photonic applications, such as optical switching, [6][7][8][9] sensing, [10][11][12][13][14][15] nonreciprocal propagation, [16][17][18][19][20][21] modulation, [22][23][24][25][26] optical logic gates, [27][28][29] and light buffering and storage. [3,[30][31][32][33][34][35] The Fano line shape profiles are related to the system's structural parameters and the electromagnetic parameters of ambient media. So, various systems, including optical cavities, [36][37][38][39] nanoclusters, [40][41][42][43] photonic crystals, [44][45] gratings, [46][47][48][49] metamaterials, [50][51][52][53] metasurfaces, [54][55][56][57] and many others, [58][59][60][61][62][63] have been proposed theoretically and observed experimentally to exhibit Fano resonance. Inspired by recent progress in numerous novel materials, [64][65][66][67][68][69] including 2D materials with exotic optoelectronic properties, [70][71][72] superconducting materials, [73,74] phase-changed materials, [75,76] low loss dielectric materials [77,78] and quantum dots, [79][80][81][82] many opportunities to investigate Fano resonance are anticipated.It is well-known that the macroscopically arrayed structures are typically too bulky for highly integrated on-chip optical circuits, thus, compact photonic structures are in high demand. [83][84][85] Optical microcavities, with high quality (high-Q) factors, convenient all-optical control, and compatibility with on-chip fabrication, have been used extensively for sophisticated optical devices, such as isolators, [86] lasers, [87] circuits, [88] sensors, [89][90][91] and buffers, [92,93] with distinctive properties. Based on the similarities between the classical electromagnetism and quantum mechanics, a single cavity and coupled-cavity can be referred to as artificial photonic atom and photonic molecule, [94][95][96] respectively. Analogous to the single atom molecule, a single cavity can also be referred to as a photonic molecule. The spectral response arising from the coherent interaction of optical modes depends heavily on the configuration of the artificial photonic molecules. [97][98][99] Over the years, sorts of photonic molecules have been implemented as key building blocks for realizing Fano resonance, [83,85,[88][89][90][91]100,101] thanks to the interactions between optical modes as illustrated in Figure 1a. The Fano profiles have been both experimentally and theoretically studied with numerous interesting physical phenomena revealed by the coupled oscillator model, [83,86,102] temporal coupled-mode theory, [103][104][105][106][107] transfer matrix method, [108][109][110] and quantumoptics approach. [111,112] In this work, we focus on reviewing the Fano resonance in artificial photonic molecules. We first introduce the properties of Fano resonance. Specifically, we discussThe spectral signatures of chemical molecules are dependent on the hybridization of electronic states. The artificial photonic molecules formed by structured optical microcavities, exhibiting Fano features with sharp asymmetric line shape a...