Dressed Photon as an Off-Shell Quantum Field

The concept of the dressed photon (DP) and the principles of its creation are reviewed on the basis of the concept of quantities off the mass-shell. Unique properties of the DP are presented. Generic novel applications of the DP are demonstrated by taking the technologies for fabrication and processing of nanomaterials, nano-optical devices, and light-emitting devices using crystalline silicon as representative examples. In addition to those experimental achievements, the present status of a developing new theory for understanding DP is presented. The working hypothesis to initiate our research was inferred from experiments which seem to show that DP is a singular off-shell field in which spacelike momentum field, arising from the interaction between incident light and nanoscale material fields dominate. Since nanoscale is an intermediate scale between quantum and classical ones, we need a comprehensive dynamical theory which connects those two scales compatibly and, in this sense, Micro-Macro duality proposed by Ojima plays a guiding role in our theory developing efforts. In order to implement Micro-Macro duality in concrete terms for our DP problem, we also employ a method of Clebsch parameterization applied to electromagnetic 4-vector potential.