Ultrafast active plasmonics

Surface plasmon polaritons, propagating bound oscillations of electrons and light at a metal surface, have great potential as information carriers for next-generation, highly integrated nanophotonic devices1,2. Since the term ‘active plasmonics’ was coined in 20043, a number of techniques for controlling the propagation of guided surface plasmon polariton signals have been demonstrated4–7. However, with sub-microsecond or nanosecond response times at best, these techniques are likely to be too slow for future applications in such fields as data transport and processing. Here we report that femtosecond optical frequency plasmon pulses can propagate along a metal–dielectric waveguide and that they can be modulated on the femtosecond timescale by direct ultrafast optical excitation of the metal, thereby offering unprecedented terahertz modulation bandwidth—a speed at least five orders of magnitude faster than existing technologies. The term ‘active plasmonics’ was introduced in 2004 in a paper reporting the concept of using optically activated phase-change waveguide materials to control propagating surface plasmon polaritons (SPPs)3. Subsequently, reversible changes in waveguide media caused by heating4,5, optical excitation of photochromic molecules6 and interactions mediated by quantum dots7 have been applied to achieve active modulation of optical-frequency plasmonic signals. However, with switching times no shorter than a few tens of nanoseconds, these are unlikely to satisfy the demands of future chip-scale data transport and integrated nanophotonic applications. Although ultrafast dynamics have been observed for certain plasmon-dependent phenomena, including the extraordinary transmission of sub-wavelength apertures8,9, the optical absorption of colloidal metal nanoparticles10 and optically induced shifting of the Wood’s anomalies of gold gratings11, the femtosecond optical switching of a propagating SPP signal, as reported here, has not previously been demonstrated. In essence, we have discovered a nonlinear interaction between a propagating SPP and light that takes place in the skin layer of the metal surface along which the plasmon wave is propagating. A femtosecond optical pulse incident on the metal surface disturbs the equilibrium in the energy–momentum distribution of electrons, thereby influencing SPP propagation along the surface. We have demonstrated the nonlinear interaction between propagating SPP waves and light in a pump–probe experiment in which a pulsed plasmonic probe signal was generated on an aluminium/ silica interface by grating coupling from a pulsed 780-nm laser beam. After travelling 5 mm across the unstructured interface (a distance comparable to the SPP decay length), the plasmon wave was decoupled to light by another grating and subsequently detected. Optical control (pump) pulses, originating from the same laser, were incident on the waveguide region between the coupling and decoupling gratings (see Fig. 1). The transient effect of control pulse excitation on the propagation of the SPP signal was monitored by varying the time delay between the SPP excitation and optical pump pulses. It was found that an optical pump fluence of about 10 mJ cm22 leads to around 7.5% modulation of the plasmon wave intensity. The experiments used nearly transform-limited 200 fs optical pulses with a spectrum centred at 780 nm. The photon energy of the optical radiation (hv 1⁄41.59 eV) was therefore close to the interband absorption peak in aluminium (hv 1.55 eV), the metal component of the plasmon waveguide. Group velocity dispersion for the plasmonic signal is close to zero in this spectral range, and pulse broadening during propagation between the gratings is estimated to be no more than a few femtoseconds. With an electron configuration of [Ne]3s23p1, aluminium is a classic example of a free-electron metal in which the absorption spectrum is modified by interband transitions. Its optical interband absorption originates mainly from transitions between parallel bands S3 2 S1 in the vicinity of the S [110] axis, near the K point (see Fig. 2a and ref. 12). Two experimental configurations were used. In the first, the linear polarization direction of the pump field was in the plane of incidence containing the SPP propagation direction and was thus predominantly in the direction of the electron oscillations in the SPP wave (Fig. 2b). In the second configuration, the pump field polarization was perpendicular to the plane of incidence and was thus perpendicular to the electron oscillations in the plasmon wave (Fig. 2c). The experimental results are summarized in Fig. 3, which shows the effect that the pump pulses have on the amplitude of the decoupled plasmonic signal as a function of pump–probe delay SPP signal Control (pump) pulse