Second Harmonic Generation Wavelength Research Articles

Precise wavelength alignment of second-harmonic generation in thin-film lithium niobate resonators.

Second-harmonic generation (SHG) plays a significant role in modern photonic technology. Integrated photonic resonators fabricated with thin-film lithium niobate can achieve ultrahigh efficiencies by combining small mode volumes with high material nonlinearity. Cavity-enhanced SHG requires accurate phase and frequency matching conditions, where fundamental and second-harmonic wavelengths are both on resonance. However, this double-resonance condition can typically be realized only at a fixed random wavelength due to the high sensitivity of photonic resonances to the device geometry and fabrication variations. Here, we propose a novel method that can achieve the double-resonance condition over a large wavelength range. We combine thermal-optic and electro-optic (EO) effects to realize the separate tuning of fundamental and second-harmonic resonances. We demonstrated that the optimum SHG efficiency can be maintained over a wavelength range that exceeds the limit achievable with only thermal tuning. With this flexible tuning capability, we further show the precise alignment of SHG wavelengths of two separate thin-film lithium niobate resonators without sacrificing efficiencies.

Double localized surface plasmon‒enhanced-second-harmonic generation behavior of equilateral triangular aluminum nanoprisms

This paper presents second-harmonic generation (SHG) behaviors for different-sized equilateral triangular aluminum nanoprisms. These behaviors were observed under the double localized surface plasmon (LSP) resonance condition. The SHG conversion efficiency was substantially enhanced when the excitation and SHG wavelengths were simultaneously tuned to the LSP-resonance wavelengths. The fundamental and higher-order LSP modes could be used to enhance the SHG conversion efficiency. The nonlinear light‒matter interaction associated with the double LSP resonance was numerically analyzed while focusing on the spatial overlap of the near-field distributions between the excitation and SHG wavelengths. The double LSP resonance enhanced SHG behaviors have been realized by using several elegantly designed plasmonic metal nanostructures consisting of multi-elements. Our present study demonstrates this double LSP resonance enhanced SHG behavior from a single, simple-shape triangular aluminum nanoprism.

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.

Highly Efficient Ultraviolet Harmonic Generation at a Nonlinear Crystal Interface

High conversion efficiency is vital for the acquisition of UV light through nonlinear processes. Here, we investigate an alternative method to realize highly efficient second-harmonic generation (SHG) based on the interface of $\ensuremath{\beta}$ barium metaborate ($\ensuremath{\beta}\mathrm{BBO}$) and magnesium-doped lithium niobate ($\mathrm{MgO}$:LN). Theoretical simulations indicate that the SHG efficiency of our method is significantly higher than other phase-matching conditions. In the experiment, we demonstrate that higher conversion efficiency can be realized compared with only employing a single BBO crystal. Besides, the shortest output wavelength of SHG can reach 205 nm in our method. The method proposed here can open another way for the integrated ultraviolet laser and may promote the development of UV micromachining in the future.

Augmentation of surface plasmon-enhanced second harmonic generation from Au nanoprisms on SiO2/Si: interference contribution.

We present an augmentation of Surface Plasmon (SP)-enhanced second harmonic generation (SHG) due to interference field enhancement in Au nanoprisms (AuNPs) on SiO2-coated Si substrates. The SiO2 spacer contributed for the optical interference and increased the coupling efficiency of the pump light with the SP polarization as well as a decoupling efficiency of the SHG waves from nonlinear polarization. The intensity of the SP-enhanced SHG signals increased 4.5-fold with respect to the AuNPs on the bare SiO2 substrate by setting the SiO2 spacer layer to the appropriate thickness. The numerical analysis revealed that the optimal SHG conversion was determined by the balance between the degree of the optical interference at the fundamental and SHG wavelengths.

High-efficiency broadband second-harmonic-generation of YCa4O(BO3)3 crystal

High-efficiency broadband second-harmonic-generation (SHG) of YCa4O(BO3)3 (YCOB) crystal was demonstrated for the first time. The theoretical calculation predicted that the retracing point wavelength for type-I SHG of YCOB crystal was at around 1650 nm, which was proved by the followed phase-matching experiment. With a tunable femtosecond laser as the light source, the type-I SHG experiments of YCOB crystal were performed in the range of 1550–1700 nm. Compared with other broadband SHG crystals, YCOB crystal presented the highest optical conversion efficiency of 58%, the largest spectral bandwidth conversion ratio of 65%, and the broadest wavelength acceptance bandwidth of 118.8 nm·cm1/2.

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.

Resonant optical effects in composite Co/opal-based magnetoplasmonic structures.

Plasmonic structures are extremely attractive for the light flow manipulation. In turn, the spectrum of the plasmon excitations can be controlled by external magnetic field, thus giving rise to magnetoplasmonics. However, in the case of traditional magnetoplasmonic structures, the enhancement of magneto-optical (MO) effects is often accompanied by the transmission damp, which constricts the area of their applications. This paper examines resonant optical effects in composite structures based on artificial opal films covered by a thin cobalt layer, which forms a 2D hexagonal lattice of nanoholes in the metal film. Such periodic structure exhibits surface plasmon polariton-assisted extraordinary transmission along with the increase of odd in magnetization intensity magnetooptical effect in the Voigt geometry. Local field enhancement accompanying the surface plasmon polaritons excitation in composite Co/opal structure provides a distinct enhancement of the magnetization-induced second harmonic generation (SHG) and relevant MO effects at the SHG wavelength that appear as Fano-type resonances. High transmission along with resonantly-high MO effects make Co/opal films promising in plasmonic applications.

UV Solar‐Blind‐Region Phase‐Matchable Optical Nonlinearity and Anisotropy in a π‐Conjugated Cation‐Containing Phosphate

Wide ultraviolet (UV) transparency, strong second-harmonic generation (SHG) response, and sufficient optical birefringence for phase-matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non-traditional π-conjugated cation-based UV NLO phosphate [C(NH2 )3 ]6 (PO4 )2 ⋅3 H2 O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH2 PO4 @ 1064 nm) and 532 nm (0.3 × β-BaB2 O4 @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar-blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π-conjugated cations [C(NH2 )3 ]+ and their favorable arrangement with [PO4 ]3- anions.

Second harmonic generation in gallium phosphide nano-waveguides.

We designed, fabricated and tested gallium phosphide (GaP) nano-waveguides for second harmonic generation (SHG). We demonstrate SHG in the visible range around 655 nm using modal phase matching. We observe phase matched SHG for different combinations of interacting modes by varying the widths of the waveguides and tuning the wavelength of the pump. We achieved a normalized internal SHG conversion efficiency of 0.4% W-1cm-2 for a continuous-wave pump at wavelength of 1283.5 nm, the highest reported in the literature for a GaP waveguide. We also demonstrated temperature tuning of the SHG wavelength with a slope of 0.17 nm/°C. The presented results contribute to the development of integrated photonic platforms with efficient nonlinear wave-mixing processes for classical and quantum applications.

Nonlinear Optical Oxythiophosphate Approaching the Good Balance with Wide Ultraviolet Transparency, Strong Second Harmonic Effect, and Large Birefringence.

Ultraviolet (UV) transparency, second harmonic effect and optical birefringence are three vital but mutually restrictive factors in the application of UV nonlinear optical (NLO) materials. It is difficult for traditional phosphates to achieve a good balance among these factors. In this communication, we propose that the structural evolution of the NLO motif from traditional phosphate to oxythiophosphate would enhance the birefringence and second harmonic generation (SHG) effect while maintaining wide UV transparency, which is confirmed by first-principles calculations and preliminary experimental measurements. Following this strategy, we predict that, compared with the well-known NLO phosphate KH2 PO4 , the oxythiophosphate KH2 PO3 S exhibits better balance for the UV NLO performance, including wide UV transparency (UV cutoff down to 203 nm), strong SHG effect (ca. 0.9 pm V-1 ), and large birefringence (ca. 0.1 at 1 μm) with short phase-matching SHG output wavelength (≈214 nm).

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.

Characterization of off-axis phase singular optical vortex and its nonlinear wave-mixing to generate control broad OAM spectra

We theoretically and numerically studied the off-axis Gaussian vortex beam (GVB) characteristics. The off-axis GVB scheme is utilized to generate tunable broad OAM modes using a single helical phase. The single helical phase can be easily created in the high peak power lasers using a spiral phase plate (SPP). The high peak power GVBs greatly enhances the efficiency of nonlinear wave-mixing to tune the wavelength as well as OAM modes of laser beams. Further, we propose type-II second harmonic generation (SHG) of off-axis GVB to tune the wavelength and to increase the number of OAM modes of vortex beam. Polarization-controlled broad OAM spectra is generated at SHG wavelength. This scheme is effective for control generation of a broad range of OAM spectra in the required wavelength for high-capacity and multi-channel optical communication.

Optimization on the frequency conversion of LiGaS2 crystal

Based on newly improved optical parameters of the excellent biaxial crystal LiGaS2, its phase-matching upon second harmonic generation and general parametric processes are investigated. In particular, the issue of optimal phase-matching is solved by a computation on figure-of-merit (FM). It is found that the effective fundamental wavelength of second harmonic generation entirely covers the 3–5 µm atmosphere window and mid-IR signals of 2–12 µm can be generated by suitable parametric processes. Analysis of the FM and dispersion verifies a quite effective type II parametric process: from a pump in the range of 0.8–1.0 µm to an ultrashort pulse generation at 6–12 µm and also suggests a type I pump at 1.7–1.9 µm for 3–5 µm pulse generation. The computation definitely renders an optimal phase-matching in the XY principal plane for type II configuration, or in a special direction for type I case, far away from the commonly used XZ principal plane.

Imaging of triboelectric charge distribution induced in polyimide film by using optical second-harmonic generation: Electronic charge distribution and dipole alignment

By using an optical second-harmonic generation (SHG) microscope, we visualized spatial distribution of triboelectric charge in polyimide films, which are negatively charged by rubbing with a cotton belt. The origins of this triboelectric charging were excessive electronic charges and aligned dipoles. The excessive electronic charges were imaged using the probe laser wavelength of 1140 nm (SHG wavelength 570 nm), whereas the aligned dipoles were visualized by choosing the laser wavelength of 570 nm (SHG wavelength 285 nm). Combining the two visualized images showed that the dipolar alignment region with a width of 10 μm was induced on the rubbed polyimide surface, and at the same time, excess electronic charges were generated in that region. We conclude that dipolar alignment and electronic charging are main contributors, and the interaction between the two contributors plays an essential role in triboelectrification.

Beam Control in an Intracavity Frequency-Doubling Semiconductor Disk Laser

In this paper, we have demonstrated a 1.3 W green laser using a V-shaped intracavity frequency doubling 1036 nm semiconductor disk laser. The beam quality of the fundamental and second harmonic generation (SHG) laser is investigated. It has been found that the output lasers at the fundamental and SHG wavelength both suffer from reduced beam quality along with power scaling. The measured beam profile is elliptical under high power operation. In order to improve the SHG laser beam, an intracavity aperture is employed to control the mode characteristics of fundamental frequency light. By fine-tuning the aperture, a nearly circular beam profile with improving beam quality and brightness is realized.

Anisotropy of nonlinear optical properties in monoclinic SmxY1-xCa4O(BO3)3 crystals

The angular noncritical phase matching (A-NCPM) second-harmonic-generation (SHG) properties of SmxY1-xCOB (x = 0, 0.44, 0.55 and 1) nonlinear optical (NLO) crystals were investigated. The type-I A-NCPM SHG wavelengths along the Y and Z axes were found to be tunable in the ranges of 725–900 nm and 826–1116 nm, respectively, by adjusting the ratio of the rare-earth elements Sm and Y in SmxY1-xCOB crystals. Taking advantages of the fundamental laser pulses generated from a mode-locked Ti:Sapphire laser and a long Y-cut Sm0.44Y0.55COB crystal sample, the single-pass SHG average output power of ∼436 mW at 397 nm was obtained with a conversion efficiency of 41.5%. In addition, the SHG phase-matching (PM) curve and the effective NLO coefficients (deff) of SmCOB crystal for 1064 nm laser were studied, where the optimal type-I SHG PM direction in principal planes was determined to exist in the XY principal plane i.e. (90°, 52.4°), rather than the ZX plane i.e. (θº, 180°), which was different with those of TmCOB (32.5°, 180°), YCOB (30.7°, 180°), TbCOB (22.6°, 180°), GdCOB (19.7°, 180°) and LaCOB (15.1°, 180°) crystals in the same crystal family.

Comparison of second harmonic microscopy images of collagen-based ocular tissues with 800 and 1045 nm.

Second harmonic generation (SHG) imaging is a well-suited multiphoton technique allowing visualization of biological tissues mainly composed of collagen with submicron resolution. Despite its inherent confocal properties, imaging of deeper layers within thick samples has still some limitations. Although the use of longer wavelengths might help to overcome this, the dependence between SHG signals and wavelength is still under discussion. We report here on the dependence with wavelength of SHG signals from collagen-based ocular tissues. The quality of SHG images for two commonly used excitation wavelengths (800 and 1045 nm) is studied. The analysis of the collagen structural information reveals that the information provided by both wavelengths is similar. It was also found that, independently of the depth location, 1045-nm SHG images presented always lower signal levels than those acquired with 800 nm. However, the contrast of the former images was higher, what may improve the visualization of certain features of interest.

Enhancing the blue shift of SHG signal in GaSe:B/Ce crystal

The influence of Ce3+ on the wavelength of second harmonic generation (SHG) signal in boron doped GaSe crystals have been investigated. We found that by substitution of Ce3+ with B3+, SHG signal shifted to lower wavelength. In addition, the nonlinear absorption (NA) properties and ultrafast dynamics of pure, 1at.% B3+ and 0.5at.% B3++0.5at.% Ce3+ doped GaSe crystals have been studied by open aperture Z-scan and ultrafast pump probe spectroscopy techniques. From the open aperture Z-scan experiments we observed that all of the crystals showed nonlinear absorption (NA). However, pump-probe experiments revealed that when GaSe crystal is doped, the NA signal turns into a bleaching signal with different lifetimes depending on the type and concentration of the dopant atoms.

Enhanced Second-Harmonic Generation from Sequential Capillarity-Assisted Particle Assembly of Hybrid Nanodimers.

We show enhanced second-harmonic generation (SHG) from a hybrid metal-dielectric nanodimer consisting of an inorganic perovskite nanoparticle of barium titanate (BaTiO3) coupled to a metallic gold (Au) nanoparticle. BaTiO3-Au nanodimers of 100 nm/80 nm sizes are fabricated by sequential capillarity-assisted particle assembly. The BaTiO3 nanoparticle has a noncentrosymmetric crystalline structure and generates bulk SHG. We use the localized surface plasmon resonance of the gold nanoparticle to enhance the SHG from the BaTiO3 nanoparticle. We experimentally measure the nonlinear signal from assembled nanodimers and demonstrate an up to 15-fold enhancement compared to a single BaTiO3 nanoparticle. We further perform numerical simulations of the linear and SHG spectra of the BaTiO3-Au nanodimer and show that the gold nanoparticle acts as a nanoantenna at the SHG wavelength.