refractive index less than air

Hi All,
Can any dielectric material (not meta-material or left-hand materials) have refractive index between 0 and 1 or in other words less than air? And also what should be the standard range of extinction coefficient (k), if known, if I want to extract k for most general materials like teflon, glass or most common dielectrics. Lets say if I run a loop for k from 0.00001, with step 0.00001, to 1, this should cover all the possible values of k, while solving complex equations. I assume k cannot be greater than 1 from my experience.
Any expert who could answer from experience?
Many Thanks
Henry797

Materials can have anomolous properties around their resonances. I am not sure there is a standard range for k, but there are a some resources for looking up the material properties. For material databases, look under "Useful Links" here:

http://emlab.utep.edu/resources.htm

For a discussion about material resonances and anomolous properties, check out Lecture 2 here:

http://emlab.utep.edu/ee5390em21.htm

Hope this helps!