An ultrashort light pulse
A carrier wave under a narrow envelope is a pulse. For a fixed envelope, raising the carrier frequency packs more optical cycles into it; an attosecond pulse needs a carrier whose period is below 1 fs (X-ray or extreme UV).
A carrier wave under a narrow envelope is a pulse. For a fixed envelope, raising the carrier frequency packs more optical cycles into it; an attosecond pulse needs a carrier whose period is below 1 fs (X-ray or extreme UV).