CNST researchers have designed a nanophotonic interface to very efficiently collect light emitted from single semiconductor quantum dots, providing an avenue for high resolution measurements needed for the development of future generations of classical and quantum information processing devices.* The system consists of a single quantum dot embedded in a GaAs channel waveguide with cross-sectional dimensions of about 250 nm x 250 nm. The channel waveguide forms one half of a directional waveguide coupler, with the second half formed by an optical fiber taper waveguide. The taper waveguide is fabricated from a standard 125 µm-diameter, single mode optical fiber that is gradually tapered down to a diameter of about 1 µm along an approximately 1 cm-long section. Through a detailed characterization of the device's electromagnetic properties, the CNST researchers have shown that this structure can be used to perform highly efficient fluorescence spectroscopy and resonant scattering measurements of a single quantum dot. The key innovation in the system is that the directional waveguide coupler provides an almost ideal mechanism for taking light from a single mode optical fiber and transforming it into a tightly confined mode within the GaAs waveguide, where it can interact with the quantum dot before transforming back into a mode of the fiber. The net result is a device in which a single semiconductor quantum dot, a structure with a characteristic length scale of 10 nm, can be effectively connected to macroscopic fiber optics that are an integral component of modern communications.
*Fiber-coupled Semiconductor Waveguides as an Efficient Optical Interface to a Single Quantum Dipole, Marcelo Davanço and Kartik Srinivasan, Optics Letters 34, 2542–2544 (2009).