Acoustic waves from mechanical impulses due to fluorescence resonant energy (Förster) transfer

Blowing a whistle with light

¹ Instituto Nacional de Astrofísica, Óptica y Electrónica - Apdo. Postal 51 y 216, Puebla, Pue. 72000, Mexico
² Institut für Physik und Astronomie, Universität Potsdam - Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany, EU

Received: 27 October 2011
Accepted: 11 January 2012

Abstract

We present a momentum transfer mechanism mediated by electromagnetic fields that originates in a system of two nearby molecules: one excited (donor D^*) and the other in ground state (acceptor A). An intermolecular force related to fluorescence resonant energy or Förster transfer (FRET) arises in the unstable D^*A molecular system, which differs from the equilibrium van der Waals interaction. Due to the its finite lifetime, a mechanical impulse is imparted to the relative motion in the system. We analyze the FRET impulse when the molecules are embedded in free space and find that its magnitude can be much greater than the single recoil photon momentum, getting comparable with the thermal momentum (Maxwell-Boltzmann distribution) at room temperature. In addition, we propose that this FRET impulse can be exploited in the generation of acoustic waves inside a film containing layers of donor and acceptor molecules, when a picosecond laser pulse excites the donors. This acoustic transient is distinguishable from that produced by thermal stress due to laser absorption, and may therefore play a role in photoacoustic spectroscopy. The effect can be seen as exciting a vibrating system like a string or organ pipe with light; it may be used as an opto-mechanical transducer.