Efficient Energy Transfer between Laser Beams by Stimulated Raman Scattering Youbo Zhao, Tana E. Witt, and Robert J. Gordon Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60680-7061, USA (Received 2 May 2009; published 23 October 2009) Efficient energy transfer between two ultrafast laser beams is reported. Energy transfer occurs when linearly polarized donor and acceptor beams are focused in air and intersect at an acute angle. This effect is attributed to plasma-mediated forward stimulated Raman scattering, facilitated by supercontinuum generation. Donor depletion as high as 57% is observed, with quantitative energy transfer from the donor to the acceptor beam. Amplification of the acceptor depends on the polarization directions of the two pulses and the delay between them. Interaction between the beams results also in compression and spectral broadening of the acceptor pulse. DOI: 10.1103/PhysRevLett.103.173903 PACS numbers: 42.65.Dr, 42.79.Ta, 52.38.-r All-optical control of light propagation is of broad in- terest in science and engineering, with application to areas such as photonic computing, optical communication, ultra- fast spectroscopy, and laser amplification [1–7]. An ex- ample is the optical Kerr effect [8], in which a pulse of light induces birefringence in a medium, thereby allowing a linearly polarized light beam to pass through a pair of crossed polarizers. Electromagnetically induced transpar- ency [9] occurs when quantum interference induced by a control beam reduces the absorption of light tuned to a resonance frequency of the medium. The group velocity of light may be slowed by using a control beam to induce a steep change in the refractive index of the medium [10]. Four wave mixing occurs when a pair of light beams create an optical grating, which scatters a third beam [11]. In all these examples, one or more light beams modify the optical properties of a medium so as to control the propagation of an independent probe beam. It is also possible to take a more active approach in which one beam transfers a substantial fraction of its energy to a second beam. Many studies have been performed using backwards stimulated Raman scattering (SRS), in which two beams counterpropagate over a distance of several mm through a preformed plasma [12]. Much higher transfer efficiencies of low energy pulses have been achieved in fibers on a length scale of meters [13]. In another two-beam coupling scheme, a pair of laser pulses intersect at a graz- ing angle to produce a dynamic phase grating in the resulting filaments, which promotes energy transfer be- tween the beams [14]. Here we report a simple and robust method of trans- ferring most of the energy from one laser to another on a fs time scale over a distance of tens of m by simply focus- ing and intersecting them in air. Two linearly polarized Ti:sapphire laser beams (790 nm, 50 fs) intersect at an angle  [Fig. 1(a)]. Near their focal points the intensity of each is sufficient to ionize the air, producing the trails shown in Fig. 1(b). If the beams intersect without over- lapping temporally, they simply pass through each other. If the pulses are timed, however, to collide at the crossing point, a large fraction of the energy from the one pulse (the ‘‘donor’’) is transferred to the other (‘‘acceptor’’) pulse. The crossing point lies before the focus of the donor and after the focus of the acceptor laser. The donor and accep- tor beams are focused with plano-convex lenses having focal lengths of 150 and 100 mm, producing 1=e 2 beam waists of 28 and 20 m and Rayleigh lengths of 920 and 420 m, respectively. The images in Fig. 1 were collected with a 4x objective and detected with a CCD camera. The energy transfer efficiency was found to depend on the intensities of the lasers, the location, angle, and timing of their intersection, and their polarizations. As shown in Fig. 2(a), the output energy of the acceptor pulse, E a;o , increases with its initial energy, E a;i , whereas the gain ratio G (defined as the ratio of E a;o with and without the donor pulse present) decreases with E a;i . As shown in the inset of Fig. 2(a), G falls off at low values of E a;i , as must be the case near the threshold for air breakdown. The decline in G FIG. 1 (color online). Experimental setup (a) and images of the plasmas produced by the intersecting lasers, with (b) 100 fs and (c) zero delay between the pulses. Apparatus components in- clude lenses (L1, L2), half wave plates (HW1, HW2), beam splitters (BS1, BS2), delay lines (DL1, DL2), a neutral density filter (ND), a power meter (PM) and spectrograph (SM). PRL 103, 173903 (2009) PHYSICAL REVIEW LETTERS week ending 23 OCTOBER 2009 0031-9007= 09=103(17)=173903(4) 173903-1 Ó 2009 The American Physical Society