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