  Measurements of the phase of two-photon matrix elements are presented for
resonant and antiresonant two-color ionization of helium. A tunable,
narrow-bandwidth, near-infrared (NIR) laser source is used for extreme
ultra-violet (XUV) high-harmonic generation (HHG). The 15th harmonic of the
laser is used within (1+1') XUV+NIR two-photon ionization, and tuned in and out
of resonance with members of the 1s$n$p $^1$P$_1$ ($n=3,4,5$) Rydberg series,
covering a broad spectral range with high spectral resolution. The technique
allows to observe characteristic rapid changes in the phase of the two-photon
matrix elements around the resonances and, previously unobserved, at the
antiresonances between the resonances. Similar effects are observed for (1+2')
XUV+NIR three-photon ionization. The experimental results are compared to a
perturbative model and to numerical solutions of the time-dependent
Schr\"odinger equation (TDSE) in the single active electron (SAE)
approximation, elucidating the origin and dependences of the observed
phenomena.
