Engineering birefringence in polymers and polymer-based nanocomposites: Mechanisms, characterization, control, and applications en en Birefringence is a key optical property of polymers and polymer nanocomposites, influencing the performance of optical, photonic, and electro-optical devices. Precise control of birefringence is essential for polarization optics, optical communication, flexible electronics, smart materials, and stress analysis. Although previous reviews have addressed specific aspects of polymer birefringence, a framework linking molecular and structural origins to processing, nanofiller engineering, characterization, and material design remains limited. This review addresses this gap by providing an integrated and critical analysis of the mechanisms, theoretical models, processing-structure relationships, nanofiller effects, characterization approaches, and emerging applications of birefringence in polymeric systems. Attention is given to polymer chain orientation, crystallinity, molecular anisotropy, residual stress, phase morphology, and the effects of nanofiller dispersion, orientation, concentration, and surface functionalization on refractive-index modulation and optical anisotropy. Theoretical approaches, including the Lorentz-Lorenz equation, stress-optical relationship, and Abbe number, are examined to clarify the relationships among material structure, processing, refractive index, and birefringence. Advanced characterization techniques, including polarized optical microscopy, interferometry, ellipsometry, polarimetry, conoscopic analysis, and electro-optic and magneto-optic methods, are compared in terms of their principles, capabilities, and limitations. Recent advances in optical sensors, waveguides, optical films, liquid crystal displays, biomedical devices, and multifunctional smart materials are highlighted. Unlike earlier reviews that primarily emphasize individual mechanisms, materials, or applications, this review establishes an integrated structure-processing-property perspective. It further provides practical guidance on polymer architectures, processing conditions, nanofiller characteristics, surface-engineering strategies, and characterization methods to achieve targeted birefringence. Finally, current challenges, knowledge gaps, and future research directions are identified to support the rational design of next-generation high-performance optical and photonic polymer nanocomposites for emerging applications in high-performance optical and photonic devices. 172 202 Ali Borchloo Center for Nanoscience and Nanotechnology, Institute for Convergence Science and Technology Sharif University of Technology, Tehran 14588-89694 Iran ali.borchloo77@sharif.edu Birefringence Polymer-based nanocomposites Optical anisotropy Refractive index Nanofillers Vol 6 No 3 Paper 2.pdf [1] J.J. Zhao, Q. 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