Excellent Nonlinear Optical Material Research Articles

Designing Strong Polarity and High Configurational Entropy Flexible Units toward Excellent Ultraviolet Nonlinear Optical Materials

AbstractPolar materials play a crucial role in numerous essential fields; however, efficiently synthesizing them remains a significant challenge. In this study, a viable approach is proposed to enhance the probability of discovering polar materials by developing new functional basic units (FBUs) with strong polarity and high configurational entropy. To validate this approach, a site‐modification strategy is applied to the high configurational entropy flexible (C3H2O4)2− FBU, resulting in the development of four new polar‐enhancing FBUs: (C3(CH3)2O4)2−, (C3F2O4)2−, (C3H(OH)O4)2−, and (C3HFO4)2−. Moreover, 11 new compounds through temperature‐driven conformational changes in FBUs are successfully synthesized. Among these, the proportion of polar structures reaches an impressive 54%. Remarkably, all of these novel polar compounds exhibit outstanding nonlinear optical (NLO) properties. The [Li2C3(CH3)2O4]2·3H2O demonstrates the rare coexistence of a moderate birefringence (0.071@514 nm), short UV cutoff edge (210 nm), and the strongest second harmonic generation (SHG) effect (6.5 × KDP) among ionic‐organic fourth harmonic generation crystals. Preliminary laser experiments have also proven its potential practical value as a fourth harmonic generation crystal. This research not only discovers a variety of exceptional UV NLO crystals but also provides a feasible method to increase the likelihood of finding polar materials.

Investigation of the third-order optical nonlinear enhancement properties of Ag doped chalcogenide glass films

Chalcogenide glass is widely recognized as an excellent nonlinear optical material. Enhancing its nonlinear optical properties through metal doping has emerged as a prominent research avenue in the field of chalcogenide glass. In this paper, silver (Ag) doped Ge28Sb12Se60 (GSS) chalcogenide glass films with different concentrations were prepared by co-evaporation method. The nonlinear absorption coefficients (β) of different samples were investigated by open-aperture top-hat Z-scan experiments and pump-probe experiments, and the nonlinear refractive index (γ) values were determined using the Tichy-Ticha relation. The results showed that the nonlinear absorption of GSS thin films can be improved by Ag doping. The nonlinear absorption coefficient β exhibits the maximum enhancement of about 2.5 times at 8.3% concentration, with the maximum β reaching 1.83×10−6 m/W. The nonlinear absorption mechanism observed in the thin films can be attributed to free carrier absorption, and the free carrier lifetime is positively correlated with the Ag doping concentration. Within the selected Ag concentration range, the nonlinear refractive index (γ) of Ag doped chalcogenide glass films continues to increase, reaching about 3 times at 30.7% concentration, with the maximum γ being 4.643×10−17 m2/W. Additionally, the third-order nonlinear polarizability (χ(3)) values can be increased from 1.014×10−11 esu to 3.314×10−11 esu with increasing Ag concentration. The research results in present paper are of great significance for further exploration of the optical nonlinear enhancement properties and carrier recombination dynamics in chalcogenide glass.

Mo4/3B2T2/TiO2 vdW heterostructure as saturable absorber for ultrafast pulse generation

Van der Waals heterostructure have garnered significant interest in ultrafast laser generation because of their attractive saturable absorption properties, holding promise for application in optics, medicine, and optoelectronics. In this study, the saturable absorption properties of TiO2, Mo4/3B2T2, and Mo4/3B2T2/TiO2 vdW heterostructure SAs are investigated. The integration of the SAs into the cavity reveals the observation of fundamentally conventional solitons. The output characteristics of conventional solitons based on three SAs are recorded. These results suggest that the saturable absorption of Mo4/3B2T2/TiO2 vdW heterostructure is enhanced obviously compared with that of Mo4/3B2T2 and TiO2. Furthermore, 2nd-, 4th-, and 5th-order harmonic solitons are achieved, corresponding to the repetition rate of 20.36 MHz, 42.71 MHz, and 53.39 MHz, respectively. We established a model in order to understand the saturable absorption properties of the Mo4/3B2T2/TiO2, which suggests the enhancements of the saturable absorption are attributed to the carrier migration between Mo4/3B2T2 and TiO2. Our results indicate that Mo4/3B2T2/TiO2 vdW heterostructure can be used as an excellent nonlinear optical material, which can be used as an SA in ultrafast pulse generation.

Wavelength-dependent optical nonlinear response of vanadium diselenide

The wavelength-dependent and ultrafast nonlinear optical response of vanadium diselenide (VSe2) in the visible to near-infrared spectrum are experimentally investigated, which shows broadband saturation absorption and self-focusing characteristics, and obtained the maximum nonlinear absorption coefficient and nonlinear absorption coefficient are −4.058 × 10−10 m/W and 6.043 × 10−17 m2/W, respectively. Meanwhile, the ultrafast carriers dynamic response of VSe2 is studied by using time-resolved pump-probe method, and the ultrafast response time of 0.65 ps is obtained. Our results show that VSe2 is an excellent nonlinear optical material and has great application potential in the field of nonlinear optics.

Discovery of excellent ultraviolet nonlinear optical materials in chlorates and bromates with highly stereochemically active lone pairs

The long-neglected stereochemically active lone pair groups ClO3− and BrO3− are demonstrated to be important UV NLO functional motifs based on the fact that AClO3 and ABrO3 possess high SHG effects and short λPM and are promising UV NLO materials.

Second harmonic generation in an anisotropic lithium niobate metasurface governed by quasi-BICs

Resonant metasurfaces can greatly trap the light fields, so that they are widely used to enhance light–matter interactions at the nanoscale, such as promoting nonlinear effects of materials. Lithium niobate (LN) is an excellent nonlinear optical material and is often employed to generate harmonic signals. In this Letter, we numerically study the second harmonic generation (SHG) characteristics of the LN metasurface based on the quasi-bound states in the continuum (QBIC). The designed BIC and excited QBIC metasurfaces always hold C4v symmetry, and the BIC is demonstrated to degenerate into two BICs due to the anisotropic characteristics of LN. Moreover, the excited two high Q-factor QBICs can effectively enhance the SHG in LN, although the device maintains C4v symmetry, the SHG signal still shows polarization dependence. In addition, with the increase of Q-factor of quasi-BIC, the power and conversion efficiency (η) of SHG increase significantly. The calculated η can reach 6.04 × 10−3 and can be further improved when the resonance mode is closer to BIC. These results have important implications for high-quality nonlinear light sources based on LN materials.

Wavelength-Dependent Nonlinear Optical Activities of Near-Infrared Boron-Dipyrromethene Derivatives

Near-infrared (NIR) organic dyes with strong ultrafast nonlinear optical (NLO) activities are important for various applications. However, the study of the wavelength-dependent NLO properties of NIR dyes is still ongoing. In this work, we present the first comparison of the NLO properties of two NIR boron-dipyrromethene (BODIPY) derivatives at different excitation wavelengths, in which strong electron-donating groups, namely 4-(N,N-dimethylamino)phenyl and 1-ethyl-1,2,3,4-tetrahydroquinoline groups, are connected through the BODIPY cores. Results obtained from Z-scan experiments show that the two BODIPY derivatives exhibit strong saturable absorption and large modulation depth when excited by femtosecond pulses at 800 nm. With 1300 nm excitation, both derivatives exhibit strong nonlinear refraction. In addition, the derivatives also display effective two-photon action cross-sections in the wavelength range of 1200–1600 nm. They are potentially excellent nonlinear optical materials. This comprehensive comparison of the NLO properties of BODIPY derivatives can suggest new possibilities for the design and development of NIR NLO materials.

Cascaded four-wave mixing all-optical wavelength converter based on highly nonlinear fiber deposited graphene oxide

The all-optical wavelength conversion using cascaded four-wave mixing (FWM) phenomena with graphene oxide (GO) and a highly nonlinear fiber (HNLF) device is demonstrated experimentally. GO has a strong third-order nonlinear effect and is an excellent nonlinear optical material for nonlinear optical wavelength conversion. The group velocity matching of pump and signal, HNLF, and GO amplification promote the cascaded nonlinear frequency mixing process. From the experimental and analytical results, the maximum spacing between signal and pump is 21 nm, and specifically, the order of cascaded FWM light increases from order 1 to order 2 with increasing GO, and the first-order FWM conversion efficiency increases to a maximum of −14.5 dB. To the best of our knowledge, this is the first time that cascaded FWM-based all-optical wavelength conversion in HNLF–GO with wide wavelength selectivity is investigated with combined pump and signal light. Our findings not only provide an effective method for achieving all-optical wavelength conversion in cascaded FWM but also offer the possibility of fabricating high-performance nonlinear optical devices.

The effect of the CPP size on the nonlinear optical properties of the new necklace-type molecules formed by carborane and [n]Cycloparaphenylenes(n = 8–11)

The new necklace-type molecules were formed by [8–13]CPP and carborane, which further manipulated the size of the macroring, revealing the effect of size on its luminescence behavior. In this work, the effects of ring size on the absorption spectrum, electron excitation and nonlinear optical properties of the compounds were investigated in detail, aiming to reveal an effective way to improve the optical properties of these necklace-type compounds. The absorption spectra of the compounds showed that the size of the CPP ring had little effect on the spectral shape and position, but the electron transition information showed that there were the significant charge transfer within the CPP ring and a gradual enhancement of interfragment charge transfer from the CPP ring to carborane. The increasing order of polarizability, first and second hyperpolarizability values of these compounds with the increase of CPP size indicated that increasing the size of the CPP ring was an effective way to increase the nonlinear optical properties of necklace-type molecules. Among the frequency dependent hyperpolarizability values, the γ(-ω;ω,0,0) value increased by a factor of 4 from complex 1 to 6 with the increase of CPP ring size, which indicated that increasing the size of the CPP ring was an effective way to increase the optical Kerr effect of necklace-type molecules. Therefore, these the new necklace-type nolecules formed by carborane and [n]Cycloparaphenylenes would be excellent nonlinear optical materials in the field of the all-optical switch.

Highly stable femtosecond pulse generation enabled by the indium tin oxide nanocrystals

The ultrafast third-order optical nonlinearity of indium tin oxide nanocrystals (ITO NCs) and their application in femtosecond laser generation have been investigated experimentally. The ITO NCs exhibit a large modulation depth of ∼25%, nonlinear refractive behavior with self-focusing, ultrafast carrier recovery time of ∼300 fs, and high damage threshold of ∼1.16 TW cm−2, and the stable femtosecond Er-doped fiber laser can be delivered with a signal-to-noise ratio over 80 dB modulated by the ITO NCs successfully. The experimental results indicate that the ITO NCs can be excellent ultrafast nonlinear optical materials for developing highly stable photonic and optoelectronic devices.

Promoting the nonlinear optical absorption of conjugated polymers by in-gap states modulation via chemical dedoping

With the development of laser technology, nonlinear optical (NLO) materials are attracting more and more attention because of their significant application potential in optical limiting, optical switching, photonic diode, sub-bandgap photoelectric detection, Q-switched/mode-locked pulse laser generation, and so on. In this paper, a convenient dedoping strategy is developed to promote the NLO response of conjugated polymer based on in-gap states modulation. The poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), PEDOT:PSS, exhibits remarkably enhanced nonlinear absorption at different dedoping levels in comparison with the pristine one. The nonlinear absorption coefficient can reach -1578 ± 49 (lightly dedoped sample) and 612 ± 43 cm GW−1 (highly dedoped sample) under 800 nm laser excitation. The performance is 6 and 2 times that of the pristine PEDOT:PSS, respectively. The optical nonlinearity of dedoped films is also superior to many excellent NLO materials. The modulation of in-gap states alters the excitation process during the NLO response and accordingly the corresponding performance. In-gap states modulation can be applied to promote the NLO performance of other conjugated polymers, e.g. poly(3-hexylthiophene).

2D janus niobium oxydihalide NbOXY: Multifunctional piezoelectric semiconductor for electronics, photonics, sensing and sustainable energy applications

Two-dimensional (2D) niobium oxydihalide NbOI2 has been recently demonstrated as an excellent in-plane piezoelectric and nonlinear optical material. Here we show that Janus niobium oxydihalide, NbOXY (X, Y = Cl, Br, I and X ≠ Y), is a multifunctional anisotropic semiconductor family with exceptional piezoelectric, electronic, photocatalytic and optical properties. NbOXY are stable and flexible monolayers with band gap around the visible light regime of ∼1.9 eV. The carrier mobility of NbOXY exhibits an exceptionally strong anisotoropy ratio of ∼400, which is larger than most of the anisotropic 2D semiconductors. The exceptionally large deformation potential of NbOXYmonolayers leads to strong piezoresistive effect useful for strain sensing application. Inversion symmetry breaking in Janus NbOXY generates sizable out-of-plane d31 piezoelectric response while still retaining a strong in-plane piezoelectricity. Remarkably, NbOXY exhibits an additional out-of-plane piezoelectric response, d32 as large as 0.55 pm/V. G0W0-BSE calculation further reveals the strong linear optical dichroism of NbOXY in the visible-to-ultraviolet regime. The optical absorption peaks of 14 ∼ 18% in the deep UV regime (5 ∼ 6 eV) outperform the vast majority of other 2D materials. The high carrier mobility, strong optical absorption, sizable built-in electric field and band alignment compatible with overall water splitting further suggest the strengths of NbOXY in solar-to-hydrogen conversion. We further propose a directional stress sensing device to demonstrate how the out-of-plane piezoelectricity can be harnessed for functional device applications. Our findings unveil NbOXY as an exceptional multifunctional 2D semiconductor for flexible electronics, optoelectronics, UV photonics, piezoelectronics and sustainable energy applications.

Achieving Large Second Harmonic Generation Effects via Optimal Planar Alignment of Triangular Units

AbstractTrigonal planar units with large polarizability anisotropy and high physicochemical stability are ideal structural units for exploring nonlinear optical (NLO) materials. Integrating the merits of two types of triangular‐like moieties, a family of second‐order NLO‐active hybrid halides, MATX (X = Cl (1), Br (2), and I (3)), are achieved. MATX crystallizes in a nonpolar space group of 2c but exhibits the optimal spatial arrangement and superior NLO performance. The low coordination planar trigonal AgX3 units enable segregation in layers of the three‐winged propeller‐like Me3TPA units. All of the layers are packed in a perfect parallel fashion, making the functional materials exhibit superior NLO performances, including the phase matchable behavior with strong SHG responses (6.2/1, 6.5/2, and 7.6/3 times that of potassium dihydrogen phosphate), large birefringence (0.232/1, 0.252/2 and 0.260/3 at 1064 nm), high laser damage threshold, wide transparent window, and easiness of crystal growth. The first‐principles calculations reveal that the coexistence of strong linear and nonlinear optical properties are ascribed to the synergistic effect of the trigonal moieties. This study points out a useful path for the rational design of excellent NLO materials.

Structure-Property Relationship of Three 2-Chloro-4-fluoro Chalcone Derivatives: A Comprehensive Study on Linear and Non-linear Optical Properties, Structural Characterizations and Density Functional Theory

This work focused on the relationship between molecular structure and non-linear optical (NLO) property of the chalcone derivatives. The NLO property is well known increased by push-pull effect, therefore we have explored donor-acceptor substitutions on both rings to develop excellent NLO materials. Three new crystals of 2-chloro-4-fluorochalcone derivatives bearing various donor end groups were synthesized in order to evaluate the donor dependence of their second-order hyperpolarizability γ. The grown crystals were spectroscopically confirmed and crystallographically analysed. Their intermolecular interactions were visualized by Hirshfeld surface analysis. The electronic absorption spectrums were investigated by experimental and theoretical methods, to discuss the intramolecular charge transfer and energy gap of solution sample, which defined significantly affected NLO properties. Theoretical electronic properties have been simulated with density functional theory (DFT) at B3LYP/6-311++G (d,p) level and compared with experimental results. Thermally-induced NLO parameters such as non-linear absorption (NLA) coefficient (α2∼10−5 cmW−1), non-linear refractive (NLR) index (n2 ∼10−9cm2W−1), third-order susceptibility [χ(3)(-ω;ω,ω,-ω) ∼10−8 esu] and γ(-ω;ω,ω,-ω)∼ 10−27 esu] were measured by Z-scan technique under continuous-wave laser excitation (532 nm). The solution samples exhibit two-photon assisted reverse saturable absorption and self-defocusing non-linear refraction, optical limiting and all-optical switching behaviours. We have determined that γ depends on the strength of donor/acceptor substituents, which is the result of the two contributions: the energy gap and the transition dipole moment between the ground and first excited states. A large value of γ was found for compound I which bearing strong donor hydroxy substituent, thus the hydroxychalcone derivatives as a potential material with excellent NLO performance.

Effect of TGS on crystal growth and properties of ammonium dihydrogen phosphate (ADP)

Ammonium dihydrogen phosphate (ADP) is an excellent inorganic nonlinear optical material having scalenohedral class of tetragonal structure provides order disorder phase tansitions used in various instrumental applications. Good quality of pure and three different wt% of TGS doped ADP single crystals have been grown by natural evaporation method at room temperature. The structural, optical and dielectric characterization of as-grown pure and doped ADP crystals was made using powder XRD, UV–Vis and dielectric spectroscopy. Band gap energies of the pure and doped crystals have been calculated at cut off frequencies using the UV–Vis spectrum analysis. Dielectric constant and dielectric loss of the grown crystals is measures using VNA with frequency ranges 200 MHz to 20 GHz at room temperature.

Static, dynamic nonlinear optical (NLO) response and electride characteristics of superalkalis doped star like C6S6Li6

In search of excellent nonlinear optical (NLO) materials, new type of M3O@C6S6Li6 (M = K, Na, Li) electrides are set forth herein. The geometric, electronic and nonlinear optical properties of these electrides are evaluated by using density functional theory calculations. All the considered complexes have interaction energies in the range of -42.86 to -45.56 kcal/mol that validate their thermodynamically stability. The NBO charge analysis is performed to confirm the charge transfer between M3O and C6S6Li6. The highest amount of electronic charge transfer (0.92 |e|) is seen in K3O@C6S6Li6 electride. The frontier molecular orbitals (FMOs) and density of states (DoS) analyses are implemented to further rationalize the electronic properties. The remarkable polarizability (αo) and static first hyperpolarizability (βo) observed for these complexes validate their excellent nonlinear optical response. The highest βo (2.98×105 au) is obtained for Li3O@C6S6Li6 electride. The frequency-dependent first and second hyperpolarizability coefficients along with dipole dependent hyperpolarizability are also analyzed to get insight of NLO response under variational frequencies. Furthermore, the Hyper-Rayleigh Scattering coefficient (βHRS) of these complexes is investigated to provide information required for experimental point of view of NLO response. This study will contribute towards the development of more stable superalkalis doped electrides having pronounced NLO response.

Graphdiyne-Coated Microfiber All-Optical Temporal Modulator Based on Saturable Absorption

A high-speed all-optical modulator is a key device in next-generation communication systems. Due to easy fabrication and an effective modulation effect, two-dimensional (2D) material–microfiber structure all-optical modulators have aroused much attention. Graphdiyne (GDY) is an excellent nonlinear optical material and is expected to be utilized in all-optical modulators. In this work, we demonstrate a GDY-coated all-optical temporal modulator according to its saturable absorption. Under the pump pulse light of 1,064 nm, the fabricated modulator successfully modulates the continuous-wave (CW) light of 1,550 nm to the waveform of pump light with a pulse width of 4 ns and a repetition rate of 5 kHz. Our results show that GDY could be used in high-speed all-optical modulators and pave a way for the research of GDY in all-optical information processing applications.

Computationally assisted design and prediction of remarkably boosted NLO response of organoimido‐substituted hexamolybdates

AbstractIn recent years, nonlinear optical (NLO) molecular material networks/frameworks have shown attractive application prospects in modern laser technology, data storage components, optical communication, and digital processing. Finding new and novel materials with excellent nonlinear optical properties has become an attractive research topic in the field of nonlinear optics. Herein, DFT calculations have been performed on hybrid organic‐inorganic composite push‐pull systems based on polyoxometalates (POMs). Systems (DPA1‐DPA4) are theoretically analyzed to study the second‐order NLO response and have been compared with reference compound. The effect of four substituent groups on NLO properties has been investigated. Based on optimized geometric configurations, dipole polarizability(α), first‐order hyperpolarization(β), transition energy (E), oscillation frequency (ƒ), and HOM0 (highest occupied molecular orbital)‐LUM0 (lowest unoccupied molecular orbital) analysis have been performed. All the studied systems possess significantly large second‐order polarizability and proven to be excellent nonlinear optical hybrid composite materials. The asymmetric polarization of charges caused by D‐π‐A combination in our studied systems is accountable for the improvement of nonlinear optical activity of these push‐pull compounds. POM cluster/cage is behaving as an electron donor, whereas charge transfer has been observed in the direction of chain length. On the bases of the electron donor and electron acceptor ability of these substituent groups, substantial influence on NLO response and charge transfer characteristics has been identified. Our studied systems are efficient NLO materials and will be helpful for the synthetic chemists to produce effective hybrid composites.

Near-infrared all-fiber mode-locked laser based on vanadium carbide nanosheets

Vanadium carbide (V2C), a newly developed MXene, has been theoretically predicted to be an excellent nonlinear optical materials. Here, few-layer V2CTx nanosheets were synthesized by liquid phase exfoliation (LPE) method. Z-scan measurement was performed to explore their third-order nonlinear optical properties, highlighting the strong nonlinearities of few-layer V2CTx nanosheets. And the effective nonlinear absorption coefficient was determined to be − 98.3 cm/MW, corresponding to the imaginary part of the third-order nonlinear effects was obtained to be − 1.106 × 10−8 esu. In addition, microfiber-based few-layer V2CTx saturable absorber (SA) was fabricated by optical deposition method and the saturable absorption characteristic was measured by I-scan technique. The Yb-doped and Er-doped fiber lasers (YDFLs and EDFLs) were performed by inserting microfiber-based few-layer V2CTx SA into the ring cavity. And the stable mode-locked operation was realized with super high signal- noise ratio (SNR). Our work systematically reveals that few-layer V2CTx nanosheets should be an excellent SA for ultrafast photonics in the near-infrared band.

Assembling of Perylene, Naphthalene, and Pyromellitic Diimide-Based Materials and Their Third-Order Nonlinear Optical Properties

π-conjugated aromatic diimides with chemical stability, heat resistance, and redox activity have attracted more attention due to their excellent fluorescence quantum yield in solution. The planar perylene diimide (PDI) derivatives generally have aggregation-induced emission quenching in the solid state, while the cyclic trimers based on pyromellitic diimides (PMDIs), naphthalene diimides (NDIs), and PDIs can increase the fluorescence quantum yield in the solid state and have large two-photon absorption cross section, which can be used as excellent nonlinear optical (NLO) materials. Therefore, this paper will study the effects of multiple assembly modes of the three monomers on the NLO responses of materials. It was found that the assembly modes of 2PMDI-1NDI and 2NDI-1PDI exhibit larger third-order NLO response (γ) values, which was due to the larger conjugate surface of PDI effectively reducing the energy gap between the HOMO and LUMO. Compared with other assembly methods, 2PMDI-1NDI and 2NDI-1PDI were conducive to causing redshifts (150 nm) in the absorption spectrum. Therefore, the larger conjugate surface of PDI and the assembly mode of the isosceles triangle were more favorable for intramolecular charge transfer, thus improving its NLO properties.