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Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton. [ABSTRACT FROM AUTHOR]

We present here a theory of Majorana excitons, photo-excited conduction electron-valence band hole pairs, interacting with Majorana Fermions in a Kitaev chain of semiconductor quantum dots embedded in a nanowire. Using analytical tools and exact diagonalization methods, we identify the presence of Majorana zero modes in the nanowire absorption spectra. [ABSTRACT FROM AUTHOR]

Fluorescence properties of quantum dots (QDs) are critically affected by their redox states, which is important for practical applications. In this study, we investigated the optical properties of MoSe2-metallic phase quantum-dots (MoSe2-mQDs) depending on the pH variation, in which the MoSe2-mQDs were dispersed in water with two sizes (Φ~3 nm and 12 nm). The larger MoSe2-mQDs exhibited a large red-shift and broadening of photoluminescence (PL) peak with a constant UV absorption spectra as varying the pH, while the smaller ones showed a small red-shift and peak broadening, but discrete absorption bands in the acidic solution. The excitation wavelength-dependent photoluminescence shows that the PL properties of smaller MoSe2-mQDs are more sensitive to the pH change compared to those of larger ones. From the time-resolved PL spectroscopy, the excitons dominantly decaying with an energy of ~3 eV in pH 2 clearly show the shift of PL peak to the lower energy (~2.6 eV) as the pH increases to 7 and 11 in the smaller MoSe2-mQDs. On the other hand, in the larger MoSe2-mQDs, the exciton decay is less sensitive to the redox states compared to those of the smaller ones. This result shows that the pH variation is more critical to the change of photophysical properties than the size effect in MoSe2-mQDs. [ABSTRACT FROM AUTHOR]

The temperature-driven plasmon-exciton coupling in thermoresponsive dextran-graft-PNIPAM/Au nanoparticle/CdTe quantum dot (D-g-PNIPAM/Au NPs/CdTe QDs) hybrid nanosystem was studied. A significant (0.84 eV) splitting of the absorption peak was observed in the absorption spectrum of the nanosystem, which reflects the fact of formation of plexcitons, occurring due to strong plasmon-exciton coupling. An increasing with time plasmonic enhancement of the photoluminescence of CdTe QDs was revealed, as a result of the penetration of quantum dots into the volume of the D-g-PNIPAM/Au NP hybrid nanosystem and bonding to it. The heating–cooling cycle of the aqueous solution of the studied nanosystem leads to a reversible quenching-recovery alteration of the QD photoluminescence. The quenching was rationalized as a result of an increased probability of nonradiative resonance energy transfer (RET) from CdTe QDs to Au NPs, which occurs due to shortening of the NP-QD distance, caused by shrinking of the macromolecule due to cooling-induced lower critical solution temperature phase transition. Increasing the NP-QD distance in the heating stage recovers the QD PL intensity. The observed effect opens up opportunities for the controlled reversible temperature-driven tuning of the photoluminescence intensity of D-g-PNIPAM/Au NP/CdTe QD nanosystem, which is highly important for its potential use in photonics and biomedical applications. [ABSTRACT FROM AUTHOR]

Low‐dimensional metal halide perovskites have been widely regarded due to their unique crystal structure and photophysical properties. Herein, blue‐emission CsPbBr3 quantum dots (QDs) or nanocrystals (NCs) with extremely small crystal size (1–2 nm) but high photoluminescence quantum yield (PLQY, 72.4%) are synthesized with a new surface ligand, single (6‐amino‐6‐deoxy) beta cyclodextrin (6A‐βCD), which also acts as the confined‐growth template for such small QDs. Detailed photoluminescence spectrum and femtosecond transient absorption spectrum study proves that the blue emission with multiple peaks originates from both band‐edge radiation and exciton recombination. Furthermore, as a result of the small size, exciton localization in the form of self‐trapped excitons (STEs) and significant quantum size confinement effect are both important reasons for the efficient and wide blue emissions of these QDs. This work provides a new strategy not only to synthesize ultrasmall perovskite QDs but also to develop blue perovskite‐QD light‐emitting devices. [ABSTRACT FROM AUTHOR]

We have studied light matter interaction in quantum dot and exciton polaritonic coupler hybrid systems. The coupler is made by embedding two slabs of an excitonic material (CdS) into a host excitonic material (ZnO). An ensemble of non-interacting quantum dots is doped in the coupler. The bound exciton polariton states are calculated in the coupler using the transfer matrix method in the presence of the coupling between the external light (photons) and excitons. These bound exciton-polaritons interact with the excitons present in the quantum dots and the coupler is acting as a reservoir. The Schrödinger equation method has been used to calculate the absorption coefficient in quantum dots. It is found that when the distance between two slabs (CdS) is greater than decay length of evanescent waves the absorption spectrum has two peaks and one minimum. The minimum corresponds to a transparent state in the system. However, when the distance between the slabs is smaller than the decay length of evanescent waves, the absorption spectra has three peaks and two transparent states. In other words, one transparent state can be switched to two transparent states when the distance between the two layers is modified. This could be achieved by applying stress and strain fields. It is alsofound that transparent states can be switched on and off by applying an external control laser field. [ABSTRACT FROM AUTHOR]

In this paper, we study the exciton absorption spectra in InGaAs prolate ellipsoidal quantum dots when a strong pump laser resonant with electron quantized levels is active. Our obtained results by renormalized wavefunction theory show that, under suitable conditions, the initial exciton absorption peak is split into two new peaks as the evidence of the existence of the three-level optical Stark effect of excitons. We have suggested an explanation of the origin of the effect as well as investigating the effect of pump field energy, size, and geometric shape of the quantum dots on effect characteristics. The comparison with the results obtained in the spherical quantum dots implies the important role of geometric shape of the quantum structures when we examine this effect. [ABSTRACT FROM AUTHOR]

The theoretical analysis of intersubband optical transitions for InAs/InGaAs quantum dots-in-a-well (DWELL) detectors are performed in the framework of effective-mass envelope-function theory. In contrast to InAs/GaAs quantum dot (QD) structures, the calculated band structure of DWELL quantitatively confirms that an additional InGaAs quantum well effectively lowers the ground state of InAs QDs relative to the conduction-band edge of GaAs and enhances the confinement of electrons. By changing the doping level, the dominant optical transition can occur either between the bound states in the dots or from the ground state in the dots to bound states in the well, which corresponds to the far-infrared and long-wave infrared (LWIR) peaks in the absorption spectra, respectively. Our calculated results also show that it is convenient to tailor the operating wavelength in the LWIR atmospheric window (8–12 μm) by adjusting the thickness of the InGaAs layer while keeping the size of the quantum dots fixed. Theoretical predictions agree well with the available experimental data. [ABSTRACT FROM AUTHOR]

Transmission spectra of the narrow-bandgap semiconductor quantum dots PbS and PbSe in the fluorine phosphate glass are experimentally studied at different temperatures. Energies of 1s exciton transitions observed in the absorption spectra are used to determine characteristic sizes of the quantum dots under study. A nontrivial temperature behavior of the ground and excited quantum confined states in the quantum dots of different sizes are observed. [ABSTRACT FROM AUTHOR]

The system formed by an electron and a hole coupled by Coulomb interactions with an ionized donor is the least known among the excitonic complexes. This complex is usually described as an exciton bound to an ionized donor. Strictly speaking, the exact mechanism of formation of this complex remains unclear. Two processes of formation giving rise to this system can be imagined. In a first process labeled A, the complex may be regarded as an exciton trapped by an ionized donor while in the second mechanism labeled B, this complex can be derived from the binding of a neutral donor and a hole. From a theoretical point of view both protocols can occur, with different probabilities and different binding energies, and consequently lead to different lines in the optical absorption spectra. In our hypothesis, we assume that the statistical mixture contains the two species coming from different origins and which were formed randomly. In this context of uncertainty and mixing processes, we propose to determine the absorption coefficient in the framework of the two possible hypotheses. In the aim to contribute with a valid description of the absorption spectrum, we report in this paper a full theoretical analysis of the optical and magneto-optical phenomena accompanying the two possible processes of formation, taking into account the dot sizes, the magnetic field strength, and the effect of the dielectric constant of the host material. [ABSTRACT FROM AUTHOR]

Ultrafast transient absorption spectroscopy is used to investigate sub-nanosecond exciton dynamics in Cd x Hg (1 − x ) Te alloy colloidal quantum dots. A bleach was observed at the band gap due to state-filling, the mono-exponential decay of which had a characteristic lifetime of 91 ± 1 ps and was attributed to biexciton recombination; no evidence of surface-related trapping was observed. The rise time of the bleach, which is determined by the rate at which hot electrons cool to the band-edge, ranged between 1 and 5 ps depending on the pump photon energy. Measuring the magnitude of the bleach decay for different pump fluences and wavelengths allowed the quantum yield of multiple exciton generation to be determined, and was 115 ± 1% for pump photons with energy equivalent to 2.6 times the band gap. [ABSTRACT FROM AUTHOR]

The electronic and optical properties of a single exciton in a CdSe/CdS/CdSe/CdS quantum dot is studied by using effective mass approximation with parabolic confinement. The Coloumbic interaction between electron and hole is included by Hartree potential. A self-consistent technique is used to calculate the energy eigenvalue and wavefunction of exciton. Based on this approximation we investigate the effect of core size, shell thickness, well width on exciton binding energy, absorption spectra, and oscillator strength. The results provide the tuning possibility of electronic and optical properties of multilayer quantum dot with layer thickness. [ABSTRACT FROM AUTHOR]

Highlights: [•] The electron and hole dynamics can be separated by probing their respective intraband transitions by the transient absorption spectroscopy. [•] The multiple exciton generation in the PbS quantum dots cannot be affected in the presence of the electron acceptors. [•] The strong electronic coupling between PbS quantum dots and TiO2 will lead to a spectral broadening. [•] The electron transfer time from PbS quantum dots to TiO2 is less than 150 fs due to the strong coupling. [Copyright &y& Elsevier]

The magnetic field-dependent heavy hole excitonic states in a strained Ga0.2In0.8As/GaAs quantum dot are investigated by taking into account the anisotropy, non-parabolicity of the conduction band, and the geometrical confinement. The strained quantum dot is considered as a parabolic dot of InAs embedded in a GaAs barrier material. The dependence of the effective excitonic g-factor as a function of dot radius and the magnetic field strength is numerically measured. The interband optical transition energy as a function of geometrical confinement is computed in the presence of a magnetic field. The magnetic field-dependent oscillator strength of interband transition under the geometrical confinement is studied. The exchange enhancements as a function of dot radius are observed for various magnetic field strengths in a strained Ga0.2In0.8As/GaAs quantum dot. Heavy hole excitonic absorption spectra, the changes in refractive index, and the third-order susceptibility of third-order harmonic generation are investigated in the Ga0.2In0.8As/GaAs quantum dot. The result shows that the effect of magnetic field strength is more strongly dependent on the nonlinear optical property in a low-dimensional semiconductor system. [ABSTRACT FROM AUTHOR]

Blue and green light emitting CsPbBr 3 and CsPb(Cl/Br) 3 perovskite quantum dots glasses (QDGs) have been fabricated in multi-component borate glass matrices by melt quenching method and following heat treatment. Absorption spectra show regular blue shift of first exciton absorption peak from CsPbBr 3 to CsPb(Cl/Br) 3 QDGs. Photoluminescence spectra show that CsPbBr 3 and CsPb(Cl/Br) 3 QDGs have adjustable emission in the wavelengths from 528 to 442 nm. The full width at half maximum (FWHM) is 15–17 nm and 21–31 nm for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs respectively. Emission map and photoluminescence excitation spectra show broad band excitation characteristics of the exciton emission. Photoluminescence decay shows bi-exponential function for both CsPbBr 3 and CsPb(Cl/Br) 3 QDGs. As the ratio of Cl/Br increases from CsPbBr 3 to CsPb(Cl/Br) 3 QDGs, the short and long lifetime components roughly decrease from 9.7 to 4.0 ns and from 87.8 to 55.2 ns respectively, and the average lifetime decreases from 72.2 to 36.5 ns. Photoluminescence quantum yield changes from 82.8% to 5.7%. Temperature dependence of PL spectra in the range of 28–120 °C shows that integrated intensity obviously decreases with increasing temperature, but the emission peak wavelength and FWHM almost do not change. • Absorption spectra show blue-shift for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs with different Cl/Br ratios. • Photoluminescence spectra show blue shift from 528 to 442 nm for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs. • Narrow FWHM is 15–17 nm and 21–31 nm for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs respectively. • With increasing Cl/Br ratio, short lifetime component and PLQY decrease with the same rule. • With increasing temperature, integrated intensity decreases, but emission peak and FWHM do not change. [ABSTRACT FROM AUTHOR]

Abstract: Numerical calculations of the excitonic absorption spectra in a strained Cd x Zn1−x O/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels. [Copyright &y& Elsevier]

Abstract: An investigation of the influence of a laser field on the exciton absorption spectra and the refractive indexes has been performed by using the matrix diagonalization and the compact density-matrix methods. Electrons and holes are confined by the parabolic potential of the quantum dot. We conclude that in quantum dots the laser field amplitude provides an important effect on the linear, third-order nonlinear, and total optical absorption coefficients as well as the refractive index changes. [Copyright &y& Elsevier]

Due to the strong exchange interaction between a single Mn spin embedded in a single semiconductor quantum dot and the quantum dot exciton, the spin state of the Mn atom can be controlled via optical manipulation of the exciton. When the hole of the exciton is of light hole type, both electron and hole can induce spin flips and thus change the Mn spin. We propose switching protocols to selectively address the Mn spin states. Depending on the polarization of the laser pulses the Mn spin can be switched by different switching schemes into all eigenstates. In contrast to previous studies involving heavy holes, we can find faster and more stable switching sequences. We discuss in detail the absorption spectrum of the lowest light hole exciton transition, which is strongly modified by the presence of the Mn spin. Furthermore, we show that spectrally resolved pump-probe measurements provide a suitable technique to monitor the Mn spin dynamics. [ABSTRACT FROM AUTHOR]

We theoretically investigate a coherent optical spectroscopy of a strongly driven quantum dot without and with the coupling between exciton and phonons. In the absence of this coupling, we achieve the experimental results obtained by Xu et al. [Science 317, 929 (2007)]. However, while taking this coupling into account, we observe some new features in the probe absorption spectrum including two sharp phonon-induced sidebands. Our results also demonstrate that the lifetime of phonons will play an important role in this coherent spectroscopy. [ABSTRACT FROM AUTHOR]

We analyze the electron-hole (exciton) ground state associated with the first peak in the optical absorption spectra of semiconductor quantum dots. We assume the effective mass approximation and a dot radius R on the order of the exciton Bohr radius aB. A Hamiltonian diagonalization method which accounts for the exciton's kinetic, direct Coulomb, and surface polarization energies is used. We obtain a representation of the exciton ground-state wavefunction and a value for its energy using a basis set consisting of only three composite infinite spherical well wavefunctions. We discuss the precision obtained by this basis set by comparing with results from a much more extended basis set. Our results are used to predict the radius-dependent energy of the first peak in visible-light absorption spectra for CdSe quantum dots. Our analysis accurately describes the experimental data for dots with radii in the range aB

Quantum dots are typically formed from large groupings of atoms and thus may be expected to have appreciable many-body behavior under intense optical excitation. Nonetheless, they are known to exhibit discrete energy levels due to quantum confinement effects. We show that, like single-atom or single-molecule two- and three-level quantum systems, single semiconductor quantum dots can also exhibit interference phenomena when driven simultaneously by two optical fields. Probe absorption spectra are obtained that exhibit Autler-Townes splitting when the optical fields drive coupled transitions and complex Mollow-related structure, including gain without population inversion, when they drive the same transition. Our results open the way for the demonstration of numerous quantum level-based applications, such as quantum dot lasers, optical modulators, and quantum logic devices. [ABSTRACT FROM AUTHOR]

The self-assembed chitosan CdSe quantum dots (QDs) and chitosan CdSe−ZnS core−shell QDs films have been prepared by using layer-by-layer electrostatic technique. The well-ordered nanostructure and the layer-by-layer deposition of the QDs are revealed by AFM and exciton absorption spectra, respectively. The optical nonlinearity of the composite films were studied by using Z-scan technique with femtosecond pulses at the wavelength of 790 nm, the value of third-order susceptibility of core−shell QDs are measured to be about 1.1 × 10-8 esu, which is about 200% larger than that of CdSe QDs of 5.3 × 10-9 esu. This has potential applications in all-optical switches in optical information processing. [ABSTRACT FROM AUTHOR]

We present the basic steps for the study of the linear near field absorption spectra of semiconductor quantum dots under magnetic field of variable orientation. We show that the application of the magnetic field alone is sufficient to induce -increasing the spot illuminated by the near field probe- interesting features to the absorption spectra. [ABSTRACT FROM AUTHOR]

We have studied infrared transient absorption spectra of CuCl QDs grown inside a NaCl matrix under tunable UV laser excitation on resonance with the confined excitons. In addition to the transient excited-state absorption, there appears induced IR absorption with lifetime much longer than that of the confined exciton. The peak energy remains at ∼ 0.3 eV regardless of the QD size. The optical density is proportional to the square root of the UV laser power. The decay of the induced absorption is logarithmic on time and is accelerated by the irradiation of light below the band gap. We assign this absorption to be originated from the electron-bound defect center called I[sub e] center generated in the NaCl side of the interface and discuss it phenomenon in relation with photo-irradiation effects, such as photofatigue, blinking of the luminescence and hole burning. [ABSTRACT FROM AUTHOR]

• Natural MoS 2 quantum dots are discovered in the MoS 2 powder in micrometer. • The MoS 2 quantum dots are separated by vacuum filtration and dispersed in water. • The MoS 2 quantum dots work as a p-type dopant toward multilayer MoS 2 flakes. In contrast to laborious efforts to gain quantum dots (QDs) of two-dimensional (2D) layered materials, less attention has been given to the discovery of natural QDs present in the powder sample. In this study, we discovered that a considerable quantity of natural QDs are present in 2D powders and demonstrated a simple way to obtain MoS 2 QDs from a bulk powder without any chemical and severe ultrasonication. The natural MoS 2 QDs were obtained by filtering the MoS 2 powders dispersed in ethanol. The separated QDs resulted in a stable blue suspension in deionized water which exhibited the absorption spectra at a wavelength of 320 nm, supporting that MoS 2 QDs were successfully separated from the bulk powder. And, the maximum photoluminescence intensity appeared in the ultraviolet region under the excitation of 320 nm. Also, the doping effect of MoS 2 QDs on MoS 2 flakes led to the enhancement of photoluminescence spectra due to a hole carrier from MoS 2 QDs. Our finding paves the way to explore natural QDs from a powder sample of other 2D materials. [ABSTRACT FROM AUTHOR]

The effects of an external electric field on the exciton polarizability and absorption spectra in CdSe/ZnS nanocrystal quantum dots have been investigated theoretically by using an exact diagonalization method within the effective-mass approximation. The results show that the application of an external electric field causes the splitting of degenerated states introducing new interband transitions (1s-1p0 and 1p0-1s), resulting in the increase of the excitonic absorption coefficients of the 1s-1p0 and 1p0-1s transitions due to the enhancement of the electric-field-induced coupling between the 1s and 1p0 states. The excitonic absorption intensity of 1s-1s, 1p-1-1p-1, and 1p1-1p1 transitions decreases with the increase of the electric field strength. A red-shift in the absorption spectra of 1s-1s, 1p-1-1p-1, 1p1-1p1, 1s-1p0, and 1p0-1s interband transitions is observed while the absorption peak of 1p0-1p0 interband transition is first blue-shifted and then red-shifted with increasing the electric field strength, which is attributed to the quantum-confined Stark effect (QCSE). The exciton polarizability increases monotonically with increasing the dot radius. The fitting expressions of the Stark shift and exciton polarizability have been proposed for the interband transitions in the strong confinement regime. Therefore, the emission wavelength and intensity of the output of optoelectronic nanodevices can be manipulated using an external electric field. [ABSTRACT FROM AUTHOR]

The Rabi splitting of the negatively charged exciton in a single InGaAs quantum dot is observed in resonance transmission spectroscopy. We use a pump laser excitation to drive strongly the unpolarized trion transition in a quantum dot and detect its modified absorption spectrum with a second weak probe laser. By tuning the pump laser near resonance, we observe an ac-Stark effect dispersion, with a power dependent Rabi splitting on resonance, both signatures of a strongly coupled two level system. Although the pump and probe laser fields are resonant with the same transition, we do not observe all features in the Mollow spectrum. We combine the results of pump probe with saturation spectroscopy data to deduce the individual contributions to the low power linewidth. [ABSTRACT FROM AUTHOR]

Self-assembled ZnO quantum dots (QDs) were achieved by a vapor phase transport process. ZnO nanodots were naturally formed on solid substrates in the Volmer-Weber growth mode. Size control of nanodots could be readily realized by varying the growth time. The as-prepared ZnO QDs are of high quality and very stable after formation. The blueshift of band gap energies derived from quantum confinement effects was confirmed by optical absorption spectra. Photoluminescence spectra revealed the tunable behavior of ultraviolet luminescence due to exciton localization. The realization of size-tuned color from ZnO QDs makes them more promising for practical applications. [ABSTRACT FROM AUTHOR]

We investigate intraband transitions in Si/SiO2 quantum dots (QD’s) by using a tridimensional quantum mechanical model that takes into account the six-valley structure of silicon. The interplay between QD orientation and shape strongly affects the infrared absorption spectra of Si QD’s. In particular, we show the orientation of the Si valleys dramatically changes the optical properties of Si QD’s. [ABSTRACT FROM AUTHOR]

Excitonic interband optical transitions within single InAs self-assembled quantum dots have been directly observed in a transmission experiment at 4.2 K. Using Stark shift, the excitonic energy levels of a single quantum dot are tuned into resonance with a narrow-band laser line. The Stark shift is also modulated at low frequencies. Relative changes in transmission can be detected this way down to one part per million. The oscillator strength as well the homogeneous linewidth of the transition is obtained. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We consider excitons in a quantum dot as q-deformed systems. The interaction of some excitonic systems with one cavity mode is considered. The dynamics of the system is obtained by diagonalizing the total Hamiltonian, and the emission spectrum of a quantum dot is derived. The physical consequences of a q-deformed exciton on the emission spectrum of a quantum dot are given. It is shown that when the exciton system deviates from Bose statistics, the emission spectra will become multi-peak. With our investigation we try to find the origin of the q-deformation of the exciton. The optical response of excitons, which is affected by the nonlinear nature of q-deformed systems, up to the second order of approximation is calculated and the absorption spectrum of the system is given. [ABSTRACT FROM AUTHOR]