  Shortcuts to isothermality provide a powerful method to speed up quasistatic
thermodynamic processes within finite-time manipulation. We employ the shortcut
strategy to design and optimize Brownian heat engines, and formulate a
geometric description of the energetics with the thermodynamic length. We
obtain a tight and reachable bound of the output power, which is reached by the
optimal protocol to vary the control parameters with a proper constant velocity
of the thermodynamic length. Our results generalize the previous optimization
in the highly underdamped and the overdamped regimes to the general-damped
situation, and are applicable for arbitrary finite-time cycles.
