Second Harmonic Generation Efficiency Research Articles

Second and cascaded harmonic generation of pulsed laser in a lithium niobate on insulator ridge waveguide.

Nonlinear crystalline ridge waveguides, e.g., lithium niobate-on-insulator ridge waveguides, feature high index contrast and strong optical confinement, thus dramatically enhancing nonlinear interaction and facilitating various nonlinear effects. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and cascaded fourth-harmonic generation (FHG) in a periodically poled lithium niobate (PPLN) ridge waveguide pumped with pulsed laser at the quasi-phase matching (QPM) wavelength, as well as simultaneous SHG and cascaded third-harmonic generation (THG) waves when pumped at the non-QPM wavelength. Furthermore, the ridge waveguide achieves an efficient single-pass SHG conversion efficiency of picosecond pulsed laser at ∼62%. These results may be beneficial for on-chip nonlinear frequency conversion.

Design, Synthesis, and Characterization of the Nonlinear Optical Material MnTeWxMo1–xO6

MnTeMoO6 is a novel nonlinear optical material in the near-mid-IR region. In order to improve the properties of the MnTeMoO6 family, W6+ is used to substitute Mo6+ in MnTeMoO6. W-doped MnTeMoO6 (MnTeWxMo1–xO6) has been designed and predicted by using the first-principles calculations. Based on the prediction of theoretical calculations, MnTeWxMo1–xO6 has been synthesized by solid-state reaction techniques. Powder X-ray diffraction, the vibrational spectra, the UV–vis diffuse reflectance spectra, and the powder second-harmonic generation efficiency of MnTeWxMo1–xO6 have been investigated. Finally, the theoretical calculation result shows that the formation of MnTeW1/16Mo15/16O6 is possible, and an attempt on the synthesis of MnTeWxMo1–xO6 should be carried out. Moreover, the experimental result proves that the doping ratio of the MnTeWxMo1–xO6 resultant with a single-crystal phase is less than 5 mol %. The band gap energy is decreased by the W-doping ratio increase, and the second-harmonic generation response and band gap energy are not significantly correlated in MnTeWxMo1–xO6 (x = 0.01–0.04). To enhance the second-order nonlinear optical properties of MnTeMoO6, it is demonstrated that W6+ is a suitable dopant to substitute Mo6+, and the suitable doping ratio is about 2 mol %.

The role of nonlocal response in second harmonic generation at metasurfaces with triangular metaatoms

Metasurface consisting of square array of obtuse isosceles triangle metaatoms is found to exhibit much more efficient second harmonic generation (SHG) compared to that of split ring resonators and its heptagonal modification, experimentally as well as numerically. All of them are designed to lack inversion symmetry in horizontal direction but to have vertical symmetry, and to meet the double resonance condition for vertically-polarized fundamental and horizontally-polarized SHG waves. SHG efficiencies were estimated numerically for each structure by evaluating an overlap integral of second order nonlinear polarization and field distribution at the SHG wavelength. The large overlap integral for the triangle metasurface is ascribed to the nonlocal response at the SHG wavelength. Current induced at the center of the triangle near the obtuse angle by horizontally polarized light at SHG wavelength flows toward two corners of acute angles. As a result, polarization charges oscillate in time at the surfaces of two acute corners, which results in strong field oscillation away from the center where the electric field is applied to majority of free electrons. Experimentally observed wavelength dependence of SHG efficiencies for the three metasurfaces are reasonably reproduced by a numerical estimation.

Growth and characterization of barium bis para-nitrophenolate para-nitrophenol tetrahydrate single crystal for NLO applications

Single crystals of barium bis para-nitrophenolate para-nitrophenol tetrahydrate (BBPNT) was grown using slow evaporation technique with the crystal dimensions of 15 × 10 × 6 mm3. The structure of the BBPNT crystal was examined by X-ray diffraction and the functional groups present in the compound were identified using FTIR spectrum. The optical and electrical properties were characterized by UV–Vis spectrum and dielectric studies respectively. Thermal properties of the grown crystal were investigated by using thermo-gravimetric (TG) and differential thermal analyses (DTA). The mechanical behavior of the grown crystal was studied using Vickers microhardness test. Using the Kurtz-Perry powder method, the second-harmonic generation efficiency was found to be 15.75 times greater than to that of KDP. Third-order nonlinear response was studied using Z-scan technique with a He-Ne laser (632.8 nm) and NLO parameters such as intensity dependent refractive index, nonlinear absorption coefficient and third-order susceptibility were also estimated. In the PL spectra, a broad band observed at 525 nm suggests its applications for fabricating green laser and new tunable laser system. The laser damage threshold value of BBPNT was found to be 5.09 GW/cm2 using an Nd: YAG laser.

Efficient second-order nonlinear response and upconversion emission from a wide-bandgap quasi-1D lead bromide perovskite

In this work, a new quasi-1D lead bromide perovskite with efficient second-harmonic generation and unexpectedly anomalous green PL phenomenon is developed. The second-order NLO coefficient is about 0.1 pm V−1, 1/4 times that of the commercial crystal KH2PO4 (KDP).

Dual bound states in the continuum enhanced second harmonic generation with transition metal dichalcogenides monolayer

The emergence of two dimensional (2D) materials has opened new possibilities for exhibiting second harmonic generation (SHG) at the nanoscale, due to their remarkable optical response related to stable excitons at room temperature. However, the ultimate atomic-scale interaction length with light makes the SHG of Transition Metal Dichalcogenides (TMDs) monolayers naturally weak. Here, we propose coupling a monolayer of TMDs with a photonic grating slab that works with doubly resonant bound states in the continuum (BIC). The BIC slabs are designed to exhibit a pair of BICs, resonant with both the fundamental wave (FW) and the second harmonic wave (SHW). Firstly, the spatial mode matching can be fulfilled by tilting FW's incident angle. We theoretically demonstrate that this strategy leads to more than four orders of magnitude enhancement of SHG efficiency than a sole monolayer of TMDs, under a pump light intensity of 0.1 GW/cm². Moreover, we demonstrate that patterning the TMDs monolayer can further enhance the spatial overlap coefficient, which leads to an extra three orders of magnitude enhancement of SHG efficiency. These results demonstrate remarkable possibilities for enhancing SHG with nonlinear 2D materials, opening many opportunities for chip-based light sources, nanolasers, imaging, and biochemical sensing.

Angular birefringence phase matching behaviour of Lithium Triborate (LBO) crystal for second harmonic generation in broad wavelength range

Second Harmonic Generation (SHG) is a nonlinear optical process in which the frequency of the incident radiation gets doubled. There are various types of applications of SHG such as ultra-short pulse measurement, high resolution optical microscopy, characterization of the structure of 2D materials and its physical properties like the thickness of interlayer stacking, stacking angle between different layers, the grain boundary etc. It has also been used to determine the structure of Langmuir-Blodgett films of low symmetry. For SHG, various commercial nonlinear optical (NLO) crystals are generally used. Lithium Triborate (LBO) is one of the best NLO material among them due to its wide transparency range (160–2600 nm), high damage threshold (19 GW/cm2 @1064 nm, 1 ns) and high second order nonlinear coefficient. It has also the ability to establish birefringence phase matching condition for efficient SHG for different wavelengths. The crystal can be used for generation for SHG in broad wavelength range through angular phase matching method. For this, in this work detail investigation has been made to find out the optimum wavelength range that can be used for efficient SHG. This has been done by finding the variation of phase matching angle, second order nonlinearity etc. with respect to wavelength using the SNLO software. Both Type I and Type II phase matching has been considered for various planes of the crystal.

Silicon nitride waveguides with directly grown WS2 for efficient second-harmonic generation.

Different functions can be directly realized by silicon (Si) in integrated electronic circuits. Although Si and silicon nitride (Si3N4) photonics have shown great potential in integrated optoelectronic devices, different functions, such as light generation, transparency for guided light, and light detection, cannot be simultaneously achieved only by Si or Si3N4. Second-order nonlinearity is another optical property they do not possess due to their centrosymmetric properties. Several kinds of 2D materials emerged recently and were transferred to specified photonic devices aimed at improving their nonlinear performance. However, the transferring methods are time-consuming, unable to achieve large-scale production, and will inevitably cause materials damage and introduce impurities at the interface. Herein, we demonstrate the direct growth of large-area homogeneous monolayer WS2via a physical vapor deposition method onto Si3N4 waveguides. The WS2 growth can be controlled mainly along the Si3N4 waveguides and the waveguides show an obvious enhancement of second-harmonic generation with the elongated WS2 coverage. The direct growth of WS2 endows Si3N4 integrated photonics with new nonlinear optical properties. As an alternative method of transferring 2D materials, the method we present here is compatible with large-scale integrated photonic fabrication, which lays the foundation for on-chip integrated optical fabrication and applications.

Giant Second Harmonic Generation Enhancement in a High-Q Doubly Resonant Hybrid Plasmon-Fiber Cavity System.

A high-quality plasmon-fiber cavity in a doubly resonant configuration can exhibit second-harmonic generation (SHG) with over 5 orders of magnitude enhancement compared to gold nanoparticles on a fused silica substrate. Through coupling to a fiber cavity with the proper diameter, a high-quality (Q ≈ 160) resonance can be achieved in combination with a single gold nanoparticle. In a classical picture, where the incident electric field travels coherently Q times around the fiber during the nonlinear process, the high Q of the coupled mode aids in highly efficient SHG. We accomplish two feats: First, we analyze the Q factor dependence of the SHG efficiency, proving the expected Q4 dependence and thus confirming coherent E-field amplification in the fiber cavity. Second, we carefully adjust the fiber size further and tune the plasmon response of a gold nanoparticle to a high-Q cavity mode. We make sure that the second harmonic wavelength is simultaneously in resonance with a higher order fiber cavity mode, fulfilling the doubly resonant condition. As a result, a giant SH response with conversion efficiency up to 1.6 × 10-5 is detected upon a pump intensity of 5 × 108 W/cm2 for 100 fs pump pulses around 840 nm incident wavelength. Additionally, the importance of the doubly resonant condition is proven by detuning the size of the fiber, which leads to a drastic drop in SHG efficiency. This disparity of the SHG efficiency can be observed even by eye, when monitoring the intensity changes of the visible SH light during detuning.

Interlayer exciton mediated second harmonic generation in bilayer MoS2

Second-harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS2, that shows strong optical oscillator strength for the intra- but also interlayer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations. Since interlayer exciton transitions are highly tunable also by choosing twist angle and material combination our results open up new approaches for designing the SHG response of layered materials.

Experimental verification of electro-optical effect in LBO crystal based on nonlinear optical process

Lithium triborate (LBO) crystal is a widely used nonlinear crystal and a potential electro-optic (EO) crystal, but there was no report on experimental verification of its EO characteristics. We verified the linear EO effect of LBO crystal experimentally by designing a second harmonic generation (SHG) experiment. In the experiment, the direction of applied voltage was determined by the analysis of LBO EO tensor matrix. By measuring the curve of voltage-tuning SHG efficiency with different angle detuning, the linear EO effect was verified, and the linear EO coefficient was calculated by the slope of phase mismatch with respect to voltage.

Second Harmonic Generation in Generalized Ferroelectric Superlattices

The goal of the work presented in this investigation is to achieve the efficient parametric interaction of narrow beams (in particular second harmonic generation) using a nonlinear (ferroelectric-LiTaO3) photonic crystal tuned to self-collimation (nondiffractive) regimes. A numerical study of second harmonic generation (SHG) in one-dimensional nonlinear photonic crystals based on a full nonlinear system of equations, implemented by a combination of the method of finite elements and fixed-point iterations, is reported. This model is derived from a nonlinear system of Maxwell’s equations that partly overcomes the known shortcoming of some existing models that rely on the undepleted pump approximation. We derive a general solution of SHG in one-dimensional nonlinear photonic crystals structures. Numerical simulations also show that the conversion efficiency of SHG can be significantly enhanced when the frequencies of the fundamental wave are located at the photonic band edges or are assigned to the designed defect states.

Second Harmonic Generation in Germanium Quantum Wells for Nonlinear Silicon Photonics

Second-harmonic generation (SHG) is a direct measure of the strength of second-order nonlinear optical effects, which also include frequency mixing and parametric oscillations. Natural and artificial materials with broken center-of-inversion symmetry in their unit cell display high SHG efficiency, however, the silicon-foundry compatible group IV semiconductors (Si, Ge) are centrosymmetric, thereby preventing full integration of second-order nonlinearity in silicon photonics platforms. Here we demonstrate strong SHG in Ge-rich quantum wells grown on Si wafers. Unlike Si-rich epilayers, Ge-rich epilayers allow for waveguiding on a Si substrate. The symmetry breaking is artificially realized with a pair of asymmetric coupled quantum wells (ACQW), in which three of the quantum-confined states are equidistant in energy, resulting in a double resonance for SHG. Laser spectroscopy experiments demonstrate a giant second-order nonlinearity at mid-infrared pump wavelengths between 9 and 12 μm. Leveraging on the strong intersubband dipoles, the nonlinear susceptibility χ(2) almost reaches 105 pm/V, 4 orders of magnitude larger than bulk nonlinear materials for which, by the Miller’s rule, the range of 10 pm/V is the norm.

Second-harmonic generation from green printed selenium structures

Plenary large-scale efficient second-harmonic generation sources remains a challenge in micro-/nanoscale research. In this work, we demonstrate the strong second-harmonic generation in arrays of selenium particles fabricated by green printing method at the pumping wavelength of 1047 nm. Second-harmonic generation mapping shows the strong signal over the whole area of selenium particle. We propose, that such structures can be a perspective platform for efficient frequency conversion systems and nonlinear sensing applications.

Advances in theoretical, experimental and applied studies on second-harmonic generation of ultrasonic Lamb waves

<p indent=0mm>Nondestructive evaluation and testing of material degradation or micro-damages in an early stage are essential for the maintenance of the safety-critical structures. Therefore, it is important to detect defects in a very early stage or even better to detect changes in the microstructure before the defect initiation even occurs. Linear ultrasonic characteristics are not sufficiently sensitive to the kind of microscopic degradation of the material. However, on the other hand, even very small imperfections can produce very significant excess nonlinearity, which can be orders of magnitude higher than the intrinsic nonlinearity of the intact material. The excess nonlinearity is produced mainly by the strong local nonlinearity of microcracks. In order to assess the damage in components at an early stage, the use of nonlinear acoustics has been shown to be the leading and one of the most promising techniques. Second harmonic generation is a typical acoustic nonlinear effect of ultrasonic waves, which can be used to assess the material degradation and detect the micro-defects in their early stages. Furthermore, ultrasonic Lamb wave has been widely used in large-scale industrial structures for health monitoring and material nondestructive testing. Possibility to inspect inaccessible or hidden areas and great cost-effectiveness are the key advantages of ultrasonic Lamb wave-based damage detection approaches. Second harmonic Lamb wave, which combines the advantages of Lamb wave and the high sensitivity of second harmonic waves to micro damage, is drawing increasing attention in recent years. The measurement of second harmonic Lamb waves is investigated as a promising feature extraction technique to characterize material nonlinearity in plate-like structures. However, the second harmonic fields of Lamb waves are much more complex due to the dispersion and multi-mode nature of propagating Lamb waves. In general, the effect of second harmonic generation is often very small and can easily be overlooked experimentally. Consequently, proper mode tuning with physically based feature is essential to enhance the efficiency of second harmonic Lamb wave generation and reception. In experimental work, it is also difficult to observe and measure the second harmonic Lamb waves. Although significant development has been made in theoretical analysis and experimental investigation of second Lamb waves, the use of second harmonic Lamb wave for structural damage assessments is still limited by a series of scientific and technical difficulties. In this paper, the theoretical and experimental advances concerning second harmonic Lamb wave are systematically reviewed. First, theoretical analysis of second harmonic Lamb waves based on nonlinear reflection of acoustic waves is reviewed. Then, a general approach of perturbation method and normal modal expansion for theoretical study of the second harmonic Lamb waves propagation in a plate is introduced. The physical insight of nonlinear feature among different Lamb modes is clarified. As revealed in these pieces of theoretical work, the second harmonic modes of Lamb waves display an accumulative effect when conditions of phase matching and non-zero power transfer from the fundamental mode to the second harmonic mode are satisfied. The conditions for strong nonlinear effect of Lamb wave are discussed. Experimental observation and measurement of second harmonic Lamb waves are reviewed. Cumulative effect and efficiency of second harmonic generation of Lamb waves are discussed. The guideline for choosing the Lamb modes with higher sensitivity to material nonlinearity is provided. Based on the theoretical and experimental studies, the applications of second harmonic Lamb waves for material damage evaluation are summarized. In addition, some obscure understanding and disputed issues during the study in this research area are clarified. Finally, prospective priorities and research trends of second harmonic Lamb waves are discussed.

Crystal structure, DFT-study and NLO properties of the novel copper(I) nitrate π,σ-coordination compound based on 1-allyl-3-norbornan-thiourea

This work is directed toward the synthesis, structural characterization and NLO properties investigation of the novel copper(I) nitrate π,σ-coordination compound [Cu(Anor)(CH3CN)](NO3) (1) based on 1-allyl-3-norbornan-thiourea (Anor). In the crystal structure of both Anor and 1, the organic part is disordered because of the superposition of (2S)-exo and (2R)-exo enantiomers. The Anor ligand in 1 acts as a bridging ligand, connecting Cu(I) ions into the cationic coordination polymer {[Cu(Anor)(CH3CN)]+}n through η2-allyl group and the thiourea S atom. The H atoms of the –NH– groups in the Anor structure participate in NH···S hydrogen bonding and form an infinite hydrogen-bonded layer, while the same ligand groups in the organometallic chain of 1 are linked by bifurcated NH···O bonds to the two O atoms of the same NO3– anion. The second-order nonlinear optical properties for Anor and coordination compound 1 were studied at 1064 nm generated by a nanosecond pulsed Nd:YAG laser. The corresponding values of the average first-order NLO hyperpolarizabilities calculated by the DFT method and the B3LYP functional for the Anor ligand molecule and the cation fragment confirm the experimentally determined much higher second-harmonic generation efficiency for compound 1.

Second‐Harmonic Generation Based on the Dual‐Band Second‐Order Topological Corner States

Coherent frequency conversions using multiband topological edge states in photonic structures made of nonlinear optical materials have been extensively investigated. In practice, however, coherent sources localized at the subwavelength scale are of great importance. Herein, efficient second‐harmonic generation (SHG) using dual‐band second‐order topological corner states is reported. By properly adjusting the structural parameters, dual‐band corner states can be obtained in the first and second bandgaps, when the eigenfrequency of the corner state in the higher bandgap is twice that of the corner one in the lower gap. In a square lattice made of tellurium as an example, high SHG efficiency in the order of 10−1% can be obtained for a peak pump intensity of around 20 GW cm−2. The results presented provide the realizability of subwavelength‐scale coherent sources, which are robust against the structural disorder or defects.

Optimization of second harmonic generation in Au/Si core-shell nanoparticles

This study numerically investigated second harmonic generation (SHG) and the linear optical responses in core–shell nanoparticles comprising a gold core and silicon shell. The effects of the core radius and shell thickness were studied based on the resonance wavelength shifts, linear responses, and SHG efficiency in numerical simulations. By accurately setting the shell thickness, a model was designed where the resonance wavelength was almost constant relative to variations in the core radius for a specific shell thickness (i.e., the resonance wavelength did not shift). Thus, it was possible to optimize the efficiency of the linear optical responses and SHG in the core–shell nanoparticles by only changing the core radius. The desired efficiency can be achieved for this nanoparticle in specific applications by first tuning the resonance wavelength by changing the shell thickness and then by optimizing the efficiency by changing the core radius. The results obtained in this study may facilitate the development and optimization of optical nanodevices for different applications in nanophotonics and nano-optics, such as nanolasers and nanosensors.

Effects of Ce3+/Dy3+ and Ce3+/Sm3+ co-doping as a luminescent modifier in alumina-borophosphate glasses for w-LED application

Ce3+/Dy3+ and Ce3+/Sm3+ co-doped alumino-borophosphate glasses (CDABP/ CSABP) were prepared and characterized for luminescence and SHG efficiency properties. FT-IR and FT-Raman spectra have been utilized to identify different functional groups of Phosphate networks. Various optical properties such as, optical absorption, optical band gap energy (Eg), Urbach energy (ΔE) and linear refractive index (n) of the glasses were determined using UV–Vis spectrometer in the spectral range 200–1200 nm. Scanning Electron Microscopy/Energy Dispersive spectrometry (SEM/EDS) reveals information about the elemental compositional homogeneity as well as the surface morphology of the prepared glasses. Influences of a luminescence co-activator of Ce3+/Dy3+ and Ce3+/Sm3+ ions in alumina-borophosphate glasses were interpreted under the excitation wavelength of 300 nm. Two emission peaks detected at 426 nm (blue) and 574 nm (Yellow) in CDABP glass corresponding to the Dy3+, whereas three emission peaks traced at 558 nm (green), 598 nm (orange) and 652 nm, (red) in CSABP glass corresponding to the Sm3+. The Influence of internal and external quantum efficiency (QE) on the glass’s photoluminescence spectrum was measured. The CIE chromaticity obtained from the luminescence spectra, confirming that the CSABP glass emits white light. The colour purity (Pc) and excitation purity (Pe) of the glasses were investigated in order to quantify the quality of light emitted by them. The second harmonic generation (SHG) efficiency of the synthesized glasses was determined without the need of a thermal polling mechanism, and the magnitude was compared to that of Urea and KDP crystal reference samples.

High power drive laser system for photocathode at IHEP.

The photocathode drive laser system in the Institute of High Energy Physics (IHEP) has been upgraded. An all-fiber drive laser system has been developed using photonic crystal fibers and photonic crystal rods as the main gain medium. This system has been operated stably. The output infrared (IR) power reaches 116.2 W. The pulse width and maximum output power of the green laser generated by the second harmonic generation (SHG) are less than 2 ps and about 39.4 W, respectively. The SHG efficiency exceeds 60%. This paper introduces the development of the drive laser system and reports the measurement results of the performance test.