Single Charged Quantum Dot in a Strong Optical Field: Absorption, Gain, and the ac-Stark Effect

Abstract

We investigate a singly-charged quantum dot under a strong optical driving field by probing the system with a weak optical field. When the driving field is detuned from the trion transition, the probe absorption spectrum is shifted from the trion resonance as a consequence of the dynamic Stark effect. Simultaneously, a gain sideband is created, resulting from the coherent energy transfer between the optical fields through the quantum dot nonlinearity. As the pump detuning is moved from red to blue, we map out the anticrossing of these two spectral lines. The optical Bloch equations for a stationary two-level atom can be used to describe the numerous spectral features seen in this nano solid state system. PACS numbers: 78.67.Hc,42.50.Hz,42.50.Gy Quantum dot (QD) nano-structures have been proposed for numerous quantum mechanical applications due to their customizable atom-like features [1]. One important application involves using these QDs as the building blocks for quantum logic devices [2]. An electron spin trapped inside a QD is a good candidate for a quantum bit (qubit) since it is known to have long relaxation [3] and decoherence times [4,5]. Recently, the electron spin coherence has been optically generated and controlled [5,6,7] in ensembles of QDs. The initialization of the electron spin state in a single QD has also been realized by optical cooling techniques [8,9].One important task is to understand and control the physical properties of a singly-charged QD in the strong optical field regime, i.e. the light-matter interaction strength is much larger than the transition linewidth, under both resonant and nonresonant excitation. Given the recent work on optically driven neutral quantum dots in strong fields [10,11] demonstrating many features similar to atomic systems, it is clear that a negatively charged quantum dot has similarities to a negative ion. However, the excited state of a dot is a many body structure comprised of two electrons and a hole. Interestingly, the results in this paper show that strong field excitation tuned near resonance in a negatively charged dot leads to changes in the absorption spectrum that are in excellent agreement with theory.In the time domain, the strong field interaction leads to the well-known Rabi oscillations [12,13,14,15]. In the frequency domain, it will introduce Rabi side bands in the absorption, and strikingly, the amplification of a probe beam. This phenomenon has been studied theoretically [16,17,18] and demonstrated experimentally in atomic systems [19,20]. Recently, these effects have also been observed in quantum dot and molecular systems [10,11,21,22]. The optical AC Stark effect has been seen by exciting a neutral QD with a detuned strong op- , respectively. However, the study of the singly-charged QDs in the strong field regime has been very limited at the single dot level [24]. In this letter, we investigate a singly-charged QD under a strong optical driving field with both on and off-resonant pumping. When the strong pump is on resonance with the t...

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