Experimental methods of post-growth tuning of the excitonic fine structure splitting in semiconductor quantum dots
© Plumhof et al.; licensee Springer. 2012
Received: 11 March 2012
Accepted: 23 April 2012
Published: 22 June 2012
Deterministic sources of polarization entangled photon pairs on demand are considered as important building blocks for quantum communication technology. It has been demonstrated that semiconductor quantum dots (QDs), which exhibit a sufficiently small excitonic fine structure splitting (FSS) can be used as triggered, on-chip sources of polarization entangled photon pairs. As-grown QDs usually do not have the required values of the FSS, making the availability of post-growth tuning techniques highly desired. This article reviews the effect of different post-growth treatments and external fields on the FSS such as thermal annealing, magnetic fields, the optical Stark effect, electric fields, and anisotropic stress. As a consequence of the tuning of the FSS, for some tuning techniques a rotation of the polarization of the emitted light is observed. The joint modification of polarization orientation and FSS can be described by an anticrossing of the bright excitonic states.
where H and V are two different polarization states. This feature can be used in the field of quantum cryptography and in advanced quantum optics experiments as quantum teleportation. QDs have the advantage, compared to other sources of entangled photon pairs, that the photons can be emitted on demand, i.e., by using optical or electrical trigger pulses [5, 6]. However, these experimental achievements hide the difficulties connected with the experiments. If the QD does not have a certain spatial symmetry (D2d, or higher), the intermediate X-state is split by the so-called fine structure splitting (FSS), leading to a difference in energy of the photons originating from the X (and also from the XX) decay [4, 7, 8]. This energy splitting prevents entanglement, since it makes the two decay paths distinguishable, unless the FSS is tuned to the values of the order of the radiative linewidth .
In the case of Figure 1a, the energy of the X is split by the so-called FSS, Δ1; the FSS in (a) is much larger than the linewidth of the emitted lines, leading to energetically well-separated states. The emission energy of the XX is clearly detuned from the one of the X. The different energies (indicated by different colors) of the possible transitions, allow to distinguish the photons originating from the left and the right cascade. This so-called which-path information destroys the polarization entanglement [4, 11].
In Figure 1b, the FSS remains unchanged compared to (a), but the average emission energy E(XX)of the two perpendicularly polarized XX lines is equal to the one of the X (E(X)). This leads within the radiative linewidth to same transition energies of H1 and V2 as well of V1 and H2. It has been proposed that the polarization entanglement can be measured when the photons are reordered in time after their emission [10, 12–15]. Although, entanglement via the time reordering scheme has not been proven yet, Ding et al. have shown that isotropic biaxial stress can be used to tune the emission energy difference of XX and X to values smaller than the respective linewidth. The observation was explained with a strain-induced tuning of the electron-hole wavefunction overlap .
The research presented in this work is motivated by the original proposal from Benson et al. , which is equivalent to the XX-X decay configuration depicted in Figure 1c. Here, the emission energies of XX and X are de-tuned with respect to each other, but the FSS is tuned to values smaller than the respective linewidth. In this case, the so-called two first generation transitions (red lines) as well as the two second generation transitions (blue lines) are degenerated in energy. The emitted photons are circularly polarized. Photons from the same generation but from different branches have opposite circular polarization . The spectral overlap removes the which-path information and creates polarization entanglement of the emitted photons [9, 11, 17, 18]. Since the as-grown QDs usually exhibit nonzero FSS, post-growth tuning methods are highly desirable. Several techniques such as thermal annealing , lateral [20, 21], and vertical electric fields , magnetic fields , optical Stark effect , and anisotropic stress [23, 24] can be utilized for this purpose. All the above mentioned techniques have their particular advantages and disadvantages, as will be discussed in this article. This work begins with an introduction on the origin of the FSS, followed by a discussion of the different tuning techniques (some parts are reproduced from the work of Plumhof ).
Excitons with the spin configuration M d are commonly known as dark excitons, since optical (single photon) transitions are forbidden due to spin conservation. The excitons with spins M b are called bright excitons, since they can decay under the emission of a circularly polarized photon. This is only the case for bright excitonic states with zero FSS. In the case of non-zero FSS, the bright states are mixed with each other, leading to the emission of linearly polarized light. In the following, the origin of the excitonic FSS and the resulting mixing of the bright excitonic states are discussed.
The origin of the FSS is strongly correlated with the spatial symmetry of the QD, the QDs of symmetry D2d or higher show zero FSS. The required spatial symmetry includes the shape of the QD and the symmetry of the underlying crystal. The anisotropy in QDs can have several origins: apart from the QD shape intrinsic strain fields, a crystal of lower symmetry, alloy fluctuations, or alloy ordering can reduce the symmetry [27–31]. For instance, it has been shown that GaAs dots grown on (111) oriented substrates generally exhibit smaller FSS values compared to dots grown on conventional (001) oriented substrates [37, 38], in line with theoretical predictions . In addition to symmetry and shape of the QD, also, the volume of the QD modifies the excitonic FSS [32, 33]. However, a pure change of the QD-volume will not allow to tune the FSS exactly to zero and will always be related to a potentially unwanted change of the QD emission energy.
The D2d symmetry describes a system  which is: (1) two-fold rotational symmetric for three different axes (perpendicular to each other); (2) symmetric under reflection at two planes (perpendicular to each other); (3) invariant under a combination of a reflection and a 90 degree rotation around an axis perpendicular to the reflection plane.
where: Δ0 = 1.5(a z + 2.25b z ), Δ2 = 0.75(b x + b y ), and Δ1 = 0.75(b x − b y ). The latter term is equivalent to the FSS. The constants describing the spin-spin coupling are given by a i , and b i , with i = x y z. If the structure exhibits a D2d symmetry, b x and b y are identical so that the FSS Δ1 is zero. In this case, the two bright excitons have eigenstates | + 1〉and | − 1〉, which are degenerated and both have the energy . The energy splitting of the dark excitonic states, which is given by Δ2 is always nonzero. The corresponding eigenstates are , with relative energies .
If the symmetry is lower than D2d, i.e., b x ≠b y , the | ± 1〉states are not anymore the eigenstates of the Hamiltonian. In this case, the eigenstates are given by , and the degeneracy of the energy is lifted. The energy is then given by , and the difference of the two energies is the FSS (Δ1). Due to the block diagonal form of the Hamiltonian, the mixing between bright and dark excitons does not occur in this simplified model. On the other hand, if the hole states are not pure but a mixture of HH and LH, also, the LH contributes to the FSS .
The exchange interaction can be split into two parts: the so-called short-range interaction (SR) part, arising from the crystal symmetry, i.e., the interaction of electron and hole in the same Wigner-Seitz unit cell, and the long-range interaction (LR) part, when both carriers are in different unit cells, originating from the macroscopic anisotropy of the QD. The LR interaction has two effects: (i) it contributes to the splitting of the bright and dark excitons and (ii) it contributes to the FSS in QDs with shapes of symmetry lower than D2d. The LR interaction does not contribute to the splitting of the dark excitons. The main effect of the SR interaction is the splitting of the bright and dark excitons, but it also contributes to the FSS.
Even using optimized growth conditions [32, 37], the stochastic processes governing the QD formation will always lead to FSS values which are on average too large to satisfy the stringent requirements for entangled-photon generation. Post-growth tuning techniques are therefore essential to increase the yield of QD-based devices capable of generating entangled photon pairs.
Tuning of the FSS
Similar results, based also on atomistic theory, were obtained by Bryant et al. . They studied the influence of stress on InAs/GaAs QDs embedded in a bendable GaAs bridge. They explain the rotation of the linear polarization by a change of the phase between the mixed bright excitonic states. Recently, the relation between magnitude of the FSS and polarization angle has been described more in detail in the work of Gong et al. .
All these theoretical works predict a non vanishing FSS for QDs which do not exhibit a certain symmetry. However, to create polarization entangled photon pairs zero FSS is not strictly required, as it is sufficient to tune the FSS down to values of several μeV, which means that also the FSS of QDs of a lower symmetry can be tuned to values small enough to create polarization entangled photon pairs.
Tuning of FSS by annealing
Thermal annealing of the QD structures allows the FSS to be modified irreversibly [19, 45]. This technique has been mainly employed on InGaAs/GaAs QDs and is based on the diffusion of In in InGaAs/GaAs QDs. Annealing of the QDs at temperatures above about 700°C leads to a net in-diffusion from the in-richer center of the QD to the in-poorer surrounding of the QD. To control this process, one can either set a fixed temperature and vary the duration of the annealing process , or one can keep the annealing time fixed and vary the temperature . The change of the QD-composition profile leads to a change of the QD-extension and to a modification of the effective shape of the confinement potential.
Tuning of FSS by magnetic fields
A very powerful technique to tune the FSS is represented by magnetic fields, where one generally distinguishes between two configurations: (i) Faraday configuration, i.e., with the magnetic field oriented along the growth direction , (ii) Voigt configuration, i.e., with the magnetic field aligned along an axis perpendicular to the growth direction [8, 48]. In the Faraday configuration, the magnetic field lifts the degeneracy of the excitonic states of positive and negative spin projections, but it does not allow to tune the FSS to zero . In the Voigt configuration, the in-plane magnetic field affects the lateral symmetry of the QDs and, thus, also the FSS. Additionally, it couples bright and dark excitonic states, M b and M d , making the dark states bright [48–50].
Tuning of FSS by the optical Stark effect
In the right panel of Figure 7, the same is shown for the excitonic spectrum. Again, both the emission lines get closer as I is increased, and they finally overlap at an intensity of I = 3I0. Interestingly, the V-polarized excitonic emission is only slightly effected by I, which can simply be explained by a coupling of the linearly polarized laser field with mainly the H-polarized excitons. It is shown that QDs with FSSs of more than 20 μeV can be tuned to values small enough to create polarization entangled photon pairs. The presented technique has the advantage that one can precisely in-situ modify the FSS, but on the other hand, an additional laser source is needed, making the on chip implementation more complicated.
Tuning of FSS by the quantum-confined stark Stark effect
Another promising approach is the so-called quantum-confined Stark effect, where static electric fields are utilized. The electric field can have different configurations, i.e., it can be applied either vertically (i.e., parallel to the growth direction)  or laterally (i.e., perpendicular to the growth direction) [20, 21, 53]. Gerardot et al.  investigated the influence of a lateral electric field on the FSS of InGaAs/GaAs QDs. They placed two Schottky contacts separated by 15 μ m on the surface of the sample in order to apply an in-plane electric field to the QDs, being located 130 nm below the surface.
The three arrows in the bottom panel depict measurements, where the polarization orientation of the emission was characterized. No rotation of the polarization was reported. The main drawback of the measurements presented in this work is that the application of a lateral electric field leads to a separation electrons and holes, which leads to a decrease of the emission intensity, making together with an increase of the emission linewidth this technique less appealing for future applications. Similar observations were reported in the work of Vogel et al. and Kowalik et al. [20, 53]. Another drawback of this method is that the roughness of the the electric contacts does not allow for a precise alignment of the electric field.
This relation explains why the FSS changes linearly with F for FSS > > s0. In the region close to F − F0 = 0 kV/cm, this simple approximation is not valid. The fact that for FSS > > s0, all QDs show a similar relative change of the FSS as F is varied indicates that all QDs have similar in-plane anisotropy. In Figure 9b, the orientation angle of the polarization of the two excitonic emission lines with respect to the crystal axes / for three different QDs (same as in Figure 9a) is shown. It can be seen that the polarization rotates faster, the smaller is the minimum value of the FSS, and that the polarization rotation mainly takes place in the region of the lowest FSS magnitudes. The joint effect of changing polarization orientation and modification of the FSS can be explained by the afore mentioned anticrossing of the bright excitonic states. Despite the clear anticrossings, it has been demonstrated that this tuning technique can be used to tune the FSS of some QDs to values small enough for the creation of entangled photon pairs.
Tuning of FSS by anisotropic stress
In Figure 10, the behavior of the emission of an X confined in a GaAs/AlGaAs quantum well potential fluctuation (QWPF) is presented as anisotropic stress is applied to the QD-membrane. (A QWPF is a local variation of the quantum well, acting as a comparably shallow confinement potential [7, 29]).
Figure 10a shows the polarization resolved color-coded PL-intensity map of the X-emission as a function of the emission energy for different electric fields F =33, 10, −6.6, −20 kV/cm (labeled in the figure by the numbers 1 to 4) applied to the PMN-PT. The intensities of each line (1-4) are normalized to their maximum. Three effects are mainly observed in (a): (i) The emission energy E(F,ϕ) shifts by approximately 4 meV. (ii) The polarization orientation of the excitonic emission, which is related to the phase of the wavy pattern, rotates by more than 60° (indicated by the dashed lines). (iii) The magnitude of the FSS, i.e., the amplitude of the oscillations of the wavy patterns first decreases (points 1→2) and then increases again (2→4).
In the following, the polarization angles are given by the orientation of the high-energy emission line of an exciton with respect to the direction of F. In Figure 10c,e, the polarization dependence of the relative peak positions ΔE = |E(ϕ,F) − E(F)| for points 1, 2, and 4 is illustrated in polar coordinates, where E(F)is the average emission energy for a certain value of F. They show clearly that FSS and polarization orientation change for different fields F. Figure 10f displays the polarization orientation vs. E(F), as F is varied from F = 33 to −20 kV/cm. Figure 10g shows the magnitude of the FSS vs. E(F). Going from low to high emission energies, i.e., from tensile to compressive strain, the FSS first decreases down to a minimum value of about 20 μeV and then increases again. Simultaneously, the polarization orientation rotates by around 70° (see Figure 10f). The origin of the oscillations of the polarization in Figure 10f is ascribed to the defects in the QWPF surrounding, which act as traps for charge carriers. The presented data describe a clear anticrossing of the bright excitonic states. In the same work, it is also shown that anisotropic stress can be used to tune the FSS of different kinds of semiconductor QDs by several tens of μeV, down to values, which are expected to be small enough to create polarization entangled photon pairs.
Strain-dependent calculations based on k·p and configuration interaction models show that the minimum reachable FSS highly depends on the alignment of stress with respect to the QD elongation direction (compare also with Figure 3b,c). Furthermore, it is demonstrated that the anisotropic stress produces a rotation of the hole wave function . The calculations show that the reason for the tuning of the FSS is not the physical deformation (which is only a few per mill) but the influence of the stress on the band structure. The stress leads to a mixing of heavy with light hole band and to an anisotropy of the effective mass, which finally modify the FSS. These results demonstrate that stress is a powerful tuning technique.
The advantage of stress is that it can easily be combined with other tuning techniques, like electric  or magnetic fields which provides another degree of freedom. An on-chip combination of QDs embedded in a light emitting diode and stress would allow to electrically trigger the QD emission and to tune the FSS by using anisotropic stress. The disadvantages of stress as a tuning technique are that the piezoelectric materials have to be actively stabilized in order to avoid creeping of the piezo, and it is difficult to apply the stress only locally and stress, as demonstrated after submission of this manuscript by Trotta et al. .
Several methods are presented, which allow the excitonic FSS of semiconductor QDs to be tuned after growth. All the presented tuning techniques like annealing, magnetic fields, optical Stark effect, electric fields, and stress are able to tune the FSS of particular QDs to magnitudes, which are sufficiently small to create polarization entangled photon pairs. All the presented methods have their particular advantages and disadvantages. Magnetic fields for example require cumbersome experimental setups, which makes an on-chip integration difficult, annealing does not lend itself to in-situ monitoring, but it has the advantage that the FSS can be permanently tuned to low magnitudes. Vertical electric fields are proven to be an effective tuning method, but the use of static electric fields for tuning makes it difficult to electrically trigger the QD emission. A problem which all the techniques presented so far share is that they are not able to tune the FSS of all the QDs to sufficiently small magnitudes. This problem could be solved by combining two or more tuning techniques, like static electric fields, optical stark effect, and stress.
We acknowledge V. Křápek and V. Fomin for fruitful discussions. This work was supported by the DFG (FOR730) and BMBF (QuaHL-Rep, Contract No. 01BQ1032)
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