Tuning emission energy and fine structure splitting in quantum dots emitting in the telecom O-band

We report on optical investigations of MOVPE-grown InGaAs/GaAs quantum dots emitting at the telecom O-band that were integrated onto uniaxial piezoelectric actuators. This promising technique, which does not degrade the optical quality or performances of the quantum emitters, enables us to tune the quantum dot emission wavelengths and their fine-structure splitting. By spectrally analyzing the emitted light with respect to its polarization, we are able to demonstrate the cancelation of the fine structure splitting within the experimental resolution limit. This work represents an important step towards the high-yield generation of entangled photon pairs at telecommunication wavelength, together with the capability to precisely tune the emission to target wavelengths.

Self-assembled semiconductor quantum dots (QDs) are the most promising candidates as sources of ondemand polarization entangled photon pairs, which are highly desired for next generation quantum information and telecommunication applications, e.g. quantum relays and repeaters. 1-3 Furthermore, this technology allows for straightforward on-chip integration 4-6 enabling rapid transfer from proof-of-concept devices to the applied system level. State-of-the-art technology for QDs is nowadays set by GaAs-based structures 7-10 which naturally emit in the NIR wavelength range 10 . In order to transmit single photon-encoded information over long distances and with limited pulse distortion, flying Qbits are expected to emit in the so-called telecommunication O-and Cbands (~1310 nm and ~1550 nm, respectively) 11 . InP-based structures reach emission wavelengths in the telecom range, hence benefitting from low loss fiber communication, 11 but suffering from a lack of effective distributed Bragg reflectors (DBRs). For this reason, efforts have been made in order to extend the GaAs-based technology up to the telecom regime, in order to transfer the leading technology from NIR to a spectral range suitable for both fiber communication and integration with silicon photonics. To this end, single-photon emission in the telecom bands has recently been demonstrated for GaAs-based devices 12,13 , as well as the resonant excitation scheme 14 and the creation of entangled photon pairs 15,16 . The strongest limitation in using a QD as source for entangled photons is given by the broken symmetry of as-grown QDs, caused by the anisotropy of strain, composition and shape, which leads to the exciton emission split into two bright excitonic sates. These two states are orthogonally polarized in the linear basis, and their energy difference is generally referred to as the finestructure splitting (FSS). [17][18][19][20] A finite FSS results in an additional phase term in the two-photon polarization state created by the biexciton-exciton radiative cascade. For time-integrated measurements, this effect has to be compensated. 21 To create the entangled states |Ψ ⟩ = 1/√2(| ⟩ + | ⟩)with H and V being the horizontal and vertical polarizations, it is preferable that the FSS is smaller than the radiative lifetime limited linewidth of ~ 1 µeV 22 .
In the past few years, several post-growth tuning approaches were developed, such as uniaxial strain induced by piezoelectric materials, 22-24 electric field induced quantum confined Stark effect, 25,26 magnetic field induced Zeeman shifts 27,28 or laser annealing techniques, 29,30 to be able to individually engineer the FSS in semiconductor QDs emitting at  < 1 µm. These techniques ultimately lead to a high yield of QDs capable of emitting polarization-entangled photon pairs effectively. Post-growth tuning of the FSS in the telecommunication range has also been demonstrated, 31 the full cancellation of the FSS, however, was not achieved in spectral ranges beyond  = 1 µm, yet.
Another advantage of such post-growth tuning techniques is the simultaneous tuning of the FSS and the emission energy, which can be decoupled by adding more than one tuning knob. Such a flexibility in emission energy is beneficial in order to increase the yield of applicable QDs, if a distinct resonance is required, e.g. in remote QD indistinguishability experiments 32 or for implementing hybrid quantum systems 33 .
Here, by means of uniaxial strain tuning, we demonstrate the successful elimination of the FSS for telecom- systems, which further support the possibility to fabricate high-quality photonic cavity devices as micropillar cavities. 34 As precursors we used TMGa, TMIn, TMAl, and AsH 3 . After the removal of the oxide at 710°C, we deposited 50 nm of GaAs to ensure a high-quality epitaxial growth surface. This buffer is followed by an Al 0.75 Ga 0.25 As sacrificial layer with a thickness of 100 nm that allows the removal of the substrate in a postgrowth processing step. The QDs are embedded in a GaAs membrane with an overall thickness of 460 nm. After the deposition of the first GaAs layer, the temperature is lowered from 710°C to 530°C and InGaAs with a nominally equal concentration of Ga and In in the gas phase is introduced for the formation of the QDs. The QDs are then capped by a strain reducing layer of In 0.16 Ga 0.84 As to achieve the desired red shift to the telecom O-band.
Subsequently, the membrane is completed after the deposition of a GaAs top layer, which eliminates the nonradiative decay channels caused by surface effects. The complete layer stack is shown in Figure 1a. Further details of the QD growth can be found in Ref. 12 , and information about its structure and morphology in Ref. 35 Figure 1b displays a broad-range spectrum of the as-grown sample before the integration onto the piezoelectric substrate. At short wavelengths the wetting layer (WL) emission is observed, while sharp emission lines originating from the QDs are found in the telecom O-band. The micro-photoluminescence (μ-PL) spectroscopy together with a low spatial QD density is sufficient to isolate the emission of single QDs. The observed spectral lines show mainly excitonic behavior, i.e. finite FSSs and linear power dependencies, and the corresponding QD asymmetry tends to align along the [110] crystal axes as observed for similar QD architectures. 36 To achieve the desirable tunability of the emission energy and FSS the as grown sample (Fig. 1)  previously been used to achieve independent energy and FSS tuning 38 by employing uniaxial or biaxial stress.
Here we used uniaxial stress as this has a stronger impact on the FSS. The final device structure is shown in Figure 2a.
For the optical characterization, the samples were mounted in a helium flow-cryostat operating at 4K and were optically excited above the GaAs band gap using a Helium-Neon continuous-wave laser. A confocal microscopy setup equipped with a near infrared objective (numerical aperture of 0.6) was used to collect the emission from single QDs. The QD light was analyzed by a standard 0.5 m spectrometer equipped with a nitrogen cooled InGaAs-CCD array suitable for telecom wavelengths. By inserting a half-wave plate (HWP) and a linear polarizer after the collection lens, polarization-resolved measurements were performed to estimate the FSS. 24 The brightness of typical QD emission lines on the final device structure appears comparable to that before the nanomembrane processing in Figure 1b and (d)), respectively. Secondly, as shown in Figs. 3 (b) and (e), the FSS values change from the initially low values to zero, i.e. below the resolution limit of the experimental setup. Similar to our previous observations, the elimination of the FSS is accompanied with a distinct phase shift of 90°, 24,39,41 The experimental data of the corresponding phase measurement on the two QDs are included in Figs. 3 (c) and (e), respectively.
The uniaxial strain tuning of the FSS has been investigated in several theoretical 17,18 and experimental 24,25 studies. It originates from the coherent coupling of the two bright excitonic states. The experimental data for the FSS and the phase presented in Fig. 3b-f  According to the same model, the polarization angle  over V P can be obtained from: In addition to the change of the FSS we observe an abrupt change of 90° for the polarization angle in the spectrum while the FSS reaches its minimum, which is evidence of its elimination. Both QDs, as presented in Figure 3, show good agreement in their tuning behavior and clearly prove the reduction of the FSS below the present resolution limit. The parameters α,  and  fitted to the experimentally obtained FSS data for both QDs according to eq. 1 with  = 0 can be found in Table I. QD 1 QD 2 a(µeV/V) 0.11 ± 0.01 0.08 ± 0.01 (µeV) 3.64 ± 0.28 -1.29 ± 0.2 (µeV) 0.36 ± 0.18 0.5 ± 0.38