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A new class of quasi-optical filters for harmonic suppression in sub-mm wave frequency multipliers with a pass-band at 300 GHz and a stop-band at 450 GHz was developed. The filters consist of 2-dimensional aperture arrays, and are made from microstructured aluminium on electrically thick, high-resistivity silicon substrates. This leads to a very good mechanical stability, low insertion loss, and permits to manufacture the structure by using standard processes available from the semiconductor industries. Two quasi-optical filters, the former with rectangular slots and the latter with dogbone-shaped holes, have been designed by using the MoM/BI-RME method. Several prototypes have been fabricated, and measured by THz-time-domain spectroscopy and CW-measurements, showing an insertion loss smaller than 1.6 dB at 300 GHz and a stop-band attenuation larger than 20 dB at 450 GHz. High reproducibility of the structures has also been proved. Moreover, it was shown that fine-tuning of the stop-band can be accomplished by small changes of the incidence angle.