  The growing requirements of pump-probe techniques and nonlinear optics
experiments greatly promote the studies of two-color free-electron lasers
(FELs). We propose a new method to generate coherent two-color pulses in a
high-gain harmonic generation (HGHG) FEL. In this scheme, an initial tilted
electron beam is sent though the modulator and dispersive section of an HGHG
FEL to generate the bunching at harmonics of the seed laser. Then a transverse
gradient undulator (TGU) is adopted as the radiator and in such radiator, only
two separated fractions of the tilted beam will resonate at two adjacent
harmonics of the seed laser and are enabled to emit the coherent two-color
pulses simultaneously. The time separation between the two pulses are on the
order of hundreds of femtoseconds, and can be precisely controlled by varying
the tilted amplitude of the electron beam and/or the transverse gradient of the
TGU radiator. Numerical simulations confirm the validity and feasibility of
this scheme in the EUV waveband.
