We have experimentally investigated the fluorescence photon emission characteristics for single dots by using an optical nanofiber. We have demonstrated that single q-dots can be deposited systematically and reproducibly along an optical nanofiber. For single q-dots on an optical nanofiber, we measured the fluorescence photon-count rates channeled into the nanofiber guided modes and the normalized photon correlations, by varying the excitation laser intensity. We have obtained a value for the fluorescence photon channeling efficiency into the nanofiber guided modes of higher than 9.4±3%.
Chapter 4
Efficient Channeling of Fluorescence Photons into the Guided modes of Optical Nanofiber
4.1 Introduction
Efficient collection of fluorescence photons from a single emitter into a single-mode fiber is a major challenge in the context of quantum information science. For that purpose various novel techniques have been proposed so far. Examples include mi-cropillar cavities [29], photonic crystal cavities [35], solid immersion lens [37], and plasmonic metal nanowires [34]. However, in these techniques the subsequent cou-pling of fluorescence photons into a single-mode fiber may reduce the actual collection efficiency. For example, in micropillar cavity experiments channeling efficiency to a single mode fiber is of the order (14-40%) [38].
In the view of the ability to directly channel the fluorescence photons into the single-mode fiber, optical nanofibers are particularly promising. It has been theoret-ically predicted that, by positioning the emitter on the nanofiber surface, one can channel the fluorescence photons into the nanofiber-guided modes with an efficiency higher than 20% [39, 40], and moreover, fibers can be tapered adiabatically to keep
39
the light transmission into the single-mode fiber higher than 90% [41, 42].
As discussed in the Chapter 3, the measurements through the guided modes pre-sented so far were able to determine a product between the channeling efficiency (ηc) and the fluorescence quantum efficiency of single q-dot (ηq) [82, 83]. These two pa-rameters have different physical nature in the sense that, the value of ηq intrinsically depends on the q-dot environment via the total decay rate of its excited state, whileηc
is determined by the interaction between the q-dot dipole moment and the nanofiber.
However, the ηc cannot be determined without accurate information on ηq.
Therfore, in this Chapter a method is devised to determine ηc by measuring the photon-count rates through the guided and radiation modes simultaneously. For measuring various diameters of the nanofiber, a maximum channeling efficiency we obtained was 22.0 (±4.8)% at a fiber diameter of 350 nm for an emission wavelength of 780 nm. The measured results completely reproduce the theoretical predictions [39,40]
within experimental error.
4.2 Experimental procedures
Figure 4.1 shows a schematic diagram of the experimental setup. Optical nanofibers are produced by adiabatically tapering commercial single-mode optical fibers (SMF1, cutoff wavelength: 1.3µm) using a heat and pull technique [41]. The diameter of the nanofiber is measured using a SEM before and after the optical experiments. The thinnest diameter of the nanofiber is 300-400 nm, and the nanofiber diameter varies along the fiber axis by 100 nm/1 mm (data is shown in Fig.2.2). The ambiguity of the diameter measurements is estimated to be 6%. The transmission through the optical nanofiber is measured to be 90% using the SLED at a wavelength of 800 nm.
4.2. Experimental procedures 41
4.2.1 Single/Two q-dots deposition at various nanofiber di-ameters
We use core-shell type colloidal CdSeTe (ZnS) q-dots having an emission wavelength at around 792 nm [72, 73](see Appendix B). We use the subpicoliter needle dispenser to deposit q-dots on nanofibers. The details of the deposition procedure were dis-cussed in the Chapter 2. We note that the method used to deposit q-dots only on the upper surface of nanofiber. The deposition is performed for several positions along the fiber axis corresponding to a fiber diameter of 300-800 nm. The transmis-sion through the optical nanofiber is drops less than 10% after the depositions. We performed depositions for three different optical nanofiber samples, and the following measurements are performed for all the deposited positions.
4.2.2 Single/Two q-dots fluorescence photon measurements procedures
The q-dots are excited using a cw laser-diode LD at a wavelength of 640 nm. The excitation beam is focused to the nanofiber by a microscope objective lens (OL) [40X, numerical aperture (NA)= 0.6].
The fluorescence photons channeled into the guided modes, in order to guaran-tee the observation through the fundamental mode (HE11), the fiber SMF1 is fusion spliced to another single-mode fiber (SMF2, cut-off wavelength: 557 nm) at both ends marked as S1, S2 in Fig. 4.1. Each end of SMF2 is angle cleaved to avoid the effect of reflection.The fluorescence light beam from each end of SMF2 is filtered out from the scattered excitation laser light with a color glass filter FL1 (FL2) (HOYA, R72) and recoupled into a multimode fiber. At one end of the multimode fiber, fluores-cence photons are detected with a fiber-coupled avalanche-photodiode APD1 (Perkin Elmer, SPCM-AQR/FC). At the other end of multimode fiber, fluorescence emission spectrum is measured using an optical multichannel analyzer (Andor, DV420A-OE).
CCD
SMF1
OMA APD1
SS22
MMF
SS11
APD2
MMF
FL1
640 nm LD
BS FM
MMF SMF2
FC1 FL2
FL3
FC2
FC3
SMF1 SMF1
OL
SMF2 SMF1
Figure 4.1: Schematic diagram of the experimental setup. The inset shows the op-tical nanofiber and microscope system. OL, BS, FM, and FL denote objective lens, beam splitter, flipper mirror, and filter, respectively. SMF, FC, and MMF denote single-mode fiber, fiber coupler, and multimode fiber, respectively. LD, APD, and OMA denote laser diode, avalanche photodiode, and optical multichannel analyzer, respectively.
Regarding the radiation modes, the fluorescence photons are collected by OL, coupled into a multimode fiber by FC3, and detected by a fiber-coupled avalanche photodiode APD2. A set of two filters FL3 (HOYA, R70/R72) is used to reject the scattered laser light from the focus point. Characteristics of APD1 and APD2 are the same, and signals from APD1 and APD2 are accumulated and recorded using a photon-counting system (Hamamatsu, M8784). Photon-counting measurements for both guided and radiation modes and spectrum measurements are performed for each deposited position simultaneously. Additionally, we performed photon-correlation measurements through the guided modes for all deposited positions [82, 83]. We also performed photon-correlation between guided and radiation modes. All the above fluorescence measurements were performed for the three nanofiber samples keeping the excitation laser intensity at a low value of 50 W/cm2 so that the q-dots did not
4.2. Experimental procedures 43
deteriorate [81, 84].
4.2.3 Measurement of fluorescence light-transmission factors for guided and radiation modes
Figure 4.2 shows the experimental concept. The value of κg was measured to be 49.6 (±2.1)%. The measurement procedure is as follows: The SLED output was fusion spliced to SMF2 at the FL1 end, and the output power is measured at the APD1 position. The input power to the optical nanofiber was measured by cleaving the SMF1 before the tapered region. The measured value is consistent with a value calculated as a product of transmission factors of the optical nanofiber (81%), the splicing point between SMF1 and SMF2 (81%), FL2 (83%), and the coupling efficiency into the multimode fiber at FC2 (90%).
SS FL2 SLED MMF
800 nm
BS FM
FL3
MMF
APD1 APD2
SLED800 nm
a)
b)
OL
Figure 4.2: Conceptual diagram of the measurement. (a) The κg value is the fluores-cence light-transmission factor from the nanofiber region to the APD1-position. (b) The κr value is a product of transmission factors of all optical components in the path and coupling efficiency into multimode fiber.
Theκr value was obtained to be 23.5 (±1.3)% as a product of transmission factors of all optical components in the path and coupling efficiency into multimode fiber.
Transmission factors are measured for OL (74%), beam splitter (63%), flipper mirror
(83%), and FL3 (75%), using the SLED light. The coupling efficiency into the mul-timode fiber at FC3 was found to be 81% by using following procedure. First, SLED light is introduced from the LD port and is focused to the nanofiber. The scattered light from the focused spot is collected through the OL, and its power is measured both at FC3 position and at APD2 position through multimode fiber.