  We present phenomenological analysis of the period and the phase of
oscillatory microwave photoresistance (OMP) and zero-resistance states (ZRS)
recently observed in 2D electron systems. The results demonstrate that as OMP
evolves into ZRS with increasing magnetic field, the absolute value of the
phase becomes progressively smaller, decreasing roughly as 1/B. Virtually
eliminating a phase-shift and resulting in different periodicities for the
maxima and minima, such specific dependence is supported by a simple model
based on oscillatory density of states. Finally, it follows that fine
structures first reported in Ref. 3 can be viewed as an experimental evidence
for multi-photon processes.
