**1. Introduction**

Materials with reduced dimensionality have become particularly important, owing to their unexpected physical and chemical properties. Since the discovery of graphene, fabricated via the Scotch tape exfoliation method from graphite in 2014 [1, 2], scientists have made intensive efforts to search two-dimensional (2D) van der Waals (vdW) materials. This family includes metals, e.g., NbSe2, semiconductors, e.g., transition-metal dichalcogenides (TMDs), insulators, e.g., hexagonal boron nitride (hBN), and other elemental 2D vdW materials [3–5]. In particular, exfoliated ultrathin 2D vdW materials down to atomic thickness (one or several unit layers) display unique and fascinating electronic and optical properties. For example, optical excitation on the monolayer TMDs will generate a strongly bound electron-hole pair called an exciton, instead of fee electrons and holes as in traditional bulk semiconductors [6]. Moreover, their extraordinary mechanical property makes these atomic thin materials with breakthrough of integration technologies accelerate the on-chip device applications [7].

Magnetism in two dimensions has been long pursued by scientists. 2D magnets are at the heart of numerous theoretical [8, 9], experimental [10, 11] and technological

advances [11], which are considered as one of the most promising solutions for nextgeneration spintronic devices in a revolution in semiconductor technology [12–14]. However, 2D vdW materials with intrinsic magnetism had been missing until quite recently. Based on the theoretical point of view, the transition of magnetism from bulk to 2D, the fluctuations of any kind will increase and can easily destroy longrange magnetic order at any finite temperature [15]. In addition, the superexchange will also be modified with the dimension reduction of the system due to the breaking of the inversion symmetry or the modification of the electronic/lattice structure [16]. Thus, the magnetic properties of 2D vdW materials are far less experimentally investigated for a long time.

With the first discovery of intrinsic magnetic order in the monolayer/few-layer limit in 2D ferromagnets, Cr2Ge2Te6 and CrI3 in 2017 [17, 18], this trend may be poised to change. Subsequently, various 2D vdW magnets have been rapidly discovered and studied [19]. In contrast to the conventional magnetic materials, the magnetic order of 2D magnets can persist down to the monolayer limit, thus possessing a vast reservoir of properties. For example, (i) 2D magnets have strong quantum confinement and mechanical flexibility [20]; (ii) 2D magnets possess good sensitivity to external fluctuations (doping, strain, electric field, et al.) [21]; (iii) 2D magnets can form various artificial heterostructures by a simple stacking process [22]; (iv) 2D magnets show thickness-dependent tuning properties [23]. Thus, 2D magnets have been demonstrated promising building blocks for next-generation information devices. Currently, intense experimental and theoretical investigations are almost devoted to explore the intrinsic magnetism, increase Curie temperatures, and design/fabricate 2D magnet-based devices with advanced functionalities. However, the investigation of light-matter interactions in these 2D magnets is still at the primary stage, there have already been reported unique optical properties in some ferromagnetic (FM) or antiferromagnetic (AFM) ones, exhibiting quite different light emission/scattering as compared with conventional nonmagnetic 2D materials. The mechanism of lightmatter interactions in 2D magnets challenges the knowledge of materials physics, which drives the rapid development of materials synthesis and device applications.

In this chapter, the special topic of light-matter interactions in atomically thin 2D magnets is mainly discussed. We begin with a brief review of the recent remarkable progress of achieved 2D vdW magnets, mainly on CrX3 (X = Cl, Br, I), MPS3 (M = Fe, Ni, Mn), CrSBr and CrPS4, with special emphasis on the magnetic properties and electronic band structures by changing the number of layers. Then, we mainly discuss light absorption, emission and scattering behaviors in 2D magnets. Additionally, the mechanism of light-matter interactions in 2D magnets determined by complex interactions between magnetic, electronic and vibrational degrees of freedom will be elaborated in detail, which plays a key role in the state-of-the-art of opto-electronic device applications.
