Single-photon sources are important elements in quantum optics and quantum information science. It is crucial that such sources be able to couple photons emitted from a single quantum emitter to a single propagating mode, preferably to the guided mode of a single-mode optical fiber, with high efficiency. Various photonic devices have been successfully demonstrated to efficiently couple photons from an emitter to a single mode of a cavity or a waveguide. However, efficient coupling of these devices to optical fibers is sometimes challenging. Here we show that up to 38% of photons from an emitter can be directly coupled to a single-mode optical fiber by utilizing the flat tip of a silica nanofiber. With the aid of a metallic mirror, the efficiency can be increased to 76%. The use of a silicon waveguide further increases the efficiency to 87%. This simple device can be applied to various quantum emitters.

We show that a hemispherically shaped tip on an air-clad optical fiber simultaneously works as a high-numerical-aperture lens and efficiently collects photons from an emitter placed near the beam waist into the fundamental guided mode. Numerical simulations show that the coupling efficiency reaches about 25%. We have constructed a confocal microscope with such a lensed fiber. The measurements are in good agreement with the numerical simulation. The monolithic structure with a high-photon-collection efficiency will provide a flexible substitute for a conventional lens system in various experiments such as single-atom trapping with a tightly focused optical trap.

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We show that the output mode of a single-mode optical fiber can be directly focused to a subwavelength waist with a finite working distance by tapering the fiber to a diameter of the order of the wavelength and terminating it with a spherically/hemispherically shaped tip. We fabricate the micro-lensed fiber and construct a probe-scanning confocal reflection microscope. Measurements on a Chrome-plated test target clearly show tip size dependence of the resolution power of the microscope. Due to their monolithic structures, these micro-lensed fibers will be flexible substitutes for conventional compound lenses in various experimental conditions such as cryogenic temperature and/or ultra-high vacuum.

We have performed numerical simulations of photon coupling devices utilizing a nanofiber tip. It is shown that proposed devices can couple up to 38% of light from a point dipole source to the fundamental guided mode of a single-mode optical fiber. These devices can be applied for various kinds of single-photon emitter, including quantum dots, neutral atoms, and defect centers in diamond crystals.