NANOPHOTONIC STRUCTURES FOR EFFICIENT CHARACTERIZATION OF SINGLE InAs QUANTUM DOTS
Marcelo Davanço, Matthew Rakher, Antonio Badolato and Kartik Srinivasan
Nanometer scale photonic structures are capable of producing a variety of unique optical effects that can greatly benefit applications in a wide range of scientific fields. Wavelength-scale light confinement, strong light-matter interaction, photonic bandgaps, and enhanced optical nonlinearities are only a few of the many possibilities currently being investigated for applications in optical communications, classical and quantum optical signal processing, wavelength conversion, spectroscopy of single emitters, etc.
We report on two nanophotonic structures designed to provide highly efficient photoluminescence collection for single, epitaxially grown InAs quantum dots. Quantum dots are nanometer-scale structures that provide three-dimensional charge confinement, being thus capable of supporting discrete excitonic states. With bright, discrete, potentially tunable transitions and relatively long dephasing times, these nanostructures are very attractive for quantum optics applications. The two nanophotonic structures to be discussed may benefit not only spectroscopic studies of isolated dots, but also the applications of such emitters as, e.g., efficient single photon sources or quantum gates.
The first structure consists of a phase-matched directional coupler formed by a suspended, ~100 nm scale GaAs waveguide containing InAs quantum dots, and a micron-scale diameter optical fiber. Detailed electromagnetic simulations predict collection efficiencies of ≈35 % into the optical fiber, with operation bandwidths of several tens of nm. Fabricated devices demonstrated photon collection, into an optical fiber, 10 to 100 times larger than typically obtained with free-space methods, over > 40 nm bandwidths. A maximum efficiency of ≈7 % was observed. The enhanced collection allowed low noise measurements of excited state lifetime and second-order correlation, which revealed evidence of non-ideal quantum dot operation. The coupler structure also offers the possibility of performing coherent, resonant, single dot spectroscopy.
The second structure consists of a suspended, circular GaAs grating cavity with embedded quantum dots, which was designed to maximize vertical extraction of photoluminescence while maintaining a near-Gaussian far-field pattern. Fabricated devices yielded collection of photoluminescence from single InAs quantum dots superior, by at least 20 times, to that obtained from dots emitting in bulk GaAs. Excited state lifetimes at least three times shorter than typically observed in InAs dots were measured, giving indication of Purcell radiative rate enhancement by the circular grating cavity.