  We propose a method based on the slice energy spread modulation to generate
strong subpicoseond density bunching in high-intensity relativistic electron
beams. A laser pulse with periodic intensity envelope is used to modulate the
slice energy spread of the electron beam, which can then be converted into
density modulation after a dispersive section. It is found that the double-horn
slice energy distribution of the electron beam induced by the laser modulation
is very effective to increase the density bunching. Since the modulation is
performed on a relativistic electron beam, the process does not suffer from
strong space charge force or coupling between phase spaces, so that it is
straightforward to preserve the beam quality for further applications, such as
terahertz (THz) radiation and resonant excitation of plasma wakefield. We show
in both theory and simulations that the tunable radiation from the beam can
cover the frequency range of 1-10 THz with high power and narrow-band spectra.
