Nonlinear Absorption Research Articles

Unleashing the nonlinear optical response of layered vanadium diselenide towards the deep mid-infrared regime

Broadband nonlinear optical materials towards the deep mid-infrared regime are highly required in medical diagnostics, environmental pollution monitoring, etc. Here, we report the ultra-broadband nonlinear optical absorption behavior of the layered vanadium diselenide (VSe2) towards deep mid-infrared regime experimentally. The layered VSe2 prepared by the liquid-phase exfoliation method exhibits broadband and wavelength-dependent nonlinear saturable absorption and a large modulation depth of over 8% in the 3-10 µm spectral range. By utilizing the layered VSe2 for optical modulation, stable nanosecond pulse with a low Q-switched threshold has been delivered successfully from an erbium-doped ZBLAN fiber laser. The experimental findings can deepen the understanding of the nonlinear optical response of the transition metal dichalcogenides towards the deep mid-infrared regime, and may make inroads for the advancement of mid-IR optoelectronic devices.

Managing Residual Heat Effects in Femtosecond Laser Material Processing by Pulse-on-Demand Operation

Femtosecond laser processing combines highly accurate structuring with low residual heating of materials, low thermal damage, and nonlinear absorption processes, making it suitable for the machining of transparent brittle materials. However, with high average powers and laser pulse repetition rates, residual heating becomes relevant. Here, we present a study of the femtosecond laser pulse-on-demand operation regime, combined with regular scanners, aiming to improve throughput and quality of processing regardless of the scanner’s capabilities. We developed two methods to define the needed pulse-on-demand trigger sequences that compensate for the initial accelerating scanner movements. The effects of pulse-on-demand operation were studied in detail using direct process monitoring with a fast thermal camera and indirect process monitoring with optical and topographical surface imaging of final structures, both showing clear advantages of pulse-on-demand operation in precision, thermal effects, and structure shape control. The ability to compensate for irregular scanner movement is the basis for simplified, cheaper, and faster femtosecond laser processing of brittle and heat-susceptible materials.

Plasmonic gold-enhanced GaAs for femtosecond laser generation

Surface plasmon resonance (SPR) can significantly enhance the local electromagnetic fields, facilitating the interaction between light and matter at the nanoscale. However, its ultrafast photonics application in lasers remains unexplored, especially for femtosecond pulsed laser generation. In this work, we investigate the femtosecond pulse generation based on SPR-enhanced nonlinear absorption characteristics in GaAs for the first time, to the best of our knowledge. The gold nanoparticles (GNPs)+GaAs bilayer saturation absorber (SA) decorated on the tapered fiber generates a modulation depth of 2.02% and a saturable intensity of 3.13 MW/cm2. Stable mode-locked pulses can be obtained in an erbium-doped fiber cavity, as demonstrated here. The signal-to-noise ratio (SNR) of the pulses is 75 dB and the shortest pulse duration can reach 384 fs, highlighting the potential of the SPR effect in manufacturing ultrafast optical devices.

Lead‐Sulfide‐Based Hybrid Inorganic‐Organic Superlattice Particles for Prominent Nonlinear Optical Absorption

AbstractSimultaneously optimizing nonlinear absorption coefficient and modulation depth is a considerable challenge for nonlinear optical materials. Here we present an effective solution through constructing PbS2‐based inorganic‐organic superlattice. PbS2/Cn superlattice particles are synthesized, where n (4, 6, and 8) denotes the number of carbon atoms in the organic component. First‐principles simulations indicate the formation of covalent bonds and van der Waals interaction between PbS2 unit and interlayer organic molecules. All samples exhibit strong nonlinear absorption under femtosecond laser excitation with a wavelength range between 515 nm and 900 nm, and the nonlinear absorption coefficient increases with interlayer distance. The optimized sample, PbS2/C8, demonstrates outstanding nonlinear absorption and substantial modulation depth under 800 nm (the third‐order nonlinear absorption coefficient βeff, 10449 ± 609 cm GW−1; modulation depth, 39.1%) and 900 nm (the fifth‐order nonlinear absorption coefficient γeff, 6465 ± 68 cm3 GW−2; modulation depth, 88.1%), which exhibits saturable absorption under 515 nm (βeff, ‐4932 ± 818 cm GW−1; modulation depth, 48.1%). It exhibits a small optical limiting threshold of 1.23 mJ cm−2. These performances surpass those of typical single‐/few‐layer metal chalcogenides. Structural and spectral analyses elucidate that the remarkable optical nonlinearity can be attributed to quantum confinement in the inorganic layer and the dielectric enhancement of the superlattice.

Generation of mode-locked pulses assisted by reduced graphene oxide/cerium oxide (rGO/CeO2) embedded in arc-shaped fiber

Abstract This study demonstrates the synthesis of a reduced graphene oxide/cerium oxide (rGO/CeO2) composite and its successful use as a saturable absorber (SA) in ultrafast photonics. The rGO/CeO2 composite was synthesized using a facile hydrothermal approach, and an arc-shaped fiber was fabricated using a polishing technique. The arc-shaped fiber employed indirect evanescent field coupling, allowing interaction between the composite material and the laser, which facilitated the generation of mode-locked pulses. An arc-shaped fiber drop-casted with rGO/CeO2 was connected to a 2.0 µm laser source, achieving a modulation depth of 50.16%. The emitted laser pulses have a duration of 1.776 ps, an operational wavelength of 1904 nm, and a spectral bandwidth (FWHM) of 2.76 nm. The pulse energy was 89.36 pJ with a high signal-to-noise ratio (SNR) of 63 dB. This study introduces rGO/CeO2 embedded in an arc-shaped fiber as an effective SA, exhibiting favorable nonlinear optical absorption in the near 2.0 µm wavelength region, making it suitable for applications in ultrafast photonics.

Synthesis, structural characterization and third-order nonlinear optical study of W/Cu/S cluster-based coordination polymers with thioether-containing ligands

The assembly of [Et4N][Tp*WS3] (Et = ethyl, Tp* = tetrakis(3,5-dimethyl-1-pyrazolyl)borate), Cu(I) with 1,3,5-tri((pyridin-4-ylthio)methyl)benzene (tptmb) or 4,4′-bis((pyridin-4-ylthio)methyl)-1,1′-biphenyl (bptmb) leads to two coordination polymer with the formulae of {[Tp*WS3Cu3(tptmb)(CH3CN)](PF6)2·CHCl3·2CH3CN}n (1·CHCl3·2CH3CN) and {[(Tp*WS3Cu3)2(bptmb)3(CH3CN)](PF6)4·5CH2Cl2·1.5CH3CN}n (2·5CH2Cl2·1.5CH3CN), respectively. Compounds 1 and 2 were characterized by elemental analysis, infrared spectroscopy, UV–vis spectroscopy, high-resolution electrospray ionization mass spectrometry, and single-crystal X-ray diffraction. In 1, each tptmb ligand connects two adjacent cluster nodes by its three coordination arms, forming a cluster-based one-dimensional zigzag chain. In 2, equal amounts of [Tp*WS3Cu3(CH3CN)]2+ and [Tp*WS3Cu3]2+ clusters serve as three-connected nodes and are alternatively bridged by the bptmb ligands, forming a 2D network. Z-scan tests of 1 and 2 in dimethylformamide (DMF) solution reveal they have reverse saturable absorption with effective nonlinear absorption coefficients of 1.50 × 10−11 m/W for 1 and 5.00 × 10−11 m/W for 2. This work could inspire the construction of coordination polymer structures using cluster nodes and flexible thioether-containing ligands, providing a new way for the development of third-order nonlinear optical materials.

Predicting the Multiphotonic Absorption in Graphene by Machine Learning

This study analyzes the nonlinear optical properties exhibited by graphene, focusing on the nonlinear absorption coefficient and the nonlinear refractive index. The evaluation was conducted using the Z-scan technique with a 532 nm wavelength laser at various intensities. The nonlinear optical absorption and the nonlinear optical refractive index were measured. Four machine learning models, including linear regression, decision trees, random forests, and gradient boosting regression, were trained to analyze how the nonlinear optical absorption coefficient varies with variables such as spot radius, maximum energy, and normalized minimum transmission. The models were trained with synthetic data and subsequently validated with experimental data. Decision tree-based models, such as random forests and gradient boosting regression, demonstrated superior performance compared to linear regression, especially in terms of mean squared error. This work provides a detailed assessment of the nonlinear optical properties of graphene and highlights the effectiveness of machine learning methods in this context.

Ultrafast carrier dynamics and transient nonlinear absorption in chalcogenide perovskite BaZrS3

The unique combination of excellent semiconducting properties in halide perovskites and the high stability and nontoxicity of oxide perovskites has led to a recent surge in interest in chalcogenide perovskite BaZrS3 for optoelectronic applications. However, to realize its potential in future device technologies, a comprehensive understanding of photoexcited carrier dynamics and transient optical response is imperative, yet it remains largely unexplored for BaZrS3. In this work, employing transient absorption spectroscopy, we have revealed that photoexcited carriers in epitaxial BaZrS3 nanofilms exhibit two exponential decay components relating to optical phonon cooling and interband recombinations. Meanwhile, our investigation unveils an intriguing transient nonlinear absorption phenomenon in BaZrS3, characterized by the ultrafast switching of the pump-induced transparency (i.e., the saturable absorption) to the absorption enhancement within a timescale commensurate with the measurement resolution (hundreds of femtosecond). This study provides crucial dynamic insights essential for leveraging chalcogenide perovskites, such as BaZrS3, in the development of advanced optoelectronic devices.

Crystallization and phase transition boosted optical linear& nonlinear and magnetic properties in transparent xCu: BaSnO3 NCs/glass-ceramic

In this study, cubic xCu: BaSnO3 (x = 0, 1, 3, and 5 mol%) perovskite nanocrystals were synthesized using a hydrothermal method. The influence of Cu doping levels on the crystal structure, morphology, size, and properties was analyzed using X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Raman, Ultraviolet-Visible Spectroscopy (UV–vis), Vibrating Sample Magnetometry (VSM), and X-ray Photoelectron Spectroscopy (XPS) techniques. Across the 1–5 % doping range, the cubic BaSnO3 structure was maintained with Cu2+ in CuO6 replacing Sn4+ in the B-site. XPS analysis confirmed the formation of oxygen vacancies due to charge imbalance, leading to a decrease in optical bandgap and an increase in ferromagnetic moment. When various contents of xCu: BaSnO3 were doped into borosilicate glass, significant impacts on the glass network, crystallization, and properties were observed. Nanocrystals (∼50 nm) transitioned from cubic to orthorhombic phase beyond 3 mol% doping, modifying the glass network by converting BO3 to BO4, AlO4 to AlO6, and CuO6 to CuO4. This phase change endowed the glass's strong ferromagnetic moment and absorption due to the t2 g→eg transitions of Cu2+ in CuO6, and reduced the optical bandgap from 3.16 to 2.2 eV. The glass also exhibited improved Vickers hardness (475 HV) and a high Verdet constant of 0.174 min/G·cm due to network modifications. Glass also shows a distinct red emission at 596 nm with a 160 µs lifetime and good thermal stability, while doping levels above 3 % resulted in emission quenching. The optical nonlinear absorption coefficient and nonlinear susceptibility increased with higher doping levels, reaching huge values of α3: 5.12×10−10 m/W and χ(3): 7.52×10−10 esu, indicating potential for self-focusing applications. The increased polarizability also resulted in a high dielectric constant (18) and a large P-E loop of glasses which are promising for advanced photonics devices.

Femtosecond Mode-Locking and Soliton Molecule Generation Based on GaAs Saturable Absorber

Abstract In the past few years, research on advanced ultrafast photonic devices has maintained great interest throughout the laser community. As a semiconductor material with excellent nonlinear saturation absorption characteristics, GaAs has been used in solid-state and fiber lasers as a mode-locker. However, pulse widths that have been reported in the searchable published literature are all longer and the shortest is also tens of picoseconds. Femtosecond pulse widths, desired for variety of applications, have not yet been reported in GaAs-based pulsed lasers. In this work, we further explore the nonlinear characteristics of GaAs that magnetron sputtered onto the surface of the tapered fiber and its application in the generation of femtosecond laser via effective dispersion optimization and nonlinearity management. With the enhanced interaction between evanescent wave and GaAs nanosheets, mode-locked soliton pulses as short as 830 fs are generated at 4.64 MHz repetition rates. As far as we know, this is the first time that femtosecond-level pulses are generated with a GaAs-based SA. In addition, soliton molecule, including dual-pulse state are also realized under stronger pumping. This work demonstrates that GaAs-based photonic device has good application prospects in effective polymorphous ultrashort pulsed laser generation.

Revealing the Role of Electronic Effect to Modulate the Photophysics and Z-Scan Responses of o-Locked GFP Chromophores.

Three novel core green fluorescent protein (GFP) chromophore analogues, based on a doubly locked conformation and variable electronic effects by replacing one hydrogen with bromine, iodine, and methyl, respectively, have been synthesized to modulate the push-pull effect. These chromophores exhibited intramolecular H-bonding, as evidenced by single-crystal X-ray and 1H NMR studies. The fluorescence quantum yields (ϕf) of all of the chromophores were found to be more than an order of magnitude higher (∼0.2) than the unlocked chromophores (∼0.01). It was found that the electronic effect did affect the nonradiative rates, as the quantum yields were found to vary with respect to different analogues in the same solvents. The effect of the push-pull effect was also evident by a higher Stokes-shifted emission in the case of the methyl derivative with respect to the bromo- and iodo-analogues. Furthermore, the emission spectra of these GFP chromophores were found to show positive solvatochromism, which was supported by a quantum chemical calculation. A detailed study, correlating the observed spectral changes with various solvent functions and supported by computational results, established a facile proton transfer, followed by twisted intramolecular charge transfer (TICT) to be the major nonradiative channels of these chromophores. Besides, a completely novel usage of these chromophores was explored for the first time by studying their third-order nonlinear optical characteristics in DMSO using a single-beam Z-scan technique. All of the chromophores exhibited tunable nonlinear refraction (NLR) and nonlinear absorption (NLA) properties that depend upon different substituent groups present in the chromophores. Here, the NLR was due to the effect of self-defocusing, whereas the NLA was triggered by reverse saturable absorption, which is attributed to the two-photon absorption (TPA) process. Surprisingly, the efficiency of the TPA ability of the chromophores was found to be a function of the induced electronic effect. Hence, this work opens a new route for the utility of the ortho-locked GFP chromophores in the field of nonlinear optical applications.

A physics-informed neural network for non-linear laser absorption tomography

Hyperspectral absorption tomography has emerged as a promising technique for combustion diagnostics due to its rich spectral measurements. However, the non-linear and ill-posed nature of the inverse problem makes obtaining accurate results challenging. This paper proposes a novel application of a physics-informed neural network to address the non-linear inverse problem in hyperspectral absorption spectroscopy. This method utilizes physical laws and measurement data to guide the neural network in finding the optimal solution, without requiring training data. To demonstrate its capabilities, the physics-informed neural network is employed to retrieve temperature and CO2 mole fraction fields in axisymmetric laminar diffusion flames via 4.3μm TDLAS (tunable diode laser absorption spectroscopy). The developed neural network is applied to resolve the spatial distributions from the spectral dimensions, requiring fewer spatial measurements for directly retrieving temperature and CO2 mole fraction profiles. We investigate the minimum radial projections needed for accurate retrievals and evaluate the model’s robustness to random noise through the inversion of a simulated flame. The developed model is further applied to reconstruct the temperature and CO2 mole fraction fields for an experimentally measured flame. Our results demonstrate that the proposed model maintains high retrieval accuracy even with limited, noisy data. This work highlights the potential of the physics-informed neural network for robust solutions to non-linear laser absorption tomography problems in scientific and engineering applications.

Para-phenylenediamine Schiff base: highly fluorescent photostable solid-state organic dye

Schiff bases derived from the condensation of primary amines with aldehydes, such as para-phenylenediamine with salicylaldehyde, exhibit unique optical and structural properties ideal for photonic applications. This study synthesizes such a Schiff base, revealing its properties through detailed HNMR1, FTIR, and XRD characterizations. The compound forms a robust π-conjugated structure, showing a fluorescence emission peak at 560 nm and significant absorbance at 380 nm. A spin-coated laser cavity displayed a critical lasing threshold at 1 MW·cm−2, which could be optimized down to 0.6 kW·cm−2. Moreover, the compounds’ acceptor-donor-acceptor configuration raised outstanding nonlinear optical properties, including a substantial two-photon absorption cross-section of 2×10−11 cm ⋅ W−1·GM, enhancing its utility in high-resolution two-photon imaging and advanced photonic applications. Other nonlinear optical characteristics determined during these studies are the saturable absorption-induced nonlinear absorption coefficient (−2×10−11 cm ⋅ W−1), saturation intensity (2.5×1011 W·cm−2), and Kerr-induced nonlinear refractive index (5×10−16 cm2 ⋅ W−1). The combined linear and nonlinear optical properties, supported by sustained emission and minimal photobleaching under intense re-excitation, establish the para-phenylenediamine Schiff base and derivatives as promising materials for high-brightness, photostable organic light emitters, and solid-state lasers.

Ultrafast Charge Transfer-Induced Unusual Nonlinear Optical Response in ReSe2/ReS2 Heterostructure.

Ultrafast charge transfer in van der Waals heterostructures can effectively engineer the optical and electrical properties of two-dimensional semiconductors for designing photonic and optoelectronic devices. However, the nonlinear absorption conversion dynamics with the pump intensity and the underlying physical mechanisms in a type-II heterostructure remain largely unexplored, yet hold considerable potential for all-optical logic gates. Herein, two-dimensional ReSe2/ReS2 heterostructure is designed to realize an unusual transition from reverse saturable absorption to saturable absorption (SA) with a conversion pump intensity threshold of approximately 170 GW/cm2. Such an intriguing phenomenon is attributed to the decrease of two-photon absorption (TPA) of ReS2 and the increase of SA of ReSe2 with the pump intensity. Based on the characterization results of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, femtosecond transient absorption spectrum, Kelvin probe force microscopy, and density functional theory calculation, a type-II charge-transfer-energy level model is proposed combined with the TPA of ReS2 and SA of ReSe2 processes. The results reveal the critical role of ultrafast interfacial charge transfer in tuning the unusual nonlinear absorption and improving the SA of ReSe2/ReS2 under different excitation wavelengths. Our finding deepens the understanding of nonlinear absorption physical mechanisms in two-dimensional heterostructure materials, which may further diversify the nonlinear optical materials and photonic devices.

Enhanced nonlinear optical properties of MXene (Ti3C2Tx) via surface-covalent functionalization with porphyrin

The surface terminations (=O, -OH, and -F) play a key role in determining the physical and chemical properties of MXenes, which have been demonstrated with significant potential in field-effect transistors, humidity sensors, energy storage, and photocatalysis, etc. It is therefore crucial to modify these active functional groups on the surface of MXenes in order to optimize the applicability of these materials. In this study, we introduce a covalent modification strategy to successfully construct a porphyrin-functionalized Ti3C2Tx organic-inorganic nanohybrid (TPP-Ti3C2Tx) by covalently attaching porphyrin molecules to the surface groups on Ti3C2Tx nanosheets for the first time. As revealed by steady-state fluorescence spectra, transient fluorescence spectra, and DFT calculations, the robust covalent bonds between TPP and Ti3C2Tx can effectively promote the photon-induced electron and/or energy transfer within the TPP-Ti3C2Tx nanohybrid. The investigation on the nonlinear optical (NLO) properties of TPP-Ti3C2Tx nanohybrid as well as its precursors, reveals that the TPP-Ti3C2Tx nanohybrid exhibits the highest nonlinear absorption coefficient and the lowest optical limiting threshold among the tested samples at both 532 and 1064 nm, indicating its great potential as a broadband optical limiter for visible and near-infrared wavelengths. This work not only demonstrates the significant promise of covalently-linked TPP-Ti3C2Tx nanohybrid in optical limiting applications but also provides a paradigm for engineering high-performance NLO MXenes-based materials through the covalent modification strategy.

Enhanced optical nonlinearity in suspensions of dual-resonance gold–copper chalcogenide core–shell nanoparticles

Plasmonic copper chalcogenide (Cu2−xE, E = S, Se, Te) nanoparticles (NPs) possess high potentials in photothermal therapy and photoacoustic tomography for their unique localized surface plasmon resonant absorption in the second near-infrared biologically transparent window. In this study, strong thermal-induced nonlinear negative-lens effect in aqueous suspensions containing Cu2−xSe NPs was observed. Furthermore, the nonlinear absorption was notably enhanced in aqueous suspensions containing Au@Cu2−xSe core–shell NPs, as the positive nonlinear absorption coefficient increased by 20-fold, attributed to the strong plasmonic coupling in Au@Cu2−xSe core–shell NPs. This finding underscores the importance of the nonlinear optical behaviors of plasmonic NPs in biomedical applications, as they offer extra strategy for optimizing systems and incorporating additional functionalities.

Localized surface plasmon resonance induced nonlinear absorption and optical limiting activity of gold decorated graphene/MoS2 hybrid

The synergistic supremacy of a hybrid nanocomposite made of reduced graphene oxide (rGO), molybdenum disulfide (MoS2) and gold nanoparticles (Au) for Q-switching and optical limiting applications is investigated. Hydrothermally synthesized Au-rGO-MoS2 hybrid with varying concentrations of Au (2.5, 5, 7.5 and 10 wt%) was subjected to interact with Q-switched Nd:YAG nanosecond green laser pulses. The intensity dependent nonlinear absorption studies revealed a switch over in the nonlinear behaviour from saturable absorption (SA) to reverse saturable absorption (RSA) behaviour. SA occurs due to ground state bleaching at comparatively lower laser irradiances serving the need for Q-switching and optical communications. The RSA trait at higher laser irradiance is attributed to sequential two-photon absorption which serves as the platform for optical limiting behaviour prompting laser safety and effective photothermal therapy applications. The strong mid-visible and UV absorption promotes the availability of multiple energy bands for energy transitions of excited state absorption (ESA). Enhanced local fields due to localized surface plasmon resonance effect, structural defects, hierarchical morphology, increased absorption cross-section and plasmon induced energy transfer promotes robust ESA process. These tunable nonlinear optical properties make Au-rGO-MoS2 hybrid promising futuristic candidates for applications in nonlinear photonic devices, ultrafast optical switches and all-optical signal processing systems.

Using Femtosecond Laser Light to Investigate the Concentration- and Size-Dependent Nonlinear Optical Properties of Laser-Ablated CuO Quantum Dots.

In this work, the nonlinear optical (NLO) properties of CuO nanoparticles (CuO NPs) were studied experimentally using the pulsed laser ablation (PLA) technique. A nanosecond Nd: YAG laser was employed as the ablation excitation source to create CuO NPs in distilled water. Various CuO NPs samples were prepared at ablation periods of 20, 30, and 40 min. Utilizing HR-TEM, the structure of the synthesized CuO NPs samples was verified. In addition, a UV-VIS spectrophotometer was used to investigate the linear features of the samples. The Z-scan technique was utilized to explore the NLO properties of CuO NPs samples, including the nonlinear absorption coefficient (β) and nonlinear refractive index (n2). An experimental study on the NLO features was conducted at a variety of excitation wavelengths (750-850 nm), average excitation powers (0.8-1.2 W), and CuO NPs sample concentrations and sizes. The reverse saturable absorption (RSA) behavior of all CuO NPs samples differed with the excitation wavelength and average excitation power. In addition, the CuO NPs samples demonstrated excellent optical limiters at various excitation wavelengths, with limitations dependent on the size and concentration of CuO NPs.

Distinct nonlinear absorption behaviors in Cu1.8S nanocrystals influenced by geometry features

Distinct nonlinear absorption behaviors in Cu_1.8S nanocrystals influenced by geometry features

Effects of multiple reflections on nonlinear absorption measurements.

Accurate measurement of nonlinear absorption coefficients by transmission-based techniques, such as Z-scan and its modifications, remains challenging due to high measurement errors. In this Letter, we claim that one of the reasons for that could lie in ignoring multiple reflections of light in plane-parallel samples in the presence of multi-photon excitation. Closed-form formulations for transmittance and reflectance coefficients, considering both linear and multiphoton absorption, are obtained in the geometrical optics approximation. The contributions of multiple reflections to the total transmittance are calculated for several II-VI materials reported in the literature. The results show a nonlinear dependence on incident intensity, ranging from 15.7% down to 7.9% for CdS, from 10.3% down to 5.8% for CdSe, and from 18.1% down to 9.1% for ZnSe. The dependence of the contribution of multiple reflections on the sample thickness is shown to exhibit a near exponential decay.