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Applied Physics Letters

Decoherence of exciton complexes in single InAlAs quantum dots measured by Fourier spectroscopy

S. Adachi, N. Yatsu, R. Kaji, Shunichi Muto, H. Sasakura

Published Oct 15, 2007• 24 citations
Publisher

Abstract

We report the single-photon Fourier spectroscopy of exciton and exciton complexes in single self-assembled InAlAs∕AlGaAs quantum dots. A Michelson interferometer was inserted in the photoluminescence path for measuring the coherence length of the time-averaged emission of neutral excitons and their complexes. The effects of exchange interactions and fluctuations in the surrounding excess charges were estimated by comparing the coherence times and the excitation energies of the excitons and their complexes.

DOI
10.1063/1.2800373

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We observe single-photon interferograms for emission of a single self-assembled GaAs quantum dot. A phase stabilizer is applied to the interferometer, enabling us to trace autocorrelations of weak emissions with a photon-counting level. At low excitation, where the average number of excitons in a dot is less than one, interferography reveals a single exponential decay, reflecting excitonic decoherence. At moderately high excitation, polarization interference between the exciton-biexciton transitions is found to appear on the interferogram. The decoherence time and the binding energy of biexcitons are determined simultaneously.
We developed a high-resolution Michelson interferometer with a He–Ne two-frequency laser positioning system, and measured the coherent carrier dynamics of a single InAs self-assembled quantum dot (SAQD) using a micro-spectroscopy system. The phase-locked double pulses were stabilized, with the maximum deviation being below 10 nm during the long measurement time of 1 h. Using this system, coherent control of an exciton in an InAs SAQD with very fine phase stabilization was demonstrated. The dephasing time of the single quantum dots was 9.5 ps which is close to that estimated from the homogeneous linewidth in the photoluminescence excitation (PLE) spectrum.
We report photon correlation measurements that reveal unique signatures of biexciton and charged exciton emissions in a single self-assembled InAs quantum dot. Biexciton emission exhibits both bunching and antibunching under continuous-wave excitation and only antibunching under pulsed excitation. Cross correlation between biexciton and single-exciton emissions reveals highly asymmetric features due to cascaded emission. Under our excitation conditions, no polarization correlations between the exciton and biexciton emissions were observed.
We measured dephasing times in InGaAlAs self-assembled quantum dots at low temperature using degenerate four-wave mixing. At 0 K, the coherence time of the quantum dots is lifetime limited, whereas at finite temperatures pure dephasing by exciton-phonon interactions governs the quantum dot coherence. The inferred homogeneous line widths are significantly smaller than the line widths usually observed in the photoluminescence from single quantum dots indicating an additional inhomogeneous broadening mechanism in the latter.
We investigate charged and neutral exciton complexes confined in a single self-assembled InAs/InGaAlAs/InP quantum dash emitting at 1.55 μm. The emission characteristics have been probed by measuring high-spatial-resolution polarization-resolved photoluminescence and cross-correlations of photon emission statistics at T = 5 K. The photon auto-correlation histogram of the emission from both the neutral and charged exciton indicates a clear antibunching dip with as-measured g(2)(0) values of 0.18 and 0.31, respectively. It proves that these exciton complexes confined in single quantum dashes of InP-based material system can act as true single photon emitters being compatible with standard long-distance fiber communication technology.