Natural-Modes Expansion of Microwave Fields Emitted by Ultra-Short Pulse Laser Illumination of a Conducting Wire

We investigate microwave emissions caused by the neutralizing current (NC) induced in a metallic wire by ultrashort pulse laser (USPL) illumination at intensities below $\sim \!{10}^{14}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$. The NC flows on the object to neutralize positive charge induced by electron ejection from the illuminated spot. The model employs the natural-mode expansion also known as the singularity expansion (SE) originally devised by C. Baum to expand the NC in terms of the natural current modes flowing on the wire. The SE technique generally applies to metallic and nonmetallic objects with general shapes. However, the application of this method to objects of general shapes and material composition requires the use of numerical methods, such as the moment method or the finite-element method, and can be computationally intensive. The objective of this article is to demonstrate that the NC current constitutes a main contribution to the USPL-induced microwave emissions, even at subrelativistic intensities and ambient conditions. To elucidate the physical phenomenon while sidestepping daunting numerical simulations, this demonstration is accomplished by applying the SE to the NC induced on a simple conducting wire where a quasi-closed form solution is available. As a model validation, we present comparisons between radiated fields predicted by the model and those measured by a laboratory.