In an earlier Letter [Opt. Lett. 36, 4821 (2011)] an error was made in an equation. That error is corrected here.
© 2013 Optical Society of America
The denominator in Eq. (6) was incorrect in , and the correct equation should be
1. J. Liu, Z. F. Shao, and Q. M. Cheng, Opt. Lett. 36, 4821 (2011). [CrossRef]