The team, led by Shaghayegh Khalilzadeh, Ehsan Koushki, and Mohammad-Reza Zamani-Meymian, synthesized two types of carbon quantum dots using simple thermal decomposition methods. The pristine sample, labeled S1, was made by heating 15 grams of citric acid at 250 degrees Celsius for 20 minutes in a solvent-free solid-phase process, yielding a carbonized powder with a consistent reaction mass yield of roughly 42 percent across three replicate batches. The co-doped sample, S2, was produced by dissolving citric acid and thiourea in water and heating the mixture at 180 degrees Celsius for 40 minutes, a gentler protocol chosen specifically to prevent the degradation of thiourea and to optimize the incorporation of nitrogen and sulfur into the carbon framework. Under ultraviolet illumination at 365 nanometers, the two samples glow distinctly: S1 emits blue light, while S2 glows blue-green.

Confirming that the doping had genuinely worked required a battery of characterization techniques. Energy-dispersive X-ray spectroscopy revealed that the pristine sample contained only carbon, oxygen, and sodium, with no detectable nitrogen or sulfur, while the co-doped sample showed weight percentages of 1.28 percent nitrogen and 3.95 percent sulfur. The carbon-to-sulfur weight ratio of approximately 2 to 1 matched the ideal values reported in prior literature. Elemental mapping went further, demonstrating that the nitrogen and sulfur signals overlapped precisely with the carbon framework, proving that the heteroatoms were chemically integrated into the lattice rather than sitting in separate clusters. X-ray diffraction backed this up: both samples showed the broad (002) peak of amorphous carbon at 2 theta equals 20 degrees, but the doped sample displayed new peaks at roughly 19, 28, and 32 degrees, signatures of lattice strain from larger sulfur atoms, graphitic carbon nitride-like domains from carbon-nitrogen bonding, and localized ordering at sulfur-decorated lattice edges.

High-resolution transmission electron microscopy of the pristine sample revealed well-dispersed spherical nanoparticles with an average diameter of about 4 nanometers and clear lattice fringes with a spacing of 0.34 nanometers, corresponding to the (200) plane of graphitic structure. Fourier-transform infrared spectroscopy completed the chemical picture. The pristine dots showed the broad hydroxyl stretching band between 3000 and 3500 inverse centimeters and carboxyl peaks expected of an oxidized carbon surface, while the co-doped sample developed intense new peaks between 1030 and 1150 inverse centimeters assigned to carbon-sulfur and sulfonic group vibrations, plus an enhanced signal at 1380 inverse centimeters from covalent carbon-nitrogen stretching. Together, these results describe what the authors call bond engineering: a stable hybrid nanostructure in which nitrogen and sulfur are covalently woven into the carbon matrix, fundamentally altering its electronic charge density.

The optical consequences of this structural surgery were striking. Ultraviolet-visible absorption spectra showed a dominant band near 230 nanometers from pi-to-pi-star transitions in the aromatic carbon domains, plus a shoulder between 270 and 320 nanometers from n-to-pi-star transitions involving lone-pair electrons of the heteroatoms. In the doped sample, the absorption edge red-shifted and a pronounced tail extended into the visible range, evidence that nitrogen and sulfur had introduced new intermediate energy states between the intrinsic frontier orbitals. Yet when the team quantified the optical bandgap using Tauc analysis with an allowed direct transition exponent, they found the gap had actually widened, from 3.20 electron volts in the pristine sample to 3.45 electron volts in the doped one. This apparent paradox, a blue-shifted bandgap alongside a red-shifted absorption tail, is explained by the Burstein-Moss effect, in which nitrogen and sulfur act as electron donors that fill the lower states of the conduction band, forcing valence electrons to absorb higher-energy photons to find unoccupied states. The larger atomic radius of sulfur also induces lattice strain that fragments the extended sp2 domains into smaller clusters, strengthening the quantum confinement effect and further widening the gap.

Photoluminescence measurements told an equally compelling story. The pristine sample displayed a dual-peak emission profile, with a higher-energy peak from the intrinsic graphitic core and a lower-energy peak from oxygen-containing surface states, plus a broad near-infrared band around 760 nanometers signaling deep defect traps. After co-doping, this complicated landscape collapsed into a single, intense green emission band centered near 500 nanometers. The dopant-induced surface states act as efficient carrier trapping centers that quench the intrinsic core emission and channel all radiative recombination through one low-energy pathway. The practical payoff was dramatic: the photoluminescence intensity of the doped sample increased more than 3.5-fold, and the fluorescence quantum yield, measured against a quinine sulfate standard, jumped from 4.2 percent to 32.5 percent. The nitrogen and sulfur atoms appear to heal surface defects and dangling bonds, suppressing non-radiative recombination and preserving excitation energy for light emission.

The centerpiece of the study, however, was the Z-scan investigation of nonlinear optics. In this technique, a sample is translated through the focus of a laser beam while its transmittance is recorded, allowing researchers to extract the nonlinear absorption coefficient and the nonlinear refractive index. The team used three continuous-wave lasers at 405, 532, and 633 nanometers, violet, green, and red, at powers of 5, 10, and 15 milliwatts, with peak intensities ranging from about 1.62 to 4.84 kilowatts per square centimeter. Control measurements on the pure solvent confirmed that the nonlinear signals originated from the nanodots themselves. In the open-aperture configuration, all samples showed positive nonlinear absorption, which the authors attribute to electrostriction: polarizable nanoparticles are drawn toward the center of the beam by the electric field gradient, increasing local concentration and absorption.

The doped sample outperformed the pristine one decisively. Under green excitation at 5 milliwatts, the nonlinear absorption coefficient of the co-doped dots reached 9.74 times ten to the minus three centimeters per watt, roughly five times higher than the pristine value and orders of magnitude above many recently reported carbon-based nanomaterials, including sugarcane-derived carbon dots and yttrium-doped carbon nanodots measured at the same wavelength. The closed-aperture measurements revealed negative nonlinear refractive indices in all cases, confirming self-defocusing behavior driven by thermal lensing, in which absorbed energy heats the solvent and lowers the refractive index at the beam center. The doped sample again showed larger on-axis phase shifts, thanks to the higher polarizability imparted by the bulky, electron-rich sulfur atoms.

Perhaps the most counterintuitive finding concerned wavelength. One might expect violet photons, with energy closest to the 3.45-electron-volt bandgap, to produce the strongest electronic response. Instead, the red laser generally yielded the largest nonlinear coefficients. The authors argue that at these low powers the response is not governed by electronic transitions at all, but by the classical mechanics of electrostriction: lower-frequency red light exerts its driving force on the nanoparticles over longer time periods, making it more effective at pulling the particles toward the beam center. This frequency matching to the mechanical response of nanometer-scale particles, rather than to their electronic energy levels, is a distinctive feature of the continuous-wave regime, where the measured coefficients are effective, thermally dominated quantities rather than the ultrafast electronic Kerr responses captured by pulsed lasers.

The implications reach well beyond the laboratory bench. Materials that change their absorption and refraction in response to light intensity are the working elements of optical limiters, which protect sensitive sensors and human eyes from damaging laser exposure, as well as optical switches and modulators for photonic circuits. Because the co-doped carbon dots deliver their strongest nonlinear response under cheap, low-power continuous-wave diode lasers, and because they are biocompatible, water-soluble, and free of the toxic heavy metals found in first-generation quantum dots, they are unusually well suited to practical, cost-effective photonic devices. The study also delivers a conceptual lesson for the field: doping does not just shift energy levels, it reshapes the entire interplay between electronic structure, polarizability, and the mechanical forces that light exerts on matter. By widening the core bandgap while simultaneously seeding dense sub-bandgap surface states, nitrogen and sulfur co-doping creates a material whose linear and nonlinear optical personalities can be tuned almost independently, a degree of control that could define the next generation of carbon-based photonics.

Subject of Research: Bandgap engineering and nonlinear optics of nitrogen and sulfur co-doped carbon quantum dots

Article Title: Synthesis, characterization and low power Z -scan study of bandgap-engineered N, S-co-doped carbon quantum dots using lasers of different colors

Article References: Synthesis, characterization and low power Z -scan study of bandgap-engineered N, S-co-doped carbon quantum dots using lasers of different colors. (n.d.). https://doi.org/10.1016/j.rio.2026.101169

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101169

Keywords: carbon quantum dots, nitrogen sulfur co-doping, nonlinear optics, Z-scan, bandgap engineering, photoluminescence, electrostriction, thermal lensing, optical limiting, nanomaterials, photonics, Burstein-Moss effect

The team grew their films by metal organic chemical vapor deposition, or MOCVD, an industrial-standard technique prized for producing high-quality, uniform coatings over large areas. In a hot-wall, low-pressure horizontal tube reactor held at 0.5 Torr, molybdenum hexacarbonyl powder vaporized at 30 degrees Celsius reacted with hydrogen sulfide gas delivered at a precisely controlled flow rate, with argon carrying the molybdenum precursor into the reaction zone. Fused quartz substrates, cleaned sequentially in acetone, alcohol and deionized water, were coated at deposition temperatures spanning 250 to 750 degrees Celsius. What emerged was a forest of nanosheets standing on edge, perpendicular to the substrate, at every temperature tested.

Temperature, however, dramatically changed what that forest looked like. At 250 degrees Celsius the films consisted of vertical sheets roughly 100 to 150 nanometers long and only a few nanometers thick, with a total film height near 180 nanometers. At 550 degrees Celsius the sheets thickened to 40 to 50 nanometers and the film grew to about 300 nanometers, while at 750 degrees Celsius sheet lengths stretched to roughly 350 nanometers and film thickness reached 600 nanometers. Crucially, sheet density dropped as temperature rose. Because the exposed edges of these vertical sheets terminate crystal planes and host defects, plasmonic behavior and dangling bonds, controlling their dimensions is effectively controlling the material’s active optical surface.

Structural analysis told a consistent story. X-ray diffraction showed the 250-degree film to be essentially amorphous or too thin to diffract, while films grown at 550 and 750 degrees matched the hexagonal 2H phase of MoS2, with reflections characteristic of semi-epitaxial growth parallel to the substrate and of the vertical sheets themselves. Rietveld refinement revealed grain sizes increasing from 31 to 47 nanometers, microstrain falling from 2 percent to 0.3 percent, and dislocation density dropping from 850 to 450 per square micrometer as temperature climbed. Raman spectroscopy reinforced the picture: the characteristic in-plane and out-of-plane vibrational modes sharpened dramatically, with the width of the out-of-plane mode narrowing from 16 to 4 inverse centimeters, signaling steadily improving crystallinity.

The linear optical properties shifted in parallel. All films showed bandgaps between 2.05 and 2.25 electronvolts, well above the 1.3 electronvolts of bulk MoS2, and the gap decreased as sheet size increased with deposition temperature. The smallest sheets, grown at 250 degrees, displayed a strong absorption peak near 475 nanometers but weak or absent A and B exciton features. The researchers attribute this to the enormous surface-to-volume ratio of tiny nanostructures, where edge defects, lattice distortions and dangling bonds create localized states that trap photo-excited carriers before coherent delocalized excitons can form. Larger sheets, with fewer such traps, showed the expected excitonic absorption in the 600 to 750 nanometer range, slightly shifted by residual strain.

The real surprises came when the team probed nonlinear behavior with a Z-scan setup, sweeping samples through the focus of a 532-nanometer continuous-wave laser and recording transmission with microwatt precision over 12,000 points per scan. Films grown at 250 degrees showed positive nonlinear absorption, with an effective photo-thermal coefficient reaching about 900 centimeters per watt at 25 milliwatts. Films grown at 550 degrees flipped behavior depending on power: positive at low power, but strongly negative at 50 and 75 milliwatts, with coefficients as large as roughly 10,000 centimeters per watt and temperature rises inferred to reach 70 units. Films grown at 750 degrees showed negative, comparatively stable values at all powers. The team links the stronger response of the 550-degree film to its higher sheet density, since sheet thicknesses at 550 and 750 degrees are nearly identical.

Here the authors exercise unusual and commendable caution. Continuous-wave illumination, unlike ultrafast pulses, pumps heat into the nanostructures continuously, far longer than the microsecond-to-millisecond thermal diffusion time of MoS2. The result is a thermal lens and temperature-dependent absorption changes that can masquerade as electronic saturable or reverse saturable absorption. The team therefore interprets their continuous-wave coefficients strictly as effective photo-thermal figures of merit tied to their exact experimental geometry, substrate thermal conductivity and exposure time, not as intrinsic electronic properties. Their analysis of transmission versus incident power showed no saturation behavior consistent with a standard two-level saturable absorption model; instead, transmission rose exponentially with power, pointing to thermally induced bleaching amplified by the films’ remarkable light-trapping absorption of roughly 97 percent.

To isolate genuine electronic effects, the researchers turned to a twin-detector configuration with 20-nanosecond laser pulses at 532 nanometers, energies from 10 to 80 millijoules. Under pulsed excitation the picture inverted cleanly: all three films, regardless of growth temperature, showed transmission decreasing as pulse intensity increased, the hallmark of reverse saturable absorption. The extracted nonlinear absorption coefficients, between 10 and 4.5 times 10 to the minus 4 centimeters per watt, were orders of magnitude smaller than the continuous-wave values and of opposite sign for the hotter films, confirming that thermal accumulation dominated the continuous-wave regime. The pulsed response likely arises from excited-state absorption or defect-assisted transitions through edge states, though the authors note that confirming carrier lifetimes and specific decay pathways will require ultrafast pump-probe spectroscopy beyond this study.

The practical implications are considerable. Reverse saturable absorption under nanosecond pulses is exactly the property sought in optical limiters, devices that become more opaque as incoming light grows more intense, protecting detectors and eyes from laser damage. Meanwhile, the giant photo-thermal coefficients observed under continuous-wave illumination, among the largest reported for MoS2 on quartz, suggest potential in thermally managed photonic components. The work also delivers a clear design rule: deposition temperature is a single, accessible knob that tunes sheet size from 100 to 350 nanometers, sheet thickness from a few to about 50 nanometers, bandgap from 2.25 down to 2.05 electronvolts, and the sign and magnitude of nonlinear absorption. For a material already central to two-dimensional electronics and optoelectronics, that kind of process-structure-property control brings practical nonlinear optical devices a meaningful step closer.

Subject of Research: Temperature-controlled MOCVD growth of vertical MoS2 nanosheet thin films and the dependence of their nonlinear optical responses on morphology

Article Title: Morphological and deposition temperature dependence of the nonlinear optical responses in nanostructures MoS2 thin films synthesized by the metal organic chemical vapor deposition

Article References: Khattab, Y., jazmati, A., & Allaham, A. (2026). Morphological and deposition temperature dependence of the nonlinear optical responses in nanostructures MoS2 thin films synthesized by the metal organic chemical vapor deposition. Results in Optics, 25, Article 101168. https://doi.org/10.1016/j.rio.2026.101168

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101168

Keywords: MoS2, two-dimensional materials, MOCVD, nanosheets, nonlinear optics, Z-scan, saturable absorption, reverse saturable absorption, optical limiting, bandgap tuning, photothermal effects, thin films

P3HT is one of the most studied semiconducting polymers in the world, prized for its strong absorption in the visible spectrum, its processability from common solvents and its ability to self-organise into ordered structures. Third-order nonlinear optics, the regime in which a material’s optical response scales with the square of the light intensity, is where such polymers can outshine conventional glass. Conjugated small molecules such as cyanoethynylethenes exhibit third-order susceptibilities roughly a thousand times greater than silica, and P3HT monolayers with improved molecular ordering have shown exceptionally high values of the complex third-order susceptibility, χ3. Yet most research has focused on molecular design rather than on a more fundamental question: what happens when the same polymer chains shift from a tangled, amorphous state into tightly packed crystalline aggregates?

To answer that question, the team exploited a remarkably simple preparation route. Regioregular P3HT was dissolved in anhydrous toluene at a 1:200 weight-to-volume ratio, stirred at 75 degrees Celsius for four hours and then left in the dark for 72 hours. During this quiet incubation the polymer chains slowly aggregated into nanowires, forming crystalline H-aggregates in which neighbouring backbones stack face to face. The dark conditions also prevented photodegradation. The resulting suspension took on a bluish hue, while amorphous P3HT dispersed in tetrahydrofuran appeared burgundy, a visible sign of the electronic reorganisation within. Electron microscopy confirmed the transformation, revealing interconnected fibrillar nanowires with a mean diameter of about 16 nanometres, in stark contrast to the diffuse, irregular nanostructures of the amorphous phase.

Optical characterisation reinforced the picture. Spectroscopic ellipsometry, fitted with a Tauc-Lorentz oscillator model, showed that the amorphous films had refractive indices of roughly 1.2 to 1.3, while the nanowire films ranged from 1.85 to 1.95, both consistent with literature values. The nanowires displayed a steep anomalous dispersion slope near 2.33 electronvolts, driven by a red-shifted central oscillator and narrower broadening coefficient, direct optical evidence of highly ordered pi-pi stacking through Kramers-Kronig relations. High-resolution Raman spectroscopy added a decisive signature: the dominant carbon ring stretching mode narrowed from a full width at half maximum of 49 wavenumbers in the amorphous phase to just 26 wavenumbers in the nanowires, indicating straighter backbones, longer effective conjugation lengths and well-formed crystalline domains.

The heart of the study was the Z-scan technique, a sensitive single-beam method for extracting both the nonlinear refractive index and the nonlinear absorption coefficient. A continuous-wave diode-pumped solid-state laser at 532 nanometres, delivering 50 milliwatts and a focal intensity of 2.63 kilowatts per square centimetre, probed ultra-dilute P3HT suspensions at five concentrations between 0.0006 and 0.0010 percent. Open-aperture detection captured nonlinear absorption, while closed-aperture detection, with a 0.42 aperture, captured nonlinear refraction. The 532-nanometre line was chosen deliberately: P3HT absorbs strongly near 520 nanometres with vibronic features extending to about 600 nanometres, so the nanowires sat close to resonant excitation while the amorphous phase sat on its absorption blue edge.

The results overturned a straightforward expectation. One might assume that the crystalline nanowires, with their dense conjugation and dielectric confinement, would deliver the stronger nonlinear response. Instead, the amorphous phase consistently showed larger effective nonlinear absorption coefficients, peaking at 1.31 times ten to the minus five centimetres per watt at the lowest concentration, compared with 1.05 times ten to the minus five for the nanowires. The explanation lies in defects. Disordered chains are riddled with torsional defects and localised electronic states that promote sequential excited-state absorption, a hallmark of reverse saturable absorption. In the crystalline nanowires, delocalised electronic states and enhanced interchain stacking suppress these defect-mediated pathways, yielding lower but more stable absorption nonlinearity.

Both materials behaved as self-defocusing media under green continuous-wave excitation, with closed-aperture traces showing the characteristic peak-valley profile of negative nonlinear refraction. Again the amorphous phase dominated, exhibiting systematically higher negative effective refractive indices across the entire concentration range. The authors attribute this to thermal lensing: defect states accelerate non-radiative decay, heating the sample locally and driving a strong negative thermo-optic response. The ordered nanowires, lacking such trap-rich landscapes, produced a moderate and stable refractive response. Under continuous-wave illumination the extracted coefficients are therefore effective parameters that blend electronic polarisation with cumulative photothermal contributions, a caveat the team stresses throughout.

Quantitatively, the total effective third-order susceptibility averaged 2.88 plus or minus 0.84 times ten to the minus nine electrostatic units for the amorphous phase, against 2.39 plus or minus 0.47 for the nanowires, a mean suppression of 16.9 percent. The imaginary component outweighed the real part by an order of magnitude, confirming that nonlinear absorption dominates the overall profile. Intriguingly, the suppression was non-uniform: at intermediate concentrations of 0.0007 to 0.0008 percent the nanowires actually outperformed the amorphous matrix by 9.6 to 26.1 percent, because disordered chains undergo a coil-contraction dip that temporarily collapses their response. A photophysical crossover at 0.0008 percent marked the transition from isolated single-chain dynamics to collective interchain behaviour in both systems.

Perhaps the most striking comparison came against conventional organic dyes. Orange G and Methylene Blue, benchmark nonlinear chromophores under identical 532-nanometre continuous-wave excitation, exhibit effective susceptibilities confined to the ten to the minus five esu regime at far higher loadings. That P3HT architectures sustain a robust ten to the minus nine esu response at concentrations below one thousandth of a percent points to an intense intrinsic microscopic hyperpolarizability in the conjugated backbone, making the polymer highly competitive with standard optoelectronic dyes even in an ultra-dilute regime where solvent properties dominate the thermal background.

X-ray photoelectron spectroscopy added a sobering note about ageing. High-resolution carbon and sulphur core-level spectra of the nanowire films, stored for fourteen days under ambient conditions before analysis, revealed that oxidised sulphur accounted for 46.87 percent of the sulphur signal, with sulphone formation indicating cumulative chemical aging. Crystalline nanostructuring preserved the macroscopic architecture but could not shield the polymer from ambient oxidation, suggesting that encapsulation or chemical optimisation will be essential for real-world devices. The authors conclude that while the amorphous phase offers a modestly higher nonlinear response, the nanowire morphology provides the long-term stability that solid-state photonics demands, a technological advantage they judge to outweigh the 16.9 percent sensitivity compromise. Their roadmap ahead includes dopant engineering to tune the electronic landscape, femtosecond transient absorption to decouple sub-picosecond electronic responses from slower photothermal mechanisms, and Z-scan measurements under ultrafast pulsed excitation to isolate the pure electronic Kerr effect, all steps toward stable, high-performance polythiophene photonic devices.

Subject of Research: Morphology-dependent third-order nonlinear optical properties of poly(3-hexylthiophene) under continuous-wave green laser excitation

Article Title: Effect of poly(3-hexylthiophene) morphology formation on third-order nonlinear optical under green continuous wave excitation

Article References: Poad, N. A., Krishnan, G., Ros, F. C., Sa'aya, N. S. N., Halim, N. A., Manaf, N. A. A., & Demon, S. Z. N. (2026). Effect of poly(3-hexylthiophene) morphology formation on third-order nonlinear optical under green continuous wave excitation. Results in Optics, 25, Article 101172. https://doi.org/10.1016/j.rio.2026.101172

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101172

Keywords: P3HT, nonlinear optics, nanowires, Z-scan, conjugated polymers, optical limiting, self-assembly, third-order susceptibility, photothermal lensing, spectroscopic ellipsometry, XPS, organic photonics