Frequencies above 275 GHz, the terahertz bands, are a new area for experimentation and propagation research. In the UK a Notice of Variation (NoV) is available to Full Licensees for such access.
See the RSGB Terahertz page for full details of >275 GHz access in the UK.
Most access to these frequencies is by a combination of stable lower frequency synthesisers and frequency multipliers. For example 288 GHz is a x3 multiple of 96 GHz, itself multiplied up by x6 from a ~13GHz synthesiser. Whilst antennas can deliver substantial gain, limitations arise from the low output powers and lack of Rx LNAs etc at present.