How to escape [ and ]

Question

Consider this script

\documentclass{report}
\usepackage{graphicx, subfig}
\begin{document}
\begin{figure}
  \subfloat[$a>5$~[a.u.]]{%                                                                                                                                                                                 
    \includegraphics[width=0.45\textwidth]{example-image-a}%                                                                                                                                                
  }
  \subfloat[$a\le5$~[a.u.]]{%                                                                                                                                                                               
    \includegraphics[width=0.45\textwidth]{example-image-b}%                                                                                                                                                
    }
  \end{figure}
\end{document}

[] inside [] is causing a distortion in the portrayal of the captions of the subfloats:

What is the best way to escape [ and ]?