Large Nonlinear Susceptibility Research Articles

Enhanced vertical second harmonic generation from layered GaSe coupled to photonic crystal circular Bragg resonators

Abstract Two-dimensional (2D) layered materials without centrosymmetry, such as GaSe, have emerged as promising novel optical materials due to large second-order nonlinear susceptibilities. However, their nonlinear responses are severely limited by the short interaction between the 2D materials and light, which should be improved by coupling them with photonic structures with strong field confinement. Here, we theoretically design photonic crystal circular Bragg gratings (CBG) based on hole gratings with a quality factor as high as Q = 8 × 103, a mode volume as small as V = 1.18 (λ/n)3, and vertical emission of light field in silicon nitride thin film platform. Experimentally, we achieved a Q value up to nearly 4 × 103, resulting in a 1,200-fold enhancement of second harmonic generation from GaSe flakes with a thickness of 50 nm coupling to the CBG structures under continuous-wave excitation. Our work endows silicon-based photonic platforms with significant second-order nonlinear effect, which is potentially applied in on-chip quantum light sources and nonlinear frequency conversion.

Phase-matched third-harmonic generation in silicon nitride waveguides.

Third-harmonic generation (THG) in silicon nitride waveguides is an ideal source of coherent visible light, suited for ultrafast pulse characterization, telecom signal monitoring and self-referenced comb generation due to its relatively large nonlinear susceptibility and CMOS compatibility. We demonstrate third-harmonic generation in silicon nitride waveguides where a fundamental transverse mode at 1,596 nm is phase-matched to a TM02 mode at 532 nm, confirmed by the far-field image. We experimentally measure the waveguide width-dependent phase-matched wavelength with a peak-power-normalized conversion efficiency of 5.78 × 10-7 %/W2 over a 660-μm-long interaction length.

Magnetic modes for enhancing second harmonic generation at ultraviolet frequencies

Enhancing nonlinear frequency conversions at ultraviolet (UV) frequencies holds immense significance in contemporary physics and nanophotonics. In this work, we have theoretically demonstrated a highly efficient UV second-harmonic generation (SHG) in the Al nanorod−LiNbO3 (LN)−Al film nanostructure by two orders of magnitude higher compared to the conventional LiNbO3 nanostructure in this range. This result is acquired by designing a simple Al nanoparticle-Al film plasmonic system that generates magnetic resonances at near-UV frequencies to confine light to the LN, which can highly enhance the electromagnetic field and lead to strong interactions with the large nonlinear susceptibility. Our research provides profound insights into the pivotal role of plasmon-induced magnetic resonance (PIMR) modes in UV-SHG processes, thereby opening up new avenues for the development of nanoscale UV nanosources and nonlinear metasurface applications at subwavelength scale.

Large in-plane negative piezoelectricity and giant nonlinear optical susceptibility in elementary ferroelectric monolayers

Large in-plane negative piezoelectricity and giant nonlinear optical susceptibility in elementary ferroelectric monolayers

Broadband Spatial Self‐Phase Modulation in Black and Violet Phosphorus and Near‐Infrared All‐Optical Switching

AbstractBlack phosphorus (BP) has attracted great research interest in nonlinear optics due to its anisotropic photoelectronic properties. As an allotrope of phosphorus, violet phosphorus (VP) has higher stability and desirable wide bandgap than BP, broadening the potential applications of phosphorus in optoelectronics. Herein, the nonlinear optical responses of layered BP and VP over broad spectral range from 405 to 1064 nm are investigated by the spatial self‐phase modulation (SSPM) method. These results show that both BP and VP exhibit excellent broadband SSPM effects. They all have large nonlinear susceptibility value (up to ≈ 10−8 e.s.u.), except for a weaker value at the near‐infrared band. Moreover, the SSPM pattern formation time becomes significantly longer with increasing wavelength. Significantly, an all‐optical switching using a 532 nm beam to control a 1064 nm near‐infrared beam is realized based on the SSPM, which demonstrate that the nonlinear optical response of signal beam (1064 nm) can be improved by adjusting the intensity of controlling beam (532 nm). This study provides a fascinating foundation for future applications of phosphorus allotropes in broadband nonlinear photonics.

Enhancing second harmonic generation of monolayer WS2 with edge plasmon

Transition-metal dichalcogenide (TMD) monolayers possess large second-order nonlinear susceptibility and have shown great potentials in on-chip nonlinear optical devices. However, the thickness of TMD monolayers is less than 1[Formula: see text]nm, resulting in a rather low second harmonic generation (SHG) efficiency. In this paper, we propose a physical scheme of enhancing SHG of TMD monolayers with edge plasmon. As a proof of concept demonstration, we constructed a hexagonal gold nanoplates/monolayer WS2 heterostructure with high interface quality. By performing SHG measurements, we show that SHG was enhanced by a factor of about 60. Our results set a foundation for on-chip nonlinear optical applications with TMD monolayers.

Berry Curvature Dipole Induced Giant Mid-Infrared Second-Harmonic Generation in 2D Weyl Semiconductor.

Due to its inversion-broken triple helix structure and the nature of Weyl semiconductor, 2D Tellurene (2D Te) is promising to possess a strong nonlinear optical response in the infrared region, which is rarely reported in 2D materials. Here, a giant nonlinear infrared response induced by large Berry curvature dipole (BCD) is demonstrated in the Weyl semiconductor 2D Te. Ultrahigh second-harmonic generation response is acquired from 2D Te with a large second-order nonlinear optical susceptibility (χ(2) ), which is up to 23.3 times higher than that of monolayer MoS2 in the range of 700-1500nm. Notably, distinct from other 2D nonlinear semiconductors, χ(2) of 2D Te increases extraordinarily with increasing wavelength and reaches up to 5.58nmV-1 at ≈2300nm, which is the best infrared performance among the reported 2D nonlinear materials. Large χ(2) of 2D Te also enables the high-intensity sum-frequency generation with an ultralow continuous-wave (CW) pump power. Theoretical calculations reveal that the exceptional performance is attributed to the presence of large BCD located at the Weyl points of 2D Te. These results unravel a new linkage between Weyl semiconductor and strong optical nonlinear responses, rendering 2D Te a competitive candidate for highly efficient nonlinear 2D semiconductors in the infrared region.

A universal route to efficient non-linear response via Thomson scattering in linear solids.

Non-linear materials are cornerstones of modern optics and electronics. Strong dependence on the intrinsic properties of particular materials, however, inhibits the at-will extension of demanding non-linear effects, especially those second-order ones, to widely adopted centrosymmetric materials (for example, silicon) and technologically important burgeoning spectral domains (for example, terahertz frequencies). Here we introduce a universal route to efficient non-linear responses enabled by exciting non-linear Thomson scattering, a fundamental process in electrodynamics that was known to occur only in relativistic electrons in metamaterial composed of linear materials. Such a mechanism modulates the trajectory of charges, either intrinsically or extrinsically provided in solids, at twice the driving frequency, allowing second-harmonic generation at terahertz frequencies on crystalline silicon with extremely large non-linear susceptibility in our proof-of-concept experiments. By offering a substantially material- and frequency-independent platform, our approach opens new possibilities in the fields of on-demand non-linear optics, terahertz sources, strong field light-solid interactions and integrated photonic circuits.

Tunable Metasurface Based on Plasmonic Quasi Bound State in the Continuum Driven by Metallic Quantum Wells

AbstractEffectual free‐space optical metasurface is essential for telecommunication and information processing. However, the lack of efficient optical nonlinearity is still an obstacle to empower its full capability and practical applications. Metallic quantum wells (MQWs) with large nonlinear susceptibility may pave a new way. Here, such MQWs are implemented into a resonance configuration of the quasi bound state in the continuum (BIC) and an efficient tunable plasmonic metasurface is proposed. Such metasurface is composed of MQWs‐based nanostructure, which supports the tunable plasmonic quasi‐BIC. The tunability is controlled through substantial change in refractive index of MQWs induced by Kerr‐type nonlinearity, which leads to around 9 dB extinction ratio and extremely high modulation speed up to terahertz level. The quasi‐BIC with narrow linewidth is obtained through symmetry‐breaking of nanoelliptical elements, further enhancing the modulation depth. This work satisfies the ultrafast‐speed and high‐efficiency requirement of free‐space all‐optical metasurfaces, including the spatial light modulators in optical computing field.

Efficient second- and higher-order harmonic generation from LiNbO3 metasurfaces.

Lithium niobate (LiNbO3) is a material that has drawn great interest in nonlinear optics because of its large nonlinear susceptibility and wide transparency window. However, for complex nonlinear processes such as high-harmonic generation (HHG), which involves frequency conversion over a wide frequency range, it can be extremely challenging for a bulk LiNbO3 crystal to fulfill the phase-matching conditions. LiNbO3 metasurfaces with resonantly enhanced nonlinear light-matter interaction at the nanoscale may circumvent such an issue. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and HHG from a LiNbO3 metasurface enhanced by guided-mode resonance. We observe a high normalized SHG efficiency of 5.1 × 10-5 cm2 GW-1. Moreover, with the alleviated above-gap absorption of the material, we demonstrate HHG up to the 7th order with the shortest generated wavelength of 226 nm. This work may provide a pathway towards compact coherent white-light sources with frequency spanning into the deep ultraviolet region for applications in spectroscopy and imaging.

Enhancing THz wave generation in silica nanofibers with Zinc Telluride nonlinear coating

This study investigates the use of Zinc Telluride (ZnTe) as a second-order nonlinear coating to enhance THz wave generation in silica nanofibers. Numerical simulations show that ZnTe coatings can significantly improve THz wave generation efficiency due to their large second-order nonlinear susceptibility and high transparency in the THz frequency range. Specifically, we observe a 2000-fold increase in THz wave generation efficiency with a 100nm thickness ZnTe coating compared to an uncoated silica nanofiber.

Milliwatt terahertz harmonic generation from topological insulator metamaterials

Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example, in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene. Here, we demonstrate room-temperature terahertz harmonic generation in a Bi2Se3 topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW—an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications.

Generation of entangled photons with a second-order nonlinear photonic crystal and a beam splitter

We discuss the generation of entangled photons using a nonlinear photonic crystal and beam splitter. In our method, the photonic crystal is assumed to be composed of a material with a large second-order nonlinear optical susceptibility . Our proposal relies on two facts: (1) A nonlinear photonic crystal changes coherent incident light into squeezed light. (2) A beam splitter transforms the squeezed light into entangled light beams flying in two different directions. We estimate the yield efficiency of pairs of entangled photons per pulse of the very weak coherent light for our method at 0.0783 for a specific concrete example. Our method is more effective because the conversion efficiency (entangled biphotons per incident pump photons) in the spontaneous parametric downconversion is of the order of . The only drawback is that it requires very fine tuning of the frequency of signal photons fed into the photonic crystal; for example, an adjustment is given by Hz for the signal light of Hz. We investigate the application of entangled photons produced by our method to the BB84 quantum key distribution protocol, and we explore how to detect an eavesdropper. We suggest that our method is promising for decoy-state quantum key distribution using weak coherent light.

NLOphoric Azo Dyes Studied Using Z‐Scan

AbstractDisperse Red 343((E)‐N‐(2‐((2,6‐dicyano‐4‐methylphenyl) diazenyl) ‐ 5 ‐ (diethylamino) phenyl)methanesulfonamide,1 a), Disperse Red177((E) ‐ methyl3‐((2‐cyanoethyl)(4‐((6‐nitrobenzo [d]thiazol‐2‐yl)diazenyl) phenyl) amino) propanoate,1 b), Disperse Blue 165((E) ‐ N ‐ (2 ‐ ((2,6 ‐ dicyano ‐ 4 ‐nitrophenyl)diazenyl)‐5‐ (diethylamino) phenyl) acetamide, 1 c), Disperse Blue 165.1((E) ‐ N ‐ (5 ‐ (benzyl (ethyl) amino) ‐ 2 ‐ ((2‐cyano ‐ 4,6‐dinitrophenyl) diazenyl) phenyl) acetamide, 1 d), Disperse Blue 366 ((E) ‐ 2 ‐ ((4‐ (diethylamino) ‐ 2 ‐ methylphenyl) diazenyl) ‐ 5 nitroisophthalonitrile, 1 e), and Disperse Blue 148((E) ‐ methyl 3‐(ethyl (4 ‐ ((5 ‐ nitrobenzo [c]I sothiazol‐3‐yl) diazenyl) phenyl) amino) propanoate, 1 f) were examined in solvents for their non‐linear optical properties using Z‐scan technique. Because of their CT characteristics, they have shown positive solvatochromism. As observed in nonpolar solvents, the ground states are more stable than their excited states. Changes in solvating the low‐lying ground state and the state above generate the bathochromic shift in vertical excitation. The dye 1 b produces the maximum due to strong NO2 donor‐ п‐ acceptors systems in DMSO. We found a positive nonlinear refractive index (η2) in both series using the closed aperture (valley‐peak) compared to the other five disperse dyes. Dye 1 c (with two cyano groups) has a ′larger nonlinear susceptibility (χ3)′ in DMSO than in the other two solvents (acetone, ethanol). Dye 1 a–1 f showed LDT (9.109, 6.553,4.641,6.44) W/m2 respectively. The “nonlinear absorptive coefficient (β), nonlinear refractive index (η2), third‐order nonlinear optical susceptibility χ(3) and optical limiting (OL)” of acetone, ethanol, and DMSO are determined using the Z‐scan technique, and the results are significantly better than earlier known dyes.

SnP2S6: A Promising Infrared Nonlinear Optical Crystal with Strong Nonresonant Second Harmonic Generation and Phase-Matchability

High-power infrared laser systems with broadband tunability are of great importance due to their wide range of applications in spectroscopy and free-space communications. These systems require nonlinear optical (NLO) crystals for wavelength up/down conversion using sum/difference frequency generation, respectively. NLO crystals need to satisfy many competing criteria, including large nonlinear optical susceptibility, large laser induced damage threshold (LIDT), wide transparency range and phase-matchability. Here, we report bulk single crystals of SnP_2S_6 with a large non-resonant SHG coefficient of d33= 53 pm/V at 1550nm and a large LIDT of 350 GW/cm^2 for femtosecond laser pulses. It also exhibits a broad transparency range from 0.54 {\mu}m to 8.5{\mu}m (bandgap of ~2.3 eV) and can be both Type I and Type II phase-matched. The complete linear and SHG tensors are measured as well as predicted by first principles calculations, and they are in excellent agreement. A proximate double-resonance condition in the electronic band structure for both the fundamental and the SHG light is shown to enhance the non-resonant SHG response. Therefore, SnP2S6 is an outstanding candidate for infrared laser applications.

Structure and principles of self-assembly of giant “sea urchin” type sulfonatophenyl porphine aggregates

Principles of molecular self-assembly into giant hierarchical structures of hundreds of micrometers in size are studied in aggregates of meso-tetra(4-sulfonatophenyl)porphine (TPPS4). The aggregates form a central tubular core, which is covered with radially protruding filamentous non-branching aggregates. The filaments cluster and orient at varying angles from the core surface and some filaments form bundles. Due to shape resemblance, the structures are termed giant sea urchin (GSU) aggregates. Spectrally resolved fluorescence microscopy reveals J- and H-bands of TPPS4 aggregates in both the central core and the filaments. The fluorescence of the core is quenched while filaments exhibit strong fluorescence. Upon drying, the filament fluorescence gets quenched while the core is less affected, showing stronger relative fluorescence. Fluorescence-detected linear dichroism (FDLD) microscopy reveals that absorption dipoles corresponding to J-bands are oriented along the filament axis. The comparison of FDLD with scanning electron microscopy (SEM) reveals the structure of central core comprised of multilayer ribbons, which wind around the core axis forming a tube. Polarimetric second-harmonic generation (SHG) and third-harmonic generation microscopy exhibits strong signal from the filaments with nonlinear dipoles oriented close to the filament axis, while central core displays very low SHG due to close to centrosymmetric organization. Large chiral nonlinear susceptibility points to helical arrangement of the filaments. The investigation shows that TPPS4 molecules form distinct aggregate types, including chiral nanotubes and nanogranular aggregates that associate into the hierarchical GSU structure, prototypical to complex biological structures. The chiral TPPS4 aggregates can serve as harmonophores for nonlinear microscopy.

A-axis oriented Zn0.72Mg0.28O epitaxial thin films with large second-order nonlinear susceptibility

Large nonlinear susceptibility that originates from strong electronic polarization enables ultrafast nonlinear optical devices. This work discovers that Mg incorporation has important contribution for enhancing second-order nonlinear susceptibility (χ (2)) and laser-induced surface-damage threshold of wurtzite ZnO epitaxial thin films deposited by radio-frequency magnetron sputtering method. Second-harmonic generation measurements derive that as-deposited Zn0.72Mg0.28O shows a gain of 48%, 77% and 33% in χ 33, χ 31 and χ 15 with respect to as-deposited ZnO. Specially, the annealed Zn0.72Mg0.28O has a χ 33 value of −57.0 ± 1.8 pm V−1, which gets comparable to that of LiNbO3 crystals. Triple-axis x-ray diffraction measurements conclude that the Mg incorporation should increase the χ (2) under optical frequency electric field by strengthening electronic polarization rather than increasing the residual strain in the film. Furthermore, the annealed Zn0.72Mg0.28O exhibits an increase of 48% in laser-induced surface-damage threshold relative to [11–20] ZnO crystals. These findings open the way of the Zn0.72Mg0.28O thin films to ultrafast nonlinear optical devices.

Comprehensive study of -Alanine passivated colloidal gold nanoparticles and GNP-PVP thin films: Linear optical properties and very large nonlinear refractive index, absorption coefficient, third-order nonlinear susceptibility measurements and effect of passivation

We synthesized gold nanoparticles (GNPs) stabilized in various concentration of l-alanine by chemical reduction method. We measured NLO responses of GNPs when they are dispersed in the l-alanine stabilizer with 1, 2.5 and 5 mM concentrations and when they are implanted into the PVP matix, by using single beam Z- Scan technique with 632 nm He–Ne CW laser for excitation. GNPs exhibited blue shift in localised surface plasmon resonance (LSPR) peak with the l-alanine concentration. The fcc structured GNPs of reducing size with the stabilizer concentration were obtained and characterized by the TEM as well as XRD. The FTIR spectral investigation revealed the formation of strong bond between the surface of GNPs and the l-alanine functional groups and established that l-alanine is very good candidate for stabilization of MNPs. It was observed that these samples show very large nonlinear refractive index, absorption coefficient and third-order nonlinear susceptibility in the order of 10−8cm2W−1, 10−6 cmW−1 and 10−9esu respectively for colloidal suspension of GNPs, and for the GNP-PVP thin films these parameter values were found in the order of 10−6cm2W−1, 10−5cmW−1and 10−6esu respectively. The thermally stimulated, l-alanine concentration dependant, very large NLO responses attributed to the local field effect from Mie resonance and thermal lensing effect originated from thermo-optic phenomenon. The obtained results validate the influence of the stabilizer (l-alanine) on NLO properties.

Ultraviolet second harmonic generation from Mie-resonant lithium niobate nanospheres

Abstract Lithium niobate (LN), as a nonlinear material with a large nonlinear susceptibility, has been widely employed in second harmonic generation (SHG) up to ultraviolet (UV) frequency range due to its broad low-absorption window. In nanophotonics, it is possible to harness the Mie resonances associated with the single dielectric particles to boost the nonlinear light–matter interactions. Here, we fabricate single Mie-resonant LN nanospheres on a SiO2 substrate via the femtosecond (fs) laser ablation technique. By exploiting the magnetic dipole (MD) Mie resonance, UV SHG from the LN nanosphere is significantly enhanced with a measured conversion efficiency of 4.45 × 10−8 under the excitation of an fs laser at 750 nm. The single LN nanospheres achieved in this work could serve as Mie resonators for building nonlinear nanophotonic devices such as frequency converters and quantum light sources, etc.

Giant Enhancement of Continuous Wave Second Harmonic Generation from Few-Layer GaSe Coupled to High-Q Quasi Bound States in the Continuum.

Two-dimensional (2D) layered materials such as GaSe recently have emerged as novel nonlinear optical materials with exceptional properties. Although exhibiting large nonlinear susceptibilities, the nonlinear responses of 2D materials are generally limited by the short interaction lengths with light, thus further enhancement via resonant photonic nanostructures is highly desired for building high-efficiency nonlinear devices. Here, we demonstrate a giant second-harmonic generation (SHG) enhancement by coupling 2D GaSe flakes to silicon metasurfaces supporting quasi-bound states in the continuum (quasi-BICs) under continuous-wave (CW) operation. Taking advantage of both high-quality factors and large mode areas of quasi-BICs, SHG from a GaSe flake is uniformly enhanced by nearly 4 orders of magnitude, which is promising for high-power coherent light sources. Our work provides an effective approach for enhancing nonlinear optical processes in 2D materials within the framework of silicon photonics, which also brings second-order nonlinearity associated with 2D materials to silicon photonic devices.