## 4 Materials and Methods  
Each microbolometer was fabricated from titanium, which was chosen among several appropriate materials, as its micromachining properties were found to be most suitable for silicon processing technology, temperature coefficient and reliability. The sensor was processed on the silicon nitride (SiN) membrane of 2 \(\upmu\)m thickness; subsequently, the titanium thermistor film is electro-deposited onto the membrane, along with the aluminum antenna and metal interconnections. The thermistor is suspended in the air by etching out the underlying membrane. The microbolometer was connected either to double-dipole or to log-periodic type THz antenna [23]. Both single pixel bolometers and their linear arrays consisting of 32 pixels were used in the experiments, where pixel size was 2.5 \(\times\) 2.5 mm\({}^{2}\), pixel pitch was 2.5 mm. Just one pixel of the linear array was active during all experiments. Sensitivity and NEP of the arrays of sensors exceeds 200 kV/W and less then 20 \(\text{pW}/\sqrt{Hz}\), respectively. Linear arrays of titanium microbolometers were produced by Luvitera, Ltd, Vilnius, Lithuania.  
Electronic multiplier sources (Virginia Diodes, Inc., Charlottesville, Virginia, United States of America) were used to generate radiation of 0.15 THz, 0.3 THz and 0.6 THz frequencies in continuous wave mode with power of 29 mW, 13.2 mW and 0.56 mW, respectively.  
Commercially available frequency-domain terahertz spectrometer (Toptica Terascan 780) based on GaAs photo-mixer coupled with silicon lens was engaged in the experiments. Its operation range is within 0.1-2 THz, it delivers radiation of circular polarization of about 1 \(\upmu\)W power.  
For the experiment a broadband photoconductive low-temperature grown gallium arsenide (LTG-GaAs) antenna coupled with silicon lens from time-domain spectrometer (Teravil, Ltd., Vilnius, Lithuania) was also used. The microbolometer was connected to the lock-in amplifier to modulate signal optically chopped at frequency of 500 kHz. The emitter was pumped with femtosecond Ti-Sapphire optical laser of 100 fs pulse duration and generating broadband THz pulses in frequency range of 0.1-2.5 THz with average emission power of 1.2 \(\upmu\)W. The THz beam was investigated by scanning it in the \(xy\) plane perpendicular to the direction of propagation with detector being fixed to \(xy\) motorized positioning stage. No additional focusing optical components are used in radiation measurements. In beam imaging recording, position synchronized raster scan technique was implemented similar to that described in Ref. [9]. Scanning parameters were selected to enable sufficient SNR with resolution up to \(0.1\times 0.25\) mm\({}^{2}\) which is comparable with the wavelength of upper frequency limit of emitted radiation (or the size of the microbolometer).  
Optically pumped continuous-wave molecular THz laser FIRL-100 (Edinburg Instruments, Ltd., Livingston, United Kingdom) was used to generate discrete spectrum at 0.76 THz, 1.63 THz and 2.52 THz frequencies with corresponding powers of 1.6 mW, 1.6 mW and 7 mW, respectively. The emitted unfocused radiation was of 11 mm in diameter; the power was artificially reduced varying the power of optical pumping from 20 W down to 9.8 W range.  
The absolute power was measured using a calibrated power meter (Thomas Keating Absolute Power Meter System, Version 2, Thomas Keating Ltd., Billingshurst, West Sussex, UK).  
Commercially available pyroelectric sensor SPH62 THz with 2 \(mm^{2}\) active area from Spectrum Detector Inc. was used in the experiments.  
All experiments were conducted in the ambient air conditions.