Nonlinear Optical Response Research Articles

Symmetry breaking in 2D materials for optimizing second-harmonic generation

Abstract Second-harmonic generation (SHG) is the generation of 2ω (or half wavelength) light from incident light with frequency ω as a nonlinear optical response of the material. Three-dimensional (3D) SHG materials are widely investigated for developing laser technology to obtain shorter wavelengths in photolithography fabrication of semiconductor devices and the medical sciences, such as for imaging techniques that do not use fluorescent materials. However, to obtain the optimized SHG intensity, the 3D material is required to have no spatial-inversion symmetry (or non-centrosymmetry) and special crystal structure (or so-called phase-matched condition). Recently, engineering symmetry breaking of thin two-dimensional (2D) materials whose 3D structure has the inversion symmetry can offer a breakthrough to enhance the SHG intensity without requiring the phase-matched condition. Over the past decade, many 2D SHG materials have been synthesized to have broken inversion symmetry by stacking heterostructures, twisted moire structures, dislocated nanoplates, spiral nanosheets, antiferromagnetic order, and strain. In this review, we focus on the recent progress in breaking inversion and rotational symmetries in out-of-plane and/or in-plane directions. The theoretical calculations and experimental setup are briefly introduced for the non-linear optical response of the 2D materials. We also present our perspectives on how these can optimize the SHG of the 2D materials.

Computational Evaluation of the Structural, Topological, and Solvent Effects on the Nonlinear Optical Properties of 1-Methylurea Butanedioic Acid Crystal

In silico investigation of the effects of a molecule’s framework and surroundings on its nonlinear optical (NLO) response is still an active topic of study in the fields of photonics and optoelectronics. NLO materials play a crucial role in modern photonics and optoelectronic technologies. Presented here is a comprehensive theoretical analysis of the structural, topological, and NLO features of 1-methylurea butanedioic acid (MUBA) alongside solvent effects (water, DMSO, and benzene) using the DFT method at the B3LYP(D4)/6–311++G(d,p) level. Geometric and infrared parameters were calculated and compared with experimental values. The analysis using atoms in molecules (AIM) and the independent gradient model (IGM) reveals the presence of two noncovalent intermolecular interactions: N4–H16⋯O12 and O6–H25⋯O3, which stabilize the crystal structure. The natural bond orbital (NBO) analysis reveals that the LP(1)N4 ⟶ π∗(C2-O3) interaction is the most stabilizing and is enhanced in solvent environments. NLO data show that the first (βtot) and second (γ) hyperpolarizability values of MUBA are approximately 0.6–1.1 and 8.3–17.0 times higher than those of urea. In addition, the quadratic and cubic responses of MUBA are significantly reduced and increased, respectively, in solvent environments. Based on its NLO susceptibilities, MUBA exhibits SHG, EOPE, OKE, and EFISHG properties, suggesting its potential application in the production of optoelectronic devices and optical limiting. This study enhances our understanding of the factors influencing the NLO behaviour of organic crystals, providing valuable insights for designing materials with enhanced NLO characteristics. The implications extend to industries such as telecommunications and computing, where faster data transmission rates are in high demand.

Investigating the Third‐Order NLO Performance of a Three‐Phase Core‐Shell PANI@MIL‐101(Cr)@CeO2 with Internal and External Synergies

AbstractCombining metal–organic frameworks (MOFs) with other functional materials can effectively improve the third‐order nonlinear optical (NLO) performances of MOFs. In this study, two distinct functional materials are selected and a PANI@MIL‐101(Cr)@CeO2 composite is successfully synthesized. The test results of third‐order NLO indicate that introducing PANI regulates the distribution of π electron clouds within the structure of the composite. Additionally, the core@shell structure formed by PANI@MIL‐101(Cr) and CeO2 promotes charge transfer and improves charge transfer efficiency under the weak heterojunction interaction, thereby enhancing nonlinear absorption and refractive signals. Under the double synergistic action of interior and exterior, a new charge transfer occurs between the components of the composite, resulting in excellent third‐order NLO performances. This study indicates that encapsulating polymer and loading metal oxides can effectively improve the NLO response of MOFs, which offers a new idea for more research on multiphase NLO materials.

Superalkalides based on stacked janus molecule with improved optical nonlinearity

Research is actively being carried out to design new materials with exceptional nonlinear optical (NLO) responses. In this context, a systematic investigation of designing novel hypothetical materials e.g., superalkalides based on stacked Janus dimer, all-cis-1,2,3,4,5,6-hexafluorocyclohexane (C6H6F6)2 has been reported using high-level density functional simulations. The superalkalides are designed here by doping alkali metals on the fluorine face and superalkalis on the hydrogen face of Janus dimer. The thermodynamic stability of newly designed complexes is demonstrated by the interaction energies (Eint), ranging between −13.64 and −33.12 kcal/mol. Moreover, the superalkalide nature of these complexes is proved by negative charge and HOMO on superalkalis in natural bond orbitals (NBO) and frontier molecular orbitals (FMO) analyses, respectively. Energy gaps (Eg) between HOMO and LUMO orbitals of the designed complexes lie in the range of 0.63–0.83 eV. UV–Vis analysis is performed to gain idea about the transparent region of electromagnetic spectrum for nonlinear activity. The nonlinear activity of the designed superalkalide(s) is studied by static first hyperpolarizability β0, dynamic first hyperpolarizability β(ω) and dynamic second hyperpolarizability γ(2- ω; ω, ω). The highest β0, β(ω), and γ(2- ω; ω, ω) ranges up to 1.1 × 107, 2.9 × 106 and 2.9 × 1012 au, respectively for designed complexes. A comprehensive examination of these superalkalides based on stacked Janus dimers with exceptionally high NLO response highlights their potential for use in nonlinear optical materials.

Optical non-linearities and applications of ZnS phosphors

Optical non-linearities play a crucial role in enabling efficient and ultrafast switching applications that are essential for next-generation photonic devices. ZnS phosphor material produces the best results in terms of increased luminescence quantum yield when doped with certain impurities. Nevertheless, the investigation of the third-order non-linear optical susceptibility of the phosphor materials can be exploited for various switching applications. In this regard, we review the recent advancements in the investigation of non-linear optical properties of ZnS phosphors, where the knowledge of absorption and refraction is utilized in various optical and detector applications. Furthermore, the review highlights strategies employed to enhance the non-linear optical response of phosphor materials as well as a general discussion of an attosecond optical switching scheme which can be used to fabricate devices with petahertz speeds. Consequently, we provide a solution to the unsolved problem of the significant extension of optical limiting applications to switching applications by developing design strategies to manipulate conventional ZnS phosphor material. The potential challenges and future prospects of utilizing phosphor materials for switching applications are also addressed. The strategies for manipulating ZnS phosphor can be generalized for a broad range of other materials by minimizing linear and non-linear losses, while enhancing the values of the non-linear refractive index coefficient. We propose that the figure-of-merit of ZnS material can be enhanced by using a suitable combination of pump and probe wavelength values, which can be useful for optical switching applications.

Investigating the Z-scan technique for quantifying circulating cell-free DNA (ccfDNA) extracted from blood plasma as a potential biomarker for various cancers.

Abstract
The Z-scan technique is a nonlinear optical method that has found applications in characterizing various materials, particularly those exhibiting nonlinear optical response (NLOR). This study applies the continuous wave (CW) Z-scan technique to examine the NLOR in terms of the nonlinear optical phase shifts (ΔΦ_0) exhibited by the ccfDNA extracted from blood plasma samples collected from a group constituting 30 cancer-diagnosed patients and another group constituting 30 non-diagnosed individuals. The cancer group exhibited significantly higher ΔΦ_0vs. incident power slopes compared to the non-cancer group (0.34 vs 0.12) providing a clear distinction between the two groups. The receiver operating characteristic (ROC) curve analysis of the results indicates a clear separation between cancer and non-cancer groups, along with a 94% accuracy rate of the data. The Z-scan results are corroborated by spectrophotometric analysis, revealing a consistent trend in the concentration values of ccfDNA samples extracted from both cancerous and non-cancerous samples, measuring 3.24 and 1.41 respectively. Additionally, more sensitive fluorometric analyses of the respective samples demonstrate significantly higher concentrations of ccfDNA in the cancer group, further affirming the correlation with the Z-scan results. The study suggests that the Z-scan technique holds promise as an effective method for cancer detection, potentially contributing to improved oncology diagnosis and prognosis in the future.&#xD.

Iodinated lutetium (III) phthalocyanine: Synthesis, photochemical and nonlinear optical properties

New octaiodo-substituted lutetium (III) phthalocyanine was prepared using a six-step synthetic procedure starting from commercially available o-xylene. The amide synthesis and dehydration step were optimized. The influence of the nature of peripheral halogen atoms on optical and photochemical properties was studied. For the complex under study, the possibility of participation in photoreactions of types I and II with the formation of two different forms of reactive oxygen species was investigated: superoxide anion radical and singlet oxygen respectively. Competition between two photoreactions was demonstrated. The Z-scan method revealed a pronounced nonlinear optical response of the iodinated complex, caused by the action of heavy atoms of peripheral iodine groups.

Observation of Anisotropic Second Harmonic Generation in Two-Dimensional Niobium Diselenide.

The intrinsic anisotropy of NbSe2 provides a favorable prerequisite of second harmonic generation (SHG) and rich possibilities for tailoring its nonlinear optical (NLO) properties. Here we report the highly efficient SHG of mechanically exfoliated NbSe2 flakes. The nonlinear optical response changes with excitation wavelengths, layer thicknesses, and polarizations of the excitation laser. The anisotropic SHG response further exhibits the intrinsic non-centrosymmetric crystal structure and could effectively assign the crystalline orientation of NbSe2 flakes. Interestingly, although NbSe2 flakes with tens of nanometers thickness experience attenuations in SHG performance, more efficient SHG anisotropy ratios were obtained, which are around 4 times higher than that of the 5-layer counterpart. The determined second-order nonlinearities of NbSe2 flakes (monolayer: ∼1.0 × 103 pm/V; 3-layer: ∼73 pm/V) are comparable to those of the commonly reported two-dimensional materials (e.g., MoS2, WSe2, graphene) with the same number of layers and much higher than those of commercial nonlinear optical crystals.

Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms.

Conjugated and processable self-standing vinylene-linked covalent organic framework membranes (COFMs) are highly demanding for photonics and optoelectronics. In this work, we have fabricated the first cyclotriphosphazene (CTP) cored vinylene-linked self-standing COFM (CTP-PDAN). For comparison purposes, we have successfully fabricated the imine-linked congener (CTP-PDA). Leveraging the inherent nonlinear optical (NLO) response of the CTP core, both membranes were directly mounted to evaluate NLO parameters using the open-aperture (OA) Z-scan technique. Direct measurement of NLO responses on membranes is advantageous and free from solvent and scattering effects, making it a more practical approach compared to the conventional dispersion mode. The OA Z-scan transmission yields a reverse saturable absorption signature exhibiting a higher NLO absorption coefficient (β) of 58.37 cm/GW for CTP-PDAN, compared to that of the imine-linked CTP-PDA COFM (β = 8.5 cm/GW). These results can be correlated to the efficient conjugation through the vinylene linkage in CTP-PDAN compared to the imine linked congener.

Two-Photon Absorption in Bioactive C4-substituted Coumarin Derivatives: Impact of Electron-Withdrawing and Donating Groups in different Environment

The versatile applications of coumarin derivatives in materials processing, fluorescence imaging, data storage, and photodynamic therapy have sparked considerable interest in scientific community. In this research paper, we explore the UV-vis spectra, IR spectra, and two photon absorption (TPA) characteristics of biological active coumarin derivatives under the influence of electron withdrawing (specifically X = CONH2, CHO, and NO2) and electron donating (X= -CH3, -OH, -NH2) functional groups at C-4 position. To delve into the absorption spectra, we employ time-dependent density functional theory calculations. Results revealed that acceptor functional groups (specifically R = CONH2, CHO, and NO2) not only lower the energy of the lowest two photon active state but also increase the corresponding TPA cross-section. Furthermore, using the Conductor-like Polarizable Continuum Model (CPCM), the influence of solvent polarity on C-4 substituted coumarin derivative was also investigated. Our findings effectively capture how substitutions and solvents impact the nonlinear optical response in coumarins, as evidenced by the measurement of the two-photon absorption cross-section (σTPA). This study offers valuable insights for identifying and designing novel coumarin derivatives with C4 substitutions, unlocking new and intriguing possibilities for their practical applications.

Diagrammatic approach to excitonic effects on nonlinear optical response

Diagrammatic approach to excitonic effects on nonlinear optical response

Amplifying the nonlinear optical response of sunflower-like C16S8 through superhalogen doping

Superhalogens, known for their higher electron affinity compared to halogens, exhibit a unique ability to accept electrons from any electron donor specie. Leveraging this property, the doping of superhalogens onto electron-rich surfaces has emerged as a potent approach to augment nonlinear optical (NLO) response. Herein, we investigated the doping of beryllium trifluoride (BeF3), calcium trifluoride (CaF3) and magnesium trifluoride (MgF3) superhalogens on thia[8]circulene (C16S8) sunflower through density functional theory (DFT) calculations, to observe the improvement in the NLO response of resultant complexes. The study explores the thermal stability of complexes through interaction energy analysis, revealing robust structural properties. Conducting properties of the resultant complexes are examined via frontier molecular orbitals (FMOs) analysis, demonstrating notable changes. Molecular electrostatic potential (MEP) and Fukui function analyses gave an idea about the reactivity of the pure C16S8 and superhalogen doped C16S8 complexes. To quantify the NLO response, we assessed the first hyperpolarizability (β o) via comprehensive analysis. Remarkably, the MgF3@C16S8 complex reflects the highest hyperpolarizability value (1.85 × 104 au), signifying its exceptional NLO enhancement potential. Ultra-violet visible (UV–vis) analysis illustrated the increase in the wavelengths (red shift) of C16S8 complexes after doping of superhalogen. To elucidate the underlying factors contributing to the improved NLO response, a rationalization is provided through a two-level model.

Correlation of quantum well periods on the 3rd order nonlinear optical responses of InGaN/GaN heterostructures

Semiconductor multi quantum well (MQW) design allows for the compression of electron-interaction dimensions, offering advantages for various applications, particularly in nonlinear optics. Despite the potential improvements in nonlinear optical responses, the influence of quantum well periods remains unexplored. In this study, we report the correlation of quantum well periods on the 3rd order nonlinear optical properties in In0.15 Ga0.85 N/GaN MQW using z-scan technique under femtosecond and continous wave laser regime. In0.15 Ga0.85 N well/GaN barrier with x1, x5, x10, x15 periods were grown by MOCVD technique. Good structural quality of grown MQWs were confirmed via HRTEM imaging, EDS, AFM and XRD. Absorption coefficient from UV-Vis coupled with PL emission spectrum shows systematic response of optical linear properties which provide an insight on the overall band gap structure. Positive nonlinear refraction, n2 and nonlinear absoprtion, β trends is observed under femtosecond an CW laser albeit differences of one order magnitude in χ3 value. The increase in χ3 value is shown to be monotonous with the increasing number of quantum well suggesting that compounding effect took place. This findings indicate the possibility of manipulating and tuning nonlinear reponses of MQW structures leading to improvement in semiconductor design in various areas such as optoelectronics, photonics, solar cell research, as well as quantum information processing.

Synthesis, physicochemical, nonlinear optical properties of azonaphtharylmide dyes: Dyeing and UV-protection performance on wool, silk, polyester

This study focuses on the synthesis and characterization of three azonaphtharylmide dyes, APAS, APASD, and APASTR, using naphthol AS, ASD, and ASTR couplers. The research also observes the presence of a hydrazone tautomer in a DMSO solution. It was characterized through spectroscopic analysis, which revealed three characteristic carbonyl carbon peaks in the 13C NMR spectrum. The calculated low LUMOs (−3.46 eV to −3.54 eV) align closely with experimental calculations (−3.45 eV to −3.48 eV), indicating the potential of these dyes as electron acceptors. Furthermore, the alignment of frontier molecular orbital (FMO) values supports their suitability for electron acceptance. When applied to wool, silk, and polyester materials, these dyes confer UV protection properties, with wool-dyed fabrics achieving a UV Protection Factor (UPF) of 50+. Additionally, these dyes exhibit significantly improved nonlinear optical responses, with beta (β0) values ranging from 11 × 10−30 to 23 × 10−30 esu and gamma (γ0) values ranging from 275 × 10−36 to 365 × 10−36 esu, indicating their functionality as nonlinear optical materials.

Design of Pentanary Mixed-Chalcogenides Ag2In2SiS6-xSex (x = 1, 2) Based on the Bucket Effect: Local Structural Difference and High-Performance Nonlinear-Optical Properties Realized by Partial Congener Substitution.

Noncentrosymmetric chalcogenides are promising candidates for infrared nonlinear-optical (NLO) crystals, and exploring high-performance ones is a hot topic and challengeable. Herein, the combination of AgQ4, InQ4, and SiQ4 (Q = S, Se) units with different S/Se ratios resulted in the discovery of the tetrahedral chalcogenides Ag2In2SiS4Se2 (1) and Ag2In2SiS5Se (2). They both crystallize in the monoclinic Cc space group with different local structures. Co-occupied S/Se sites only exist in 2, and the arrangement of [In2SiQ3] six-membered rings builds different helical chains and 3D [(In2SiQ6)2-]n polyanionic frameworks in 1 and 2. They show balanced NLO performances, including phase-matchable moderate NLO responses (0.7 and 0.5 × AGS) and enhanced laser-induced damage thresholds (4.5 and 5.1 × AGS). Theoretical calculations reveal that their NLO responses are predominantly contributed by the AgQ4 and InQ4 units.

Strongly Interacting Photons in 2D Waveguide QED.

One-dimensional confinement in waveguide quantum electrodynamics (QED) plays a crucial role to enhance light-matter interactions and to induce a strong quantum nonlinear optical response. In two or higher-dimensional settings, this response is reduced since photons can be emitted within a larger phase space, opening the question whether strong photon-photon interaction can be still achieved. In this study, we positively answer this question for the case of a 2D square array of atoms coupled to the light confined into a two-dimensional waveguide. More specifically, we demonstrate the occurrence of long-lived two-photon repulsive and bound states with genuine 2D features. Furthermore, we observe signatures of these effects also in free-space atomic arrays in the form of weakly subradiant in-band scattering resonances. Our findings provide a paradigmatic signature of the presence of strong photon-photon interactions in 2D waveguide QED.

The behavior of low-symmetry penta(chloro)cyclotriphosphazene-substituted monophthalocyanines in the oriented external electric fields

Electric field-mediated chemistry provides fertile ground for interdisciplinary research, where theoretical ideas and experimental data converge to unravel the complexities of molecular behavior under the influence of electric fields, offering unprecedented opportunities for scientific research and technological innovation. Thus, this work is devoted to the study underscoring the importance of oriented external electric fields (OEEFs) in modulating the structural and electronic properties of low-symmetry monophthalocyanines, which is crucial for enhancing their reactivity and nonlinear optical response. The highlight of this article is that the OEEFs lead to molecular conductivity due to reduced energy gap and electron density shifting towards the phosphazene ring. Additionally, there is a tendency for the elimination of chlorine atoms in the trans- position relative to the phthalocyanine macrocycle. Also, the use of the previously created CORRELATO algorithm has allowed the demonstration of the relationship between structural changes and microscopic parameters responsible for the appearance of NLO properties of final dye materials.

Nonlinear optical response in multiple-mode coupling nanomechanical system

Nonlinear optical response in multiple-mode coupling nanomechanical system

Ultrafast nonlinear optical response of layered violet phosphorus for femtosecond noise-like pulse generation

As a new member of two-dimensional (2D) phosphorene, 2D layered violet phosphorus (VP) has unique optoelectronic properties and good environmental stability, showing its huge advantages in optoelectronic applications. In this paper, the ultrafast nonlinear optical (NLO) properties of layered VP nanosheets at 1 µm band were explored, which exhibit an obvious saturable absorption response with a modulation depth of ∼1.97%. Meanwhile, the fast and slow carrier lifetimes of VP nanosheets at 1µm band were also determined as 295.9 fs and 2.36 ps, respectively, which are much shorter than that of most reported 2D materials. The excellent saturable absorption response combined with ultrashort carrier lifetimes indicate the prospect of layered VP nanosheets as a fast saturable absorber (SA) for ultrafast laser modulation. Then we demonstrated a Yb-doped fiber laser based on the VP-deposited taper-shaped fiber (TSF) SA, which delivers stable Q-switched mode-locked (QSML) pulses, dual-wavelength mode-locked pulses and 404-fs noise-like pulses. This work fully demonstrates the great potential of 2D VP materials for 1 µm ultrashort laser pulse generation.

Boosted Second Harmonic Generation and Cascaded Sum Frequency Generation from a Surface Crystallized Glass Ceramic Microcavity.

The development of novel materials and structures for efficient second-order nonlinear micro/nano devices remains a significant challenge. In this study, we demonstrate the remarkable enhancement of second-harmonic generation (SHG) and even cascaded sum frequency generation (CSFG) in whispering gallery mode microspheres made of surface-crystallized glass with a 6-μm Ba2TiSi2O8 crystal layer. Attributed to the core-shell design, the Ba2TiSi2O8 located on the surface can be efficiently coupled with whispering gallery modes, resulting in a highly efficient micron-scale cavity-enhanced second-order optical nonlinearity. Greatly enhanced SHG of the microcavity was observed, which is up to 80 times stronger than that of non-resonant bulk sample. Furthermore, owing to the wavelength non-selectivity of random quasi-phase matching, we demonstrated ultra-wideband SHG with a strong response ranging from 860nm to 1600nm and high-contrast polarization characteristics. The glass-ceramic based microsphere cavity also boosts the cascading optical nonlinearity, manifested by a two-magnitude enhancement of CSFG. This work delineates an efficient strategy for boosting nonlinear optical response in glass-ceramics, which will open up new opportunities for applications in photonics and optical communications. This article is protected by copyright. All rights reserved.