An optical cavity is defined by a first mirror and a second mirror. The first mirror has a front surface that includes a first concave mirror. The second mirror has a front surface that includes a second concave mirror. The optical cavity has a resonant optical mode with a small focal spot size. The optical cavity may be used in a method of enhancing phase contrast in an electron beam image, and associated system for electron beam imaging or electron-beam spectroscopy, with a transmission electron beam microscope.