1. Institute for Terahertz Science and Technology, University of California, Santa Barbara, Santa Barbara, California 93106, USA
2. Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA
3. Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA
4. Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, USA