Magneto-optical spectroscopy of single charge-tunable InAs/GaAs quantum dots emitting at telecom wavelengths

We report on the optical properties of single InAs/GaAs quantum dots emitting near the telecommunication O band, probed via Coulomb blockade and nonresonant photoluminescence spectroscopy, in the presence of external electric and magnetic fields. We extract the physical properties of the electron and hole wave functions, including the confinement energies, interaction energies, wave-function lengths, and $g$ factors. For excitons, we measure the permanent dipole moment, polarizability, diamagnetic coefficient, and Zeeman splitting. The carriers are determined to be in the strong confinement regime. Large range electric field tunability, up to 7 meV, is demonstrated for excitons. We observe a large reduction, up to one order of magnitude, in the diamagnetic coefficient when rotating the magnetic field from Faraday to Voigt geometry due to the unique dot morphology. The complete spectroscopic characterization of the fundamental properties of long-wavelength dot-in-a-well structures provides insight for the applicability of quantum technologies based on quantum dots emitting at telecom wavelengths.

Figure 1

(a) Schematic of the energy diagram of the charge-tunable structure under study, including a single layer of telecom wavelength quantum dots (QDs) grown in a quantum well. $E_f$ indicates the Fermi energy level. (b), (c) Photoluminescence spectra collected as a function of the applied gate voltage $V$ under nonresonant excitation ($\lambda =830$ nm) at $T=4$ K. The external magnetic field $B$ applied along the quantum dot growth axis $z$ is 0 T in (b) and 9 T in (c). The emission lines corresponding to the different charge states of a single quantum are labeled accordingly ($X^0=\text{neutral}$ exciton, $X^{1-}=\text{negatively}$ charged exciton, etc.).

Figure 2

The Coulomb blockade model (see main text) is applied to extract the electron-electron and electron-hole interaction energies ($E_{ee}^{ss}$ and $E_{eh}^{ss}$, circles and triangles respectively), as well as the electron and hole confinement energies ($E_C$ and $E_V$, squares and triangles respectively) and the electron and hole wavefunction extension ($l_e$ and $l_h$, squares and triangles respectively). The wavelength on the $x$-axis corresponds to the emission wavelength for the $X^0$ line in the middle of the emission tuning range.

Figure 3

(a) The energies of different charged states from a single quantum dot as a function of the applied electric field. Here the multiparticle Coulomb interaction energies have been subtracted. The solid black line is a parabolic fit to the data. (b) Permanent dipole moments $p$, measured from several single quantum dots, plotted as a function of polarizability $\beta$. The solid red line is a linear fit with slope $3.2\pm 0.2$ nm/[meV/(kV/cm)${}^2$] and the error bars are obtained from the errors in the fits.

Figure 4

(a) Faraday configuration: Normalized photoluminescence (PL) spectra (shifted for clarity) of a negatively charged exciton state from a single quantum dot, collected under nonresonant excitation at a temperature of 4 K, for applied magnetic fields ranging from 0 to 9 T (with 0.5 T increments). The solid lines are Lorentzian fits to the data. (b) Energy position of the peaks, as found from the Lorentzian fits of panel (a), plotted as a function of the applied magnetic field. The solid lines are quadratic fits. (c) Voigt configuration: Energy position of the negatively charged exciton peaks, as found from the Lorentzian fits of the spectra collected for two orthogonal polarizations [horizontal (H) and vertical (V)], plotted as a function of the applied magnetic field. The error bars (often within the symbol size) in panels (b) and (c) are obtained from the Lorentzian fits of the photoluminescence peaks.