Abstract
We present a novel class of CMOS-compatible devices aimed to perform the solid-state-biased coherent detection of ultrashort terahertz pulses, i.e., featuring a gap-free bandwidth at least two decades-wide. Such a structure relies on a 1-µm-wide slit aperture located between two parallel aluminum pads, embedded in a 1-µm-thick layer of silicon nitride, and deposited on a quartz substrate. We show that this device can detect ultra-broadband terahertz pulses by employing unprecedented low optical probe energies of only a few tens of nanojoules. This is due to the more than one order of magnitude higher nonlinear coefficient of silicon nitride with respect to silica, the nonlinear material employed in the previous generations. In addition, due to the reduced distance between the aluminum pads, 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. These results pave the way to the integration of solid-state ultra-broadband detection in compact and miniaturized terahertz systems fed by high repetition-rate laser oscillators and low-noise, low-voltage generators.
Highlights
The realization of innovative techniques able to manage ultra-broadband terahertz (THz) pulses, i.e., electromagnetic waves, the spectra of which cover a two-decade wide frequency range (0.1-10 THz) or more,[1,2] has always been of interest due to the advantages that such a wide spectral range brings about with respect to conventional THz systems.[3]
We started the characterization comparing the spectral response of the devices against standard air-biased coherent detection21 (ABCD), the latter being performed by focusing the THz and the 50-μJ-probe beam together in air through a 2-in.-parabolic mirror and a 100-mm-lens, respectively
With our simulations, the field enhancement (FE) induced by the sub-λ slit somewhat emphasizes the lower frequencies, as pointed out by the slight red-shift observed for the solid-state biased coherent detection (SSBCD) spectrum
Summary
By modulating the bias electric field and performing heterodyne detection via a lock-in amplifier, it is possible to isolate and record such a linear term, reconstructing both amplitude and phase of the THz transient Building on this detection approach, we achieved an important breakthrough, demonstrating an ultra-broadband detection scheme relying on the EFISH generation process in a thin film of UV fused silica, which we named solid-state biased coherent detection (SSBCD).[2] SSBCD requires up to three orders of magnitude lower probe energy (
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