Significance Organic light-emitting diodes (OLEDs) have emerged as a key technology for next-generation displays owing to their self-emissive operation, high contrast ratio, fast response, wide viewing angle, thin form factor, and mechanical flexibility. They are now widely used in smartphones, televisions, wearable electronics, and virtual- and augmented-reality display systems. Nevertheless, the manufacturing yield and operational reliability of OLED devices remain highly sensitive to microscopic defects introduced during fabrication, assembly, and encapsulation. The complex multilayer architecture of OLEDs, comprising electrodes, charge injection and transport layers, emissive layers, and thin-film encapsulation, makes defects such as line breaks, scratches, foreign particles, interfacial delamination, local stress concentration, microcracks, and material degradation difficult to detect comprehensively. These defects can induce non-uniform emission, pixel failure, electrical instability, and accelerated device degradation. Conventional inspection approaches, including automated optical inspection, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy, provide valuable structural information but are constrained by optical diffraction, limited probing depth, destructive or complex sample preparation, low throughput, and potential secondary damage. Femtosecond-laser-based nonlinear optical microscopy offers an alternative route by exploiting nonlinear interactions confined to the focal volume, thereby providing intrinsic optical sectioning, depth-resolved imaging, and reduced dependence on conventional contrast mechanisms. Multi-harmonic femtosecond-laser microscopy is consequently attracting increasing attention as a high-resolution and low-damage approach for inspecting complex OLED structures.

Progress This review summarizes the fundamental mechanisms, representative applications, and emerging developments of multi-harmonic femtosecond-laser microscopy for OLED defect inspection. Femtosecond excitation combines high peak intensity with a relatively low average thermal load, enabling efficient generation of nonlinear optical signals while restricting excitation to localized regions. Second-harmonic generation (SHG) is particularly sensitive to non-centrosymmetric structures and symmetry breaking, making it suitable for probing material interfaces, structural discontinuities, and local strain. In multilayer OLED structures, SHG therefore provides a useful optical contrast for evaluating interfacial integrity, delamination, and stress distributions. Third-harmonic generation (THG), in comparison, is strongly influenced by local variations in refractive index and nonlinear susceptibility. At material boundaries, microstructural discontinuities, and defect-induced optical inhomogeneities, the balance of nonlinear contributions and destructive interference can be disturbed, resulting in enhanced THG signals. This mechanism enables THG to sensitively reveal microcracks, scratches, foreign particles, and other localized structural defects. In indium tin oxide (ITO) anode patterns, for example, line breaks and damaged edges introduce abrupt optical discontinuities that can produce localized THG enhancement, providing a route toward sub-micrometer-scale detection of patterning defects.For thin-film encapsulation, SHG and THG offer complementary sensitivity to interfacial and boundary-related features. SHG is more closely associated with symmetry breaking and interfacial structural changes, whereas THG responds strongly to physical boundaries and refractive-index discontinuities. Their combination could therefore provide depth-resolved information on crack propagation and delamination, with potential for three-dimensional reconstruction of complex defects. For deeply embedded foreign particles, the joint analysis of SHG, THG, and multiphoton photoluminescence can provide complementary information on defect morphology, optical response, and material-dependent emission, offering a possible means of detecting and preliminarily classifying contaminants within multilayer structures. Two-photon excited fluorescence (TPEF) and three-photon excited fluorescence (3PEF) further extend the available contrast mechanisms by providing molecular- and material-specific information from organic functional layers. These modalities can be used to investigate local material states and photochemical degradation. Electric-field-induced second-harmonic generation (EFISHG) extends nonlinear optical characterization from structural imaging to functional analysis by enabling the investigation of local electric fields, charge accumulation, carrier injection, and transport dynamics. In parallel, terahertz time-domain spectroscopy (THz-TDS) combined with near-field enhancement has demonstrated the capability to monitor the degradation of blue-emitting OLED materials in real time, indicating the complementary role of ultrafast optical and terahertz techniques in assessing material evolution. Recent multimodal femtosecond microscopy platforms integrating TPEF, 3PEF, SHG, and THG further demonstrate the feasibility of simultaneously acquiring structural and functional information. The integration of these optical modalities with machine vision and deep-learning algorithms may enable more comprehensive defect detection, localization, classification, and source analysis.

Conclusions and prospects Multi-harmonic femtosecond-laser microscopy provides a versatile optical framework for high-resolution and low-damage characterization of OLED defects. The complementary responses of SHG, THG, TPEF, 3PEF, and EFISHG to interfaces, cracks, foreign particles, organic materials, electric fields, and carrier-related processes create opportunities for multidimensional structural and functional inspection. However, several challenges must be addressed before these approaches can be translated into high-throughput industrial manufacturing. These include the complexity and stability requirements of femtosecond optical systems, efficient collection of weak nonlinear signals, high-speed scanning and three-dimensional imaging, laser safety and power management, quantitative calibration of nonlinear signals against defect parameters, and real-time processing of large multimodal datasets. In addition, some proposed applications, particularly the use of combined harmonic signals for specific OLED encapsulation defects, remain at an exploratory stage and require systematic experimental validation under realistic device conditions. Future research should focus on compact and robust femtosecond sources, high-efficiency large-area nonlinear detection, rapid three-dimensional imaging, quantitative signal calibration, and artificial-intelligence-assisted multimodal analysis. Closed-loop control of excitation power and automated defect-recognition algorithms could further improve inspection safety, stability, and reproducibility. With continued advances in ultrafast laser sources, nonlinear optical detection, computational imaging, and intelligent manufacturing, multi-harmonic femtosecond microscopy is expected to progress from laboratory-scale studies toward compact, automated, and high-throughput inspection systems for OLED production. The underlying principles may also be extended to emerging display technologies, including Micro-LEDs, quantum-dot light-emitting diodes, and perovskite light-emitting diodes, providing a broader optical inspection framework for next-generation display manufacturing.