Modulation of High-Intensity Optical Properties in CdS/CdSe/CdS Spherical Quantum Wells by CdSe Layer Thickness.

Spherical quantum wells (SQWs) have proven to be excellent materials for suppressing Auger recombination due to their expanded confinement volume. However, research on the factors and mechanisms of their high-intensity optical properties, such as multiexciton properties and third-order optical nonlinearities, remains incomplete, limiting further optimization of these properties. Here, a series of CdS/CdSe (xML)/CdS SQWs with varying CdSe layer thicknesses were prepared. The modulation effects of CdSe shell variations on the PL properties, defect distribution, biexciton binding energy, and third-order optical nonlinearities of the SQWs were investigated, and their impact on the material's multiexciton properties was further analyzed. Results showed that the typical CdS/CdSe(3ML)/CdS sample exhibited a large volume-normalized two-photon absorption cross-section (18.17 × 102 GM/nm3) and favorable biexciton characteristics. Optical amplification was observed at 12.4 µJ/cm2 and 1.02 mJ/cm2 under one-photon (400 nm) and two-photon (800 nm) excitation, respectively. Furthermore, different amplified spontaneous emission spectra were observed for the first time under one/two-photon excitation. This phenomenon was attributed to thermal effects overcoming the biexciton binding energy. This study provides valuable insights for further optimizing multiexciton gain characteristics in SQWs and developing optical gain applications.

Impact of Tamm plasmon structures on fluorescence and optical nonlinearity of graphene quantum dots.

Graphene Quantum Dots (GQDs) are crucial in biomedicine for sensitive biosensing and high-resolution bioimaging and in photonics for their nonlinear optical properties. Integrating GQDs with photonic structures enhances optical properties by optimizing light-matter interactions and enabling precise control over their emission wavelengths. In this work, we explore a facile synthesis method for GQDs by pulsed laser irradiation in chlorobenzene and highlight the transformative potential of Tamm Plasmon Cavity (TPC) structures for tuning and amplifying the photoluminescence and nonlinear optical properties of GQDs. The characterization of GQDs revealed their exceptional properties, including efficient optical limiting and stable photoluminescence. The study demonstrated that the TPC structure significantly amplifies nonlinear optical effects due to the high light-matter interaction, indicating the potential for advanced optical systems, including optical limiters and nonlinear optical devices. Furthermore, introducing GQDs into the TPC structure leads to a significant enhancement and tuning of fluorescence emission. The Purcell effect, in combination with the confined electromagnetic fields within the TPC, increases the spontaneous emission rate of GQDs and subsequently enhances the fluorescence intensity. This enhanced and tunable fluorescence has exciting implications for high-sensitivity applications such as biosensing and single-molecule detection.

Asymmetric Laser Field Interaction with MXene Coated on the Side Surface of Optical Fibers for Ultrafast Nonlinear Switches.

In recent years, there has been significant interest in researching ultrafast nonlinear optical phenomena involving light-matter interactions in two-dimensional (2D) materials, owing to their potential applications in optics and photonics. MXene, a recently developed 2D material, has garnered considerable attention due to its graphene-like properties and highly tunable electronic/optical characteristics. Herein, we demonstrate ultrafast all-optical switches based on four-wave-mixing (FWM) utilizing the nonlinear optical property of MXene Ti3C2Tx. In order to realize the device, we deposited multilayered Ti3C2Tx in the form of a supernatant solution onto the polished surface of a side-polished optical fiber, enabling the interaction of Ti3C2Tx with the asymmetric evanescent field of the incident input. We systematically characterized the nonlinear optical responses derived from the Ti3C2Tx layers. The fabricated device exhibits notable performance metrics, an enhancement of the extinction ratio, and a conversion efficiency of the newly generated signal, displaying 5.3 and 5.2 dB, respectively. Additionally, the device operates at high modulation frequencies, reaching up to 20 GHz, and demonstrates high-resolution detuning with channel distances of up to 15 nm. Our findings highlight the potential of MXene-based materials for ultrafast optical data management systems.

Optical Bistability in Nanosilicon with Record Low Q-Factor.

We demonstrated optical bistability in an amorphous silicon Mie resonator with a size of ∼100 nm and Q-factor as low as ∼4 by utilizing photothermal and thermo-optical effects. We not only experimentally confirmed the steep intensity transition and the hysteresis in the scattering response from silicon nanocuboids but also established a physical model to numerically explain the underlying mechanism based on temperature-dependent competition between photothermal heating and heat dissipation. The transition between the bistable states offered particularly steep superlinearity of scattering intensity, reaching an effective nonlinearity order of ∼100th power over excitation intensity, leading to the potential of advanced optical switching devices and super-resolution microscopy.

Intensity-Dependent Optical Response of 2D LTMDs Suspensions: From Thermal to Electronic Nonlinearities.

The nonlinear optical (NLO) response of photonic materials plays an important role in the understanding of light-matter interaction as well as pointing out a diversity of photonic and optoelectronic applications. Among the recently studied materials, 2D-LTMDs (bi-dimensional layered transition metal dichalcogenides) have appeared as a beyond-graphene nanomaterial with semiconducting and metallic optical properties. In this article, we review most of our work in studies of the NLO response of a series of 2D-LTMDs nanomaterials in suspension, using six different NLO techniques, namely hyper Rayleigh scattering, Z-scan, photoacoustic Z-scan, optical Kerr gate, and spatial self-phase modulation, besides the Fourier transform nonlinear optics technique, to infer the nonlinear optical response of semiconducting MoS2, MoSe2, MoTe2, WS2, semimetallic WTe2, ZrTe2, and metallic NbS2 and NbSe2. The nonlinear optical response from a thermal to non-thermal origin was studied, and the nonlinear refraction index and nonlinear absorption coefficient, where present, were measured. Theoretical support was given to explain the origin of the nonlinear responses, which is very dependent on the spectro-temporal regime of the optical source employed in the studies.

Determination of Light-Induced Deformation Characteristics of the Director Field of Nematic Liquid Crystal by Combining the Methods of Aberrational Self-Action of Light Beam and Polarization Microscopy.

Light-induced director field deformation of a nematic liquid crystal in the field of an obliquely incident laser beam is experimentally studied with aberrational self-action and polarization microscopy. Each of the methods has features associated with the geometry of the light interaction with the director. The combination of methods significantly expands the possibilities of reconstructing the light-induced nonlinear phase shift profile and the director field deformation.

Third-Order Optical Nonlinearities of 2D Materials at Telecommunications Wavelengths.

All-optical signal processing based on nonlinear optical devices is promising for ultrafast information processing in optical communication systems. Recent advances in two-dimensional (2D) layered materials with unique structures and distinctive properties have opened up new avenues for nonlinear optics and the fabrication of related devices with high performance. This paper reviews the recent advances in research on third-order optical nonlinearities of 2D materials, focusing on all-optical processing applications in the optical telecommunications band near 1550 nm. First, we provide an overview of the material properties of different 2D materials. Next, we review different methods for characterizing the third-order optical nonlinearities of 2D materials, including the Z-scan technique, third-harmonic generation (THG) measurement, and hybrid device characterization, together with a summary of the measured n2 values in the telecommunications band. Finally, the current challenges and future perspectives are discussed.

Nanophotonic reservoir computing for COVID-19 pandemic forecasting.

The coronavirus disease 2019 (COVID-19) has spread worldwide in unprecedented speed, and diverse negative impacts have seriously endangered human society. Accurately forecasting the number of COVID-19 cases can help governments and public health organizations develop the right prevention strategies in advance to contain outbreaks. In this work, a long-term 6-month COVID-19 pandemic forecast in second half of 2021 and a short-term 30-day daily ahead COVID-19 forecast in December 2021 are successfully implemented via a novel nanophotonic reservoir computing based on silicon optomechanical oscillators with photonic crystal cavities, benefitting from its simpler learning algorithm, abundant nonlinear characteristics, and some unique advantages such as CMOS compatibility, fabrication cost, and monolithic integration. In essence, the nonlinear time series related to COVID-19 are mapped to the high-dimensional nonlinear space by the optical nonlinear properties of nanophotonic reservoir computing. The testing-dataset forecast results of new cases, new deaths, cumulative cases, and cumulative deaths for six countries demonstrate that the forecasted blue curves are awfully close to the real red curves with exceedingly small forecast errors. Moreover, the forecast results commendably reflect the variations of the actual case data, revealing the different epidemic transmission laws in developed and developing countries. More importantly, the daily ahead forecast results during December 2021 of four kinds of cases for six countries illustrate that the daily forecasted values are highly coincident with the real values, while the relevant forecast errors are tiny enough to verify the good forecasting competence of COVID-19 pandemic dominated by Omicron strain. Therefore, the implemented nanophotonic reservoir computing can provide some foreknowledge on prevention strategy and healthcare management for COVID-19 pandemic.

Effects of external field wavelength and solvation on the photophysical property and optical nonlinearity of 1,3-thiazolium-5-thiolates mesoionic compound.

The photophysical property and optical nonlinearity of an electronic push-pull mesoionic compound, 2-(4-trifluoromethophenyl)-3-methyl-4-(4-methoxyphenyl)-1,3-thiazole-5-thiolate, were theoretically investigated with a reliable computing strategy. The essence of the optical properties were then explored through a variety of wavefunction analysis methods, including the natural transition orbital analysis, hole-electron analysis, (hyper)polarizability density analysis, decomposition of the (hyper)polarizability contribution, and (hyper)polarizability tensor analysis, at the level of electronic structure. The influences of the electric field and solvation on the absorption spectrum and (hyper)polarizability of the molecule are highlighted and clarified. The results are expected to provide guidance for people to understand the effects of external field wavelength and solvation on the optical properties of mesoscopic molecules.

Large Optical Nonlinearity of Dielectric Nanocavity-Assisted Mie Resonances Strongly Coupled to an Epsilon-near-Zero Mode.

Strong coupling provides a powerful way to modify the nonlinear optical properties of materials. The coupling strength of the state-of-the-art strongly coupled systems is restricted by a weak-field confinement of the cavity, which limits the enhancement of the optical nonlinearity. Here, we investigate a strong coupling between Mie resonant modes of high-index dielectric nanocavities and an epsilon-near-zero mode of an ultrathin indium tin oxide film and obtain an anticrossing splitting of 220 meV. Static nonlinear optical measurements reveal a large enhancement in the intensity-independent effective optical nonlinear coefficients, reaching more than 3 orders of magnitude at the coupled resonance. In addition, we observe a transient response of ∼300 fs for the coupled system. The ultrafast and large optical nonlinear coefficients presented here offer a new route towards strong coupling-assisted high-speed photonics.

Poly-lysinated nanoscale carbon probe for low power two-photon bioimaging.

Effective outcome from dynamic live-cell-imaging requires utilization of a probe with high emission intensity and low photobleaching. It would be preferable to achieve such properties at a low power of the applied laser to avoid any probable damage to biological cells or tissue. Most of the used small-molecule fluorophores have been reported to show significant photobleaching in a time-dependent manner and require high laser power to gain significant intensity for bioimaging. Carbon nanoparticles have recently been successfully used for cell imaging with low bleaching characteristics but require high laser power and lack optical nonlinearity at low power levels. Here, we report the preparation, characterization, and application of a Nanoscale Carbon (NC) which, on being surface decorated with crescent-shaped poly-lysine (PLNC), provides two-photon fluorescence (TPF) and low bleaching properties. PLNC was found to stain the cytoplasm of C2C12 muscle cells in the first four-hours of incubation with high TPF in the infrared range and can be useful for deep tissue imaging with further improvements.

Linear Electro-Optic Modulation in Highly Polarizable Organic Perovskites.

Electrical-to-optical signal conversion is widely employed in information technology and is implemented using on-chip optical modulators. State-of-the-art modulator technologies are incompatible with silicon manufacturing techniques: inorganic nonlinear crystals such as LiNbO3 are integrated with silicon photonic chips only using complex approaches, and hybrid silicon-LiNbO3 optical modulators show either low bandwidth or high operating voltage. Organic perovskites are solution-processed materials readily integrated with silicon photonics; and organic molecules embedded within the perovskite scaffold allow in principle for high polarizability. However, it is found that the large molecules required for high polarizability also require an increase of the size of the perovskite cavity: specifically, using the highly polarizable DR2+ (R = H, F, Cl) in the A site necessitates the exploration of new X-site options. Only by introducing BF4 - as the X-site molecule is it possible to synthesize (DCl)(NH4 )(BF4 )3 , a material exhibiting a linear EO coefficient of 20 pm V-1 , which is 10 times higher than that of metal halide perovskites and is a 1.5 fold enhancement compared to reported organic perovskites. The EO response of the organic perovskite approaches that of LiNbO3 (reff  ≈ 30 pm V-1 ) and highlights the promise of rationally designed organic perovskites for use in efficient EO modulators.

Recent Advances of Spatial Self-Phase Modulation in 2D Materials and Passive Photonic Device Applications.

Optical nonlinearity in 2D materials excited by spatial Gaussian laser beam is a novel and peculiar optical phenomenon, which exhibits many novel and interesting applications in optical nonlinear devices. Passive photonic devices, such as optical switches, optical logical gates, photonic diodes, and optical modulators, are the key compositions in the future all-optical signal-processing technologies. Passive photonic devices using 2D materials to achieve the device functionality have attracted widespread concern in the past decade. In this Review, an overview of the spatial self-phase modulation (SSPM) in 2D materials is summarized, including the operating mechanism, optical parameter measurement, and tuning for 2D materials, and applications in photonic devices. Moreover, some current challenges are also proposed to solve, and some possible applications of SSPM method are predicted for the future. Therefore, it is anticipated that this summary can contribute to the application of 2D material-based spatial effect in all-optical signal-processing technologies.

Room-Temperature Ferroelectric LiNb6Ba5Ti4O30 Spinel Phase in a Nanocomposite Thin Film Form for Nonlinear Photonics.

Tetragonal tungsten bronze (TTB) materials are one of the most promising classes of materials for ferroelectric and nonlinear optical devices, owing to their very unique noncentrosymmetric crystal structure. In this work, a new TTB phase of LiNb6Ba5Ti4O30 (LNBTO) has been discovered and studied. A small amount of a secondary phase, LiTiO2 (LTO), has been incorporated as nanopillars that are vertically embedded in the LNBTO matrix. The new multifunctional nanocomposite thin film presents exotic highly anisotropic microstructure and properties, e.g., strong ferroelectricity, high optical transparency, anisotropic dielectric function, and strong optical nonlinearity evidenced by the second harmonic generation results. An optical waveguide structure based on the stacks of α-Si on SiO2/LNBTO-LTO has been fabricated, exhibiting low optical dispersion with an optimized evanescent field staying in the LNBTO-LTO active layer. This work highlights the combination of new TTB material designs and vertically aligned nanocomposite structures for further enhanced anisotropic and nonlinear properties.

Phase Structure Recording in a Nematic Side-Chain Liquid-Crystalline Polymer.

Dye-doped nematic side-chain liquid-crystalline polymers possess extraordinary large optical nonlinearity and ability to store the induced orientational deformations in a glassy state, which makes them a very promising material for photonic applications. In this study, the phase structures were generated and recorded in the bulk of a 50-µm layer of a nematic liquid-crystalline side-chain polymer, containing polyacrylate backbone, spacer having five methylene groups, and phenyl benzoate mesogenic fragment. The polymer was doped with KD-1 azodye. The director field deformations induced by the light beam close to the TEM01 mode were studied for different geometries of light-polymer interaction. The phase modulation depth of 2π was obtained for the 18-µm spacing between intensity peaks. The experimental data were analyzed based on the elastic continuum theory of nematics. The possibility to induce and record positive and negative microlenses in the polymer bulk was shown experimentally.

Neuromorphic Functions of Light in Parity-Time-Symmetric Systems.

As an elementary processor of neural networks, a neuron performs exotic dynamic functions, such as bifurcation, repetitive firing, and oscillation quenching. To achieve ultrafast neuromorphic signal processing, the realization of photonic equivalents to neuronal dynamic functions has attracted considerable attention. However, despite the nonconservative nature of neurons due to energy exchange between intra- and extra-cellular regions through ion channels, the critical role of non-Hermitian physics in the photonic analogy of a neuron has been neglected. Here, a neuromorphic non-Hermitian photonic system ruled by parity-time symmetry is presented. For a photonic platform that induces the competition between saturable gain and loss channels, dynamical phases are classified with respect to parity-time symmetry and stability. In each phase, unique oscillation quenching functions and nonreciprocal oscillations of light fields are revealed as photonic equivalents of neuronal dynamic functions. The proposed photonic system for neuronal functionalities will become a fundamental building block for light-based neural signal processing.

Gap-Induced Giant Third-Order Optical Nonlinearity and Long Electron Relaxation Time in Random-Distributed Gold Nanorod Arrays.

The third-order optical nonlinearities and the hot electron relaxation time (τ) of random-distributed gold nanorods arrays on glass (R-GNRA) have been investigated by using Z-scan and optical Kerr effect (OKE) techniques. Large third-order optical susceptibility (χ(3)) with the value of 2.5 × 10-6 esu has been obtained around the plamsonic resonance peak under the excitation power intensity of 0.1 GW/cm2. Further decrease of the excitation power intensity down to 0.3 MW/cm2 will lead to the significant increase of χ(3) up to 6.4 × 10-4 esu. The OKE results show that the relaxation time of R-GNRA around the plasmonic peak is 13.9 ± 0.4 ps, which is more than 4 times longer than those of the individual gold nanostructures distributed in water solutions. The Finite-difference time domain simulations demonstrate that this large enhancement of χ(3) and slow down of τ are caused by the gap-induced large local field enhancement of GNRs dimers in R-GNRA. These significant results offer great opportunities for plasmonic nanostructures in applications of photonic and photocatalytic devices.

Giant and Tunable Optical Nonlinearity in Single-Crystalline 2D Perovskites due to Excitonic and Plasma Effects.

Materials with large optical nonlinearity, especially in the visible spectral region, are in great demand for applications in all-optical information processing and quantum optics. 2D hybrid Ruddlesden-Popper-type halide perovskites (RPPs) with tunable ultraviolet-to-visible direct bandgaps exhibit large nonlinear optical responses due to the strong excitonic effects present in their multiple quantum wells. Using a microscopic Z-scan setup with femtosecond laser pulses tunable across the visible spectrum, it is demonstrated that single-crystalline lead halide RPP nanosheets possess unprecedentedly large nonlinear refraction and absorption coefficients near excitonic resonances. A room-temperature insulator (exciton)-metal (plasma) Mott transition is found to occur near the exciton resonance of the thinnest qunatum-well RPPs, boosting the nonlinear response. Owing to the rapidly changing refractive index near resonance, a single RPP crystal can exhibit different nonlinear functionalities across the excitation spectrum. The results suggest that RPPs are efficient nonlinear materials in the visible waveband, indicating their potential use in integrated nonlinear photonic applications such as optical modulation and switching.

High spectral resolution second harmonic generation microspectroscopy at thin layer interfaces with broadband continuum pulses.

We demonstrate an effective microspectroscopy technique by tracing the dispersion of second order nonlinear optical susceptibility χ (2) in single atomic layer materials. The experimental method relies on the detection of single-shot second harmonic (SH) spectra from the materials and the subsequent data normalization. The key point in our study is that we used a broadband (˜350 nm) near-infrared femtosecond continuum pulses generated at high repetition rates in a photonic crystal fiber with superior spatial quality and stable spectral power density. This is opposite to the point-by-point laser tuning method in SH generation spectroscopy that was applied extensively in the past and has shown limited precision in obtaining χ (2) dispersion. The continuum pulse technique produces spectral resolution better than 2 meV (<0.3 nm at 450 nm) and shows low (<5-6% rms) signal detection noise allowing the detection of subtle features in the χ (2) spectrum at room temperatures. Fine sub-structure features within the main peak of χ (2) spectra indicate the impact of broadened resonances due to exciton transitions in the single layer materials. • Tailored continuum pulses are used to generate second harmonic signal in non-centrosymmetric semiconductors. • SHG spectrum carries fingerprints of the bandstructure around the direct gap states. • The technique produces fine spectral resolution and much better signal-to-noise ratio compared to point-by-point wavelength tuning methods.

Nonlinear Errors Resulting from Ghost Reflection and Its Coupling with Optical Mixing in Heterodyne Laser Interferometers.

Even after the Heydemann correction, residual nonlinear errors, ranging from hundreds of picometers to several nanometers, are still found in heterodyne laser interferometers. This is a crucial factor impeding the realization of picometer level metrology, but its source and mechanism have barely been investigated. To study this problem, a novel nonlinear model based on optical mixing and coupling with ghost reflection is proposed and then verified by experiments. After intense investigation of this new model's influence, results indicate that new additional high-order and negative-order nonlinear harmonics, arising from ghost reflection and its coupling with optical mixing, have only a negligible contribution to the overall nonlinear error. In real applications, any effect on the Lissajous trajectory might be invisible due to the small ghost reflectance. However, even a tiny ghost reflection can significantly worsen the effectiveness of the Heydemann correction, or even make this correction completely ineffective, i.e., compensation makes the error larger rather than smaller. Moreover, the residual nonlinear error after correction is dominated only by ghost reflectance.