Public defence, Radio Science and Engineering, MSc Maxim Masyukov

Title of the thesis: Towards Quasioptical Characterization of Cryogenically Cooled, Active THz Components.

Thesis defender: Maxim Masyukov
Opponent: Dr. Goutam Chattopadhyay, NASA’s Jet Propulsion Laboratory, California Institute of Technology, United States
Custos: Prof. Zachary Taylor, Aalto University School of Electrical Engineering 

Terahertz frequencies, ranging from 100 GHz to 10 THz, are crucial for basic science. Submillimeter waves and far‑infrared radiation, whose overlap defines the THz band, can reveal molecular clouds, star‑forming regions, and galaxy formation. However, this region remains largely unexplored due to instrumentation constraints. Traditional waveguides struggle at these frequencies, portable sources are scarce, and low‑noise amplifiers are hard to scale here for fundamental reasons. Operating at cryogenic temperatures—which are common in astronomy to reduce thermal noise—makes matters even more challenging.

This thesis focuses on free‑space, quasioptical characterization techniques in the THz band. Quasioptics, relying on focusing and collimating reflectors, provide a powerful advantage at these frequencies. Such techniques can be used at room temperature and, more importantly, enable contactless coupling of radiation to devices in a cryostat. In the first part, room‑temperature active devices—specifically semiconductor THz sources based on the field‑domain collapse phenomenon—were investigated quasioptically. It was shown that quasi‑optics can probe current dynamics in these circuits and significantly increase the peak output power.

In the second part, a quasi‑optical system was designed for passive, cryogenically cooled devices, and characterization approaches were developed for both room‑temperature and cryogenic environments. To test the calibration, frequency‑selective surfaces based on niobium films—becoming superconducting below about 9–10 K—were fabricated for both room‑temperature and cryogenic operation. Experimental results matched simulations, making this, to our knowledge, the first demonstrable calibration and de‑embedding of cryogenically cooled quasi‑optical devices at THz frequencies.

The main outcome is a technique for calibrating and de‑embedding quasioptical devices under cryogenic conditions. This allows measurement of their intrinsic properties, such as S‑parameters, without contamination from room‑temperature instrumentation, thereby directly assessing the behavior of the device itself. Precise characterization of each component enables building complex THz instruments from individual room‑temperature and cryogenic parts. As superconducting technology advances, such systems are in high demand and will transform future THz space‑observation technologies by enabling low‑noise, high‑sensitivity instruments with higher resolution.

Key words: THz frequency range, cryogenics, calibration

Thesis available for public display 7 days prior to the defence at Aalto University's public display page.

Contact:
email (aalto): maxim.masyukov@aalto.fi 
email (personal): maxim.masyukov@gmail.com 
phone number (personal): 040 2569926