staircase

\documentclass{article}

\usepackage{geometry}

\usepackage{subcaption}
\usepackage{tikz}

\tikzset{%
    pics/staircase/.style={code={
  \draw[mydash] (-0.5*#1,-0.5*#1) -| (0.5*#1,0.5*#1);
  \foreach \XX in {1,...,#1}
   {\draw[mydash] (-0.5*#1+\XX-1,-0.5*#1+\XX-1) |- ++ (1,1);
   \ifnum\XX<#1
   \draw[mydash,red] (-0.5*#1+\XX,-0.5*#1) -- ++ (0,\XX);
   \draw[mydash,blue] (0.5*#1,0.5*#1-\XX) -- ++ (-\XX,0);
   \fi}}},mydash/.style={thick,shorten <=1mm,dash pattern=on 8mm off 2mm}
}
\newcommand{\staircase}[1]{%
    \begin{tikzpicture}
        \path (0,0) pic{staircase=#1} (0,1);
    \end{tikzpicture}
}
\begin{document}

\begin{figure}[!h]
    \centering
    \begin{subfigure}[b]{0.09\textwidth}
        \centering
        \staircase{1}
        \caption*{$F_1$}
    \end{subfigure}
    \begin{subfigure}[b]{0.19\textwidth}
        \centering
        \staircase{2}
        \caption*{$F_2$}
    \end{subfigure}
    \begin{subfigure}[b]{0.29\textwidth}
        \centering
        \staircase{3}
        \caption*{$F_3$}
    \end{subfigure}
    \begin{subfigure}[b]{0.39\textwidth}
        \centering
        \staircase{4}
        \caption*{$F_4$}
    \end{subfigure}
    \caption{}
\end{figure}

\end{document}