Exciton-polaritons are quasi-particles arising from the strong coupling between cavity photons and semiconductor excitons, exhibiting a half-light half-matter nature. The inherited photonic component enables polaritons with a low effective mass, which can overcome the bottleneck effect and achieve Bose-Einstein condensation at room temperature. The photonic component also allows their wavefunctions extend over macroscopic distances, resulting in long-range coherence and facilitates energy transfer and transport. This paves new ways for ultra-low-threshold polariton lasers and coherent photonic circuits. The excitonic component inherited by polaritons enables them with stronger nonlinear compared to conventional photonic systems, such as parametric scattering, showing significant application potential in ultrafast optical switches and all-optical circuits. The temporal evolution of polaritons can be characterized by ultrafast spectroscopic techniques. A key dynamical feature of polaritons is their Rabi oscillations, which can be tracked on the femtosecond scale by femtosecond transient absorption (reflection) spectroscopy and two-dimensional electronic spectroscopy (2DES). In 2DES, excitation and detection frequencies are simultaneously resolved to generate two-dimensional spectra. By analyzing the frequency cross peaks in the two-dimensional maps, the Rabi oscillation of polaritons can be observed, thereby revealing the dynamics of polariton generation, energy exchange, and dephasing , which helps to gain a deeper understanding of polariton physics. On the picosecond and longer timescales, transient absorption