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Quantum Electronics and Laser Science Conference

Persistent hole-burning spectroscopy in semiconductor quantum dots

Yasuaki Masumoto

Published May 22, 1995• 0 citations
OpenAlex

Abstract

Nanometer-size crystals abbreviated to semiconductor nanocrystals are known as zero-dimensional dots. Their optical properties have been characterized by the quantum confinement effect and the lowest excited states show blue shifts depending on their size. Quantum dots are sharply different from other low-dimensional quantum structures, such as quantum wells and quantum wires, because they are made of as small as 103~106 atoms. Considerable fraction of atoms face the surface or the interface of quantum dots in the surrounding materials, so that the energy of quantum dots is expected to be easily modified by the surroundings.

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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.

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Optical absorption saturation of semiconductor quantum dots

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Nanometer-size crystals abbreviated to semiconductor nanocrystals are known as zero-dimensional dots. Their optical properties have been characterized by the quantum confinement effect and the lowest excited states show blue shifts depending on their size. Quantum dots are sharply different from other low-dimensional quantum structures, such as quantum wells and quantum wires, because they are made of as small as 103~106 atoms. Considerable fraction of atoms face the surface or the interface of quantum dots in the surrounding materials, so that the energy of quantum dots is expected to be easily modified by the surroundings.
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.
The persistent spectral hole-burning (PSHB) phenomenon was observed in semiconductor quantum dots, CdSe , CuCl , and CuBr , embedded in glass and crystal. In inhomogeneously broadened exciton absorption spectra of these dots, the narrow bleaching hole and its associated structure are made by the narrowband laser excitation and are conserved for more than several hours at 2 K after the laser irradiation. The observation of the PSHB phenomenon in these four kinds of samples shows the generality of the phenomenon in semiconductor quantum dots and requires the existence of more than one ground-state configurations of the total system consisting of quantum dots and surrounding matrix. It means that not only the size distribution but also these ground-state configurations give inhomoge-neous broadening to semiconductor quantum dots. Thermally annealing and light-induced hole-filling phenomena were observed. Hole-burning and hole-filling mechanisms are discussed. Quantum dots consisting of 103–104 atoms behave like molecules or ions in matrix to give the PSHB phenomenon.
Different aspects of the photoluminescence from semiconductor quantum-confined structures are studied in this dissertation, for a better understanding of fundamental physics of semiconductors.The precursor of any photoluminescence study is the characterization of the linear optical properties of the semiconductor structure. High resolution absorption measurements were performed in order to study the interplay of disorder and acoustic phonon scattering in a quantum well. Also, reflectivity measurements, together with a fitting procedure based on the transfer matrix formalism, are used to determine the thickness of samples.Excitons are atom-like quasi-particles, formed from a bound electron-hole pair. They follow a Bose-Einstein statistic, so in principle it is possible to achieve an excitonic Bose-Einstein condensate. Time resolved photoluminescence measurements were performed over an extensive range of lattice temperatures and carrier concentrations, in order to determine the fraction of excitons formed from the electron-hole plasma in a quantum well, after non-resonant excitation. The experimental spectra were compared to a pure plasma calculation first, then excitons were taken into account. The highest fraction of formed excitons is found for low temperatures and intermediate carrier densities. This fraction is found to be very small, and this has clear implications on the excitonic Bose-Einstein condensation studies.The photoluminescence emitted left and right from a quantum well is interfered in a modified Mach-Zender interferometer. It is shown that the light emitted on the two paths will interfere for a V-shape geometry and will not for any other paths.A structure formed by placing a quantum well in a field antinode of a resonant planar microcavity exhibits normal mode coupling: splitting of the resonance spectral line. The coherence properties of the photoluminescence from a normal-mode-coupling microcavity are studied using another version of the Mach-Zender interferometer. The degree of coherence measured in this way depends greatly on the pump wavelength and intensity, ranging from zero to 0.8. However, direct observation of the emission speckle shows significant coherence in all cases. The difference is explained by the different methods used to evaluate the coherence.The strong coupling between a quantum dot and a photonic crystal nanocavity is investigated by observation of photoluminescence. A new method of tunning the cavity wavelength by deposition of a thin film of solid Xenon on all the surfaces of the sample is presented. The method allows the scanning of the cavity wavelength with about 5 nm without a decrease in the quality factor and without changing the temperature.Finally, an extensive study of the quality factors of quantum dot photonic crystal nanocavities is presented. The role of the quantum dot ensemble absorption is investigated. At higher excitation levels, lasing is observed and discussed.
Quantum dot structures confine electrons in a small region of space. Some properties of semiconductor quantum dots, such as the discrete energy levels and shell filling effects visible in addition spectra, have analogies to those of atoms and indeed dots are sometimes referred to as 'artificial atoms'. However, atoms and dots show some fundamental differences due to electron correlations. For real atoms, the kinetic energy of electrons dominates over their mutual Coulomb repulsion energy and for this reason the independent electron approximation works well. For quantum dots the confining potential may be shallower than that of real atoms leading to lower electron densities and a dominance of mutual Coulomb repulsion over kinetic energy. In this strongly correlated regime the independent electron picture leads to qualitatively incorrect results. This thesis concentrates on few-electron quantum dots in the strongly correlated regime both for quasi-one-dimensional and two-dimensional dots in a square confining potential. In this so-called 'Wigner' regime the ground-state electronic charge density is localised near positions of classical electrostatic minima and the interacting electronic spectrum consists of well separated spin multiplets. In the strongly correlated regime the structure of low-energy multiplets is explained by mapping onto lattice models with extended-Hubbard and Heisenberg effective Hamiltonians. The parameters for these effective models are calculated within a Hartree approximation and are shown to reproduce well the exact results obtained by numerical diagonalisation of the full interacting Hamiltonian. Comparison is made between square dots and quantum rings with full rotational symmetry. In the very low-density regime, direct diagonalisation becomes impractical due to excessive computer time for convergence. In this regime a numerical renormalisation group method is applied to one-dimensional dots, enabling effective spin-interactions to be pr
Quantum spectroscopy utilizes the quantum fluctuations of the light source to characterize and control matter. More specifically, desired many-body states can be directly excited to the semiconductor by adjusting light source's quantum fluctuations. The method is experimentally realizable by projecting an extensive set of classical measurements into a quantum-optical response resulting from any possible quantum source. In this work, quantum spectroscopy is used to identify new classes of many-body states and quantum processes in semiconductor nanostructures. In the first part of this Thesis, the optical properties of semiconductor quantum wells are analyzed with quantum spectroscopy by projecting high-precision optical measurements into quantum-optical responses. It is shown that quantum spectroscopy can characterize the properties of specific stable electron-hole cluster – called quasiparticles – much more sensitively than traditional ultrafast laser spectroscopy. In particular, unambiguous evidence is demonstrated for the identification of a new highly correlated quasiparticle in direct-gap Galliumarsenide quantum wells, the dropleton, that is a quantum droplet consisting of four-to-seven electron-hole pairs. To determine the detectable excitation energetics of such correlated quasiparticles in optically excited semiconductor quantum wells, a new theoretical framework is presented which allows for the computation of the excitation spectrum based on a pair-correlation function formulation of the quasiparticle state. Another study in this Thesis deals with the emission properties of optically pumped quantum-dot microcavities. Experimental and theoretical evidence is shown for a new intriguing quantum-memory effect that is controllable by adjusting pump source's quantum fluctuations. The last part of this Thesis presents a fundamental study about the general applicability of quantum spectroscopy in dissipative systems.