(WP1) We devised and implemented an efficient electron-terahertz velocity-matched interaction platform for 70-keV femtosecond electron bunches on a silicon prism. i) finite-difference time domain (FDTD) numerical simulations were used to calculate the optimal angle and dimensions of the silicon prism. A prism with the desired dimensions was acquired from a commercial supplier and installed in the interaction chamber of an ultrafast electron diffraction beamline. ii) The THz radiation with p-polarization was generated and delivered onto the prism, providing a streaking gradient of 400 as per camera pixel, and sub-femtosecond overall resolution. iii) a classical trajectory Monte Carlo simulation code was developed to understand the electron-terahertz velocity-matched interaction in detail and to further optimize the device. iv) we designed the Schottky diode in several variations (gold and aluminum plated) with the combined effort of the Fraunhofer Institute for Solar Energy Systems and University of Konstanz Nanolab. We deposited additional gold layers on the Schottky diode to study the effect of THz penetration depth and screening. v) we obtained the electron streaking deflectograms with and without the Schottky diode and developed a 1 kHz lock-in detection scheme for enhanced sensitivity of potential rectification features.
(WP2) The frequency limits were directly explored on integrated circuits that also have metal-semiconductor (Schottky and Ohmic) contacts. i) We devised and fabricated a printed-circuit-board microchip, driven by a picosecond photodiode. Femtosecond laser pulses were generated via bulk compression, coupled into the vacuum chamber and aligned onto the fast photodiode. Synchronized electron pulses then probe parts of the circuit in a stroboscopic manner. Here, bending of the electron beam trajectories is recorded for different time delays between the laser-triggered voltage pulses and the probing electron beam. ii) In a proof-of-principle experiment, we demonstrate the ability to measure voltage pulses on a printed circuit board via the femtosecond electron beam (fs-eBeam). Compared to a modern GHz oscilloscope, our femtosecond eBeam allows to see local voltage transients the oscilloscope cannot resolve. Further, we show that eBeam is effectively impedance free, local probe that also gives superior resolution is space without disturbing the circuit. iii) We repeat the measurements with a photoconductive switch with a femtosecond rise time. We show how the dispersion of the printed circuit board limits the maximum speed of its operation, by recording its time-dependent voltage response with a picosecond temporal resolution.
(WP3) A 4D pulsed electron oscilloscope was realized. i) we designed and fabricated a terahertz-supporting microchip. We deposit on a gallium arsenide wafer an elementary circuit with a coplanar waveguide, photoconductive switch, bias and termination contacts. We perform basic calculation showing that the microchip supports terahertz frequency of operation. ii) We mount the microchip in the vacuum chamber of the electron beamline and arrange a tightly-focused optical excitation. iii) We record the generation, propagation, dispersion and reflection of the electric field pulse with micrometer, millivolt and femtosecond accuracy, by utilizing terahertz compression of the electron pulses. We further explore the nonlinearities of the system by varying the bias potential and observe mechanisms of the electric pulse amplitude.
