Vanishing fine-structure splittings in telecommunication-wavelength quantum dots grown on (111)A surfaces by droplet epitaxy

The emission cascade of a single quantum dot is a promising source of entangled photons. A prerequisite for this source is the use of a symmetric dot analogous to an atom in a vacuum, but the simultaneous achievement of structural symmetry and emission in a telecom band poses a challenge. Here we report the growth and characterization of highly symmetric InAs/InAlAs quantum dots self-assembled on $C_{3v}$ symmetric InP(111)A. The broad emission spectra cover the O $\left(\lambda \sim 1.3 \mu \mathrm{m}\right)$, C $\left(\lambda \sim 1.55 \mu \mathrm{m}\right)$, and $L \left(\lambda \sim 1.6 \mu \mathrm{m}\right)$ telecom bands. The distribution of the fine-structure splittings is considerably smaller than those reported in previous works on dots at similar wavelengths. The presence of dots with degenerate exciton lines is further confirmed by the optical orientation technique. Thus, our dot systems are expected to serve as efficient entangled photon emitters for long-distance fiber-based quantum key distribution.

Figure 1

(a) AFM image of the sample surface of InAs dots on ${\mathrm{In}}_{0.52}{\mathrm{Al}}_{0.48}\mathrm{As}$/InP(111)A. (b) Cross sections along [$01-1$] and [$-211$] for a relatively large dot (top) and a small dot (bottom).

Figure 2

(a) PL spectrum of the ensemble of dots. The blue broken lines show the results of multiple-peak fit with assuming Gaussian broadening for each peak. (c) Time-resolved PL signals after picosecond pulsed excitation. The gray broken line shows the instrumental response function (IRF). The red line shows a single exponential fit to the decay data with a lifetime of 1.43 ns.

Figure 3

(a) Typical PL spectra of a single QD with an excitation power of $4 \mu \mathrm{W}$ and $0.8 \mu \mathrm{W}$. (b) PL intensity as a function of excitation power for each exciton complex. The solid lines show power lows, $I\propto P^{\alpha }$, where $\alpha =0.8,0.8,1.1$, and $1.8$ for $X,X^{-},X^{+}$, and $XX$, respectively. (c) Evolution of the binding energy with the $X$ energy for $X^{-}$ (red triangles), $XX$ (black circles), and $X^{+}$ (blue diamonds).

Figure 4

(a), (b) Energy shifts of the $X$ line with rotating the axis of linear polarization for two dots. The blue lines are sinusoidal fits to the data. (c) Absolute value of FSS measured on single QDs (red circles). The broken line shows the ensemble PL spectrum. The shaded thick lines are guides to the eye. (d) Polarization axis with respect to the emission energy. Two orthogonal in-plane axes [$01-1$] and [$-211$] are shown by arrows. The equivalent directions appear with every ${120}^{\circ }\mathrm{rotation}$ due to the $C_{3v}$ point-group symmetry.

Figure 5

Circularly polarized emission signals with a nonresonant circularly polarized pump at a wavelength of 705 nm for (a) a QD without a detectable FSS, and (b) a QD with a finite value of FSS. Upper panels show unpolarized spectra. Lower panels show differential spectra between cocircular signals and cross-circular signals. The degree of circular polarization, $(I_{\sigma }-I_{{\sigma }^{*}})/(I_{\sigma }+I_{{\sigma }^{*}})$, of each emission line is also indicated. The acquisition time was 100 s for (a) and 20 s for (b).