Semiconductor quantum dots (QDs) can be used as nonclassical light sources with applications in next-generation quantum communication
and computing. Electrically pumped QDs are all solid-state sources of single photons that are not limited by Poisson statistics
(1). Recent work by the group of Dieter Bimberg at Technische Universitšt-Berlin has demonstrated the selective pumping of
a single InAs QD with emission from a single exciton (1–3).
Measurement of electro- and photoluminescent signals from nanoscale structures requires both highly sensitive detectors and
excellent spectral resolution. Using the Princeton Instruments TriVista triple monochromator and Spec-10 CCD camera, the researchers
measured the fine structure splitting of the QD exciton emission line at 952 nm to a resolution of 55 μeV.
The single QD emitter device consisted of a pin diode, where InAs QDs are in contact with p and n-type GaAs junctions. An 850 nm AlGaO current aperture restricted electrical
pumping to a single QD (2). The device was maintained at 10 K by an He cryostat. An XY mirror scanner offered precise spatial
control. Electroluminescence was induced using 870 pA of current, while photoluminescence was induced by a Nd:YAG laser operating
at 532 nm. 10 nW of laser light was focused on the sample to a 1 μm spot with a 100X N.A. 0.9 IR enhanced microscope objective.
Light emission was collected and dispersed by a TriVista 500 mm triple monochromator operating in additive mode, with 600
groove/mm gratings in the first two stages and a 1200 groove/mm grating in the final stage, providing a nominal resolution
of 0.025 nm per pixel. The detector was an LN cooled Spec10:100 BR CCD camera, with an active area of 1340 ◊ 100 20 μm square
pixels. A Hanbury-Brown-Twiss interferometer was coupled to a second exit port of the first stage, allowing the setup to be
switched over to perform photon correlation antibunching measurements with a single mouse click.
Photoluminescence measurements showed numerous sharp lines in the region between 900 and 960 nm, arising from both excitonic
and biexcitonic states of different QDs. In contrast, electrical pumping at currents below 1 nA produced only a single line
at 952 nm, with no bulk emission. At higher currents, additional lines assigned to biexcitons and trions appeared. The 952
nm transition was assigned to a single exciton based on the fine structure splitting (FSS). The confinement symmetry causes
the excitonic states of these QDs to be split into different levels with orthogonal polarizations parallel to the (110) and
(110) crystal planes (3). The FSS was found to be independent of bias voltage over the range tested (Figure 1). The magnitude
of the FSS was 55 μeV, corresponding to 0.44 cm–1 or 0.040 nm at the emission wavelength. The spectra shown were obtained with a 1 s acquisition time.
Figure 1: Electroluminescence spectra of a single QD, showing that the fine structure splitting is insensitive to bias voltage.
Normally, a polarizer is used to detect the emission FSS, but the high resolution of the TriVista allowed the FSS to be detected
directly. The magnitude of the FSS depends on the size and shape of the QD structures (3). When the FSS is sufficiently small,
the exciton and biexciton emission can couple to produce an entangled photon pair source for quantum computing applications
(2). These experiments demonstrate the fabrication of a simple, solid-state single-photon or entangled pair emission device
with outstanding spectral purity.
We thank Erik Stock and Dieter Bimberg for their kind assistance with this note. All data and images are copyright Institute
of Semiconductor Physics, TU-Berlin.
(1) M. Schotz, et al., Optics Express
15, 9107–9112 (2007).
(2) A. Lochmann, et al., Electron. Lett. 42, 774–775 (2006).
(3) R. Seguin, et al., Phys. Rev. Lett. 95, 257402 (2005).
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