Second Harmonic Generation Efficiency Research Articles

Second-harmonic generation in nonlinear plasmonic lattices enhanced by quantum emitter gain medium.

We report on a theoretical study of second-harmonic generation (SHG) in plasmonic nanostructures interacting with two-level quantum emitters (QEs) under incoherent energy pump. We generalize the driven-dissipative Tavis-Cummings model by introducing the anharmonic surface plasmon-polariton (SPP) mode coupled to QEs and examine physical properties of corresponding SPP-QE polariton states. Our calculations of the SHG efficiency for strong QE-SPP coupling demonstrate orders of magnitude enhancement facilitated by the polariton gain. We further discuss time-domain numerical simulations of SHG in a square lattice comprising Ag nanopillars coupled to QEs utilizing a fully vectorial nonperturbative nonlinear hydrodynamic model for conduction electrons coupled to Maxwell-Bloch equations for QEs. The simulations support the idea of gain enhanced SHG and show orders of magnitude increase in the SHG efficiency as the QEs are tuned in resonance with the lattice plasmon mode and brought above the population inversion threshold by incoherent pumping. By varying pump frequency and tuning QEs to a localized plasmon mode, we demonstrate further enhancement of the SHG efficiency facilitated by strong local electric fields. The incident light polarization dependence of the SHG is examined and related to the symmetries of participating plasmon modes.

Wavelength-scale optical parametric oscillators

Despite recent progress in nonlinear optics in wavelength-scale resonators, there are still open questions on the possibility of parametric oscillation in such resonators. We present a general approach to predict the behavior and estimate the oscillation threshold of multi-mode subwavelength and wavelength-scale optical parametric oscillators (OPOs). As an example, we propose an OPO based on Mie-type multipolar resonances, and we demonstrate that due to the low- Q nature of multipolar modes in wavelength-scale resonators, there is a nonlinear interaction between these modes. As a result, the OPO threshold, compared to the single-mode case, can be reduced by a factor that is significantly larger than the number of interacting modes. The multi-mode interaction can also lead to a phase transition manifested through a sudden change in the parametric gain as well as the oscillation threshold, which can be utilized for enhanced sensing. We establish an explicit connection between the second-harmonic generation efficiency and the OPO threshold. This allows us to estimate the OPO threshold based on measured or simulated second-harmonic generation in different classes of resonators, such as bound states in the continuum and inversely designed resonators. Our approach for analyzing and modeling miniaturized OPOs can open unprecedented opportunities for classical and quantum nonlinear photonics.

A Polar Material with Facility of Crystal Growth and a Large Second-Harmonic Generation Response

Compounds with a polar space group are widely interesting as they usually exhibit polar properties, such as a second-harmonic generation (SHG) response. Herein, we describe a hexahydrated aluminum iodate, which crystallizes into the polar space group P63. As expected, this polar material is SHG-active, with a powder SHG efficiency of about 8.0× that of the benchmark KH2PO4 (KDP) at 1064 nm in the 30–40 μm particle size range. The UV–vis-NIR spectrum shows that AHIO is transparent from the UV region to the NIR region (0.31–1.63 μm). The measured laser-induced damage threshold (LIDT) of AHIO is 647MW/cm2 (1HZ, 10 ns, 1064 nm). Notably, AHIO has a good crystal growth behavior, and its large single crystals have been readily obtained from its aqueous solution.

Investigation of factors affecting on the efficiency of second harmonic ‎generation of the continuous-wave focused Gaussian beam of 1064 nm inside the ‎periodically poled nonlinear crystals

In this research, the nonlinear propagation equations of the fundamental and second harmonic waves coupled to the heat equation are solved for a focused Guassian beam inside two quasi-phase matched periodically poled MgO: PPLN and MgO: PPLT crystals, and the optimum focal length and radius of the continuous-wave laser beam with the wavelength of 1064 nm and with Gaussian profile, and also, the optimal phase-mismached parameter are calculated and the dependence of the second harmonic efficiency on these parameters is shown. Also, the effects of the thermal distribution inside the crystal on the second harmonic generation efficiency are investigated and it is shown how the thermal distribution reduces the second harmonic efficiency and, accordingly, the optimal crystal length for different incident powers is determined and shown that the lower crystal lengths should be selected for higher incident powers. Finally, a comparison between the second harmonic efficiencies in two crystals of MgO: PPLN and MgO: PPLT is performed considering the thermal distribution for different insident powers and it is shown that for the lower/ higher incident power than 21 watts, the crystal of MgO: PPLN / MgO: PPLT is more suitable and more efficient for second harmonic generation.

Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces.

Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high Q-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative Q-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter α of the system. Furthermore, high Q-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a Q-factor of ∼8×104, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of 3.3kW/cm2. This suggests that such high Q-factor LN metasurfaces may be good candidates for practical blue-ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.

Influence of l-Phenylalanine doping on potassium dihydrogen phosphate: crystal growth, structural, optical and mechanical traits

Potassium dihydrogen phosphate (KH2PO4; KDP) is a well known nonlinear optical (NLO) material for optoelectronic, laser technology and fiber optical communication device applications. In a view to yeild better device properties, l-Phenylalanine (l-Phe) aminoacid doped KDP (LKDP) crystals with different dopant concentrations (0.5 mol%, 1 mol% and 2 mol %) were grown by slow evaporation solution technique (SEST). The grown crystals were powdered of micron size crystallites and subjected to powder X-ray diffraction (PXRD). In PXRD spectra, a few peaks with increasing intensity were observed in LKDP crystals when compared to pure KDP, revealing the effect of incorporation of dopants in the crystalline matrix of KDP and the enhancement of growth rate along [100] direction. FT-IR spectrum confirms the incorparation of dopant in KDP crystalline matrix with the presence of different vibrational modes of assignments belongs to L-Phe. To study the crystalline perfection of pure and doped crystals, the high-resolution X-ray diffraction (HRXRD) was employed and the crystalline perfection was found to be reasonably good with relatively low values of FWHM. UV–Vis-NIR spectroscopy results show significant increase in optical transmission up to ~ 80% from ~ 60% (pure KDP) in the entire visible spectrum of l-Phe doped KDP at low doping concentration. However, it was decreased to ~ 70% when we increase the dopant concentration by 2 mol% and suggests that if we increase the dopant (l-Phe) concentration above an optimal value in KDP, the transmission decreases. The Photoluminescence (PL) spectrum reveals high crystalline perfection at low concentration of dopants as observed by the sharp peaks. The weak green emission peak for sample with higher dopant concentrations indicates the segregation of doped molecules at the structural grain boundaries as observed in the HRXRD studies. The mechanical strength of pure and doped KDP crystals with increasing dopant concentration has been studied and found that the hardness was increased slightly due to doping. The powdered sample of micron size particle made from as grown crystals were subjected to Kurtz powder technique to observe the relative second harmonic generation (SHG) efficiency in comparison with the pure KDP. These results reveal significant enhancement of SHG efficiency with increasing doping concentration up to 2 mol% in KDP crystals which is 1.31 times to that of pure KDP.

Enhanced Nonlinear Emission from Single Multilayered Metal–Dielectric Nanocavities Resonating in the Near-Infrared

Harmonic generation mechanisms are of great interest in nanoscience and nanotechnology, since they allow generating visible light by using near-infrared radiation, which is particularly suitable for its endless applications in bio-nanophotonics and opto-electronics. In this context, multilayer metal-dielectric nanocavities are widely used for light confinement and waveguiding at the nanoscale. They exhibit intense and localized resonances that can be conveniently tuned in the near-infrared and are therefore ideal for enhancing nonlinear effects in this spectral range. In this work, we experimentally investigate the nonlinear optical response of multilayer metal-dielectric nanocavities. By engineering their absorption efficiency and exploiting their intrinsic interface-induced symmetry breaking, we achieve one order of magnitude higher second-harmonic generation efficiency compared to gold nanostructures featuring the same geometry and resonant behavior. In particular, while the third order nonlinear susceptibility is comparable with that of bulk Au, we estimate a second order nonlinear susceptibility of the order of 1 pm/V, which is comparable with that of typical nonlinear crystals. We envision that our system, which combines the advantages of both plasmonic and dielectric materials, might enable the realization of composite and multi-functional nano-systems for an efficient manipulation of nonlinear optical processes at the nanoscale.

Inversion symmetry broken in 2H phase vanadium-doped molybdenum disulfide.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received much attention in nonlinear optical applications due to their unique crystal structures and second harmonic generation (SHG) efficiency. However, SHG signals in TMDs show a layer-dependent behavior, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer) of TMDs. Herein, we synthesized monolayer and bilayer 2H and 3R phase vanadium (V)-doped MoS2 crystal. Raman spectroscopy, XPS, and STEM were used to identify the chemical composition and crystalline structure of as-grown nanoflakes. SHG measurement was used to research the symmetry of V-doped MoS2 crystals with different stacking orders. Significantly, the SHG efficiency in bilayer 2H phase V-doped MoS2 is equivalent to the 3R phase, indicating an inversion symmetry broken lattice structure caused by the in situ V substitute for Mo sites. This study will be conducive to promote the development of promising nonlinear optical devices based on 2D material.

Расчет эффективности удвоения частоты излучения субтерагерцового гиротрона за счет решеточной нелинейности в монокристаллической пластине InP

In this work, we calculated the efficiency of second harmonic generation for sub-THz gyrotron radiation in a single-crystal InP plate embedded in a conical metal waveguide. For the values of the losses at the doubled frequency (depending on the degree of purity of the InP crystal), the dimensions of the wafer are revealed that provide the optimal effective frequency conversion. Thus, the conversion factor f –>2f can reach 5 W/kW^2 when pumped at a frequency of 263 GHz and a loss of alpha_2f = 0.1 cm^-1. The possibility of a noticeable increase in the efficiency of frequency doubling with fine tuning of the phase matching due to the selection of the size of the submicron gap between the InP plate and the plates of the metal waveguide is shown. The results obtained can be used to solve applied problems in the field of dynamic nuclear polarization spectroscopy and nuclear magnetic resonance studies.

Effect of malachite green dye doping in tristhiourea zinc (II) sulphate single crystal—a potential nonlinear optical material

Tristhiourea zinc (II) sulphate (ZTS) is a semi-organic and highly nonlinear optical material used for second harmonic generation (SHG) and other nonlinear optical (NLO) applications. The good quality of pure ZTS (PZTS) and 0.05 mol% and 0.1 mol% malachite green (MG) dye-doped ZTS crystals were harvested by employing low cost effective slow evaporation solution growth technique. The as-grown crystals were subjected to different characterization techniques to see the effect of MG doping on structural, mechanical, thermal, optical and NLO properties. The sharp and intense peaks observed in powder X-ray diffraction and high-resolution X-ray diffraction of PZTS and doped ZTS confirmed the good crystalline quality of pure and doped ZTS. A comparison of FT-IR spectroscopy studies of pure and doped crystals confirmed the incorporation of MG dopant in ZTS from the observation of shifting of various absorption bands. The optical bandgap energies and cut off wavelengths for pure and doped ZTS crystals were studied using UV–Vis.-NIR spectroscopy over an extended spectral range of 200–1100 nm at room temperature. The photoluminescence study revealed that microscopic defects like vacancies and self interstitials are reduced. Thermal stability of the grown crystals were studied by thermogravimetric analysis and found that the thermal stability is not affected much by doping. Vicker’s hardness test was employed to pure and MG-doped ZTS crystals and elucidated that the doped ZTS crystals were found to be slightly softer than pure ZTS. The frequency dependent dielectric constants i.e. ɛr (real), ei (imaginary), dielectric loss (tan δ) and ac electrical conductivity (σac) of pure and doped ZTS crystals were studied by subjecting the grown crystals with impedance analyzer. To observe the NLO property, the grown crystals were subjected to Kurtz powder technique. The results of SHG studies expounded that the SHG efficiency was increased by 2.75 times with MG doping in ZTS crystals.

Growth, ultraviolet‐visible, vibrational, photoluminescence and second harmonic generation studies of Amaranth and EDTA co‐doped KDP crystals

KDP (Potassium dihydrogen phosphate) single crystals were grown from Amaranth dye and EDTA (Ethylenediaminetetraacetic acid disodium salt) mixed KDP solutions by the temperature lowering method and Sankaranarayanan-Ramasamy technique. The molar concentration of EDTA in the solutions was 1 mol%. The Amaranth concentrations were 0.001, 0.005, 0.01, 0.03 and 0.05 mol%. The growth of KDP crystals was oriented in type I and type II phase-matching directions which bring maximum second harmonic generation (SHG) efficiency to the crystals. Optical properties of Amaranth and EDTA co-doped KDP crystals (KAE) were investigated by Ultraviolet-Visible (UV-Vis) spectroscopy, FT-Raman spectroscopy, and photoluminescence. SHG efficiency of the crystals was measured by shining 1064 nm Nd:YAG laser directly into them and receiving output signals. The most important result in the study showed that KAE crystals had SHG efficiency of 1.7 and 2.4 times higher than EDTA doped KDP crystal (KE) and pure KDP crystal respectively. Then second-order nonlinear optical coefficients of these crystals were calculated. The simultaneous doping of two organic substances into KDP crystals has further enhanced their nonlinear optical characteristics.

Second and third harmonic generation from gold nanolayers: experiment versus theory

The use of semiconductors, metals, or ordinary dielectrics in the process of fabrication of nanodevices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. Understanding accurately how light interacts with these materials at the nanoscale is crucial if one is to properly engineer nano-devices. When the nanoscale is reached, light-matter interactions display new phenomena where conventional approximations may not always be applicable and they should be either revised or voided. In this work, we measure the efficiency of second and third harmonic generation from gold nanolayers. The experimental results are compared with numerical simulations based on a detailed microscopic hydrodynamic model that considers different effects playing a role in the nonlinear response, not usually considered by more generic models. The agreement between experimental and theoretical results proves the importance of all these contributions.

Effect of dimension variation for second-harmonic generation in lithium niobate on insulator waveguide [Invited

We study the effect of dimension variation for second-harmonic generation (SHG) in lithium niobate on insulator (LNOI) waveguides. Non-trivial SHG profiles in both type-0 and type-I quasi-phase matching are observed during the wavelength tuning of the fundamental light. Theoretical modeling shows that the SHG profile and efficiency can be greatly affected by the waveguide cross-section dimension variations, especially the thickness variations. In particular, our analysis shows that a thickness variation of tens of nanometers is in good agreement with the experimental results. Such investigations could be used to evaluate fabrication performance of LNOI-based nonlinear optical devices.

Structural, mechanical, thermal, optical and antifungal properties of pure and nickel doped ninhydrin non liner single crystals

A conventional method was adopted to prepare pure ninhydrin (NH) and nickel doped ninhydrin (N1NH) single crystals. The present investigation focused on various properties of as grown NH and NINH crystals. Monoclinic crystal structure with P21 space group which is attractive phenomena were noticed in structural mode of grown NH and NINH crystals. Various vibrations of functional groups related to NH and nickel doped NINH crystals were assigned from Fourier-transform infrared peaks of good resolution. Energy-dispersive X-ray spectrum presents the percentage of carbon, oxygen and nickel occurs in the sample. The strength of the samples have deliberated with the help of Vicker’s microhardness tester. The decomposition point of both NH and N1NH crystals has been measured by thermal analysis. The dielectric constant of both NH and N1NH crystals was also determined. Green emission which denotes second harmonic generation (SHG) process for the samples generally confirms the nonlinear optical (NLO) activity of the NH and N1NH crystals and it has been tested successfully based on the technique of Kurtz-Perry. The SHG efficiency values of NH and N1NH samples are comparable with KDP which has been used as reference sample as it is being used commercially. Basically, NH and NINH crystal samples were used for many medical applications. Therefore, the antifungal activity was also focused for investigation against two famous fungi like Aspergillus niger, Aspergillus flavus.

Robust modal phase matching in subwavelength x-cut and z-cut lithium niobate thin-film waveguides

We use the nonlinear coupled-mode theory to theoretically investigate second-harmonic generation (SHG) in subwavelength x-cut and z-cut lithium niobate (LN) thin-film waveguides and derive the analytical formula to calculate SHG efficiency in x-cut and z-cut LN thin-film waveguides explicitly. Under the scheme of optimal modal phase matching (MPM), two types of LN thin films can achieve highly efficient frequency doubling of a 1064 nm laser with a comparable conversion efficiency due to very consistent modal field distribution of the fundamental wave and second-harmonic wave with efficient overlap between them. Such a robust MPM for high-efficiency SHG in both the subwavelength x-cut and z-cut LN thin-film waveguides is further confirmed in a broad wavelength range, which might facilitate design and application of micro–nano nonlinear optical devices based on the subwavelength LN thin film.

Inch-Scale Ball-in-Bowl Plasmonic Nanostructure Arrays for Polarization-Independent Second-Harmonic Generation.

Second-harmonic generation (SHG) in plasmonic nanostructures has been investigated for decades due to their wide applications in photonic circuit, quantum optics and biosensing. Development of large-scale, uniform, and efficient plasmonic nanostructure system with tunable modes is desirable for their feasible utilizations. Herein, we design an efficient inch-scale SHG source by a solution-processed method instead of traditional high-cost processes. By assembling the gold nanoparticles with the porous anodic alumina templates, multiresonance in both visible and near-infrared regions can be achieved in hexagonal plasmonic nanostructure arrays, which provide strong electric field enhancement at the gap region. Polarization-independence SHG radiation has been realized owing to the in-plane isotropic characteristic of assembled unit. The tilt-angle dependent and angle-resolved measurement showed that wide-angle nonlinear response is achieved in our device because of the gap geometry of ball-in-bowl nanostructure with nonlinear emission electric dipoles distributed on the concave surface, which makes it competitive in practical applications. Our progress not only makes it possible to produce uniform inch-scale nonlinear arrays through low-cost solution process; and also advances the understanding of the SHG radiation in plasmonic nanostructures.

Chiral Template‐Driven Macroscopic Chirality Control: Structure‐Second‐Harmonic Generation Properties Relationship

AbstractChirality is one of the most important topics in chemistry attributable to the peculiar stereochemical properties. Since the chirality belongs to one of the noncentrosymmetric (NCS) crystal classes, chiral materials may reveal second‐harmonic generation (SHG) properties, one of the unique structure‐related properties. The high efficiency of SHG can be achieved by the well‐controlled alignment of NCS chromophores such as d0 transition metal cations in distorted octahedra, lone pair cations, highly polarizable d10 cations, π‐delocalized anionic groups, etc. Chiral organic structure‐directing agents can also serve as chiral templates by aligning inorganic units in hybrid solid‐state materials. In this Minireview, recently reported ten chiral materials containing both chiral templates and SHG active unsymmetrical metal cations are introduced. The relationship between the asymmetric arrangement of the NCS chromophores achieved by the interaction with chiral templates and the resulting SHG properties are investigated, which might provide further insight for designing novel chiral materials.

Realizing high efficiency 532 nm laser by optimizing the mode- and impedance-matching

Increasing the conversion efficiency of second harmonic generation (SHG) is an area of interest in research. We report a high-efficiency 532 nm laser generation, with a conversion efficiency of 94.04 ± 0.115% from the pump depletion of 98.1% ± 0.1%, by accurately quantifying the round-trip loss and the transmissivity of the input mirror using our proposed scheme. The optimal conversion efficiency of the cavity-enhanced frequency doubling process is independent of the waist and is determined by the pump depletion, round-trip loss, and transmissivity of the input mirror. These results show that the cavity-enhanced frequency doubling process is not necessary to set the focusing parameter at the optimal single-pass conversion. These results provide a guide for future research on high-efficiency SHG.

Phase-matched optical second harmonic generation in a hyperbolic metamaterial based on silver nanorods

Artificial hyperbolic metamaterials (HMM) are perspective for the nonlinear optical applications, their exciting functionality being due to the hyperbolic dispersion induced by a strong shape anisotropy. Here we study the second harmonic generation (SHG) in HMM formed by arrays of silver nanorods in anodic alumina. In the hyperbolic dispersion regime, giant SHG is observed associated with the fulfillment of the phase-matching conditions supported by the epsilon-near-pole resonance at the SHG wavelength, and an increase in the pump field in the epsilon-near-zero (ENZ) spectral range. We predict a strong increase in the SHG efficiency for metamaterials with the ENZ resonance at the second harmonic frequency.

Molecular structure, Hirshfeld surface and density functional theoretical analysis of a NLO active chalcone derivative single crystal—A quantum chemical approach

Nonlinear optically active thienyl chalcone, with molecular formula C16H15ClO4S has been synthesized by adopting claisen-schimdt condensation reaction method. FT-IR and NMR spectral study was used to identify the functional groups present in the newly synthesized compound and confirm the molecular structure. Based on the solubility study DMF is chosen as solvent to grow single crystals of the compound using slow evaporation (solution growth) technique. The single crystal XRD study reveals that the grown crystal belongs to the orthorhombic crystal system with Pbca space group and the associated cell parameters are, a = 16.6575 (6) Å, b = 8.1147 (3) Å, c = 23.6618 (9) Å, α = β = γ = 90°. The linear optical properties were studied using UV-–visible, Photoluminescence method. The energy band gap of the material was calculated using tauc's plots. The refractive index (1.47) of the crystal was determined using Brewster's angle method. Second harmonic generation (SHG) test was carried out using Kurtz and Perry powder technique and SHG efficiency of 1.82 times that of urea was estimated. The thermal stability of the compound was tested by thermo gravimetric and differential thermal analysis. Various inter contacts present within the crystal were visualized through 3-D molecular hirshfeld surface analysis and their percentage contribution towards the total hirshfeld surface area has been quantized using 2D finger print plot method. The theoretical investigation was carried out using DFT method and HOMO-LUMO energy band gap was determined. The different global chemical reactivity descriptors (GCRDs) parameters were calculated. The molecular electrostatic potential map of the molecule was analyzed and the sites of electrophilic and nucleophilic attacks were predicted. The molecular NLO properties such as static dipole moment, mean polarizability, anisotropy of polarizability and first order hyper polarizability values were theoretically determined.