H. P. Porte

Ultrafast release and capture of carriers in InGaAs/GaAs quantum dots observed by time-resolved terahertz spectroscopy

We observe ultrafast release and capture of charge carriers in InGaAs/GaAs quantum dots in a room-temperature optical pump-terahertz probe experiment sensitive to the population dynamics of conducting states. In case of resonant excitation of the quantum dot ground state, the maximum conductivity is achieved at approximately 35 ps after photoexcitation, which is assigned to release of carriers from the quantum dots. When exciting carriers into the conduction band of the barriers, depletion of the conductivity via carrier capture into the quantum dots with a few picosecond pump fluence-dependent time constant was observed.

On Ultrafast Photoconductivity Dynamics and Crystallinity of Black Silicon

We investigate the carrier dynamics of thin films of black silicon, amorphous hydrogenated silicon which under laser annealing forms a microstructured surface with extremely high broadband optical absorption. We use Raman spectroscopy to determine the degree of crystallinity of the annealed surfaces, and investigate the dependence on crystallinity and fabrication method of the photoconductivity. Time-resolved THz spectroscopy is used to determine the evolution of the carrier scattering time and confinement of carriers on the picosecond time scale. We conclude that a fabrication method with high energy leading edge of the annealing laser results in black silicon with the largest photon-to-electron conversion efficiency, largest mobility, and longest carrier lifetime.

Terahertz study of ultrafast carrier dynamics in InGaN/GaN multiple quantum wells

Ultrafast carrier dynamics in InGaN/GaN multiple quantum wells is measured by time-resolved terahertz spectroscopy. The built-in piezoelectric field is initially screened by photoexcited, polarized carriers, and is gradullay restored as the carriers recombine. We observe a nonexponential decay of the carrier density. Time-integrated photoluminescence spectra have shown a complete screening of the built-in piezoelectric field at high excitation fluences. We also observe that the terahertz conductivity spectra differs from simple Drude conductivity, describing the response of free carriers, and are well fitted by the Drude-Smith model.

Capture and release of carriers in InGaAs/GaAs quantum dots

We observe the ultrafast capture and release of charge carriers in InGaAs/GaAs quantum dots (QDs) at room-temperature with time-resolved terahertz spectroscopy. For excitation into the barrier states, a decay of the photoinduced conductivity, due to capture of carriers into the nonconducting QD states is observed. The increase of the decay time constant with increasing pump fluence is attributed to filling of the QD states. In the case of resonantly excitation into the QD ground state a maximum conductivity is reached 35 ps after photoexcitation, which is assigned to the release of carriers from the QDs into the wetting layer and barrier states.

Time-resolved terahertz spectroscopy of black silicon

The ultrafast photoconductivity dynamics of black silicon is measured by time-resolved terahertz spectroscopy. Black silicon is produced by laser annealing of an a-Si:H film. We show that the decay time of the photoconductivity depends on the annealing method and fluence used in the production process.

Ultrafast conductivity dynamics in optically excited InGaN/GaN multiple quantum wells observed by transient THz spectroscopy

We investigate ultrafast carrier dynamics in photoexcited InGaN/GaN multiple quantum wells by time-resolved terahertz spectroscopy. The initially very strong built-in piezoelectric field is screened upon photoexcitation by the polarized carriers, and is gradually restored as the carriers recombine. The conductivity related to the presence of photoexcited carriers, sensed by the THz probe pulses, shows a non-exponential, slowing-down decay with time, which is explained by the gradual restoration of the built-in field in the QWs and consequent quenching of recombination. Screening and restoration of the built-in field are confirmed by the photoluminescence measurements.

Observation of trapping and release of carriers in InGaAs/GaAs quantum dots by ultrafast THz spectroscopy

Depending on the photoexcitation wavelength, we either observe the trapping of the free carriers into quantum dots, or release of carriers from quantum dot ground state into conducting states of the quantum dot sample.

Transient photoconductivity in InGaN/GaN multiple quantum wells, measured by time-resolved terahertz spectroscopy

Terahertz conductivity of InGaN/GaN MQWs was studied by time-resolved terahertz spectroscopy. Restoration of the built-in piezoelectric field leads to a nonexponential carrier density decay. Terahertz conductivity spectrum is described by the Drude-Smith model.