Dual mode microwave deflection cavities for ultrafast electron microscopy

\u3cp\u3eThis paper presents the experimental realization of an ultrafast electron microscope operating at a repetition rate of 75 MHz based on a single compact resonant microwave cavity operating in a dual mode. This elliptical cavity supports two orthogonal TM\u3csub\u3e110\u3c/sub\u3e modes with different resonance frequencies that are driven independently. The microwave signals used to drive the two cavity modes are generated from higher harmonics of the same Ti:Sapphire laser oscillator. Therefore, the modes are accurately phase-locked, resulting in periodic transverse deflection of electrons described by a Lissajous pattern. By sending the periodically deflected beam through an aperture, ultrashort electron pulses are created at a repetition rate of 75 MHz. Electron pulses with τ = (750 ± 10) fs pulse duration are created with only (2.4 ± 0.1) W of microwave input power; with normalized rms emittances of ϵ\u3csub\u3en,x\u3c/sub\u3e = (2.1 ± 0.2) pm rad and ϵ\u3csub\u3en,y\u3c/sub\u3e = (1.3 ± 0.2) pm rad for a peak current of I\u3csub\u3ep\u3c/sub\u3e = (0.4 ± 0.1) nA. This corresponds to an rms normalized peak brightness of B n p , rms = ( 7 ± 1 ) × 10 6 A/m\u3csup\u3e2\u3c/sup\u3e sr V, equal to previous measurements for the continuous beam. In addition, the FWHM energy spread of ΔU = (0.90 ± 0.05) eV is also unaffected by the dual mode cavity. This allows for ultrafast pump-probe experiments at the same spatial resolution of the original TEM in which a 75 MHz Ti:Sapphire oscillator can be used for exciting the sample. Moreover, the dual mode cavity can be used as a streak camera or time-of-flight electron energy loss spectroscopy detector with a dynamic range >10\u3csup\u3e4\u3c/sup\u3e.\u3c/p\u3