Nonlinear Optical Conversion Efficiency Research Articles

Angular Engineering Strategy for Enhanced Surface Nonlinear Frequency Conversion in Centrosymmetric Topological Semimetal HfGe0.92Te.

Surface nonlinear optics are essential for developments in integrated photonics and micro/nano optoelectronics. However, the nonlinear optical conversion efficiency on a surface is restricted by the finite nonlinear susceptibility of matter and the intrinsic atomic-layered interaction length between light and matter. In this work, based on an angular engineering strategy, it is demonstrated that the centrosymmetric topological semimetal HfGe0.92Te crystal has a giant and anisotropic surface second-order nonlinear susceptibility up to 5535 ± 308pm V-1 and exhibits efficient and unprecedented second-harmonic generation (SHG). The maximum optical conversion efficiency is found to be up to 3.75‰, which is 104 times higher than that obtained from a silicon surface. Because of the linear dispersion over a wide range of energies around the Dirac points, this high conversion efficiency can be maintained with SHG wavelengths ranging from the visible region (779nm) to the deep-UV region (257.5nm). This study can facilitate the development of topological photonics and integrated nonlinear photonics based on topological semimetals.

Study on interface engineering and chemical bonding of the ReS2@ZnO heterointerface for efficient charge transfer and nonlinear optical conversion efficiency.

Rhenium sulfide (ReS2) has emerged as a promising two-dimensional material, demonstrating broad-spectrum visible absorption properties that make it highly relevant for diverse optoelectronic applications. Manipulating and optimizing the pathway of photogenerated carriers play a pivotal role in enhancing the efficiency of charge separation and transfer in novel semiconductor composites. This study focuses on the strategic construction of a semiconductor heterostructure by synthesizing ZnO on vacancy-containing ReS2 (VRe-ReS2) through chemical bonding processes. The ingeniously engineered built-in electric field within the heterostructure effectively suppresses the recombination of photogenerated electron-hole pairs. A direct and well-established interfacial connection between VRe-ReS2 and ZnO is achieved through a robust Zn-S bond. This distinctive bond configuration leads to enhanced nonlinear optical conversion efficiency, attributed to shortened carrier migration distances and accelerated charge transfer rates. Furthermore, theoretical calculations unveil the superior chemical interactions between Re vacancies and sulfide moieties, facilitating the formation of Zn-S bonds. The photoluminescence (PL) intensity is increased by the formation of VRe-ReS2 and ZnO heterostructure and the PL quantum yield of VRe-ReS2 is improved. The intricate impact of the Zn-S bond on the nonlinear absorption behavior of the VRe-ReS2@ZnO heterostructure is systematically investigated using femtosecond Z-scan techniques. The charge transfer from ZnO to ReS2 defect levels induces a transition from saturable absorption to reverse saturable absorption in the VRe-ReS2@ZnO heterostructure. Transient absorption measurements further confirm the presence of the Zn-S bond between the interfaces, as evidenced by the prolonged relaxation time (τ3) in the VRe-ReS2@ZnO heterostructure. This study offers valuable insights into the rational construction of heterojunctions through tailored interfacial bonding and surface/interface charge transfer pathways. These endeavors facilitate the modulation of electron transfer dynamics, ultimately yielding superior nonlinear optical conversion efficiency and effective charge regulation in optoelectronic functional materials.

Defect and interface/surface engineering synergistically modulated electron transfer and nonlinear absorption properties in MoX2 (X = Se, S, Te)@ZnO heterojunction.

Systematic interface and defect engineering strategies have been demonstrated to be an effective way to modulate the electron transfer and nonlinear absorption properties in semiconductor heterojunctions. However, the role played by defects and interfacial strain in electron transfer at the interface of the MoX2 (X = Se, S, Te)@ZnO heterojunction remains poorly understood. Herein, using the MoX2@ZnO heterojunction, we reveal that vacancies play a critical role in the interfacial electron transfer of heterojunctions. Specifically, we present the defect and interface engineering of the MoX2@ZnO heterojunction for controlled charge transfer and electron excitation-relaxation. The experimental characterization combined with first-principles calculations showed that the presence of defects promoted the transport of photogenerated carriers at the heterojunction interface, thereby inhibiting their rapid recombination. The DFT calculation confirmed that the electron band structure, density of states and charge density distribution in the system changed after the formation of Mo-O bonds. On the basis of defects and interfacial stress and the effective charge transfer, the MoX2@ZnO heterojunction exhibited excellent excitation and emission behaviors. The nonlinear optical regulation behavior of TMDs is expected to be realized with the help of the defects and interface/surface synergistically modulated effect of ZnO nanoparticles. The local strain generation on the MoX2@ZnO heterojunction boundary provides a new method for the design of new heterogeneous materials and will be of great significance to investigate the contact physical behavior and application of metals and two-dimensional (2D) semiconductors. This work provides some inspiration for the construction of heterojunctions with rich defects and surface/interface charge transfer channels to promote tunable electron transfer dynamics, thereby achieving a good nonlinear optical conversion efficiency and efficient charge separation in optoelectronic functional materials.

Stacking-Controlled Growth of rBN Crystalline Films with High Nonlinear Optical Conversion Efficiency up to 1.

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of 2Dmaterials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers are controllably synthesized. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single-crystal monolayer and the continuous precipitation of (B,N) source for thick layers are guaranteed. The preserved in-plane inversion asymmetry in rBN films keeps the in-phase second-harmonic generation field in every layer and leads to a record-high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6µm. The work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on-demand fabrication of integrated photonic and compact quantum optical devices.

Multimodal Optically Nonlinear Nanoparticles Exhibiting Simultaneous Higher Harmonics Generation and Upconversion Luminescence for Anticounterfeiting and 8‐bit Optical Coding

AbstractNonlinear optical materials are essential in areas such as nanophotonics, optical information processing, and biomedical imaging. However, nanomaterials employed for these diverse applications to date are efficient only for one type of nonlinear optical activity. Herein, the first multimodal nonlinear optically active class of nanomaterials based on lanthanide‐doped lithium niobate nanoparticles, which simultaneously exhibit unprecedentedly efficient second and third harmonic generation, as well as up‐conversion photoluminescence, is reported. These dielectric nanoparticles retain their high nonlinear optical conversion efficiency both as powder and as aqueous colloidal solution. The high stability also allows for the fabrication of optically active biocompatible micron‐sized fibers and polymer‐based 3D‐printable objects, as well as for fingerprint detection. Finally, the first 8‐bit coding platform purely based on multimodal nonlinear optical activity originating from different parametric and nonparametric processes is demonstrated, showcasing the technological potential of these materials for both anti‐counterfeiting and advanced optical information processing.

Tailoring the bound states in the multi-channel nonlinear plasmonic metasurfaces

Metasurfaces offer a paradigm shift in the nonlinear optics, controlling and enhancing of nonlinear effects at the nanoscale, and offering new opportunities for applications in nonlinear optics, plasmonics and electromagnetics. Particularly, nonlinear optical conversion efficiency is limited due to the inherent losses in metallic materials. Nonlinear metasurfaces based on bound states in the continuum(BIC) can effectively increase the Q value and are expected to further enhance the nonlinear efficiency. In this paper, a tri-capacitance-like metasurface supporting both Friedrich–Wintgen quasi-BIC and symmetry-protected quasi-BIC is proposed to generates multiple high-Q resonances in the spectrum. The coupling between perfect BIC and quasi-BIC mode in the multi-channel optical system corresponds to the storage and release of electromagnetic waves, respectively. At the same time, the electromagnetic field is further enhanced at the resonant frequency, which can significantly improve the second order nonlinear efficiency. In addition, due to the different generation mechanism, the frequency and amplitude of the two resonances are easy to tune independently, which facilitates the sum and difference frequency generation (SFG and DFG). Therefore, the proposed nonlinear metasurfaces in this paper are of wide research value in the fields of nonlinear optics, multi-channel communication and memory.

Topological insulator-based nonlinear optical effects and functional devices

Topological insulators (TIs) are a new type of electronic material with the insulating bulk state and topologically protected conducting surface/edge state. TI materials exhibiting outstanding optical properties containing high carrier mobility, ultrahigh refractive index, wide operating spectral range, and dynamic tunability play an important role in optical functional devices. Especially, the TI materials with special topological energy bands possess high nonlinear optical coefficient, superior to conventional semiconductor and two-dimensional materials. The nonlinear optical characters of TIs are expected to realize highly efficient nonlinear optical conversion, nonlinear absorption, and all-optical switching, terahertz emission. TIs will provide a new platform for the interaction between light and matter as well as the development of nonlinear optics. In this review, we conclude current advances about some research on TI-based nonlinear optical effects and related devices, and give a prospect of potential applications for TIs in nonlinear optics.

Broadband Plasmon-Enhanced Four-Wave Mixing in Monolayer MoS2.

Two-dimensional transition-metal dichalcogenide monolayers have remarkably large optical nonlinearity. However, the nonlinear optical conversion efficiency in monolayer transition-metal dichalcogenides is typically low due to small light–matter interaction length at the atomic thickness, which significantly obstructs their applications. Here, for the first time, we report broadband (up to ∼150 nm) enhancement of optical nonlinearity in monolayer MoS2 with plasmonic structures. Substantial enhancement of four-wave mixing is demonstrated with the enhancement factor up to three orders of magnitude for broadband frequency conversion, covering the major visible spectral region. The equivalent third-order nonlinearity of the hybrid MoS2-plasmonic structure is in the order of 10–17 m2/V2, far superior (∼10–100-times larger) to the widely used conventional bulk materials (e.g., LiNbO3, BBO) and nanomaterials (e.g., gold nanofilms). Such a considerable and broadband enhancement arises from the strongly confined electric field in the plasmonic structure, promising for numerous nonlinear photonic applications of two-dimensional materials.

Crystal growth, structural, optical, thermal, dielectric, mechanical and NLO studies of L-tyrosine zinc carbonate single crystals

L-Tyrosine Zinc Carbonate (LTZC) a semi-organic nonlinear optical single crystal has been grown by slow evaporation solution technique. The lattice parameters of the grown crystals are found out by single crystals X- ray diffraction technique. The modes of vibrations of different functional groups present were identified by using Fourier Transform Infrared (FTIR) spectroscopic analysis. The UV–Visible optical absorption spectrum shows that the crystal has high transmission with lowest UV cutoff wavelength of 230 nm. The calculated band gap of LTZC crystal was found to be 5.13 eV. Dielectric constant and dielectric loss for various frequencies and temperatures were performed on LTZC crystal and the results were discussed in detail. The frequency dependence of ac conductivity for different temperatures was studied. The photoconductivity studies confirm that the crystal LTZC has negative photo conducting nature. The mechanical and thermal properties of the grown crystals have been studied. Surface morphological changes are observed in the grown LTZC crystal. The Non-linear optical second harmonic generation conversion efficiency of LTZC was determined using Q-switched Nd: YAG laser. The second harmonic generation conversion efficiency of LTZC is 2.12 times that of KDP.

Grating-Graphene Metamaterial as a Platform for Terahertz Nonlinear Photonics.

Nonlinear optics is an increasingly important field for scientific and technological applications, owing to its relevance and potential for optical and optoelectronic technologies. Currently, there is an active search for suitable nonlinear material systems with efficient conversion and a small material footprint. Ideally, the material system should allow for chip integration and room-temperature operation. Two-dimensional materials are highly interesting in this regard. Particularly promising is graphene, which has demonstrated an exceptionally large nonlinearity in the terahertz regime. Yet, the light–matter interaction length in two-dimensional materials is inherently minimal, thus limiting the overall nonlinear optical conversion efficiency. Here, we overcome this challenge using a metamaterial platform that combines graphene with a photonic grating structure providing field enhancement. We measure terahertz third-harmonic generation in this metamaterial and obtain an effective third-order nonlinear susceptibility with a magnitude as large as 3 × 10–8 m2/V2, or 21 esu, for a fundamental frequency of 0.7 THz. This nonlinearity is 50 times larger than what we obtain for graphene without grating. Such an enhancement corresponds to a third-harmonic signal with an intensity that is 3 orders of magnitude larger due to the grating. Moreover, we demonstrate a field conversion efficiency for the third harmonic of up to ∼1% using a moderate field strength of ∼30 kV/cm. Finally, we show that harmonics beyond the third are enhanced even more strongly, allowing us to observe signatures of up to the ninth harmonic. Grating-graphene metamaterials thus constitute an outstanding platform for commercially viable, CMOS-compatible, room-temperature, chip-integrated, THz nonlinear conversion applications.

Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7-Fold to 8-Fold Interpenetrated Metal-Organic Frameworks (MOFs).

Interpenetration in metal-organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7-fold interpenetrated MOF (1) is transformed into an 8-fold interpenetrated MOF by the loss of DMF in a single-crystal-to-single-crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two-photon photoluminescence (ca. 14 times). The strengthened π-π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30-150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single-crystal NLO devices.

Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7‐Fold to 8‐Fold Interpenetrated Metal–Organic Frameworks (MOFs)

AbstractInterpenetration in metal–organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7‐fold interpenetrated MOF (1) is transformed into an 8‐fold interpenetrated MOF by the loss of DMF in a single‐crystal‐to‐single‐crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two‐photon photoluminescence (ca. 14 times). The strengthened π–π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30–150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single‐crystal NLO devices.

Optical Frequency Metrology Study on Nonlinear Processes in a Waveguide Device for Ultrabroadband Comb Generation

Waveguide-based devices are essential for highly efficient nonlinear optical conversion. However, the nonlinearity associated with the conversion in such devices is not self-evident. The authors present a method using optical frequency metrology to reveal the nonlinearity inside a waveguide, and establish that the spectral broadening in a periodically poled lithium niobate waveguide is due to quadratic nonlinearity. They also demonstrate absolute frequency measurement using the generated ultrabroadband comb. This study will contribute to the design of chip-scale, fully integrated devices for efficient nonlinear optical conversion.

Composite‐Assisted Phase‐Matching: Efficient Wavelength Conversion in Nonlinear Optical Composite Materials Containing Metal Nanoparticles

AbstractA novel phase‐matching scheme which is based on the dispersion compensation in the nonlinear optical composite materials containing metal nanoparticles is proposed. Anomalous dispersion originating from the plasmon resonance in metal nanoparticles compensates the dispersion of the host nonlinear material, leading to the perfect phase‐matching and high efficiency of nonlinear optical wavelength conversion. The effectiveness of this approach is theoretically demonstrated, taking third‐order nonlinear processes such as the direct third‐harmonic generation and four‐wave mixing in ZnO composites containing silica‐core–silver‐shell nanoparticles as examples. The results show that with the proposed phase‐matching scheme, unprecedentedly high conversion efficiency can be obtained compared with preceding results in third‐order nonlinear optical solid‐state materials.

Recovery of wave-mixing conversion efficiency in weakly scattering nonlinear crystals.

Nonlinear optical wave mixing is a widely used method to produce light with new frequencies that has a significant impact on laser technology and optical imaging. The most important figure of merit in wave-mixing processes, i.e., high conversion efficiency, is always required in laser applications. We demonstrate a method to recover high conversion efficiency of second harmonic generation in a BaMgF4 single crystal with weakly scattering defects via feedback-based wavefront shaping under birefringent phase-matching condition. By optimizing the fundamental wavefront, a typical second harmonic output with an enhancement factor of 1.14 and a corresponding recovery efficiency of 86.3% is displayed. This investigation may modify the wide understanding of scattering in crystals and provide an avenue to recover the nonlinear optical conversion efficiency in crystals with various defects without special fabrications.

Hydrothermal single crystal growth and second harmonic generation of Li2SiO3, Li2GeO3 and Li2Si2O5

The growth of large high quality single crystals of three lithium silicates and germanates Li2SiO3, Li2SiO5, and Li2GeO3 is described. The crystals are all grown using hydrothermal fluids at 650 °C and 1 kbar using simple oxide feedstock and 3–6 M LiOH serving as both the lithium ion source and mineralizer. The crystals could be grown by spontaneous transport without any specific seeding, and grew between 2 and 10 mm/edge and without evidence of cracking, physical twinning or multiple domain structures. The crystals were characterized by single crystal diffraction, UV–vis spectroscopy and Raman spectroscopy. All the crystals are acentric and polar. Preliminary nonlinear optical properties were investigated using the Kurtz method and indicate that the crystals exhibit Type-1 phase matching ability, and Li2SiO3 demonstrates a significant nonlinear optical conversion efficiency, making it of potential interest as a UV NLO crystal.

Highly Efficient Nonlinear Optical Conversion in Waveguiding GaSe Nanoribbons with Pump Pulses Down to a Femto‐Joule Level

AbstractLayered GaSe flakes have revealed strongest second‐harmonic (SH) generation among all the two‐dimensional (2D) atomic crystals measured up to now. However, the unique feature of physical atomic thickness at nanometer scale prevents their practical applications in efficient nonlinear optical conversion due to limited light–matter interaction length. In this work, high quality single‐crystal GaSe nanoribbons (NRs) are fabricated and demonstrated as waveguides with good performance. By taking advantage of the strong confinement and long interaction length of the waveguiding approach, significantly enhanced nonlinear light–matter interaction is observed in GaSe NRs. Based on transverse SH generation, a single GaSe NR‐configured optical auto‐correlator is constructed for ultrashort pulse characterization. Because of the large second‐order nonlinear susceptibility, high sensitivity down to few femto‐joule (fJ) is realized. To the best of our knowledge, this is the highest energy sensitivity among the reported nanoscale auto‐correlators. Such NR waveguides and devices are highly compatible with standard optical fiber systems. The presented results may enable the construction of future energy efficient nonlinear photonic circuitry, chips, and devices.

Nonlinear optical conversion in synthetic opal

This paper presents an experimental study of third optical harmonic generation processes in synthetic opal. Excitation was provided by ultrashort (∼10−13 s) laser pulses (λ = 1026 and 513 nm) with a high repetition rate (105 Hz), high peak power (up to 1 GW), and low pulse energy (10−5 J). The third optical harmonic intensity was shown to depend significantly on the diameter of the silica spheres, which determines the spectral position of the band gaps in the opal photonic crystal. The highest efficiency of the nonlinear optical conversion of excitation light to the third optical harmonic was observed when the excitation or optical harmonic wavelength was close to the spectral position of the band gap of the photonic crystal. This behavior can be accounted for by the presence of surface photon (Tamm) states of the optical field in opal photonic crystals in the spectral region corresponding to the band gap of the photonic crystal. The presence of photon Tamm states leads to the formation of high spectral intensity regions localized near the surface of the photonic crystal. This causes a considerable increase in nonlinear optical conversion efficiency, in particular, in the case of the third optical harmonic generation in the centrosymmetric structure of opal lattices.

Synthesis, growth and optical properties of an efficient nonlinear optical single crystal: L-alanine DL-malic acid

Single crystals of L-alanine DL-malic acid (LADLMA) have been grown from aqueous solution by slow-cooling technique. Powder X-ray diffraction studies reveal the structure of the crystal to be orthorhombic. The nonlinear optical conversion efficiency test was carried out for the grown crystals using the Kurtz powder technique. The third order nonlinear refractive index and the nonlinear absorption coefficient where evaluated by Z-scan measurements. As the material have a negative refractive index it could be used in the protection of optical sensors such as night vision devices.

Relaxed damage threshold intensity conditions and nonlinear increase in the conversion efficiency of an optical parametric oscillator using a bi-directional pump geometry

A novel bi-directional pump geometry that nonlinearly increases the nonlinear optical conversion efficiency of a synchronously pumped optical parametric oscillator (OPO) is reported. This bi-directional pumping method synchronizes the circulating signal pulse with two counter-propagating pump pulses within a linear OPO resonator. Through this pump scheme, an increase in nonlinear optical conversion efficiency of 22% was achieved at the signal wavelength, corresponding to a 95% overall increase in average power. Given an almost unchanged measured pulse duration of 260 fs under optimal performance conditions, this related to a signal wavelength peak power output of 18.8 kW, compared with 10 kW using the traditional single-pass geometry. In this study, a total effective peak intensity pump-field of 7.11 GW/cm(2) (corresponding to 3.55 GW/cm(2) from each pump beam) was applied to a 3 mm long periodically poled lithium niobate crystal, which had a damage threshold intensity of 4 GW/cm(2), without impairing crystal integrity. We therefore prove the application of this novel pump geometry provides opportunities for power-scaling of synchronously pumped OPO systems together with enhanced nonlinear conversion efficiency through relaxed damage threshold intensity conditions.