Speaker
Description
In transmission electron microscopy (TEM), biological samples are typically weak phase objects. High-contrast imaging of such specimens can be achieved with a Zernike phase plate, which shifts the phase of the unscattered wave by 90 degrees relative to the scattered waves, converting a microscope into an imaging interferometer. A promising approach to developing a phase plate for TEM is to use the ponderomotive potential of a cavity-enhanced continuous-wave laser beam tightly focused at the center of a TEM's diffraction plane.
We set out to develop the next-generation laser phase plate for TEM. Achieving a near-perfect contrast transfer function requires developing a high-power, high-finesse optical resonator with a tight focus. This quest, however, runs into a series of severe challenges: focusing cavities are inherently near-degenerate and therefore critically sensitive to cavity misalignment and imperfections or deformations of the optical elements. Moreover, at high intracavity power, thermoelastic deformations significantly alter the mode shape, leading to a dynamical instability beyond a certain power threshold. Finally, tightly focused cavities are non-paraxial systems, ruling out the use of the conventional paraxial cavity model.
To address these challenges, we developed a framework that describes non-paraxial tightly-focusing cavities with aberrations in a simple and intuitive mathematical form. This insight allowed us to develop a resonator design that overcomes these challenges by suppressing degeneracy, replacing the thermoelastic instability with inherent dynamical stabilization, and using engineered aberrations for enhanced robustness and mode shape control. Experimental realization of this new design is currently underway.