Designing a THz passive device

If someone with good background knowledge of RF, microwave and a little bit of millimetre designs wants to design for 10-1000x times higher frequencies, like tera-hertz antenna (somewhere in 1 ... 10THz range), what would you recommend and suggest?

Do materials still have the same properties at terahertz as they have in microwave? Are metals still metals? What is the lowest-loss substrate material in this range? Diamond?

Silly questions I am sure...

What do you want to design?
I am also interested in design of microwave circuits up to 200GHz that is whole microwave transmitter & receiver circuits operating at this range.
If you see IEEE papers you will find people used GaAsFET, InP etc. etc. mostly semiconductors are used.
SOme people used old techniques such as valves to generate frequencies up to 1500GHz! I have seen pictures of this circuits.

My design is roughly 1THz planar antenna. I don't want you to suggest me a direct antenna design, but rather the difference in thinking about THz antenna design in comparison with microwave antenna design.

Probably at THz one must really carefully select materials because of high losses.

SO I think there is book available on internet
named " Gigahertz & Terahertz technologies". i have not read that book but I suppose It may help you in understanding the difference & lot of things you want to know.

The latest version (fourth edition) of Jasik's Antenna Engineering Handbook, now edited by John Volakis has a chapter on mm and THz antennas. If nothing else the list of references at the end of the chapter should keep you busy doing research for a while.

Your antenna will certainly be physically small. Are you thinking of an on-chip application? If you can find a really short RF technician it should be easier to fabricate.