Using electromagnetic simulations, researchers from the Center for Nanoscale Science and Technology have shown that fiber taper waveguides can be a very efficient tool for optical spectroscopy on individual light emitters, such as single molecules, atoms, or semiconductor quantum dots, deposited on the surface of a dielectric membrane.* Optical fiber taper waveguides, sometimes called micro- or nanofiber waveguides, are single mode optical fibers whose diameter is slowly and symmetrically reduced to a wavelength-scale minimum. CNST researchers are developing these structures for a range of light measurement solutions. In this work, the team simulated how, when placed in proximity to single emitters on a dielectric membrane, the fiber probe would form an optical waveguide supporting modes in the low refractive index gap between the fiber and the membrane. The high concentration of the optical field of these gap modes, when interacting with vertical electric dipoles, is predicted to enhance light emission rates by a factor of 20. Additionally, efficient power transfer from the gap modes into the optical fiber should improve collection efficiencies to values of more than 20%. These two factors combined should significantly improve the signal to noise ratio in light detection from single emitters compared with standard lens-based collection methods.
The strong interaction between gap modes and emitters also led the researchers to propose a new technique to performing spectroscopic measurements based on the ability of a single emitter to strongly modify light transmission through the fiber probe. In this technique, an individual emitter would be illuminated with light from a narrow bandwidth, tunable laser whose wavelength is swept with high spectral resolution. This approach should allow the determination of dipole transitions lines with considerably higher resolution than possible with a grating spectrometer, potentially leading to a better understanding of the interaction between an emitter and its surrounding environment. Calculations performed for two slightly different configurations predicted similar performance at both visible and infrared wavelengths, suggesting the technique will be widely applicable. Finally, light can be efficiently collected with a fiber taper waveguide from a wide variety of structures without complex sample preparation.
*Hybrid Gap Modes Induced by Fiber Taper Waveguides: Application in Spectroscopy of Single Solid-state Emitters Deposited on Thin Films, M. Davanco and K. Srinivasan, Optics Express 18, 10995–11007 (2010). [NIST Publication Database Entry] [Journal Web Site]