## 3 Measurement Results and Discussion
### Mode Control in Time-Domain Spectrometer  
In the context of the obtained results in THz frequency-domain, it is reasonable to turn the research into time-domain spectrometry and check a suitability of Ti-mB for this kind of THz systems. In contrast to frequency domain, inherent feature of time-domain spectroscopic systems is their broadband radiation; on the other hand, power of the radiation is in the same range as in THz FDS, i.e., of a few microwuts. Furthermore, THz TDS systems require precise alignment to measure spectra correctly or record images of acceptable quality. In both cases the polarisation of the incident light was parallel to the bolometer axis.  
Figure 4: THz beam spatial profiles emitted by low-temperature grown GaAs-based emitter in frequency domain spectrometer at 0.2 THz (**a**) and 0.4 THz (**b**) recorded using a linear array of the bolometers coupled with antennas. White lines indicate intensity cross sections along the relevant axes. Spectral characteristics of the detectors with different resonant dipole antennas and spectra normalized to the spectrometer signal-to-noise ratio (SNR) (**c**); full-width half-maximum (FWHM) dependency on the wavelength with linear approximation depicted by the dark yellow line (**d**), configuration of measurement scheme and polarization are depicted in the inset of (**d**). A photo of dual frequency THz linear array, where the top line of pixels is designed for frequencies around 0.2 THz and the bottom line for frequencies around 0.4 THz. The pixel pitch is 2 mm (**e**).  
To reveal picture of operation, two types of microbolometers were designed for the experiments: the first one was coupled with resonant antennas for 0.3 THz and the second for 0.7 THz frequency. The choice of the resonant frequencies was motivated by features of the emission--the 0.3 THz line is close to the maximal value in the red side of the spectrum, while 0.7 THz at the blue one, as it can be seen in the measured THz emission spectrum results presented together with experiment schematics in Figure 5a. Sharp lines gracing with the emission spectrum are due to the water vapour absorption in the air. Spatial mode profiles recorded in parallel polarization in respect to microbolometer axis using different coupling antennas are displayed in Figure 5b,c. As it can be seen, the mode profiles can be precisely recorded using both types of antennas. The microbolometer coupled to 0.3 THz resonant antenna removes high-frequency components of the radiation; however, its spatial profile, due to strong low-frequency constituents, reproduces the shape of the mode quite well. The distribution registered at other resonant frequency, around 0.7 THz, shows quite similar picture; however, the spatial mode distribution is narrower in comparison with that at 0.3 THz frequency. One can presume that these data also reproduce correctly the spatial mode distribution despite of the absence of relatively strong low-frequency components recorded. The data obtained allow to infer that the antenna-coupled bolometers can be used for mode profiles control in various optoelectronic THz time-domain spectrometers excited by femtosecond laser pulses [27].