  Efficient and compact frequency converters are essential for frequency
stabilization of terahertz sources. In this paper, we present a 3.5-THz,
x6-harmonic, integrated Schottky diode mixer operating at room temperature. The
designed frequency converter is based on a single-ended, planar Schottky diode
with a sub-micron anode contact area defined on a suspended 2-$\mu$m ultra-thin
GaAs substrate. The dc-grounded anode pad was combined with the radio frequency
E-plane probe, which resulted in an electrically compact circuit. At 200 MHz
intermediate frequency, a mixer conversion loss of about 59 dB is measured and
resulting in a 40 dB signal-to-noise ratio for phase locking 3.5-THz
quantum-cascade laser. Using a quasi-static diode model combined with
electromagnetic simulations, good agreement with the measured results was
obtained. Harmonic frequency converters without the need of cryogenic cooling
will help in the realization of highly sensitive space and air-borne heterodyne
receivers.
