Colour and spectral imaging systems typically use fi lters and glass prisms to disperse light of different wavelengths. With the miniaturization of integrated devices, current research on imaging sensors focuses on novel designs aiming at high effi ciency, low power consumption and slim dimension, which poses great challenges to the traditional colourant-based fi ltering and prismbased spectral splitting techniques. In this context, surface plasmon-based nanostructures are attractive due to their small dimensions and the ability to effi ciently manipulate light. In this article we use selective conversion between free-space waves and spatially confi ned modes in plasmonic nanoresonators formed by subwavelength metal -insulator -metal stack arrays to show that the transmission spectra through such arrays can be well controlled by using simple design rules, and high-effi ciency colour fi lters capable of transmitting arbitrary colours can be achieved. These artifi cial nanostructures provide an approach for high spatial resolution colour fi ltering and spectral imaging with extremely compact device architectures.

We report coherent optical control of a biexciton (two electron-hole pairs), confined in a single quantum dot, that shows coherent oscillations similar to the excited-state Rabi flopping in an isolated atom. The pulse control of the biexciton dynamics, combined with previously demonstrated control of the single-exciton Rabi rotation, serves as the physical basis for a two-bit conditional quantum logic gate. The truth table of the gate shows the features of an all-optical quantum gate with interacting yet distinguishable excitons as qubits. Evaluation of the fidelity yields a value of 0.7 for the gate operation. Such experimental capability is essential to a scheme for scalable quantum computation by means of the optical control of spin qubits in dots.