Nonlinear Nano-Optics: Probing One Exciton at a Time

Abstract

Coherent nonlinear optical spectroscopy of individual excitons in GaAs quantum dots is demonstrated and shows strong similarities with atoms in their coherent optical interaction, unlike higher dimensional heterostructures. The nonlinear response is dominated by an incoherent contribution (saturation) and a coherent contribution (population pulsations) of the single dot and compares well with present theory. The data shows that energy relaxation and dephasing rates are comparable, reflecting the absence of significant pure dephasing, and also demonstrate the presence of interdot energy transfer.[S0031-9007(98)07287-1] PACS numbers: 73.20.Fz, 78.47. + p, 78.66.Fd Recent studies of electron-hole excitation in quantum dots (QD) show the consequences of three-dimensional confinement including atomiclike d-function density of states and increased exciton binding energy and oscillator strength. Using photoluminescence (PL) and various microprobing methods to eliminate spectral blurring, numerous groups have now reported sharp line spectra and excited states of individual QD's similar to atoms [1][2][3][4]. In this paper, we combine the breakthrough of single QD probing with the power of nonlinear optical spectroscopy in semiconductors [5] to observe the coherent nonlinear optical response of a single exciton confined in a QD potential. The measurements show the coherent nonlinear interaction with light is atomiclike in nature revealing that the dominant nonlinearity is state filling as predicted in earlier theoretical studies [6]. The work opens up a new direction of research for direct measurements of exciton dynamics in QD's (e.g., dephasing, energy relaxation, interdot coupling, etc.) and optical nonlinearities as well as taking the first step in the direction of demonstrating coherent control and fast switching for novel applications [7]. The data provide a measure of the energy relaxation rates and the dephasing rate and show the absence of significant pure dephasing processes at low temperature. The data also show the contribution to energy relaxation in some dots due to interdot energy transfer. Probing a single exciton demonstrates coherent optical coupling leading to population pulsations of single dots at frequencies above 15 GHz. The measurements show the profound effects of the complete discretization of the energy spectra to the resonant nonlinear response [6] demonstrating the similarity of these dots with atomic systems beyond linear response.Measurements are made at T 6 K on a narrow ͑42 Å͒ single molecular-beam epitaxy (MBE) grown GaAs quantum well with 250 Å Al 0.3 Ga 0.7 As barriers. Growth interrupts at the interfaces lead to the formation of large monolayer-high islands [8] which localize excitons in QD potentials that are 15 monolayers thick with lateral dimensions on the order of 40 nm [3]. Excitons in isolated QD's are probed by exciting through a 0.2-2.5 mm aperture in a 100-nm-thick Al mask deposited directly on the sample surface [3]. The sample is attached to a sapphire disk (c-axis nor...

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