Harmonic Generation Research Articles

Potential of data-driven discovery of nonlinear wave equations

Data-driven partial differential equation discovery techniques have been growing in importance in recent years. This paper discusses its possible applications in the field of finite-amplitude sound propagation and the related phenomena (such as generation of higher harmonics and shock formation). Two cases are investigated, namely the propagation of pressure pulses as travelling waves and the interference of pressure pulses, the former leading to the Westervelt equation and the latter to the Kuznetsov equation. It is shown how representative wave equations can be extracted from data obtained by numerically solving the compressible Navier-Stokes equations (up to the third order smallness in entropy changes) and subsequently employing the sparsity promoting regression techniques. The resulting error of the equation coefficients is about 7% in the nonlinear terms. The limitations of this approach are discussed (e.g., the appropriate capturing of thermal and viscous loss terms).

Profiling the thermal damage gradient in concrete using linear and non-linear ultrasonics

Processes such as carbonation and thermal damage result in a depth-dependent variation in concrete mechanical properties, which are typically characterized by invasive sampling and destructive testing. In this study, several ultrasonic testing methods are employed to nondestructively measure gradients in elastic properties within concrete and mortar slabs, induced by varying amount of exposure to elevated temperatures. The slabs are subjected to two distinct heat exposures, carefully chosen to create different gradients within the first few centimeters below the surface. The first exposure scenario following the ISO fire standard led to superficial damage. Deeper damage was achieved with radiant panels. We use linear and non-linear ultrasonic testing, with the aim of characterizing the resulting property gradients including refracted P-waves, analysis of surface wave dispersion and higher harmonic generation. Additionally, we employ NRUS (nonlinear resonant ultrasound spectroscopy) to test slices extracted from the different depths within the slabs. The ultrasonic test results are benchmarked and interpreted in relation to a suite of other multiphysical measurements (resistivity, capacity, temperature). This work represents a first crucial step towards the development of a new method for measuring gradients of nonlinear elastic properties within concrete through nondestructive ultrasonic testing.

The influence of resonant light pulses on high harmonic generation in solids

Abstract We have theoretically studied the high harmonic generation (HHG) in solids driven simultaneously by a mid-infrared (MIR) laser and a high-order harmonic pulse with energy around the band gap between the valence band and conduction band. By adding this resonant harmonic light pulse with the relative intensity ratio of 4\%, the high-order harmonic emission from the crystal is enhanced by 1-2 orders of magnitude. The yield of HHG in solid increases monotonically with the relative strength of the resonant harmonic pulses. In addition, we also found that HHG dynamics from the $k$ channel around the boundary of the Brillouin zone can be selectively enhanced by adjusting the frequency of the resonant high-order harmonic pulse. The resonance-enhanced HHG and $k$ channel selection effect in solids is also investigated by using the three-band semi-conductor Bloch equation for HHG in ZnO. We also find that the harmonic in the plateau region driven by adding a resonant light field to the strong MIR driving field has less red-shifted compared with the case driven by the MIR driving field alone.

Circularly Polarized High-Harmonic Beams Carrying Self-Torque or Time-Dependent Orbital Angular Momentum.

In the rapidly evolving field of structured light, self-torque has been recently defined as an intrinsic property of light beams carrying time-dependent orbital angular momentum. In particular, extreme-ultraviolet (EUV) beams with self-torque, exhibiting a topological charge that continuously varies on the subfemtosecond time scale, are naturally produced in high-order harmonic generation (HHG) when driven by two time-delayed intense infrared vortex beams with different topological charges. Until now, the polarization state of such EUV beams carrying self-torque has been restricted to linear states due to the drastic reduction in the harmonic up-conversion efficiency with increasing the ellipticity of the driving field. In this work, we theoretically demonstrate how to control the polarization state of EUV beams carrying self-torque, from linear to circular. The extremely high sensitivity of HHG to the properties of the driving beam allows us to propose two different driving schemes to circumvent the current limitations to manipulate the polarization state of EUV beams with self-torque. Our advanced numerical simulations are complemented with the derivation of selection rules of angular momentum conservation, which enable precise tunability over the angular momentum properties of the harmonics with self-torque. The resulting high-order harmonic emission, carrying time-dependent orbital angular momentum with a custom polarization state, can expand the applications of ultrafast light-matter interactions, particularly in areas where dichroic or chiral properties are crucial, such as magnetic materials or chiral molecules.

Tunneling displacement of electron-hole pairs in high-order harmonic generation for monolayer hexagonal boron nitride

Tunneling displacement of electron-hole pairs in high-order harmonic generation for monolayer hexagonal boron nitride

Investigations on the growth, spectral, optical, electrical, thermal and nonlinear optical properties of L-valine itaconic acid single crystal

A nonlinear optical single crystal of L-Valine Itaconic Acid (LVIA) was grown at room temperature from an aqueous solution by using the slow evaporation method. The grown crystal was undergone X-ray diffraction investigation, which confirm that the crystal belongs to orthorhombic system with the non-centro symmetric Pbca space group. This single crystal contains several functional groups, according to FTIR spectroscopic analysis there were identified. By analyzing the crystal's UV–visible spectrum, based on its optical transparency this crystal may use for optoelectronic device manufacturing. The emission happens at 553 nm wavelength, according to the photoluminescence spectrum. Thermo gravimetric and differential thermal analytical studies helps us to measure title crystal melting point and thermal stability. At various temperatures, the dielectric property of the crystal was found out. The Kurtz-powder method has been used to study the phenomenon of Second Harmonic Generation (SHG) in the crystal.

High-harmonic generation by a bright squeezed vacuum

AbstractHigh-harmonic generation has been driving the development of attosecond science and sources. More recently, high-harmonic generation in solids has been adopted by other communities as a method to study material properties. However, so far high-harmonic generation has only been driven by classical light, despite theoretical proposals to do so with quantum states of light. Here we observe non-perturbative high-harmonic generation in solids driven by a macroscopic quantum state of light, a bright squeezed vacuum, which we generate in a single spatiotemporal mode. The process driven by a bright squeezed vacuum is considerably more efficient in the generation of high harmonics than classical light of the same mean intensity. Due to its broad photon-number distribution, covering states from 0 to 2 × 1013 photons per pulse, and strong subcycle electric field fluctuations, a bright squeezed vacuum gives access to free carrier dynamics within a much broader range of peak intensities than accessible with classical light.

Real-space perspective on dephasing in solid-state high harmonic generation

We develop and demonstrate a fully real-space perspective on high harmonic generation (HHG) in crystals. Due to the Wannier-Stark localization induced on subcycle timescales in the presence of a strong field, real-space descriptions are natural for strongly driven solids. Our approach allows us to address the origin of the extremely short dephasing times, which appear necessary for agreement between experimental HHG measurements and theoretical calculations generally performed in reciprocal space. We develop a physically transparent model of real-space dephasing which relates its rate to the distance between different sites in a laser-driven lattice. Our approach leads to well-structured high harmonic spectra at the microscopic level, reproduces the results of macroscopic propagation, and demonstrates that the requirement for ultrafast dephasing times stems from the need for suppressing recombination events with large electron-hole separations during radiative recombination. Published by the American Physical Society 2024

A systematic evaluation of Random Lasing, Third Harmonic Generation, and quantum chemical calculations in multifunctional perylene derivatives

The quest for multifunctional materials in the field of optoelectronics has become increasingly imperative, as it offers a pathway toward the development of highly versatile and efficient devices. In this study, we present a comprehensive overview of our achievements in the characterization of perylene derivative-based materials. This group of chromophores exhibits remarkable multifunctionality including Third Harmonic Generation (THG), Z-scan results, and excellent Random Lasing (RL) capabilities. Theoretical quantum chemical calculations (QCC) were conducted to investigate dipole moments, frontier molecular orbital HOMO and LUMO energies, optical band gap energies, as well as second hyperpolarizabilities (γ). The study revealed how groups attached to the perylene core influence the chromophore's dipole moment and, consequently, the nonlinear optical (NLO) properties of the investigated compounds. The combined theoretical and experimental analyses provide valuable insights for the design of various optoelectronic devices using thin films of perylene derivatives, enhancing the understanding of their nonlinear and laser action behavior.

Broadband and widely tunable second harmonic generation in suspended thin-film LiNbO3 rib waveguides

Our study demonstrates second harmonic generation (SHG) in a high confinement LiNbO3 rib waveguide through type-I birefringence phase matching of fundamental modes. The combination of micro-waveguide dispersion and material birefringence reveals unique SHG characteristics that complement the performance of standard LiNbO3 on insulator (LNOI) components. Dual-pump wavelength phase matching in the near-infrared and mid-infrared regions is shown in a given waveguide. These two wavelengths can be positioned in the 1.1–3.5 μm range or converge near 1.5 μm by adjusting the core waveguide size or through temperature tuning. A 25 °C temperature change enables a broad pump tunability band of 300 nm, ranging from 1350 to 1650 nm, with a conversion efficiency exceeding 40 %/W/cm2 within a single waveguide. A temperature tuning range of up to 900 nm is foreseen by tailoring the waveguide core size. In addition, a broadband response of 150 nm within the telecom window is demonstrated experimentally. The nonlinear waveguide, etched in a thin film membrane, is combined with titanium-indiffused waveguides to form a monolithic LiNbO3 component. This configuration provides low coupling losses of 0.8 dB and single-mode operation. It paves the way for a new generation of versatile and cost-effective frequency conversion components with wide-band spectral responses suitable for optical communications, environmental sensing, and quantum information processing.

Phase Synchronism in the Generation of Harmonics in Gas Taking into Account Changes in the Wave Vector Detuning during the Propagation of an Intense Femtosecond Laser Pulse

Phase Synchronism in the Generation of Harmonics in Gas Taking into Account Changes in the Wave Vector Detuning during the Propagation of an Intense Femtosecond Laser Pulse

Above-Room-Temperature Ferroelectricity and Giant Second Harmonic Generation in 1D vdW NbOI3.

The realization of spontaneous ferroelectricity down to the one-dimensional (1D) limit is both fundamentally intriguing and practically appealing for high-density ferroelectric and nonlinear photonics. However, the 1D vdW ferroelectric materials are not discovered experimentally yet. Here, the first 1D vdW ferroelectric compound NbOI3 with a high Curie temperature TC>450K and giant second harmonic generation (SHG) is reported. The 1D crystalline chain structure of the NbOI3 is revealed by cryo-electron microscopy, whereas the 1D ferroelectric order originated from the Nb displacement along the Nb-O chain (b-axis) is confirmed via obvious electrical and ferroelectric hysteresis loops. Impressively, NbOI3 exhibits a giant SHG susceptibility up to 1572pmV-1 at a fundamental wavelength of 810nm, and a further enhanced SHG susceptibility of 5582pmV-1 under the applied hydrostatic pressure of 2.06GPa. Combing in situ pressure-dependent X-ray diffraction, Raman spectra measurements, and first-principles calculations, it is demonstrated that the O atoms shift along the Nb─O atomic chain under compression, which can lead to the increased Baur distortion of [NbO2I4] octahedra, and hence induces the enhancement of SHG. This work provides a 1D vdW ferroelectric system for developing novel ferroelectronic and photonic devices.

Table-top source for x-ray absorption spectroscopy with photon energies up to 350 eV.

We present a table-top setup for x-ray absorption spectroscopy (XAS) based on high harmonic generation (HHG) in noble gases. Using sub-millijoule pump pulses at a central wavelength of 1550nm, broadband HHG in the range of 70-350eV was demonstrated. The HHG coherence lengths of several millimeters were achieved by reaching the nonadiabatic regime of harmonic generation. Near edge x-ray absorption fine structure spectroscopy experiments on the boron K edge of a boron foil and a hexagonal boron nitride (hBN) 2D material demonstrate the capabilities of the setup. Femtosecond pulse duration makes pump-probe XAS experiments with corresponding time resolution possible.

Table-top laser-based terahertz high harmonic generation spectroscopy under magnetic fields and low temperatures.

We have developed a terahertz (THz) nonlinear spectrometer at low temperatures (1.5-300K) and under high magnetic fields (up to 10T) by combining the laser-driven table-top intense THz source with a superconducting magnet. The strong-field THz pump pulse was generated from LiNbO3 crystal using the tilted-pulse-front technique and tightly focused into the center of the magnet by an off-axis parabolic mirror and a THz lens. The electric fields at the focus can achieve 500kV/cm with a monocycle waveform and 30kV/cm with a multicycle waveform at 0.5THz. The sample was mounted on a low-temperature motorized rotation stage, which enables performing the polarization dependent measurements of the third harmonic generation (THG) intensity without rotating the incident THz pulses. The magnetic field direction can be rotated using a mechanical rotator, allowing for a convenient switch between Faraday and Voigt geometry. We demonstrate the excellent performance of our instrument by conducting THG measurements in the two-band superconductor MgB2 as a function of temperature, sample azimuth angle, as well as in-plane and out-of-plane magnetic fields. The successful combination of the strong field THz source with magnetic fields enables us to study a variety of materials with magnetic-field-dependent properties of interest.

Growth of high-density Zinc Oxide (ZnO) nanoflowers in all aqueous two-step method and their application in second harmonic generation of femtosecond laser

This study presents a novel two-step all aqueous solution method for the growth of high-density Zinc Oxide (ZnO) nanoflowers, utilizing ZnO powder as a precursor. For this, initially, a seed layer of ZnO nanostructures is prepared using a non-conventional sol-gel technique in an all-aqueous medium. Subsequently, high-density ZnO nanoflowers are synthesized on these seed layers through the chemical bath deposition method. Four samples, each with varying numbers of seed layers (ranging from one to four) are prepared and analyzed using field emission scanning electron microscopy (FESEM). FESEM results indicate that the sample with three seed layers exhibits optimized morphological characteristics. Consequently, further detailed investigation is conducted exclusively on this optimized sample. Furthermore, a preliminary study is done on second harmonic generation (SHG) of this optimized sample.

Efficient third harmonic generation in an all-dielectric metasurface based on tunable bound states in the continuum

Efficient third harmonic generation in an all-dielectric metasurface based on tunable bound states in the continuum

A 25–32-GHz Frequency Doubler With 21% Efficiency and 41-dBc Fundamental Rejection

A 25–32-GHz Frequency Doubler With 21% Efficiency and 41-dBc Fundamental Rejection

Three-dimensional perovskite (1,4-3.2.2-H2dabcn)CsBr3·H2O showing reversible phase transition, dielectric anomaly and second harmonic generation effect

Three-dimensional perovskite (1,4-3.2.2-H2dabcn)CsBr3·H2O showing reversible phase transition, dielectric anomaly and second harmonic generation effect

Spectral analysis of high-order harmonic generation in carbon plasma using linearly, radially, and azimuthally polarized femtosecond pulses

We demonstrate high-order harmonic generation in carbon plasma using linearly, radially, and azimuthally polarized beams from a femtosecond laser (1030nm, 40 fs, 50kHz). The conditions for carbon plasma formation during laser ablation were optimized by adjusting the heating pulse energy in the cases of the single-color (1030nm) and two-color (1030nm + 515nm) pump schemes to generate harmonics up to the 39th order. The application of radial and azimuthally polarized annular single-color (1030nm) beams at optimal conditions of plasma formation allowed the demonstration of the spectral division and inclination of the vector harmonics. An asymmetric distribution of harmonics was obtained using the vortex beam generated during the propagation of laser radiation through the quarter-wave plate and S-waveplate.

Machine learning-based diagnostics of capsular invasion in thyroid nodules with wide-field second harmonic generation microscopy

Papillary thyroid carcinoma (PTC) is one of the most common, well-differentiated carcinomas of the thyroid gland. PTC nodules are often surrounded by a collagen capsule that prevents the spread of cancer cells. However, as the malignant tumor progresses, the integrity of this protective barrier is compromised, and cancer cells invade the surroundings. The detection of capsular invasion is, therefore, crucial for the diagnosis and the choice of treatment and the development of new approaches aimed at the increase of diagnostic performance are of great importance. In the present study, we exploited the wide-field second harmonic generation (SHG) microscopy in combination with texture analysis and unsupervised machine learning (ML) to explore the possibility of quantitative characterization of collagen structure in the capsule and designation of different capsule areas as either intact, disrupted by invasion, or apt to invasion. Two-step k-means clustering showed that the collagen capsules in all analyzed tissue sections were highly heterogeneous and exhibited distinct segments described by characteristic ML parameter sets. The latter allowed a structural interpretation of the collagen fibers at the sites of overt invasion as fragmented and curled fibers with rarely formed distributed networks. Clustering analysis also distinguished areas in the PTC capsule that were not categorized as invasion sites by the initial histopathological analysis but could be recognized as prospective micro-invasions after additional inspection. The characteristic features of suspicious and invasive sites identified by the proposed unsupervised ML approach can become a reliable complement to existing methods for diagnosing encapsulated PTC, increase the reliability of diagnosis, simplify decision making, and prevent human-related diagnostic errors. In addition, the proposed automated ML-based selection of collagen capsule images and exclusion of non-informative regions can greatly accelerate and simplify the development of reliable methods for fully automated ML diagnosis that can be integrated into clinical practice.