Time-resolved photoluminescence and photoluminescence excitation spectroscopy on single quantum systems

Semiconductor nanocrystals with dimensions below approximately 10 nm confine charge carriers in all three spatial directions. This size is in the order of the de Broglie wavelength, leading to quantum effects and discretized energy spectra of the electronic states (Fig. 1). Therefore, these artificial atoms are termed quantum dots (QD) [1, 2, 3, 4, 5, 6].

Figure 1: (left) TEM micrograph of a chemically-synthesized semiconductor nanocrystal. (right) Scheme of the discrete eigenenergies of a quantum dot.

The systems studied in our group are chemically synthesized CdSe/CdS quantum dots provided by the chair of Stefan Mecking in the Department of Chemistry. These nanocrystals are available in colloidal suspension.They can easily be spin-coated onto a substrate and manipulated with an atomic force microscope, e.g. to place them into photonic structures [7, 8, 9].

To protect the quantum dots against external influences like e.g. photo-oxidization, they are coated with a thick polymer shell [10] (see Figure 2).

Figure 2: (left) TEM image of a chemically-synthesized semiconductor nanocrystal with polymer shell. (right) Scheme of the electronic structure of a chemically-synthesized CdSe/CdS core/shell quantumdot with polymer shell.

The interaction of QDs with light is investigated in a micro-photoluminescence setup (Figure 3). It is a confocal microscope with sub-micrometer spatial resolution and sub-nanometer spectral resolution. For excitation, a mode locked Erbium-doped fiber laser (link Faserlasergruppe) [11, 12, 13, 14, 15, 16, 17] is used. After frequency doubling, the wavelength is tunable from 430 nm to 700 nm with a pulse duration around one picosecond.

Figure 3: (left) Photograph of the micro-photoluminescence setup. (right) Scheme of the confocal microscope with tunable Er:fiber laser source.

For high resolution spectroscopy of single nanocrystals, the colloidal suspension is highly diluted and spin-coated onto a substrate. A Hanbury-Brown and Twiss setup allows us to study the photon statistics of the emitter. Individual quantum dots can be addressed with an xy translation stage at temperatures ranging from 4 K to 300 K. High-resolution spectra are recorded with an electronically magnified CCD camera. The luminescence lifetime is determined with an avalanche photodiode. To measure lifetimes longer than 12 ns, the repetition rate of the fiber laser can be adjusted from 40 MHz down to 2.5 MHz with the help of a fiber-coupled electro-optic modulator located between the oscillator and the amplifier.

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