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Applied Physics Letters

Single-photon emission of CdSe quantum dots at temperatures up to 200 K

Kathrin Sebald, Peter Michler, T. Passow, D. Hommel, Gerd Bacher, A. Forchel

Published Oct 10, 2002• 194 citations
Publisher

Abstract

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.

DOI
10.1063/1.1515364

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Applied Physics Letters•Feb 12, 2004•48 citations
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.
Abstract We report on the generation of triggered single photons and photon pairs relying on pulsed optical excitation of epitaxially grown self‐assembled CdSe quantum dots (QDs). Single‐photon emission is studied on individual CdSe/Zn(S,Se) QDs for temperatures up to 200 K. At low temperatures nearly perfect single‐photon emission is achieved whereas at higher temperatures an increasing multi‐photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring QDs is observed. The multi‐photon emission probability of a bare QD (background subtracted) is strongly suppressed. Furthermore, the polarization cross correlation of the biexciton–exciton cascade has been investigated in a CdSe/ZnSe QD at low temperatures. A strong polarization correlation of the emitted photon pairs of ∼74% is observed in a linear detection basis.
Quantum information processing demands efficient quantum light sources (QLS) capable of producing high-fidelity single photons or entangled photon pairs. Single epitaxial quantum dots (QDs) have long been proven to be efficient sources of deterministic single photons; however, their production via molecular-beam epitaxy presents scalability challenges. Conversely, colloidal semiconductor QDs offer scalable solution processing and tunable photoluminescence, but suffer from broader linewidths and unstable emissions. This leads to spectrally inseparable emission from exciton (X) and biexciton (XX) states, complicating the production of single photons and triggered photon pairs. Here, we demonstrate that colloidal semiconductor quantum shells (QSs) achieve significant spectral separation (∼75-80 meV) and long temporal stability of X and XX emissive states, enabling the observation of exciton-biexciton bunching in colloidal QDs. Our low-temperature single-particle measurements show cascaded XX-X emission of single photon pairs for over 200 s, with minimal overlap between X and XX features. The X-XX distinguishability allows for an in-depth theoretical characterization of cross-correlation strength, placing it in perspective with photon pairs of epitaxial counterparts. These findings highlight a strong potential of semiconductor quantum shells for applications in quantum information processing.
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.
We report on the single photon emission from single InAs/GaAs self-assembled Stranski–Krastanow quantum dots up to 80 K under pulsed and continuous wave excitations. At temperature 80 K, the second-order correlation function at zero time delay, g (2) (0), is measured to be 0.422 for pulsed excitation. At the same temperature under continuous wave excitation, the photon antibunching effect is observed. Thus, our experimental results demonstrate a promising potential application of self-assembled InAs/GaAs quantum dots in single photon emission at liquid nitrogen temperature.
We report the observation of photon antibunching from a single, self-assembled InGaAs/GaAs quantum dot at temperatures up to 135 K. The second-order intensity correlation, g(2)(0), is measured to be less than 0.260 for temperatures up to 100 K. At 120 K, g(2)(0) increases to about 0.471, which is slightly less than the second-order intensity correlation expected from two independent single emitters. At 135 K, g(2)(0) is 0.667, which still indicates nonclassical light emission that is equivalent to having three independent single emitters.