%\begin{center}
\begin{figure}
\begin{tikzpicture}
  [scale=0.8,line width=.8pt,
    decoration={%
      markings,
      mark=at position 0.25 with {\arrow[line width=1pt]{>}},
      %mark=at position 0.5 with {\arrow[line width=1pt]{>}},    
      mark=at position 0.75 with {\arrow[line width=1pt]{>}}    
    }
  ]
  \draw [help lines,->] (-4,0) -- (4,0) coordinate (xaxis);
  \draw [help lines,->] (0,-1)coordinate(-yaxis) -- (0,4) coordinate (yaxis);
  \node  at (1.5,-0.5) {$\bullet$};
  \node  at (-1.5,-0.5) {$\bullet$};
  \draw [postaction=decorate] (3,0) arc (0:180:3) ;
  %\draw [postaction=decorate] (-3,0) --(-2,0) arc  (180:0:.5) --(1,0) arc  (180:0:.5)--(3,0) ;
  \draw [postaction=decorate] (-3,0) --(3,0) ;
  \node [below] at (xaxis) {$k_0$};
  \node [above] at (yaxis) {$ik_0'$};
  %\node [below left] {$O$};
  %\node at (-1.5,.8) {$\gamma_{1}$};
  %\node at (1.5,.8) {$\gamma_{2}$};
  %\node at (2,3) {$\Gamma_{1}$};
  %\node at (3,-.5){$R$};
  %\node at (-3,-.5){$-R$};
  \node at (1.5,-0.95){$\scriptstyle (\omega_k-i\epsilon)$};
  \node at (-1.5,-0.95){$\scriptstyle -(\omega_k+i\epsilon)$};
  \node at (2,3.2){$x_0<0$};
\end{tikzpicture}
\begin{tikzpicture}
  [scale=0.8,yscale=-1,line width=.8pt,
    decoration={%
      markings,
      mark=at position 0.25 with {\arrow[line width=1pt]{>}},
      %mark=at position 0.5 with {\arrow[line width=1pt]{>}},    
      mark=at position 0.75 with {\arrow[line width=1pt]{>}}    
    }
  ]
  \draw [help lines,->] (-4,0) -- (4,0) coordinate (xaxis);
  \draw [help lines,->] (0,-1)coordinate(-yaxis) -- (0,4) coordinate (yaxis);
  \node  at (1.5,-0.5) {$\bullet$};
  \node  at (-1.5,-0.5) {$\bullet$};
  \draw [postaction=decorate] (3,0) arc (0:180:3) ;
  %\draw [postaction=decorate] (-3,0) --(-2,0) arc  (180:0:.5) --(1,0) arc  (180:0:.5)--(3,0) ;
  \draw [postaction=decorate] (-3,0) --(3,0) ;
  \node [below] at (xaxis) {$k_0$};
  \node [above] at (-yaxis) {$ik_0'$};
  \node [below left] {$O$};
  %\node at (-1.5,.8) {$\gamma_{1}$};
  %\node at (1.5,.8) {$\gamma_{2}$};
  %\node at (2,3) {$\Gamma_{2}$};
  \node at (2,3) {$x_0>0$};
  %\node at (3,-.5){$R$};
  %\node at (-3,-.5){$-R$};
  \node at (1.5,-.95){$\scriptstyle (\omega_k+i\epsilon)$};
  \node at (-1.5,-.95){$\scriptstyle -(\omega_k-i\epsilon)$};
\end{tikzpicture}
\caption{Contornos de integración para el propagador retardado \label{contornos.propagador.retardado}}
\end{figure}
%\end{center}
