Harmonic Generation Research Articles

Efficient Second Harmonic Generation via Plasmonic-Photonic Mode Matching in Hybrid Waveguide.

Hybrid nonlinear plasmonic waveguides, characterized by a small mode area and large nonlinear susceptibility, present an intriguing and practical platform for the minimization of nonlinear photonic devices. Nevertheless, the intrinsic Ohmic loss associated with surface plasmon polaritons (SPPs) and modal dispersion imposes constraints on the effective interaction length and, consequently, the ultimate efficiency of nonlinear processes. In this study, we demonstrate an efficient second harmonic generation (SHG) within a hybrid plasmonic waveguide by leveraging SPP-like modes at the fundamental wave and photonic-like modes at the SHG under phase matching conditions. The presence of photonic-like modes significantly reduces the SHG losses, while the SPP-like modes improve the modal overlap, resulting in SHG conversion efficiency up to 8.5% W-1 cm-2 in a 55 μm-long waveguide. Our findings offer new avenues for the minimization of nonlinear photonic devices on a chip.

A novel workflow for multi-modal imaging of musculoskeletal tissues.

According to the World Health Organization (WHO) musculoskeletal conditions are a leading contributor to disability worldwide. This fact is often somewhat overlooked, since musculoskeletal conditions are less likely to be associated with mortality. Nonetheless, treatments, therapies and management of these conditions are extremely costly to national healthcare systems. As with all systemic conditions, biomedical imaging of relevant tissues plays a major role in understanding the fundamental biological processes involved in musculoskeletal health. However, the skeletal system with its relatively large proportion of dense, opaque (often mineralised) tissues can often be more challenging to image, and recently important advances have been made in imaging these complex musculoskeletal tissues. Thus, we here describe a novel workflow in which recent advanced imaging techniques have been modified and optimised for use in musculoskeletal tissues (specifically bone and cartilage). This will allow for investigations, of different phases of these tissues, at new and higher resolutions. Furthermore, the process has been designed to fit with the existing and standard processes which are typically used with these samples (i.e. μCT imaging and standard histology). The additional modalities which have been included here are second harmonic generation (SHG) imaging, tissue clearing, specifically the Passive Clear Lipid-exchanged Acrylamide-hybridised Rigid Imaging Tissue hYdrogel (CLARITY) method known as PACT, and then imaging of these tissues with confocal, multiphoton and light-sheet microscopy. This paper serves to introduce a combination of existing new methods and improvements in imaging of musculoskeletal tissues.

High harmonic generation in organic molecules: a time-dependent density functional theory (TDDFT) approach

High harmonic generation in organic molecules: a time-dependent density functional theory (TDDFT) approach

Amplify high harmonic generation via anharmonic phonons in PbTe

Amplify high harmonic generation via anharmonic phonons in PbTe

Second harmonic generation from bound-state in the continuum-hosted few-layers van der Waals metasurface

Abstract Monolayer transition metals dichalcogenides (TMDs) have been coupled to bound-state in the continuum (BIC) hosted dielectric structures to attain high second harmonic generation (SHG). However, the transvers electric modes are strongly localized in the waveguides result in fairly weak exciton-photon coupling in monolayer TMD placed on the surface. To achieve SHG in few-layers TMDs based BIC-inspired structure is a challenge. Here, we report BIC in few-layers TMDs metasurface with high quality factor (Q-factor), tunability, and modes-upholding in different environments. The metasurface sustains BIC at different thickness of the meta-atoms, which is highly desired for maintaining the accuracy in fabrications. Next, we calculate the SHG efficiency from few-layers TMD metasurface around BIC wavelengths. The high conversion efficiency in this work is 1.47 × 10−4 for 6 mW incident power. Moreover, our design is highly thin and can be used for various linear and non-linear applications in optics. This study will provide a new route to next generation post-silicon metasurfaces.

High-harmonic generation from subwavelength silicon films

Abstract Recent years have witnessed significant developments in the study of nonlinear properties of various materials at the nanoscale. Often, experimental results on harmonic generation are reported without the benefit of suitable theoretical models that allow assessment of conversion efficiencies compared to the material’s intrinsic properties. Here, we report experimental observations of even and odd harmonics up to the 7th, generated from a suspended subwavelength silicon film resonant in the UV range at 210 nm, the current limit of our detection system, using peak power densities of order 3 TW/cm2. We also highlight the time-varying properties of the dielectric function of silicon, which exhibits large changes under intense illumination. We explain the experimental data with a time domain, hydrodynamic-Maxwell approach broadly applicable to most optical materials. Our approach accounts simultaneously for surface and magnetic nonlinearities that generate even optical harmonics, as well as linear and nonlinear material dispersions beyond the third order to account for odd optical harmonics, plasma formation, and a phase locking mechanism that makes the generation of high harmonics possible deep into the UV range, where semiconductors like silicon start operating in a metallic regime.

Quantum control in size selected semiconductor quantum dot thin films

Abstract We introduce a novel technique for coherent control that employs resonant internally generated fields in CdTe quantum dot (QD) thin films at the L-point. The bulk band gap of CdTe at the L-point amounts to 3.6 eV, with the transition marked by strong Coulomb coupling. Third harmonic generation (λ 3 = 343 nm, hν = 3.61 eV) for a fundamental wavelength of λ 1 = 1,030 nm is used to control quantum interference of three-photon resonant paths between the valence and conduction bands. Different thicknesses of the CdTe QDs are used to manipulate the phase relationship between the external fundamental and the internally generated third harmonic, resulting in either suppression or strong enhancement of the resonant third harmonic, while the nonresonant components remain nearly constant. This development could pave the way for new quantum interference–based applications in ultrafast switching of nanophotonic devices.

XUV yield optimization of two-color high-order harmonic generation in gases

Abstract We perform an experimental two-color high-order harmonic generation study in argon with the fundamental of an ytterbium ultrashort pulse laser and its second harmonic. The intensity of the second harmonic and its phase relative to the fundamental are varied while keeping the total intensity constant. We extract the optimum values for the relative phase and ratio of the two colors which lead to a maximum yield enhancement for each harmonic order in the extreme ultraviolet spectrum. Within the three-step model, the yield maximum can be associated with a flat electron return time versus return energy distribution. An analysis of different distributions allows to predict the required relative two-color phase and ratio for a given harmonic order, total laser intensity, fundamental wavelength, and ionization potential.

A gene-encoded bioprotein second harmonic generation (SHG) probe from Autographa californica nuclear polyhedrosis virus (AcMNPV) polyhedrin for live cell imaging.

Compared to fluorescence, second harmonic generation (SHG) has recently emerged as an excellent signal for imaging probes due to its unmatched advantages in terms of no photobleaching, no phototoxicity, no signal saturation, as well as the superior imaging accuracy with excellent avoidance of background noise. Existing SHG probes are constructed from heavy metals and are cellular exogenous, presenting with high cytotoxicity, difficult cellular uptake, and the limitation of non-heritability. We, therefore, initially propose an innovative gene-encoded bioprotein SHG probe derived from Autographa californica nuclear polyhedrosis virus (AcMNPV) polyhedrin. The primitive gene of AcMNPV polyhedrin was codon-optimized and mutated in its nuclear localization sequence to achieve cytoplasmic expression in mammalian cells. While providing strong SHG signals, this gene-modified AcMNPV (GM-AcMNPV) polyhedrin could be utilized as an SHG probe for cell imaging. Our experimental results demonstrated successful expression of GM-AcMNPV polyhedrin in the cytoplasm of HEK293T cells and bone mesenchymal stem cells (BMSCs), and verified its characteristic features as an SHG probe. Such SHG probes exhibit high biocompatibility and showed no hindering of central physiological activities such as the differentiation of stem cells. Most importantly, our SHG probes may be successfully used for imaging in living cells. This work will inspire the development of gene encoding-derived bioprotein SHG probes, for long-term tracing of cells/stem cells along with their division, to understand stem cell cycles, reveal stem cell-based therapy mechanisms in regenerative medicine, and unravel cell lineage origins and fates in developmental biology, among other potential applications.

Leveraging Optical Anisotropy of the Morpho Butterfly Wing for Quantitative, Stain-Free, and Contact-Free Assessment of Biological Tissue Microstructures.

Changes in the density and organization of fibrous biological tissues often accompany the progression of serious diseases ranging from fibrosis to neurodegenerative diseases, heart disease and cancer. However, challenges in cost, complexity, or precision faced by existing imaging methodologies and materials pose barriers to elucidating the role of tissue microstructure in disease. Here, we leverage the intrinsic optical anisotropy of the Morpho butterfly wing and introduce Morpho-Enhanced Polarized Light Microscopy (MorE-PoL), a stain- and contact-free imaging platform that enhances and quantifies the birefringent material properties of fibrous biological tissues. We develop a mathematical model, based on Jones calculus, which describes fibrous tissue density and organization. As a representative example, we analyzed collagen-dense and collagen-sparse human breast cancer tissue sections and leverage our technique to assess the microstructural properties of distinct regions of interest. We compare our results with conventional Hematoxylin and Eosin (H&E) staining procedures and second harmonic generation (SHG) microscopy for fibrillar collagen detection. Our findings demonstrate that our MorE-PoL technique provides a robust, quantitative, and accessible route toward analyzing biological tissue microstructures, with great potential for application to a broad range of biological materials.

Measurement of the electric field distribution in streamer discharges

Using electric field induced second harmonic generation (E-FISH), we performed direction-resolved absolute electric field measurements on single-channel streamer discharges in 70 mbar (7 kPa) air with 0.2 mm and 2 ns resolutions. In order to obtain the absolute (local) electric field, we developed a deconvolution method taking into account the phase variations of E-FISH. The acquired field distribution shows good agreement with the simulation results under the same conditions, in direction, magnitude, and shape. This is the first time that E-FISH is applied to streamers of this size (>0.5 cm radius), crossing a large gap. Achieving these high resolution electric field measurements benefits further understanding of streamer discharges and enables future use of E-FISH on cylindrically symmetric (transient) electric field distributions. Published by the American Physical Society 2025

Frequency shifts of high-order harmonics from ZnO crystals by the chirped laser pulses

Abstract We investigate theoretically the effects of chirped laser pulses on high-order harmonic generation (HHG) from solids. We find that the harmonic spectra display redshifts for the driving laser pulses with negative chirp and blueshifts for those with positive chirp, which is due to the change in the instantaneous frequency of the driving laser for the different chirped pulses. The analysis of the crystal-momentum-resolved (kresolved) HHG reveals that the frequency shifts are equal for the harmonics generated by different crystal momentum channels. The frequency shifts in the cutoff region are larger than the shifts in the plateau region. With the increase of the absolute value of the chirp parameters, the frequency shifts of HHG become more significant, leading to the shifts from odd- to even-order harmonics. We also demonstrate that the frequency shifts of harmonic spectra are related to the duration of the chirped laser field, but are insensitive to the laser intensity and dephasing time.

Harmonic generation with topological edge states and electron-electron interaction

Harmonic generation with topological edge states and electron-electron interaction

Resonant enhancement of high-order harmonic generation by Ba and Cs atoms

Resonant enhancement of high-order harmonic generation by Ba and Cs atoms

Thermally Controlled A-site Cation Ordering and Coupled Polarity in Double Perovskite NaLaZr2O6.

A-site cation ordering in double perovskites is crucially important for their physical properties. In this study, polycrystalline samples of Zr-based double perovskite NaLaZr2O6 were synthesized via high-temperature solid-state reactions, and the influence of the heating temperature and cooling rate on their crystal structures was investigated using synchrotron X-ray diffractometry and optical second harmonic generation. The samples prepared at 1200 °C, followed by slow cooling to room temperature, crystallize in a polar P21am structure, exhibiting partial A-site cation ordering, with Na- and La-rich A-site layers alternately stacked along the c axis. Remarkably, this structure transforms to a previously reported nonpolar Pnam phase with a disordered A-site cation arrangement when the slow-cooled samples are reheated at 1300 °C or higher and then rapidly quenched to room temperature. Theoretical analyses reveal that in the P21am phase, the a-a-c+ octahedral rotations trigger antiparallel displacements of the A-site cations in the Na- and La-rich A-site layers to induce spontaneous polarization. This is in contrast to the case of the Pnam phase, in which the rotation-induced antiparallel displacements of the equivalent A-site cations are antipolar. This work represents a rare example of AA'B2O6 perovskites exhibiting layered A-site ordering and polarity and also demonstrates a novel mechanism for reversible thermal switching from polar to nonpolar states in perovskites.

Berry curvature and shift vector effects at high-order wave mixing in biased bilayer graphene

In this paper, we present a microscopic quantum theory that elucidates the nonlinear and nonperturbative optical response of biased bilayer graphene subjected to bichromatic strong laser fields. This response is analyzed using a four-band Hamiltonian derived from calculations. For the laser-stimulated dynamics, we employ structure gauge-invariant evolutionary equations to accurately describe the evolution of the single-particle density matrix across the entire Brillouin zone. The resonant generation of electron-hole pairs by the high-frequency component of the field, combined with the induction of high-order harmonic generation and high-order wave mixing by the strong low-frequency field component, leads to significant alterations in the resulting spectra. These changes are driven by the effects of Berry curvature and the shift vector, which modify the relative contributions of interband and intraband channels, thereby fundamentally reshaping the radiation spectra at high-order frequency multiplication. The numerical results are further supported by approximate analytical calculations, demonstrating that high-order wave mixing can be modeled using the classical trajectory analysis of electron-hole pairs, with Berry curvature and the shift vector significantly influencing the saddle-point equations. Published by the American Physical Society 2025

Modeling Femtosecond Beam Propagation in Dielectric Hollow-Core Waveguides

The propagation of femtosecond pulses in guided structures is a matter of both fundamental and practical interest in nonlinear optics. In particular, hollow-core waveguides (HCWs) filled with a gas medium are fabricated and used as devices for the generation of attosecond pulses from high-order harmonics. In this process, the configuration of the laser field (intensity and phase) inside the waveguide is of crucial importance for enhancing the (well-known, low) efficiency of high-order harmonic generation (HHG). Here, we present numerical calculations which demonstrate the main features of the propagation process in fabricated HCWs. We consider a variety of experimental parameters like gas pressure, waveguide size, laser wavelength, and pulse energy and duration. In particular, the beam profile at the fiber input is found to be a sensitive parameter which influences the whole evolution of the laser field along the propagation. Our model is based on a split-step method modified to account for propagation in ionized media and is validated against experimental and theoretical data from the literature. Our results contribute to the description of the main features of beam propagation in HCWs and provide guiding directions for designing efficient configurations for HHG.

Resolving and routing magnetic polymorphs in a 2D layered antiferromagnet.

Polymorphism, commonly denoting diverse molecular or crystal structures, is crucial in the natural sciences. In van der Waals antiferromagnets, a new type of magnetic polymorphism arises, presenting multiple layer-selective magnetic structures with identical total magnetization. However, resolving and manipulating such magnetic polymorphs remain challenging. Here, phase-resolved magnetic second harmonic generation microscopy is used to elucidate magnetic polymorphism in 2D layered antiferromagnet CrSBr, demonstrating deterministic and layer-selective switching of magnetic polymorphs. Using a nonlinear magneto-optical technique, we unambiguously resolve the polymorphic spin-flip transitions in CrSBr bilayers and tetralayers through both the amplitude and phase of light. Remarkably, the deterministic routing of polymorphic spin-flip transitions originates from a 'layer-sharing' effect, where the transitions are governed by laterally extended layers acting as 'control bits'. We envision that such controllable magnetic polymorphism could be ubiquitous for van der Waals layered antiferromagnets, enabling new designs and constructions of spintronic and opto-spintronic devices for probabilistic computation and neuromorphic engineering.

Systematic analysis of an attosecond pulse generation by a subcycle laser field

We investigated the influence of subcycle driving fields on high-order harmonic generation (HHG), with a focus on driver's intrinsic chirp, carrier-envelope phase (CEP), and number of laser cycles. Our findings reveal that the center frequency of a laser pulse scales as τ−5/4 with pulse duration τ, and that attochirp exhibits a similar dependence on pulse duration. Additionally, we identified CEP-specific trends in harmonic yield: it increases as τ5/4 for ϕ0=0∘ and decreases as τ−4.1 for ϕ0=−90∘. Although subcycle pulses can generate intense isolated attosecond pulses (IAPs), they also tend to produce higher attochirp and reduced cutoff energies. However, effective compensation for attochirp can mitigate these drawbacks, thereby increasing the capability of subcycle pulses to generate short-duration, high-intensity IAPs. These results offer valuable insights into HHG using subcycle pulses and have important implications for the advancement of ultrafast light sources and the understanding of ultrafast phenomena at the attosecond timescale. Published by the American Physical Society 2025

Quantitative analysis on the optical kerr impact and third harmonic generation in beltrami-shaped curved graphene

We present a comprehensive analysis of the optical attributes of graphene sheets with charge carriers residing on a curved substrate. In particular, we focus on the fascinating case of Beltrami geometry and provide an explicit parametrization for this curved two-dimensional surface. By employing the massless Dirac description that is characteristic of graphene, we investigate the impact of the curved geometry on the optical properties within the sample. Our findings reveal that the optical properties of the system are highly sensitive to several key factors, namely the Beltrami radius, surface radius, chemical potential and relaxation time. The numerical findings demonstrate that the optical characteristics of Beltrami-shaped graphene differ significantly from those of regular graphene due to the curved geometry. This distinction opens up exciting possibilities for exploring new optical phenomena and designing graphene-based devices with customized optical properties.