Single quantum dot spectroscopy using a fiber taper waveguide near-field optic

Abstract

Photoluminescence spectroscopy of single InAs quantum dots at cryogenic temperatures ͑ϳ14 K͒ is performed using a micron-scale optical fiber taper waveguide as a near-field optic. A lower bound on the measured collection efficiency of quantum dot spontaneous emission into the fundamental guided mode of the fiber taper is estimated at 0.1%, and spatially resolved measurements with ϳ600 nm resolution are obtained by varying the taper position with respect to the sample and using the fiber taper for both the pump and collection channels. 4 have been implemented to improve the efficiency of light collection, while near-field scanning optical microscopy ͑NSOM͒ has been used to achieve sub-100 nm spatial resolution. 5 In this letter, we examine the use of optical fiber taper waveguides as a near-field optic for performing single QD spectroscopy. These micron-scale silica waveguides have been used in many studies of optical microcavities, beginning as an efficient coupler to silica microspheres. 6 More recently, we have shown that they can effectively probe the spatial and spectral properties of small mode volume ͑V eff ͒, high refractive index semiconductor cavities. 7 Other researchers have proposed 8,9 and realized their use as a collection tool for spontaneous emission from atomic vapors. 10 Here, we show that a fiber taper may be used to channel emission from single selfassembled QDs embedded in a semiconductor slab directly into a standard single-mode fiber with high efficiency ͑ϳ0.1% ͒, and to provide submicron spatial resolution of QDs.The QDs we study consist of a single layer of InAs QDs embedded in an In 0.15 Ga 0.85 As quantum well, a so-called dotin-a-well ͑DWELL͒ structure. 11 The DWELL layer is grown in the center of a GaAs waveguide ͑total waveguide thickness of 256 nm͒, which sits atop a 1.5 m thick Al 0.7 Ga 0.3 As buffer layer. The resulting peak of the ground state emission of the ensemble of QDs is located at = 1.35 m at room temperature. To limit the number of optically pumped QDs, microdisk cavities of diameter D =2 m were fabricated using electron beam lithography and a series of dry and wet etching steps. 12 Although the QDs physically reside in a microcavity, they are nonresonant with the cavity whispering gallery modes ͑WGMs͒. In other words, our primary interest here is general single QD spectroscopy through the fiber taper, without enhancement through interaction with the high quality factor ͑Q͒ microdisk WGMs. The samples were mounted in a continuous-flow liquid He cryostat that has been modified to allow sample probing with optical fiber tapers while being held at cryogenic temperatures ͑T ϳ 14 K͒, as described in detail in Ref. 13. The cryostat setup provides any combination of free-space and fiber taper pumping and collection ͓see Fig. 1͑a͒ for details͔.The inset of Fig. 1͑b͒ shows the emission spectrum from an ensemble of QDs in one of the microdisks. Here, the device is optically pumped through an objective lens at normal incidence ͑free-space pumping͒, with a spot size of 3 m and wa...

Q-factor limits for far-field detection of whispering gallery modes in active microspheres

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Efficient Single-Mode Photon-Coupling Device Utilizing a Nanofiber Tip

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Highly Efficient Coupling of Photons from Nanoemitters into Single-Mode Optical Fibers

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