Nonlinear Optical Measurements Research Articles

The phonon-modulated Jahn–Teller distortion of the nitrogen vacancy center in diamond

The negatively charged nitrogen vacancy (NV) center in diamond is an optically accessible material defect with a unique combination of spin and optical properties that has attracted interest in quantum-information sciences and as a design candidate for nanoscale quantum sensors. Here, we present time-resolved nonlinear optical spectroscopy measurements, conducted with ultrabroadband laser pulses, that reveal strong modulation of the excited-state by the longitudinal optical (LO) phonon of the diamond lattice. The LO phonon and its overtones geometrically distort neighboring NV centers, driving long lived (3.5 ps) excited state relaxation of coupled NV centers after the initial excitation and ultrafast (<150 fs) decay of the Jahn–Teller distortion. These observations elevate the LO phonon to an important tuning mode of the Jahn–Teller conical intersection and help resolve previous spectroscopy experiments that noted longer-lived excited-state dynamics.

Synthesis and Optical Characterization of Hydrazone-Substituted Push-Pull-Type NLOphores.

Two distinct families of NLOphores featuring hydrazone donors were synthesized using click-type [2 + 2] cycloaddition retroelectrocyclizations (CA-RE). Despite the limitations in the substrate scope, it was shown for the first time that hydrazone-activated alkynes could undergo reactions with TCNE/TCNQ. The electrochemical, photophysical, and second-order nonlinear optical (NLO) characteristics of the chromophores were analyzed utilizing experimental and computational approaches. Chromophores 17-21 and 23-27 exhibited two reduction waves, along with one oxidation wave that can be attributed to the hydrazone moiety. All chromophores exhibit charge-transfer bands extending from the visible to the near-infrared region. The λmax of hydrazone-based chromophores falls within the range of 473 to 725 nm. Additionally, all chromophores exhibited positive solvatochromism. Computational studies have been performed to elucidate the origin of the low-energy absorption bands. Parameters such as dipole moment, band gaps, electronegativity, global chemical hardness/softness, average polarizability, and first hyperpolarizability were calculated to obtain information about NLO properties of the target structures. The thermal stabilities of the NLOphores were assessed through TGA. Experimental NLO measurements were conducted using the electric field-induced second harmonic generation (EFISHG) technique. The studied structures demonstrated NLO responses, with μβ values between 520 × 10-48 esu and 5300 × 10-48 esu.

Improvement in optical nonlinearity of spiral square Mn-based nanostructures with increasing the number of square sides

Spiral square Mn-based nanostructures with different numbers of square sides were grown on glass substrate using the glancing angle deposition method. The crystallinity of the products was determined by X-ray diffraction. The surface properties of the synthesized nanostructures were analyzed by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). 2D AFM images have been used for more detail on grain growth evolution and height distribution. The band gaps of the nanostructures were determined using the Tauc formula in the range of 2.72–3.06 eV. It was observed a decrease in band gap energy of spiral square Mn-based nanostructures with increasing the number of square sides. Nonlinear optical measurements of synthesized samples were performed using a Z-scan setup under a CW laser irradiated at 532 nm. Values of the order of 10−4 cm W−1 and 10−7 cm2 W−1 were calculated for nonlinear absorption coefficients and refraction indices of spiral square nanostructures, respectively. This investigation revealed an increase of about 1.7 times for nonlinear optical parameters of Mn-based nanostructures with increasing the number of square sides from 8 up to 10 that is significant increment. These results suggest spiral square Mn-based nanostructures as a potential material for application in optics.

Spectrally resolved nonlinearities within a laser pulse in a single-scan and spectrometer-based nonlinear optical probing.

The intricate spectrally resolved optical nonlinearities resulting from a spectrally broad femtosecond Gaussian laser pulse have been unraveled using a single-scan and spectrometer-based nonlinear optical probing technique. The interaction of the broad femtosecond laser pulse with a strongly absorbing organic dye has unveiled a remarkably distinct nonlinear absorption behavior across the broad spectral window. The nonlinear absorption behavior unveils an unusual transition from the reverse saturation absorption (RSA) to the saturation absorption (SA) as we sweep the wavelength on both sides of the central wavelength of the excitation laser pulse. A competition between the band-filling and excited-state absorption results in such a dramatic switch-over from the RSA to the SA due to the variation of the intensity distribution across the Gaussian pulse spectrum. On the other hand, the nonlinear refraction studies dictate more over the constant Kerr-type positive nonlinear refractive indices across the entire laser pulse, with a pronounced contribution from the nonlinear absorption phase dominating at the center of the pulse. The presented technique establishes a robust and simple spectrometer-based technique that offers new, to the best of our knowledge, avenues for estimating optical nonlinearities for rapid nonlinear optical measurements.

Influence of defects on the linear and nonlinear optical properties of Cu-doped rutile TiO2 microflowers.

Defects and disorder work as controlling parameters to alter the electronic structure of nanostructures and significantly influence their electronic, magnetic, and nonlinear optical (NLO) properties. In this study, we found that defect engineering is an effective strategy for tailoring the linear and nonlinear optical properties of Cu-doped titanium oxide (TiO2) flower-shaped nanostructures. The concentration of chemical doping of Cu in the TiO2 lattice creates intermediate defect states that impact electronic bandgap reduction and tunable defect luminescence. The estimation of the bandgap from density functional theory calculation follows the same trend of bandgap narrowing with Cu doping. The XPS study reveals that oxygen defects are responsible for bandgap narrowing and quenching of the PL intensity. A single-beam Z-scan technique with open and closed aperture configurations using ultrashort pulses centered at 532 nm excitation wavelength was used to study the NLO measurements. The open aperture reveals saturable absorption, whereas the closed aperture shows self-focusing behavior. The nonlinear absorption coefficient and refractive index extracted from NLO measurements demonstrate the linear dependence on the defect concentration and bandgap. The effects of heterogeneous dopants and lattice disorder on the nonlinear absorption behavior of these nanostructures are discussed in comparison with the figure of merit, non-linear refractive index, and absorption coefficient. The tunable NLO properties achieved by controlling such dopant-induced defects boost the scope of these nanostructures as optical limiting, optical switching, and optical photodiode applications.

Carbon nanomaterials as nonlinear optical modulators for pulsed laser generation at near-infrared region

In this study, carbon nanoparticles (CNPs) were experimentally investigated as novel saturable absorbers (SAs) capable of delivering strong ultrashort pulses in a passively Q-switched fiber laser. Carbon nanoparticles produced by candle burning and laser ablation can act as saturable absorbers (SAs). In addition, the nonlinear saturable absorption properties of the two synthetic CNPs in telecommunication region (1.5 μm) are investigated. Nonlinear optical measurements show that both SAs have good saturable absorption properties with average modulation depths of about ∼1 and 5 % at 1560 nm. These candle soot carbon nanoparticles (CSC-np) and laser ablation carbon nanoparticles (LAC-np) based SAs, can be used as a building blocker for the generating of stable Q-switched pulses in the near-infrared region. The shortest pulse duration measured with CSC-np and LAC-np are 5.62 and 3.87 μm, respectively. The pulse energy, peak power and SNR were also compared with both SA types at the highest pump power. The damage threshold and long-term stability of the CSC-cn SA and LAC-np were also explored. To the best of the author's knowledge, it is the first comparison of performance and long-term stability of EDFL based on two experimental techniques CSC-np and LAC-np based SAs. These results indicate that LAC-np SA provide have several advantages, including a high damage threshold, good saturable absorption, and optimum stability over a long time compared to CSC-np SA.

Two different coordination polyhedra in the crystal structure of a novel Zn(II) hybrid complex: DFT investigation, biological activities and emission properties

This work centers around the search for sophisticated hybrid compounds with versatile applications. A new compound, denoted as (C6H8NO)3[ZnCl4][ZnCl3(H2O)] (1), was synthesized and thoroughly characterized. The structural features were determined through the use of various analytical techniques, including spectroscopy (FT-IR, Raman), thermo-gravimetric analysis, and optical studies encompassing ultraviolet-visible spectroscopy, nonlinear optical (NLO), and fluorescence measurements. The crystalline arrangement was explored through single crystal X-ray diffraction analysis, revealing a triclinic crystal system with the P1¯ space group. The molecular composition comprises three distinct mono-protonated p-aminophenol (C6H8NO)+ cations alongside two isolated tetrahedral anions ([ZnCl4]2− and [ZnCl3(H2O)]−). The stabilization of this hybrid compound accounts for a wide array of interactions, including hydrogen bonds and π–π (ring-ring) interactions, as deduced from comprehensive analyses. Theoretical investigations were conducted employing density functional theory (DFT) at the Cam-B3LYP/Lanl2dz level, accompanied by a detailed examination of the atoms in molecules (AIM) utilizing the BCP point. Furthermore, the nonlinear optical (NLO) properties were assessed, encompassing the evaluation of dipole moment, linear polarizability, and second hyperpolarizability. Notably, the compound exhibited pronounced solid-state fluorescence when excited at 350 nm, displaying intense emission within the blue spectrum. The correlated color temperature (CCT) value was measured at 7011 K, indicative of a "cool" white light emission. Basically, the synthesized compound exhibited antibacterial efficiency against several strains of human pathogenic bacteria in vitro and considerable enzyme inhibitory activity against pathogenic microorganisms. This research underscores the characteristics of (1), making it an appealing contender for a variety of applications.

Enhanced NLO response and switching self-focussing in benzodiazepine derivative with –NO2 and -Br substitution

Optoelectronic and the cubic nonlinear optical properties of 4-(4-Bromophenyl)-2-(4-nitrophenyl)-2, 3-dihydro-1H-1, 5-benzodiazepine have been studied. Z-scan technique was used for the third-order nonlinear optical measurements namely, nonlinear absorption, nonlinear refraction, and optical power limiting behaviour employing an Nd: YAG laser of 532 nm wavelength having 5 ns Gaussian pulses. B3LYP/6–311 ++ G (d, p) level of theory was employed for structural optimization, vibrational wavenumber, frontier molecular orbitals, natural bond orbital and population analysis. The MOLVIB programme was used to perform unambiguous vibrational assignments based on potential energy distribution values acquired from normal coordinate analysis. B3LYP and CAM-B3LYP hybrid functions have been employed at the DFT level to calculate the theoretical second-order hyperpolarizability. The substitution of –NO2 and -Br in this benzodiazepine compound enhances the second-order hyperpolarizability (γ) to the order of 10−34 esu and, switching of self-defocussing to self-focussing phenomenon. The HOMO-LUMO and optical band gap analysis illustrates that polarizing nature of the molecule vary with substituents. The obtained results indicate that this compound has potential applications in optoelectronics and photonics.

Extrinsic Nonlinear Kerr Rotation in Topological Materials under a Magnetic Field.

Topological properties in quantum materials are often governed by symmetry and tuned by crystal structure and external fields, and hence, symmetry-sensitive nonlinear optical measurements in a magnetic field are a valuable probe. Here, we report nonlinear magneto-optical second harmonic generation (SHG) studies of nonmagnetic topological materials including bilayer WTe2, monolayer WSe2, and bulk TaAs. The polarization-resolved patterns of optical SHG under a magnetic field show nonlinear Kerr rotation in these time-reversal symmetric materials. For materials with 3-fold rotational symmetric lattice structure, the SHG polarization pattern rotates just slightly in a magnetic field, whereas in those with mirror or 2-fold rotational symmetry, the SHG polarization pattern rotates greatly and distorts. These different magneto-SHG characters can be understood by considering the superposition of the magnetic field-induced time-noninvariant nonlinear optical tensor and the crystal-structure-based time-invariant counterpart. The situation is further clarified by scrutinizing the Faraday rotation, whose subtle interplay with crystal symmetry accounts for the diverse behavior of the extrinsic nonlinear Kerr rotation in different materials. Our work illustrates the application of magneto-SHG techniques to directly probe nontrivial topological properties, and underlines the importance of minimizing extrinsic nonlinear Kerr rotation in polarization-resolved magneto-optical studies.

Formula omitted] thin-films as efficient optical limiters: A study on impact of annealing

We present a detailed investigation on the impact of thermal annealing on the third-order nonlinear optical (NLO) properties of sputtered WSe2 thin films. The process of annealing results in an appreciable change in morphology and texture of WSe2 films that is governed by the annealing temperature and duration of annealing. These modifications are apparent from the electron micrographs as well as X-ray diffraction and Raman spectrum. The visible absorption spectroscopy point towards a gradual shrinkage of electronic bandgap which motivated us to explore the impact of NLO properties of the annealed WSe2 films. The NLO measurements were carried out using a single-beam Z-scan technique using ultrashort pulses centered at 1030 nm excitation wavelength. The investigations reveal a distinct two-photon absorption (TPA) signature for the annealed WSe2 films along with a self-focusing effect. The observations indicate the strengthening of optical limiting behavior exhibited by WSe2 films when they are annealed for a longer duration. The outcomes suggest that the process of annealing in WSe2 films allows controlling the NLO properties and renders them more suitable candidates as optical limiters for high-power applications.

Nonlinear optical and luminescence studies on Zinc Alumino-Boro-Silicate glasses co-doped with Dy3+ and Sm3+ ions for optical limiting and W-LEDs applications

In the present work, Dy3+ and Sm3+ co-doped Zinc Alumino-Boro-Silicate (ZABS) glasses doped with an optimized Dy3+ concentration (0.5 mol%) and varying Sm3+ concentrations are prepared using a conventional melt-quenching method. Optical absorption spectra showed the signature of Dy3+ and Sm3+ peaks through possible transitions in the range of 300–1800 nm. The optical bandgap of the glasses is obtained in the range of 4.2 to 3.9 eV. The emission curves aswellas decay lifetimes of Dy3+ and Sm3+ ions under 350 nm and 402 nm excitation were investigated using photoluminescence studies. Through the Inokuti-Hirayama (I-H) energy fitting model, the nature of energy transfer between Dy3+ and Sm3+ is found to be a ‘dipole–dipole’ type. Neutral to near warm white light emissions from the co-doped glasses were evaluated through colour chromaticity and correlated colour temperature calculations. The nonlinear optical measurements showed the reverse saturable absorption behaviour of the glasses whose nonlinear absorption coefficients (β) decreased with higher concentrations of Sm3+. Among the prepared glasses, ZABSDS1 (Sm3+ 0.1 mol%) glass showed the highest nonlinear coefficient (3.56 × 10−12 W cm−2) and lowest optical limiting (37.5 × 1011 W cm−2) threshold. From the overall studies, the suitability of the prepared glasses for White LEDs and optical limiting laser device applications were realized.

Determination of optical nonlinearity with photothermal effect within a layered bismuth telluride

In this work, the photothermal nonlinearity within a layered bismuth telluride (Bi2Te3) is studied in detail. The dynamics of the optical nonlinearity with increasing photothermal effect are investigated via pump-probe spectroscopy. The difference between the thermally-induced and photon-induced nonlinearities was clarified and the entire photothermal nonlinearity was therefore sketched. Furthermore, by removing the thermally-induced background signal, the n2 without accumulated heat can then be estimated. For example, the n2 value is 2.80 × 10−8 cm2/W at a peak intensity of 12.81 MW/cm2. The results are compared with those obtained from another measurement to validate the reliability of the estimation. The working range of the n2 value is also discussed. To provide further validation of the feasibility of the proposed method, the photothermal nonlinearity within monolayer graphene is provided and discussed. This work provides an alternative method to deal with optical nonlinearity measurements that are subject to the photothermal effect, paving the way toward further related research.

Synthesis route and structural, magnetic, optical, and non-linear optical properties of triboluminescent [Cu(NCS)(py)2(PPh3)] complex

Transition-metal coordination compounds exhibit many unique properties, which can be used in a broad range of state-of-the-art technologies. One of these interesting complexes is copper(I) thiocyanate bipyridine triphenylphosphine [Cu(NCS)(py)2(PPh3)]. Herein, we report detailed structural, magnetic, spectroscopic, and nonlinear optical properties, as well as the optimized synthesis route of the [Cu(NCS)(py)2(PPh3)] complex. Performed Raman, infrared (IR), X-ray photoelectron (XPS), and X-ray diffraction (XRD) spectroscopy provide valuable data about the synthesis time dependence and general structural characteristics. Optical studies describe in detail the photoluminescence properties and emission temperature dependence behavior. For the first time, the magnetic and third-order nonlinear optical measurements of the [Cu(NCS)(py)2(PPh3)] complex were performed. Additionally, thermogravimetric analysis was conducted to detail the thermal stability of the complex. The results constitute a detailed description of the physical properties of the [Cu(NCS)(py)2(PPh3)] complex, which is an essential tool for designing sensing solutions based on the triboluminescent properties of the investigated compound.

Z-scan and degenerate four wave mixing studies on newly synthesized copper complexes of curcumin

Designing novel metal complexes with high nonlinear optical (NLO) susceptibilities is a special interest for coordination compounds. They include a wider selection of metals with a variety of ligands and oxidation states, which can result in tuneable electronic capabilities. Paramagnetic Copper (II) complexes of curcumin were synthesized using different metal precursors and keeping curcumin as the base ligand. All the complexes were characterized by elemental analysis, magnetic susceptibility, and spectroscopic methods (UV–vis, FT-IR, ESR, and mass spectrometry). Nonlinear optical transmission measurements were carried out using nanosecond laser pulses with 532 nm wavelength, via a Z-scan and Degenerate four-wave mixing (DFWM) techniques. CuAcCur, CuClCur, and CuThCur complexes provided with 1.34 × 1024, 3.61 × 1024, and 4.05 × 1024 m3/W2, proved that these complexes possess excellent NLO properties and CuThCur being best among them. The NT dropped from 100 % to 78 %, 65 %, and 75 % in CuAcCur, CuClCur, and CuThCur respectively, showing that all the complexes have great potential for optical limiting (OL) application. The Conceptual-Density-Functional-Theory (CDFT) were employed to perform the theoretical analyses of complexes. Calculations on the various CDFT-based parameters such as electronegativity, HOMO-LUMO energy gap, molecular hardness, softness, dipole moment, and electrophilicity index of both species are performed. This work introduces three copper complexes with curcumin that show an excellent NLO and OL behavior.

Post-2000 nonlinear optical materials and measurements: data tables and best practices

In its 60 years of existence, the field of nonlinear optics has gained momentum especially over the past two decades thanks to major breakthroughs in material science and technology. In this article, we present a new set of data tables listing nonlinear-optical properties for different material categories as reported in the literature since 2000. The papers included in the data tables are representative experimental works on bulk materials, solvents, 0D–1D–2D materials, metamaterials, fiber waveguiding materials, on-chip waveguiding materials, hybrid waveguiding systems, and materials suitable for nonlinear optics at THz frequencies. In addition to the data tables, we also provide best practices for performing and reporting nonlinear-optical experiments. These best practices underpin the selection process that was used for including papers in the tables. While the tables indeed show strong advancements in the field over the past two decades, we encourage the nonlinear-optics community to implement the identified best practices in future works. This will allow a more adequate comparison, interpretation and use of the published parameters, and as such further stimulate the overall progress in nonlinear-optical science and applications.

Gold nanoparticles enhanced femtosecond nonlinear optical properties of sodium borate oxide glasses

We present results from our experimental measurements on the effect of annealing and intensity–dependent femtosecond (MHz) third–order nonlinear optical (NLO) attributes of gold nanoparticles (NPs) embedded sodium borate glass specimens. A melt–quench procedure was used to fabricate the gold nanoparticle–loaded sodium antimony containing borate glasses, which were then subjected to different schedules of thermal treatment at 500 °C. The highest saturation intensity and nonlinear refraction magnitude were observed for the 7–hour annealed glass specimen (i.e., for NBAu–7 glass). Thus, on the NBAu–7 glass, intensity–sensitive NLO measurements were performed. The saturable absorption behavior that was detected at lower peak intensities switched to reverse saturable absorption during the exposure of NBAu–7 glass with high fluence laser pulses. While the sign of the nonlinear refraction remained unchanged, the magnitude increased. The methodology examined in this work may enable the fabrication of borate–based glasses loaded with gold NPs with high saturation intensities and robust nonlinear refractive index magnitudes to be used in optical switching operations. The results of the current study support that the NBAu–7 glass is advantageous for optical switching functions when exposed to low–energy laser pulses. In contrast, the same glass benefits optical limiting functions when exposed to high–fluence laser pulses.

Nonlinear Optical Properties of Several π Conjugated Bridge Groups based on Diketo-Pyrrole-Pyrrole: Study on the Length and Symmetry of N-atom Position Substituted Alkyl Chains

In this paper, three modified π-conjugated bridges, were selected to study the relationship between substituted alkyl chains and two photon absorption ability as well as intramolecular charge transfer property. The modification is on the N-atom position of Diketo-Pyrrole-Pyrrole bridges. Some nonlinear optical measurements as well as several spectra methods, were used. DPP-J0, DPP-J1 and DPP-J2 exhibit cross-section of 5.6 GM, 8.1 GM, and 6.4 GM. The results exhibit obvious relation on the length and symmetry of N-atom position substituted alkyl chains. A pump-probe measurement was operated at 800 nm. Ultrafast dynamics results exhibit slight difference on the intramolecular charge transfer of these conjugated bridge derivatives. The quantum chemical calculations results are well accorded with the nonlinear optical properties. The results obtained may provide some useful information on molecular design and optimization of new advanced materials.

Understanding the conditions for the optimum nonlinear refraction of epsilon-near-zero films based on transparent conducting oxides.

Transparent Conducting Oxides (TCOs) exhibit a large and ultrafast intensity-dependent refractive index in their Epsilon-Near-Zero (ENZ) spectral region, which depends dramatically on the material properties and measurement arrangement conditions. Therefore, attempts to optimize the nonlinear response of ENZ TCOs usually involve extensive nonlinear optical measurements. In this work, we show that significant experimental work can be avoided by carrying out an analysis of the material's linear optical response. The analysis accounts for the impact of thickness-dependent material parameters on the absorption and field intensity enhancement under different measurement conditions and estimates the incidence angle required for achieving the maximum nonlinear response for a given TCO film. We perform measurements of angle-dependent and intensity-dependent nonlinear transmittance for Indium-Zirconium Oxide (IZrO) thin films with different thicknesses and demonstrate a good agreement between the experiment and theory. Our results also indicate that the film thickness and the excitation angle of incidence can be adjusted simultaneously to optimize the nonlinear optical response, allowing a flexible design of TCO-based highly nonlinear optical devices.

Quadratic electro-optic effect in the nonconjugated conductive polymer iodine-doped polyethylene terephthalate

Quadratic electro-optic effect has been measured in the nonconjugated conductive polymer iodine-doped polyethylene terephthalate (PET). Films of PET were doped with iodine in the vapor phase using standard procedure. The optical absorption and FTIR spectroscopic results show that iodine doping causes a charge-transfer from the terephthalic ring to iodine, thus forming a metallic quantum dot of subnanometer dimension. Nonlinear optical measurements were performed on the iodine-doped PET samples using field-induced birefringence method at 633 nm with lock-in detection. The magnitude of the Kerr coefficient for varying doping levels was determined. The magnitudes were shown to increase with doping level. For an iodine molar concentration of 0.28, a Kerr coefficient of ∼1.5 × 10−11 m/V2 was measured. For a molar concentration of iodine of 0.83, the measured Kerr coefficient was ∼5 × 10−11 m/V2. These magnitudes are comparable to (slightly smaller than) the Kerr coefficients measured in other iodine-doped nonconjugated conductive polymers representing metallic quantum dots of subnanometer dimensions.

Crystal Engineering to Avoid Pairing Dipolar Moments: The Case of 5-Nitrouracil, a Highly Polarizable Molecule

5-nitrouracil is a polarizable molecule with a permanent electric dipole moment. Its molecular properties caught the attention of physicists working on nonlinear optics or optoelectronics, but the translation of the molecular assets to the crystalline solid has not been straightforward. This review compares all the known crystal structures incorporating the neutral or ionic 5-nitrouracil, or the two species concomitantly, discussing the effect of the packing in the optimization of the crystalline optical properties. Two new centrosymmetric 5-nitrouracilate salts are also reported for the first time, showing extensive hydrogen bonding between anions and cations. This review also gathers data from nonlinear optical measurements of non-centrosymmetric crystals and thermal stabilities of known polymorphs, showing that a neutral 5-nitrouracil molecule in acentric crystalline environment allows efficient blue-light generation.