High-energy-density electron-positron pair plasma production and its dynamics in a thin foil illuminated by two counter-propagating laser pulses are investigated through multi-dimensional particle-in-cell simulations. We compare the production of electron-positron pairs and gamma-photons via quantum electrodynamics processes in the relativistic transparent and opaque regimes, and find that the target transparency can significantly enhance the electron-positron pair production due to the formation of stable standing wave (SW). An optimum foil density of 200 - 280 n_c (n_c is the laser critical density) is found for enhancing electron-positron pair production when laser intensity reaches a few 10e23 W/cm2. At such foil density, laser energy conversion to electron-positron pairs is approximately four times higher than at foil density of 710n_c, whereas laser energy conversion to gamma-photons keeps almost the same. Consequently, high dense electron-positron plasma with a maximum intensity above 10e20 W/cm2 is produced. Modulation dynamics of created pair plasmas is further observed when target foil becomes transparent. It is shown that stable SWs formed directly by two counter-propagating lasers, not only trap the created electron-positron pairs to their nodes, but also modulate periodically average energy and phase-space and angular distributions of trapped particles. However, similar trapping and modulation effects become obscure in the opaque regime due to the absence of stable SW field.
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