Gold nanowire plasmonic mode photo-thermal modulation at telecom wavelengths

International audienceThermo-optical switches relying on polymer based plasmonic waveguides have been recently demonstrated for high-bit rate routing. In these devices, the active thermo-optical medium is polymer with a large thermo-optical coefficient. However, so far the role of the thermo-optical properties of the metal sustaining the plasmon mode has not been considered in details. In this study, we investigate the thermo-optical properties of plasmonic modes sustained by different purely metallic structures at telecom wavelengths. We first consider the basic situation of a SPPs mode traveling along a simple gold/air interface. By measuring the modulation depth of the SPPs signal when photo-thermally modulated, we extract so-called thermo-plasmonic coefficients of the SPPs mode from which we compute the thermo-optical coefficients (TOCs) of gold at telecom wavelengths. Knowing these coefficients, the thermo-optical modulation depth of any plasmonic modes dominantly damped by ohmic losses can be obtained. Next, we investigate the photo-thermal modulation of gold nanowires sustaining highly confined plasmonic mode at telecom wavelengths. The nanowires are fabricated by electron-beam lithography and are optically characterized by using a fiber-to-fiber configuration. The efficiency of this fiber-to-fiber set-up for the optical characterization of nanowires with widths from 150nm to 400nm is demonstrated by a 10GHz NRZ signal transmission experiment at telecom wavelengths. Finally, the photo-thermal modulation of the plasmonic nanowire modes is investigated by illuminating nanowires with an electro-optically modulated CW visible laser. We show that for a given pumping power, the modulation depth is larger for narrower nanowires. This result arises from the larger damping and hence larger field penetration into the metal for the SPPs mode sustained by narrow nanowires. We also show that the photo-thermal modulation can be controlled by the polarization localized surface plasmon resonance if the incident field contains a component perpendicular to the longitudinal axis of the waveguide