We describe how the finite-difference time-domain (FDTD) technique can be used to compute the quasinormal mode (QNM) for metallic nano-resonators, which is important for describing and understanding light-matter interactions in nanoplasmonics. We use the QNM to model the enhanced spontaneous emission rate for dipole emitters near a gold nanorod dimer structure using a newly developed QNM expansion technique. Significant enhanced photon emission factors of around 1500 are obtained with large output β-factors of about 60%. [7]. While the optical properties of MNRs are being actively pursued, numerically modelling of the basic cavity physics is extremely demanding, and analytical solutions of the modes only exists for very simple structures such as spheres. For resonant cavity structures, the natural modes of the system are called quasinormal modes (QNMs) [8,9], defined as the frequency domain solutions to the wave equation with open boundary conditions (the Silver-Müller radiation condition). Kristensen et al. [10] first used the QNMs to introduce a rigorous definition of the "generalized effective mode volume" and Purcell factor [11], and applied these results to photonic cavity structures. For MNRs, the QNMs also form the natural starting point for developing analytical theories of lightmatter interactions in nanoplasmonics [12][13][14].One of the most common numerical techniques for obtaining the cavity mode for dielectric cavities is the finite-difference time-domain (FDTD) technique [15]. The FDTD technique allows one to simulate open boundary conditions with "perfectly matched layers" (PMLs) located at the leaky mode region outside the cavity. For dielectric cavities, this open-boundary FDTD approach has been shown to yield excellent agreement with direct integral equation methods [10]. Other timedomain techniques such as the discontinuous Galerkin time-domain approach can also use PMLs [16]. For metals, a major problem occurs when using the usual mode calculation approach with a dipole excitation source, since the extracted mode depends sensitively on the dipole position [17], and is therefore incorrect. Thus it has been common practice to excite the MNR with a plane wave source and obtain the scattered field. However, this scattered field is not the same field as the QNM and it cannot be properly normalized for use in quantum optics, e.g., for obtaining the Purcell factor and effective mode volume [12]-two well known quantities that help describe the underlying physics of cavity light-matter interactions. While some frequency-domain techniques exist for computing the QNMs of MNRs [18,19], it is highly desirable to be able to compute the QNMs using the commonly employed and general FDTD technique.The FDTD method is already widely used by the plasmonics community, and its accuracy for obtaining the enhanced field has been verified against other numerical techniques such as the multipole expansion technique [20]. In addition, the LDOS can be calculated by employing a dipole excitation source [21][22][23], wh...