  Here we present a semiclassical analysis of spontaneous and stimulated
radiative emission from unmodulated and optically-modulated electron quantum
wavepackets. We show that the radiative emission/absorption and corresponding
deceleration/acceleration of the wavepackets depend on the controllable
'history-dependent' wavepacket size. The characteristics of the radiative
interaction when a wavepacket of size (duration) is short relative to the
radiation wavelength, are close to the predictions of the classical
point-particle modeling. On the other hand, in the long-sized wavepacket limit,
the interaction is quantum-mechanical, and it diminishes exponentially at high
frequency. We exemplify these effects through the scheme of Smith-Purcell
radiation, and demonstrate that if the wavepacket is optically-modulated and
periodically-bunched, it exhibits finite radiative emission at harmonics of the
modulation frequency beyond the limit of high-frequency cutoff. Besides, the
radiation analysis is further extended to the cases of superradiant emission
from a beam of phase-correlated modulated electron wavepackets. This
wavepacket-dependent emission process shows the radiative features of
classical-to-quantum transition, indicates a way for measuring the quantum
electron wavepacket size and suggests a new direction for exploring
light-matter interaction.
