Continuous 40 GW/cm$^2$ laser intensity in a near-concentric optical   cavity
  Manipulating free-space electron wave functions with laser fields can bring about new electron-optical elements for transmission electron microscopy. In particular, a Zernike phase plate would enable high-contrast imaging of soft matter, leading to new opportunities in structural biology and materials science. A Zernike plate can be implemented using a tight, intense CW laser focus that shifts the phase of the electron wave by the ponderomotive potential. Here, we use a near-concentric cavity to focus 7.5 kW of circulating laser power at 1064 nm into a 7 $\mu$m waist, setting a record for CW laser intensity and establishing a pathway to ponderomotive phase contrast TEM. 
Osip Schwartz, Jeremy J. Axelrod, Philipp Haslinger, Colin Ophus,   Robert M. Glaeser, and Holger M\"uller
physics.optics