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Excitonrelated nonlinear optical properties in cylindrical quantum dots with asymmetric axial potential: combined effects of hydrostatic pressure, intense laser field, and applied electric field
Nanoscale Research Letters volume 7, Article number: 508 (2012)
Abstract
Abstract
The exciton binding energy of an asymmetrical GaAsGa_{1−x}Al_{ x }As cylindrical quantum dot is studied with the use of the effective mass approximation and a variational calculation procedure. The influence on this quantity of the application of a directcurrent electric field along the growth direction of the cylinder, together with that of an intense laser field, is particularly considered. The resulting states are used to calculate the excitonrelated nonlinear optical absorption and optical rectification, whose corresponding resonant peaks are reported as functions of the external probes, the quantum dot dimensions, and the aluminum molar fraction in the potential barrier regions.
Background
Exciton states in quantum dot (QD) systems are relevant to the understanding of a number of their optical and electronic features. There have been a significant number of works dealing with excitonic effects in these nanodimensional structures (see, for instance,[1]). Among the external probes affecting the spectrum of localized states in semiconducting nanostructures, we can mention the electric field. There is a work from Peter and Lakshminarayana on the influence of the electric field on donor binding energies in QDs with parabolical, spherical, and rectangular confinement[2]. Besides, the asymmetric potential quantum well (QW) configuration can lead to the enhancement of the interband oscillator strength through the obtention of larger values of the dipole moments of the optical transitions. It is possible to predict the obtention of important excitonrelated nonlinear optical responses in these confined semiconducting structures. We can mention some theoretical studies regarding the optical nonlinearities[3, 4].
The application of intense laser fields (ILFs) to lowdimensional semiconductor systems has allowed the appearance of new and interesting features in their electronic structures. One of them is the ILFinduced transition to a doubleQW configuration in an otherwise singlewell heterostructure[5]. This phenomenon occurs when the value of the socalled laserdressing parameter[6] becomes larger than the halfwidth of the QW. The mathematical description goes through deriving a modified form of the confining potential energy function[7]. Niculescu and Burileanu have put forward calculations on shallow impurity states in QW wires and QWs of different geometries, combining the ILF effects and the application of static magnetic and electric fields[8]. We have reported on the influence of the laserinduced transition from single to doublewell potential on impurity states in a GaAsbased QW[9], whereas the optical response of semiconducting nanostructures subject to the radiation of highintensity laser fields has also been a matter of some studies in the last few years (see, for instance,[10–12]).
With this motivation, in this work, we investigate the properties of the excitonrelated nonlinear optical absorption (NOA) and nonlinear optical rectification (NOR) in cylindrical GaAsGa_{1−x}Al_{ x }As QDs under ILFs, directcurrent (dc) electric fields, and hydrostatic pressure.
Methods
Theoretical framework
In what follows, we shall express energies in effective Rydbergs\left({R}_{0}=\frac{\mu \phantom{\rule{0.3em}{0ex}}{e}^{4}}{2\phantom{\rule{0.3em}{0ex}}{\hslash}^{2}\phantom{\rule{0.3em}{0ex}}{\epsilon}^{2}}\right) and lengths in effective Bohr radii\left({a}_{0}=\frac{{\hslash}^{2}\phantom{\rule{0.3em}{0ex}}\epsilon}{\mu \phantom{\rule{0.3em}{0ex}}{e}^{2}}\right). According to the model proposed by Le Goff and Stébé, within the effective mass approximation, the Hamiltonian for exciton states in a cylindrical GaAsGa_{1−x}Al_{ x }As QD under ingrowthdirection applied electric field (F) is given by[1]
wherer=\sqrt{{\rho}^{2}+{({z}_{\text{e}}{z}_{\text{h}})}^{2}},\rho =\overrightarrow{{\rho}_{\text{e}}}\overrightarrow{{\rho}_{\text{h}}}, and ± stands for electrons and holes;{m}_{\text{e}}^{\ast} ({m}_{\text{h}}^{\ast}) labels electron (hole) effective mass, while μ is the electronhole reduced mass, e is the electron charge, ε is the GaAs static dielectric constant, and V_{e}(ρ_{e}z_{e})V_{h}(ρ_{h}z_{h})] is the QD confinement potential function for the electron (hole) carrier. We assume that the applied electric field is oriented along (0, 0, −z).
Our model considers a QD of cylindrical shape with radius R and height L. For the confinement potential of the carriers, we have considered infinite and finite confinement potentials in the ρ and zdirections, respectively. In such an approximation, it is valid to consider the separation{V}_{i}({\rho}_{i},{z}_{i})={V}_{i}^{1}\left({\rho}_{i}\right)+{V}_{i}^{2}\left({z}_{i}\right) (i = e,h). Here,{V}_{i}^{1}\left({\rho}_{i}\right)=0 for ρ_{ i }≤ R and{V}_{i}^{1}\left({\rho}_{i}\right)\to \infty for ρ_{ i }> R. In the case of the zdependent asymmetric confinement, we have{V}_{i}^{2}\left({z}_{i}\right)=0 for z_{ i } ≤ L/2,{V}_{i}^{2}\left({z}_{i}\right)={V}_{0}^{i}\left({x}_{\text{Al}}^{1}\right) when z_{ i }≤ −L/2, and{V}_{i}^{2}\left({z}_{i}\right)={V}_{0}^{i}\left({x}_{\text{Al}}^{2}\right) when z_{ i }≥ + L/2. We use the value of the aluminum contents, ({x}_{\text{Al}}^{1},{x}_{\text{Al}}^{2}), in the confining barriers to monitor their corresponding heights. For electrons (holes), the axial confinement potential barrier heights{V}_{0}^{\text{e}} ({V}_{0}^{\text{h}}) are obtained from the 60% (40%) of the bandgap between the well (GaAs) and barrier (Ga_{1−x}Al_{ x }As) materials (\Delta {E}_{g}^{0}=(1,155\phantom{\rule{0.3em}{0ex}}x+370\phantom{\rule{0.3em}{0ex}}{x}^{2}) meV).
In order to obtain the exciton eigenfunctions, and the corresponding energies (E), we adopt a variational scheme[13] which consists minimizing the functional E(Ψ)=〈ΨHΨ〉 by using the trial wave function, Ψ, as[1]
where{N}_{{n}_{e},{n}_{h}} is the normalization constant,{\alpha}_{{n}_{e},{n}_{h}} and{\beta}_{{n}_{e},{n}_{h}} are variational parameters, and{{\rm Y}}_{{n}_{e},{n}_{h}}\phantom{\rule{0.3em}{0ex}}({\rho}_{\text{e}},{\rho}_{\text{h}},{z}_{\text{e}},{z}_{\text{h}})=F\left({\rho}_{\text{e}}\right)\phantom{\rule{0.3em}{0ex}}F\left({\rho}_{\text{h}}\right)\phantom{\rule{0.3em}{0ex}}{g}_{{n}_{e}}\left({z}_{\text{e}}\right)\phantom{\rule{0.3em}{0ex}}{g}_{{n}_{h}}\left({z}_{\text{h}}\right) is the eigenfunction of the Hamiltonian in Equation 1 without the Coulomb term at the righthand side. The integer numbers n_{ e } and n_{ h }identify the quantized states for both the electron and the hole, respectively, associated with the confinement along the zdirection. The uncorrelated radial wave functions for single particles are given by F(ρ_{ i }) = J_{0}(θ_{0}ρ_{ i }/R), where J_{0} represents the Bessel function of the zeroth order and θ_{0} = 2.4048 is its first root on the real axis. The way of obtaining the uncorrelated axial wave functions for single particles relies in the method developed by Xia and Fan[14]. The energy of the exciton,{E}^{{n}_{e},{n}_{h}}, is calculated by minimizing the Hamiltonian with the trial wave function. The exciton binding energy ({E}_{\text{b}}^{{n}_{e},{n}_{h}}) is obtained from the definition{E}_{\text{b}}^{{n}_{e},{n}_{h}}={E}_{0}^{{n}_{e},{n}_{h}}{E}^{{n}_{e},{n}_{h}}, where{E}_{0}^{{n}_{e},{n}_{h}} is the eigenvalue associated to{{\rm Y}}_{{n}_{e},{n}_{h}}({\rho}_{\text{e}},{\rho}_{\text{h}},{z}_{\text{e}},{z}_{\text{h}}). In the present work, we are interested in the two lowest excitonic states: ϕ_{1}≡Ψ_{1,1} and ϕ_{2}≡Ψ_{2,1}, with energies{E}^{1,1}\equiv {E}_{1} and{E}^{2,1}\equiv {E}_{2}, respectively.
Looking to include the nonresonant intense laser effects (with the choice of the polarization of laser radiation to be parallel to the zdirection), we have followed the Floquet method. To briefly summarize the outcome of such a procedure, it is possible to say that the information regarding laser influence will appear in the zdependent part of the confinement potential in Equation 1 by substituting{V}_{i}^{2}\left({z}_{i}\right)\to \u3008{V}_{i}^{2}\u3009({z}_{i},{\alpha}_{0i})[5, 6, 15, 16]. In that expression, the quantity{\alpha}_{0i}=\left(e{A}_{0}\right)/({m}_{i}^{\ast},c\varpi )=({\mathbb{I}}^{1/2}/{\varpi}^{2})(e/{m}_{i}^{\ast}){(8\Pi /c)}^{1/2} is the laserdressing parameter. Here,\mathbb{I} and ϖ are, respectively, the average intensity and the frequency of the laser field wave. A_{0} represents the amplitude of the vector potential of the incident radiation. In the case of the Coulomb interaction, the last term in Equation 1 must be replaced as
with z_{eh} = z_{e}−z_{h} and{\alpha}_{0}=\left(e\phantom{\rule{0.3em}{0ex}}{A}_{0}\right)/\left({\mu}^{\ast}\phantom{\rule{0.3em}{0ex}}c\phantom{\rule{0.3em}{0ex}}\varpi \right).
After the energies and the corresponding envelope wave functions are obtained, the magnitude of the resonant peaks of the{E}_{1}\leftrightarrows {E}_{2} excitonrelated NOA and NOR coefficients can be derived under a density matrix approach. They are given, respectively, by[17]
and
where{M}_{\mathrm{ij}}=\left.\right)" close=">">\n \n \n \n \varphi \n \n \n i\n \n \n \n \n \n z\n \n \n e\n \n \n \u2212\n \n \n z\n \n \n h\n \n \n \n \n \n \varphi \n \n \n j\n \n \n \n. Also, ε_{0} is the vacuum permittivity, n is the refractive index of the QD material, N is the electron density in QD, and{\omega}_{21}=({E}_{2}{E}_{1})/\hslash. The quantities T_{1,2}represent the lifetimes that associate with the damping in the system. Moreover, the hydrostatic pressure effects are introduced via the pressure dependence of the effective masses and the dielectric constant. In the calculations, we have used{m}_{\text{e}}^{\ast}={[1+15,020/{E}_{g}+7,510/({E}_{g}+341\left)\right]}^{1},{m}_{\text{h}}^{\ast}=0.340.1\times 1{0}^{3}\phantom{\rule{0.3em}{0ex}}P, and\epsilon =12.7\phantom{\rule{0.3em}{0ex}}exp(1.67\times 1{0}^{3}\phantom{\rule{0.3em}{0ex}}P). Here, E_{ g }= (1,519 + 10.7 P) meV, P is the hydrostatic pressure (in kbar), and m_{0}is the free electron mass[18].
Results and discussion
Let us now go back to giving the energy values in units of millielectron volts and the lengths in nanometers. The calculations use T_{1} = 1 ps, T_{2} = 0.2 ps, and N = 3×10^{22}m^{−3}[17, 19]. We have chosen to report the calculated quantities as functions of the Al molar fraction in the lefthand barrier. The composition of aluminum in the righthand potential barrier remains fixed at{x}_{\text{Al}}^{2}=0.33
Figure1 shows the dependence of the ground (n_{ e }= 1 and n_{ h }= 1) exciton binding energy as a result of the variation of the lefthandbarrier Al composition{x}_{\text{Al}}^{1}. The results correspond to the geometric configuration: L = 15 nm; R = 10 nm. In Figure1a,b, we have considered several values of the hydrostatic pressure with different setups of the dc electric field (F = 0 kV/cm (a); F = 20 kV/cm (b)), taking a constant value of the intense laser field parameter α_{0}(0) = 3L(0)/4 = 11.3 nm. Figure1c contains the results of{E}_{\text{b}}\left({x}_{\text{Al}}^{1}\right) with P = 0 and α_{0} = 0, for two values of the applied dc electric field. Results for E_{b}in a system with equal geometric and external configurations, but for the firstexcited exciton state, are presented in Figure2.
Let us first explain the results for E_{b}of Figures1c and2c. In both figures, we notice an increasing variation of E_{b} as a result of the increment in{x}_{\text{Al}}^{1}. This happens with and without applied dc electric field and relates with the growing degree of the carrier confinement. Augmenting the Al molar fraction in the lefthand barrier leads to the effective deepening of the quantum potential well for electrons and holes. The electron (both the ground and the excited state) and hole wave functions become more spatially localized in the well region, and the expected electronhole distance diminishes. This has the effect of strengthening the Coulombic interaction, with the observed growth in the binding energy.
The inversion in energy position of the curves with F = 0 and 20 kV/cm, seen when going from Figure1c to Figure2c is due to the dcelectricfieldinduced linear decrement of the height of one of the potential barriers and the linear increase of the other. This, together with the inclination in the QW bottom position, makes the carriers to be less localized inside the well, displacing the groundstate density of probability towards one side of the system. This influence is opposite in sign for electrons and holes. The expected electronhole distance augments and the Coulombic attraction weakens for nonzero F. In the case of the firstexcited exciton complex, the effect of an augmenting dc field on the holeconfined level is the same, but the wave function of the conduction electron with n_{ e }= 2 is deformed in such a way that most of its probability density has a greater spatial coincidence with that of the hole in n_{ h }= 1. Thus, the quantity 〈ϕ_{2}z_{e}−z_{h}ϕ_{1}〉 will have a lower value, and the Coulombic attraction will be stronger.
Figure3 contains a picture of the confining potential for both electrons and holes for different geometric configurations  related with the intensity of the laser field. Schematic representations of the ground hole state and the ground and firstexcited electron states are provided as well. The aim of this figure is to help understand the features of the binding energy depicted in Figures1 and2:

1.
By comparing Figures 1a and 2a, it becomes clear that for finite values of the hydrostatic pressure, the binding energy is larger in the excited state compared with the ground state ( {E}_{\text{b}}^{2,1}>{E}_{\text{b}}^{1,1}). This situation is contrary to the one observed in the case of P = 0 ( {E}_{\text{b}}^{1,1}>{E}_{\text{b}}^{2,1}). We can observe from Figure 3a,b, which corresponds to the case of P=0, that the density of probability of the electron excited state (n _{ e }= 2) is more extended along the structure. This bound (excited) state corresponds, in fact, to the infinite barrier system and not to our QW, but the electron ground state (n _{ e }= 1) is actually a confined state of the QW. Consequently, according to these arguments, {E}_{\text{b}}^{1,1}>{E}_{\text{b}}^{2,1}, given that the average electronhole distance is smaller if the electron moves with the energy of the confined ground state. In Figure 3c,d, which is valid when P = 50 kbar (Figure 3e,f, valid for P = 100 kbar), it is possible to observe that the state with n _{ e }= 2 tends to become localized within the QW region; in fact, it is bound to the QW when P = 100 kbar, whereas the ground state remains with the same zeropressure character. As a result, we obtain {E}_{\text{b}}^{2,1}>{E}_{\text{b}}^{1,1}.

2.
The comparison between the results in Figures 1b and 2b yields clearly to the situation {E}_{\text{b}}^{2,1}>{E}_{\text{b}}^{1,1}. In Figure 3g,h, we are depicting the densities of probability that correspond to the electron and hole states involved in the formation of the two excitons. The application of an electric field will push the electron and hole wave functions to opposite directions in the case of both uncorrelated ground states with the consequent increment of the electronhole distance and reduction of the binding energy. However, a very different situation is that involving the coupling of the ground hole state and the firstexcited electron one, whose wave functions are displaced towards the same space region, decreasing the electronhole distance and augmenting the corresponding exciton binding energy.

3.
When we compare the results in Figures 1c and 2c, it can be clearly observed that the condition {E}_{\text{b}}^{1,1}>{E}_{\text{b}}^{2,1} is always fulfilled. The zerolaserfield and zeropressure case corresponds to the situation seen in Figure 3i,j. We can, therefore, extract the following information: Although the action of the electric field rises the proximity of the hole and firstexcited electron wave functions and makes the wave functions of the hole and ground electron states to move further apart, it is a fact that under the particular conditions for the obtention of those results, one finds that {E}_{\text{b}}^{1,1}>{E}_{\text{b}}^{2,1}. This is a consequence of the strong confinement of the electron ground state within the QW region, spatially close to the maximum of the ground hole probability density, with really little influence of the electric field. Given that the firstexcited electron state lies near the potential barrier edge, the application of an electric field largely modifies it, generating a bigger separation between the electron and the hole and a decrease of the binding energy.
Now, we come back to the discussion of Figures1 and2. Regarding the results of Figures1a and2a, we can say that the significant distinction in the potential profiles that confine electrons and heavy holes is a consequence of the important difference between the effective masses of both kinds of carriers. The potential profile for the conduction electrons, given their lightness, is strongly modified by the effect of the laser field, whereas the deformation of the heavyhole valence band is smaller. The descent observed for{E}_{\text{b}}\left({x}_{\text{Al}}^{1}\right) in Figure1a is due, then, to the diminishing of the carrier confinement. Augmenting{x}_{\text{Al}}^{1} produces a higher potential barrier at the lefthand side. This is reflected in a progressively more homogeneous raising of the QW bottom caused by the change in the laser fieldinduced change in the QW shape. Thus, the energy position of the electron ground state shifts upwards, and its density of probability spreads over a larger region. On the contrary, the hole state remains more localized when the lefthand barrier is higher, in spite of the laserinduced valence band bending deformation that narrows the QW’s lower part. This pushes the energy levels upwards, and larger values of the expected electronhole distance are then obtained.
With hydrostatic pressure applied, the ground staterelated exciton binding energy keeps decreasing along the whole range of the lefthandbarrier Al composition values. However, as we notice from Figure1a, the starting and ending values of the curves are increasingly higher with P. There are two main reasons for this phenomenon: (1) The hydrostatic pressure induces growth in the electron effective mass, the magnitude of the conduction band groundstate energy becomes smaller, with an associated increment of the wave function confinement. This shifting effect on heavy holes is less pronounced, and the change of m_{e}(P) becomes the dominant effect. As a result of this, there will be a reduction in the expected electronhole distance, as well as an increase in the binding energy, for a given value of{x}_{\text{Al}}^{1} if compared with the zeropressure situation. (2) The dielectric constant is a decreasing function of the hydrostatic pressure. Hence, again fixing a value of the Al composition in the lefthand barrier, a larger value of the pressure will affect the Coulombic interaction in the system by increasing its magnitude. This latter effect is the most important.
Let us analyze now the same problem but for the firstexcited exciton state when F = 0 (Figure2a). The argument related with the pressureinduced changes in the effective masses and dielectric constant keeps its validity to justify the higher values of E_{b} for nonzero pressures compared with those of the P = 0 case. However, unlike the groundstate exciton, the firstexcited binding energy is not a monotonically decreasing function of{x}_{\text{Al}}^{1} over the range of values of this quantity considered. Considering first the zeropressure regime, we observe a slight growth in E_{b} when the Al concentration in the lefthand barrier augments from 0 to approximately 0.18. Such an increment associates with the higher localization of the n_{ e }= 1 wave function inside the QW region due to the presence of higher confining barriers. If the value of{x}_{\text{Al}}^{1} grows beyond that point, a very smoothly decreasing variation of E_{b}(P = 0) will start, due to the loss in electron confinement, with the corresponding augmenting rate for the expected electronhole distance. For finite pressures, we find the same growing behavior for the smaller values of the Al molar fraction, but the ulterior decrease is much more pronounced, resembling that exhibited by the exciton binding energy of the ground state. In this case, the explanation can be found in the pressure effect on the n_{ e }= 1 state localization.
The results presented in Figures1b and2b correspond to the variation of the exciton binding energy due to the increment in the Al composition in the lefthand confining barrier in the situation in which there is an additional applied dc electric field with the amplitude fixed to F = 20 kV/cm. The configuration includes the application of laser radiation with the intensity characterized by α_{0}(P) = 3L(P)/4. The distribution of E_{b}curve positions with the pressure value as a parameter is the same as in the zerodcfield case and can be explained by the arguments above. The monotony of the curves is significantly different in this case. The groundexciton binding energy as a function of{x}_{\text{Al}}^{1} (Figure1b) shows initially a decreasing behavior until the lefthandbarrier Al composition is approximately 0.18. Above this value, all curves change their variation to an increasing one. The reduction in E_{b} for the smaller values of the Al composition is mostly due to the increment in the expected electronhole distance associated to the additional electronhole polarization induced by the dc field. The subsequent growth in E_{b} relates with the effect of the higher lefthand barrier on the displaced electron wave function. A higher potential barrier repels the shifted probability density of the ground electron state away from the corresponding interface. This causes a reduction in the average electronhole distance and the observed growth of E_{b} for the larger values of{x}_{\text{Al}}^{1}.
The variations of the firstexcited exciton binding energy with respect to the increment in the lefthandbarrier Al alloy composition, considering three values of the hydrostatic pressure as parameters and the presence of intense laser and applied dc fields (with intensity F=20 kV/cm) also show a decreasing monotony when{x}_{\text{Al}}^{1} augments in the region of its smaller values. The upper value for the decrease of E_{b} depends on the hydrostatic pressure. If P = 0, then the binding energy is an all the way decreasing function of the Al concentration. The increase in the expected electronhole distance induced by the dc field  which is predominant for the lower alloy compositions  can be understood by remembering that the excited state is less affected by barrier repulsion. Augmenting the lefthandbarrier height  tending to convert the system into a symmetric QW  causes the pushing effect of the raising in the QW bottom to have a greater influence on the delocalization of the n_{ e }= 2 state, leading to the weakening of the Coulombic interaction. When the pressure goes up and reaches 50 kbar, there will be a slight growth in E_{b}above the value of{x}_{\text{Al}}^{1} at which the repulsive barrier interaction becomes predominant. The application of hydrostatic pressure augments the electron effective mass and the firstexcited electron level occupies a lower energy position, thus granting a higher sensitivity to the barrier rising. If the pressure value is even bigger (P = 100 kbar), E_{b} recovers the increasing variation for larger values of the lefthand Al composition, as what happens for the ground exciton state.
In order to have a support for the discussion of the excitonrelated nonlinear optical properties, Figures4,5, and6 show, respectively, the variations of the transition energy\mathrm{\Delta E}={E}_{2}{E}_{1}={E}_{1}^{\text{e}}{E}_{0}^{\text{e}}+{E}_{\text{b}}^{1,1}{E}_{\text{b}}^{2,1} ({E}_{0}^{\text{e}} and{E}_{1}^{\text{e}} are the energies of the ground and firstexcited states, respectively, of the noncorrelated electron due to the confinement in the conduction band, and{E}_{\text{b}}^{1,1} and{E}_{\text{b}}^{2,1} are the binding energies in Figures1 and2, respectively), the transition dipole moment matrix element M_{21}, and the absolute difference M_{22}−M_{11} as functions of{x}_{\text{Al}}^{1}. In all these figures, panel (a) corresponds to the zerodcfield case for a fixed magnitude of the intense laser field strength α_{0}=3L(P)/4 and the QD dimensions already adopted. The values of P used in previous figures are once again taken as curve parameters. The features exhibited by ΔE, M_{21} and M_{22}−M_{11} due to the growth in the lefthand Al molar fraction can be explained by the same type of arguments  related with the changes in the potential profile and wave function overlapping  given above and with the help of Figures1 and2. The most striking result is the vanishing of M_{22}−M_{11} for certain values of{x}_{\text{Al}}^{1}. This has something to do with the combination of a certain lefthand barrier height with the deformation of the potential band profile. There will be a particular configuration at which both exciton intrasubband matrix elements are equal in magnitude or both 0. This effect is reinforced by the action of the dc field, which makes it to appear at lower{x}_{\text{Al}}^{1}.
Figures7 and8 contain the evolution of the NOA and NOR resonant peaks due to the increment in the lefthandbarrier Al concentration. In the case of the absorption, one realizes that its variations, under the different circumstances considered in this work, are a direct consequence of the combination in the evolutions of ΔE and M_{21} as functions of{x}_{\text{Al}}^{1} (see Equation 4). Even though the application of an axial dc field in a QD without applied intense laser implies a reduction in the amplitude of the NOA response at zero pressure (Figure7c), the same situation but with an applied intense laser field results in the opposite effect. That is, with an intense laser acting on the QD, the application of an axially oriented electric field acts to enhance the resonant amplitude of the nonlinear absorption, which is only compared in magnitude to the nonlaser case if the pressure becomes as high as 100 kbar. Therefore, the dc electric field is a tool for enhancing the optical absorption in cylindrical GaAsGa_{1−x}Al_{ x }As QDs under intense laser radiation. Analogously, the shape of the dependence of the NOR coefficient on the variation of{x}_{\text{Al}}^{1} follows mainly from the one corresponding to M_{22}−M_{11} according to Equation 5. By observing Figure8c, we readily conclude that the cylindrical QD with an asymmetric axial potential barrier configuration at zero pressure and without the presence of static or intense laser electric fields is a rather worse optical rectifier if compared, for instance, with an asymmetric double QW[20], although the influence of a nonzero dc field slightly improves this property. If F = 0, but there is an intense laser applied, the tendency of the axial potential configuration to become symmetric causes the vanishing of the optical rectification coefficient, given the even and odd symmetries acquired by the ground and firstexcited exciton states, respectively. In that case, the factor M_{22}−M_{11} identically vanishes because each of the two dipole matrix elements becomes equal to 0 (Figure8a). The application of a dc field changes this situation given that, even if the Al concentrations in both barriers are the same, the presence of the linear dc fieldrelated potential term prevents the carrier densities of probability to be symmetric (Figure8b). The other values at which one detects the vanishing of the NOR correspond to potential configurations that, even with an asymmetric barrier profile, will present ground and firstexcited probability density distributions that acquire a symmetry, thanks to the changes in the conduction and valence band profiles induced by the application of the intense laser field. As a result, the involved intrasubband exciton dipole matrix element may either simultaneously vanish, or they can become equal in magnitude, when these probability densities are integrated together with z_{e}−z_{h}.
Conclusions
We have studied the cases of two 1slike exciton complexes formed by the coupling of a groundstate hole with a groundstate electron and with an electron occupying the firstexcited state in the axial motion of a GaAsGa_{1−x}Al_{ x }As cylindrical quantum dot with symmetrical and asymmetrical potential energy profiles in the axial direction. Also, the amplitudes of the groundtoexcited excitonrelated nonlinear optical absorption and nonlinear optical rectification coefficients are reported as functions of different external probes in the system, keeping a fixed geometric configuration for it, but changing the aluminum concentration in one of the axial potential barriers. Our work shows that hydrostatic pressure induces growth in both calculated binding energies. Increasing the Al concentration towards a symmetric axial potential profile configuration results in the reduction of the exciton binding energy in the zerodcfield case, for given laser field intensities, whereas it results in the increase of this quantity when there is an additional static onaxis field applied, with the only exception of the firstexcited exciton binding energy in the lowpressure regime.
The amplitude of the optical absorption resonant peak is a rather smooth function of the varying Al composition. The application of a dc field, in addition to the intense laser one, inverts the rate of variation of the optical absorption coefficient with respect to the increase in the hydrostatic pressure. The nonlinear optical rectification coefficient shows oscillations in its resonant peak amplitude due to the increment in the aluminum concentration of the lefthand axial barrier. In the zerodcfield case, this amplitude can happen for the symmetric barrier profile case (equal Al composition in each of the barriers) and zero dc field or in cylindrical quantum dots with a particular axial lefthandbarrier Al concentration at which the effect of the laser on the shape of the confining potential causes the equality in the intrasubband exciton dipole matrix elements. This latter case can be present in the presence or absence of an applied dc electric field.
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Acknowledgements
CAD thanks the partial support from Colombian agencies COLCIENCIAS, CODIUniversidad de Antioquia (project: E01553Propiedades ópticas no lineales en heteroestructuras semiconductoras de baja dimensionalidad (nitruros y arseniuros)), and Facultad de Ciencias Exactas y Naturales Universidad de Antioquia (CADexclusive dedication project 20122013). MEMR acknowledges the support from CONACYT through grant CB/2008101777 and through sabbatical grant 20112012 no.180636. He also thanks Universidad de Antioquia and Escuela de Ingeniería de Antioquia for the hospitality.
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AZ carried out the numerical work. REA carried out the analytical work. MEMR carried out the discussion of results. CAD carried out the numerical work. All authors read and approved the final manuscript.
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Zapata, A., Acosta, R.E., MoraRamos, M.E. et al. Excitonrelated nonlinear optical properties in cylindrical quantum dots with asymmetric axial potential: combined effects of hydrostatic pressure, intense laser field, and applied electric field. Nanoscale Res Lett 7, 508 (2012). https://doi.org/10.1186/1556276X7508
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DOI: https://doi.org/10.1186/1556276X7508
Keywords
 Exciton binding energy
 GaAsGa_{1−x}Al_{ x }As cylindrical quantum dot
 Effective mass approximation
 78.67.De; 71.55.Eq; 32.10.Dk