https://peeterjoot.com/2018/11/27/phy2403h-quantum-field-theory-lecture-20-dirac-lagrangian-spinor-solutions-to-the-kg-equation-dirac-matrices-plane-wave-solution-helicity-taught-by-prof-erich-poppitz/
PHY2403H Quantum Field Theory. Lecture 20: Dirac Lagrangian, spinor solutions to the KG equation, Dirac matrices, plane wave solution, helicity. Taught by Prof. Erich Poppitz