In 0.1 - 10 THz regime, how to reduce the path loss?, what are issues associated with path loss?
The answer is to restrict yourself to a very narrowband frequency in that range that has a long path length.
If I remember correctly, 0.8 THz is not too bad.
If you increase the bandwidth, the spectral width will increase, the dispersion will be more, so if you want to reduce the dispersion the ideal will be use the signal which is temporally & spatially coherent. Temporal coherence & spatial coherence means very low bandwidth.
If you are referring to free-beam transmission of terahertz rays (i.e. not via waveguides), then you'd better stay away from any water vapor absorption lines. There are transmission windows, e.g. at 0.3 THz, 0.6 THz or, as Derek already suggested, 0.8 THz. You can find the relevant water lines e.g. in the HITRAN database.
As Anselm said, you have to stay away from frequencies that have high water vapor absorption. You can find the absorption lines in any THz channel modeling paper. Secondly, you have to use high gain directional antenna to combat the high path loss (spreading + absorption loss) at THz frequency.
As said by Zahed Hossain you should not select THz frequencies for wireless communication that have high water vapor absorption. In addition to wise selection of THz frequencies and high gain antennas you can overcome high pathloss penality by beamforming with ultra-massive MIMO antennas.
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