Second Harmonic Generation Research Articles

Evolution of the surface phase transitions in IrTe2

The phase transitions in IrTe2 have been extensively studied but the symmetry at each phase is yet to be settled. Employing second harmonic generation (SHG) measurements over a temperature range of 4 –300 K, we probe the evolution of the symmetry of IrTe2. Our results indicate shifts in two distinct transition temperatures (Ts1 and Ts2 with Ts1 > Ts2) through thermal cycling, providing an explanation for the variations of reported values in literature. The SHG polarimetry measurements identify symmetries in different temperature ranges, confirming the trigonal symmetry above Ts1, the triclinic symmetry between Ts1 and Ts2, and the coexistence of multiple stripe phases below Ts2. The most striking feature is the reemergence of a trigonal phase as reflected by six-fold symmetry below ~ 10 K which is likely responsible for phenomena observed at low temperatures.

Temperature Effects on the Optical Properties of Bismuth Nanoparticles Prepared by PLAL for Antibacterial Activity

Bismuth nanoparticles (Bi NPs) constitute a promising technology for combating infectious illnesses and bacterial resistance to antibacterial treatments. Bi NPs were synthesized by Pulsed Laser Ablation in Liquid (PLAL) using a 532 nm second-harmonic generation Nd:YAG laser. Bi NPs samples were prepared in 10, 35, 50, 75, and 90 ˚C water for various laser energies and number of laser pulses. Ultraviolet-visible (UV-Vis) spectra measurements revealed a characteristic plasmonic absorption band indicative of metallic bismuth nanosized particles. Field emission scanning electron microscopy (FESEM) analysis showed that the water temperature during the ablation process affected the size and morphology of Bi NPs. The average Bi NP size at 60 ˚C was 28 nm, considerably smaller than the 55 nm average observed at 10 ˚C. The antimicrobial properties of Bi NPs against two opportunistic pathogens, E. coli and S. aureus, were assessed. For S. aureus, the inhibition zone of Bi NPs prepared at 60 ˚C was greater than that of samples prepared at 10 ˚C. Further, Bi NPs exhibited lower potency against E. coli than S. aureus in both samples.

Designing a 2D van der waals oxide with lone-pair electrons as chemical scissor

Abstract 2D van der Waals (vdW) materials are known for their intriguing physical properties, but their rational design and synthesis remain a great challenge for chemists. In this work, we successfully synthesized a new non-centrosymmetric oxide, i.e. InSbMoO6, with Sb3+ lone-pair electrons serving as chemical scissor to generate its 2D vdW crystal structure. Monolayer and few-layer InSbMoO6 flakes are readily obtained via facile mechanical exfoliation. They exhibit strong second-harmonic generation (SHG) response with an effective second-order nonlinear optical susceptibility $\chi _{{\rm{eff}}}^{{\rm{(2)}}}\ $of 32.4 pm·V−1. Meanwhile, the SHG response is in-plane anisotropic and directly proportional to the layer thickness, independent of layer parity. In addition, the InSbMoO6 flakes exhibit excellent thermal and atmospheric stability, along with pronounced anisotropy in Raman spectroscopy. This work implies that using lone-pair electrons as chemical scissor is an effective strategy for designing and synthesizing new 2D vdW materials for integrated photonic applications.

Switchable Photovoltaic Effect and Robust Nonlinear Optical Response in a High-Temperature Molecular Ferroelectric [C8N2H22][PbI4

Hybrid organic-inorganic molecular ferroelectrics (HOIMFs) have garnered significant attention for their potential applications in nonvolatile memory and spintronic devices. However, few efforts have been devoted to the photoelectric properties of lead halide molecular ferroelectrics, despite the fact that robust ferroelectricity and flexibility are desirable for thin-film photoelectric devices. Herein, we present a novel lead halide molecular ferroelectric [C8N2H22][PbI4] (1) synthesized hydrothermally. A polar monoclinic structure of 1 was solved by single-crystal X-ray diffraction and second-harmonic generation (SHG) tests. A direct band gap of 2.36 eV was confirmed by UV-vis spectrum and theoretical calculation. Hysteresis measurements demonstrated inherent room-temperature (RT) ferroelectricity in 1 with a spontaneous polarization (Ps) of 3.2 μC/cm2. The 1-based photoelectric device shows a notable photovoltaic (PV) effect with Voc ∼ 0.27 V, Jsc ∼ 38 nA/cm2 under AM 1.5 G illumination, and a rapid response time of ∼1.5 ms. A considerable enhancement in PV performance has been achieved by adjusting the ferroelectric polarization, resulting in a maximum Voc ∼ 0.75 V, Jsc ∼ 2.28 μA/cm2. Notably, 1 exhibits a rather large SHG signal, which is approximately 2.61-fold higher than that of KH2PO4 (KDP) upon a 1064 nm laser radiation. This study offers a bright avenue for lead halide molecular ferroelectrics as promising optoelectronic devices and SHG materials.

Microscopic view on the polarization-resolved S-SHG intensity of the vapor/liquid interface of pure water.

Second harmonic generation (SHG) is a nonlinear optical phenomenon where two photons at the frequency ω combine to form a single photon at the second-harmonic frequency 2ω. Since that second-order process is very weak in bulk isotropic media, optical SHG responses of interfaces provide a powerful and versatile technique to probe the molecular structure and dynamics of liquid interfaces. Both local dipole contributions and non-local quadrupole contributions can be interesting to investigate different properties of the interface, such as the molecular orientation or the charge density. However, a major difficulty is to comprehend the link between the S-SHG intensity and molecular details. This article reports a numerical approach to model the polarization-resolved SHG intensities of a model vapor/liquid interface of pure water. The influence of the interfacial local environment on the hyperpolarizability is taken into account using quantum mechanical/molecular mechanics calculations. The numerical predictions are in very good agreement with experiments. We detail the hypotheses made during the modeling steps and discuss the impact of various factors on the modeled SHG intensities, including the description of the exciting field in the interfacial layer, the effect of neighboring molecules on the second-harmonic polarization, and the presence of an additional static electric field at the interface.

Harnessing physics-guided neural networks for tailoring the orbital angular momentum spectrum in second harmonic generation

The management of orbital angular momentum (OAM) in frequency conversion processes is essential for numerous applications such as quantum and classical optical communications. This paper presents a wavefront modulation approach for the fundamental beam in second harmonic generation (SHG) to efficiently control the OAM spectrum. We employ an inverse design method to derive the necessary wavefront shape of the fundamental beam for achieving a desired SHG OAM spectrum. Specifically, we introduce an efficient inverse design technique based on physics-guided neural networks (PGNNs) that incorporates the coupled equations governing SHG, aimed at tailoring the OAM spectrum of SHG. Utilizing the proposed PGNN, we design the phase pattern for a spatial light modulator (SLM) to shape the wavefront of the fundamental beam. Furthermore, we present a novel loss function, to our knowledge, that effectively links the OAM of the SHG spectrum and efficiency to the SLM phase pattern and crystal temperature, independent of empirical weight coefficients. The proposed PGNN facilitates the purification of the SHG OAM spectrum, even when the fundamental beam comprises mixed Laguerre–Gaussian (LG) modes. Additionally, we demonstrate the generation of desired SHG spectra using the proposed PGNN framework. This study introduces what we believe to be a groundbreaking inverse design method for developing photonic devices with customized functionalities, addressing challenges associated with traditional data-driven deep learning techniques.

Individual domain characteristics determined by second-harmonic generation diffractometer for multi-domain multiferroics

We investigated the multi-domain states of a multiferroic La-doped BiFeO3 (BLFO) thin film by examining diffraction patterns in optical second-harmonic generation (SHG) measurement. By directing a laser onto the domain wall within the domain-patterned sample, we observed clear diffraction signatures of SHG waves generated from two ferroelectric domains. We explained the experimental results of the diffraction patterns, including the intensity distribution and the polarization characteristics, using Fresnel propagation of SHG waves. From this, we could determine the amplitude and phase of the SHG waves generated from each domain, and figure out not only the polarization direction of each ferroelectric domain but also the phase related to the complex-valued second-order susceptibility tensor. Consequently, we could present SHG diffractometry as an effective measurement method to reveal the phase details of electric polarization of the multi-domain states of ferroic materials.

Two-Dimensional Lead Iodide Perovskites with Extremely Reduced Dielectric Confinement: Embedded Self-Erasing Second-Harmonic Generation Switching, Thermochromism, and Photoluminescence

Two-Dimensional Lead Iodide Perovskites with Extremely Reduced Dielectric Confinement: Embedded Self-Erasing Second-Harmonic Generation Switching, Thermochromism, and Photoluminescence

Enhancement of second-harmonic generation in a lithium niobate metasurface by exploring the bound states in the continuum

The nonlinear lithium niobate (LiNbO3), characterized by transparency across a broad spectral range from ultraviolet to mid-infrared, stands out as an ideal material for second-harmonic generation (SHG). The concept of bound states in the continuum (BIC) represents a non-radiative mode embedded within the radiation continuum, offering the capability to confine the electromagnetic field within the nanostructure. Here, we propose the design of a LiNbO3 metasurface utilizing the BIC mechanism to enhance SHG at fundamental wavelengths above 2 µm, which injects new thoughts into the field of integrated optics and on-chip photonics. Notably, we rigorously accounted for the influence of the side-wall angle θ and the corner rounding radius R of the LiNbO3 metasurface, which arises from fabrication tolerances. Considering those influences in the simulation, we achieved a quasi-BIC (q-BIC) with a quality factor (Q-factor) up to 1.44 × 105. Moreover, by considering the depleted pump model, the absolute efficiency of SHG reached 4.09% with a corresponding normalized efficiency of 8.19 × 10−10m2/W under a radiation intensity of 5 kW/cm2. Our research aims to establish a predictive framework through numerical simulations, specifically addressing realistic fabrication problems. This approach is intended to optimize the parameters for LiNbO3 metasurface fabrication, ensuring that the subsequent experiment efforts are efficient. Our findings provide an approach to predicting the optical response in actual structures and inspire new applications in photonics.

Magnetic properties and linear negative magnetocapacitance in rutile like LiFe2SbO6

Abstract We report the high-temperature magnetic ordering and the observation of linear negative magnetocapacitance mediated by a low-temperature magnetic phase crossover in the compound LiFe2SbO6. The combined observations of the absence of a second harmonic generation (SHG) signal, and Rietveld refinements of X-ray and neutron powder diffraction (NPD) patterns of the sample confirm that LiFe2SbO6 crystalizes in a centrosymmetric Pnnm structure. The DC magnetization measurements reveal that an antiferromagnetic ordering sets in at high temperature (TN= 850). The magnetic anomaly in the DC magnetization at a lower temperature T = 13 K, is corroborated by AC susceptibility and the specific heat measurements. We observe a linear negative magnetocapacitance below T = 13 K, demonstrating an indirect magneto-dielectric coupling. Further, the neutron powder diffraction study reveals a collinear antiferromagnetic ordering with a wavevector k = (0, 0, 0).

InGaP χ(2) integrated photonics platform for broadband, ultra-efficient nonlinear conversion and entangled photon generation

Nonlinear optics plays an important role in many areas of science and technology. The advance of nonlinear optics is empowered by the discovery and utilization of materials with growing optical nonlinearity. Here we demonstrate an indium gallium phosphide (InGaP) integrated photonics platform for broadband, ultra-efficient second-order nonlinear optics. The InGaP nanophotonic waveguide enables second-harmonic generation with a normalized efficiency of 128, 000%/W/cm2 at 1.55 μm pump wavelength, nearly two orders of magnitude higher than the state of the art in the telecommunication C band. Further, we realize an ultra-bright, broadband time-energy entangled photon source with a pair generation rate of 97 GHz/mW and a bandwidth of 115 nm centered at the telecommunication C band. The InGaP entangled photon source shows high coincidence-to-accidental counts ratio CAR > 104 and two-photon interference visibility > 98%. The InGaP second-order nonlinear photonics platform will have wide-ranging implications for non-classical light generation, optical signal processing, and quantum networking.

Visible-wavelength and monostatic coherent transceiver with independent second harmonic generation on transmitted and local lights using commercially available lithium niobate phase modulator for fundamental wavelength

We propose what we believe to be a new monostatic coherent transceiver at visible wavelength using a commercially available lithium niobate (LiNbO3) phase modulator (LNPM), which is capable of high-speed, wide-bandwidth, and multi-functional operation. The proposed coherent transceiver needs successful heterodyne detection with independent second harmonic generation (SHG) on transmitted and local lights. Experimental proof of the above-mentioned heterodyne detection is shown. The proof of concept for the application to continuous-waves coherent Doppler lidar (CW-CDL) is shown in both free-space and underwater sensing. Preliminary experiment regarding high-speed and wide-bandwidth modulation is additionally shown.

The influence of spin-orbit coupling on the electronic structure and optical properties of α-BiB3O6 crystals: a first-principles investigation

The α-BiB3O6 (BIBO) crystal is highly regarded in the field of nonlinear optical (NLO) due to its excellent ultraviolet (UV) NLO properties, including a short ultraviolet cutoff edge (270 nm), appropriate birefringence (Δn = 0.175@546 nm), and strong second harmonic generation (deff = 3.20 pm V−1). In this study, the first-principles computational software PWmat was employed to conduct theoretical calculations on BIBO crystals, including the non-negligible spin-orbit coupling (SOC) effects induced by the presence of the heavy element bismuth (Bi). The results show that after the inclusion of the SOC, the calculated birefringence (0.173@546 nm) and effective second harmonic generation (SHG) coefficient (deff = 3.16 pm V−1) are in good agreement with experimental data. Furthermore, comparative analysis has demonstrated that the changes in the band structure induced by the SOC effect are significant factors influencing the optical properties of the material.

Investigating pump harmonics generation in a SNAIL-based traveling wave parametric amplifier

Abstract Traveling wave parametric amplifiers (TWPAs) are extensively employed in experiments involving weak microwave signals for their highly desirable quantum-limited and broadband characteristics. However, TWPAs’ broadband nature comes with the disadvantage of admitting the activation of spurious nonlinear processes, such as harmonics generation, that can potentially degrade amplification performance. Here we experimentally investigate a Josephson TWPA device with superconducting nonlinear asymmetric inductive element-based unit cells focusing on the amplification behaviour along with the generation of second and third harmonics of the pump. By comparing experimental results with transient numerical simulations, we demonstrate the influence of Josephson junctions’ fabrication imperfections on the occurrence of harmonics and on the gain behaviour.

Structure and optical properties of alkali rare-earth double phosphates of M3RE(PO4)2 (M = K, Rb; RE = Y, La, and Lu)

Designing and synthesizing phosphate nonlinear optical materials with novel structures and excellent properties remains challenging. In this work, the electronic structure and optical properties (second harmonic generation (SHG) response or birefringence) of alkali rare-earth double phosphates M3RE(PO4)2 (M = K, Rb; RE = Y, La, and Lu) are systematically investigated. It is worth mentioning that P31m-Rb3Lu(PO4)2 was successfully synthesized by a flux-method, and the UV–vis–NIR diffuse reflectance spectroscopy showed that Rb3Lu(PO4)2 exhibits a short absorption edge at 207 nm and a high transmittance of 77.9 %. Based on first-principles calculations, the results show that the birefringence of the M3RE(PO4)2 series of crystals exhibits significant differences (0.009-0.028@1064 nm). Subsequently, analysis of the electronic structure and the real-space atom cutting method reveals that rare-earth polyhedra play a major role in enhanced birefringence. Overall, this work enriches the study of nonlinear optical crystals of rare earth phosphates and provides a case for further exploration.

Enhanced Nonlinear Optical Responses in MoS2 via Femtosecond Laser‐Induced Defect‐Engineering

Abstract2D materials are a promising platform for applications in many fields as they possess a plethora of useful properties that can be further optimized by careful engineering, for example, by defect introduction. While reliable high‐yield defect engineering methods are in demand, most current technologies are expensive, harsh, or non‐deterministic. Optical modification methods offer a cost‐effective and fast mechanism to engineer the properties of 2D materials at any step of the device fabrication process. In this paper, the nonlinear optical responses of mono‐, bi‐, and trilayer molybdenum disulfide (MoS2) flakes are enhanced by deterministic defect‐engineering with a femtosecond laser. A 50‐fold enhancement in the third harmonic generation (THG) and a 3.3‐fold increase in the second harmonic generation (SHG) in the optically modified areas is observed. The enhancement is attributed to resonant SHG and THG processes arising from optically introduced mid‐band gap defect states. These results demonstrate a highly controllable, sub‐micrometer resolution tool for enhancing the nonlinear optical responses in 2D materials, paving the way for prospective future applications in optoelectronics, quantum technologies, and energy solutions.

Structural design and characterization of a new type of organic nonlinear optical flavanone crystal

Two new crystals were designed and obtained: the chalcone derivative HMMP (1-(2‑hydroxy-4-methoxyphenyl)-3-(4-(methylthio)phenyl)prop‑2-en-1-one) and the flavanone derivative 7M4MC (7‑methoxy-2-(4-(methylthio)phenyl)chroman-4-one). Yellow bulk 7M4MC single crystal with dimensions of 3 × 2 × 1 mm3 and orange-yellow needle-like HMMP single crystal were grown by solution evaporation. 7M4MC crystal belongs to the P21 space group of the monoclinic structure, which has a non-centrosymmetric structure. HMMP crystal monoclinic, belongs to the P21/m space group with a centrosymmetric structure. In the 7M4MC molecule, the methoxy and methylthio groups act as electron donors (D) and the carbonyl group acts as an electron acceptor (A), forming a d-π-A-π-D structure with an electron gradient. 7M4MC exhibits a high degree of second-harmonic generation intensity (11 × KDP), a broad transmission range (600–1600 nm, >88 %), an elevated optical energy bandgap (Eg = 2.57 eV), high thermal stability (Tm = 105 °C) and excellent dielectric properties (εr ≈ 2.1). This work not only provides two new crystals but also offers a viable molecular design option by changing the degree of intramolecular distortion by altering the backbone structure.

Carbonatoperoxovanadates with Strong Second-Harmonic Generation: Insights from First-Principles Calculations on Anisotropic Structures and Optical Parameters.

Carbonatoperoxovanadates are considered as promising functional materials in optoelectronic devices due to their excellent optical properties, particularly strong second-harmonic generation (SHG) response. However, the relationship between their geometric structures and optical properties remains unclear. Herein, the structural, electronic, and optical properties of carbonatoperoxovanadates A3VO(O2)2CO3 (A=K, Rb, and Cs) were investigated using first-principles calculation. Results suggest that high density and parallel arrangement of nonlinear optical active [VO(O2)2CO3] units are conducive to generating large SHG response in A3[V(O2)2O]CO3. Optical anisotropy was observed. Birefringence values for A3[V(O2)2O]CO3 were comparable to those of commonly used infrared nonlinear optical materials. Specifically, results of tiny optical characteristics (local dipole moments, HUMO-LUMO gap, polarizability anisotropy, and hyperpolarizability) indicate that asymmetry [VO(O2)2CO3] is an excellent nonlinear optical active functional unit, owing to the synergistic effect between its non-centrosymmetric nonlinear optical elements. This study elucidates the structure-property relationship of carbonatoperoxovanadates, offering valuable insights for designing novel high-performance SHG materials.

On‐Demand Generation and Control of Giant Terahertz Nonlinearity with Metasurfaces

AbstractSimultaneous control of the optical linearity and nonlinearity of the materials plays key roles in next‐generation electronics and optics as it offers the potential of manipulating the fundamental properties while generating new frequencies of light. Such control is exceedingly challenging at the technologically important terahertz (THz) frequencies due to the severe scarcity of high‐performance nonlinear materials. Here, a unique method is introduced for achieving THz second‐order nonlinearity through Lorentz force‐driven anharmonic oscillations of free electrons in metasurfaces. It does not depend on natural nonlinear materials and provides ultrahigh design freedom. Using this approach, efficient THz second harmonic generation (SHG) is experimentally demonstrated with remarkably large values of the effective nonlinear susceptibility at room temperature. By engineering the geometry of the metasurfaces, the generation and suppression of the second harmonics can be controlled and customize their polarization states at will. This efficient, configurable, room‐temperature, and ultra‐compact platform for THz SHG provides new perspectives for developing high‐performance THz sources, frequency multipliers, and other nonlinear devices.

Growth, characterization and cognition of 2-cyanopyridinium perchlorate single crystals for nonlinear optical applications.

The fascinating electronic applications attracted researchers to explore the field of nonlinear optical (NLO) materials. The slow evaporation of solvent technique (SEST) was employed to grow the 2-cyanopyridinium perchlorate (2-CPPC) NLO single crystals. The cell parameters of the grown 2-CPPC crystal are confirmed by the single crystal X-ray diffraction (SCXRD) study. The powder X-ray diffraction studies confirm the crystallinity of 2-CPPC crystals, and the peaks were indexed. The computation for the geometry optimization, HOMO-LUMO energy gap, global reactivity parameters, natural bond orbital (NBO) analysis, polarizability, and hyperpolarizability of the 2-CPPC molecule was done using B3LYP (6-311G basis set) functional of DFT method. The experimental FTIR and UV-Vis results of the 2-CPPC compound were compared with the simulated results. The second harmonic generation (SHG) study for the 2-CPPC crystal was employed using Kurtz-Perry powder technique. Single beam Z-scan technique using He-Ne laser is used to study the third-order NLO properties.