Here is Mark Thoma quoting Josh Lehner:
What we see here is strong job growth at both the top and bottom ends of the wage spectrum. Yes, food preparation and personal care account for a disproportionately large share of jobs gained in recent years, but so too have business and financial services, healthcare practitioners, computer and mathematical occupations and management. Where we have seen slower growth is in the middle. The light blue bars, which I term lower middle-wage jobs account for about 40% of all occupations in 2012 yet account for just 26% of the growth. The dark blue bars, which I term upper middle-wage jobs, account for another 19% of all occupations and 0% of the growth. This, by definition, is job polarization.
There are useful pictures at the link.