On-chip spectrometers, such as DESHIMA and
SuperSpec, require transmission lines with very low loss of tanδ <
10-4 to achieve sufficient system efficiency. Transmission lines
with higher loss would introduce too much signal attenuation in the line from
antenna to fi
...
On-chip spectrometers, such as DESHIMA and
SuperSpec, require transmission lines with very low loss of tanδ <
10-4 to achieve sufficient system efficiency. Transmission lines
with higher loss would introduce too much signal attenuation in the line from
antenna to filter and in the filters themselves. Data regarding the losses of
transmission lines at THz frequencies and sub-K temperatures is severely lacking.
In this report an on-chip Fabry-Pérot resonator concept is demonstrated that
can be used to measure the losses of a transmission line with high sensitivity
at high frequencies. To create the in-line Fabry-Pérot resonator, a
transmission line of certain length is coupled to a THz source via a twin-slot lens
antenna on one side and to an Al-NbTiN hybrid MKID on the other side. The goal
of this work is to measure the losses of microstrip lines at frequencies >
300 GHz, at a temperature of about 250 mK, with dielectric dominated loss in
the range of 10-3 > tanδ >
10-5. There are several experimental challenges for measuring tanδ. The
first challenge is the limited frequency resolution of the source, due to which
resolving low tanδ can become impossible. Secondly it was experimentally
found that there is stray light coupled to the detector which causes a spurious
response with a level of −30dB with respect to the peak (unity) transmission of
the Fabry-Pérot resonator. Taking these experimental challenges into account
results in a Fabry-Pérot resonator design where the length, the mode number,
and the coupler quality factor Qc of the resonator are
optimized. Furthermore multiple resonators on a single chip are used, each
coupled to a separate antenna and detector, with different Qc
values. This design method is applicable for different dielectric materials and
different transmission line configurations. Using this method a chip was
designed and fabricated for a microstrip line based Fabry-Pérot resonator
fabricated from sputter deposited superconducting NbTiN metal and a PECVD deposited
a-Si layer. Using this chip a tanδ ≈ 10-4 @ 350 GHz was
measured, which represents the lowest loss values of a microstrip line at
frequencies > 10 GHz ever measured.
