Nonlinear Optical Effects Research Articles

Efficient application of an all-fiber saturable absorber for the generation of heterogeneous and vector dissipative solitons in fiber lasers

A fully fiber-based structural saturable absorber (SA) incorporating graded-index multimode fiber, stepped-index multimode fiber, and graded-index multimode fiber (GIMF–SIMF–GIMF) has been fabricated and applied to a fiber laser. The modulation depth and saturation intensity are measured to be 1.32 % and 26.79 MW/cm2, respectively. A heterogeneous soliton with an adjustable optical spectrum interval with 4.6 nm and 3 nm is generated in the anomalous dispersion regime. The main pulse widths are approximately 41.80 ps and 35.56 ps. Additionally, a switchable single vector soliton and bound states of vector dissipative solitons are achieved in the normal dispersion regime. The pulse width of the single vector soliton is measured to be 6.16 ps, while the bound state exhibits a pulse width of 6.49 ps with a temporal separation of 15.22 ps. By employing external cavity compression, the minimum pulse widths for the single and bound states of vector dissipative solitons are reduced to 2.18 ps and 1.55 ps, respectively. These experimental results demonstrate that the GIMF–SIMF–GIMF SA exhibits unique nonlinear optical effects and offers promising potential for soliton generation applications.

Unusual Thermo-Enhanced Second Harmonic Generation in Organic Configurationally-Locked Polyene Crystals.

To modulate nonlinear optical (NLO) effects of crystalline material holds great application potential in the photoelectronic and optical fields. Organic configurationally-locked polyene represents an exciting NLO family with large second harmonic generation (SHG) effects, whereas it is a huge blank to switch and modulate their NLO property through external stimuli. For the first time, here present unusual thermo-enhanced SHG activities are presented in a polyene-based NLO compound, 2-{3-[2-(4-pyrrolidinphenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (1), giving a record-high magnitude of SHG enhancement up to ≈170% during its isomorphic phase transition. Theoretical analysis discloses this behavior stems from the reduced degree of torsion in the π-conjugated structures in 1, as verified by dihedral angles between its pyrrolidine and phenyl planes. As the first study on thermo-enhanced SHG properties of organic crystals, this work affords a new avenue of modulating physical properties to fabricate high-performance photoelectronic and optical devices.

Effects of self-limiting oxide layer on optical and electronic properties of WTe2 for optoelectronic applications

The study focuses on the optical and electrical properties of Tungsten Ditelluride (WTe2), a type II Weyl semimetal, as well as the influence of its self-limiting oxide (SLO) layer that forms during natural oxidation. WTe2 exhibits promising applications in photodetection and energy harvesting due to its unique gapless linear dispersion and Berry-field-enhanced nonlinear optical effects. However, surface oxidation poses a challenge as it degrades the performance of WTe2. By employing spectroscopic ellipsometry and Raman spectroscopy, the progression of the oxide layer’s thickness and its impact on the optical constants of WTe2 were examined. The results revealed a rapid increase in oxide thickness within the first 24 h, and it reached saturation at ∼10 nm after 45 h of atmospheric exposure. Electrical properties were explored using Kelvin Probe Force Microscopy, uncovering a modification in surface potential and Fermi level following oxidation. Additionally, a SLO/WTe2 heterojunction device exhibited a wide region of positive and negative coexisting photocurrent, highlighting the potential for logical operations in ambient air. Finally, the mechanism of operation of the device is discussed. The electrical and optical properties of the pristine, partially oxidized and fully oxidized WTe2 are analyzed using density functional theory calculations. It shows that the SLO layer has a significant effect on the optical and electronic properties, which is instructive for future wavelength-modulated device optoelectronic logic operations.

Strongly enhanced shift current at exciton resonances in a noncentrosymmetric wide-gap semiconductor

Excitons are fundamental quasiparticles that are ubiquitous in photoexcited semiconductors and insulators. Despite causing a sharp and strong photoabsorption near the interband absorption edge, charge-neutral excitons do not yield photocurrent in conventional photovoltaic processes unless dissociated into free charge carriers. Here, we experimentally demonstrate that excitons can directly contribute to photocurrent generation through a nonlinear light−matter interaction in a noncentrosymmetric semiconductor CuI. Epitaxial thin films of CuI exhibit a substantial enhancement of photocurrent at exciton resonance energies even below the bandgap. From the light polarization dependence, this photocurrent is identified to be shift current, a nonlinear photocurrent driven by the change in the geometric Berry phase of electron wave functions upon the optical transition. The shift current at the exciton resonance is much larger than that induced above the band gap by free electron−hole excitation, and their signs are opposite. First-principles calculations elucidate that the sign and magnitude of the exciton shift current are strongly dependent on the strain in the thin film. The present study reveals the crucial role of excitons in enhancing the shift current magnitude and its strain sensitivity, and will open an unprecedented route for efficient manipulation of nonlinear optical effects.

Non-linear optics for an online probing of the specific surface area of nanoparticles in the aerosol phase

In the present study the generation of non-linear optical (NLO) effects, such as second harmonic generation (SHG), by black carbon particles, also named soot, and by other types of nanoparticles in aerosol phase is quantified and analysed. Its potential for measuring the specific surface area of an aerosol is put forward. SHG is a Non Linear Optical phenomenon that is typically used in biosciences and fundamental physics and has shown to have large potential for the investigation of surface sensitive phenomena. It exists in two forms, coherent SHG and incoherent SHG, also named Hyper Rayleigh Scattering (HRS). While applications on particles in solution or organic molecules located on the surface of droplets exist, the SHG naturally induced by solid nanoparticles in aerosol phase without any SHG enhancing additive has neither been detected nor quantified yet. The present work aims at narrowing this gap by exposing a jet of well-characterized nanoaerosols to a femtosecond laser featuring high peak pulse energies allowing to induce NLO phenomena. The experiments are carried out in an innovative optical setup allowing to analyse the NLO response resolved in time, wavelength and angle, thus having the capability to isolate SHG from other phenomena, such as laser filamentation. The optical setup was calibrated in order to quantify the generated signal power and optimized in order to have a high sensitivity and in order to avoid NLO generation from its own optical elements. The results confirm that soot particles, as well as DEHS droplets and arc generated carbon nanoparticles, feature SHG at intensities that are more than 7 orders of magnitude smaller than that of static light scattering. SHG depends in particular on aggregate and/or monomer size. On the other hand, SHG induced by soot does not seem to depend on the organic or elementary carbon content. The experiments also show that the detected NLO signal increases linearly with particle surface area, independently of the particle shape or composition. Finally, the angular response of NLO signal is fundamentally different from that of linear scattering. Due to the isotropic nature of the angular response, the observed SHG signal is probably non-coherent and thus related to Hyper Raleigh Scattering. These findings show the potential of non-linear optics, in particular to quantify in situ the specific surface of an aerosol. Giving access to this information which is crucial in the evaluation of toxicity of aerosols, the present work can thus give way to a new class of laser based diagnostics for aerosols.

Polar-Layer-Driven Reconstruction of a Noncentrosymmetric Vandate Mid-Infrared Nonlinear-Optical Crystal.

Noncentrosymmetry (NCS) is essential for a second-harmonic nonlinear-optical (NLO) crystal; however, the tailored synthesis of NCS materials has historically posed significant challenges because the adjacent dipole moments of constituted NLO-active units in the structure tend to align in opposite directions, resulting in the NLO effect being canceled as in a centrosymmetric (CS) crystal. In this work, we propose a polar-layer-driven strategy, wherein a polarization-layered framework is constructed to constrain the dipole moment to align in the same direction, thereby facilitating the formation of the NCS structure. Taking the layered structure CS K2ZnV2O7 as a prototype compound, a novel vandate K4ZnV5O15Br (KZVB) was rationally synthesized via a multiple sites-oriented cosubstition method. KZVB containing two types of NLO-active units of V5+ d0 and Zn2+ d10 cations can be utilized as a mid-infrared NLO crystal with a remarkable NLO response comparable to that of nonoxide AgGaS2. This work not only broadens the effective strategy for designing novel NCS compounds but also provides a progressive development of NLO materials.

Advancing Optoelectronics with Benzonitriles: Theoretical Understanding, Experimental Realization, and Single‐Molecule White Light Emission

AbstractBenzonitrile derivatives (BDs) are very promising for applications in materials science, photonics, and optoelectronics due to their intriguing electronic and optical properties. This study comprehensively investigates BDs, aiming to uncover their fundamental characteristics and potential applications. Using advanced theoretical techniques like Gaussian software, it is gained unprecedented insights into the photoinduced isomerization phenomenon for all‐optical switching in these compounds. The theoretical framework clarifies molecular transition states and explores a range of properties, providing a comprehensive understanding of BDs. Empirical data on the emission properties of BDs, from fluorescence analyses in liquid solutions to light amplification in solid‐state PMMA thin films, complement the theoretical examinations. Notably, white light emission from a single benzonitrile compound is achieved, showcasing its potential for data transmission through Li‐Fi technology. Finally, the all‐optical switching phenomenon in BDs using 3rd order nonlinear optical effects is experimentally validated. This complementary and comprehensive study advances understanding of BDs and demonstrates their potential for practical applications in emerging photonic and optoelectronic technologies.

2D Material Sensors with Light Excitation

Abstract2D materials are highly regarded for their exceptional sensing application prospects, stemming from their distinctive atomic layer structure and exceptionally sensitive surfaces. Over the past decade, numerous high‐performance 2D material sensors are extensively developed; however, challenges related to sensitivity, selectivity, and stability continue to impede their industrial advancement. The interaction between light and 2D materials has introduced unique properties, including absorption and emission characteristics, photoelectric effects, nonlinear optical effects, surface‐enhanced Raman scattering, and light response enhancement. Consequently, exciting and adjusting the electronic structure and carrier concentration of 2D materials through light with specific wavelength ranges is an effective strategy for enhancing sensing performance. This strategy has yielded remarkable breakthroughs in applications such as photodetectors, semiconductor gas sensors, and fiber optic sensors. Moreover, it demonstrates extraordinary potential in emerging applications such as image sensors, flexible electronics, and biomedical sensors. However, the sensing mechanism, device structure design, and specific applications of 2D materials under light excitation remain unclear. This perspective endeavors to elucidate the intrinsic photophysical mechanisms between light‐excited 2D materials and their target sensing analytes. Furthermore, it aims to explain the evolutionary pattern of sensing applications and provide novel insights and inspiration to advance this burgeoning field.

Chiral cadmium-amine complexes for stimulating non-linear optical activity and photoluminescence in solids based on aurophilic stacks.

The design of high-performance optical materials can be realized using coordination polymers (CPs) often supported by non-covalent interactions, such as metallophilicity. The challenge is to control two or more optical effects, e.g., non-linear optics (NLO) and photoluminescence (PL). We present a new strategy for the combination of the NLO effect of second-harmonic generation (SHG) and the visible PL achieved by linking dicyanidoaurate(i) ions, which form luminescent metallophilic stacks, with cadmium(ii) complexes bearing chiral amine ligands, used to break the crystal's symmetry. We report a family of NLO- and PL-active materials based on heterometallic Cd(ii)-Au(i) coordination systems incorporating enantiopure propane-1,2-diamine (pda) ligands (1-S, 1-R), their racemate (2), and enantiopure trans-cyclopentane-1,2-diamine (cpda) ligands (3-S, 3-R). Due to acentric space groups, they exhibit the SHG signal, tunable within the range of 11-24% of the KDP reference, which was correlated with the dipole moments of Cd(ii) units. They show efficient blue PL whose energy and quantum yield, the latter ranging from 0.40 to 0.83, are controlled by Cd(ii) complexes affecting the Au-Au distances and vibrational modes. We prove that chiral Cd(ii)-amine complexes play the role of molecular agents for the stimulation of both the NLO and PL of the materials based on aurophilic stacks.

A Brief Look Into Nonlinear Optical Properties Of MXenes (M2X) Ti2C, Nb2C, And V2C Through Four Wave Mixing Generation In 2 µm Fiber Laser

Generation of four wave mixing (FWM) effect in MXenes with chemical ratio of M2X, particularly Ti2C, Nb2C, and V2C were reported in this work. The setup was operated at 2 µm wavelength region with tapered fiber as the material host. The experiment managed to show conversion efficiencies of −43.93 dB, −42.60 dB, and −41.20 dB and idlers’ SNR of 7.73 dB, 9.10 dB, and 9.80 dB for Ti2C, Nb2C, and V2C, respectively. The relation of nonlinear optical properties such as nonlinear absorption coefficient and nonlinear refractive index to FWM generation were explored. The result of the investigation successfully showed the relationship of third order nonlinear optical properties and FWM effect in the specified MXenes with comparisons.

Emergence and transformation of polar skyrmion lattices via flexoelectricity

As analogies to magnetic skyrmions, polar skyrmions in ferroelectric superlattices and multilayers have garnered widespread attention for their non-trivial topology and novel properties like negative capacitance and nonlinear optical effect. So far, they have only been theoretically predicted to be able to assemble ordered hexagonal skyrmion lattices (SkLs) in ferroelectric thin films. Here, based on phase-field simulations, we report the critical roles of flexoelectricity playing in the stabilization and transformation of polar SkLs. Different polar SkL patterns can emerge in the ferroelectric thin films, including tetragonal-SkL, and hexagonal-SkLs with diverse orientations, as summarized by phase diagrams. These emergent SkL states are attributed to the material anisotropy modified by the flexoelectric effect. Interestingly, we further found that the hexagonal-SkLs can be rotated by applying strain gradient or in-plane electric field to the films. Moreover, a nonreciprocal bending response of tetragonal-SkL is also induced by the flexoelectric effect. Our results provide useful guidelines for the implementation of polar skyrmion lattices in experiments.

Analysis on optical, thermal and third-order nonlinear properties of L-histidine hydrochloride monohydrate single crystal

This research contributes valuable insights into the understanding of third-order nonlinear optical effects in L-histidine hydrochloride monohydrate (LHHC) single crystal, offering a foundation for future work in optimizing its properties for practical applications in the field of nonlinear optics. It presents a systematic analysis into the LHHC crystal properties harvested by slow evaporation solution (SEST) method. Originally, structure of the compound was confirmed through single crystal x-ray diffraction and powder x-ray diffraction analysis. FTIR analysis was performed to get information about molecular composition of the compound. The assessment of the optical quality of LHHC single crystal was done by UV–Visible and photoluminescence spectroscopies. TG-DTA was employed to assess the stability of the single crystal under different temperature conditions. Z-scan technique, with an open aperture (OA) & closed aperture (CA) configuration, was employed to comprehensively explore the third-order nonlinear response of LHHC single crystal. The nonlinear absorption coefficient β is determined to be 5.32 × 10−10 cm W−1. The NLR index (n2) is calculated as −2.72 × 10−15 cm2 W−1.

Cavity Floquet engineering

Floquet engineering is a promising tool to manipulate quantum systems coherently. A well-known example is the optical Stark effect, which has been used for optical trapping of atoms and breaking time-reversal symmetry in solids. However, as a coherent nonlinear optical effect, Floquet engineering typically requires high field intensities obtained in ultrafast pulses, severely limiting its use. Here, we demonstrate using cavity engineering of the vacuum modes to achieve orders-of-magnitude enhancement of the effective Floquet field, enabling Floquet effects at an extremely low fluence of 450 photons/μm2. At higher fluences, the cavity-enhanced Floquet effects lead to 50 meV spin and valley splitting of WSe2 excitons, corresponding to an enormous time-reversal breaking, non-Maxwellian magnetic field of over 200 T. Utilizing such an optically controlled effective magnetic field, we demonstrate an ultrafast, picojoule chirality XOR gate. These results suggest that cavity-enhanced Floquet engineering may enable the creation of steady-state or quasi-equilibrium Floquet bands, strongly non-perturbative modifications of materials beyond the reach of other means, and application of Floquet engineering to a wide range of materials and applications.

The Study on the Nonlinear Effects of Soliton in Optical Fibre

Abstract: In optical fibres, the chirps generated by GVD and SPM, which both limit the system's performance when operating independently, balance each other out to produce soliton formation. To comprehend how such a balance is possible, we shall study the nonlinear optical effects and the dispersion-induced pulse broadening. If an optical pulse is not adequately chirped before it propagates inside an optical fibre, the GVD broadens the pulse. More specifically, an early stage of transmission compresses a chirped pulse whenever β2 and the chirp parameter C have opposite signs and β2C is negative. The optical pulse chirps due to SPM, ensuring that C > 0. If β2 < 0, it is easy to meet the criteria β2C < 0. Moreover, because the SPM-induced chirp depends on power, it is possible that in some situations, SPM and GVD could cooperate to the extent that the SPM-induced chirp is precisely the right amount to cancel out the pulse broadening caused by GVD. Here, an optical pulse propagates distortion-free as a soliton [27].

Ferroelectric nematic fluids as spontaneous polarization field for enhanced orientational-dependent second-harmonic generation

ABSTRACT Second-order nonlinear optical effects have piqued researchers’ interest in fundamental physics and practical perspectives. The ferroelectric nematic liquid crystals (NF LCs) have recently drawn attention to LC nonlinear optical applications due to their relatively high second-harmonic generation output. Further improving the nonlinear optical properties to get closer to the performance of inorganic nonlinear optical materials is challenging. Here, we take a blending strategy to enhance the nonlinear optical response of a prototype NF LC, DIO. We mixed an organic nonlinear optical crystalline material, 3,5,5-trimethyl (cyclohex-2-enylidene) malononitrile (TCM), with DIO and investigated the effects of TCM doping at various concentrations. We quantitatively determined the second-harmonic generation and spontaneous polarization as functions of temperature and concentration. We found that, compared with the pure DIO, the second-harmonic generation performance increased by about 130% at maximum, 93% in maximum for the nonlinear optical coefficient, and 10% in maximum for spontaneous polarization.

Enhancement of nonlinear optic effect of the second harmonic generation in plasmonic waveguide using graphene and pyramid-shape gold nanoparticles

Enhancement of nonlinear optic effect of the second harmonic generation in plasmonic waveguide using graphene and pyramid-shape gold nanoparticles

The Directional Design of the Quasi‐Phase‐Matching Short‐Wave Ultraviolet Nonlinear Optical Crystal

AbstractSince the study of short‐wave ultraviolet nonlinear optical crystals based on the quasi‐phase‐matching principle will effectively expand the development field of nonlinear optical materials, it is urgent to achieve their directional design. Here, it is proposed and implemented on account of the hexagonal wurtzite structure. Thus, the LiRbSeO4 crystal is obtained with [SeO4] groups as the main tetrahedra and [LiO4] as the separation. It exhibits a wide transparency window (0.22–5.57 µm) and an excellent nonlinear optical effect (≈1.7 × KH2PO4), which originates from the synergy of distorted tetrahedral groups and their uniform arrangement. More importantly, the LiRbSeO4 crystal is confirmed to be a ferroelectric with a typical polarization‐electric field hysteresis loop (a coercive field of 27.71 kV cm−1, a remanent polarization of 8.60 µC cm−2 at 373 K). Its ferroelectricity is further identified by the domain engineering studies, involving piezoelectricity and the chirality. Besides, ferroelectricity is attributed to the ordered arrangement of tetrahedra with the polarization reversal involving atomic displacements. Therefore, the LiRbSeO4 crystal is a promising short‐wave ultraviolet nonlinear optical candidate based on the quasi‐phase‐matching principle. This work also provides a new horizon for the design of nonlinear optical crystals.

Mie-resonant metaphotonics

Mie-resonant metaphotonics is a rapidly developing field that employs the physics of Mie resonances to control light at the nanoscale. Mie resonances are excited in high-refractive-index transparent nanoparticles and voids created in dielectric media, and they can be used to achieve a wide range of optical effects, including enhanced light–matter interaction, nonlinear optical effects, and topological photonics. Here, we review the recent advances in Mie-resonant metaphotonics, with a focus on the physics of Mie resonances and their applications in metaphotonics and metasurfaces. Through a comprehensive multipolar analysis, we demonstrate the complex interplay of electric and magnetic multipoles that govern their interaction with light. Recent advances have unveiled a diverse spectrum of scattering phenomena that can be achieved within precisely engineered structures. Within this framework, we review the underlying mechanics of the first and second Kerker conditions and describe the intricate mechanisms guiding these nanostructures’ light-scattering properties. Moreover, we cover intriguing phenomena such as the anapole and bound or quasi-bound states in the continuum. Of profound interest are the numerous practical applications that result from these revelations. Ultrafast processes, the emergence of nanolasers, and advancements in magneto-optic devices represent just a fraction of the transformative applications.

The nonlinear optical properties of meso-substituted porphyrins using spatial self-phase modulation

Herein, we studied the nonlinear optical properties of four meso-substituted porphyrins (H2MHTPP, H2THTPP, H2DETPP, and H2TETPP) by the spatial self-phase modulation (SSPM) method, using CW lasers with wavelengths of 532 nm and 780 nm. All samples exhibit strong nonlinear response due to the thermal optical nonlinear effect at 532 nm, which corresponds to the strong absorption band. In addition, four porphyrin samples were calculated that have nonlinear refractive index coefficients of the same order of magnitude (10−10 m2/W) at 532 nm. However, four porphyrins exhibited different optical nonlinear responses affected by the type and numbers of peripheral substituents at 780 nm. This work is of significance for reasonably adjusting the structure of compounds and improving their third-order nonlinear optical coefficients.

(Invited) On the Optical Response of Layered Materials Exfoliated in Liquid Phase: Evidence of Functional Groups and Size-Dependent Features

Liquid phase exfoliation of layered materials has proven successful for a range of applications [1]. However, the range of organic solvents needed, and the broad distribution of thicknesses [2] can be aided by applying intercalation steps such as electrochemical or solvothermal expansion which leads to thinner crystals in the dispersion [2, 3]. In this talk I will present the effect of electrochemical exfoliation on the basal functionalization of graphene and a proposal for a more accurate determination of the optical extinction coefficient based on oxidation levels for standardization [4]. I will also present a few examples of MoS2 and WS2 exfoliated via ultrasonication which display non-linear optical effects in fluorescence spectroscopy [5] and the effect of crystal-size selection on the piezoresistive and photoconduction response of MoS2 and WS2 exfoliated via lithium intercalation.[1] Yenny Hernandez, et al, Nature Nanotechnology, 3, 563–568 (2008)[2] Jonathan N. Coleman et al. Science 331, 568-571 (2011)[3] Zhongfen Ding, et al, J. Mater. Chem., 2009, 19, 2588-2592 (2009)[4] Khaled Parvez, et al, ACS Nano 2013, 7, 4, 3598–3606 (2013)[5] J Rico, et al, J. Phys.: Condens. Matter 34 105701 (2022)[6] Gabriel Cardenas-Chirivi, et al, Optical Materials, 133,112890 (2022)