The paper demonstrates results of our experimental study of THz emission from the plasma of a single-color laser filament for two laser wavelengths of 740 and 940 nm. Particular attention is paid to the study of a frequency-angular distribution of the THz emission within 0.1 – 3 THz spectral range. It is observed that different spectral components of THz radiation have different angular distribution, and an increase in the laser beam numerical aperture leads to a growth of the THz emission divergence, especially in its low-frequency range. The study revealed a significant effect of a laser pulse initial wavelength on the THz emission characteristics. Transformation of frequency-angular THz emission spectrum produced by a single-color (740 nm) laser filament plasma under an external electrostatic field of various strength is also experimentally studied. If there is no static electric field, THz emission is predominantly generated in the low-frequency spectral range around 0.1 THz and propagates within a hollow cone. When the electric field is applied, the transition from the hollow cone to a filled one is observed with the field strength rise, THz emission frequency being within of 0.3 - 0.5 THz. Higher frequency emission of ~1 THz fills the whole cone with the emission maximum along the laser filament axis. Furthermore, the angular distribution for the low-frequency THz emission depends significantly on the laser pulse energy in contrast to the case of no electric field. Namely, the laser pulse energy rise results in a decrease of the propagation angles for low-frequency THz emission and disappearance of the local minimum in the angular distribution on the propagation axis.
