Signatures of single-photon interaction between two quantum dots located in different cavities of a weakly coupled double microdisk structure

We report on the radiative interaction of two single quantum dots (QDs) each in a separate InP/GaInP-based microdisk cavity via resonant whispering gallery modes. The investigations are based on as-fabricated coupled disk modes. We apply optical spectroscopy involving a $4f$ setup, as well as mode-selective real-space imaging and photoluminescence mapping to discern single QDs coupled to a resonant microdisk mode. Excitation of one disk of the double cavity structure and detecting photoluminescence from the other yields proof of single-photon emission of a QD excited by incoherent energy transfer from one disk to the other via a mode in the weak-coupling regime. Finally, we present evidence of photons emitted by a QD in one disk that are transferred to the other disk by a resonant mode and are subsequently resonantly scattered by another QD.

Figure 1

(a) Exemplary SEM image of a microdisk dimer with a nominal interdisk spacing of 50 nm. (b) $4f$ setup for PL scans and maps. The lens configuration in combination with a piezo driven tiltable mirror enables us to scan detection and excitation independently from each other over the disk pair. (c) Schematic of the PL map scanning procedure, here, for fixed excitation (red) and scanning detection (blue). The setup also allows for other configurations, i.e., scanning both the excitation and detection, or fixed detection and scanning excitation. (d) Schematic of the PL side scanning procedure. The cryostat geometry makes it possible to optically access the sample also in plane. (e) Comparison of spectra taken from the disk side (red solid line) and disk top (green solid line).

Figure 2

(a) Normalized color-scale plot of a one-dimensional scan displaying emission lines from both disks. Two emission lines signified in the plot are selected for further investigations: a presumably coupled mode at $\approx 669$ nm and an uncoupled mode at $\approx 666$ nm. The emission lines display a slight wavelength shift. This is probably due to an artefact as the angle of incidence with respect to the spectrometer slit changes as the position is varied on the sample. (b)–(e) Real-space images of the disk pair evaluated with a bandpass filter at 669 nm (666 nm) for the (un)coupled mode. The lower disk is excited in (b) and (c) (see inset). Only in the case of the supposedly coupled mode does the nonexcited disk contribute PL. In the CCD images of (d) and (e) the upper disk is excited. Again, the nonexcited disk remains dark for the uncoupled mode. The red arrows in the insets show the spatial position of the excitation laser on the disk.

Figure 3

(a) Color-scale plot of a two-dimensional $\mu$-PL scan applying an excitation power of $94.5\mathrm{nW}$ with the excitation spot fixed on the upper disk of the dimer, indicated by the dotted white circle. The map shows the complete integrated intensity. A distinct mode profile can be seen from both the excited and nonexcited disk as well as possible field enhancement at the interdisk gap. The dashed circles serve as guides to the eye and indicate the disk edges. (b) Spectrally postselected intensity map at $670.9\mathrm{nm}$ of the same measurement as in (a). The nonexcited lower disk shows a bright spot indicating a single QD excited by energy transfer via a resonant mode. (c) A one-dimensional $\mu$-PL scan from the side of the disks reveals a resonant mode at $670.9\mathrm{nm}$ responsible for the energy transfer that excites QD1. The inset shows a close up view of the coupled mode. (d) The spectrum taken at the spatial position of QD1 depicts a sharp emission line. An autocorrelation measurement on this line is shown in the inset. Note: The excitation laser is still focused on the upper disk. The antibunching dip indicates a single QD excited via a coupled microdisk mode.

Figure 4

(a) Normalized linear color-scale plot of a two-dimensional $\mu$-PL scan with both excitation and detection moving across the microdisk dimer. The postselected intensity for a spectral region of $668.4\pm 0.2\mathrm{nm}$ is displayed. Two QDs with almost the same emission wavelength are found in the different disks, labeled by QD2 and QD3 [see panel (c) for their spectra]. (b) Normalized logarithmic color-scale plot of a two-dimensional (2D) excitation scan, i.e., the detection is spatially fixed on QD2 while the excitation laser is scanned across the cavity structure [see panel (d)]. We plot the intensity for the same spectral region and indicate a strong intensity increase when scanning over QD3 in the lower disk. (c) Comparison of the two ordinary $\mu$-PL spectra for QD2 (upper disk) and QD3 (lower disk) with the spectrum recorded in the upper disk when exciting QD3 in the lower disk. The highlighted area in grey outlines the filtered spectral region in the $\mu$-PL maps of panels (a) and (b). The inset numbers denote the full width at half maximum (FWHM) values of each peak. (e),(f) Autocorrelation measurements indicating antibunching on QD2 and QD3, respectively. Both measurements have been taken at the spectral peak on the energetically lower side of (c).