Spin properties in droplet epitaxy-grown telecom quantum dots

TL;DR

Problem: telecom-wavelength quantum dots require long spin coherence for quantum repeater networks. Approach: quantify spin properties of MOVPE DE-grown InAs/InGaAs/InP QDs via pump-probe Faraday ellipticity to extract electron and hole g-factors and spin lifetimes. Key findings: electron and hole g-factors are $|g_e| = 0.934$ and $|g_h| = 0.471$, long longitudinal lifetime $T_1 = 2.95 μs$, and spin dephasing $T_2^* \,\sim\, 3.06$ ns; reduced g-factor anisotropy ($\ au \approx 0.33$) and smaller spreads compared with SK-grown telecom QDs; the hyperfine-nuclear fluctuations give ΔB_e and ΔB_h values and activation energies $E_a^{(e)} ≈ 15$ meV and $E_a^{(h)} ≈ 3.6$ meV. Significance: demonstrates MOVPE DE as a viable path to enhanced spin coherence and symmetry in telecom QDs, with strong potential for quantum repeater implementations.

Abstract

We investigate the spin properties of InAs/InGaAs/InP quantum dots grown by metalorganic vapor-phase epitaxy (MOVPE) deposition using droplet epitaxy, which emit in the telecom C-band. Using pump-probe Faraday ellipticity measurements, we determine electron and hole $g$-factors of $|g_e| = 0.934$ and $|g_h| = 0.471$, with the electron $g$-factor being nearly twice as low as typical molecular beam epitaxy Stranski-Krastanov (SK) grown samples. Most significantly, we measure a longitudinal spin relaxation time $T_1 = 2.95\,μs$, representing an order of magnitude improvement over comparable MBE SK grown samples. Despite significant electron $g$-factor anisotropy, we observed that it is reduced relative to similar material composition samples grown with MBE or MOVPE SK methods. We attribute these g-factor anisotropy and spin lifetime improvements to the enhanced structural symmetry achieved via MOVPE droplet epitaxy, which mitigates the inherent structural asymmetry in strain-driven growth approaches for InAs/InP quantum dots. These results demonstrate that MOVPE droplet epitaxy-grown InAs/InGaAs/InP quantum dots exhibit favorable spin properties for potential implementation in quantum information applications.

Figures (5)

• Figure 1: MOVPE droplet epitaxy grown InAs/InGaAs QD spin properties of carriers in transverse magnetic field. Excitation energy for all measurements is 0.785 eV. Measurements are done at 6K temperature, 20 mW pump power and 2 mW probe power. (a) Photoluminescence of the sample (green) and the excitation laser spectrum (red). (b) QD oscillatory behavior is displayed when a transverse magnetic $B_V = 0.4$ T is applied and the Faraday ellipticity signal is measured. The black curve is the raw data, and the red curve is the fit. The red and black curves correspond to hole and electron contributions, respectively. (c) Larmor precession frequencies depending on the transverse magnetic field for electron (black) and hole (red). The $g$ factor calculated for both carriers is displayed in the plot. (d) Spin dephasing times as a function of transverse magnetic field. The calculated $g$-factor and $B$ field spreads for each component are displayed in the top right.
• Figure 2: Layer structure of MOVPE droplet epitaxy grown InAs/InGaAs/InP quantum dots showing the substrate, buffer layer, and three repetitions of the quantum dot active region consisting of InGaAs interlayer, InAs QDs, InP capping, and InP burial layers.
• Figure 3: PL spectra, spectral dependence of $g$-factors and Faraday ellipticity amplitudes at $B_V = 0.4$ T of electron and hole components in MOVPE droplet epitaxy grown InAs/InGaAs QDs. Measurements are done at 6 K temperature, 20 mW pump power and 2 mW probe power. (a) PL spectrum of the sample (green markers) fitted with a bimodal distribution (black curve, with red and blue dashed curves showing components of the bimodal). (b) Absolute values of $g$-factor as a function of excitation energy for electron (black) and hole (red) components. The normalized PL spectrum is shown as a green line (plotted on the right axis). (c) Normalized Faraday ellipticity amplitudes for electrons (black) and holes (red) as a function of excitation energy.
• Figure 4: a) Temperature dependence of spin dephasing times at a transverse magnetic field of 0.4 T for electrons (black) and holes (red), fitted with Arrhenius equation. b) Anisotropy of $g$-factors measured at $B = 0.4$ T as a function of magnetic field rotation angle. c) Magnetic field dependence of electron spin polarization showing the polarization recovery curve (red) and Hanle effect (black). Measurements are done at 6K temperature, 2 mW probe power, 17 mW pump power for PRC and 15 mW for Hanle. d) Sample rotation geometry showing magnetic field B at angle $\theta$ relative to the [001] growth axis, enabling measurement of g-factor anisotropy between transverse and parallel orientations.
• Figure 5: Spin dynamics measurements of quantum dots showing: (a) Half-width at half maximum (HWHM) of the Hanle curve as a function of pump power, showing electron spin lifetime at zero power extrapolation. (b) Inverse electron spin lifetime versus pump power measured with a $B_F=0.1$ T parallel magnetic field applied, with linear extrapolation to zero to obtain electron longitudinal spin relaxation time $T_1$. (c) Temperature dependence of electron spin lifetime with a 0.1 T parallel magnetic field applied, fitted with an Arrhenius equation (dashed line).