1. Institute for Lasers, Photonics and Biophotonics, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, USA

Correspondence to: Paras N. Prasad Correspondence and requests for materials should be addressed to P.N.P. (e-mail: Email: pnprasad@acsu.buffalo.edu).

Abstract

Multiphoton processes, predicted¹ theoretically in 1931, were for a long time considered to be mainly of academic interest. This view changed when it was shown^{2, 3} that a two-photon absorption process could, because of a quadratic dependence of excitation on intensity, produce a spatially confined excitation useful for three-dimensional data storage and imaging. Two-photon absorption has received considerable attention recently because of the development of highly efficient two-photon-sensitive materials, leading to numerous technological applications^{4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28}. These successes have created interest in exploring applications based on three-photon excitations²⁹. For a three-photon process, a longer excitation wavelength such as those common in optical communications can be used. Also, the cubic dependence of the three-photon process on the input light intensity provides a stronger spatial confinement, so that a higher contrast in imaging can be obtained. Here we report the observation of a highly directional and up-converted stimulated emission as an amplified spontaneous emission, produced in an organic chromophore solution by a strong simultaneous three-photon absorption at 1.3 m. This achievement suggests opportunities for a three-photon process in frequency-upconversion lasing, short-pulse optical communications, and the emerging field of biophotonics.