All-optical modulation by plasmonic excitation of CdSe quantum dots

Photonics is a promising candidate technology for information processing, communication and data storage. Essential building blocks, such as logic elements and modulators, have been demonstrated. However, because of weak nonlinear light –matter interactions, these components typically require high power densities and large interaction volumes, limiting their application in dense chip-based integration. A solution may be found in surface plasmon polaritons (SPPs), guided electromagnetic waves that propagate with high field confinement along a metal –dielectric interface. We demonstrate an all-optical modulator in which efficient interaction between two light beams at different wavelengths is achieved by converting them into co-propagating SPPs interacting by means of a thin layer of CdSe quantum dots (QDs). The high SPP field confinement and high QDabsorption cross-section enable optical modulation at low power densities ( 10 W cm) in micrometre-scale planar devices. Optical transmission through subwavelength apertures in metal films can be strongly influenced by the presence of nearby surface features, such as grooves or holes. Recent studies performed at a specific wavelength (852 nm) have shown that light transmission through a slit milled in an uncoated Ag film can be passively enhanced or suppressed as a result of constructive and destructive interference with an SPP launched by a nearby groove. Here we explore the possibility of launching SPPs at various wavelengths (in the range 400–1,500 nm), and demonstrate active control of light transmitted through the slit by optically modulating the amplitude of the SPP. Amplitude modulation of the SPP is enabled by excited-state carrier absorption in CdSe QDs pumped by another SPP at a different wavelength. A schematic depicting the approach is shown in Fig. 1. We coated with CdSe QDs (ref. 12) a surface-wave interferometer featuring a subwavelength-width slit in a thin metallic film on a transparent substrate, flanked by a single parallel groove etched partially through the metal, with a slit–groove distance D. Two incident beams at different wavelengths (‘signal’ and ‘control’ beams, respectively) uniformly illuminate the structure. Diffractive scattering by the groove converts the incident beams into co-propagating SPPs that interact with an active layer of QDs. The total transmitted intensity of the signal beam through the slit is the result of the interference between the incident field (E0) and the propagating SPP (ESPP) at the slit position; the resulting total transmitted intensity IT normalized to that of an isolated slit I0 is given by