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Optimizing the parameters that control the InAsP/InP quantum well (QW) system is of utmost importance for this system to give the best yield. Beginning from QW energy levels and momentum matrix element calculation, this work studies linear, nonlinear absorption and refractive index change, second harmonic generation (SHG), and third-harmonic generation (THG) in InAsP/InP QWs. Parameters controlling the structure, like hydrostatic pressure, temperature, well width, and phosphor mole fraction, are studied. The results show that the difference between energy levels increases with increasing temperature or reducing pressure. These differences are essential in obtaining SHG and THG. Pressure is more effective than the temperature in changing energy differences while increasing phosphor reduces the energy difference. So, one can choose the adjusted parameter (pressure or composition) according to the change in energy required. The pressure reduces absorption while temperature increases it. Increasing phosphor mole fraction reduces absorption; at high mole fraction, the absorption is approximately constant. The total refractive index change (RIC) is reduced with pressure or increasing molarity, but the higher molarity increases RIC. Increasing phosphor mole fraction reduced SHG and vice versa to THG. Increasing temperature increases SHG, and blue shifts its peak. For the pressure, it reduces SHG and redshifts its peak. Pressure increases THG and redshifts its peak while the temperature blue shifts THG peak. The effect of the temperature on the peak of the THG depends on the pressure: the temperature reduces the peak at low pressure, while the temperature increases it at high pressure. The results show the importance of such a study because the optical properties do not have a single-parameter effect; it has an interference effect of many parameters to produce the result. [ABSTRACT FROM AUTHOR]

Optical vortices have the tremendous potential to increase data capacity by leveraging the extra degree of freedom of orbital angular momentum. On the other hand, anisotropic 2D materials are promising building blocks for future integrated polarization‐sensitive photonic and optoelectronic devices. Here, highly anisotropic third‐harmonic optical vortex beam generation is demonstrated with fork holograms patterned on ultrathin 2D germanium arsenide flakes. It is shown that the anisotropic nonlinear vortex beam generation can be achieved independent of the fork grating orientation with respect to the crystallographic orientation. Furthermore, 2D fork hologram is designed to generate multiple optical vortices having different topological charges with strong anisotropic responses. These results pave the way toward the advancement of 2D material‐based anisotropic nonlinear optical devices for future applications in photonic integrated circuits, optical communication, and optical information processing. [ABSTRACT FROM AUTHOR]

We report on the theoretical/numerical investigation of simultaneous second- and third-harmonic generation from a single wavelength input in quasi-phase-matched crystals. The presented technique consists of a quadratic crystal with two first-order quasi-phase-matched sections: one designed for quasi-phase-matching to second-harmonic generation and the other for quasi-phase-matching to third-harmonic generation via sum-frequency generation. We identify an optimal length ratio (optimal number of domains) for these sections in order to enhance the conversion to the third harmonic, achieving nearly complete energy transfer. The advantages of the method are demonstrated both numerically and analytically, with a specific example using periodically poled lithium niobate. Quadratic cascading with quasi-phase-matching proves to be an effective approach for achieving cubic-like effects with high conversion efficiencies. [ABSTRACT FROM AUTHOR]

Nonlinear nanophotonic devices have shown great potential for on‐chip information processing, quantum source, 3D microfabrication, greatly promoting the developments of integrated optics, quantum science, nanoscience and technologies, etc. To promote the applications of nonlinear nanodevices, improving the nonlinear efficiency, expanding the spectra region of nonlinear response and reducing device thickness are three key issues. Herein, this study focuses on the nonlinear effect of third‐harmonic generation (THG), and present a thin Si meta‐sructure to improve the THG efficiency in the ultraviolet (UV) region. The measured THG efficiency is up to 10−5 at an emission wavelength of 309 nm. Also, the THG nanosystem is only 100 nm in thickness, which is two–five times thinner than previous all‐dielectric nanosystems applied in THG studies. These findings not only present a powerful thin meta‐structure with highly efficient THG emission in UV region, but also provide a constructive avenue for further understanding the light–matter interactions at subwavelength scales, guiding the design and fabricating of advanced photonic devices in future. [ABSTRACT FROM AUTHOR]

Effective interplay between the uterus and the embryo is essential for pregnancy establishment; however, convenient methods to screen embryo implantation success and maternal uterine response in experimental mouse models are currently lacking. Here, we report 3DMOUSEneST, a groundbreaking method for analyzing mouse implantation sites based on label-free higher harmonic generation microscopy, providing unprecedented insights into the embryo--uterine dynamics during early pregnancy. The 3DMOUSEneST method incorporates second-harmonic generation microscopy to image the three-dimensional structure formed by decidual fibrillar collagen, named 'decidual nest', and third-harmonic generation microscopy to evaluate early conceptus (defined as the embryo and extra-embryonic tissues) growth. We demonstrate that decidual nest volume is a measurable indicator of decidualization efficacy and correlates with the probability of early pregnancy progression based on a logistic regression analysis using Smad1/5 and Smad2/3 conditional knockout mice with known implantation defects. 3DMOUSEneST has great potential to become a principal method for studying decidual fibrillar collagen and characterizing mouse models associated with early embryonic lethality and fertility issues. [ABSTRACT FROM AUTHOR]

Si wafers are vital substrate materials in semiconductor manufacturing and require precise non-destructive thickness measurements. However, the conventional electrical and optical measurement techniques are limited by depth selectivity and system complexity. Here, we propose a simple, high-precision, non-destructive thickness measurement method based on surface-sensitive optical third-harmonic generation at both sides of Si wafers. We irradiated a highly stabilized near-infrared femtosecond pulse laser with a broad spectrum and central wavelength of 1550 nm on the Si wafers, which are non-transparent in the visible to ultraviolet wavelength range. Using the proposed system, the thickness of the certified reference wafer was measured, yielding results that fall within the certified uncertainty.

Nonlinear dielectric metasurfaces provide a promising approach to control and manipulate frequency conversion optical processes at the nanoscale, thus facilitating both advances in fundamental research and the development of new practical applications in photonics, lasing, and sensing. Here, we employ symmetry-broken metasurfaces made of centrosymmetric amorphous silicon for resonantly enhanced second- and third-order nonlinear optical response. Exploiting the rich physics of optical quasi-bound states in the continuum and guided mode resonances, we comprehensively study through rigorous numerical calculations the relative contribution of surface and bulk effects to second-harmonic generation (SHG) and the bulk contribution to third-harmonic generation (THG) from the meta-atoms. Next, we experimentally achieve optical resonances with high quality factors, which greatly boosts light-matter interaction, resulting in about 550 times SHG enhancement and nearly 5000-fold increase of THG. A good agreement between theoretical predictions and experimental measurements is observed. To gain deeper insights into the physics of the investigated nonlinear optical processes, we further numerically study the relation between nonlinear emission and the structural asymmetry of the metasurface and reveal that the generated harmonic signals arising from linear sharp resonances are highly dependent on the asymmetry of the meta-atoms. Our work suggests a fruitful strategy to enhance the harmonic generation and effectively control different orders of harmonics in all-dielectric metasurfaces, enabling the development of efficient active photonic nanodevices.

We investigate resonant third-harmonic generation in near-zero index thin films driven out-of-equilibrium by intense optical excitation. Adopting the Landau weak coupling formalism to incorporate electron–electron and electron–phonon scattering processes, we derive a novel set of hydrodynamic equations accounting for collision-driven nonlinear dynamics in sodium. By perturbatively solving hydrodynamic equations, we model third-harmonic generation by a thin sodium film, finding that such a nonlinear process is resonant at the near-zero index resonance of the third-harmonic signal. Thanks to the reduced absorption of sodium, we observe that third-harmonic resonance can be tuned by the impinging pump radiation angle, efficiently modulating the third-harmonic generation process. Furthermore, owing to the metallic sodium response at the pump optical wavelength, we find that the third-harmonic conversion efficiency is maximised at a peculiar thin film thickness where evanescent back-reflection provides increased field intensity within the thin film. Our results are relevant for the development of future ultraviolet light sources, with potential impact for innovative integrated spectroscopy schemes. [ABSTRACT FROM AUTHOR]

Solid‐state hybrid materials with designability in topology and tunable photo‐responsiveness hold great promise for multitudinous applications from optoelectronics to information storage/communication. However, the judicious design and synthesis of metal–organic frameworks (MOFs) with intrinsically custom‐made nonlinear optical (NLO) properties including multiple photon absorption and harmonic generation effects suitable for miniaturized devices remain highly challenging. Herein, the design and synthesis of a novel pillared framework named after Zn‐TCPE‐DPNI MOF coordinated by zinc ions (Zn2+), aggregation induced emission (AIE) featured [1,1′‐biphenyl]‐4‐carboxylic acid (H4TCPE), and N,N′‐di(4‐pyridyl)‐1,4,5,8‐naphthalenetetracarboxydiimide (DPNI) are reported. The highly ordered Zn‐TCPE‐DPNI MOF single crystal exhibits tunable NLO responses by switching incident excitation wavelengths of a femtosecond laser from 900 to 1500 nm. The noticeable transformation from two‐ or three‐photon excited photoluminescence (2PL or 3PL) to the concurrent occurrence of 3PL and third harmonic generation (THG), and eventually to the mere emergence of THG with a high effective susceptibility χ(3) (3ω) of 2.9 × 10−12 esu is observed. This study paves a novel avenue for the design and synthesis of photoluminescent MOF crystals with tunable NLO responses toward the fabrication of multifunctional NLO devices targeted for optoelectronics and information communication. [ABSTRACT FROM AUTHOR]

Nonlinear photonic crystals with 3D orthorhombic lattice structures were fabricated using the femtosecond laser-poling technique in ferroelectric Sr0.28Ba0.72Nb2O6 (SBN) crystals. The crystals were used to demonstrate the possibility of generating cascaded third-harmonic waves in optically poled ferroelectric structures. The spectral response and conversion efficiency of the third-harmonic process were experimentally investigated. While the nonlinear cascading processes can be commonly realized in electric-field-poled ferroelectric crystals, their generation in optically poled ferroelectric domain structures have not been reported elsewhere. In addition to the fully phase-matched nonlinear interaction, Cherenkov-type third-harmonic generation that fulfills the longitudinal phase-matching condition was also experimentally studied. Our study contributes to exploring the full potential of optically induced nonlinear photonic crystals and provides a new choice of materials for third-harmonic generation. [ABSTRACT FROM AUTHOR]

Dielectric metasurfaces play an increasingly important role in enhancing optical nonlinear generations owing to their ability to support strong light-matter interactions based on Mie-type multipolar resonances. Compared to metasurfaces composed of the periodic arrangement of nanoparticles, inverse, so-called, membrane metasurfaces offer unique possibilities for supporting multipolar resonances, while maintaining small unit cell size, large mode volume and high field enhancement for enhancing nonlinear frequency conversion. Here, we theoretically and experimentally investigate the formation of bound states in the continuum (BICs) from silicon dimer-hole membrane metasurfaces. We demonstrate that our BIC-formed resonance features a strong and tailorable electric near-field confinement inside the silicon membrane films. Furthermore, we show that by tuning the gap between the holes, one can open a leaky channel to transform these regular BICs into quasi-BICs, which can be excited directly under normal plane wave incidence. To prove the capabilities of such metasurfaces, we demonstrate the conversion of an infrared image to the visible range, based on the Third-harmonic generation (THG) process with the resonant membrane metasurfaces. Our results suggest a new paradigm for realising efficient nonlinear photonics metadevices and hold promise for extending the applications of nonlinear structuring surfaces to new types of all-optical near-infrared imaging technologies.

As new states of matter currently attracting broad attention in the optics field, topological insulators (TIs) present topologically‐protected conducting surface states surrounding insulating bulk and prominent third‐order optical nonlinearities in an ultrawide frequency range. Optical frequency conversion in TI nanofilms plays a crucial role in TI nonlinear optical functionalities, whose enhancement is in demand for high‐performance devices. Herein, an optical Tamm state is observed between the Sb2Te3 TI nanofilm and 1D photonic crystal (PC) with a self‐enhancement behavior of optical nonlinearity in the TI nanofilm. The experiments demonstrate that there exists a distinct reflection dip in the forbidden band of the PC, which is attributed to the appearance of Tamm state in the TI/PC structure. The wavelength of the TI‐based Tamm state possesses a nearly linear redshift with increasing TI film thickness. The measurements are in excellent agreement with simulation and theoretical calculations. Moreover, a strong third‐harmonic generation reinforcement of 30‐fold is experimentally observed in a 90 nm Sb2Te3 film, which stems from the field enhancement induced by the TI self‐excited Tamm state at the fundamental frequency. This work opens a new door for generation of optical Tamm states and applications of TIs in high‐efficiency nonlinear optical functionalities, especially frequency conversion. [ABSTRACT FROM AUTHOR]

The third harmonic generation of cosh‐Gaussian laser pulses in inhomogeneous magnetized plasmas is investigated. The plasma is considered inhomogeneous with a sinusoidal form, and the external magnetic field has an oscillating wiggler structure. Various ponderomotive forces are obtained considering the external magnetic field, the magnetic field of the laser beam, and different orders of electron velocity. These forces are nonlinear, which leads to a nonlinear current density. Using linear and nonlinear current densities and Maxwell's equations, the wave equation of the third harmonic wave is obtained. Results show that by increasing the deviation parameter from the center and the external magnetic field, the third harmonic electric field increases. It is also indicated that by increasing the laser beam width, the excited electric field decreases. Finally, it is found that by increasing the wavenumber and amplitude of the rippled electron density, the amplitude of the third harmonic electric field increases. [ABSTRACT FROM AUTHOR]

Metal‐organic frameworks (MOFs) have emerged as candidate materials for nonlinear optics due to their enhanced optical and chemical stability in comparison with conventional organic crystals. However, producing large single crystals that support perfect phase matching conditions for frequency conversion is a long‐standing challenge due to the highly metastable conditions in which MOF crystals tend to self‐assemble in solution. By modulating the synthesis and growth conditions, this limitation is overcome to produce millimeter–sized Zn(3‐ptz)2 uniaxial MOF single crystals. Optimized MOF crystals with large birefringence in the visible Δn ≈ −0.3 and high transparency allow for the observation of strong second‐harmonic (SHG) and third‐harmonic generation (THG) signals for the first time, using femtosecond near‐infrared pump pulses. For conditions of type‐I SHG phase‐matching, the measured effective nonlinear coefficient of Zn(3‐ptz)2 is deff ≈ 0.10 pm V−1, the largest measured nonlinearity for MOF materials to date. The experiments quantitatively agree with first‐principles simulations based on the crystal lattice structure. The damage threshold is estimated on the order of 0.2 TW cm−2 for raw single crystals, which can be further increased with additional crystal engineering steps. The demonstration of efficient frequency up‐conversion of light with long‐range phase coherence establishes MOF single crystals as promising materials for nonlinear optical devices. [ABSTRACT FROM AUTHOR]

The nonlinear second-harmonic generation (SHG) and third-harmonic generation (THG) in "12–6" tuned GaAs/GaAlAs double quantum wells (QWs) are calculated in the presence of applied electric field under the effective mass approximation. The influence of the structural parameter of the double QW is also considered, which is the main means to adjust its nonlinear optical properties. The eigenvalues and corresponding eigenfunctions of the double QWs system are obtained via the finite difference technique. The nonlinear SHG and THG are presented as a function of the photon energy, structural parameter, and applied electric field. It is shown that the SHG and THG are sensitive to both the applied electric field and the structure parameters, which can tune the nonlinear optical properties efficiently by adjusting the symmetry and confined potential of the double QWs. [ABSTRACT FROM AUTHOR]

The skin is heterogeneous and exerts strong scattering and aberration onto excitation light in multiphoton microscopy (MPM). Shifting to longer excitation wavelengths may help reduce skin scattering and aberration, potentially enabling larger imaging depths. However, previous demonstrations of skin MPM employ excitation wavelengths only up to the 1700 nm window, leaving an open question as to whether longer excitation wavelengths are suitable for deep-skin MPM. Here, in order to explore the longer-wavelength territory, first, we demonstrate characterization of the broadband transmittance of excised mouse skin, revealing a high transmittance window at 2200 nm. Then, we demonstrate third-harmonic generation (THG) imaging in mouse skin in vivo excited at this window. With 9 mW optical power on the skin surface operating at 1 MHz repetition rate, we can get THG signals of 250 μ m below the skin surface. Comparative THG imaging excited at the 1700 nm window shows that as imaging depth increases, THG signals decay even faster than those excited at 2200 nm. Our results thus uncover the 2200 nm window as a new, promising excitation window potential for deep-skin MPM. [ABSTRACT FROM AUTHOR]

Quantum dots can modify the properties of the whispering gallery mode resonators (WGMRs) used in various potential applications. A deposition of a suitable nanomaterial for the surface functionalization of WGMRs allows for the achievement of high quality (Q) factors. Here, we show that the WGMR surface can be functionalized using quantum dots. We demonstrate that WGMRs covered with thin layers of HgS and PbS quantum dots are suitable for third-harmonic generation due to the high Q factor of the developed microresonators, thus significantly lowering the pumping power required for nonlinear optical interactions. [ABSTRACT FROM AUTHOR]

Plasmonic nanoantennas have been extensively explored to boost nonlinear optical processes due to their capabilities to confine optical fields on the nanoscale. In harmonic generation, nanoantenna array architectures are often employed to increase the number of emitters in order to efficiently enhance the harmonic emission. A small laser focus spot on the nanoantenna array maximizes the harmonic yield since it scales nonlinearly with the incident laser intensity. However, the nonlinear yield of the nanoantennas lying at the boundary of a focused beam may exhibit significant deviations in comparison to those at the center of the beam due to the Gaussian intensity distribution of the beam. This spatial beam inhomogeneity can cause power instability of the emitted harmonics when the lateral beam position is not stable which we observed in plasmon-enhanced third-harmonic generation (THG). Hence, we propose a method for deterministically designing the density of a nanoantenna array to decrease the instability of the beam position-dependent THG yield. This method is based on reducing the ratio between the number of ambiguous nanoantennas located at the beam boundary and the total number of nanoantennas within the beam diameter to increase the plasmon-enhanced THG stability, which we term as the ratio of ambiguity (ROA). We find that the coefficient of variation of the measured plasmonic THG yield enhancement decreases with the ROA. Thus, our method is beneficial for designing reliable sensors or nonlinear optical devices consisting of nanoantenna arrays for enhancing output signals.

Herein, significant enhancement of second- and third-harmonic generation efficiencies in a 1 mol% Er3+ and 0.07 mol% Fe2+-doped lithium niobate single-crystal plate were achieved after ablating periodic cylindrical pit arrays on the surface. Enhanced absorption and reduced transmittance of light were measured when the incident light signal passed through the patterned sample. Enhanced photoluminescence and two-photon-pumped upconversion emission spectra were also explored to obtain more details on the efficiency gains. The excitation-energy-dependent second-harmonic generation efficiency was measured, and an enhancement as high as 20-fold was calculated. The conversion efficiency of second-harmonic generation is 1 to 3 orders higher than that from other lithium niobite metasurfaces and nanoantennas. This work provides a convenient and effective method to improve the nonlinear conversion efficiency in a thin lithium niobite plate, which is desirable for applying to integrated optical devices. [ABSTRACT FROM AUTHOR]

An atomically precise ultrasmall Au(I)6 nanocluster where the six gold atoms are complexed by three sterically interlocking stabilizing ligands is reported, allowing a unique combination of efficient third harmonic generation (THG), intense photoluminescence quantum yield (35%), ultrafast quantum coherence, and electron accepting properties. The reaction of 6‐(dibutylamino)‐1,3,5‐triazine‐2,4‐dithiol (TRZ) with HAuCl4 leads to complexation by thiolation. However, intriguingly, another reduction step is needed to form the centrosymmetric Au(I)6TRZ3 clusters with the multifunctional properties. Here, ascorbic acid is employed as a mild reducing agent, in contrast to the classic reducing agents, like NaBH4 and NaBH3CN, which often produce mixtures of clusters or gold nanoparticles. Such Au(I)6 nanocluster films produce very strong THG response, never observed for nanoclusters. The clusters also produce brilliant single and multiphoton luminescence with exceptional stability. Density functional theory calculations and femtosecond transient absorption studies suggest ultrafast ligand‐to‐metal charge transfer, quantum coherence with long decoherence time 200–300 fs, and fast propagation of excitation from the core to the surrounding solvent. Finally, novel electron‐accepting ground state properties allow p‐doping of 2D field‐effect transistor devices. Summarizing, the potential of ultrasmall sterically interlocked Au(I) clusters, i.e., complexes allowed by the new sequential reduction protocol, towards multifunctional devices, fast photoswitches, and quantum colloidal devices is shown. [ABSTRACT FROM AUTHOR]

Nonlinear photonic crystals with 3D orthorhombic lattice structures were fabricated using the femtosecond laser-poling technique in ferroelectric Sr0.28Ba0.72Nb2O6 (SBN) crystals. The crystals were used to demonstrate the possibility of generating cascaded third-harmonic waves in optically poled ferroelectric structures. The spectral response and conversion efficiency of the third-harmonic process were experimentally investigated. While the nonlinear cascading processes can be commonly realized in electric-field-poled ferroelectric crystals, their generation in optically poled ferroelectric domain structures have not been reported elsewhere. In addition to the fully phase-matched nonlinear interaction, Cherenkov-type third-harmonic generation that fulfills the longitudinal phase-matching condition was also experimentally studied. Our study contributes to exploring the full potential of optically induced nonlinear photonic crystals and provides a new choice of materials for third-harmonic generation.

We investigate the generation of ultrabroadband femtosecond ultraviolet (UV) radiation via third-order harmonic generation in highly confined gas media. A dual-stage differential-pumping scheme integrated into a glass microfluidic chip provides an exceptional gas confinement up to several bar and allows the apparatus to be operated under high-vacuum environment. UV pulses are generated both in argon and neon with up to ∼0.8 μ J energy and 0.2% conversion efficiency for spectra that cover the UVB and UVC regions between 200 and 325 nm. Numerical simulations based on the unidirectional pulse propagation equation reveal that ionization plays a critical role for extending the spectral bandwidth of the generated third-harmonic pulse beyond the tripled 800 nm driving laser pulse bandwidth. By delivering UV supercontinua supporting Fourier transform limits below 2 fs, as well as comparable pulse energies with respect to capillary-based techniques that typically provide high spectral tunability but produce narrower bandwidths, our compact device makes a step forward towards the production and application of sub-fs UV pulses for the investigation of electron dynamics in neutral molecules.

Polarization control through all-dielectric metasurfaces holds great potential in different fields, such as telecommunications, biochemistry and holography. Asymmetric chiral metasurfaces supporting quasi-bound states in the continuum may prove very useful for controlling and manipulating the polarization state of light. A crucial quantity for characterizing the optical chirality is the circular dichroism (CD). In this work we analyse how the CD and quality factor of the optical mode can be strongly influenced by a nanofabrication error. Modelling the nanofabrication uncertainties on the gaps of the chiral metasurface, the imperfections of the etchings process or the modification of the asymmetry factor, we found that the proper engineering of the gap between the nanostructures of the unit cell is the most important parameter to achieve a high-quality factor and enhanced optical dichroism. An optimization of the nanofabrication processes, such as dose factor, dwell time and plasma etching demonstrates that, for a writing field of 100 μm2, it is possible to obtain morphologically precise chiral metasurfaces, with fabrication uncertainties lower than those that would limit Q factor and chirality property.

The third‐order susceptibility χ(3) of lithium niobate (LiNbO3) is calculated within a Berry‐phase formulation of the dynamical polarization based on the electronic structure obtained within density‐functional theory (DFT). Maximum |χzzzz(3)| values of the order of 10−19 m2 V−2 are calculated for photon energies between 1.2 and 2 eV, i.e., in the lower half of the optical bandgap of lithium niobate. Both free and bound electron (bi)polarons are found to lead to a remarkable enhancement of the third‐order susceptibility for photon energies below 1 eV. [ABSTRACT FROM AUTHOR]

Plasmonic nanoantennas have been extensively explored to boost nonlinear optical processes due to their capabilities to confine optical fields on the nanoscale. In harmonic generation, nanoantenna array architectures are often employed to increase the number of emitters in order to efficiently enhance the harmonic emission. A small laser focus spot on the nanoantenna array maximizes the harmonic yield since it scales nonlinearly with the incident laser intensity. However, the nonlinear yield of the nanoantennas lying at the boundary of a focused beam may exhibit significant deviations in comparison to those at the center of the beam due to the Gaussian intensity distribution of the beam. This spatial beam inhomogeneity can cause power instability of the emitted harmonics when the lateral beam position is not stable which we observed in plasmon-enhanced third-harmonic generation (THG). Hence, we propose a method for deterministically designing the density of a nanoantenna array to decrease the instability of the beam position-dependent THG yield. This method is based on reducing the ratio between the number of ambiguous nanoantennas located at the beam boundary and the total number of nanoantennas within the beam diameter to increase the plasmon-enhanced THG stability, which we term as the ratio of ambiguity (ROA). We find that the coefficient of variation of the measured plasmonic THG yield enhancement decreases with the ROA. Thus, our method is beneficial for designing reliable sensors or nonlinear optical devices consisting of nanoantenna arrays for enhancing output signals. [ABSTRACT FROM AUTHOR]

We investigate the generation of optical third-harmonic frequency in quadratic crystals with a nonlinear domain lattice optimized with the aid of a random number generator. In the developed Monte Carlo algorithm and numerical experiments, we consider domain thicknesses to be taking either the values d1 or d2, with d1 and d2 being the coherence lengths for the cascaded parametric interactions 2 ω = ω + ω and 3 ω = 2 ω + ω , respectively. We focus on the cases with single segments formed by equal and/or different domains, showing that frequency tripling can be achieved with high conversion efficiency from an arbitrary input wavelength. The presented approach allows one to accurately determine the optimized random alternation of domain thicknesses d1 and d2 along the propagation length. [ABSTRACT FROM AUTHOR]

Recent advancements in nonlinear nanophotonics are driven by the exploration of sharp resonances within high-index dielectric metasurfaces. In this work, we leverage doubly degenerate quasi-bound states in the continuum (quasi-BICs) to demonstrate the robust enhancement of third-harmonic generation (THG) in silicon metasurfaces. These quasi-BICs are governed by C 4 v symmetry and therefore can be equally excited with the pump light regardless of polarization. By tailoring the geometric parameters, we effectively control Q -factors and field confinement of quasi-BICs and thus regulate their resonantly enhanced THG process. A maximum THG conversion efficiency up to 1.03 × 10 -5 is recorded under a pump intensity of 5.85 GW/cm 2 . Polarization-independent THG profiles are further confirmed by mapping their signals across the polarization directions. This work establishes foundational strategies for the ultracompact design of robust and high-efficiency photon upconversion systems.

Nonlinear nanophotonic devices have brought about great advances in the fields of nano-optics, quantum science, biomedical engineering, etc. However, in order to push these nanophotonic devices out of laboratory, it is still highly necessary to improve their efficiency. Since obtaining novel nanomaterials with large nonlinearity is of crucial importance for improving the efficiency of nonlinear nanodevices, we propose the two-dimensional (2D) perovskites. Different from most previous studies which focused on the 2D perovskites in large scale (such as the bulk materials or the thick flakes), herein we studied the 2D perovskites nanosheets with thickness of ∼50 nm. The high-order nonlinear processes including multi-photon photoluminescence and third-harmonic generation (THG) have been systematically investigated, and it is found the THG process can have a high conversion efficiency up to ∼8 × 10−6. Also, it is observed that the nonlinear responses of 2D perovskites have large optical anisotropy, i.e., the polarization ratio for the incident polarization dependence of nonlinear response can be as high as ∼0.99, which is an impressive record in the perovskite systems. Our findings reveal the properties of high efficiency and huge optical anisotropy in the nonlinear processes of 2D perovskite nanosheets, shedding light on the design of advanced integrated nonlinear nanodevices in future.

Non-linear metamaterials hold great promise for enhanced terahertz harmonic generation. Here, we numerically investigate enhanced terahertz third-harmonic generation (THG) by exploiting the symmetry-protected bound states in the continuum (BICs) in graphene grating-like metamaterial. By symmetry breaking of metamaterials, BICs transform into quasi-BICs. The high Q-factor and strong localized field enhancement is achieved at quasi-BICs, leading to a boosted THG process with low incident intensity of fundamental frequency. The THG conversion efficiency reaches 3.1% at an incident intensity of 100 kW/cm2. The remarkably enhanced non-linear optical process in the proposed non-linear metamaterial constitutes an outstanding platform for on-chip terahertz non-linear conversion applications.

We discuss a formalism that allows for the calculation of a higher-harmonic-current response to a strong applied electric field for disordered superconducting systems described on the basis of tight-binding models with on- and/or intersite interactions. The theory is based on an expansion of the density matrix in powers of the field amplitudes, where we solve the equation of motion for the individual components. This allows the evaluation of higher-order response functions on significantly larger lattices than one can achieve with a previously used approach, which is based on a direct temporal integration of the equation of motion for the complete density matrix. In the case of small lattices, where both methods can be applied by including also the contribution of collective modes, we demonstrate the agreement of the corresponding results.

Nano-optics explores linear and nonlinear phenomena at the nanoscale to advance fundamental knowledge about materials and their interaction with light in the classical and quantum domains in order to develop new photonics-based technologies. In this perspective article, we review recent progress regarding the application of nonlinear optical methods to reveal the links between photonic structures and functions of natural photonic geometries. Furthermore, nonlinear optics offers a way to unveil and exploit the complexity of the natural world for developing new materials and technologies for the generation, detection, manipulation, and storage of light at the nanoscale, as well as sensing, metrology, and communication. [ABSTRACT FROM AUTHOR]

Multiphoton microscopy (MPM) imaging relies on the nonlinear interaction between ultrashort optical pulses and the samples to achieve image contrast. Featuring larger penetration depth, less phototoxicity, 3-dimensional sectioning capability, no need for labeling, MPM become a powerful medical imaging technique that can identify structural characteristics of tissues at the cellular and subcellular levels. In this review paper, we introduce the working principle of MPM imaging, present the current results of MPM imaging applied to the study of gastric tumors, and discuss the future prospects of this interdisciplinary research field. [ABSTRACT FROM AUTHOR]

The control of nonlinear optical signals in nanostructured systems is pivotal to develop functional devices suitable for integration in optical platforms. A possible control mechanism is exploiting coherent interactions between different nonlinear optical processes. Here, this concept is implemented by taking advantage of the strong field enhancement and high optical nonlinearity provided by plasmonic nanostructures. Two beams, one at the angular frequency ω, corresponding to the telecom wavelength λ = 1551 nm, and the other at 2ω, are combined to generate a sum‐frequency signal at 3ω from single asymmetric gold nanoantennas. This nonlinear signal interferes with the third‐harmonic radiation generated by the beam at ω, resulting in a modulation up to 50% of the total signal at 3ω depending on the relative phase between the beams. Such a large intensity modulation of the nonlinear signal is accompanied by a rotation of its polarization axis, due to the lack of central symmetry of the nanostructure. The demonstration that the nonlinear emission can be coherently controlled through the phase difference of the two‐color illumination represents a promising route toward all‐optical logic operations at the nanoscale through nonlinear optical signal manipulation. [ABSTRACT FROM AUTHOR]

Optically resonant dielectric and semiconductor metasurfaces are an emerging and promising area of nanophotonics and light–matter interaction at the nanoscale. Recently, active tuning of the linear response and nonlinear effects of these components has received an increasing amount of interest. However, so far these research directions have remained separated with only few sporadic works that study their combination beginning to appear in the literature. The evolution of nonlinear metasurfaces based on dielectric and semiconductor materials toward reconfigurable and dynamic components could potentially answer the demand of integrated on-chip components that realize essential functionalities such as frequency conversion, active switching, optical isolation, and all-optical routing. This review provides an overview of recent investigations in this field, reviews the main physical phenomena enabling the dynamic control of the nonlinear response and compares the temporal dynamics of the diverse approaches that have been explored so far. Finally, future directions of dynamic nonlinear metasurfaces are outlined.

Despite remarkable progress in passive nonlinear nanophotonics, dynamically controlled ultracompact nonlinear optical sources with high efficiency remain elusive. Nonvolatility, large refractive index contrast between amorphous and crystalline phases, low thermal threshold for crystallization, and particularly high‐third‐order nonlinear optical susceptibility of phase‐change alloy Ge2Sb2Te5 (GST) make it a promising candidate for active subwavelength metaphotonic structures for realization of third‐harmonic generation (THG) devices. Herein, a GST‐based asymmetric Fabry–Perot cavity is numerically designed and experimentally demonstrated to show a dynamically reconfigurable structure with a large shift of the THG resonant band. In addition, continuous resonant spectral shifting bridged by a precisely controlled semicrystalline phase of GST is realized. Tunable THG with the fundamental wavelength ranging from 1150 to 1400 nm is presented, which corresponds to a broadband THG source in the violet‐blue visible wavelength range. Herein, the potential of GST subwavelength structures as a reliable platform for realization of the broadband‐tunable frequency‐conversion sources for applications such as THG microscopy and optical communication is indicated.

The overall effectiveness of nonlinear optical processes along extended nonlinear media highly depends on the fulfillment of the phase‐matching condition for pump and generated fields. This is traditionally accomplished by exploiting the birefringence of nonlinear crystals requiring long interaction lengths (cm‐scale). For nonbirefringent media and integrated photonic devices, modal phase matching can compensate the index mismatch. Here, the various interacting waves propagate in transverse modes with appropriate phase velocities, but they suffer from a low refractive index contrast and cm‐scale interaction lengths. This work harnesses modal phase matching for third‐harmonic generation (THG) in plasmonic waveguides using an organic polymer (poly[3‐hexylthiophene‐2,5‐diyl]) as the nonlinear medium. One demonstrates experimentally an effective interaction area as small as ≈ 0.11 µm2 and the phase‐matched modal dispersion results in THG efficiency as high as ≈ 10–3 W‐2 within an effective length scale of ≈ 4.3 µm. THG also shows a strong correlation with the polarization of the incident laser beam, corresponding to the excitation of the antisymmetric plasmonic modes, corroborating that plasmonic modal phase matching is achieved. This large reduction in device area of orders of magnitude is interesting for various applications where space is critical (e.g., device integration or on‐chip applications). [ABSTRACT FROM AUTHOR]

Subwavelength silicon nanoparticles are known to support strongly localized Mie-type modes, including those with resonant electric and magnetic dipolar polarizabilities. Here we compare experimentally the efficiency of the third-harmonic generation from isolated silicon nanodiscs for resonant excitation at the two types of dipolar resonances. Using nonlinear spectroscopy, we observe that the magnetic dipolar mode yields more efficient third-harmonic radiation in contrast to the electric dipolar (ED) mode. This is further supported by full-wave numerical simulations, where the volume-integrated local fields and the directly simulated nonlinear response are shown to be negligible at the ED resonance compared with the magnetic one.This article is part of the themed issue New horizons for nanophotonics.

The authors report the third-harmonic generation, nonlinear refraction, and nonlinear absorption in HgS quantum dot (QD) suspensions and films using the nanosecond and femtosecond pulses. High conversion efficiency (7 × 10−4) towards the third harmonic (TH) of the 900–1700 nm, 150 fs laser in the thin (70 nm) films containing HgS QDs deposited on the glass substrates is obtained. The authors analyze spectral dependencies of the TH, nonlinear refractive indices, and nonlinear absorption coefficients of QDs in the 500–1700 nm range and discuss the relation between the TH process and the low-order nonlinear optical properties of these quantum dots. [ABSTRACT FROM AUTHOR]

We demonstrate all-optical modulation with a near-unity contrast of nonlinear light generation in a dielectric metasurface. We study third-harmonic generation from silicon Fano-resonant metasurfaces excited by femtosecond pulses at 1480 nm wavelength. We modulate the metasurface resonance by free carrier excitation induced by absorption of an 800 nm pump pulse, leading to up to 93% suppression of third-harmonic generation. Modulation and recovery occur on (sub)picosecond time scales. According to the Drude model, the pump-induced refractive index change blue-shifts the metasurface resonance away from the generation pulse, causing a strong modulation of third-harmonic conversion efficiency. The principle holds great promise for spatiotemporal programmability of nonlinear light generation.

A series of luminescent frameworks was synthesized from the selective combination of aggregation induced emission (AIE)‐linker tetra‐(4‐carboxylphenyl)ethylene (H4TCPE) and Zn2+. Complex 1 was formed by the close packing of Zn‐TCPE hinge, and isostructural complexes 2–5 were constructed by the linkage of Zn‐TCPE layer and pillar ligands. These complexes exhibit highly efficient multiphoton excited photoluminescence (MEPL) and concomitant third‐harmonic generation (THG). The multiphoton absorption (MPA) parameters of 1 are superior to other multiphoton emission materials including the perovskite nanocrystals. The incorporation of pillar linkers slows down the charge transfer between layers of Zn‐TCPE, and the aromatic core of pillar linkers has a great influence on the MPA performance of the corresponding frameworks. [ABSTRACT FROM AUTHOR]

Nowadays, realizing miniaturized nonlinear optical (NLO) device is crucial to meet the growing needs in on-chip nanophotonics as well as compact integrated devices. The strong optical nonlinearities, ultrafast photoexcitation dynamics, available exciton effects as well as without lattice matching make two-dimensional (2D) layered materials potential candidates for integrated and nano-scale NLO devices. Herein, a novel and inversion symmetry broken 2D layered SnP2S6 with strong second-harmonic and third-harmonic response has been reported for the first time. The second-order susceptibility (χ(2)) of SnP2S6 flakes can reach up to 4.06 × 10−9 m·V−1 under 810 nm excitation wavelength, which is around 1–2 orders of magnitude higher than that of most reported 2D materials. In addition, the NLO response of 2D SnP2S6 can break through the limitation of odd/even layers and exhibit broadband spectral response. Moreover, since the second-harmonic signal is closely related to structure variation, the second-harmonic response in 2D SnP2S6 is extremely sensitive to polarization angle and temperature, which is beneficial to some specific applications. The excellent NLO response in 2D SnP2S6 provides a new arena for realizing miniaturized NLO devices in the future. [ABSTRACT FROM AUTHOR]

The asymmetric potential profiles are focus of interest for researchers who study semiconductor optoelectronic devices. Therefore, in this study, the effects of structure parameters (σ and V0) and applied external perturbations such as electric (F), magnetic (B), and non-resonant monochromatic intense laser field on the nonlinear optical properties of the asymmetric semi-exponential quantum well (ASEQW) are theoretically investigated. The nonlinear optical rectification (NOR), second-harmonic generation (SHG), and third harmonic generation (THG) coefficients of ASEQW structure are considered. Theoretical investigations on these optical coefficients are performed in two steps. Step 1: The energy eigenvalues and eigenfunctions of a confined single electron in the ASEQW are obtained by using the diagonalization method within the framework of the effective mass and parabolic band approaches. Step 2: The compact density matrix approximation has been used to calculate the coefficients of the nonlinear optical response in the structure. The numerical results reveal that the structural parameters and external fields change the confinement potential profile of the system, and these effects significantly affect the NOR, SHG, and THG coefficients based on intersubband (intraband or subband) transitions in a low-dimensional quantum system. As a result, the amplitude and peak position of these optical coefficients are easily controlled by the applied external fields and can be adjusted according to the purpose. [ABSTRACT FROM AUTHOR]

Heavily doped semiconductors have emerged as low-loss and tunable materials for plasmonics at mid-infrared frequencies. We analyze the nonlinear optical response of free electrons and show how nonlinear optical phenomena associated with high electron concentration are influenced by the intrinsic properties of semiconductors, namely background permittivity and effective mass. We apply our recently developed hydrodynamic description that takes into account nonlinear contributions up to the third order, usually negligible for noble metals, to compare third-harmonic generation from InP, Ge, GaAs, Si, ITO and InSb. We show how free electron nonlinearities may be enhanced with a proper choice of the semiconductor. [ABSTRACT FROM AUTHOR]

We investigate the generation of optical third-harmonic frequency in quadratic crystals with a nonlinear domain lattice optimized with the aid of a random number generator. In the developed Monte Carlo algorithm and numerical experiments, we consider domain thicknesses to be taking either the values d1 or d2, with d1 and d2 being the coherence lengths for the cascaded parametric interactions 2ω=ω+ω and 3ω=2ω+ω, respectively. We focus on the cases with single segments formed by equal and/or different domains, showing that frequency tripling can be achieved with high conversion efficiency from an arbitrary input wavelength. The presented approach allows one to accurately determine the optimized random alternation of domain thicknesses d1 and d2 along the propagation length.

Nagyágite is a naturally occurring layered van der Waals heterostructure composed of alternating layers of [Pb(Pb,Sb)S2] and [(Au,Te)], where the component lattices are commensurately modulated. The weak van der Waals stacking between the heterolayers facilitates mechanical exfoliation. Due to its monoclinic crystal structure, nagyágite exhibits structural anisotropy which induces strong optical anisotropy. Here, the anisotropic optical properties of ultrathin nagyágite flakes mechanically exfoliated from a natural mineral are demonstrated through angle‐resolved polarized Raman scattering, linear dichroism, and polarization‐dependent anisotropic third‐harmonic generation. The study establishes nagyágite as a new type of natural van der Waals heterostructure based 2D material, which can be exploited for realizing ultrathin anisotropic optical devices for future on‐chip photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Ultrathin ternary 2D materials have recently gained significant attention in the context of tailoring physical properties of materials via stoichiometric variation, which are crucial to many applications in optoelectronics, thermoelectrics, and nanophotonics. Herein, sulfide mineral getchellite is identified as a new type of ternary layered material and large‐area getchellite thin flakes are prepared through mechanical exfoliation. The highly anisotropic linear and nonlinear optical responses of getchellite thin flakes facilitated by the reduced in‐plane crystal symmetry are reported, including anisotropic Raman scattering, wavelength‐dependent linear dichroism transition, and anisotropic third‐harmonic generation (THG). Furthermore, the third‐order nonlinear susceptibility for getchellite crystal is retrieved from the thickness‐dependent THG emission. The demonstrated strong anisotropic linear and nonlinear optical properties of van der Waals layered getchellite will have implications for future technological innovations in photodetectors, optical sensors, nonlinear optical signal processors, and other on‐chip photonic device prototypes. [ABSTRACT FROM AUTHOR]

A series of silicate materials with noncentrosymmetric structures, namely, Na3ScSi3O9 (NSSO), NSSO:Ce3+, NSSO:Eu2+, and NSSO:Tb3+, have been successfully synthesized by a microwave‐assisted solid‐state reaction. The materials reveal interesting second‐ and third‐harmonic generation and two‐photon absorption (2PA) properties attributable to the unusual connectivity and unique coordination environments around the cations, as evidenced by detailed structural analysis and maximum entropy method calculations. Upon irradiation with 980 nm laser light, the activator‐doped NSSO materials show an unusual upconversion emission property without using Yb3+ sensitizer ions, because of nonlinear absorption processes, as evidenced from 2PA spectroscopy results. The materials also show good chemical stability compared to NaYF4:Er3+, Yb3+. The findings of this work can stimulate the creation of a new scientific field devoted to upconversion luminescence in solid‐state materials. [ABSTRACT FROM AUTHOR]