1. Consiglio Nazionale delle Ricerche (CNR)–Istituto Nazionale di Ottica and European Laboratory for Non-Linear Spectroscopy, Via Carrara 1, 50019 Sesto Fiorentino (Firenze), Italy.
2. Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78712, USA.
3. National Enterprise for nanoScience and nanoTechnology (NEST), CNR–Istituto Nanoscienze and Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy.
4. Central Research Laboratory, Hamamatsu Photonics K.K., Hamakita-ku, Hamamatsu 434-8601, Japan.