.414 eV) for [7]HC-5. The higher NLO response is explained based on frontier molecular orbitals (FMOs), electron density difference (EDD), and molecular electrostatic potentials (MEPs). There is a red shift for studied compounds based on UV-visible results. The C-N stretching at 1372 cm-1 for [7]HC-5 indicates the presence of -N(CH3)2. Additionally, we studied photovoltaic parameters and found that [7]HC-5 has the maximum open circuit voltage and the lowest dye regeneration of 2.71 eV and 0.06 eV, respectively.

A first principles based prediction of electronic and nonlinear optical properties towards cyclopenta thiophene chromophores with benzothiophene acceptor moieties

In the current work, organic cyclopenta-thiophene (CPT) based derivatives (FICR and FICD1–FICD5) were designed by the modulation of end-capped acceptor group of the reference molecule i.e., FICR, to explore their nonlinear optical (NLO) response. The effect of terminal acceptor and donor groups in the tailored compounds was explored by using DFT based quantum calculations. The UV–Vis analysis, frontier molecular orbitals (FMOs), transition density matrix (TDM), natural bond orbitals (NBOs), density of states (DOS), nonlinear optical (NLO) analyses were performed at M06/6-311G(d,p) functional. The LUMO–HOMO band gaps of FICD1–FICD5 were found to be smaller (1.75–1.92 eV) comparative to FICR (1.98 eV). Moreover, the global reactivity parameters (GRPs) were correlated with the results of other analyses. FICD2 and FICD5 with lowest band gap 1.73 and 1.75 eV showed less hardness (0.86 and 0.87 eV, respectively), high softness (0.58 and 0.57 eV−1), and larger absorption spectrum (815 and 813 nm) in gaseous phase and (889 and 880 nm) in solvent phase among all entitled compounds. All the designed chromophores (FICD1–FICD5) demonstrated a significant NLO response as compared to FICR. Particularly, FICD2 and FICD5 exhibited the highest average linear polarizability (<α>) [2.86 × 10−22 and 2.88 × 10−22 esu], first hyperpolarizability (βtot) (8.43 × 10−27 and 8.35 × 10−27 esu) and second hyperpolarizability (γtot) (13.20 × 10−32 and 13.0 × 10−32 esu) values as compared to the other derivatives. In nutshell, structural modeling of CPT based chromophores with extended acceptors, can be significantly utilized to achieve potential NLO materials.

Comprehensive investigation of 2-methylquinolinium-5-sulphosalicylate monohydrate: Synthesis, characterization, multifaceted analysis of molecular structure, optical, and nonlinear optical properties

We introduce the synthesis and characterization of a novel molecular salt crystal, 2-methylquinolinium-5-sulphosalicylate monohydrate (MQSSA), obtained through an equimolar reaction between donors (D) and acceptors (A) employing crystal engineering principles. The resulting proton transfer complex, MQSSA, crystallized into a monoclinic crystal system with space group P21/C, as revealed by SCXRD analysis. Significant peaks observed in PXRD analysis confirmed the crystallinity and purity of the material. FT-IR and FT-Raman spectroscopy were employed to analyze various functional groups present in the molecule. UV–Vis-NIR spectroscopy provided insights into absorbance transmittance, lower cutoff wavelength, and optical band gap, suggesting it's suitability for diverse optical applications, while characteristic emission was identified using PL measurements. Thermal stability was assessed via TG-DTA and TG-DSC techniques. Nonlinear absorption coefficient (β), nonlinear refractive index (n2), and third-order nonlinear susceptibility (χ3) were quantified using the Z-scan technique. Hirshfeld surface analysis and fingerprint plots were utilized to gain insight into intermolecular interactions and atomic arrangements within the MQSSA crystal, emphasizing the roles of crystal structure in H...H and O...H/H...O interactions. Additionally, quantum computational methods were employed to evaluate various molecular-level electronic characteristics, including molecular orbitals, molecular electrostatic potential maps, and first and second-order nonlinear optical (NLO) polarizability, providing insights into the molecular-level NLO response properties.

Nonlinear optical absorption of Sb2Se3: Dual response and [Sb4Se6]n ribbon orientation dependence

The nonlinear optical (NLO) absorption response of Sb2Se3 polycrystalline film was systemically studied in this work. Both saturable absorption (SA) and reverse satuarable absorption (RSA) are found in Sb2Se3 polycrystalline film with short pulse and long pulse laser, respectively. The transient absorption spectrum of Sb2Se3 reveal the origin of dual NLO absorption response from the view of carrier dynamics. The dispersion of NLO absorption response of Sb2Se3 and its substrate dependence were also investigated in this work. By simulation of open aperture Z-scan data under the femtosecond laser excitation, the NLO absorption coefficient β and absorption cross section difference ∆σbetween ground state and excited state were calculated in spectral region of 700⁓860 nm. The NLO absorption response in these film samples were not only tuned by the excitation wavelength but also dependent on laser pulse width and their substrate. The wavelength-tailored, pulse width-dependent and substrate-dependent NLO properties allow them to be potentially applied in the field of photonics.

Theoretical investigation of the electronic and second-order non-linear optical properties of [n]helicene derivatives

Designing and synthesizing nonlinear optical materials with excellent performance has always been a persistent goal pursued by material chemists. In paper, based on the reported [6]heliceno-bis(naphthalimides) 2, four new molecules are designed by introducing 2-dicyanomethylene-1,3-dithiole on the two middle benzene rings of the naphthalimide unit and replacing CH3 units on the R1 and R2 substituents with NH2/NO2 moieties, or their combination. The electronic spectra, reorganization energy and second-order nonlinear optical properties of the reported derivatives 1 and 2, as well as four newly designed derivatives, have been systematically investigated by means of (time-dependent) density functional theory calculations. Frontier molecular orbital analysis, electron excitation analysis, reorganization energy, and the calculation of the static and dynamic first hyperpolarizability all indicate that the studied system is greatly influenced by the number of benzene rings in helicene core and the terminal substituents attached at both ends of helicene core. Further analysis of depolarization, hyperpolarizability density and unit sphere representation reveals the nature of the nonlinear optical response of the studied system. Considering that the studied derivatives all have large static first hyperpolarizabilities (e.g., 210 × 10-30 esu for derivative 5) and small reorganization energy (e.g., 0.092 eV of hole reorganization energy and 0.118 eV of electron reorganization energy for derivative 3), they are expected to become potential nonlinear optical materials with excellent performance and bipolar transport materials. This work contributes to the rational design of high-performance molecules based on [n]helicenes derivatives and further explores their potential applications in optical materials.

Giant nonlinear optical response of fullerene polymer fragments: a DFT perspective

Organic π-conjugated materials exhibit exceptional nonlinear optical (NLO) properties due to their unique electronic structures, characterized by short response times and large NLO responses. Fullerene (C60) is one of the few polymer species that possess a rich π-conjugated system, making it a promising material with significant NLO responses. In the present paper, we designed three C60 polymer fragments and employed density functional theory to estimate their molecular static first and second hyperpolarizabilities. Compared to previously reported fullerene derivatives, the designed C60 polymer fragments can exhibit notable second-order and third-order molecular NLO responses. The study shows that the hyperpolarizabilities and energy gaps of the investigated C60 polymer fragments are greatly influenced by their topological structures and bonding modes. These findings provide new insights for the design of novel NLO materials based on C60 polymers, and the realization of tunable NLO responses in C60 cluster-based molecular systems, which may have significant applications in nanophotonic devices.

Thermal nonlinear optical responses of native and oxidized low-density lipoprotein solutions at visible and infra-red wavelengths: complementary approaches

Single beam Z-scan (ZS) experiments at 532 nm (visible) and 979 nm [infra-red (IR)] wavelengths were used to determine photothermal responses of native and oxidized aqueous suspensions of human low-density lipoproteins (LDLs). The wavelengths employed in the measurements were chosen according to the optical absorption solute (LDL particles) and solvent (water) of the suspension. At 532 nm, water presents negligible absorbance, and the LDL is responsible for the light absorption. On the other hand, at 979 nm, the water is the main light absorber. In the visible light case, the particles absorb the laser light and, by conduction, transfer heat to water to form the thermal lens. In the IR experiments, water is the main absorber and transfers the heat to the particles to form the thermal lens. We show that with the IR light it is possible to investigate high degrees of oxidation of LDL, not possible with the usual visible light experiments. Differently from the usual ZS experiments with LDL at visible light, the magnitude of the thermal lens formed in the IR experiments was shown to be bigger in oxidized samples with respect to that of the native samples. For both wavelengths, all samples whose response was measured presented negative nonlinearity (self-defocusing behavior). It was also observed, in experiments with IR light, that the formation time of the thermal lens tends to decrease with the increase in the degree of oxidation of the sample.

Numerical investigation of high-order harmonic generation by gold nanorod arrays in hybrid nanostructures: effect of number of gold nanorod

Hybrid metal-dielectric nanostructures are highly effective and helpful in generating and enhancing linear and nonlinear optical phenomena due to their unique optical properties. In this study, the linear optical responses and second- and third-order harmonics have been calculated numerically for this structure. Nonlinear optical response efficiency has been studied at the resonance wavelengths of the nanostructure by varying its parameters (radius and length). The desired efficiency can be achieved by adjusting the radius and length of the nanorods and matching the resonance wavelengths with the fundamental and generation frequencies of nonlinear optical phenomena in specific applications. Moreover, the effect of the number of gold nanorods on harmonic generation has been investigated. By increasing the number of nanorods in pairs, it is possible to generate high-order harmonics. The acquired results can be used to improve optical devices such as biomedical equipment, optical integrated circuits, and sensors.

Flexible epitaxial LiNbO3 thin film with ferroelectricity and nonlinear optical response

Flexible or wearable devices are of great interests in the modern technologies, and oxide thin films are critical components for such applications. In this work, epitaxial LiNbO3 (LNO) thin film integrated on flexible mica substrate has been achieved. Al-doped ZnO (AZO) layer has been first deposited as buffer layer to obtain the epitaxial LNO film, furthermore AZO serves as bottom electrode for electrical measurement. The as-deposited LNO film exhibits high epitaxial quality with c-direction growth. The transmittance measurement indicates the high transparency of the film, ～85 % in visible range and ～90 % in near infrared region up to 1500 nm. The second harmonic generation (SHG) measurement confirms the nonlinearity of the flexible LNO film. The LNO/AZO/mica film also presents apparent ferroelectric response. Overall, the high-quality LNO integrated on flexible mica could be potentially applied in wearable optical and electrical devices.