  As a key component for next-generation wireless communications (6G and
beyond), terahertz (THz) electronic oscillators are being actively developed.
Precise and dynamic phase control of ultrafast THz waveforms is essential for
high-speed beam steering and high-capacity data transmission. However,
measurement of such ultrafast dynamic process is beyond the scope of
electronics due to the limited bandwidth of the electrical measuring
instruments. Here we surpass this limit by applying photonic technology. Using
a femtosecond laser, we generate offset-free THz pulses to phase-lock the
electronic oscillators based on a resonant tunneling diode. This enables us to
perform phase-resolved measurement of the emitted THz electric field waveform
in time-domain with sub-cycle time resolution. Ultrafast dynamic response such
as anti-phase locking behaviour is observed, which is distinct from in-phase
stimulated emission observed in laser oscillators. We also show that the
dynamics follows the universal synchronization theory for limit cycle
oscillators. This provides a basic guideline for dynamic phase control of THz
electronic oscillators, enabling many key performance indicators to be achieved
in the new era of 6G and beyond.
