High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion

We demonstrate for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than 3 times the NC band gap energy. The generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) time scale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells.     

Superradiance of a degenerate exciton gas in semiconductors with indirect fundamental absorption edge

It is predicted that superradiant states can be formed in a degenerate exciton gas in a semiconductor with an indirect fundamental absorption edge. The superradiance results from four-particle recombination processes and occurs at photon energies approximately twice as high as the band gap energy. Experimental results supporting the possibility of the observation of superradiance from SiGe/Si quantum wells are presented.     

Laser field induced interband absorption in a strained GaAs/GaAlAs double quantum well system

Effect of laser field intensity on exciton binding energies is investigated in a GaAs/ GaAlAs double quantum well system. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of laser field. The laser field induced photoionization cross-section for the exciton placed at the centre of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross-section on photon energy is carried out for the excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The intense laser field dependence of interband absorption coefficient is investigated. The results show that the exciton binding energy, interband emission energy, the photoionization cross-section and the interband absorption coefficient depend strongly on the well width and the laser field intensity. Our results are compared with the other existing literature available.     

Photoionization of a charged exciton in a GaAs/GaAlAs corrugated quantum well

Binding energies of a charged exciton as a function of well width of a GaAs/GaAlAs corrugated quantum well are investigated. The calculations have been performed by the variational method based on a two parametric trial wave function within a single band effective mass approximation. We have also included the effect of nonparabolicity of the conduction band of GaAs. We study the spectral dependence of the charged exciton in a GaAs/GaAlAs corrugated quantum well as a function of well width. The photoionization cross section for the charged exciton placed at the center of the quantum well is computed as a function of normalized photon energy. The cross-section behavior as a function of incident energy is entirely different in the two cases of radiation being x-direction (along the growth direction) or z-direction. The interband emission energy as a function of well width is calculated and the dependence of the photoionization cross section on photon energy is carried out for the charged excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The results show that the charged exciton binding energy, interband emission energy and the photoionization cross section depend strongly on the well width. Our results are compared with the other existing literature available.     

Structure Dependence of Excitonic Effects in Chiral Graphene Nanoribbons

We explore the excitonic effects in chiral graphene nanoribbons(cGNRs), whose edges are composed alternatively of armchair-edged and zigzag-edged segments. For cGNRs dominated by armchair edges, their energy gaps and exciton energies decrease with increasing chirality angles, and they, as functions of widths, oscillate with the period of three, while the exciton binding energies do not have such distinct oscillation. On the other hand,for cGNRs dominated by zigzag edges, all the energy gaps, exciton energies, and exciton binding energies show oscillation properties with their widths, due to the interactions between the edge states localized at the opposite zigzag edges. In addition, the triplet excitons are energy degenerate when the electrons are spin-unpolarized,while the degeneracy split when the electrons are spin-polarized. All the studied cGNRs show strong excitonic effects with the exciton binding energies of hundreds of meV.     

Exciton-phonon coupling in indirect gap AgBr nanocrystals

The resonance Raman scattering observed in the exciton region of indirect AgBr nanocrystals with mean particle radius R ≈ 5 nm is analyzed. Considering the processes as absorption followed by emission and calculating the size dependent exciton energies by the donor-like exciton model of Ekimov et al. [1], the 1TO(L) and 2TO(L)&LO(Γ) resonance behavior can be well-reproduced. The analysis of the Raman spectral lineshape based on linear exciton-phonon coupling shows a substantial increase of coupling strength with excitation photon energy. It is attributed to enhanced acoustic and surface phonon interaction of higher exciton states.     

Exciton binding energies in organic semiconductors

The exciton binding energy is one of the key parameters that govern the physics of many opto-electronic organic devices. It is shown that the previously reported values for the exciton binding energies in many organic semiconductors, which differ by more than an order of magnitude, can be consistently rationalized within the framework of the charging energy of the molecular units, with a simple dependence of the exciton binding energy on the length of these units. The implications of this result are discussed. PACS 71.35.-y; 78.20.-e; 78.66.Qn     

Four-wave mixing signals from extended exciton states above the mobility edge in GaAs/(Ga,Al)As multiple quantum wells

We investigate the dependence of four-wave mixing response on the photon energy close to the fundamental exciton (X) resonance in GaAs quantum wells. We find that cross-polarised incident fields give rise to a non-linear signal which decays faster at energies below the X line centre than above. We show that this behaviour cannot be assigned to biexciton transitions alone but rather suggests that the delocalised X states above the mobility edge are excited off-resonantly by the laser light having slightly lower energies.     

Effects of hydrostatic pressure on intrawell and interwell excitons in a strained GaAs/GaAlAs double quantum well system

Binding energies of intrawell and interwell excitons are investigated in a GaAs/GaAlAs double quantum well system in the presence of hydrostatic pressure applied in the z-direction. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of pressure. The pressure dependent photoionization cross section for a charged exciton placed at the center of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross section on photon energy is carried out for the charged excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The results show that the charged exciton binding energy, interband emission energy and the photoionization cross section depend strongly on the well width and the hydrostatic pressure. Our results are compared with the other existing literature available.     

Resonant Photon Drag of Dipolar Excitons

The theory of the photon drag of dipolar excitons in double-quantum-well nanostructures is presented. It is shown that the exciton-drag flux density features a resonant behavior if the photon frequency is close to some transition frequency in the discrete exciton spectrum. When the structure is irradiated with polarized light, the resonant enhancement of the drag current occurs when the photon energy coincides with the energy of an excited level of the exciton internal motion and the components of the angular momentum of internal motion in the initial and final states differ by one. The proposed effect can be used to control exciton transport in nanostructures based on a two-dimensional exciton gas.     

Primary photoexcitations and the origin of the photocurrent in rubrene single crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.     

Pressure-induced threshold optical pump intensity in a CdTe/ZnTe quantum dot

Pressure-induced binding energies of an exciton and a biexciton are studied taking into account the geometrical confinement effect in a CdTe/ZnTe quantum dot. Coulomb interaction energy is obtained using Hartree potential. The energy eigenvalue and wave functions of exciton and the biexciton are obtained using the self-consistent technique. The effective mass approximation and BenDaniel-Duke boundary conditions are used in the self-consistent calculations. The pressure-induced nonlinear optical absorption coefficients for the heavy hole exciton and the biexciton as a function of incident photon energy for CdTe/ZnTe quantum dot are investigated. The optical gain coefficient with the injection current density, in the presence of various hydrostatic pressure values, is studied in a CdTe/ZnTe spherical quantum dot. The pressure-induced threshold optical pump intensity with the dot radius is investigated. The results show that the pressure-induced electronic and optical properties strongly depend on the spatial confinement effect.     

EXCITON ENERGY OF THE InAs/GaAs SELF-ASSEMBLED QUANTUM DOT IN A SEMICONDUCTOR MICROCAVITY

We report on the theoretical study of the interaction of the quantum dot (QD) exciton with the photon waveguide models in a semiconductor microcavity. The InAs/GaAs self-assembled QD exciton energies are calculated in a microcavity. The calculated results reveal that the electromagnetic field reduces the exciton energies in a semiconductor microcavity. The effect of the electromagnetic field decreases as the radius of the QD increases. Our calculated results are useful for designing and fabricating photoelectron devices.     

An alternative approach to exciton binding energy in a GaAs-AlxGa1-x as quantum well

A variational-perturbative method is used to calculate the binding energy of an exciton in quantum well structure of Al_xGa_1-xAs-GaAs-Al_xGa_1-xAs. The fitness of potential well heights and differences of electron or hole effective mass in barrier region are both taken into considerations. The binding energies as a function of GaAs well sizes and as a function of alloy compositions, and a photon energy emitted in the recombination of an exciton, are presented. Validity of the calculation is discussed.     

Double resonance in Raman scattering on single phonon modes of GaSe

Resonance in the Raman cross section for one-phonon modes of GaSe at 80°K has been observed for both incident and scattered photon energies equal to the direct exciton energy. The Raman efficiency spectrum within the region of resonance is shown to fit well with the shape computed from existing theories.     

Exciton binding energy and the optical absorption coefficient in a strained CdxZn1−xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd_xZn_1?xO/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.     

Interband optical absorption in a strained CdxZn1−xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd_xZn_1−xO/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.     

Spontaneous generation of entangled exciton in quantum dot systems

We calculate the time evolution of single-exciton states prepared for ensembles of two to four quantum dots. Each dot is considered a two-level system, but with slightly different excitation energies and dipole moments. The dots interact via a tunnel coupling which induces excitation transfer between single emitters, but conserves the total occupation of the system. We show that the initial exciton may evolve towards a steady state where the energy is partially trapped due to the formation of the subradiant (dark) states of the system. In the steady state the individual populations of each dot have permanent oscillations with frequencies given by the energy separation between the subradiant eigenstates.     

Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

Within the range of photon energies illuminating the Earth's surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron‐hole pair; energy of a photon in excess of the semiconductor's bandgap is efficiently converted to heat through interactions between the electron and hole with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron‐hole pair generation from a single photon with energy greater than twice the effective band gap. This multiple carrier pair process, referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the near‐term future for both fundamental research and the development of working devices which exploit MEG.