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We present a modular quasi-optical pulse slicer designed for use at terahertz (THz) frequencies. Given a quasi-cw input, the two outputs of a module are (1) a pulse with programmable duration and (2) its complement. The quasi-optical design incorporates a laser-driven silicon switch at Brewster's angle to the incoming THz beam, which limits undesired reflections before the switch is activated such that THz power is only transmitted when the switch is “on.” An “off” switch ensures that no power is leaked after the pulse and that the switching profile is sharp. The slicer's small footprint (0.048×0.072×0.162 m3) and small insertion loss (1.2 dB at 320 GHz) as well as high switching efficiency (∼70%) allows modules to be stacked to create multiple pulses. The output channel that is not used for experiments can be used for concurrent analysis of beam parameters. Stacking modular assemblies will enable more complex sequences of kW-level pulses than are currently achievable for applications including free-electron-laser or gyrotron-powered pulsed electron spin resonance at high magnetic fields.