[1] P. B. Corkum,\u00a0Phys. Rev. Lett.\u00a071<\/strong>, 1994 (1993)<\/a>.
[2] J. Itatani\u00a0et al.<\/em>,\u00a0Nature\u00a0432<\/strong>, 867 (2004)<\/a>.
[3] N. L. Wagner\u00a0et al.<\/em>,\u00a0PNAS\u00a0103<\/strong>, 13279 (2006)<\/a>
[4] W. Li\u00a0et al.<\/em>,\u00a0Science\u00a0322<\/strong>, 1207 (2008)<\/a>
[5] H. J. W\u00f6rner\u00a0et al.<\/em>,\u00a0Nature\u00a0466<\/strong>, 604 (2010)<\/a>.
[6] H. J. W\u00f6rner\u00a0et al.<\/em>,\u00a0Science\u00a0334<\/strong>, 208 (2011)<\/a>.<\/p><\/div>