Nonlinear Conversion Research Articles

Accurate and Efficient Large-Scale Multi-Label Learning With Reduced Feature Broad Learning System Using Label Correlation.

Multi-label learning for large-scale data is a grand challenge because of a large number of labels with a complex data structure. Hence, the existing large-scale multi-label methods either have unsatisfactory classification performance or are extremely time-consuming for training utilizing a massive amount of data. A broad learning system (BLS), a flat network with the advantages of succinct structures, is appropriate for addressing large-scale tasks. However, existing BLS models are not directly applicable for large-scale multi-label learning due to the large and complex label space. In this work, a novel multi-label classifier based on BLS (called BLS-MLL) is proposed with two new mechanisms: kernel-based feature reduction module and correlation-based label thresholding. The kernel-based feature reduction module contains three layers, namely, the feature mapping layer, enhancement nodes layer, and feature reduction layer. The feature mapping layer employs elastic network regularization to solve the randomness of features in order to improve performance. In the enhancement nodes layer, the kernel method is applied for high-dimensional nonlinear conversion to achieve high efficiency. The newly constructed feature reduction layer is used to further significantly improve both the training efficiency and accuracy when facing high-dimensionality with abundant or noisy information embedded in large-scale data. The correlation-based label thresholding enables BLS-MLL to generate a label-thresholding function for effective conversion of the final decision values to logical outputs, thus, improving the classification performance. Finally, experimental comparisons among six state-of-the-art multi-label classifiers on ten datasets demonstrate the effectiveness of the proposed BLS-MLL. The results of the classification performance show that BLS-MLL outperforms the compared algorithms in 86% of cases with better training efficiency in 90% of cases.

Neodymium laser-pumped ultra-low noise tunable Brillouin fiber laser around 920 nm

We have developed an ultra-low noise tunable Brillouin fiber laser exhibiting three orders of magnitude better frequency noise performance than the Neodymium-doped fiber laser pump and remarkable optical signal-to-noise ratio exceeding 80 dB suitable for immediate applications in coherent nonlinear conversion, quantum computing and underwater communications. In addition, we have implemented a custom optical phase-locked loop to ensure long-term stable operation and have investigated its impact on frequency noise. We demonstrate the power scalability of the single frequency (Hz-class) Brillouin laser, delivering over 500 mW with tunability across the 900 nm to 930 nm range in an all-fiber fully polarization-maintaining architecture.

Uncovering the Mechanisms of Triplet-Triplet Annihilation Upconversion Enhancement via Plasmonic Nanocavity Tuning.

The nonlinear conversion of photons from lower to higher energy is important for a wide range of applications, from quantum communications and optoelectronics to solar energy conversion and medicine. Triplet-triplet annihilation upconversion (TTA UC), which utilizes an absorber/emitter molecular pair, is a promising tool for upconversion applications requiring low intensity light such as photovoltaics, photocatalysis, and bioimaging. Despite demonstrations of efficient TTA UC in solution, practical applications have proven difficult, as thin films retard the necessary energy transfer steps and result in low emission yields. In this work, TTA UC emission from a thin film is greatly enhanced through integration into plasmonic nanogap cavities consisting of a silver mirror, a nanometer-scale polymer spacer containing a TTA molecular pair, and colloidally synthesized silver nanocubes. Mechanistic studies performed by varying the nanocube side length (45-150 nm) to tune the nanogap cavity resonance paired with simulations reveal absorption rate enhancement to be the primary operative mechanism in overall TTA UC emission enhancement. This absorption enhancement decreases the TTA UC threshold intensity by an order of magnitude and allows TTA UC emission to be excited with light up to 120 nm redder than the usable wavelength range for the control samples. Further, combined nanogap cavities composed of two distinct nanocube sizes result in surfaces which simultaneously enhance the absorption rate and emission rate. These dual-size nanogap cavities result in 45-fold TTA UC emission enhancement. In total, these studies present TTA UC emission enhancement, illustrate how the usable portion of the spectrum can be expanded for a given sensitizer-emitter pair, and develop both mechanistic understanding and design rules for TTA UC emission enhancement by plasmonic nanostructures.

Continuously wavelength tunable, continuous wave laser ideal for UV Raman spectroscopy

AbstractWe utilize a novel, high‐power, tunable, continuous wave (CW) deep UV laser to measure resonance Raman spectra of phenolate solutions with high signal‐to‐noise ratios (SNR). In UV resonance Raman (UVRR), increased coupling of the excitation light with a chromophore can transfer molecules into excited states that cause increased heating and photochemistry. Deep UV lasers have traditionally utilized high peak powers to enable efficient single‐pass nonlinear conversion from visible into near infrared light. Nonlinear phenomena such as the formation of transient radical species, Raman saturation, thermal heating, and dielectric breakdown can introduce extraneous light sources that can complicate the interpretation of the Raman spectrum. Dielectric breakdown can increase the baseline, increase noise, and sometimes saturate the detector, preventing Raman detection. Spontaneous Raman scattering intensities should scale linearly with the excitation light intensity. However, this linear behavior does not always occur with pulsed laser excitation. This occurs because stimulated Raman scattering can cause a superlinear intensity response, or transient absorption can cause sublinear intensity responses. CW laser excitation excites samples with electric fields that are much lower than typical pulsed laser excitation. This eliminates the nonlinear responses. The geometry of our new CW laser enables high gain in the harmonic generation cavities that achieve high harmonic generation efficiencies. Average power in the deep UV is >30 mW for wavelengths as short as 206 nm. In the work here, we demonstrate that CW excitation is ideal for resonance Raman measurements in general to reduce spectral complexity.

A novel ANFIS-controlled customized UPQC device for power quality enhancement

Power quality is crucial for the reliable and efficient operation of domestic and industrial loads. Nonlinear power electronic converter loads have increased in usage, which has led to a decline in both voltage and current quality. To overcome these power quality issues, a universal power quality compensator (UPQC) has been developed and integrated into the power distribution network. The UPQC comprises dual active power filters which are equipped with a DC-link capacitor. The voltage of the DC-link capacitor is controlled by a DC-link voltage controller. However, the conventional PI controller is not suitable for regulating voltage at a defined level due to inappropriate gain values. In this work, we propose an intelligent adaptive neuro-fuzzy inference system-based DC voltage controller for customized UPQC devices. The proposed control scheme aims to mitigate the existing power quality challenges by reducing the issues in classical controllers and incorporating intelligence knowledge with subjective decisions. Using the MATLAB/Simulink software tool, we have tested a customized UPQC device controlled by ANFIS for critical operation and performance. The simulation results are presented along with valid comparisons.

Wavelength-accurate nonlinear conversion through wavenumber selectivity in photonic crystal resonators

Wavelength-accurate nonlinear conversion through wavenumber selectivity in photonic crystal resonators

Frequency conversion of orbital angular momentum with optimized efficiency and modal purity

The use of nonlinear optics for the creation, control, and detection of structured light has gained prominence of late, but it is plagued by low efficiency and variable modal purity. Here, we show how to optimize the efficiency and modal purity in the nonlinear conversion of structured light modes. Using difference frequency generation of orbital angular momentum carrying Laguerre–Gaussian modes as an example, we achieved more than a 40% rise in efficiency while maintaining the modal purity above 90% compared to conventional methods. To demonstrate the versatility of our approach, we show the frequency conversion of perfect vortex beams up to ℓ=40 in a topological charge, achieving a new state of the art. We believe this theoretical treatment will facilitate our work’s translation to other modal forms and nonlinear processes.

Command-Filter-Based Region-Tracking Control for Autonomous Underwater Vehicles with Measurement Noise

This paper investigates the AUV region-tracking control problem with measurement noise and transient and steady-state constraints. To achieve the fluctuation of AUV tracking error within an expected region while satisfying the transient and steady-state performance constraints, this paper proposes an improved nonlinear tracking error transformation method. This method converts the tracking error into a new virtual error variable through nonlinear conversion, thus transforming the above performance requirements for the tracking error into boundedness requirements for the new virtual error variable. In addition, aiming at the problem of measurement noise causing strong fluctuation of the control signal, this paper proposes a finite-time AUV control method based on a two-stage command filter. This method utilizes a finite-time sliding mode differentiator to filter the virtual control signal during the derivation of the control law using the backstepping technique. In light of the signal loss incurred by two-stage filtering and its potential impact on system stability, a finite-time compensator is designed to compensate the signal loss and achieve finite-time stability of the closed-loop system. Finally, simulations conducted using ODIN AUV demonstrate that the proposed method exhibits smooth control signal and low energy consumption characteristics. Furthermore, the tracking error meets the requirements for both transient and steady-state performance, as well as regional tracking.

Nonlinear-optical piezoelectric electrospun nanofibers

This paper presents a nonlinear optical piezoelectric electrospun nanocomposite of polyvinylidene fluoride (PVDF) nanofibers with embedded nanomaterial of poly {1-[p-(3’-carboxy-4’- hydroxyphenylazo) benzenesulfonamido]-1, 2-ethandiyl} (PCBS). PCBS of different weight ratios are added in-situ to the synthesized PVDF nanofibers. The generated nanofibers mats have been proved to possess both properties of piezoelectric response and nonlinear optical conversion due to the presence of both PVDF and PCBS, respectively. Innovatively, embedding PCBS within the PVDF nanofibers increases the piezoelectric response of the nanocomposite, compared to the case of neat PVDF nanofibers, at certain optimum weight concentrations of PCBS. This has been proved by different piezoelectric measurements at different applied forces and vibration frequencies, along with d33 coefficient measurements and supported by FTIR analysis of beta-sheets. Our results support the evidence of the coupling between the polarized diploes of PCBS and PVDF, which can enhance the overall piezoelectric response. Furthermore, the synthesized nanocomposite shows an optical second harmonic generation (SHG) at optimum concentration of added PCBS within the PVDF nanofibers of around 1 wt.%. The synthesized nonlinear optical nanocomposite of PVDF/PCBS can be multi-functionalized sensors/transducer membranes for biomedical and sensing applications.

Robust Control Algorithm for Virtual Synchronous Generator in Intermittent Generation

This work presents a comparison of two control algorithms for the model of a nonlinear converter system used as a virtual synchronous generator (VSG). The converter connects between the electrical grid and an intermittent renewable system, such as photovoltaic or wind, with backup for sudden disturbances in electricity generation. The proposal of the VSG arises from the fact that, on one hand, the conventional generation system consists of large power plants such as combined-cycle power plants that employ generators and turbines with significant physical inertia. On the other hand, when connecting renewable systems such as wind and solar, which use converters to integrate with the grid, they lack physical inertia. It is necessary to couple this inertia to the system in order to compensate for the disturbances of intermittent systems and thereby maintain the frequency of the electrical power network. It should be clarified that physical inertia is not available when renewable systems such as wind or photovoltaic are connected. Recent studies have shown that it is possible to compensate for the disturbance caused by this lack of inertia through a backup system and the virtual synchronous generator (power converter) using control algorithms. In this work, linearization and super twisting algorithms are employed for handling disturbances, and the main differences between the algorithms are demonstrated when facing sudden changes in active or reactive power.

Analytical solution to incident angle quasi-phase-matching engineering for second harmonic generation in a periodic-poled lithium niobate crystal

Phase matching or quasi-phase matching (QPM) is of significant importance to the conversion efficiency of second harmonic generation (SHG) in artificial nonlinear crystals like lithium niobate (LN) crystal or microstructured nonlinear crystals like periodic-poled lithium niobate (PPLN) crystals. In this paper, we propose and show that the incident angle of pump laser light can be harnessed as an alternative versatile tool to engineer QPM for high-efficiency SHG in a PPLN crystal, in addition to conventional means of period adjusting or temperature tuning. A rigorous model is established and analytical solution of the nonlinear conversion efficiency under the small and large signal approximation theory is obtained at different incident angles. The variation of phase mismatching and walk-off length with incident angle or incident wavelength are also explored. Numerical simulations for a PPLN crystal with first order QPM structure are used to confirm our theoretical predictions based on the exact analytical solution of the general large-signal theory. The results show that the narrow-band tunable SHG output covers a range of 532 nm–552.8 nm at the ideal incident angle from 0° to 90°. This theoretical scheme, fully considering the reflection and transmission at the air-crystal interface, would offer an efficient theoretical system to evaluate the nonlinear frequency conversion and help to obtain the maximum SHG conversion efficiency by selecting an optimum incident wavelength and incident angle in a specially designed PPLN crystal, which would be very helpful for the design of tunable narrow-band pulse nanosecond, picosecond, and femtosecond laser devices via PPLN and other microstructured LN crystals.

Convex optimization of PV-battery system sizing and operation with non-linear loss models

To mitigate climate change, households are increasingly incentivized to install PV systems in combination with battery energy storage systems to increase their self-sufficiency and flexibility as well as to relieve the stress caused by the high penetration of distributed generation on the grid. This paper aims to assess the need for non-linear loss models as opposed to linear loss models found in the literature when optimizing the sizing and operation of PV-battery systems. Therefore, an optimization model is presented which implements non-linear, convex and linear converter and battery loss model formulations. The relaxed convex formulation is equivalent to the original non-linear formulation and can be solved more efficiently, decreasing the run time by a factor of 4. The impact of non-linear as opposed to linear loss models on the optimal solution is illustrated for a residential DC-coupled PV-battery system. The linear loss model is shown to result in an underestimation of the cost by 14.7% and the battery size by 12.4%. Further, the battery utilization is underestimated by a third. The proposed method is useful to accurately model the losses when optimizing the sizing and operation of a PV-battery system in exchange for a slightly higher computational time compared to linear loss models, though far below that of solving the non-relaxed non-linear problem.

Lossless Monochromator in an Ultrafast Electron Microscope Using Near-Field THz Radiation.

The ability to form monoenergetic electron beams is vital for high-resolution electron spectroscopy and imaging. Such capabilities are commonly achieved using an electron monochromator, which energy filters a dispersed electron beam, thus reducing the electron flux to yield down to meV energy resolution. This reduction in flux hinders the use of monochromators in many applications, such as ultrafast transmission electron microscopes (UTEMs). Here, we develop and demonstrate a mechanism for electron energy monochromation that does not reduce the flux-a lossless monochromator. The mechanism is based on the interaction of free-electron pulses with single-cycle THz near fields, created by nonlinear conversion of an optical laser pulse near the electron beam path inside a UTEM. Our experiment reduces the electron energy spread by a factor of up to 2.9 without compromising the beam flux. Moreover, as the electron-THz interaction takes place over an extended region of many tens of microns in free space, the realized technique is highly robust-granting uniform monochromation over a wide area, larger than the electron beam diameter. We further demonstrate the wide tunability of our method by monochromating the electron beam at multiple primary electron energies from 60 to 200keV, studying the effect of various electron and THz parameters on its performance. Our findings have direct applications in the fast-growing field of ultrafast electron microscopy, allowing time- and energy-resolved studies of exciton physics, phononic vibrational resonances, charge transport effects, and optical excitations in the mid IR to the far IR.

A new nonlinear optical crystal Li0.81Ag0.19InSe2 with balanced properties for efficient nonlinear conversion in the mid-IR region

The achievement of balanced properties among nonlinear optical (NLO) effect, laser induced damage threshold (LIDT) and optical birefringence is the key to obtain the excellent NLO crystals for harmonic mid-IR coherent radiation generation. In this work, we successfully synthesize and grow a new NLO chalcogenide crystal Li0.81Ag0.19InSe2 by utilizing a molecular design strategy with rational replacement between Li and Ag. This crystal not only enlarges the very low birefringence in AgInSe2 from ∼ 0.017 to ∼ 0.047, but also improves the rather small NLO effect in LiInSe2 from 11.8 pm/V to ∼26 pm/V while maintaining the LIDT almost unchanged. Based on the large size Li0.81Ag0.19InSe2 single crystals of optical quality, the spectroscopic properties are measured and the main parameters important for NLO output are determined. All these results demonstrate that Li0.81Ag0.19InSe2 is a promising mid-IR NLO crystal with balanced properties.

Brillouin lasers in a graphene microresonator for multispecies and individual gas molecule detection

Optical microcavities offer a promising platform for highly efficient light–matter interactions. Recently, the combination of microresonators and 2D materials in nanoscale has further enriched the optoelectronics of the microcavity geometries, spurring broad advances ranging from lasers, nonlinear converters, modulators to sensors. Here, we report the concept of a Brillouin laser sensor, by depositing graphene on an over-modal microsphere. Driven by a single continuous-wave pump at 1550 nm, multiple Brillouin lasers from distinct mode families are co-generated in a single device. The Brillouin lasers excited in the high Q cavity produce heterodyne beating notes with phase noise down to −161 dBc/Hz at 1 MHz offset, not only enabling label-freely identifiable detection of multispecies gas molecules adsorbed on the graphene in situ but also rendering it possible to trace individual molecules. Such a combination of graphene optoelectronics and Brillouin lasers in microcavities demonstrates a novel physical paradigm and offers insights into powerful tools for fast and precise optical sensing.

Investigation of experimental setup and simulation results of 3–5 μm mid-infrared ZnGeP2 OPO pumped by Q-switched Ho:YLF laser

In this paper, the result of mid-infrared ZGP-OPO nonlinear conversion from a Q-switched Ho:YLF laser at 2.056 μm wavelength was investigated. First, the linear OPO resonator was analyzed using simulation process. To improve the output result of the ZGP-OPO structure, two models of cavity resonance were used with a double-resonance oscillator (DRO) and double-resonance pump enhancement oscillator (PE-DRO) performances. By changing the physical length of the linear Plano-Plano resonator, the optimum output energy and far field profile of the beam were recorded in the SNLO simulation. Subsequently, 21.5 mJ total energy of 3–5 μm wavelength corresponding to 53.5 % optical-to-optical efficiency was measured in experiment approach of the PE-DRO resonance regimes. At maximum output energy, the peak power of OPO waves was 0.96 MW. The experimental OPO performance of mentioned regimes confirms SNLO simulation results. The central wavelength of the signal and idler wave’s envelope were measured at 3779.5 and 4507.6 nm, and they were extracted by tuning the ZGP crystal angle.

Resonant nonlinear nanostructured grating in an unstructured lithium niobate on insulator platform

Lithium niobate (LN) is a versatile material for applications in integrated nonlinear optics; however, due to its stable physical and chemical properties, efficient nonlinear conversion with nanostructured LN is not easy to implement. Here, we propose a resonant nanostructure composed of a SiO2 grating and a lithium niobate on insulator (LNOI) layer, which has been proven to support bound states in the continuum (BIC) modes through simulations. By experimentally measuring the angle-resolved transmittance spectra of the fabricated sample, we confirm that our proposed nanostructure exhibits quasi-BIC modes transitioning from BIC modes, resulting in strong localized field confinement. In the nonlinear experiment, our results show the significant potential of the nanostructure in enhancing second harmonic generation (SHG). By pumping at the resonant wavelength, we achieve a SHG enhancement factor of 6400, compared to a flat LNOI thin film under a fundamental wave (FW) peak intensity of 460 MW/cm2. The SHG conversion efficiency reaches up to 1.741 × 10−7, leading to a normalized conversion efficiency of 3.785 × 10−7 cm2/GW. Although the experimental results are two orders of magnitude lower than the theoretical maximum SHG conversion efficiency of approximately 10−5, we believe that our proposed etchless LNOI nanostructure highlights the tremendous potential for applications in nonlinear resonant nanophotonics.

Generation of femtosecond pulses with an energy of ~10 μJ at a wavelength of 1.56 μM and nonlinear conversion of their frequency in a hollow revolver fiber

A fiber source of ultrashort pulses (USPs) with a powerful hybrid amplifier based on erbium and erbium-ytterbium fibers connected in series with a double cladding and an increased mode diameter, pumped by multimode laser diodes at a wavelength of 976 nm, was created and studied. Pulses from the output of a powerful amplifier at a central wavelength of about 1.56 μm with a duration of ~200 ps and an energy of up to 21.9 μJ, following with a frequency of 198.5 kHz, were compressed using a compressor based on a pair of volumetric polarization-independent diffraction gratings operating at transmission, with an efficiency of ≈73%. A minimum duration of compressed pulses of 742 fs was achieved, while their maximum energy and peak power were 15.8 μJ and ≈13 MW, respectively (maximum average power - 3.14 W) at a central wavelength of 1.56 μm in a beam with a Gaussian profile and measured quality parameters = 1.58 and = 1.38. When pumped by pulses from an erbium fiber source of various durations, the SRS transformation of 1.56 μm → 1.71 μm was realized in a hollow revolver light guide with a core of 75 μm in diameter filled with molecular hydrogen. Pulses with a maximum energy of about 200 nJ at a wavelength of 1.71 μm were obtained. The developed ultrashort pulse source can be useful in creating advanced systems for precision processing of transparent materials, especially silicon, as well as for other applications that require high energy and peak power of ultrashort pulses.

High-fidelity parametric beamsplitting with a parity-protected converter

Fast, high-fidelity operations between microwave resonators are an important tool for bosonic quantum computation and simulation with superconducting circuits. An attractive approach for implementing these operations is to couple these resonators via a nonlinear converter and actuate parametric processes with RF drives. It can be challenging to make these processes simultaneously fast and high fidelity, since this requires introducing strong drives without activating parasitic processes or introducing additional decoherence channels. We show that in addition to a careful management of drive frequencies and the spectrum of environmental noise, leveraging the inbuilt symmetries of the converter Hamiltonian can suppress unwanted nonlinear interactions, preventing converter-induced decoherence. We demonstrate these principles using a differentially-driven DC-SQUID as our converter, coupled to two high-Q microwave cavities. Using this architecture, we engineer a highly-coherent beamsplitter and fast (~100 ns) swaps between the cavities, limited primarily by their intrinsic single-photon loss. We characterize this beamsplitter in the cavities’ joint single-photon subspace, and show that we can detect and post-select photon loss events to achieve a beamsplitter gate fidelity exceeding 99.98%, which to our knowledge far surpasses the current state of the art.

Multimodal Optically Nonlinear Nanoparticles Exhibiting Simultaneous Higher Harmonics Generation and Upconversion Luminescence for Anticounterfeiting and 8‐bit Optical Coding

AbstractNonlinear optical materials are essential in areas such as nanophotonics, optical information processing, and biomedical imaging. However, nanomaterials employed for these diverse applications to date are efficient only for one type of nonlinear optical activity. Herein, the first multimodal nonlinear optically active class of nanomaterials based on lanthanide‐doped lithium niobate nanoparticles, which simultaneously exhibit unprecedentedly efficient second and third harmonic generation, as well as up‐conversion photoluminescence, is reported. These dielectric nanoparticles retain their high nonlinear optical conversion efficiency both as powder and as aqueous colloidal solution. The high stability also allows for the fabrication of optically active biocompatible micron‐sized fibers and polymer‐based 3D‐printable objects, as well as for fingerprint detection. Finally, the first 8‐bit coding platform purely based on multimodal nonlinear optical activity originating from different parametric and nonparametric processes is demonstrated, showcasing the technological potential of these materials for both anti‐counterfeiting and advanced optical information processing.