Physics Chat
Coherent coupling of excitons between site-controlled vertically stacked pyramidal quantum dots pairs revealed by two-dimensional four-wave mixing spectroscopy
Speaker: Vikramdeep Singh (WLangbein)
Date: Friday 21 May 2021
Time: 15:00
Venue: Zoom
Semiconductor quantum dots (QDs) can play the role of quantum bits (qubits) in quantum information processing applications (QIP). Coherent coupling between spatially separated QDs is of importance for the QIP applications to create entanglement and control. Additionally, the underlying mechanism changes with distance from electronic coupling to dipole-dipole coupling. Four-wave mixing (FWM) spectroscopy probes the third-order non-linearity, and using heterodyne spectral interferometry (HSI) it is possible to perform transient FWM measurements on individual quantum systems [1]. Coherent coupling has been investigated using HSI on excitons localized in the thin quantum well [2], identifying the coherent coupling type using the off-diagonal signal information. Here, we report on using HSI on site-controlled epitaxially grown pyramidal quantum dots (PQDs) [3,4] to demonstrate coherent coupling between a pair of vertically stacked QDs. Multiple QDs can be vertically stacked with different interdot barrier thicknesses, and thus different coupling mechanisms can be examined by tuning the interdot distance. We show results at 5K temperature on individual double QD system with 10nm separation. We use degenerate two-beam FWM with HSI detection measuring the FWM field in amplitude and phase with a high spectral resolution (15µeV) [1,2]. Fourier transforming the pulse delay time axis, time-frequency domain transforms into frequency-frequency domain providing 2D representations. We also report photoluminescence on the same QDs for comparison to assign the emission lines. Coherent coupling between vertically stacked QDs is found, and attributed to static dipole-dipole coupling, providing interaction energies of a few meV within a QD, and few 100µeV between QDs. An example of measured data is shown in Fig.1. Ground state to exciton state transitions (X1,2) appear on the diagonal at equal excitation and emission energy (#_3=#_1), and local biexciton transitions (X1X1, X2X2) are blue shifted off the diagonal by the respective interaction energies. The excitons X1, X2 of the stacked QDs 1,2 are split by 5.38 meV. They show individually two-exciton states X1X1 and X2X2 with interaction energies of 1.00 meV and 1.76 meV respectively. The off-diagonal signals connecting X1 and X2 show their coherent coupling. The observed 2# phase shift over the off-diagonal signal along #_3 indicates an underlying double peak, identifying [2] the coupling to be due to exciton density as compared to transition-dipole (Förster). The mixed two-exciton state X1X2 has an interaction energy of about 130 µeV.
References: [1] W. Langbein and B. Patton, Opt. Lett., 31, 1151 (2006) [2] J. Kasprzak, B. Patton, V. Savona, and W. Langbein, Nat. Photonics 5, 57 (2011) [3] M. H. Baier, S. Watanabe, E. Pelucchi, and E. Kapon, Appl. Phys. Lett., 84, 1943 (2004) [4] S. T. Moroni, T. H. Chung, G. Juska, A. Gocalinska, and E. Pelucchi, Appl. Phys. Lett. 111, 083103 (2017) .
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