Ultrafast Polariton-Phonon Dynamics of Strongly Coupled Quantum Dot-Nanocavity Systems

We investigate the influence of exciton-phonon coupling on the dynamics of a strongly coupled quantum dot-photonic crystal cavity system and explore the effects of this interaction on different schemes for nonclassical light generation. By performing time-resolved measurements, we map out the detuning-dependent polariton lifetime and extract the spectrum of the polariton-to-phonon coupling with unprecedented precision. Photon-blockade experiments for different pulse-length and detuning conditions (supported by quantum optical simulations) reveal that achieving high-fidelity photon blockade requires an intricate understanding of the phonons’ influence on the system dynamics. Finally, we achieve direct coherent control of the polariton states of a strongly coupled system and demonstrate that their efficient coupling to phonons can be exploited for novel concepts in high-fidelity single-photon generation.

Popular Summary

Photons, which are the quantized excitations of light, can be coupled to the electronic excitations of atoms using resonators. This coupling can be enhanced up to the so-called strong-coupling regime in which photons and electronic excitations hybridize to form new states called polaritons. In the solid state, this coupling can be realized on a chip using photonic crystal nanocavities as resonators and semiconductor quantum dots—also called artificial atoms—as quantum emitters. Such systems are promising for future integrated quantum optical hardware: A nuanced understanding of the effect of phonons on strongly coupled emitter-cavity systems is essential for developing an integrated quantum optical network in the solid state.

Here, we investigate how the coupling to the semiconductor environment impacts the dynamics of such systems and how these dynamics can be exploited to generate unusual states of light such as streams of single photons. We focus on a system that consists of a self-assembled InAs quantum dot strongly coupled to a photonic crystal cavity. Because of the solid-state environment, there exists an efficient coupling between electronic excitations and phonons (i.e., vibrational modes). Using time-resolved luminescence measurements with a temporal resolution of better than 5 picoseconds, we map out the detuning-dependent polariton lifetime and extract the spectrum of this polariton-to-phonon coupling. From this process, we choose a pulse length that results in the strongest photon blockade. Additionally, using the coupling map, we optimize our excitation conditions to coherently control the polariton population and dramatically improve the quality of single-photon generation.

Our results have revealed that phonons can provide an important and useful degree of freedom for controlling quantum optical systems.

Figure 1

(a) Schematic illustration of the energy structure of a strongly coupled system as given by the Jaynes-Cummings ladder obtained from Eq. (2). The blue arrow illustrates resonant excitation of UP1, the red arrows show photon decay, and the purple arrow is the phonon-assisted transfer. (b) Cross-polarized reflectivity spectra of the coupled QD-cavity system obtained by temperature tuning the QD through the cavity resonance. An anticrossing of the peaks clearly demonstrates the strong coherent coupling.

Figure 2

(a) Decay rates and (b) resulting lifetimes of the polariton branches as functions of the QD-cavity detuning, obtained from Eq. (2). (c) Time-resolved spectrally integrated luminescence intensity at a detuning of $\mathrm{\Delta }=2.8g$ for excitation of UP1. The lifetime is extracted from a mono-exponential fit (shown in red). (d) Measured and simulated lifetime of UP1 as a function of the detuning. (e) Phonon-assisted state transfer rates extracted from the lifetime at each detuning. (f) Spectrally and temporally resolved luminescence at a QD-cavity detuning of $\mathrm{\Delta }=7g$. The delayed onset of emission from LP1 for resonant excitation of UP1 is consistent with phonon-assisted population transfer.

Figure 3

(a) Measured second-order coherence $g^{(2)}(0)$ as a function of the laser detuning for different QD-cavity detunings (top to bottom) and excitation pulse lengths (left to right). In each case, the red vertical line indicates the bare cavity frequency, the green vertical line indicates the bare QD frequency, and the black vertical lines indicate the polariton frequencies. (b-c) Simulated photon-blockade values of $g^{(2)}(0)$ as a function of the laser detuning and pulse length including (b) and excluding (c) the effect of phonons, both at a QD-cavity detuning of $\mathrm{\Delta }=4g$. To aid in comparison with the experimental results in (a), the dashed lines in (b) indicate the simulated antibunching values for the photon blockade at the experimental pulse lengths.

Figure 4

(a) Rabi oscillation observed upon exciting UP1 and detecting LP1. (b) Measurement of the second-order coherence for exciting UP1 with a $\pi$ pulse and detecting LP1 yielding $g^{(2)}(0)=0.03\pm 0.01$.