A new technique for nanoscale spectroscopy

Free electrons in electron microscopes provide a means to map optical excitations in a specimen with atomic spatial resolution through measurements of electron energy losses and cathodoluminescence emission associated with electron-specimen interactions. Ms. Leila Prelat, Dr. Eduardo Dias, and ICREA Prof. Javier García de Abajo (Nanophotonics Theory Group at ICFO) have theoretically established the basis of a new technique capable of resolving low-frequency excitations down to the terahertz regime and below. This is achieved through nonlinear cathodoluminescence arising from the mixing of visible laser light with the electromagnetic field carried by free electrons. Such mixing gives rise to visible light at a frequency different from that of the incident laser, with the frequency difference corresponding to the measured excitation. Detection of this nonlinearly scattered light therefore provides spectral information at low frequencies, while retaining the atomic-scale spatial resolution inherited from the strong focusing properties of free electrons. The proposed technique, termed wave-mixing cathodoluminescence (WMCL), overcomes long-standing limitations by relying only on visible light and instrumentation already available in electron microscopes. In WMCL, an electron beam excites low-frequency vibrations in a material while a visible laser illuminates the same region. Owing to the material’s nonlinear optical response, the vibrations and laser light interact, producing a small frequency shift in the scattered visible light. This shift encodes information about otherwise invisible low-frequency excitations. The researchers showed that WMCL could recover molecular fingerprints at the nanometer scale, enabling identification of different chemical components in thin molecular layers (e.g., in retinal).