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Journal of Physics Conference Series

Emission characteristics for a single CdSe quantum dot on an optical nanofiber at cryogenic temperatures

K. Muhammed Shafi, Wei Luo, Ramachandrarao Yalla, K. Hakuta

Published May 1, 2019• 1 citation
Publisher

Abstract

Abstract We investigate the emission characteristics of a single CdSe quantum dot on an optical nanofiber at cryogenic temperatures from the viewpoint of quantum photonics. We show that the charged exciton (trion) of the quantum dot is a promising quantum emitter for both aspects of spectral and temporal characteristics. The nanofiber/quantum-dot system may give a promising work bench for the future quantum photonic network.

DOI
10.1088/1742-6596/1220/1/012025

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Abstract We investigate the emission characteristics of a single CdSe quantum dot on an optical nanofiber at cryogenic temperatures from the viewpoint of quantum photonics. We show that the charged exciton (trion) of the quantum dot is a promising quantum emitter for both aspects of spectral and temporal characteristics. The nanofiber/quantum-dot system may give a promising work bench for the future quantum photonic network.
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We experimentally investigate the fluorescence photon emission characteristics for single q-dots by using optical nanofibers. We demonstrate that single q-dots can be deposited along an optical nanofiber systematically and reproducibly with a precision of 5 μm. For single q-dots on an optical nanofiber, we measure the fluorescence photon numbers coupled into the nanofiber and the normalized photon correlations, by varying the excitation laser intensity. We estimate the fluorescence photon coupling efficiency into the nanofiber guided modes.
We report on the generation of triggered single photons obtained from epitaxially grown self-assembled CdSe/Zn(S,Se) quantum dots for temperatures up to 200 K. At low temperatures (T<40 K) photon correlation experiments under pulsed excitation show nearly perfect single-photon emission from an individual CdSe quantum dot. For higher temperatures (T>40 K) an increasing multi-photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring quantum dots is observed. We found that the multi-photon emission probability of a bare quantum dot (background subtracted) is strongly suppressed at 200 K if compared to a Poissonian light source of the same average intensity. Our results demonstrate the large potential of self-assembled CdSe/Zn(S,Se) quantum dots for nonclassical light generation at temperatures up to 200 K.
Single-photon sources (SPS) are essential for secure quantum communication and quantum computing. Solid-state systems based on quantum dots (QDs) are particularly attractive for emitting pure, on-demand single photons, making them ideal candidates for flying qubits in quantum key distribution protocols. Integrating a SPS into photonic circuits is a key step toward scalable and "plug-and-play" quantum technologies. However, the SPS with the best performances (III-V QDs) operate only at cryogenic temperatures, a significant drawback for practical implementation. Alternatively, CdSe QDs (II-VI) are operational at 300 K and emit in the blue-green range, thus allowing for free-space long-distance communication in seawater and air.The thesis aims to develop an on-chip single-photon source that can operate at non-cryogenic temperatures. The SPS consists of a CdSe QD embedded within a ZnSe nanowire (NW) shell grown by molecular beam epitaxy on a patterned GaAs (111)B substrate. The NW geometry enhances light extraction by acting as a single-mode waveguide, while the conical ending of the shell improves emission control.We investigated the optical performance of the as-grown QD-NW emitter at room temperature, achieving a promising brightness of 0.17 photons per pulse and antibunching behavior with g(2)(0) < 0.3 within a reduced spectral window. However, the emission spectrum at 300 K features overlapping broad lines, leading to a trade-off between brightness and purity. At 6 K, we used a broadband filter to capture entirely the narrow exciton line and measured directly the photon count rate with an avalanche photodiode. In addition, we observed strong antibunching with a few percent purity through cathodoluminescence.Phonons impact the performance of the SPS at high temperatures, where broad lines overlap, and at cryogenic temperatures, where they reduce the photon indistinguishability. Understanding and controlling the influence of phonons has increased the need for high-accuracy temperature measurements at cryogenic temperatures. We present a calibration-free method to extract the local temperature from the emission spectrum, which we used to investigate the heating effects induced by non-resonant excitation. Additionally, we analysed the line-broadening mechanisms from cryogenic to room temperature. An extension of the Huang-Rhys model is developed to analytically describe the acoustic exciton-phonon interactions, accounting for multi-phonon processes. The model accurately fits emission spectra from 6 K to 300 K using parameters obtained from low-temperature data.The tapered nanowire shape is also an advantage for integration into a photonic device, enabling efficient evanescent coupling when the emitter is positioned next to a waveguide. The direct transfer using micromanipulators in a scanning electron microscope onto pre-fabricated waveguides is very challenging due to the tiny size of the waveguide at an emission wavelength of 550 nm. We developed a new approach, where the emitter is deposited within a large zone and then embedded within a resist, which serves as a mask during waveguide etching. This method significantly reduces the need for micro-manipulation and allows for promising prospects. We used finite-difference time-domain simulations to support the design and fabrication of the waveguides for optimal coupling.
This thesis presents single photon generation and spectroscopy using quantum dots (q-dots) on optical nanofibers. Optical nanofibers are essentially vacuum-clad silica fibers with sub-wavelength diameter, which can support only the propagation of the fundamental mode through the guided mode. Optical nanofibers are realized using adiabatically tapered single mode optical fibers. Experiments were performed by depositing colloidal core-shell type CdSeTe (ZnS) q-dots on the optical nanofiber surface. A method is developed to deposit single q-dots systematically and reproducibly along the optical nanofiber. In the view point of investigation of fluorescence photon emission characteristics for single q-dots on optical nanofibers, the absolute fluorescence photon-count rates channeled into the nanofiber guided modes and the normalized photon correlations at various excitation laser intensities are measured. The results are able to obtain the product between the fluorescence photon channeling efficiency and the quantum efficiency of the single q-dots. However, the channeling efficiency cannot be determined without accurate information on the quantum efficiency of the single q-dots. Therefore, a method is devised to determine the channeling of fluorescence photons from single q-dots into the guided modes by measuring the fluorescence photon-count rates through the guided and radiation modes simultaneously. By measuring for various nanofiber diameters, a maximum channeling efficiency obtained to be 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 within the experimental error. We demonstrate in situ measurements of fluorescence quantum efficiency of single q-dots using the present nanofiber technique. We determine fluorescence quantum efficiency of single q-dots accurately by assuming the theoretical values for the channeling efficiency from the product of the channeling efficiency and the quantum efficiency. We demonstrate that one can pick up one q-dot on the nanofiber with fluorescence quantum efficiency higher than the average value measured for an ensemble. Thus, the optical nanofiber method offers a promising way for generating and manipulating single photons. This work should provide a basis for extending the optical nanofiber method to a general technique in quantum optics, which may open new possibilities in quantum information technologies.
We demonstrate cavity QED conditions in the Purcell regime for single quantum emitters on the surface of an optical nanofiber. The cavity is formed by combining an optical nanofiber and a nanofabricated grating to create a composite photonic crystal cavity. By using this technique, significant enhancement of the spontaneous emission rate into the nanofiber guided modes is observed for single quantum dots. Our results pave the way for enhanced on-fiber light-matter interfaces with clear applications to quantum networks.