AbstractDue to the non-linear nature of relativistic laser induced plasma processes, the development of laser-plasma accelerators requires precise numerical modeling. Especially high intensity laser-solid interactions are sensitive to the temporal laser rising edge and the predictive capability of simulations suffers from incomplete information on the plasma state at the onset of the relativistic interaction. Experimental diagnostics utilizing ultra-fast optical backlighters can help to ease this challenge by providing temporally resolved inside into the plasma density evolution. We present the successful implementation of an off-harmonic optical probe laser setup to investigate the interaction of a high-intensity laser at $5.4\times 10^{21}\,\hbox {W/cm}^{2}$ 5.4 × 10 21 W/cm 2 peak intensity with a solid-density cylindrical cryogenic hydrogen jet target of ${5}\,{\upmu }\mathrm{m}$ 5 μ m diameter as a target test bed. The temporal synchronization of pump and probe laser, spectral filtering and spectrally resolved data of the parasitic plasma self-emission are discussed. The probing technique mitigates detector saturation by self-emission and allowed to record a temporal scan of shadowgraphy data revealing details of the target ionization and expansion dynamics that were so far not accessible for the given laser intensity. Plasma expansion speeds of up to $(2.3 ± 0.4)\times 10^{7}\,\hbox {m/s}$ ( 2.3 ± 0.4 ) × 10 7 m/s followed by full target transparency at ${1.4}\,{\mathrm{ps}}$ 1.4 ps after the high intensity laser peak are observed. A three dimensional particle-in-cell simulation initiated with the diagnosed target pre-expansion at ${-0.2}\,{\mathrm{ps}}$ - 0.2 ps and post processed by ray tracing simulations supports the experimental observations and demonstrates the capability of time resolved optical diagnostics to provide quantitative input and feedback to the numerical treatment within the time frame of the relativistic laser-plasma interaction.