Dielectric Loss Measurements at Sub-K Temperatures and Terahertz Frequencies

Dielectric Loss Measurements at Sub-K Temperatures and Terahertz Frequencies

Kouwenhoven, Kevin (TU Delft Electrical Engineering, Mathematics and Computer Science)

Baselmans, Jochem (mentor)
Endo, Akira (graduation committee)
Vollebregt, Sten (graduation committee)

Delft University of Technology

2019-08-15

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 Q_c of the resonator are optimized. Furthermore multiple resonators on a single chip are used, each coupled to a separate antenna and detector, with different Q_c 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.

Dielectric loss
Sub-K
Terahertz
Transmission line
Microstrip
Fabry–Pérot resonator
Resonator
Loss measurements
On-chip spectrometers
Lab-on-a-chip
a-Si
NbTiN
Dielectric

http://resolver.tudelft.nl/uuid:f13dda2a-f1ca-4ecc-b45b-0650ed1fc869

2020-08-15

Student theses

master thesis

© 2019 Kevin Kouwenhoven