Third Harmonic Generation Intensity Research Articles

Anisotropic Third Harmonic Generation in 2D Tin Sulfide

AbstractThe in‐plane anisotropic properties of 2D group IV monochalcogenides offer an additional degree of freedom which can be advantageous in optoelectronic applications. Here, it is demonstrated that the third harmonic generation (THG) produced by ultrathin tin (II) sulfide (SnS) is in‐plane anisotropic with respect to the direction of the excitation linear polarization. The polarization‐resolved THG (P‐THG) measurements are described with a nonlinear optics model, which accounts for the orthorhombic crystal structure of the 2D SnS. The relative magnitudes of the χ(3) tensor components are calculated by recording and simultaneously fitting two orthogonal polarization components of the P‐THG signals. Furthermore, the “THG anisotropy ratio” is introduced, which compares the THG intensity produced when the excitation linear polarization is along the armchair crystallographic direction, with the case when it is along the zigzag direction. The experimental findings provide quantitative information on the anisotropic nature of the THG process in various SnS flakes. Nonlinear anisotropic 2D materials can be used in future optoelectronic devices, where anisotropic polarization sensitivity is required. Having an optical tool that quantitatively probes the effect of the in‐plane anisotropy among different 2D materials, could be of considerable importance for evaluating the performance of such materials and devices.

Polarization-controlled third harmonic generation by quasi-bound states in the continuum in silicon nanopillar metasurfaces

All-dielectric metasurfaces have received widespread attention due to their low intrinsic loss compared to the metallic counterparts. However, relatively high radiation loss and low quality (Q) factor of the dielectric nanostructures may significantly degrade their nonlinear optical efficiency. Here, we demonstrate that silicon-nanopillar-based metasurfaces supporting quasi bound states in the continuum (BICs) can greatly enhance the efficiency of third harmonic generation (THG). We show that symmetry-protected BICs form at the Γ-point of doubly degenerate band or nondegenerate band can be selectively transformed into quasi BIC through symmetry breaking of our silicon metasurfaces. Benefiting from the high Q and resonance enhancement by the quasi BICs, the THG conversion efficiencies of the silicon metasurface reaches 4.3 × 10-4 with a peak pump intensity of 1.3 GW/cm2, which can be four orders higher than that of a silicon planar film with the same thickness. In addition, the THG intensity is polarization-dependent for the single quasi BIC while it is polarization-independent for the doublet quasi BIC. Our results provide a new strategy for the design of high-efficiency silicon-based optical nonlinear devices.

Third Harmonic Generation in Thin NbOI2 and TaOI2.

The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI2 and TaOI2. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS2, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency.

Leukocyte differentiation in bronchoalveolar lavage fluids using higher harmonic generation microscopy and deep learning.

In diseases such as interstitial lung diseases (ILDs), patient diagnosis relies on diagnostic analysis of bronchoalveolar lavage fluid (BALF) and biopsies. Immunological BALF analysis includes differentiation of leukocytes by standard cytological techniques that are labor-intensive and time-consuming. Studies have shown promising leukocyte identification performance on blood fractions, using third harmonic generation (THG) and multiphoton excited autofluorescence (MPEF) microscopy. To extend leukocyte differentiation to BALF samples using THG/MPEF microscopy, and to show the potential of a trained deep learning algorithm for automated leukocyte identification and quantification. Leukocytes from blood obtained from three healthy individuals and one asthma patient, and BALF samples from six ILD patients were isolated and imaged using label-free microscopy. The cytological characteristics of leukocytes, including neutrophils, eosinophils, lymphocytes, and macrophages, in terms of cellular and nuclear morphology, and THG and MPEF signal intensity, were determined. A deep learning model was trained on 2D images and used to estimate the leukocyte ratios at the image-level using the differential cell counts obtained using standard cytological techniques as reference. Different leukocyte populations were identified in BALF samples using label-free microscopy, showing distinctive cytological characteristics. Based on the THG/MPEF images, the deep learning network has learned to identify individual cells and was able to provide a reasonable estimate of the leukocyte percentage, reaching >90% accuracy on BALF samples in the hold-out testing set. Label-free THG/MPEF microscopy in combination with deep learning is a promising technique for instant differentiation and quantification of leukocytes. Immediate feedback on leukocyte ratios has potential to speed-up the diagnostic process and to reduce costs, workload and inter-observer variations.

Dielectric loading method for doubly resonant enhancement of third-harmonic generation from complementary split-ring resonators

Nonlinear optical response could be greatly enhanced when metasurfaces support plasmonic resonances at both fundamental and harmonic wavelengths. However, it is still challenging to fulfill the doubly resonant condition. Here, we propose a dielectric-loading method, which simply coats a conformal thin dielectric layer onto the plasmonic metasurfaces, to introduce an additional degree of freedom and make the doubly resonant condition easily fulfilled. We demonstrate that by simultaneously tuning the thickness of the coated dielectric layer and the geometrical parameters of the gold complementary split-ring resonators (CSRRs), the doubly resonant enhancement of third harmonic generation (THG) could be achieved for any given fundamental wavelengths. We also experimentally verify this concept and show that the THG intensity in the dielectric-loaded CSRRs under the doubly resonant condition could be further increased about 3 times as compared with the case of the conventional CSRRs.

Third-harmonic generation in excitonic insulators

We study third-harmonic generation (THG) in an excitonic insulator (EI) described in a two-band correlated electron model. Employing the perturbative expansion with respect to the external electric field, we derive the THG susceptibility taking into account the collective dynamics of the excitonic order parameter. In the inversion-symmetric EI, the collective order parameter motion is activated at second order of the external field and its effects arise in THG. We find three peaks in the THG susceptibility at energies $\hbar \Omega = \Delta_g/3$, $\Delta_g/2$, and $\Delta_g$, where $\Delta_g$ is the band gap. While the THG response at $\Delta_g/3$ is caused by bare three-photon excitation of the independent particle across the band gap, the latter two peaks involve the motion of the order parameter activated at second order. The resulting resonant peaks are prominent in particular in the BCS regime but they become less significant in the BEC regime. We demonstrate that the resonant peak originated by the collective excitation is observable in the temperature profile of the THG intensity. Our study suggests that the THG measurement should be promising for detecting the excitonic collective nature of materials.

Band-selective third-harmonic generation in superconducting MgB2 : Possible evidence for the Higgs amplitude mode in the dirty limit

We report on time-resolved linear and nonlinear terahertz spectroscopy of the two-band superconductor $\mathrm{Mg}{\mathrm{B}}_{2}$ with a superconducting transition temperature ${T}_{c}\ensuremath{\approx}36$ K. Third-harmonic generation (THG) is observed below ${T}_{c}$ by driving the system with intense narrow-band THz pulses. For the pump-pulse frequencies $f=0.3$, 0.4, and 0.5 THz, the temperature-dependent evolution of the THG signals exhibits a resonance maximum at the temperatures with the resonance conditions $2f=2{\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\pi}}(T)$ fulfilled, for the dirty-limit superconducting gap $2{\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\pi}}$. In contrast, for $f=0.6$ and 0.7 THz with $2f>2{\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\pi}}(T\ensuremath{\rightarrow}0)=1.03$ THz, the THG intensity increases monotonically with decreasing temperature. Moreover, for $2f<2{\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\pi}}(T\ensuremath{\rightarrow}0)$ the THG is found nearly isotropic with respect to the pump-pulse polarization. These results suggest a predominant contribution of the driven Higgs amplitude mode of the dirty-limit $\ensuremath{\pi}$-band superconducting gap, pointing to the importance of scattering for observation of the Higgs mode in superconductors.

Enhancing third-harmonic generation by quasi bound states in continuum in silicon nanoparticle arrays

Subwavelength artificial structures of high refractive index dielectrics provide an effective way to control and manipulate light on a nanoscale by enhancing electric and magnetic fields. This kind of structure usually has low absorption loss, but its performance is also limited by radiation loss, which will reduce the efficiency of its nonlinear response. This problem can be solved by using bound states in the continuum (BICs). The BICs are a kind of unconventional state which is in continuous domain but remains localized. They exist within a light cone and have an infinite quality factor. By combining BICs with nonlinear optics, high-<i>Q</i> resonances from quasi-BICs are used to excite and enhance the nonlinear response. The simulation shows that when the symmetry of the unit cell of the silicon nanoparticle arrays is broken, the BIC become the quasi-BIC, and the transmission spectrum will produce a high-<i>Q</i> narrow resonance valley. The resonance has polarization dependence of electric field. With the change of pump wavelength, the third-harmonic generation (THG) intensity first increases and then decreases gradually. The pump wavelength changed by several nanometers can change THG intensity by at least one order of magnitude. When the pump wavelength is adjusted to the resonance wavelength, the nonlinearity is significantly enhanced as a result of the strong field localization. The THG intensity is highly sensitive to the variation of asymmetric parameters. Only a change of 75 nm will result in a decrease of THG intensity by at least one order of magnitude. There is a third-order relationship between pump power and THG power. For the proposed structure, the factors affecting the conversion efficiency of THG include pump power, pump wavelength, polarization angle of pump light, and asymmetry parameter. When the polarization direction of electric field is along the short axis of the structure and the pump light at resonance wavelength is vertically incident to the structure with an asymmetric parameter of 0.125, the conversion efficiency of THG can be increased to ～2.6 × 10<sup>–6</sup> and the intensity of THG is increased by six orders of magnitude. The results are expected to be applied to designing the silicon-based optical nonlinear devices.

Enhanced third harmonic generation from sinusoidal graphene gratings

We investigate the nonlinear optical third harmonic generation of sinusoidal curved graphene which is placed on the dielectric substrate. Under transverse magnetic polarization, the third harmonic generation (THG) from graphene is dramatically enhanced at fundamental-mode and several higher-order modes of surface plasmon of graphene. The enhancement factor of THG is up to106 compared to that from graphene on a planar dielectric. We study in detail THG intensity affected by the Fermi level of graphene, grating period and amplitude, and the angle of incidence of light. The tunable and enhanced THG response at multiple wavelengths show that the sinusoidal-shaped graphene grating has potential applications for graphene based broadband nonlinear photonics.

Study on melanin enhanced third harmonic generation in a live cell model.

Melanin dominates the endogenous contrasts of in vivo third harmonic generation (THG) imaging of human skin. A recent study investigated the THG in melanin solution and a linear relationship between melanin concentration and THG intensity was reported, in contrast to the expected nonlinear relationship. Since melanin hydrocolloid solution is very different from the skin tissue, here we report our study on the origin of the melanin-enhanced THG by using a live cell model. Different from the previous conclusion, our live cell study has indicated an initial nonlinear process where the THG intensity was enhanced according to the 3.5th power of melanin mass density (MMD). When the MMD is higher than 11 mg/ml, a transition from the resonance-enhanced THG to the high-order hyper-Rayleigh scattering process occurs. This saturation phenomenon of the virtual-transition-based THG nonlinear process is attributed to the multi-melanosome-induced scattering within the sub-femtoliter focal volume.

Enhanced third harmonic generation from graphene embedded in dielectric resonant waveguide gratings

We propose a structure of graphene embedded in dielectric resonant waveguide gratings to enhance the nonlinear interactions in graphene. Third harmonic generation (THG) from graphene is significantly enhanced in the waveguide mode under transverse electric polarization. Compared with the graphene on a flat dielectric film, the enhancement factor is up to 109. We in detail studied the THG intensity influenced by the Fermi level of graphene, the position of graphene, and multimode of waveguide. Our investigation has great significance for design and experimental fabrication of graphene-based nonlinear photonic devices.

New Templated Ostwald Ripening Process of Mesostructured FeOOH for Third-Harmonic Generation Bioimaging.

Emerging advances in iron oxide nanoparticles exploit their high magnetization for various applications, such as bioseparation, hyperthermia, and magnetic resonance imaging. In contrast to their excellent magnetic performance, the harmonic generation and luminescence properties of iron oxide nanoparticles have not been thoroughly explored, thus limiting their development as a tool in photomedicine. In this work, a seed/growth-inspired synthesis is developed combined with primary mineralization and a ligand-assisted secondary growth strategy to prepare mesostructured α-FeOOH nanorods (NRs). The sub-wavelength heterogeneity of the refractive index leads to enhanced third-harmonic generation (THG) signals under near-infrared excited wavelengths at 1230 nm. The as-prepared NRs exhibit an 11-fold stronger THG intensity compared to bare α-FeOOH NRs. Using these unique nonlinear optical properties, it is demonstrated that mesostructured α-FeOOH NRs can serve as biocompatible and nonbleaching contrast agents in THG microscopy for long-term labeling of cells as well as in angiography in vivo by modifying lectin to enhance the binding efficiency to the glycocalyx layers on the wall of blood vessels. These results provide a new insight into Fe-based nanoplatforms capable of emitting coherent light as molecular probes in optical microscopy, thus establishing a complementary microscopic imaging method for macroscopic magnetic imaging systems.

Third-harmonic generation modulation achieved through ground-state depletion.

Without real-state transition, third-harmonic generation (THG) cannot be modulated simply by fluorescence-based methods. However, utilizing the absorption-enhancement property of THG, the modulation of THG intensity has been demonstrated through ground-state depletion (GSD) in this Letter. By suppressing the absorption of materials with GSD, the THG intensity can be reduced due to a decreasing of absorption enhancement. The ability to modulate THG intensity can benefit from applying super-resolution techniques to THG microscopy.

Detection of the T cell activation state using nonlinear optical microscopy.

The ability to monitor the activation state of T-cells during immunotherapy is of great importance. Although specific activation markers do exist, their abundance and complicated regulation cannot definitely define the activation state of the cells. Previous studies have shown that Third Harmonic Generation (THG) imaging could distinguish between activated versus resting microglia and healthy versus cancerous cells, mainly based on their lipid-body profiles. In the present study, mitogen or antigen-stimulated T-cells were subjected to THG imaging microscopy. Qualitative and quantitative analysis showed statistically significant increase of THG mean area and intensity in activated versus resting T-cells. The connection of THG imaging to chemical information was achieved using Raman spectroscopy, which showed significant differences between the activation processes and controls, correlating of THG signal area with cholesterol and lipid compounds, but not with triglycerides. The obtained results suggested a potential employment of nonlinear microscopy in evaluating of T-cell activation, which is expected to be largely appreciated in the clinical practice.

Third harmonic generation in ZnO semiconductor using the simplified bond hyperpolarizability model

We describe the fourth rank tensor and the related third harmonic generation (THG) light intensity profile in a (0002) wurtzite structure using the simplified bond hyperpolarizability model (SBHM). We show that the resulting THG intensity is isotropic e.g., does not depend on the azimuthal rotation angle of the material. Assuming that THG inside wurtzite structures are dominated solely by bulk dipoles, only one fitting parameter in terms of the effective THG hyperpolarizability is required to generate the Rotational Anisotropy THG (RATHG) experiment result.

Third harmonic generation on exciton-polaritons in bulk semiconductors subject to a magnetic field

We report on a comprehensive experimental and theoretical study of optical third harmonic generation (THG) on the exciton-polariton resonances in the zinc-blende semiconductors GaAs, CdTe, and ZnSe subject to an external magnetic field, representing a topic that had remained unexplored so far. In these crystals, crystallographic THG is allowed in the electric-dipole approximation, so that no strong magnetic-field-induced changes of the THG are expected. Therefore, it comes as a total surprise that we observe a drastic enhancement of the THG intensity by a factor of fifty for the $1s$-exciton-polariton in GaAs in magnetic fields up to 10 T. In contrast, the corresponding enhancement is moderate for CdTe and almost neglectful for ZnSe. In order to explain this strong variation, we develop a microscopic theory accounting for the optical harmonics generation on exciton-polaritons and analyze the THG mechanisms induced by the magnetic field. The calculations show that the increase of THG intensity is dominated by the magnetic field enhancement of the exciton oscillator strength which is particularly strong for GaAs in the studied range of field strengths. The much weaker increase of THG intensity in CdTe and ZnSe is explained by the considerably larger exciton binding energies, leading to a weaker modification of their oscillator strengths by the magnetic field.

Optical Third Harmonic Generation Using Nickel Nanostructure-Covered Microcube Structures.

We investigated the optical third harmonic generation (THG) signal from nanostructure-covered microcubes on Ni. We found that the hierarchical structures greatly change the third-order optical nonlinearity of the metallic surface. While the symmetry and lightning rod (LR) effects on microstructures did not significantly influence the THG, the localized surface plasmon (LSP) effect on the nanostructures enhanced it. By removing the nanostructures on the hierarchical structures, THG intensity could be strongly suppressed. In the present paper, we also discuss the mechanism that enhances THG in nano/micro structures.

Quasiclassical Theory on Third-Harmonic Generation in Conventional Superconductors with Paramagnetic Impurities

We investigate the third-harmonic generation (THG) of s-wave superconductors under microwave pulse irradiation. We consider the effect of paramagnetic impurities on the THG intensity of dirty superconductors. The nonlinear response function is calculated using the method of the quasiclassical Green function. It is shown that the amplitude mode is included as the vertex correction and makes a predominant contribution to the THG intensity. When the effect of paramagnetic impurities is weak, the THG intensity shows a peak at the temperature at which the superconducting gap is about the same as the frequency of the incident pulse, similarly to in experiments. As the effect of paramagnetic impurities is strengthened, the peak of the THG intensity disappears. This indicates that time-reversal symmetry breaking due to paramagnetic impurities eliminates the well-defined amplitude mode. The result of our calculation shows that the existence of the amplitude mode can be confirmed through the THG intensity. The result of a semiquantitative calculation is in good agreement with the experimental result, and it also shows that the diamagnetic term is negligible.

Examination of Drosophila eye development with third harmonic generation microscopy.

Third harmonic generation (THG) microscopy can exploit endogenous harmonophores such as pigment macromolecules for enhanced image contrast, and therefore can be used without exogenous contrast agents. Previous studies have established that carotenoid compounds are ideal harmonophores for THG microscopy; we therefore sought to determine whether THG from endogenous carotenoid-derived compounds, such as retinal in photoreceptor cells, could serve as a new label-free method for developmental studies. Here we study the development of the pupal eye in Drosophila melanogaster and determine the localization of rhodopsin using THG microscopy technique. Additionally, by altering the chromophore or the opsin protein we were able to detect changes in both the retinal distribution morphology and in THG intensity age-dependent profiles. These results demonstrate that THG microscopy can be used to detect altered photoreceptor development and may be useful in clinically relevant conditions associated with photoreceptor degeneration.

Third order nonlinear optical effects of ZnO–ZrO2–B2O3 glass ceramics embedded with ZnZrO3 perovskite crystal phases

Zinc zirconium borate glasses crystallized with varying concentrations of ZrO2 by heat treating at the onset of the predicted crystallization temperature for prolonged times have been fabricated. The prepared samples were characterized by XRD and SEM techniques. The results of these measurements have indicated that the bulk samples are entrenched with ZnZrO3 (tetragonal/cubic) crystal phases with varying sizes and concentration, depending upon the content of ZrO2 present in the bulk material. The IR spectral studies have further confirmed that the zirconium ions mainly exist in octahedral coordination that are predicted to play crucial role in inducing third harmonic generation (THG). Later, the samples were optically poled by cw 532 nm green laser along with illumination by 9.4 μm CO2 laser. The infrared laser (CO2 laser) treatment was performed before the 532 nm laser treatment in order to activate the phonon modes (both harmonic and anharmonic modes). Later, intensity of the induced THG (of wavelength 355 nm) of the pre- and post-heated samples was measured using Nd:YAG 1064 nm 20 ns pulsed laser with the fundamental beam power varying up to 200 J/m2. The measurements were performed as functions of power of photoinduced 532 nm cw laser beam and also the concentration of the crystallizing agent ZrO2. Quantitative analysis of these results indicated that the samples crystallized with 5.0 mol% of ZrO2 exhibited the maximal intensity of THG. Further, the comparison of the obtained results with those of corresponding pre-crystallized samples pointed out that the crystallization has enhanced the THG intensity by many folds. The enhancement is attributed to the presence of ZnZrO3 perovskite crystal phases embedded in the residual glass phase of the bulk crystallized samples. The obtained results may be useful for considering the material for different optoelectronic devices especially for nonlinear optical triggering devices.