Nonlinear Excitation of Surface Plasmon Polaritons by Four-Wave Mixing Stefano Palomba and Lukas Novotny * Institute of Optics, University of Rochester, Rochester, New York, USA (Received 4 April 2008; revised manuscript received 17 June 2008; published 1 August 2008) We demonstrate nonlinear excitation of surface plasmons on a gold film by optical four-wave mixing. Two excitation beams of frequencies ! 1 and ! 2 are used in a modified Kretschmann configuration to induce a nonlinear polarization at a frequency of ! 4wm  2! 1  ! 2 , which gives rise to surface plasmon excitation at a frequency of ! 4wm . We observe a characteristic plasmon dip at the Kretschmann angle and explain its origin in terms of destructive interference. Despite a nonvanishing bulk response, surface plasmon excitation by four-wave mixing is dominated by a nonlinear surface polarization. To interpret and validate our results, we provide a comparison with second-harmonic generation. DOI: 10.1103/PhysRevLett.101.056802 PACS numbers: 73.20.Mf, 42.65.Ky, 78.47.nj The geometry-dependent excitations of a strongly coupled plasma (surface plasmons) give rise to unique optical properties, which are currently being explored for various photonic and electronic applications [1– 3]. On planar or cylindrical metal interfaces, the momentum (wave vector) of surface plasmons is larger than the mo- mentum associated with free propagating electromagnetic radiation [4,5] making it necessary to excite surface plas- mons with evanescent waves [6 – 9]. The most widely employed surface plasmon excitation scheme is the Kretschmann configuration [8]. In this configuration, sur- face plasmon excitation is associated with characteristic dips in the angular reflectivity of a laser beam incident on a metal film deposited on the surface of a prism. Nonlinear studies of surface plasmon excitation on metal films have concentrated predominantly on second-order processes, such as second-harmonic generation (SHG) and sum- frequency generation [10 –12]. The reason for using second-order processes lies in its simplicity and the ability to discriminate the bulk response from the surface response using the inversion symmetry of most metals [10,13]. For third-order processes, it can be expected that the bulk response strongly dominates over the surface response, and hence it is not a priori clear how effective third-order surface plasmon excitation is. In this Letter, we show that, despite the third-order bulk response, surface plasmons on metal films can be efficiently excited by four-wave mixing in the Kretschmann configuration. This result holds prom- ise for applications in nonlinear plasmonics, e.g., surface plasmon amplification, surface plasmon switching, and manipulation. We consider a gold film of thickness 52 nm (rough- ness 1 nm rms, prepared by e-beam evaporation on glass) excited in a modified Kretschmann configuration (cf. Fig. 1) by two incident laser beams of frequencies ! 1 and ! 2 , respectively. The metal’s third-order susceptibility  3 gives rise to a nonlinear polarization at frequency ! 4wm  2! 1  ! 2 , which leads to scattered radiation at the same frequency. The two laser beams are generated by a Ti:sapphire laser, providing pulses of duration 200 fs and wavelength  1  810 nm, and an optical parametric oscillator, providing pulses of the same duration and wave- length  2  1162 nm. Both lasers have a repetition rate of 76 MHz, and the average powers are 3 and 12 mW, re- spectively. The spot diameters at the gold surface are 1.2 and 4 m, respectively. The scattered light at the four- wave mixing frequency ! 4wm has a wavelength of  4wm  613 nm. As shown in Fig. 1, we use an oil-immersion objective of numerical aperture NA  1:3 and magnification M  40 to focus the incident laser beams on the metal surface and to collect the generated light at the four-wave mixing frequency. The different wavelengths are spectrally sepa- rated by optical filters (two bandpass filters 500 nm << 700 nm, two shortpass filters < 750 nm, a line filter   620  20 nm, and a dichroic beam splitter reflecting > 725 nm). The angular resolution of the experiments is NA k sp E 4WM Dichroic CCD Gold film beam splitter n E 1 E 2 FIG. 1 (color online). Nonlinear excitation of surface plas- mons. Two incident beams at frequencies ! 1 and ! 2 give rise to four-wave mixing at a gold film and generate an outgoing beam at frequency ! 4wm  2! 1 -! 2 , which is projected onto a CCD. The lateral CCD coordinates correspond to the transverse wave vector k x . PRL 101, 056802 (2008) PHYSICAL REVIEW LETTERS week ending 1 AUGUST 2008 0031-9007= 08=101(5)=056802(4) 056802-1  2008 The American Physical Society