Broadband Detection of Two-Photon Absorption by Intracavity Dye Laser Spectroscopy

Abstract

Traditionally, two-photon transitions have been observed by excitation spectroscopy. However, since these techniques exploit single photon fluorescence to signal the two-photon absorption, they are not especially general. In the work reported here, direct observation of a two-photon transition, in absorption, has been achieved using intracavity dye laser spectroscopy (IDLS). By placing Na inside the cavity of a broad-band probe laser and injecting a second pump photon from a narrowed dye laser, the Na 3S ²S_1/2 → 5S ²S_1/2 two-photon transition was observed. The wavelength of this absorption varied with the changing wavelength of the pump laser such that the sum of the energy of the induced absorption feature and of the pump photon was always equal to that of the 3S → 5S transition. Although the pump laser wavelength was chosen so as to resonantly enhance the two-photon cross-section, the two-photon absorption feature was observed even with detunings as large as 40 cm^-1. Since the probe laser bandwidth is typically > 10 nm, and since a high-resolution spectrum can be recorded in a single, sub-microsecond flash, this technique may be uniquely suited to the study of two-photon absorption in molecules, and perhaps even in transient species.

Work supported in part by the U. S. Air Force Office of Scientific Research, (Grant #74–2616-ABC).

J. Robert Oppenheimer Research Fellow. Work performed under the auspices of the U. S. Department of Energy.