Nonlinear Devices Research Articles

First-principles screening of metal–organic frameworks for entangled photon pair generation

The transmission of strong laser light in nonlinear optical materials can generate output photons sources that carry quantum entanglement in multiple degrees of freedom, making this process a fundamentally important tool in optical quantum technology. However, the availability of efficient optical crystals for entangled light generation is severely limited in terms of diversity, thus reducing the prospects for the implementation of next-generation protocols in quantum sensing, communication and computing. To overcome this, we developed and implemented a multi-scale first-principles modeling technique for the computational discovery of novel nonlinear optical devices based on metal–organic framework (MOF) materials that can efficiently generate entangled light via spontaneous parametric down-conversion (SPDC). Using collinear degenerate type-I SPDC as a case study, we computationally screen a database of 114 373 synthesized MOF materials to establish correlations between the structure and chemical composition of MOFs with the brightness and coherence properties of entangled photon pairs. We identify a subset of 49 non-centrosymmetric mono-ligand MOF crystals with high chemical and optical stability that produce entangled photon pairs with intrinsic G(2) correlation times τc∼10−30 fs and pair generation rates in the range 104−1010s−1mW−1mm−1 at 1064 nm. Conditions for optimal type-I phase matching are given for each MOF and relationships between pair brightness, crystal band gap and optical birefringence are discussed. Correlations between the optical properties of crystals and their constituent molecular ligands are also given. Our work paves the way for the computational design of MOF-based devices for optical quantum technology.

Broad Learning System Using Rectified Adaptive Moment Estimation for Harmonic Detection and Analysis

The extensive application of nonlinear components and power electronic devices in power systems has led to increasingly deteriorating harmonic pollution. The rapid and high-precision harmonic detection and analysis technology will set the prerequisite for mitigating the harmonic crisis. This paper presents a robust, stable and real-time approach based on broad learning system (BLS) for detecting and analyzing the amplitudes of fundamental and harmonic components, which only requires half or even quarter cycle input. Additionally, a fast-converging adaptive parameter learning strategy using rectified adaptive moment estimation (RAdam) for BLS is proposed. To obtain the actual distorted signals, the signal acquisition system is built on the basis of the designed multi-pulse rectifier power supply (6/18 pulse). With the fundamental frequency deviation, interharmonics and noise, etc., the simulated signals synthesized by MATLAB tool and the actual signals acquired from the built system are attained to verify the performance of methods. The proposed approach differentiates from other existing methods not only due to its superior precision and robustness in the presence of multitudinous interference factors but also its real-time performance because of low average computing time.

Design and realization of plasmonic nanostructures with large third-order optical nonlinearity

We designed an optical structure comprising plasmonic and dielectric materials that exhibits large third-order optical susceptibility and low-power consumption. Finite-difference time-domain simulation results were used to select tightly-packed gold nanosphere (AuNS) film, indium tin oxide (ITO) films, and gold (Au) thin film to construct AuNS/ITO/Au sandwiched nanostructures. Local-field enhancement factors up to 425 and large third-order optical susceptibility (χ(3)) with an amplitude order of 4.4 × 10−2 esu were achieved under an excitation power of 0.1 mW. These χ(3) results are exceptionally high, thereby rendering AuNS/ITO/Au a promising material for future applications in optical switches and related nonlinear optical devices.

Nonlinear spin-orbit coupling in optical thin films

Tunable generation of vortex beams holds relevance in various fields, including communications and sensing. In this paper, we demonstrate the feasibility of nonlinear spin-orbit interactions in thin films of materials with second-order nonlinear susceptibility. Remarkably, the nonlinear tensor can mix the longitudinal and transverse components of the pump field. We observe experimentally our theoretical predictions in the process of second-harmonic generation from a thin film of aluminum gallium arsenide, a material platform widely spread for its role in the advancement of active, nonlinear, and quantum photonic devices. In particular, we prove that a nonlinear thin film can be used to produce vector vortex beams of second-harmonic light when excited by circularly-polarized Gaussian beams.

Symmetry transformation of nonlinear optical current of tilted Weyl nodes and application to ferromagnetic MnBi2Te4

A Weyl node is characterized by its chirality and tilt. We develop a theory of how nth-order nonlinear optical conductivity behaves under transformations of anisotropic tensor and tilt, which clarifies how chirality-dependent and -independent parts of optical conductivity transform under the reversal of tilt and chirality. Built on this theory, we propose ferromagnetic MnBi2Te4 as a magnetoelectrically regulated, terahertz optical device, by magnetoelectrically switching the chirality-dependent and -independent DC photocurrents. These results are useful for creating nonlinear optical devices based on the topological Weyl semimetals.

High Electric Field Enhancement Induced by Modal Coupling for a Plasmonic Dimer Array on a Metallic Film

A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice resonance (SLR) and surface plasmon polariton (SPP). Mode coupling is demonstrated by observing anti-crossing in reflection spectra, which corresponds to Rabi splitting. Although the resonance coupling does not enter the strong coupling regime, an improved quality factor (Q~350) and stronger electric field enhancement in the gap region of the dimer (i.e., hot spot) in our hybrid structure are obtained compared to those of the single dimer or dimer array only. Remarkably, the magnitude of electric field enhancement over 500 can be accessible. Such high field enhancement makes our hybridized structure a versatile platform for the realization of ultra-sensitive biosensing, low-threshold nanolasing, low-power nonlinear optical devices, etc.

CVD Synthesis of Twisted Bilayer WS2 with Tunable Second Harmonic Generation.

The introduction of rotational freedom by twist angles in twisted bilayer (TB) transition metal dichalcogenides (TMDCs) can tailor the inherent properties of the TMDCs, which provides a promising platform to investigate the exotic physical properties. However, direct synthesis of high-quality TB-TMDCs with full twist angles is significantly challenging due to the substantial energy barriers during crystal growth. Here, a modified chemical vapor deposition strategy is proposed to synthesize TB-WS2 with a wide twist angle range from 0° to 120°. Utilizing a tilted SiO2/Si substrate, a gas flow disturbance is generated in the furnace tube to create a heterogeneous concentration gradient of the metal precursor, which provides an extra driving force for the growth of TB-WS2. The Raman and photoluminescence results confirm a weak interlayer coupling of the TB-WS2. High-quality periodic Moiré patterns are observed in the scanning transmission electron microscopy images. Moreover, owing to the strong correlation between the nonlinear optical response and the twisted crystal structure, tunable second harmonic generation behaviors are realized in the TB-WS2. This approach opens up a new avenue for the direct growth of high-crystalline-quality and pristine TB-TMDCs and their potential applications in nonlinear optical devices.

Third Harmonic Generation Enhancement and Wavefront Control Using a Local High-Q Metasurface.

High quality factor optical nanostructures provide a great opportunity to enhance nonlinear optical processes such as third harmonic generation. However, the field enhancement in these high quality factor structures is typically accompanied by optical mode nonlocality. As a result, the enhancement of nonlinear processes comes at the cost of their local control as needed for nonlinear wavefront shaping, imaging, and holography. Here we show simultaneous strong enhancement and spatial control over third harmonic generation with a local high-Q metasurface relying on higher-order Mie resonant modes. Our results demonstrate third harmonic generation at an efficiency of up to 3.25 × 10-5, high quality wavefront shaping as illustrated by a third harmonic metalens, and a flatband, angle independent, third harmonic response up to ±11° incident angle. The demonstrated high level of local control and efficient frequency conversion offer promising prospects for realizing novel nonlinear optical devices.

Enhanced optical third-harmonic generation in phase-engineered nanostructured Zn1−x Cd x S thin films for optoelectronic device applications

A polycrystalline nanostructured thin film of zinc cadmium sulfide was meticulously fabricated on a glass substrate using the thermal evaporation method physical vapor deposition within a vacuum chamber. Different doping concentrations were introduced by varying the cadmium (Cd) content, resulting in Zn1-x Cd x S films with Cd concentrations ranging from x = 0.00–0.20 wt %. The impact of Cd doping on the third-order nonlinear optical (TONLO) properties of these films was thoroughly studied using the Z-scan method, employing a diode-pumped solid-state continuous-wave laser. To gain insight into the structural characteristics, the Zn1-x Cd x S thin films underwent analysis through x-ray diffraction. Optical studies confirmed the tunability of the optical band gap (Eg) in the Zn1-x Cd x S films, ranging from 3.88 eV for undoped ZnS to 2.80 eV for the film fabricated with 20 wt. % of Cd-content. This significant reduction in ‘Eg’ renders the films highly suitable for use as absorbing layers in applications such as solar cells and optoelectronics. Surface morphology analysis, performed via field emission scanning electron microscopy, revealed noticeable alterations with increased Cd doping. Significantly, the doped films exhibited a substantial redshift in the band edge and an increase in transmittance within the visible and near-infrared regions. The investigation of TONLO properties, including the nonlinear absorption coefficient (β), nonlinear refractive index (n 2) and susceptibility χ(3), yielded values ranging from 3.15 × 10−3 to 8.16 × 10−3 (cm W−1), 1.65 × 10−8 to 7.45 x 10–8 (cm2 W−1), and 3.12 × 10−5 to 7.86 × 10−5 (esu), respectively. These results indicate the presence of self-defocusing nonlinearity in the films. Overall, the outcomes underscore the potential of Cd-doped ZnS nanostructures in modifying surface morphology and enhancing NLO characteristics. Zn1-x Cd x S thin films exhibit promise for applications in nonlinear optical devices, as evidenced by these encouraging findings.

Modal Phase-Matched Bound States in the Continuum for Enhancing Third Harmonic Generation of Deep Ultraviolet Emission.

Coherent deep ultraviolet (DUV) light sources are crucial for various applications such as nanolithography, biomedical imaging, and spectroscopy. DUV light sources can be generated by using conventional nonlinear optical crystals (NLOs). However, NLOs are limited by their bulky size, inadequate transparency at the DUV regime, and stringent phase-matching requirements for harmonic generation. Recently, dielectric metasurfaces support high Q-factor resonances and offer a promising approach for efficient harmonic generation at short wavelengths. In this study, we demonstrated a crystalline silicon (c-Si) metasurface simultaneously exciting modal phase-matched bound states in the continuum (BIC) resonance at the fundamental wavelength of 840 nm with a higher degree of freedom for precise control of the BIC resonance and a plasmonic resonance at the wavelength of 280 nm in the DUV to enhance third harmonic generation (THG). We experimentally achieved a Q-factor of ∼180 owing to the relatively large refractive index of the c-Si and the geometric symmetry breaking of the structure. We realized THG at a wavelength of 280 nm with a power of 14.5 nW by using a peak power density of 15 GW/cm2 excitation. The measured THG power is 14 times higher than the state-of-the-art THG dielectric metasurfaces using the same peak power density in the DUV regime, and the maximum obtained THG power enhancement factor is up to 48. This approach relies on the significant third-order nonlinear susceptibility of c-Si, the interband plasmonic nature of the c-Si in the DUV, and the strong field confinement of BIC resonance to boost overall nonlinear conversion efficiency to 5.2 × 10-6% in the DUV regime. Our work shows the potential of c-Si BIC metasurfaces for developing efficient and ultracompact DUV light sources using high-efficacy nonlinear optical devices.

Enhancement of electro‐optical and non‐linear optical parameters of halogenated hexahexyloxytriphenylene (HAT6) molecule: A computational approach

AbstractAn investigation of electro‐optical and nonlinear optical properties of pure and halogenated (−Cl, −Br) hexahexyloxytriphenylene (HAT6) molecule is presented employing a density functional theory (DFT) approach. The effect of halogenation on HAT6 molecule is studied by introducing single and double halogens (Cl, Br) in its molecular geometry. The geometries are optimized progressively to achieve stability using B3LYP method with 6‐311 basis sets. The physical properties encompassing electro‐optical, thermodynamic, and electronic properties of single molecule of HAT6 are computed using DFT method. The introduction of halogens to discotic HAT6 molecule to an augmentation in molecular polarizability, rendering halogenated HAT6 a suitable candidate for nonlinear optical devices due to its π–π conjugation and improved optical parameters. The UV‐Vis spectra indicate the predominant absorptions and changes in absorption depend on the halogenations of the HAT6 molecule. Also, the halogenation effect leads to a significant reduction in the energy bandgap directly associated with electron‐electron correlation, facilitating straightforward tuning.

Timing synchronization of LACO-OFDM under non-linear distortions using ELM

Layered asymmetrically-clipped optical orthogonal frequency division multiplexing (LACO-OFDM) is a modulation technique for visible light communication (VLC) that provides power and spectral efficiencies. The LACO-OFDM technique utilizes all the available subcarriers and will have a relatively higher peak-to-average power ratio (PAPR). Hence, the modulated samples are prone to distortions caused by the non-linear devices and diffused channels, resulting in reduced accuracy of delay offset estimation for timing synchronization. In this paper, we propose a method to improve the timing synchronization accuracy of LACO-OFDM modulated systems using extreme learning machines (ELM). The ELM is trained offline to identify the delay offset using the timing metric (TM) as a feature identification problem. Among all types of neural networks that are used as classifiers and non-linear estimators, the ELMs are the least complex and require significantly less computational capacity. The simulations are performed to evaluate the accuracy of delay offset estimation in the presence of unknown non-linear effects of high-powered light-emitting diodes (LEDs) and diffused channel characteristics. The results show that the probability of error (PoE) of delay offset estimation and bit error rate (BER) is significantly reduced for scenarios like different frame lengths, channel coefficients, and unknown LED parameters in contrast to conventional methods. The proposed approach attains a PoE of 10−1 at signal-to-noise ratios (SNR)s of 4 dB and 6 dB with various TM, surpassing conventional methods which only achieve this beyond 15 dB. Moreover, the PoE remains below 10−2 for SNRs exceeding 7 dB and 10 dB in the proposed method, while conventional methods require SNRs above 25 dB to achieve comparable results.

Enhanced terahertz third-harmonic generation based on the graphene-assisted meta-grating structure

The nonlinear response of optical materials provides a valuable approach for optical switching and nonlinear frequency conversion. However, the nonlinear responses of bulk crystals are often very weak, requiring higher pump energy for pumping. In this work, we investigate the significant enhancement of third harmonic generation (THG) at terahertz frequencies using an ultra-thin nonlinear meta-grating metasurface assisted by graphene strips. In this structure, the incident pump light experiences strong confinement and enhancement along the graphene surface caused by the generation of strongly localized surface plasmon resonances. Utilizing the significant third-order nonlinear conductivity of graphene, it becomes possible to achieve high THG conversion efficiency even at low pump intensities. The research results indicate that, at low incident intensity of 10 kW/cm2, the third harmonic wave conversion efficiency can reach approximately 2.8 %. We also thoroughly investigate the impact of meta-grating structural parameters, graphene Fermi level, incident angle, and pump light intensity on THG operation characteristics. Our study findings provide an effective approach for greatly boosting the THG conversion efficiency in the terahertz frequency bands, opening new avenues in the development of novel compact frequency converters and modulators that are emerging in terahertz chip integration and nonlinear devices.

Assessment of the Performance of Frequency Domain Models Based on Different Reference Points for Linearization

Power electronic-based loads are increasingly used in modern distribution systems. Such appliances inject harmonic currents into the power system and thus, they can have serious impacts on the power quality of the system. Therefore, it is necessary to find suitable frequency domain models of these loads to predict their contribution to the harmonic voltage levels. So far, various modeling approaches have been proposed in the relevant literature. Usually, they are parameterized based on laboratory measurements. In this paper, however, a circuit-based time domain model of a Compact Fluorescent Lamp (CFL) is implemented in MATLAB®/Simulink® and used for the parameter identification of three different frequency domain models. These models usually rely on a linearization of their emission characteristic and therefore, it is required to choose an appropriate reference voltage. As the supply voltage in real distribution systems is already distorted, the commonly made assumption of a sinusoidal reference voltage may increase the inaccuracy of models for strongly non-linear devices. Therefore, different reference points for the model parameterization are specified and their impact on the model performance under typical voltage distortions is studied.

Design, Optimization, and Application of Nonlinear Energy Sink in Energy Harvesting Device

Vibration control has been of great interest to scientists and engineers for many years. Although linear vibration absorbers have been shown to be effective in mitigating vibrations at specific frequencies, their vibration reduction effect is usually limited to a narrow frequency bandwidth. Nonlinear energy sinks have attracted attention due to their better vibration reduction effect over a wider frequency bandwidth. In practical applications, the nonlinear energy sink devices can effectively absorb, dissipate, and convert energy from broadband excitation, so as to achieve vibration reduction and energy harvesting. However, research on energy harvesting based on the nonlinear energy sink is less mature than in linear systems. Multiple parameters of device design (e.g., damping size) affect the actual performance of the nonlinear energy sinks, but there is no exact method to simplify the design of the multiparameter nonlinear energy sinks. Since it is more difficult to implement electromagnetic and electrostatic energy harvesters, more research has focused on piezoelectric energy harvesters. This paper summarizes the research on the nonlinear energy sink and energy harvesting technology, including the introduction of the nonlinear energy sink, energy harvesting based on the nonlinear energy sink, and its application in various fields of energy harvesting. The paper also summarizes some important methods for solving the dynamical equations, as well as their advantages and disadvantages. The conclusions provide an outlook on the subsequent research of the nonlinear energy sink technology, such as the introduction of piezoelectric materials with high energy density, the benefits of balanced vibration suppression and harvesting of vibration energy, and the self-tuning of parameters in complex environments. It provides a powerful reference for the popularization and application of energy harvesting technology based on nonlinear energy sinks.

An algorithm for solving a system of linear equations with Z-numbers based on the neural network approach

Mathematical modeling of many natural and physical phenomena in industry, engineering sciences and basic sciences lead to linear and non-linear devices. In many cases, the coefficients of these devices, taking into account qualitative or linguistic concepts, show their complexity in the form of Z-numbers. Since Z-number involves both fuzziness and reliability or probabilistic uncertainty, it is difficult to obtain the exact solution to the problems with Z-number. In this work, a method and an algorithm are proposed for the approximate solution of a Z-number linear system of equations as an important case of such problems. The paper is devoted to solving linear systems where the coefficients of the variables and right hand side values are Z-numbers. An algorithm is presented based on a ranking scheme and the neural network technique to solve the obtained system. Moreover, two examples are included to describe the procedure of the method and results.

Demonstration of Spatial Asymmetric Light Propagation Performance Using Violet Phosphorus Quantum Dots with Tunable Bandgap

AbstractThe spatial self‐phase modulation (SSPM) of phosphorus‐based nanomaterials is widely studied and developed as passive nonlinear photonic devices for applications in all‐optical switches, logic gates, information converters, etc. In this work, violet phosphorus quantum dots (VP QDs) are prepared and characterized in three different solvents, their SSPM is investigated, and their spatial asymmetric light propagation performances are demonstrated. It is shown that VP QDs prepared in three different solvents exhibit different bandgaps, mainly due to the interaction between the dangling bonds of VP QDs and functional groups in different solvents. The SSPM experiment characterizes the nonlinear optical response of solvent‐dependent VP QDs. It is found that VP QDs exhibit strong nonlinear optical effect and their nonlinear refractive indexes are comparable to other phosphorus‐based 2D materials. Unlike the previously reported principle of spatial asymmetric light propagation (i.e., a cascaded sample based on SSPM composed of nanomaterials with excellent nonlinear refraction and SnS2 with reverse saturation absorption), the spatial asymmetric light propagation performance based on SSPM is demonstrated using a cascaded sample of VP QDs with significantly different nonlinear optical response prepared in two different solvents.

Refined Synthesis and Single Crystal Growth of PbGa2Se4 by Chemical Vapor Transport Method for Photodetection

AbstractTernary chalcogenide PbGa2Se4 with high resistivity, photosensitivity, and excellent nonlinear properties, exhibits a potential application in optoelectronic and nonlinear optical devices. However, the preparation of large‐sized pure PbGa2Se4 crystals is challenging due to the presence of peritectic reaction and a narrow homogeneity region. Here, a “quenching‐annealing” method (quenching at 850 °C in ice water, and then annealing at 650 °C for 250 h by reheating) is developed to eliminate the PbSe second phase during the synthesis. Subsequently, the PbGa2Se4 single crystals are successfully grown using chemical vapor transport (CVT) with the I2 as the transport agent. The resulting crystal exhibits the crystal structure belonging to Fddd space group with the lattice parameters of a = 12.7192 Å, b = 21.2831 Å, and c = 21.5226 Å. Additionally, it possesses a wide bandgap (≈2.26 eV), high resistivity (6.59 × 1012 Ω·cm), and defect density calculated via space charge limited current measurement (SCLC) as 2.46 × 1011 cm−3. Photodetectors based on these as‐grown crystals demonstrate exceptional photosensitivity along with a high detectivity (3.2 × 108 Jones).

Bimetallic solid solution MAX phase Ti2NbAlC2 as saturable absorber for passively Q-switched Er3+-doped fiber laser

In this paper, we demonstrated a stable Q-switched fiber laser operation with a central wavelength of 1531.43 nm using a bimetallic solid solution MAX phase Ti2NbAlC2 saturable absorber. The minimum pulse duration of 3.67 μs and the maximum repetition rate of 32.56 kHz were acquired at the pump power of 300 mW. Furthermore, the Q-switched pulse has a maximum output power of 1.08 mW and a maximum pulse energy of 33.14 nJ. Our results indicate that the Ti2NbAlC2 is a promising saturable absorber material. The bimetallic solid solution MAX phase materials may be developed as novel nonlinear photonic devices with outstanding performance.

On the optimum multicarrier performance with memoryless nonlinearities

One of the major problems associated with multicarrier signals is their high envelope fluctuations that lead to implementation difficulties. Therefore, multicarrier signals are usually submitted to nonlinear operations such as clipping to simplify the power amplification, DAC (Digital-to-Analog Conversion) and other digital signal processing operations. However, these nonlinear operations lead to nonlinear noise that usually is associated with performance degradation. Recently, it was shown that the nonlinear distortion associated to bandpass memoryless nonlinear devices not only does not necessarily means performance degradation, but can even lead to performance gains. In this paper we study the impact of non-bandpass memoryless nonlinearities on the performance of multicarrier signals. We consider both nonlinear devices operating on the real-valued multicarrier signals as well as Cartesian nonlinear devices (also denoted as I-Q nonlinearities) operating separately on the real and imaginary parts of the complex envelope of multicarrier signals. We present a theoretical analysis of the potential performance improvements associated with the optimum detection of multicarrier signals, by deriving accurate theoretical expressions for both the average asymptotic gains in ideal AWGN (Additive White Gaussian Noise) channels and the asymptotic gain distribution for frequency-selective channels with multipath Rayleigh fading.