Until recently , the range was rarely studied .
Few sources existed for microwave energy at the high end of the band ( sub-millimeter waves or so - called terahertz waves ) , but applications such as imaging and communications are now appearing .
Until recently , the range was rarely studied and few sources existed for microwave energy at the high end of the band ( sub-millimeter waves or so - called terahertz waves ) , but applications such as imaging and communications are now appearing .