  We investigate high-order harmonics spectra radiated from a two-level model
system driven by strong, ultrabroadband half- and single-cycle pulses, which
are shorter than the inverse of the transition frequency. In this driving
regime, the plateau in frequency spectra typical for radiation from strongly
driven systems, has noticeable modulation in amplitude due to interference
between waves of a same frequency and emitted at different time instants.
Specifically, there is a characteristic `dips' structure at a set of
frequencies in the radiation spectra, where the corresponding amplitudes are
suppressed by several orders of magnitude. Understanding of this structure is
required for applications such as generation of attosecond pulse, where number
of composing modes and their relative phases are important. Therefore, we
demonstrate a systematic way to find frequencies at which the dips are formed.
To further illustrate the interference mechanism, we extract the phase
information with the help of time-frequency distribution functions, namely the
Husimi and Wigner functions. Especially, we found that the negativity structure
of the Wigner function corresponds to each dip frequency and that the
information regarding the type of interference is encoded in the pattern of the
negative region of the Wigner function. Since such time-frequency Wigner
function can actually be measured, we envisage utilizing its negativity
structure to extract the phase information between radiation components emitted
at time points within a subcycle time scale. This should provide an efficient
tool for understanding and designing photonic applications, including
short-wavelength coherent light sources.
