Significance  Micro-nano optics, an emerging interdisciplinary field at the intersection of optics and nanotechnology, leverages the dimensional effects and material properties of micro/nanostructures to achieve precise control over various aspects of light fields, such as polarization, phase, and spatiotemporal distribution. It can significantly advance the miniaturization and intelligent transformation of optical systems, thereby demonstrating immense application potential. Among the diverse design approaches in micro-nano optics, full-wave simulation methods offer high accuracy but require iterative optimization, which generates large volumes of data and results in substantial consumption of computational resources and time. In contrast, effective medium theory (EMT) facilitates more efficient design and analysis of specific parameters by simplifying physical models appropriately. Serving as a bridge between macroscopic and mesoscopic scales, EMT has been widely applied in fields such as condensed matter physics, geophysics, and acoustics due to its equivalence-based perspective and intuitive physical interpretation. In micro-nano optics, EMT also plays a crucial role in obtaining equivalent electromagnetic parameters for optical systems and guiding the design and analysis of micro/nanostructures.

Progress  Firstly, a detailed introduction to EMT, which aims to predict the macroscopic electromagnetic properties of mesoscopic composite materials, is provided. Centered on the concept of equivalent averaging, the theory is reviewed from fundamental physical principles. Based on Lorentz local field analysis and the classical Clausius-Mossotti relation, the Maxwell-Garnett model is derived to describe dilute dispersive systems. For more general two-phase mixtures, the Bruggeman EMT model—characterized by higher symmetry—is presented. The discussion then extends to the application and development of EMT in anisotropic media, analyzing how component or structural orientation affects the effective permittivity and permeability tensors. Finally, overcoming the limitations of the quasi-static approximation is highlighted as a critical direction in EMT development. This section explores how corrections or alternative approaches can be introduced to modify classical EMT, enabling more accurate characterization of electromagnetic responses when the structural feature size approaches or exceeds the operating wavelength. These theoretical foundations underpin the subsequent exploration of EMT applications in micro/nano-optics.

　　The paper highlights recent advancements in the application of EMT across several key research areas within micro/nano-optics, including thermal conduction and radiative parameter studies, spectral analysis, waveguide design, and artificial electromagnetic materials. It reviews the use of EMT in various research directions, emphasizing its applicability to different topics and describing how researchers have improved and extended the theory through modifications or integration with other methods. For instance, the combination of EMT with techniques such as Mie scattering analysis and Bloch mode analysis enables rapid prediction and optimization of device performance, provided that the assumptions of effective medium validity are maintained. The review also offers insights into the development and extension of novel EMT approaches.

　　Compared with full-wave simulations, effective medium theory enables more efficient simplified analysis of micro/nano-optical systems, allowing rapid estimation of equivalent system parameters to guide design. This review discusses the application and development of effective medium theory across various research directions in micro/nano-optics. Finally, potential future pathways for further improving the design efficiency and performance of micro/nano-optical systems are outlined, aiming to provide valuable references for ongoing research and development in the field.

Conclusions and Prospects  EMT has found broad applications in the field of micro/nano-optics, playing significant roles in studies including thermal conductivity and radiative parameters, spectral analysis, waveguide design, photonic crystals, and metasurfaces. Despite EMT’s intuitive and simplified modeling, its assumptions restrict usage, while full-wave simulations are computationally intensive. Strategic utilization and improvement of effective medium theory, or its integration with other methods, can enhance the design efficiency of micro/nano-optical systems. In recent years, machine learning-based optimization methods have attracted considerable attention. EMT can complement data-hungry machine learning methods to extend generalizability by providing cost-effective approximations. Moreover, advances in nanofabrication will further support high-performance, compact photonic device development.