Abstract
The features of the electron energy spectrum in eccentric twodimensional GaAsAlGaAs quantum rings of circular shape are theoretically investigated taking into account the effect of externally applied magnetic and intense laser fields. Analytical expressions for the laserdressed confining potential in this kind of quantum ring geometry are reported for the first time. Finite element method is used to solve the resulting singleparticle effective mass twodimensional partial differential equation. It is shown that the allowed level spectrum is greatly influence by the external probe as well as by the breaking of geometric symmetry related to the changes in eccentricity. In presence of an intense laser field, the conduction band confining profile suffers strong modifications along the structure, with an additional contribution to symmetry breaking. These modifications of electronic quantum states reflect in the intraband optical absorption. Accordingly, the features of the intraband transitions are discussed in detail, revealing the significant influence of the magnetic field strength and laser field intensity and polarization, together with eccentricity, in the allowing of groundtoexcited states transitions and their corresponding intensities.
Introduction
Semiconductor quantum rings (QRs) are nanoscopic structures which have emerged as promising systems for applications in physics and technology due to their particular electronic and optical properties. Being doublyconnected structures, their physical properties can be largely different from singlyconnected ones such as quantum dots (QDs) or quantum disks. A comprehensive review on QRs and their features can be found in ref.^{1} (see also references therein for some other reviews on various aspects on the physics of QRs).
The practical realization of semiconductor QRs has been reported in a number of works. It is possible to find references on selfassembled InAsGaAs or InAsInP QR structures^{2,3,4,5,6,7,8,9,10} as well as on (mostly GaAsbased) dropletepitaxy or molecularbeamepitaxy ones^{11,12,13,14,15,16,17}. Different image techniques used in those investigations reveal that in some cases the obtained doublyconnected complexes are actually not homogeneously shaped as circular nor do they have a constant rim height^{15}. Instead, eccentrical geometries and single or multiple hilllike tops are noticed also including the formation of concentric doublering (threefoldconnected) configurations^{16}. Ovalshape rings are shown in refs^{5,8} whilst asymmetrically lobed QRs appear in ref.^{8}. Nonetheless, QRs with almost perfect circular or slightly oval shape were produced as well^{4,7}.
From the theoretical point of view, the electronic states in QRs of arbitrary shape and nonuniform width were investigated in ref.^{18} with the inclusion of threading magnetic field effects. Studies on the AharonovBohm effect, optical responses, spinorbit interaction effects, influence of pressure and temperature, excitons, impurity states, and conduction band nonparabolicity in this kind of systems have been put forward^{19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34}. Several other investigations dealing with deviations of the QR geometrical shape from perfect circular, homogeneous rim height or thickness have also been published^{35,36,37,38,39}. In particular the dependency of electronic and optical properties on the shape eccentricity are discussed in refs^{38,39}.
The influence of an externally applied intense laser field (ILF) on the electron states of quantum systems showing the discrete energy spectrum can be studied from the works by Gavrila^{40,41,42}. In the particular situation of quantum semiconductor nanostructures, the intense laser field has a significant effect onto the electronic properties, thus affecting the optical transitions as well^{43}. For instance, Cunha Lima et al. reported on the unexpected transition from single to double quantum well potential induced by ILF in a semiconductor quantum well^{44}. More recently, the properties of the electron states under intense laser fields in circular and elliptical QRs^{45} and in double ring structures^{46} have been the subject of research.
As it is seen from cited literature, a lot of studies were devoted to different aspects of QRs. Nevertheless, the joint action of the ILF and strong magnetic field on eccentric QRs was not yet considered. Therefore, our work is focused on studying electron energy levels in an eccentric twodimensional QR taking into account the effects of magnetic and intense nonresonant laser fields. After obtaining a set of analytical expressions for the ILFrelated twodimensional confining potential, the spectrum is obtained by solving the 2D effective mass partial differential equation with the use of the finite element method (FEM). A description of the theoretical environment is shown in section II. The section III is presented in two subsections; the first of them dedicated to show the external effects on electronic levels, and the second one to investigate the intensities of the intraband optical transitions with different polarizations of the incident laser fields. Finally, the section IV contains the conclusions.
Theoretical Model
The model studied in this paper is that of an eccentric 2D GaAsAl_{0.3}Ga_{0.7}As semiconductor quantum ring under the simultaneous actions of a nonresonant intense laser radiation and an externally applied magnetic field. The ring will be considered as a twodimensional quantum structure of D_{1}symmetry class confined in the x–y plane. The effective mass and single particle approximations are used to solve the Schrödinger equation for the confined electron into the region of the heterostructure.
The ring geometric parameters are the radii R_{1} and R_{2} of the circular inner and outer borders, respectively, and the weccentric displacement, defining the shift along the xdirection of the inner border (Fig. 1(a)). Between the two borders (inside the QR) the semiconductor material has a smaller band gap and, therefore, different electronic properties. Therefore we will further use the notations \({D}_{in}=\{(x,\,y){R}_{1}^{2}{(xw)}^{2}\le {y}^{2}\le {R}_{2}^{2}{x}^{2}\}\) and \({D}_{out}=\{(x,\,y){R}_{1}^{2}{(xw)}^{2} > {y}^{2}\,{\rm{or}}\,{R}_{2}^{2}{x}^{2} < {y}^{2}\}\) for the inside (blue area in Fig. 1(b)) and the outside (green area in Fig. 1(b)) plane domains of the QR, respectively. The nonresonant laser radiation is a linearly polarized monochromatic field that arrives perpendicularly to the ringplane described by:
where A_{0} and ω are the amplitude and frequency of the laser field’s vector potential \({\overrightarrow{A}}_{l}\), respectively, whereas β gives the orientation of the laser field polarization with respect to xaxis. Besides, \(\hat{x}\) and \(\hat{y}\) denote the inplane unit vectors. The static external magnetic field will be considered perpendicular to the ring, i.e. \(\overrightarrow{B}=B\hat{z}\), and we choose the symmetric gauge \({\overrightarrow{A}}_{m}=\frac{B}{2}(y\hat{x}x\hat{y})\), with \(\overrightarrow{B}=\,\nabla \times {\overrightarrow{A}}_{m}\) ^{47}. On the other hand, the QR electron confinement potential can be written as
where V_{0} is the conduction band discontinuity at the ring borders. Figure 1(b) presents the color gradient plot of this potential. The conduction band electron dynamics in the considered electromagnetic field is described by the two dimensional Schrödinger equation solution
where m^{*} is the electron effective mass:
where \(\hat{{\bf{p}}}=\,i\hslash \nabla \) is the inplane momentum operator, and e is the elementary charge. By using the KramersHenneberger unitary transformation^{40,41,42,48}, Eq. (3) can be written as a timeindependent equation:
here, U_{d} is the laserdressed diamagnetic term of the potential
with
and \({\alpha }_{0}=\frac{e{A}_{0}}{{m}_{in}^{\ast }\omega }\sim \frac{\sqrt{{I}_{0}}}{{\omega }^{2}}\) with I_{0} representing the laser field intensity is the laser parameter inside the ring^{49}. The function V_{d}(x, y) is the laserdressed confinement potential^{40,41,42} which in this particular case can be analytically expressed by solving the integral \({V}_{d}=\frac{\omega }{2\pi }{\int }_{0}^{\frac{2\pi }{\omega }}\,V(X(t),\,Y(t))dt\) (the same integral form also lets us to deduce U_{d} in Eq. (6)), where X(t) = x + α_{0} cos β cos(ωt) and Y(t) = y + α_{0} sinβ cos(ωt). By naming \(\tilde{x}(x,\,y)=x\,\cos (\beta )+y\,\sin (\beta )\) and \(\tilde{y}(x,\,y)=y\,\cos (\beta )x\,\sin (\beta )\), one may obtain:
where \({\bar{\sigma }}_{i}(x,y)=({\sigma }_{i}(x,y)+{\sigma }_{i}(x,y\mathrm{))/2}\) and we make use of the following notations:
and
For practical purposes, α_{0}(x, y) of Eq. (7) can be expressed by means of Heaviside step functions:
It should be noted that the validity of the lowest order approximation (the zeroth order in 1/ω) is defined as \(\hslash \,\omega \gg E({\alpha }_{0})\), which is called the high frequency condition, where E(α_{0}) is the average excitation energy. Furthermore, there is no limitation on α_{0} apart from the high frequency condition. The details for dressed potential in Eq. (8) and the nonperturbative approach based on the KramersHenneberger translation transformation developed to describe the atomic behavior in intense highfrequency ILF can be found in refs^{40,41,42,50,51,52}. Technically speaking, experimentally the ILFeffect can be implemented with an Argon laser, commercially available, which satisfies the condition that its emission energy is much higher than the energy of the confined states reported in this study.
Solution of the Eq. (5) must be carried out numerically. However, in the limiting case corresponding to a quantum disk (R_{1} = 0), it has an analytical form and the transcendental equation with BenDaniel Duke boundary conditions to calculate the energy eigenvalues with finite confinement potential and radius R_{2} is given by^{53}:
where \(\alpha =\sqrt{\frac{2{m}_{in}^{\ast }}{{\hslash }^{2}}\,{E}_{mn}}\), \(\beta =\sqrt{\frac{2{m}_{out}^{\ast }}{{\hslash }^{2}}\,({V}_{0}{E}_{mn})}\), J_{m}(x) is the Bessel function of the first kind, K_{m}(x) is the Bessel function of second kind, m = 0, ±1, ±2, ±3, ..., and n = 1, 2, 3, .... Such equation will be used to test our results in the limiting case of an isolated quantum disk.
The color gradient plot of Eq. (8) is presented in Fig. 1(c). One may notice that the D_{1}type symmetry of the confinement potential is destroyed by laser dressing excepting the cases β = nπ/2, where n is an integer. We note that the applied magnetic field is not going to alter the symmetry of the system. In Fig. 1(d) we represent the 2Dimages containing the shape of the confinement potential along the special directions depicted in panels 1(b) and 1(c). The black lines correspond to the potential in panel 1(b), without ILFeffects, while the red lines correspond to the potential in panel 1(c) where the ILFeffects have been included with α_{0} = 7 nm and β = π/3. From the figures portraying the distinct d_{n} (n = 1, ..., 6) projections some specific comments are worthy: (i) for energies below the halfheight of the potential barrier (V_{0}/2) there is an effective decrease of the quantum well widths, while for energies greater than the halfheight of the potential barrier the effective sizes of the quantum wells actually increase, (ii) in those special directions where a double well structure appears (d_{1}, d_{4}, and d_{6}), the presence of ILF induces a coupling between the wells–this effect is associated both to the drop of the barrier height and the decreasing of the effective width of the central barrier, (iii) note the appearance of a double quantum well structure located on the right side of panel d_{6}, which corresponds to the phenomenon previously explained in the above mentioned work by Cunha Lima et al.^{44} and finally (iv) the Fig. d_{3} is obtained from Fig. d_{2} by making a reflection with respect to x = 0.
For clarity purposes, a 3Dcolor gradient for the undressed and dressedlaser potential is presented in Fig. 2. The panels 2(a) and 2(b) are 3Dviews of the panels 1(b) and 1(c), respectively. The color scales vary from the zero value in the potential, corresponding to the blue color, to its maximum value of 262 meV, represented by the red color. The transparent design lets us to observe the complex shape of potential when the laser is turned on in Fig. 2(b).
By solving Eq. (5) one may obtain the eigenvalues and eigenfunctions of the confined electron into the region of the eccentric quantum ring under the perpendicular applied magnetic field and ILFeffects. We denote by ϕ_{j} and E_{j} the normalized eigenfunction and eigenvalue corresponding to j^{th} electronic level in the conduction band. With the information for the spectrum, we shall further consider the intraband absorption transitions that may occur from the ground state to some of the electronic excited levels. The transition energies are labeled as ΔE_{j1} = E_{j} − E_{1} and the related intensities of the optical response will be calculated for ξpolarization of the incident resonant light from a probe laser (where the options ξ = (x, 0) and ξ = (0, y) are considered in the present work) via the expression
Results and Discussion
External fields effects on the electron energy levels
To perform out calculations we have considered a common pair of semiconductor materials, GaAs/Al_{0.3}Ga_{0.7}As. We used the following numerical values^{11}: V_{0} = 0.262 eV, \({m}_{in}^{\ast }=0.067\,{m}_{0}\), and \({m}_{out}^{\ast }=0.093\,{m}_{0}\), where m_{0} is the free electron mass. Eq. (5) is numerically solved by the FEM using a 2D model in COMSOL Multiphysics software^{54}. The Schrödinger equation has been put in the general partial differential equation (PDE) form. The eigenvalue solver was used with a zeroflux boundary condition. The boundary was taken to be a circle with radius equals to 2R_{2}. In our calculations we used an extra refined mesh with a number of degrees of freedom \(\sim 50000\). By adapting parameters numerical values to those used in ref.^{45} we were able to reproduce the results reported by Chakraborty et al. for the circular ring case. The observed differences between our solutions and those in ref.^{45} are not larger than 1 meV, for all values of the ILFparameter and the magnetic field, which in ref.^{45} was maximum 20 T. However, in the present study the magnetic field strength will be extended up to 30 T, in consonance with experimental values available in the literature^{55}. The outer border of the QR was fixed at R_{2} = 28 nm. Then, the number of electron energy levels in the conduction band is further determined by R_{1}.
Figure 3(a) shows the first thirty lowlying levels as functions of the inner border radius R_{1}. Calculation was done for the concentric GaAsAl_{0.3}Ga_{0.7}As QR in the absence of the ILFeffects (α_{0} = 0). The ground state is nondegenerated, corresponding to a slike orbital. The second nondegerate state corresponds to a slike one, related to another energy subspectrum with a different value of the mquantum number (see Eq. (12)). It can be seen as the first highslope line that crosses several excited levels. We use label “1” to point at the positions of slike states. So,a third level of that kind appears as it is indicated. Our results follow the same tendency as those reported in ref.^{27}. All other levels in Fig. 3(a) are doublydegenerated. At the left extreme of Fig. 3(a), for small values of R_{1}, the spectrum will correspond to that of a quantum disk. The full symbols close to the vertical axis of the figure correspond to the analytical calculation obtained by means of the transcendental Eq. (12)^{53}. Such equation is appropriate to obtain the limiting case when R_{1} = 0. It is observed that our numerical results are in excellent correspondence with those analytical solutions. The notation in parenthesis shown for the lowest eight states, represents the pair of quantum numbers (m, n), in relation with the discrete spectrum of a quantum disk. There, n corresponds to n^{th} zero of the transcendental equation for a fixed value of m. The observation of several subspectra related to the n number is in agreement with the calculations made by LLorens et al.^{56} for this kind of systems. The increasing character of the energies as R_{1} goes from zero to 22 nm is due to the magnification of the confinement due to the reduction of the ring’s area, \(\pi \,({R}_{2}^{2}{R}_{1}^{2})\). Notice that the excited energy levels are almost constant up to specific value of \({R}_{1}^{L}\), which increases as the energy levels are higher. In the range \(0 < {R}_{1} < {R}_{1}^{L}\) the results correspond to typical QRs energy spectra. At the right extreme of the Fig. 3(a) with R_{1} → R_{2} the spectrum resembles a quasione dimensional quantum wire system.
The Fig. 3(b) presents the results obtained under the same conditions for the structural parameters as in Fig. 3(a), but in the presence of an applied magnetic field (B = 30 T). It is observed that the number of the discrete states per unit energy increases due to the parabolictype magnetic potential in Eq. (6). In the same way as it was described in Fig. 3(a), in this case it is observed that the energy levels have a range of R_{1} (\(0 < {R}_{1} < {R}_{1}^{L}\)) where they behave as constants. Then, from \({R}_{1} > {R}_{1}^{L}\), each state presents a high rate of growth in energy. Besides, \({R}_{1}^{L}\) increases as the order of the state under consideration grows. For example, for the state labeled with α_{1}, \({R}_{1}^{L}=15.7\) nm and for the state labeled with α_{2}, \({R}_{1}^{L}=16.7\) nm. This behavior leads to a very characteristic multiplecrossing aspect of the conduction band. It is also visible that for relatively small values of R_{1} the lowlying energy levels are strongly increased by the magnetic field in comparison with Fig. 3(a), although for large R_{1} values the increase is less obvious. A second subspectrum appears close to 75 meV and it is a consequence of the axial symmetry of the QR. The inset in Fig. 3(b) shows the first five energy levels for small R_{1}. The oscillations observed for the ground state are of AharonovBohm type, due to the magnetic field effect. This means that, for example, the ground state changes its symmetry from stype to p (or d)type state as a function of R_{1}. At this point, it is worth to highlight that, for further calculations in this paper, we choose to fix the value of the inner radius at R_{1} = 12 nm.
For fixed R_{1} and R_{2} values, we investigate the effect of the eccentric wdisplacement on the conduction band structure. Figure 4(a) presents the energy levels as functions of w in the absence of external fields. One may observe that the eccentricity splits all degenerated levels into two ones, since the circular symmetry of the ring is altered. Such a degeneracy breaking takes place at larger values of the wparameter as the order of the excited levels increases. The ground state energy is a decreasing function of the wparameter due to the increasing of the effective area of the region where the electron is confined. Note that the lowest two nondegenerated levels, produced by the symmetry breaking, are generally decreasing functions of w, whereas the higher excited levels are generally increased by the growth of the asymmetry. The α_{1} state has a p_{x}symmetry when w = 0 and behaves in a similar way to stype states when the symmetry is broken; i.e., it decreases when w augments. In the case of the α_{2}state, for zero and finite values of the wparameter, it has a p_{y}symmetry and its energy increases due to the decreasing of the spatial extension of the region over which the state extends (for w finite values, the α_{2}state is pushed to the left where the yextension of the structure decreases due to the circular shape of the outer radius of the ring). The increasing character in the energy of the α_{3} and α_{4} states, which have dsymmetry, is also explained by the breaking of the axial symmetry of the QR and by the decrease in the vertical distance between its lobes as they are pushed towards the lefthand part of the ring. Note the presence of the n = 2 subspectrum with energies larger than 55 meV at w = 0 for which the energy levels physics is the same as for n = 1. Figure 4(b–d) show the same kind of results without ILFeffects but for different values of the externally applied magnetic field. It is observed that the magnetic field produces an overall rising of the allowed energy values as well as a decrease of the interlevel energy difference, and also prevents the crossings of the lowlying levels, this effect is due to the extra magnetic field confinement. From Fig. 4(d) one may conclude that in the strong magnetic field regime the conduction band structure is less sensitive to the asymmetry induced by the eccentric wparameter; i.e., in this case the breaking of the symmetry can be treated as a perturbation on the system, with the magnetic field being the dominant factor.
We have also investigated the influence of the ILF polarization direction on the energy spectrum of the eccentric QR. In particular, our calculation has focused on the first ten energy levels in a highly eccentric system, for different values of the applied magnetic field and ILFparameter. It is clear but not obvious that all energy levels are even functions of the βangle with respect to β = 90° (see inset in Fig. 5(b), which corresponds to a dressed potential V_{d} for values β = 45°, 135°, with α_{0} = 7 nm). Figure 5(a,b) show the results obtained for B = 0. As it was observed in Fig. 4(a), with w = 10 nm, the energies of the ten lowest states are smaller than V_{0}/2 = 131 meV. In consonance with Fig. 2(b), for finite values of the α_{0}parameter which induces a narrowing of the potential profile, these ten states should exhibit a blue shift, as it is actually depicted in Fig. 5(a,b). At the ILFparameter α_{0} = 3.5 nm Fig. 5(a), the effect of the laser polarization rotation is quite interesting, producing a slow increase of the energies (except for the those of slike states). For the larger value of the ILFparameter, the further asymmetry introduced by laserdressing becomes more obvious when increasing the value of β. Figure 5(b) also illustrates the complex behavior of the conduction band with the rotation of laser polarization. Several anticrossings between higher excited levels can be observed. They are also present in Fig. 5(a), but become less evident and locate closer to the β = 90° symmetry point of the curves. The inset in Fig. 5(a) shows the exchange of symmetry between the fourth and fifth excited states at the anticrossing point occurring for β = 120°.
Figure 5(c,d) present the ILF rotation effect in the strong magnetic field regime. For the smaller value of α_{0}, the values of the lowest energy levels are almost unaffected showing that the magnetic field has the dominant effect. The interlevel energies are very slowly increasing with the order of the energy levels (note in 5(c) for instance that E_{9} − E_{8} > ... > E_{2} − E_{1} > E_{1} − E_{0}, where E_{0} is the ground state). We may conclude that for relatively weak ILFparameter, in the strong magnetic field regime the eccentric ring energy levels are practically invariant under the laser polarization rotation. However, for larger values of α_{0} (Fig. 5(d)) the highly excited energy levels are more sensitive to the laser polarization rotation. Multiple anticrossings occur in the strong magnetic field case. Comparing Fig. 5(a,c) we note that in the presence of the applied magnetic field all the states are concentrated in a range of 20 meV with gravity centre at 40 meV (the lower/higher states increase/decrease their energies); whereas for the configurations corresponding to Fig. 5(b,d), the magnetic field acts by inducing a blueshift for all the states.
We are particularly interested in observing the changes of the confined conduction states induced by the continuous variation of the magnetic field and laser parameter. So, we start by presenting the concentric (noneccentric) QR case where some of the energy levels are degenerated in the absence of external fields (B = 0 and α_{0} = 0), on the vertical axis, and then connect the externally applied fields. Consequently, Fig. 6 shows the first ten energy levels for an electron confined in a GaAsAl_{0.3}Ga_{0.7}As QR as function of the perpendicular applied magnetic field for three values of the laser parameter. For α_{0} = 0 there is no inplane asymmetry and the degenerated levels are split by a Zeemanlike effect (Fig. 6(a)). The higher the degenerate energy, the stronger the split will be, as a direct consequence of higher values of the mquantum number (see discussion of Fig. 3). This behavior creates the multiple crossing pattern observed in this figure; the crossings can be seen as accidentally degeneracy between energy levels. As a consequence of the crossings the energy levels are oscillating with B. One may observe that some particular energy level oscillates and also changes the symmetry of its wave function. For example, the ground energy level, which exhibits a clear oscillation pattern, is globally increasing with B in a parabolic manner with oscillations passing from slike symmetry of the wave function for B = 0 to manymaxima aspect for higher values of the magnetic field (see inset labelled as (B) in Fig. 6(a)). The envelope quadratic function which describes the energy of the ground state, in meV, is given by the fit E_{0} = 14.39 + 0.06B + 0.0124B^{2}. Using the symmetry of the quantum disk, in Fig. 6(a) at B = 0, 10, 20, and 30 T the pairs of quantum numbers (m,n) for the ground state are (0, 1), (3, 1), (6, 1), and (8, 1), respectively. This is the reason for the number of antinodes in the inset (B) is 0, 6, 12, and 16 as the magnetic field increases.
Figure 6(b,c) show how the above described behavior changes when the laser field is added to the model. The entire band structure is expanded along the energy axis and degeneracy is broken even for B = 0. Anticrossings occur, beginning at lower energy levels for relative small laser parameter (Fig. 6(b)) and continuing with higher energy levels as α_{0} increases (Fig. 6(c)). Energy levels oscillate in pairs with reduced amplitude for lowlying levels and increased amplitude for highly excited levels. In the strong laser field studied, energy levels are generally blueshifted by the increase of the magnetic field, this effect being more obvious for lowest levels. In Fig. 6(b) the quasidegenerated ground state can be associated to a situation with two interacting quantum dots, whereas in Fig. 6(c), for very large value of the ILFparameter, it turns out as a two isolated quantum dot configuration. Then, the ILFeffect lies in the division of the space into two well defined quantum dot regions, as shown in the inset in Fig. 6(c). We have to stress that in the Fig. 6(b,c) there appears the degeneracy between states that conform a pair (1–2, 3–4, 5–6, …, etc). Additionally, and due to the broken symmetry at B = 0, which is associated with the ILFparameter different of zero, the mixing between states with different values of the mquantum number, we can observe the anticrossing behavior between states which belongs to different pairs (2–3, 4–5, 6–7, …, etc). This behavior is more evident as the asymmetry of the ring increases, which can be seen by comparing Fig. 6(b,c).
It may be also instructive to consider the behavior of the energy levels in a concentric QR with respect to the ILFparameter for different applied magnetic field values. In the results presented in Fig. 7 α_{0} was continuously modified between zero and 7 nm. Figure 7(a) shows the ones obtained in the absence of magnetic field. The lowest calculated level is nondegenerate but all others are doubly degenerated for α_{0} = 0. This is due to the rotational symmetry of the QR. Increasing α_{0} leads to the splitting of degenerate levels as a consequence of the laserinduced symmetry breaking. All energies augment with α_{0}, the increment being larger in absolute value for highlyexcited levels. The increasing behavior of the energy levels with respect to the variation of the α_{0}parameter is due to the narrowing of the potential profile for energies below V_{0}/2. Note that the maximum energy of all depicted states in Fig. 7(a) is 70 meV < V_{0}/2. The splitting of degenerate levels is stronger for lowerlying levels and occurs only for relatively high values of the laser parameter for highly excited levels. The inset in Fig. 7(a) shows the color gradient plot of the confinement potential at α_{0} = 7 nm and reveals that the QR circular symmetry was transformed by laserdressing into a configuration of two QDlike structures. This behavior explains why, for large α_{0} values, the energy levels redistribute pairwise: 1–2, 3–4, 5–6, etc., and become either symmetric or antisymmetric states. Figure 7(b,c) present the same kind of results but for medium and high magnetic field values. The degeneracy is broken even for α_{0} = 0 due to Zeemanlike effect. The entire group of energy levels is compressed on vertical axis and the laser induced absolute shift in energy is augmented by the presence of the magnetic field.
We further investigated the effect of the magnetic field on the energy levels in eccentric QRs with and without laser field. This case exhibits a more complex behavior since three different types of asymmetry occur: magneticfieldinduced, laserinduced, and geometrical. Results presented in Fig. 8 are for medium eccentric parameter (w = 5 nm). Figure 8(a) shows a very interesting change of the oscillating behavior of levels by passing from ground state to highly excited states in the absence of the laser field. The first several lowlying levels are oscillating with the magnetic field at small amplitude values. Then, by going to higher energy levels one may observe that oscillations increase in amplitude and finally they lead to the kind of crossing pattern discussed above for Fig. 6. This transition is entirely due to the geometric asymmetry of the confinement potential (see inset). In fact, eccentricity tends to quench the AhoronovBohm oscillations, resembling the effect of the application of a constant electric field to the system. The lowlying levels are more affected by the broken symmetry and under the magnetic field they behave as typical anticrossing states. The high excited levels are less affected by the geometric asymmetry and therefore they still exhibit the characteristics of degenerate levels. At zero magnetic field they couple in pairs with almost the same energy and Zeemanlike splitting for increasing magnetic field. This explains the slow but obvious transition from anticrossing behavior at small energies to crossing behavior for higher energies. Figure 8(b,c) present the same calculation for xpolarized and ypolarized laserdressing, respectively. From Fig. 8(b) it is possible to observe that oscillating behavior completely disappears because the laser field enhances the asymmetry produced by eccentric confinement. As a result all degeneracies are broken including those of highly excited levels. Figure 8(c) shows a somewhat more complicated effect of the magnetic field on the energy levels. Some oscillations are visible for highly excited states which indicates that laserdressing in ydirection partially restore the symmetry broken by eccentric geometry.
Figure 9 contains the results for a highly eccentric QR with w = 10 nm; organized similarly to those in Fig. 8. One may observe from Fig. 9(a) that oscillations are not present as in Fig. 8(a), the obvious reason being the complete lifting of degeneracy induced by a highly asymmetric geometry. However, some excited levels are still crossing each other. As compared with Figs 8(b) and 9(b) shows a more pronounced increase of the energy levels for augmenting magnetic field strength. However, the general coalescent aspect of the levels with the increase of B is preserved. In addition, Fig. 9(c) shows some anticrossings between levels but contrary to Fig. 8(c) no clear oscillator behavior may be observed. This could be explained by the fact that geometric asymmetry is so important that laser field in this range is not able to restore a magneticlike behavior.
Intraband optical transitions under magnetic and laser fields
In this subsection we investigate the transition energies and intensities associated to intraband absorption of linearly polarized light. We only take into account transitions from the ground level to several excited levels. This approach is justified when only the first level is significantly populated which may be the low temperature case. Calculations were made for nine excited levels and transition energies are plot as functions of the magnetic field for different configurations of the laser field. Transition intensities are estimated in arbitrary units by using Eq. (13) with a natural logarithmic rescaling:
The intensity as given by Eq. (14) will be doublecoded on the plots: by linewidth and by color. Thin and red line will represent very weak, near zero absorption. Thick and blue line will represent very strong, near maximum absorption.
Figure 10 presents the results for concentric QR with and without laser field dressing. One may observe that in the absence of the nonresonant laser field (Fig. 10(a)), the transitions 1–2 and 1–3 are very intense but with low transition energy (<3 meV). All other transitions are forbidden, being the QR practically transparent for optical radiation. The zero transition energies in the figure correspond to transitions between accidentally degenerated states in Fig. 6(a). Figure 10(b,c) present absorption in the presence of the laser dressing. In Fig. 10(b) the resonant incident radiation is polarized along the xdirection of the heterostructure parallel polarization (see inset) and in Fig. 10(c) it is polarized along the ydirection perpendicular polarization. In the parallel polarization case (Fig. 10(b)), the transitions 1–3 and 1–4 occur with relatively high intensity and higher energy transitions 1–5 and 1–6 are almost forbidden. It may be observed that allowed transitions are redshifted with the increase of magnetic field and some intensity oscillation occur, simultaneously with slow decrease of the absorption (see the inset in Fig. 10(b)). In the perpendicular polarization case (Fig. 10(c)) transitions 1–3 and 1–4 are almost forbidden and transitions 1–5 and 1–6 are allowed for relatively weak magnetic field and are significantly redshifted and almost vanish for large values of B. Transitions 1–3, 4 are at the limit of the far infrared domain (~2 THz) and transitions 1–5,6 are middle infrared. The horizontal transitions at ΔE = 0 in Fig. 10(b,c) are due to the two fold degenerated ground state.
Figure 11 shows the intraband transition energies and intensities for an eccentric QR, where absorbed light is polarized in the eccentricity direction (x). Two cases are considered: laser dressing on x and yaxis. It may be observed that only a few transitions are allowed (keep in mind that red lines are for I → 0 whereas the blue ones are for the maximum calculated dipole moment values). Generally the high energy transitions are blueshifted by the increase of magnetic field and keep their intensities almost constant. Near to B = 16.5 T and ΔE = 35.82 meV appears the anticrossing highlighted by the circle in the Fig. 11(c). Such anticrossing shows up as a symmetry interchange between the wave functions and consequently the growing energy transition associated to the green line in Fig. 11(c) with almost constant intensity comes from transitions between the ground state and many different states which, depending on the magnetic field strength, keep the same symmetry. This argument is also valid for the growing transitions represented by green lines in Fig. 11(a,b). Lowenergy allowed transitions are redshifted by the magnetic field and they decrease and/or increase in intensity; ie., a mixed behaviour can be exhibited depending on the final state of the optical transition. For example, the lowest energy transition in Fig. 11(c) goes from 10 meV to zero when B increases from 0 to 30 T and it is forbidden in the range from zero to 5 T (red line); then such transition becomes permitted up to 30 T with increasing intensity. By comparing Fig. 11(b,c) one may conclude that x–polarization of the laser field allows a higher energy transition (>40 meV) than y–polarized laser radiation.
Finally, Fig. 12 shows intraband transition energies as in Fig. 11, but with y–polarized absorbed light. In comparison with Fig. 11, two conditions may be highlighted: (i) the y–polarization favors higher intensities for the lowest intraband transitions; (ii) the highest transitions become not allowed for small applied magnetic field but with magnetic field increase the transitions appear.
Conclusions
We have performed the investigation of the electronic properties of eccentric GaAsbased twodimensional quantum rings taking into account the variation of the geometry due to changes in eccentricity, together with the influence of externally applied magnetic and intense nonresonant laser fields. In this context, we present for the first time analytical expressions for the laserdressed twodimensional confining potential in this kind of heteroestructures which are valid for all values of the inplane polarization of the incident laser field. The theoretical study is based on the numerical solution of the effective mass Schrödingerlike partial differential equation with the use of the finite element method.
As a result, a detailed discussion of the properties of the conduction band electron spectrum is given. The changes in the energy level range and distribution, as well as the presence and breaking of state degeneracies are particularly analysed, highlighting the sole or combined influence of the modification in the ring’s geometry and the application of the external electromagnetic fields. Then, with the information on the energy structure at hand, the work extended towards the discussion of the groundtoexcitedstates intraband transitions, related to the corresponding optical absorption. This study revealed a broad range of different behaviors, with responses going from few THz to mid infrared. So, the nanostructures considered could become a suitable model for explaining the optoelectronic properties of quantumringbased devices^{53}.
Data Availability Statement
All the files with tables, figures, and codes are available. The corresponding author will provide all the files in case they are requested.
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Acknowledgements
J.A. Vinasco acknowledges the support and fruitful cooperation with Prof. Dr. A. Radu during his doctoral stay at the Department of Physics, “Politehnica” University of Bucharest, where part of this work was done. This research was partially supported by Colombian Agencies: Universidad EIA, CODIUniversidad de Antioquia (Estrategia de Sostenibilidad de la Universidad de Antioquia and projects “Efectos de capas delta dopadas en pozos cuánticos como fotodetectores en el infrarrojo” and “Efectos ópticos intersubbanda, no lineales de segundo orden y dispersión Raman, en sistemas asimétricos de pozos cuánticos acoplados”), and Facultad de Ciencias Exactas y NaturalesUniversidad de Antioquia (CADexclusive dedication project 2018–2019 and ALMexclusive dedication project 2018–2019). This work used resources of the Centro Nacional de Processamento de Alto Desempenho em São Paulo (CENAPADSP). J.A. Vinasco acknowledges the doctoral financial support from the Colombian Agency El Patrimonio Autónomo Fondo Nacional de Financiamiento para la Ciencia, la Tecnologa y la Innovación, Francisco José de CaldasCOLCIENCIAS. M.E. Mora Ramos acknowledges support from Mexican Conacyt through Grant 206772. The authors are grateful to the Universidad EIA and Universidad de Antioquia for financial support through the EIAUdeAUdeMproject “Generación de segundo armónico, rectificación óptica y dispersión Raman entre estados electrónicos en pozos cuánticos acoplados”.
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The contributions of the authors are as follows: J.A. Vinasco: was responsible of the numerical calculations; A. Radu: was responsible of the implementation of differential equations; E. Niculescu proposed the angular dependece of the light polarization and the corresponding analysis; M.E. MoraRamos: participated in writing of full manuscript and discussion of the results; E. Feddi: participated in writing of the manuscript and discussion of results; V. Tulupenko was responsible for the discussion related to combined effects between ILF and applied magnetic field; R.L. Restrepo: was responsible of the analysis associated to the optical properties; E. Kasapoglu: was responsible of the dresedpotential comparisons; A.L. Morales: was responsible of discussions associated to energy levels and boundary conditions and also in writing of the manuscript; and C.A. Duque: proposed the problem and was responsible of the discussion and writing of the manuscript.
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Vinasco, J.A., Radu, A., Niculescu, E. et al. Electronic states in GaAs(Al,Ga)As eccentric quantum rings under nonresonant intense laser and magnetic fields. Sci Rep 9, 1427 (2019). https://doi.org/10.1038/s41598018381140
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DOI: https://doi.org/10.1038/s41598018381140
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