Hi Li:

You can calculate the penetration depth d of light of wavelength lambda as
follows:

d = lamdba/(4*Pi*k)

with k the absorption constant. d describes the penetration depth of the light
intensity. You can find k in some literature. I used the following reference:

J.H. Weaver, C. Krafka, D.W. Lynch, E.E. Koch, Optical properties of metals,
Physics Data, Fachinformationszentrum, Karlsruhe, Germany, 18, 1-2, 1981.

But I guess this book is hard to get and I don't have a copy of it anymore...

Remember that you can calculate k also from the dielectric constant e1+I*e2 (I =
imaginary number) of Al:

n+I*k = Squareroot[e1+I*e2]

Hope this helps,
Daniel