Transient intraband light absorption by quantum dots: Pump-probe spectroscopy

Capture, relaxation and recombination in quantum dots

In our experiments we superimpose a near-infrared excitation laser field with a mid infrared field on an InAs quantum dot ensemble or on a single quantum dot. In the latter case a cw infrared laser excitation and a glow-bar mid infrared field are superimposed in a confocal micro-photoluminescence configuration on a single quantum dot. By comparing the recombination spectra for the superimposed spectra with the spectra from the laser excitation we detect changes in the count rates for the different excitonic states. The decrease of the luminescence intensity in the lower states together with an increasing intensity in higher excited states suggests a direct electron transfer between these states due to absorption of mid infrared light. For the quantum dot ensembles we use interband pump – intraband probe-time domain spectroscopy to study the electron capture and relaxation dynamics within the ensemble. By tuning the femtosecond infrared pulses into resonance with transitions between confined quantum dot states and the wetting layer continuum we gain knowledge about the electron population in the quantum dots ground state and first excited states as a function of delay time between pump and probe pulse. Our experiments indicate the stepwise relaxation through the excited dot states to be the most efficient relaxation pathway into the dot ground state on a timescale between 1.5 ps and 4.7 ps depending on temperature and excitation density. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Multiple layers of GaN/AlN quantum dot (QD) ensembles were grown by the Stranski–Krastanov method on Si(111) using molecular beam epitaxy. During the subsequent cooling from growth temperature, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing the vertically stacked QDs leads to an additional biaxial tensile stress of 20–30 kbar in the III-nitride film. We have selectively modified the thermal stress in the QD layers by etching a cross-hatched pattern into the as-grown sample using inductively coupled Cl2/Ar plasma reactive ion etching. The results show that a suitable choice of stripe width from ∼2 to 10 µm and orientation along [11−20] and [1−100] can create regions of in-plane uniaxial stress that enable a selective and local control of the polarized luminescence from ensembles of QDs which were probed with cathodoluminescence. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing e–h pair excitation for the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼46 K). A theoretical modelling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schrödinger and Poisson equations using the and effective-mass methods for calculations of the e–h wavefunctions and electron and hole quasi-Fermi levels for varying levels of state filling. We attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during e–h pair excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing e–h pair excitation density at T = 300 K.

Self-organized formation of shell-like InAs/GaAs quantum dot ensembles

This letter presents a technique for the investigation of the fine structure and spin properties of quantum dot (QD) ensembles, allowing measurement of QD parameters previously accessible only from studies of individual QDs. We show how ∼μeV splittings can be deduced from information contained in the shape of the ensemble polarization spectra and demonstrate the effectiveness of this technique by measuring Zeeman splittings, g-factors, and sensitivity to QD fine structure effects.

For a better understanding of the physical properties of semiconductor quantum dot ensembles, we have followed the behaviors of the transport and photoluminescence above, at, and below the percolation threshold of ensembles of Si quantum dots that are embedded in a $\mathrm{Si}{\mathrm{O}}_{2}$ matrix. Our study revealed the roles of the interdot conduction, the single dot charging, and the connectivity in such systems. We conclude that while the first two determine the global transport, a connectivity dependent migration determines the coupling between the electrical and optical properties.

We report millisecond-range spin memory of resident electrons in an ensemble of InP quantum dots (QDs) under a small magnetic field of 0.1 T applied along the optical excitation axis at temperatures up to about 5 K. A pump-probe photoluminescence (PL) technique is used for optical-orientation of electron spins by the pump pulses and for study of spin relaxation over the long time scale by measuring the degree of circular polarization of the probe PL as a function of pump-probe delay. Dependence of spin decay rate on temperature and magnetic field suggests two-phonon processes as the dominant spin relaxation mechanism in these QDs at low temperatures.

Charge trapping due to defects in semiconductor quantum dots (QDs) is expected to challenge the applicability of QDs in future technologies. The efficient elimination of defects from QDs demands an understanding of their origin and of their impact on (photo)electronic properties. Here, we identify the presence of two charge states of a defect in CdSe QDs using electron paramagnetic resonance (EPR), combined with electronic tuning of QDs via chemically induced Ag doping. From light-induced EPR, we show that these defects have a central role in Fermi level pinning in ensembles of intrinsic QDs. By analyzing the dependence of the EPR signal of the defects on the concentration of Ag dopants, we further demonstrate that the defects act as effective electron traps in the QDs. Our study also provides support to the proposed behavior of Ag dopants in CdSe QDs, according to which Ag atoms are n-type dopants at concentrations below 2 Ag atoms per QD and become p-type dopants for higher Ag concentrations. From tempe...

Persistent spectral-hole-burning (PSHB) phenomena have been observed in many kinds of semiconductor quantum dots (QDs). Luminescence hole burning and charged excitons have been observed in CuCl QDs in NaCl crystals. The observed PSHB phenomena are explained by the ionization of QDs. The energy of photoionized QDs is changed from the original energy and their new energies depend on the spatial arrangement of trapped carriers. Quantum confinement of carriers and the resulting strong Coulomb interaction between confined carriers and trapped carriers cause the energy change. PSHB has been widely applied to site-selective spectroscopy of QDs. Size-dependent excited exciton quantum states in CuCl quantum cubes, size-dependent excited electron-hole quantum states in CdTe QDs and renormalization of longitudinal optical phonons confined in CuCl QDs have been revealed by studying sideband holes. This also allowed the measurement of accumulated photon echo, which clarified the very sharp homogeneous spectral linewidth of QDs under weak excitation conditions.

An ensemble of quantum dot excitons may be used for coherent information manipulation. Due to the ensemble inhomogeneity any optical information retrieval occurs in form of a photon echo. We show that the inhomogeneity can lead to a significant deviation from the conventional echo timing sequence. Variation of the area of the initial rotation pulse, which generates excitons in a dot sub-ensemble only, reveals this complex picture of photon echo formation. We observe a retarded echo for {\pi}/2 pulses, while for 3{\pi}/2 the echo is advanced in time as evidenced through monitoring the Rabi oscillations in the time-resolved photon echo amplitude from (In,Ga)As/GaAs self-assembled quantum dot structures and confirmed by detailed calculations.

A review devoted to the theory of excitonic quasimolecules (Biexcitons) (formed of spatially separated electrons and holes) in a nanosystems that consists of semiconductor and dielectric quantum dots synthesized in a dielectric matrix.It is shown that exciton quasimolecules formation is of the threshold character and possible in nanosystems, in with the spacing between the quantum dots surfaces is larger than a certain critical spacing.It was found that the binding energy of singlet ground state of exciton quasimolecules, consisting of two semiconductor quantum dots is a significant large values, larger than the binding energy of the biexciton in a semiconductor single crystal almost two orders of magnitude.

<p dir="ltr">Ultrafast Transient Absorption Spectroscopy is a powerful tool to reveal excited state dynamics and semiconductor photophysics of semiconducting materials on femtosecond (10-15 s) timescales, including carrier recombination, hot carrier cooling, bandgap renormalisation and charge transfer. This technique uses a pump pulse to excite electrons within a material into a higher energy state and measures the state of the material by way of transmission of a probe pulse. This has provided insight into the photoexcitation and charge carrier dynamics of a broad range of materials such as organic semiconductors and halide perovskite materials.</p><p dir="ltr">This technique, however, faces two major obstacles. The first obstacle is the inability to distinguish the effects of the excited state absorption and reflection on the transmission of a material. With Transient Absorption measurements, the assumption is made that the change in transmission is the result of the imaginary part of the refractive index alone and does not consider the effect of the real part of the refractive index, which relates to the reflection. This is not an issue for materials with a low real refractive index, however, reflection effects can create significant artefacts on the Transient Absorption signal for materials with a high refractive index, such as perovskite materials. Attempts have been made to measure the excited state refractive index, however, these techniques are model dependent and therefore may not reflect the true excited state refractive index.</p><p dir="ltr">The second obstacle is the different requirements in spectrum and pulse energy for the pump and probe pulses used in this experiment. This results in the use of a combination of light sources, such as bulk supercontinuum generation, (Non)collinear Optical Parametric Amplifiers, and/or nonlinear optical fibres to generate these distinct pump and probe pulses. The use of multiple light sources, each with their own multitude of optical components, add to the complexity of the transient absorption spectroscopy system. In this thesis, we address these problems by implementing a Frequency Domain Interferometry system and a Multiple Plate Compression light source.</p><p dir="ltr">Firstly, the Frequency Domain Interferometer based on the Michelson interferometer is implemented in an existing Transient Absorption spectrometer. This is discussed in chapter 3, where this system is used to measure the optically induced change of the real part of the refractive index of CsPbBr3 perovskite. From this, the excited state extinction spectrum is accessed, which provides valuable information regarding the suspected reflection artefacts observed in the Transient Absorption signal of this material.</p><p dir="ltr">Secondly, the Multiple Plate Compression lightsource is implemented in a Transient Absorption spectrometer in chapter 4. The Multiple Plate Compression is a novel light source capable of generating intense, stable, temporally compressed and spectrally broadband pulses, but has never been applied in the field of ultrafast spectroscopy before. The shot-to-shot stability and spectra of the pulses generated by two multiple plate compression systems are analysed. The Multiple Plate Compression Transient Absorption spectrometer is used on MAPbI3 perovskite to demonstrate the application of this light source in ultrafast spectroscopy. Finally, in chapter 5, the Multiple Plate Compression is used as a single light source for an improved Frequency Domain Interferometer system, based on the Sagnac interferometer. This technique is applied to obtain the optically induced change of the real part of the refractive index of pentacene. These results are then compared to the refractive index change obtained by a novel differential dielectric functions model.</p><p dir="ltr">Understanding of the complex refractive index change of materials has great implications for future optoelectronic device architecture. The analysis of the pulses generated by the multiple plate compression show that it is an excellent light source in the field of ultrafast Transient Absorption Spectroscopy. This paves the road for the implementation of this system as a light source in other spectroscopic techniques such as 2-Dimensional Electronic Spectroscopy and Impulsive Stimulated Raman Spectroscopy.</p>

We propose a new idea of generating and controlling quantum coherences among semiconductor quantum dots by using the spatial phase of one single-mode incident laser pulse. Two or three identical quantum dots are employed as qubit gates. Our strategy is to use the spatial phase of the laser pulse at the different positions of the qubits for quantum operations, rather than to utilize dipole-dipole interactions between them. In this work, Hadamard transformation and creation of entanglements in the bipartite and tripartite systems are examined. We predict that with much simpler experimental setup the spatial phase of the laser pulse makes it possible to perform elementary quantum operations that are infeasible by the present-day quantum computer architecture.

The dynamics of exciton hopping in an array of inhomogeneously broadened CdSe/ZnS quantum dot (QD) ensembles is examined by measuring time- and spectrally resolved fluorescence intensities. We have found a decrease in the fluorescence decay time as well as a dynamic redshift of the fluorescence spectrum originating from exciton transfer. Both show the characteristic temperature dependence reflecting the peculiar exciton dynamics in the QD ensemble. We propose a model of coupled QD arrays where inhomogeneous distribution and dark QDs that are related to a long-lasting off-state of blinking QDs are taken into account. Experimental results together with numerical calculations based on this model suggest that at low temperatures, an exciton transfers to a local low-energy site and tends to be trapped, whereas at high temperatures, thermally activated hopping of the exciton occurs repeatedly. Furthermore, we show that the decay time decrease of the QD array is attributable to exciton hopping to dark QDs.

CHAPTER VI - Towards 1D and 0D Physics and Devices