Light-hole Excitons Research Articles

A light-hole exciton in a quantum dot

A light-hole exciton is a quasiparticle formed from a single electron bound to a single light hole. This type of fundamental excitation, if confined inside a semiconductor quantum dot, could be advantageous in quantum information science and technology. However, it has been difficult to access it so far, because confinement and strain in conventional quantum dots favour a ground-state single-particle hole with a predominantly heavy-hole character. Here we demonstrate the creation of a light-hole exciton ground state by applying elastic stress to an initially unstrained quantum dot. Its signature is clearly distinct from that of the well-known heavy-hole exciton and consists of three orthogonally polarized bright optical transitions and a fine-structure splitting of hundreds of microelectronvolts between in-plane and out-of-plane components. This work paves the way for the exploration of the fundamental properties and of the potential relevance of three-dimensionally confined light-hole states in quantum technologies. An electron and a hole trapped in the same quantum dot couple together to form an exciton. Conventionally the hole involved is a heavy hole. Light-hole excitons are now observed by applying elastic stress to initially unstrained gallium arsenide-based dots. The quasiparticles are identified by their optical emission signature, and could be used in future quantum technologies.

Effects of electric field and shape on the ground state energy of an exciton bound to an ionized donor impurity in ellipsoidal quantum dots

The effects of electric field and shape on the ground state energy of an exciton bound to an ionized donor are studied in a weakly prolate (oblate) GaAs ellipsoidal quantum dot within the effective-mass approximation using a variational method in the framework of perturbation theory. Both the heavy-hole and light-hole excitons are considered. The results show that the energy shift not only depends on the electric field but also on the dot shape and size. The influence of the dot shape on the light hole case is more pronounced than that of the heavy hole case, but the influence of the electric field on the heavy hole case is more pronounced.

Density of states for a light-hole exciton in a microtube of GaAs/AlGaAs with two quantum well and different potential shape: theoretical model

We consider a simple method for calculating the ground-state energy of light hole exciton and density of states confined in a microtube of GaAs/AlGaAs recently fabricated and studied experimentally, with a double quantum well and large radius of curvature. The exciton trial function is taken as a product of the ground state wave functions of both the unbound electron and hole in the heterostructure, with an arbitrary correlation function that depends only on electron-hole separation. A renormalized Schrödinger equation for the correlation function is derived and coincides with the corresponding equation for a hydrogen atom in an effective and space-isotropic homogeneous. The binding energy of the ground state to an exciton in this heterostructure, the contribution to the energy given by the sublevels and the density of states is determined as a function of the width of the well, the aluminium concentration and confinement potential profile is obtained by solving the equation calculated by the variational model proposed.

Fine structure of light-hole excitons in nanowire quantum dots

Quantum dots with light-hole ground states could find numerous applications including faster quantum bit operations or coherent conversion of photons into electron spins. Typically, however, holes confined in epitaxial quantum dots are of heavy-hole character. I show, by use of atomistic tight-binding theory, that the hole ground state undergoes a transition from heavy holelike to light holelike with increasing height of a nanowire InAs/InP quantum dot. The fine structure of the light-hole exciton consists of a dark ground state and three bright states. Two of the bright states are quasidegenerate and are in-plane polarized, whereas, the third energetically higher bright state is polarized in the perpendicular out-of-plane direction. The light-hole exciton fine structure is robust against alloying.

Non-local coherent coupling between excitons in a disordered quantum well

We investigated coherent coupling among multiple exciton resonances formed in a single disordered quantum well using the powerful electronic two-dimensional Fourier transform spectroscopy. Our experiment revealed clear signatures of non-local coherent coupling between the heavy-hole and light-hole excitons residing in regions that differ in thickness by one atomic layer. The experimental observation is qualitatively explained by spatial overlap between exciton linear response functions calculated within a single defect model.

Magneto-optical Kerr effect spectroscopy based study of Landé g-factor for holes in GaAs/AlGaAs single quantum wells under low magnetic fields

Results from a magneto-optical Kerr effect (MOKE) spectroscopy study of ground state heavy-hole and light-hole excitons in GaAs/Al0.3Ga0.7As single quantum wells, with widths ranging from 4.3 nm to 14 nm, are presented. A novel setup and signal analysis procedure was adopted whereby polar MOKE measurements in magnetic fields up to 1.8 T could be performed with a conventional H-frame electromagnet. A first principles simulation based procedure used for simultaneously analyzing both the measured Kerr rotation and Kerr ellipticity spectral lineshapes is described in detail. The Zeeman splitting obtained from the above analysis helped to determine the longitudinal Landé g-factors. The hole g-factors were found to vary with well width, ranging from −0.6 to 1.1 for heavy-holes and 6.5 to 8.6 for light-holes. While the heavy-hole g-factor values are in fair agreement with values expected from k·p perturbation theory, no evidence is found to support theoretical predictions of possible well width dependent giant light-hole g-factors in GaAs/AlxGa1–xAs single quantum wells.

Effect of substrate temperature on optical properties and strain distribution of ZnTe epilayer on (100) GaAs substrates

The photoluminescence properties of the ZnTe epilayers grown on (100) GaAs substrates with various substrate temperatures are investigated. The spectral characteristics of both bound and free excitonic emissions measured at different measurement temperatures and excitation power, show that compared with the too low (390°C) or too high (440°C) substrate temperature, the moderate substrate temperature (around 420°C) is suitable to obtain a high-quality epilayer due to its narrowest spectral width and smallest strain. With increasing excitation power the peak energy of the free excitonic emission decreases linearly, while the spectral width remains an approximate constant. This implies that the decreasing peak energy should be attributed to an increasing strain with increasing the effective excited depth, and the increasing strain only reduces the bandgap, but does not cause much influence on the crystalline quality for each epilayer. Furthermore, the increasing strain also causes the ground state free excitons splitting into the light hole and heavy hole free excitons.

Exciton optical transitions in quantum wells with spin-orbit coupling

The energy spectrum of light-hole and heavy-hole excitons and optical absorption in a quantum well have been analyzed taking into account Rashba spin-orbit coupling. Interband and intraband exciton transitions have been considered. It has been shown that, in the presence of spin-orbit coupling, the probabilities of the interband and intraband photoelectric effects diverge in the vicinity of the threshold if the electron-hole interaction is neglected. The threshold probabilities of the interband and intraband photoelectric effects become finite when Coulomb interaction is taken into account.

Three-dimensional electronic spectroscopy of excitons in asymmetric double quantum wells

We utilize a three-dimensional (3D) visible spectroscopy to reveal and explore coherent coupling between excitons localized to GaAs quantum wells separated by barriers 4 and 6 nm wide. The coupled excitons were energetically separated by 43 meV, close to the longitudinal optical phonon energy, due to the different widths of the quantum wells to which they were localized. The ability to isolate coherence pathways in 3D spectroscopy has made it possible to not only identify weak coherent coupling, but also to explore the nature of the coupling through analysis of the peak shapes. This peak shape analysis showed inhomogeneous broadening of the excitons localized to different wells to be uncorrelated, as expected, while coupling between heavy-hole and light-hole excitons localized to the same well was shown to be correlated. To gain some insight into the coupling mechanism we explored the dependence of the coupling strength on the barrier width and hence spatial separation. Based on these results we discuss the possibility of phonon-assisted dipole coupling.

All-optically induced currents resulting from frequency-modulated coherent polarization

We employ polarization-shaped ultrafast optical pulses to generate photocurrents, which only arise if the optically induced coherent polarization is frequency modulated. This frequency modulation is obtained via detuned excitation of light-hole excitons in (110)-oriented GaAs quantum wells. The observed photocurrents vanish for resonant excitation of excitons and reverse their direction with a change of the sign of detuning. Moreover, the currents do not exist for continuous-wave excitation. Our work reveals the existence of a new class of photocurrents and visualizes the complexity of current response tensors. This is helpful for the better understanding of optically induced microscopic transport in semiconductors.

Quantum size effect on the luminous properties of excitonic states in a quasi-one-dimensional wurtzite GaN nanowire: Two-parameter variational approach

In order to reflect the anisotropic confined situations of quasi-one-dimensional (Q1D) wurtzite nanowires (NWs) in axial and radial directions and wurtzite GaN crystal, a two-parameter variational approach is brought forward and applied to investigate luminous properties of excitonic states in wurtzite NW systems. Numerical calculations on a GaN NW are performed. The quantum size effect on the excitonic binding energies of ground states, the ground state energies as well as the emission spectra is analyzed in detailed. Both the light hole excitons and heavy hole excitons in wurtzite NWs are taken in account. The calculated excitonic binding energies in the wurtzite GaN NWs are far larger than those in cubic GaAs-based quantum wires with the same radius. This is mainly ascribed to the large effective masses of electron and hole and relatively small dielectric constants in GaN material. The excitons emission wavelengths calculated here qualitatively agree with recent experimental results of exciton luminous spectra in Q1D GaN NWs. This confirmed the reasonability and reliability of the present theoretical scheme and numerical results. The numerical results also show that the two-parameter variational approach is reasonable and necessary for the description of excitonic states in Q1D wurtzite GaN NWs.

Performance prediction of an electroabsorption modulator at 1550 nm using GeSn/SiGeSn Quantum Well structure

The absorption spectra in strain-free Ge0.992Sn0.008/Si0.3Ge0.61Sn0.09 Quantum Well structure due to direct gap excitons formed in Ge0.992Sn0.008 wells reported by Chang and Chang are reproduced by an empirical expression. Two Gaussian distributions for heavy hole and light hole excitons and two exponential functions describing continuum transitions and The Sommerfeld factors are used for the fit. The expressions are then used to obtain the change in refractive index, Δn, with field (electrorefraction) using the Kramers–Kronig relation. With the calculated changes in absorption and refraction with bias, other performance parameters of an Electro-Absorption Modulator (EAM), like extinction ratio, chirp parameter, and a figure of merit (FoM) defined as the ratio of extinction coefficient and insertion loss, are evaluated. The FoM takes the largest value for 1μm length of the EAM with 2V bias applied to the p–i–n structure. The parameters are also calculated for different wavelengths as well as detuning lengths. For applications as short pulse generation the transmission T as a function of bias V should be linear. We have evaluated the best value of bias voltage that makes the sum of second and third derivatives of T with V equal to zero so that the variation of T with V becomes maximally linear for optimum operation as a short pulse generator.

Effect of light-hole tunnelling on the excitonic properties of GaAsP/AlGaAs near-surface quantum wells

Light-hole tunnelling to the surface states is studied using photoluminescence (PL) spectroscopy and transient reflectivity measurements in the tensile-strained GaAsP/AlGaAs near-surface quantum well (NSQW) samples by reducing the top barrier layer thickness from 275 to 5 nm. The ground state transition (e1–lh1) remains excitonic even at room temperature (RT) for a buried quantum well sample with 275 nm thick top barrier. When the top barrier thickness is reduced to 50 nm the same transition is found to be excitonic only at low temperatures but changes to free-carrier recombination at higher temperatures. When the top barrier layer thickness is further reduced to 5 nm, the ground state transition is no longer excitonic in nature, where it shows free-carrier behaviour even at 10 K. We therefore find a clear relationship between the character of the ground state transition and the top barrier layer thickness. Light-hole excitons cannot be formed in NSQW samples when the top barrier layer thickness is kept reasonably low. This is attributed to the quantum mechanical tunnelling of free light holes to the surface states, which is found to be faster than the exciton formation process. A tunnelling time of ∼500 fs for light holes is measured by the transient reflectivity measurements for the NSQW sample with a 5 nm top barrier. On the other hand, heavy-hole-related transitions in NSQW samples are found to be of excitonic nature even at RT because of the relatively large tunnelling time. It supports the dominance of excited state feature over the ground state transition in PL measurements at temperatures higher than 150 K.

Control of Rabi-splitting energies of exciton polaritons in CuI microcavities

We have investigated the active-layer-thickness dependence of exciton-photon interactions in CuI microcavities. The active layer thickness was changed from λ/2 to 2λ, where λ corresponds to an effective resonant wavelength of the lowest-lying exciton. In the CuI active layer, thermal strain removes the degeneracy of the heavy-hole (HH) and light-hole (LH) excitons at the Γ point. Angle-resolved reflectance spectra measured at 10 K demonstrate the strong coupling between the HH and LH excitons and cavity photon, resulting in the formation of three cavity-polariton branches: the lower, middle, and upper polariton branches. The energies of the three cavity-polariton modes as a function of incidence angle are reasonably explained using a phenomenological Hamiltonian to describe the exciton-photon strong coupling. It is found that the interaction energies of the cavity-polariton modes, the so-called vacuum Rabi-splitting energies, are systematically controlled from 29 (50) to 48 (84) meV for the LH (HH) exciton by changing the active layer thickness from λ/2 to 2λ. The active-layer-thickness dependence of the Rabi-splitting energies is semi-quantitatively explained by a simple model.

Third-order nonlinear optical susceptibility and photoionization of an exciton in quantum dots

In this study, an investigation of an exciton in a disk-like quantum dot has been carried out by using the matrix diagonalization method and the compact density-matrix approach. The dependences of binding energies of excitons for the light and heavy hole masses on the size of the confining potential are analyzed explicitly. The third-order nonlinear optical susceptibility of third harmonic generation as well as the photoionization cross section has been calculated for the light-hole and heavy-hole excitons. The results are presented as a function of the incident photon energy. The results show that the third-order nonlinear optical susceptibility and the photoionization cross section of an exciton in a quantum dot are strongly affected by the dot size and the hole mass.

Observation of a dynamical mixing process of exciton-polaritons in a ZnSe epitaxial layer using four-wave mixing spectroscopy

We have observed a coherent spectral change of exciton-polaritons in a ZnSe epitaxial layer through spectrally resolved four-wave mixing spectroscopy. The spectra exhibit an exchange of the dominant peak position between the different polariton branches depending on the delay time of the second pulse. This result reflects the initial creation process of polaritons with many-body interactions. The calculation based on the exciton-photon microscopic model reveals that the spectral change occurs due to the four-particle correlations between heavy-hole and light-hole excitons; it clearly shows the dynamical mixing process of exciton-polaritons in the initial creation.

Quantum beats of type-I and type-II excitons in an InxGa1−xAs/GaAs strained single quantum well

We have investigated the quantum beat of the type-I heavy-hole (HH) and the type-II light-hole (LH) excitons in an In0.15Ga0.85As/GaAs strained single quantum well (SQW) using a reflection-type pump-probe technique. The type-II LH exciton consists of the electron and LH located in the In0.15Ga0.85As and GaAs layers, respectively. The energies of the type-I and the type-II excitons were evaluated with photoreflectance spectroscopy. The exciton states were calculated based on a variational method. The time-domain signals clearly show the oscillatory structure with the period corresponding to the splitting energy of the HH and the LH excitons. From the pump-energy dependence of the Fourier transform spectrum and intensity, it is concluded that the oscillation originates from the quantum beat of the type-I HH and the type-II LH excitons.

Dynamic Stark effects in a semiconductor quantum dot exhibiting a two-hole species valence band

We have analytically explored the nature of Stark shift and self energy correction in the presence of two discrete excitonic states i.e, the heavy-hole and light-hole valence bands, below the fundamental absorption edge of semiconductor quantum dots. The time dependent perturbation technique for a three-level system, under the near band gap resonant excitation regime, has been employed; with the incorporation of the relaxation and dephasing mechanisms phenomenologically. The detuning spectra of Stark shift and self energy correction and their dependence on a moderately strong electric field, have been analyzed. The numerical and analytical estimations in a weak noncentrosymmetric CdTe quantum dot show a decrease in Stark shift and an enhancement in self energy correction for both heavy-hole and light-hole excitons.

Comparison of measurement techniques for electron spin relaxation time in a GaAs/AlGaAs multiple quantum well

We compared the performance of three measurement techniques for electron spin relaxation time, i.e., polarization- and time-resolved photoluminescence (PTRPL) measurement, time-resolved reflectance (TRR) measurement, and time-resolved Kerr rotation measurement. With all three techniques similar electron spin relaxation times (0.83–1.2ns) were obtained in a (110) GaAs/AlGaAs multiple quantum well with excitation wavelengths near the heavy-hole exciton resonance at 77K. We extracted the spin-independent contribution of the Coulomb interaction which obscures TRR signals as an offset without limiting the excitation wavelength to heavy-hole exciton resonance, using the degree of electron spin polarization simultaneously obtained by the PTRPL measurement. It was found that the Coulomb interaction contribution is dominant in TRR signals over both heavy- and light-hole exciton resonances.

Valence band structure of polytypic zinc-blende/wurtzite GaAs nanowires probed by polarization-dependent photoluminescence

We conducted temperature-dependent measurements of the photoluminescence (PL) polarization on GaAs nanowires (NWs) with polytypic zinc-blende/wurtzite structure in order to probe the symmetry and energy structure of the valence band in the wurtzite segments of the NWs. The low-temperature measurements revealed that in most of the investigated cases, the ground level of the interface excitons responsible for the PL is formed by the heavy hole. To describe the observed temperature dependence of the degree of PL polarization, we developed a theoretical model that allows an estimation of the splitting between the heavy hole and light hole exciton subbands in these NWs. This splitting is smaller than expected in pure wurtzite on the basis of recent first-principles calculations, which may be attributed to the multiple twinned nature of the wurtzite sections, which effectively behave as a polytype of lower hexagonality.