Two-photon absorption by a quantum dot pair

The biexciton absorption spectrum of a pair of InAs/GaAs quantum dots is being studied by photoluminescence excitation spectroscopy. An absorption resonance with the characteristics of an instantaneous two-photon process reveals a coherent interdot two-photon transition. Pauli-selective tunneling is being used to demonstrate the transduction of the two-photon coherence into a nonlocal spin singlet state. The two-photon transition can be tuned spectrally by electric field, enabling amplification of its transition strength.

Figure 1

(a) Top: Sample geometry with the dot color indicating the relative band-gap energy of the two dots (left: red $\to$ lower band gap; right: blue $\to$ higher band gap). Bottom: Energy level diagram with resonant optical single-photon, ${\text{X}}_d$ and ${\text{iXX}}_i$, and monochromatic two-photon, ${\text{TP}}_i$, transitions between the crystal ground state (cgs), exciton (X), and molecular biexciton (iXX). The static dipole moments of the different states, $p_{\text{cgs}}, p_{\text{X}}$, and $p_{\text{iXX}}$, are illustrated. (b) Schematic exciton-biexciton transition spectrum with single- and two-photon transitions. (c) Predicted EFD absorption spectrum from a CQD pair for the states and transitions depicted in (a).

Figure 2

(a) Illustration of the generation of an EFD-PLE absorption spectrum. (b) Combined EFD-PL (bottom) and PLE (top) spectrum. The PL spectrum was obtained at the energy marked by the arrow for the ${\mathrm{TP}}_{d/i}$ label. The region in the dashed box was taken with 10 times smaller energy steps than the rest. (c) Expanded view of the region in the dashed box in (b).

Figure 3

(a) Fine structure of the biexciton level resonance and (b) the exciton level resonance. The singlet-triplet splitting, ${\delta }_{ST}$, and the bright-dark splitting, ${\delta }_{BD}$, are marked in both branches of the respective anticrossings. (c) Schematic EFD biexciton transition spectrum with transitions labeled by the exciton and biexciton spin states. Inset: High-resolution EFD-PL spectrum of the boxed region of the biexciton X pattern. This spectrum was obtained with the triple spectrometer in additive mode under nonresonant excitation at 1.374 eV. The intensity was normalized at each energy. The lines are guides to the eye.

Figure 4

EFD-PLE absorption spectra in the vicinity of the resonance of the ${\mathrm{X}}_d$ and ${\mathrm{iXX}}_i$ transitions obtained by monitoring (a) the ${\mathrm{XX}}_d$ and (b) the ${\mathrm{iXX}}_d$ PL transitions. Yellow markers are placed in the exact same positions in (a) and (b). Inset to (b): Summary of all transitions; identified by the spin states they generate. (c) Energy level structure of the exciton-biexciton system at a fixed bias in the region seen in (a) and (b). Thin arrows indicate the excitation paths; wide gray arrows indicate the detection transitions used in (a) and (b).