  The creation of attosecond pulses via laser-plasma interaction has been a
subject of great scientific interest for more than three decades. This process
is investigated by using particle-in-cell simulation with varying the plasma
and laser parameters. The steepness of the density gradient at the
plasma-vacuum interface is examined to see how this parameter affects the
high-order harmonic generations and isolated-attosecond pulse creation. The
optimal density gradient lengths $L$ are explored within the relativistic
oscillating mirror mechanism. Although the ideal gradient lengths for the full
width at half maximum and peak intensity of an isolated-attosecond pulse depend
on the driving laser intensity independently, they are both found near
$L$=0.2$\lambda$ for high laser intensities.
