Ultra-broadband terahertz coherent detection via silicon nitride-based deep sub-wavelength metallic slit

We present a novel class of CMOS-compatible devices aimed to operate the solid-state-biased coherent detection of ultrashort terahertz pulses, i.e. featuring gap-free bandwidth at least two decades-wide. Such a structure relies on 1-μm-wide slit aperture between two parallel aluminum pads and embedded in a 1-μm-thick layer of silicon nitride, deposited on a quartz substrate. We show that such a device can detect ultra-broadband terahertz pulses by employing unprecedented low optical probe energies of only a few tens of nanojoule, due to the more than one order of magnitude higher nonlinear coefficient of silicon nitride with respect to silica, which was employed in the previous generations. In addition, very high static electric fields can be generated within the slit by applying extremely low external bias voltages (in the order of few tens of volts), which strongly enhance the dynamic range of the detected THz waveforms. Finally, we show how this new device potentially enables to operate the solidstate- biased technique with a homodyne scheme, in a similar manner to electro-optic sampling. These results pave the way to the integration of the solid-state ultra-broadband detection in compact and miniaturized terahertz systems fed by high repetitionrate laser oscillators and low-noise, low-voltage generators.