Nonlinear Optical Devices Research Articles

Layer-Dependent Nonlinear Optical Properties of WS2, MoS2, and Bi2S3 Films Synthesized by Chemical Vapor Deposition.

Two-dimensional (2D) layered materials have shown layer-dependent optical properties in both linear optical and nonlinear optical (NLO) regimes due to prominent interlayer coupling and quantum confinement in an atomic scale. However, the NLO properties become more complicated as both saturable absorption (SA) and reverse saturable absorption (RSA) easily happen in 2D materials, which results in a significant challenge to understand the evolution of nonlinear absorption with layers. Motivated by this, chemical vapor-deposited chalcogenide compounds (WS2, MoS2, and Bi2S3) are used to investigate the pump intensity and layer number-dependent NLO properties. The values of nonlinear absorption coefficients of these chalcogenide compounds increase with the pump intensity by an 800 nm femtosecond laser, which can be described by an empirical power law function. The SA process due to the large transition probability of the ground state readily takes place in thick samples, while RSA occurs easily in thin samples due to the two-photon absorption (TPA). The transition from TPA to SA is deduced to occur at 13L-WS2, 15L-MoS2, and 5L-Bi2S3, which is related to the layer-dependent band gaps. Our results provide an efficient way to tune optical nonlinearities with a controlled layer number and to design corresponding NLO devices such as optical switches and saturable absorbers.

Ultrafast third-order nonlinear optical response of Ag modified ZnSe/ZnS core-shell structure quantum dots composite films

ZnSe/ZnS/Ag composite films were prepared on quartz substrates by magnetron sputtering and spin coating techniques.The morphology of Ag nanoparticles (NPs) with different sputtering powers and the dispersion of ZnSe/ZnS quantum dots (QDs) at different concentrations were observed by SEM and TEM. The linear absorption spectra shows the composite films which modified by Ag NPs had a blue shifted compared with the single ZnSe/ZnS QDs.The Z-scan results showed that the saturated absorption(SA) characteristics of the composite films modified by Ag NPs were significantly enhanced.The SA intensity of the composite films can be adjusted by appropriate increase the concentration of QDs, increasing the sputtering power of Ag NPs and incident laser power.By calculating the nonlinear absorption coefficients of the composite films under different conditions, it is found that increasing the excitation power is the most effective way to improve the SA property of the composite films. Compared with the single ZnSe/ZnS QDs, the βeff value of the composite films is increased by 104 orders of magnitude, which makes the ZnSe/ZnS QDs more widely used in the field of nonlinear optical devices.

Manipulating Optical Scattering of Quasi-BIC in Dielectric Metasurface with Off-Center Hole.

Bound states in the continuum (BICs) correspond to a particular leaky mode with an infinitely large quality-factor (Q-factor) located within the continuum spectrum. To date, most of the research work reported focuses on the BIC-enhanced light matter interaction due to its extreme near-field confinement. Little attention has been paid to the scattering properties of the BIC mode. In this work, we numerically study the far-field radiation manipulation of BICs by exploring multipole interference. By simply breaking the symmetry of the silicon metasurface, an ideal BIC is converted to a quasi-BIC with a finite Q-factor, which is manifested by the Fano resonance in the transmission spectrum. We found that both the intensity and directionality of the far-field radiation pattern can not only be tuned by the asymmetric parameters but can also experience huge changes around the resonance. Even for the same structure, two quasi-BICs show a different radiation pattern evolution when the asymmetric structure parameter d increases. It can be found that far-field radiation from one BIC evolves from electric-quadrupole-dominant radiation to toroidal-dipole-dominant radiation, whereas the other one shows electric-dipole-like radiation due to the interference of the magnetic dipole and electric quadrupole with the increasing asymmetric parameters. The result may find applications in high-directionality nonlinear optical devices and semiconductor lasers by using a quasi-BIC-based metasurface.

Nonlinear absorption of CsPbBr3/antimonene blend materials prepared by laser ablation in liquid

Recently, through the continuous research on perovskite materials, the excellent nonlinear optical properties of perovskite have been shown to the world. Nevertheless, people still try their best to improve its optical nonlinearity to make it better applied to nonlinear optical devices. In this work, the nonlinear optical performance of the perovskite quantum dot (PQD) has been studied and tested, and its reverse-saturable absorption property and optical limiting potential have been discovered. On this basis, we tried to further improve the nonlinearity of the PQD by compounding other materials that has excellent optical properties especially highly nonlinearity. It is found that the new two-dimensional (2D) Group-VA material antimonene He meets our needs. Although antimonene has excellent nonlinear optical performance, its dispersibility is poor and not easy to store. Here, we tried to incorporate PQD and antimonene to further improve its nonlinear optical performance. With the method of laser ablation in liquid (LAL), CsPbBr3/antimonene blend materials (CPBAB) with stronger nonlinear absorption and higher dispersibility were successfully prepared. The nonlinear absorption properties of CPBAB with different ablation time were studied by Z-scan technology. It was found that by adjusting the laser ablation time parameters, CPBAB with higher optical limiting ability could be prepared. In addition, this CPBAB can be encapsulated in a polymer for curing, which improves its practical application to protect human eyes and optical equipment from high-power laser damage.

Tunable giant nonlinear optical susceptibility in BaSnO3 quantum wells

We investigate theoretically the giant nonlinear optical susceptibilities based on intersubband transitions in the $n$-type ${\mathrm{BaSnO}}_{3}$ quantum well heterostructures. Validity of the effective mass theory, used to compute susceptibility is demonstrated using first-principles calculations. We predict ${\ensuremath{\chi}}^{(2)}$ values up to 200 nm/V and ${\ensuremath{\chi}}^{(3)}$ values up to $8000\phantom{\rule{0.28em}{0ex}}\mathrm{n}{\mathrm{m}}^{2}/{\mathrm{V}}^{2}$, both over a much broader spectral range. ${\ensuremath{\chi}}^{(2)}$ is estimated from the THz region to the boundary with visible light, a range four times broader than current semiconductor quantum wells. This opens a possibility to make tunable nonlinear optical devices operating in the important range from THz to visible light.

Theoretical Study of Solvent Effects on the Electronic and Thermodynamic Properties of Tetrathiafulvalene (TTF) Molecule Based on DFT

Tetrathiafulvalene () is an organosulfur compound used in the production of molecular devices such as switches, sensors, nonlinear optical devices and rectifiers. In this work, a theoretical study on the effects of solvent on TTF molecule was investigated and reported based on Density Functional Theory (DFT) as implemented in Gaussian 03 package using B3LYP/6-31++G(d,p) basis set. Different solvents were introduced as a bridge to investigate their effects on the electronic structure. The HUMO, LUMO, energy gap, global chemical index, thermodynamic properties, NLO and DOS analysis of the TTF molecule in order to determine the reactivity and stability of the molecule were obtained. The results obtained showed that the solvents have effects on the electronic and non-linear-optical properties of the molecule. The optimized bond length revealed that the molecule has strong bond in gas phase with smallest bond length of about 1.0834Å than in the rest of the solvents. It was observed that the molecule is more stable in acetonitrile with HOMO-LUMO gap and chemical hardness of 3.6373eV and 1.8187eV respectively. This indicates that the energy gap and chemical hardness of TTF molecule increases with the increase in polarity and dielectric constant of the solvents. The computed results agreed with the results in the literature. The thermodynamics and NLO properties calculation also indicated that TTF molecule has highest value of specific heat capacity (Cv), total dipole moment () and first order hyperpolarizability () in acetonitrile, while acetone has the highest value of entropy and toluene has a slightly higher value of zero point vibrational energy (ZPVE) than the rest of the solvents. The results show that careful selection of the solvents and basis sets can tune the frontier molecular orbital energy gap of the molecule and can be used for molecular device applications.

Linear and nonlinear optical characteristics effected by Na+ ions of low concentration for potassium dihydrogen phosphate crystal

For KDP crystal, linear and nonlinear optics as the important physical features are susceptible to be influenced by impurities, and the metallic element of sodium as a common impurity ion is practically impossible to remove. Hence, the dependences of linear and nonlinear optical properties on Na+ are necessary, and fully studied for as-grown KDP crystals by rapid growth technique. The UV–VIS–NIR spectrums indicate that the transmittance ranging from 200 to 800 nm, especially in the ultraviolet region, decrease with the increase of sodium concentration, while the corresponding refractive index presents a trend of increase. By Z-scan method, the characteristics of nonlinear absorption and nonlinear refraction demonstrate that the electron-cloud distortion of H2PO4- group play a major part in generating this nonlinear effect under picosecond laser pulses. Moreover, a more pronounced nonlinear response can be induced by the concentration of Na+ ions, implying that this variation is mainly attributable to the distortion of crystal lattice and electronic structure, originating from the substituting effect of sodium positive ions. All results are extremely useful to the growth and application as nonlinear optical devices for KDP crystal.

Controllable synthesis of high-quality two-dimensional tellurium by a facile chemical vapor transport strategy

SummaryRecently, as an elementary material, tellurium (Te) has received widespread attention for its high carrier mobility, intriguing topological properties, and excellent environmental stability. However, it is difficult to obtain two-dimensional (2D) Te with high crystalline quality owing to its intrinsic helical chain structure. Herein, a facile strategy for controllable synthesis of high-quality 2D Te nanoflakes through chemical vapor transport in one step is reported. With carefully tuning the growth kinetics determined mainly by temperature, tellurium nanoflakes in lateral size of up to ∼40 μm with high crystallinity can be achieved. We also investigated the second harmonic generation of Te nanoflakes, which demonstrates that it can be used as frequency doubling crystals and has potential applications in nonlinear optical devices. In addition, field effect transistor devices based on the 2D Te nanoflakes were fabricated and exhibited excellent electrical properties with high mobility of 379 cm2 V−1 s−1.

Impact of the Structural Modification of Diamondoid Cd(II) MOFs on the Nonlinear Optical Properties.

A change in the degree of interpenetration (DOI) in metal-organic frameworks (MOFs) prompted by heat, pressure, or exchange of solvents is a fascinating phenomenon that can potentially impact the functional properties of MOFs. Structural transformation involving two noncentrosymmetric MOFs with different DOIs provides a rare opportunity to manipulate their optical properties. Herein, we report an unusual single-crystal-to-single-crystal (SCSC) transformation of a noncentrosymmetric 7-fold interpenetrated diamondoid (dia) Cd(II) MOF into another noncentrosymmetric but 8-fold interpenetrated dia MOF upon the removal of guest solvents. A hydrogen-bond network formed between the lattice solvents and linker trans-2-(4-pyridyl)-4-vinylbenzoate (pvb) in a 7-fold interpenetrated noncentrosymmetric MOF results in a significant increase in the two-photon absorption cross-section (11 times) as compared to that in the desolvated 8-fold interpenetrated MOF. Also, an increase in the DOI in the noncentrosymmetric crystals strengthened the π···π interaction between the individual diamondoid networks and enhanced the second-order nonlinear optical (NLO) coefficient (deff) by 4.5 times. These results provide a way to manipulate the optical properties of MOFs using a combined strategy of the formation of hydrogen bonds and interpenetration for access to tunable single-crystal NLO devices in an SCSC manner. By changing the experimental conditions, another dia Cd(II) MOF with 4-fold interpenetration can be isolated. In this centrosymmetric MOF, the olefin groups in the backbone of the ligand (pvb) undergo a [2 + 2] cycloaddition reaction quantitatively under UV light but in a non-SCSC fashion.

Fine-Tuning of Nonlinear Optical Contrasts of Hexaphyrin-Based Molecular Switches Using Inverse Design.

In the search for new nonlinear optical (NLO) switching devices, expanded porphyrins have emerged as ideal candidates thanks to their tunable chemical and photophysical properties. Introducing meso-substituents to these macrocycles is a successful strategy to enhance the NLO contrasts. Despite its potential, the influence of meso-substitution on their structural and geometrical properties has been scarcely investigated. In this work, we pursue to grasp the underlying pivotal concepts for the fine-tuning of the NLO contrasts of hexaphyrin-based molecular switches, with a particular focus on the first hyperpolarizability related to the hyper-Rayleigh scattering (β HRS ). Building further on these concepts, we also aim to develop a rational design protocol. Starting from the (un)substituted hexaphyrins with various π-conjugation topologies and redox states, structure-property relationships are established linking aromaticity, photophysical properties and β HRS responses. Ultimately, inverse molecular design using the best-first search algorithm is applied on the most favorable switches with the aim to further explore the combinatorial chemical compound space of meso-substituted hexaphyrins in search of high-contrast NLO switches. Two definitions of the figure-of-merit of the switch performance were used as target objectives in the optimization problem. Several meso-substitution patterns and their underlying characteristics are identified, uncovering molecular symmetry and the electronic nature of the substituents as the key players for fine-tuning the β HRS values and NLO contrasts of hexaphyrin-based switches.

Impact of nonlocal response in plasmonic metasurfaces on four-wave mixing

We report the impact of nonlocal response in metallic nanostructures on four-wave mixing (FWM) process in nonlinear plasmonic metasurfaces which consist of Au nanostrips coupled with Au film spaced by an ultrathin dielectric film. When the dielectric film is linear and FWM only from the Au nanostructures, the FWM efficiency of around two orders of magnitude enhancement is obtained when the nonlocal effect of Au, not the traditionally local Drude model of Au, is considered. However, when the dielectric film is nonlinear and FWM from the Au nanostructures is negligible, the almost half FWM response from the nonlinear metasurface under the nonlocal, not the local model, of Au is confirmed. These results are both ascribed to the different local electric field distributions near the surface of Au nanostructures and in the gap at the local and nonlocal response of Au. The results have an important significance to design ultra-compacted integrated nonlinear optical devices or to explain the experimental measurements of nonlinear response involving plasmonic nanostructures of ultra-small gaps.

Tunable nonlinear optical enhancement of α-FeOOH nanorods/RGO composites

High-quality α-FeOOH nanorods/reduced graphene oxide (RGO) composites were prepared by a facile hydrothermal method. Crystal structure and optical properties of the composites were characterized by XRD, SEM, TEM, FT-IR, Raman, UV–vis, PL. These results of characterization showed that the composites were successfully prepared. The third-order nonlinear optical (NLO) properties of the composites were investigated by Z-scan technique under 30 ps laser at 532 nm wavelength with 10 Hz repetition. The composites had larger third-order NLO susceptibility compared to pure α-FeOOH nanorods and RGO nanosheets. The enhancement of NLO performance benefited from synergistic effect of local field effect and charge transfer between α-FeOOH nanorods and RGO. Moreover, the composite with the α-FeOOH nanorods:RGO weight ratio of 2:1 (S3) showed the most robust NLO response. Comparing with RGO, the value of third-order NLO susceptibility χ(3) of S3 was increased by around 60 times and the saturation intensity Isat was nearly decreased by 6 times as the modulation depth M highly increased, respectively. The improved NLO property of α-FeOOH nanorods/RGO composite indicated its possible application in optical switches, mode lockers and other NLO devices.

Third-order nonlinear properties and reverse absorption behavior in layered Ti3C2 MXene in the near infrared region

Two-dimensional transition metal carbides/nitrides (MXenes) have attracted a lot of attention as optical materials owing to their unique electronic and optical features. In this paper, we demonstrated the nonlinear optical response of Ti3C2 MXene in the near infrared (NIR) regime at 1 and 1.3 µm. The nonlinear optical absorption behavior was systematically studied by the open/closed aperture (OA/CA) Z-scan techniques. Under a low excitation intensity, the saturable absorption dominated the optical nonlinear process, and the maximum effective nonlinear absorption coefficients βeff were −0.59 cm/MW and −0.68 cm/MW at 1 and 1.3 µm, respectively. The nonlinear refractive index n2 and the third-order susceptibility of Ti3C2 Mxene at 1 µm were −2.18 ×10−2 cm2/GW and 3.65×10−3 esu, respectively. While with the excitation laser at 1.3 µm, the nonlinear refractive index n2 was −1.64×10−2 cm2/GW and the third-order susceptibility was 2.75×10−3 esu. With the increase of incident optical intensity, the two-photon absorption (TPA) process was observed, leading to the reverse absorption phenomenon. Our results confirmed that Ti3C2 MXene possessed intensity-depended nonlinear optical properties, which could be applied in various nonlinear optical devices.

Tuning nonlinear second-harmonic generation in AlGaAs nanoantennas via chalcogenide phase-change material

The ability to engineer nonlinear optical processes in all-dielectric nanostructures is both of fundamental interest and highly desirable for high-performance, robust, and miniaturized nonlinear optical devices. Herein, we propose a paradigm for the efficient tuning of a second-harmonic generation (SHG) process in dielectric nanoantennas by integrating with chalcogenide phase-change material. In a design with ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ (GST) film sandwiched between the AlGaAs nanoantennas and ${\mathrm{AlO}}_{x}$ substrate, the nonlinear SHG signal from the AlGaAs nanoantennas can be boosted via the resonantly localized field induced by the optically induced Mie-type resonances, and further modulated by exploiting the GST amorphous-to-crystalline phase change in a nonvolatile, multilevel manner. The tuning strategy originates from the modulation of resonant conditions by changes in the refractive index of GST. With a thorough examination of tuning performances for different nanoantenna radii, a maximum modulation depth as high as $540%$ is numerically demonstrated. This work not only reveals the potential of GST in optical nonlinearity control, but also provides a promising strategy in smart designing of tunable and reconfigurable nonlinear optical devices, e.g., light emitters, modulators, and sensors.

Geometry-dependent two-photon absorption followed by free-carrier absorption in AlGaAs waveguides

Nonlinear absorption can limit the efficiency of nonlinear optical devices. However, it can also be exploited for optical limiting or switching applications. Thus, characterization of nonlinear absorption in photonic devices is imperative for designing useful devices. This work uses the nonlinear transmittance technique to measure the two-photon absorption coefficients ( α 2 ) of AlGaAs waveguides in strip-loaded, nanowire, and half-core geometries in the wavelength range from 1480 to 1560 nm. The highest α 2 values of 2.4, 2.3, and 1.1 c m / G W were measured at 1480 nm for 0.8-µm-wide half-core, 0.6-µm-wide nanowire, and 0.9-µm-wide strip-loaded waveguides, respectively, with α 2 decreasing with increasing wavelength. The free-carrier absorption cross section was also estimated from the nonlinear transmittance data to be around 2.2 × 10 − 16 c m 2 for all three geometries. Our results contribute to a better understanding of the nonlinear absorption in heterostructure waveguides of different cross-sectional geometries. We discuss how the electric field distribution in the different layers of a heterostructure can lead to geometry-dependent effective two-photon absorption coefficients. More specifically, we pinpoint the third-order nonlinear confinement factor as a design parameter to estimate the strength of the effective nonlinear absorption, in addition to tailoring the bandgap energy by varying the material composition.

Improved near‒infrared nonlinear optical properties of Sm3+ containing borate glasses: Effect of silver nanoparticles concentration

Efficacy of silver nanoparticles on tuning the optical nonlinear characteristics of Sm3+ doped borate‒based glasses has been investigated comprehensively and the outcomes are provided. The nonlinear optical (NLO) attributes were retrieved using the Z‒scan technique under an ultrafast pulse excitation with the wavelength ranging from 700 nm to 1000 nm. Reverse saturable absorption (RSA), ascribed to two‒photon absorption, was realized in open aperture Z‒scan profiles whereas refraction nonlinearity of positive type due to the self‒focusing was observed in closed aperture Z‒scan data. Based on the Z‒scan results along with the figure of merit condition (higher than unity), it was identified that the silver nanoparticles embedded Sm3+ containing glass hosts are competing materials for NLO device applications to function in near‒infrared region.

Mechanical and nonlinear optical properties of two-dimensional LiXY2 (X=Al, Ga, In; Y[dbnd]S, Se, Te) monolayers

Two-dimensional (2D) nonlinear optical (NLO) materials with large flexibility possess application advantages for phototransistors and flexible laser fields. In this work, we studied the electronic, mechanical and NLO properties of 2D LiXY2 (X = Al, Ga, In; YS, Se, Te) monolayers via first-principles calculations. Theoretical results suggest that their bandgaps, which determine laser damage threshold (LDT) and harmful two-photon absorption (TPA), are as wide as that of typical 2D NLO materials single-layer GaSe and MoS2. The large energy gap between In-5s and Y-pz states will help single-layer LiXY2 to avoid TPA in a wide range of optical energies. Their NLO coefficients are comparable to that of bulk AgGaS2 and single-layer GaSe. Moreover, 2D LiXY2 monolayers are stable under mechanical distortion, and even more flexible than single-layer GaSe and MoS2. Overall, this work supplies a group of 2D materials exhibiting superiority as flexible NLO devices in nanotechnology.

Two-Dimensional Gallium Sulfide as a Novel Saturable Absorber for Broadband Ultrafast Photonics Applications.

Two-dimensional (2D) gallium sulfide (GaS) offers a plethora of exceptional electrical and optical properties, allowing it to be used in a wide range of applications, including photodetectors, hydrogen generation, and nonlinear optical devices. In this paper, ultrathin 2D GaS nanosheets are synthesized using the liquid-phase exfoliation method, and the structure, morphology, and chemical composition of the as-prepared nanosheets are extensively investigated. After depositing 2D GaS nanosheets on side polished fibers, successful saturable absorbers (SAs) are fabricated for the first time. The realized modulation depths are 10 and 5.3% at 1 and 1.5 μm, respectively, indicating the wideband saturable absorption performance of the prepared SAs. By integrating GaS-SAs into three different wavelength-based fiber laser cavities, stable mode-locked pulses are achieved, having pulse durations of 46.22 ps (1 μm), 614 fs (1.5 μm), and 1.02 ps (2 μm), respectively. Additionally, different orders of harmonic mode-locked pulses with the highest repetition rate of 0.55 GHz (45th order) and Q-switched pulses with the shortest pulse duration of 2.2 μs are obtained in the telecommunication waveband. These findings suggest that 2D GaS has a lot of potential for broadband ultrafast photonics in nonlinear photonics devices.

Chemistry, Functionalization, and Applications of Recent Monoelemental Two-Dimensional Materials and Their Heterostructures.

The past decades have witnessed a rapid expansion in investigations of two-dimensional (2D) monoelemental materials (Xenes), which are promising materials in various fields, including applications in optoelectronic devices, biomedicine, catalysis, and energy storage. Apart from graphene and phosphorene, recently emerging 2D Xenes, specifically graphdiyne, borophene, arsenene, antimonene, bismuthene, and tellurene, have attracted considerable interest due to their unique optical, electrical, and catalytic properties, endowing them a broader range of intriguing applications. In this review, the structures and properties of these emerging Xenes are summarized based on theoretical and experimental results. The synthetic approaches for their fabrication, mainly bottom-up and top-down, are presented. Surface modification strategies are also shown. The wide applications of these emerging Xenes in nonlinear optical devices, optoelectronics, catalysis, biomedicine, and energy application are further discussed. Finally, this review concludes with an assessment of the current status, a description of existing scientific and application challenges, and a discussion of possible directions to advance this fertile field.

All-optical switching based on interacting exciton polaritons in self-assembled perovskite microwires.

Ultrafast all-optical switches and integrated circuits call for giant optical nonlinearity to minimize energy consumption and footprint. Exciton polaritons underpin intrinsic strong nonlinear interactions and high-speed propagation in solids, thus affording an intriguing platform for all-optical devices. However, semiconductors sustaining stable exciton polaritons at room temperature usually exhibit restricted nonlinearity and/or propagation properties. Delocalized and strongly interacting Wannier-Mott excitons in metal halide perovskites highlight their advantages in integrated nonlinear optical devices. Here, we report all-optical switching by using propagating and strongly interacting exciton-polariton fluids in self-assembled CsPbBr3 microwires. Strong polariton-polariton interactions and extended polariton fluids with a propagation length of around 25 μm have been reached. All-optical switching on/off of polariton propagation can be realized in picosecond time scale by locally blue-shifting the dispersion with interacting polaritons. The all-optical switching, together with the scalable self-assembly method, highlights promising applications of solution-processed perovskites toward integrated photonics operating in strong coupling regime.