Ultrafast carrier dynamics in InGaAs/GaAs quantum dots, observed by THz spectroscopy

Prof. Dr. Dmitry Turchinovich

Ultrafast THz pulses are efficient ultrafast conductivity probes. Carriers confined in the QDs do not contribute to the conductivity. Only the free carriers are sensed by the THz pulses.

Optical pump – THz probe spectroscopy allows to study the population dynamics of joint conducting states in QD structures - wetting layers and barriers.

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Fig. 1.

1. Schematic of InGaAs/GaAs QD sample: multiple QD layers grown on a Bragg reflector with the stop-band around the ground state wavelength of QDs. The Bragg reflector is needed to block the propagation of pump light at QD ground state wavelength into the substrate, and to prevent the photoconductivity due to two-photon absorption
2. Carrier dynamics processes in photoexcited QDs. CS - conducting state. GS - QD ground state. The population of CS (marked in yellow) is probed by the THz pulses.
3. Small signal reflectivity and room temperature optical emission spectra at 800 nm excitation of the studied QD SESAM.

1. Resonant excitation of QD ground states: Population of conducting states due to carrier release from the QD ground states.
2. Peak conductivity as a function of pump fluence at QD ground state wavelength: clear saturable absorption in the QDs.

1. Resonant excitation of conducting states: De-population of conducting states due to carrier capture into the QDs. Long-living contribution is due to charge-separation in the Bragg stack, accessible to the pump at this wavelength. Transient photoconductivity in bulk GaAs (dashed black line) is shown for control.
2. 10% decay time of photoconductivity - a measure of carrier capture dynamics into the QDs. The carrier capture slows down with growth in excitation fluence due to increased population of the QD states. Inset: dynamics of capture and release of carriers in a QD structure on a normalized conductivity scale.

H. P. Porte, P. Uhd Jepsen, N. Daghestani, E. U. Rafailov, and D. Turchinovich
"Ultrafast release and capture of carriers in InGaAs/GaAs quantum dots observed by time-resolved terahertz spectroscopy"
Appl. Phys. Lett. 94, 262104 (2009)