Enhancement Of Optical Nonlinearity Research Articles

Efficient High-Order Harmonic Generation from the van der Waals Layered Crystal Copper Indium Thiophosphate.

Layered metal thio- and selenophosphates (MTPs) are a family of van der Waals gapped materials that exhibit a multitude of functionalities in terms of magnetic, ferroelectric, and optical properties. Despite the recent progress in terms of understanding the material properties of these compounds, the potential of MTPs as a material class yet needs further scrutiny, especially in terms of nonlinear optical properties. Recent reports of efficient low-order harmonic generation and extremely high third-order nonlinear optical properties in MTPs suggest the potential application of these materials in integrated nanophotonics. In this article, we investigate the high-order nonlinear response of bulk and exfoliated thin-film crystals of copper indium thiophosphate (CIPS) to intense mid-infrared fields through experimental and computational studies of high-order harmonic generation (HHG). From a driving laser source with a 3.2 μm wavelength, we generate odd and even harmonics up to the 10th order, exceeding the bandgap of the material. We note conversion efficiencies as high as 10-7 measured for the fifth and seventh harmonics and observe that the harmonic intensities follow a power law scaling with the driving laser intensity, suggesting a perturbative nonlinear optical origin of the observed harmonics for both bulk and thin flakes. Furthermore, first-principles calculations suggest that the generation of the highest harmonic orders results from electron-electron interactions, suggesting a correlation-mediated enhancement of the high-order optical nonlinearity.

Boosting third-order optical nonlinearity in ITO/Au multilayer films via interfacial effects

We present the enhancement of third-order optical nonlinearity in indium tin oxide (ITO)/Au multilayer films via interfacial effects. The overall thickness of prepared ITO and Au layer was kept as 200 nm and 14 nm, respectively, and thus multilayers are 214 nm, i.e., for the sandwich structure ITO/Au/ITO, the thickness of ITO and Au is 100 nm and 14 nm, respectively, while the thickness of ITO and Au is 40 nm and 3.5 nm in the nine-layer films composed of five layers of ITO and four layers of Au. The measured nonlinear refractive index (n2) and absorption coefficient (β) of the multilayers rise as the number of layers increases. The maximum n2 and β in the nine-layer film are 2.6×10−14 m2/W and −3.7×10−8 m/W, which are 3.8 and 2.3 times larger than the values in the pure ITO film, respectively. Such enhancement of optical nonlinearity as the number of layers increases originates from the increase of carrier concentrations in multilayers due to contact of metals/semiconductors (interfacial effects), not following the traditional effective media theory and epsilon-near-zero effect. The results pave a way to modulate the optical nonlinearity in special metal-dielectric multilayers of interfacial effects and indicates the promising applications in nonlinear photonics.

Band-gap tailoring and third-order optical nonlinearity enhancement through B-site V doping in SrSnO3 films

Perovskite oxide vanadium-doped strontium stannate (SrSn1-xVxO3) thin films with different concentrations x = 0.1–0.9 were fabricated by using pulsed-laser deposition, and the effects of V doping on the structure, optical band gap and the third-order optical nonlinearity were systematically investigated. With the increase of x value, the lattice parameters of SrSn1-xVxO3 decrease from 3.997 to 3.862 Å gradually, while the optical band gaps firstly increase from 4.18 to 4.26 eV, and then decrease to 3.66 eV with a boundary at x = 0.3. The third-order nonlinear optical responses were studied via the z-scan technique. The closed-aperture measurements show a negative nonlinear refractive index n2, and the open-aperture measurements demonstrate a saturable absorption β. Both n2 and β responses vary with the increase of V doping level. The maximum values of n2and β are 2.01×10−13 m2/W and 7.66×10−8 m/W, respectively, which are observed in the SrSn0.5V0.5O3sample. The metal-oxygen chemical bond along with the localized V5+Sn2+V5+complex contribute to the enhancement of optical nonlinearity.

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand.

Chiral hybrid organic-inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second-order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self-trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second-order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1-ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V-1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second-order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand

AbstractChiral hybrid organic–inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second‐order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self‐trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second‐order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1‐ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V−1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second‐order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

Investigation of the third-order optical nonlinear enhancement properties of Ag doped chalcogenide glass films

Chalcogenide glass is widely recognized as an excellent nonlinear optical material. Enhancing its nonlinear optical properties through metal doping has emerged as a prominent research avenue in the field of chalcogenide glass. In this paper, silver (Ag) doped Ge28Sb12Se60 (GSS) chalcogenide glass films with different concentrations were prepared by co-evaporation method. The nonlinear absorption coefficients (β) of different samples were investigated by open-aperture top-hat Z-scan experiments and pump-probe experiments, and the nonlinear refractive index (γ) values were determined using the Tichy-Ticha relation. The results showed that the nonlinear absorption of GSS thin films can be improved by Ag doping. The nonlinear absorption coefficient β exhibits the maximum enhancement of about 2.5 times at 8.3% concentration, with the maximum β reaching 1.83×10−6 m/W. The nonlinear absorption mechanism observed in the thin films can be attributed to free carrier absorption, and the free carrier lifetime is positively correlated with the Ag doping concentration. Within the selected Ag concentration range, the nonlinear refractive index (γ) of Ag doped chalcogenide glass films continues to increase, reaching about 3 times at 30.7% concentration, with the maximum γ being 4.643×10−17 m2/W. Additionally, the third-order nonlinear polarizability (χ(3)) values can be increased from 1.014×10−11 esu to 3.314×10−11 esu with increasing Ag concentration. The research results in present paper are of great significance for further exploration of the optical nonlinear enhancement properties and carrier recombination dynamics in chalcogenide glass.

The band gap and nonlinear optical susceptibility of SrSn1-xVxO3 films

Perovskite-type oxide SrSn1-xVxO3 thin films with different concentrations x = 0.1–0.9 were fabricated by using pulsed-laser deposition, and the effects of V doping on the structure, optical band gap and the third-order optical nonlinearity were systematically investigated. With the increase of x value, the lattice parameters of SrSn1-xVxO3 decrease from 3.997 to 3.862 Å gradually, while the optical band gaps firstly increase and then decrease with boundary at x = 0.3. The third-order nonlinear optical responses were studied via the z-scan technique. The closed-aperture measurements show a negative nonlinear refractive index n2, and the open-aperture measurements demonstrate a saturable absorption β. Both the n2 and β responses vary with the increase of V doping level. The metal-oxygen chemical bond along with the localized V5+Sn2+V5+ complex contribute to the enhancement of optical nonlinearity, and the highest value of third-order susceptibility χ(3) is observed in SrSn0.5V0.5O3 film.

Enhancement of third-order optical nonlinearity of VSe2 by hybridizing with CoSe2 as flower-like nanoparticles.

Through one-pot hydrothermal synthesis, we synthesized VSe2-CoSe2 nanostructures with a distinctive flower-like morphology. The structural characterizations were conducted using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR). By modulating the proportion of the CoSe2 phase, we transformed these hybrid nanostructures into nanoparticles, thereby influencing their nonlinear optical properties. The third-order nonlinear optical parameters (n 2 and β) were rigorously examined via the precise Z-scan technique under various laser power conditions. The nanostructures demonstrated a self-focusing refractive response and exhibited pronounced two-photon absorption during the nonlinear absorption process. The magnitudes of n 2 and β were determined to be in the order of 10-8 (cm2 W-1) and 10-4 (cm W-1), respectively. The exceptional figure of merit (FOM) of these composite flower-like structures underscores their superior nonlinear optical characteristics, establishing them as promising candidates for photonic applications, particularly in optical switching devices.

All-optical switching based on spatial self-phase modulation of Ag/violet phosphorus heterojunctions.

With the increasing demand for high-performance passive nonlinear photonic devices, significant progress has been made in spatial self-phase modulation (SSPM) based on 2D nanomaterials in all-optical switches, logic gates, and information converters in recent years. However, there are still challenges in improving the responsiveness of photonic devices. In this work, we prepared a heterojunction of Ag nanoparticles deposited on the surface of violet phosphorus nanosheets (VP Ns), investigated their SSPM, and demonstrated their performance in all-optical switches. The SSPM experimental results show that compared with pure VP Ns at the same light intensity, both the maximum number and the formation time of self-diffraction rings in Ag/VP heterojunctions increase, and the nonlinear refractive index is approximately doubled. The main reason for optical nonlinearity enhancement is that the internal electric field in the heterojunction strengthens the mobility of the photogenerated carrier, thereby enhancing its optical nonlinearity. In particular, we demonstrate the performance of all-optical switches based on SSPM by utilizing the superior optical nonlinearity of Ag/VP heterojunctions. It is shown that with the increase of low-dose Ag content in heterojunctions, the switching time in the all-optical switch becomes shorter and the maximum number of self-diffraction rings of the signal light increases, although the quality of self-diffraction rings slightly decreases due to the scattering of Ag particles. The results contribute to the design and implementation of high-performance nonlinear photonic devices, based on the use of heterojunctions with low-cost preparation and a high nonlinear refractive index.

Bi2Te3/Sb2Te3-graphene bidirectional heterostructures: Enhancement of optical nonlinearity and short pulse generation of solid-state laser

Bi2Te3/Sb2Te3-graphene bidirectional heterostructures: Enhancement of optical nonlinearity and short pulse generation of solid-state laser

Enhanced Nonlinearity of Epsilon‐Near‐Zero Indium Tin Oxide Nanolayers with Tamm Plasmon‐Polariton States

AbstractRecently, materials with vanishingly small permittivity, known as epsilon‐near‐zero (ENZ) media, emerged as promising candidates to achieve nonlinear optical effects of unprecedented magnitude on a solid‐state platform. In particular, the ENZ behavior of indium tin oxide (ITO) thin films resulted in Kerr‐type nonlinearity with non‐perturbative refractive index variations that are key to developing more efficient Si‐compatible devices with sub‐wavelength dimensions. In this contribution, exceptional enhancement of the nonlinear index variation of 30 nm‐thick ITO nanolayers by silicon dioxide/silicon nitride (SiO2/SiN) Tamm plasmon‐polariton structures fabricated by radio‐frequency magnetron sputtering on transparent substrates under different annealing conditions is proposed and demonstrated. In particular, the linear and nonlinear optical properties of ITO thin films and resonant photonic structures are investigated using broadband spectroscopic ellipsometry and intensity‐dependent Z‐scan nonlinear characterization, demonstrating enhancement of optical nonlinearity with refractive index variations as large as Δn ≈ 2 in the non‐perturbative regime. This study reveals that the efficient excitation of strongly confined plasmon‐polariton Tamm states substantially boosts the nonlinear optical response of ITO nanolayers, providing a stepping stone for the engineering of more efficient infrared devices and nanostructures for a broad range of applications, including all‐optical data processing, nonlinear spectroscopy, sensing, and novel quantum detection modalities.

An experimental and computational study for enhancement in optical nonlinearity and dielectric behaviour in thermal annealing induced Ag diffusion at Ag/Sb-S-Se interface

The annealing induced Ag diffusion into Sb40Se30S30 film brings enormous changes in its linear-nonlinear optical along with the structural and dielectric properties which is reported in the current study. The prepared Ag/Sb40Se30S30 film was annealed at different temperatures like 100,150,200 and 250 °C. The structure remained amorphous one as confirmed from XRD while the particle size increased with annealing as observed from FESEM images. The Raman study depicts the microstructural change in terms of shift in vibrational peaks. The optical study was done by UV–Vis-NIR spectrometer from which different optical parameters were evaluated. The decrease in optical bandgap with increase in linear refractive index is accompanied by the increase in optical density, extinction coefficient and decrease in skin depth. The dielectric loss factor increased while the quality factor decreased with annealing. The static dielectric constant increased with the increase in carrier concentration. Both the electrical as well as optical conductivity are found to be increased while optical electronegativity and plasma frequency decreased with annealing. The 3rdorder nonlinear susceptibility and non-linear refractive index are doubled with annealing while there is decrease in nonlinear absorption coefficient. The surface wettability study infers the hydrophilic nature sustains with annealing. The computational analysis using the DFT method was carried out to provide some necessary information about the structure and physical properties of the prepared materials.

3rd order optical nonlinearity enhancement of halochromic phenol red dye immobilized in PVA/silica hybrid host matrix

3rd order optical nonlinearity enhancement of halochromic phenol red dye immobilized in PVA/silica hybrid host matrix

Enhancement of second-order optical nonlinearities and nanoscale periodic domain patterning in ferroelectric boron-substituted aluminum nitride thin films

The discovery and development of CMOS-compatible, nonlinear optical materials is essential to produce integrated photonic devices with advanced functionalities. AlN is a strong candidate for on-chip device demonstration due to its intrinsic second-order optical nonlinearities, large bandgap, and well-established fabrication techniques. However, AlN is not easily phase matched for the largest coefficient d33; the coefficients that could potentially be dispersion phase-matched, d31 and d15, have weak nonlinearities. This work investigates ferroelectric Al1-xBxN (x = 0 to 0.11) for viability as a large bandgap nonlinear optical material with unique suitability towards ultraviolet light generation using second harmonic generation. The linear and nonlinear optical properties are characterized accounting for material anisotropy. With increasing B concentration, a large enhancement from near negligible values to d31 = 0.9 ± 0.1 pm/V and d15= 1.2 ± 0.1 pm/V is observed. This compares favorably to other large bandgap materials like β-Ba(BO2)2, where the largest nonlinear coefficient is d22 ∼ 2.3 pm/V at 800 nm. This is accompanied by a change in the bandgap from 6.1 eV to 5.8 eV as B substitution goes from 0 to 11%. A periodically poled, quasi-phase-matched ferroelectric domain pattern with 400 nm domain size and a wall roughness of <16 nm is demonstrated.

Silver-Based Surface Plasmon Sensors: Fabrication and Applications.

A series of novel phenomena such as optical nonlinear enhancement effect, transmission enhancement, orientation effect, high sensitivity to refractive index, negative refraction and dynamic regulation of low threshold can be generated by the control of surface plasmon (SP) with metal micro-nano structure and metal/material composite structure. The application of SP in nano-photonics, super-resolution imaging, energy, sensor detection, life science, and other fields shows an important prospect. Silver nanoparticles are one of the commonly used metal materials for SP because of their high sensitivity to refractive index change, convenient synthesis, and high controllable degree of shape and size. In this review, the basic concept, fabrication, and applications of silver-based surface plasmon sensors are summarized.

Broadband Enhancement of Optical Nonlinearity in a Plasmonic Nanocavity Coupled with an Epsilon-Near-Zero Film

Nonlinear optics plays a crucial role in various fields, including microscope, all-optical data processing, and quantum information. However, nonlinear optical responses in natural materials are generally very weak, and therefore it is extremely difficult to design all-optical active devices at the subwavelength scale. Here, a dielectric-metal hybrid system is proposed to acquire optical nonlinearity enhancement through the strong coupling between the epsilon-near-zero mode in an indium tin oxide (ITO) nanofilm and the localized surface plasmon in a nanocavity. Such a coupled system exhibits a nonlinear refractive index n2 up to 10.31 cm2/GW and an absorption coefficient β up to 1.0 × 106 cm/GW, over three orders and two orders of magnitude larger than those of bare ITO, respectively. In addition, enhanced nonlinear responses can be observed in a wide wavelength range (∼1400 nm bandwidth) in the near infrared region. Our results provide an efficient way for the development of low-power all-optical modulation devices.

Modulation of epsilon-near-zero wavelength and enhancement of third-order optical nonlinearity in ITO/Au multilayer films

We report the modulation of epsilon-near-zero (ENZ) wavelength and enhanced third-order nonlinearity in indium tin oxide (ITO)/Au multilayer films. The samples consisting of five-layer 40 nm ITO films spaced by four-layer ultrathin Au films of different thickness, i.e., ITO(40 nm)/[Au(x)/ITO(40 nm)]4, were prepared by magnetron sputtering at room temperature. The ENZ wavelength in the multilayer films is theoretically calculated and experimentally confirmed. The nonlinear refractive index and nonlinear absorption coefficient of the samples of x=0, 2, 3, 4 nm were determined using the Z-scan method at a wavelength of 1.064 µm. The large nonlinear refractive index n2=1.12×10-13 m2/W and nonlinear absorption coefficient β=-1.78×10-7 m/W in the sample of x = 4 nm are both four times larger than those in the single-layer ITO film. The large optical nonlinearity due to the ENZ enhancement and carrier concentration is discussed. The results indicate that the ITO/Au multilayer films are promising for advanced all-optical devices.

On-chip light trapping in bilayer moiré photonic crystal slabs

The optical response of bilayer moiré photonic structures can be precisely controlled by varying the lattice geometry. Bilayer moiré photonic crystal slabs exhibit flat bands in the optical band structure, where the optical modes have zero group velocity. They also give rise to momentum-independent light-trapping of Bloch waves in both transverse and vertical directions, leading to high quality-factors (Q=109) and small mode volumes (V=0.12 λ2). The large Q and small V lead to a large Purcell enhancement (FP=1035), providing opportunities for low-threshold lasing, enhancement of optical nonlinearities, and quantum information processing.

Tunable luminescence and nonlinear optical properties of nano colloidal Ge20Te75Bi5 chalcogenide glass

In this article, we report spectral studies and optical nonlinearities of optical grade thin films fabricated from nano colloidal solutions of Ge20Te75Bi5 chalcogenide glass. Emission peaks of varying concentrations of Ge20Te75Bi5 nanocomposites shift from 426 nm to 492 nm. Bandgap Eg also varies in the proportion tunable to emission peaks depending on the grain size of the films. The existence of reverse saturable absorption (RSA) is confirmed by the open aperture z-scan traces of nanocomposites with a valley-like pattern. Enhancement of third-order optical nonlinearities in Ge20Te75Bi5 chalcogenide nano colloids studied using the z-scan technique can be exploited to improve the desirable features for usage in nonlinear optical systems.

Transient Super‐/Sub‐Linear Nonlinearities in Silicon Nanostructures

AbstractNonlinear silicon nano‐photonics has recently attracted significant attention due to the plethora of electric and magnetic Mie resonances that offer substantial enhancement of optical nonlinearities. Conventionally, the characterization of nonlinearity and its transient nature rely on intensity‐scan methods (z‐scan) in the spatial domain and pump‐probe techniques in the temporal domain. However, most studied ultrafast nonlinear effects are instantaneous, that is, strongest at zero pump‐probe delay, and have a solitary nonlinear power dependency (square, cubic, etc.). Here the authors found that when relaxation lifetime is dependent on pump fluence, transient nonlinearity appears. The effect is exemplified via Auger‐based nonlinear carrier dynamics of a nano‐silicon Mie‐resonator. The Auger‐induced transient nonlinearity not only locates at the time delay of several tens of picoseconds, but also displays diverse nonlinearities, including sub‐linear, super‐linear, and surprisingly full saturation, which features a “crossing point” where the probe scattering is pump‐fluence independent. The crossing point exists when the relaxation lifetime is inversely dependent to second power of carrier density. Combining confocal intensity‐scan (x‐scan) and pump‐probe temporal scan, the authors demonstrate that sub‐linearity and super‐linearity lead to swelled and reduced full‐width‐at‐half‐maximum (FWHM) of single‐nanostructure images, further confirming the nonlinearity as well as the potential of sub‐diffraction microscopy. The results open up a new avenue in nonlinear silicon nano‐photonics by adding new degrees of freedom in temporally tuning the types of transient nonlinearities, which are valuable in all‐optical signal processing and nano‐imaging.