**3.1 Light absorption and emission of 2D magnets**

The excitonic absorption and emission properties have been widely studied in **CrX3 (X = Cl, Br, I)**, which can be tuned by changing their magnetic ordering. **Figure 5a** presents the optical absorption and photoluminescence (PL) properties of bulk CrBr3 in FM state (10 K) [27]. It is clear that two excitonic absorption peak X1 and X2 are located in the visible region, and PL is in the near-infrared region. As explained in this work PL originates from the *d*-*d* transitions modified by polaronic effect in CrBr3, resulting in a ultra long emission lifetime of around 4.3 μs [27, 80]. Besides, the *d*-*d* transition in the out-of-plane magnetic field shows circularly selective PL [71]. **Figure 5b** shows the PL from monolayer CrBr3 at 2.5 K in −0.5 and 0.5 T with σ<sup>+</sup> σ+ and σ**−**σ**−** excitation-collection configurations, where σ+ (σ**−**) represents the left (right) circularly polarized light. The PL at ±0.5 T shows opposite helicity, indicating that the spin of electrons in CrBr3 is coupled to the circularly polarized light [80]. **NiPS3** exhibits strongly correlated electrons, where the excitons' unique coupling to spin and orbital degrees of freedom in this AFM 2D magnet. The non-equilibrium driven of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phased beyond thermo-dynamic equilibrium. Recently, spin-orbital-entangled excitons that arise from Zhang-Rice state have been achieved and observed in this vdW correlated insulator NiPS3 under photo excitation. As shown in **Figure 5c** (left), NiPS3 has a charge-transfer (CT) gap of ~1.8 eV at 20 K (AFM state) [81]. Below the charge excitation lies a rich spectrum of sub-gap absorption resonances, including on-site *d*-*d* transitions at around 1.1 and 1.7 eV [81]. More importantly, there is a complex of spin-orbit-entangled excitons at around 1.5 eV, which has been assigned to the transition from a ZRT to ZRS state [77]. Besides, PL spectrum of a few-layer NiPS3 sample with a thickness of about 10 nm shows two broadened emission peaks at

#### **Figure 5.**

*a, Optical absorption and overlaid PL spectrum of CrBr3 thick samples measured at 10 K (left). The d-d (X1 and X2) and CT transitions are marked and illustrated in a simplified energy diagram in the inset. Time-resolved PL decay curve obtained at 4 K by integrating the PL intensity over the spectral range of 1.25–1.4 eV (right). The fitting (solid line) yields an averaged lifetime of 4.3 μs. Inset: schematic of light emission from Cr3+ metal ion in the octahedral crystal field [27] Copyright 2022, American Chemical Society. b, Spontaneous circularly polarized PL spectra for σ<sup>+</sup> σ+ (red) and σ***−***σ***−** *(blue) circular polarization components from monolayer CrBr3 at 2.7 K under the applied out-plane magnetic field of - 0.5 T (left) and 0.5 T (right) [80] Copyright 2019, American Chemical Society. c, Optical absorption (α) of NiPS3 below the CT gap at 20 K (left). The features around 1.5 eV at the spin-orbital-entangled exciton transitions. The broad structures around 1.1 and 1.7 eV are on-site d-d transitions [81] Copyright 2021, Springer Nature. PL spectra of the band-edge emissions of a few-layer NiPS3 sample at 300 K (right) [82]. Copyright 2021, Springer Nature. The fitting curves of the PL peaks are also included. The left inset shows a microscopic image of the few-layer sample, and the right inset shows the possible band-edge scheme for the observed PL emissions. d, Differential reflectance spectrum of a few-layer CrPS4 flake (black dots), and the liner fitting (red line) reveals the absorption edge at 2.00 eV (left). Inset: the zoomed-in spectrum. PL spectrum of CrPS4 at 4 K fitted by the Fano formula (right). The continuum band is composed of four Gaussian peaks and one Lorentzian peak. Inset: PL spectra obtained at 10 K with applied field of 0 T and 7 T [51] Copyright 2020, American Chemical Society.*

#### *Novel Light-Matter Interactions in 2D Magnets DOI: http://dx.doi.org/10.5772/intechopen.112163*

*E*3d ~1.23 eV and at B ~1.825 eV at 300 K, as shown in **Figure 5c** (right). A detailed analysis of the PL line-shape fitting of the PL spectrum reveals that the A1 peak (~1.366 eV) is still involved in the broadened *E*3d peak [82]. Further thicknessdependent PL intensity results indicate *E*3d peak may be from the Ni 3*d* eg \* band to the *E*v transition assisted by phonons at 300 K. The inset of **Figure 5c** (right) shows the probable band scheme of the band edge for PL band-edge emissions. The *E*3d peak is an indirect-like emission caused by the intermediate band of Ni 3*d* eg \* existed in NiPS3. The A1 and B emissions are originated from the direct *E*c bottom to the *E*<sup>v</sup> top and to the spin-split-off band below *E*v, respectively [82]. **CrPS4** is a promising ternary AFM semiconductor with PL in the near-infrared wavelength region. Recently, a Fano resonance, arising from quantum interference between a discrete optical transition and a continuous background, is observed in PL from CrPS4 flakes with decreasing temperature below the Néel temperature. **Figure 5d** (left) shows the plot of (αhν) 2 versus the photon energy hν, which can determine the absorption edge is ~2.00 eV ascribed to the CT transitions from the 3*p* of the S band to unoccupied 3*d* band of the Cr atom. In the energy below the absorption edge, two weak peaks can be observed at 1.54 and 1.65 eV, as show in the inset of **Figure 5d** (left) [51]. At the low temperature 4 K, PL emission in the near-infrared region is observed, where PL shows several narrow peaks with two asymmetric profiles located at 1.333 and 1.367 eV emerging [79]. Such an asymmetric line shape is a typical feature of the Fano resonance, which can be well described by a Fano formula (the fitted red solid line). This work explains that the continuous background responsible for the Fano resonances is attributed to the *d*-*d* transition of the Cr3+ Center, predominantly the spin-forbidden <sup>2</sup> Eg to <sup>4</sup> A2g transition with contributions of the broad-band <sup>4</sup> T2g to 4 A2g transition [51]. Since this Fano resonance is only observed with the temperature below the Néel temperature, it indicates that AFM order of CrPS4 is of great importance to the occurrence of this quantum inference. However, the applied out-ofplane magnetic field shows no obvious influence on the PL spectra, as demonstrated by the inset of **Figure 5d** (right), which may be due to the saturation of the discrete state transition at 10 K.
