Fermi polaron-polaritons in charge-tunable atomically thin semiconductors

The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultracold Fermi gases. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation6 and exciton–electron interactions leading to polaron or trion formation. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe2 exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive exciton-polaron resonance to the lower-energy attractive exciton-polaron state. Simultaneous observation of polariton formation in both attractive and repulsive branches indicates a new regime of polaron physics where the polariton impurity mass can be much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose–Fermi mixtures.Physic

Optically controlled locking of the nuclear field via coherent dark-state spectroscopy

A single electron or hole spin trapped inside a semiconductor quantum dot forms the foundation for many proposed quantum logic devices 1-6. In group III-V materials, the resonance and coherence between two ground states of the single spin are inevitably affected by the lattice nuclear spins through the hyperfine interaction 7-9, while the dynamics of the single spin also influence the nuclear environment 10-15. Recent efforts 12,16 have been made to protect the coherence of spins in quantum dots by suppressing the nuclear spin fluctuations. However, coherent, control of a single spin in a single dot with simultaneous suppression of the nuclear fluctuations has yet. to be achieved. Here we report the suppression of nuclear field fluctuations in a singly charged quantum dot to well below the thermal value, as shown by an enhancement, of the single electron spin dephasing time T 2 *, which we measure using coherent dark-state spectroscopy. The suppression of nuclear fluctuations is found to result from a hole-spin assisted dynamic nuclear spin polarization feedback process, where the stable value of the nuclear field is determined only by the laser frequencies at fixed laser powers. This nuclear field locking is further demonstrated in a three-laser measurement, indicating a possible enhancement of the electron spin T 2 * by a factor of several hundred. This is a simple and powerful method of enhancing the electron spin coherence time without use of 'spin echo'-type techniques 8,12. We expect that our results will enable the reproducible preparation of the nuclear spin environment for repetitive control and measurement of a single spin with minimal statistical broadening. ©2009 Macmlllan Publishers Limited. All rights reserved.link_to_subscribed_fulltex

Magneto-optical properties of trions in non-blinking charged nanocrystals reveal an acoustic phonon bottleneck

Charged quantum dots provide an important platform for a range of emerging quantum technologies. Colloidal quantum dots in particular offer unique advantages for such applications (facile synthesis, manipulation and compatibility with a wide range of environments), especially if stable charged states can be harnessed in these materials. Here we engineer the CdSe nanocrystal core and shell structure to efficiently ionize at cryogenic temperatures, resulting in trion emission with a single sharp zero-phonon line and a mono exponential decay. Magneto-optical spectroscopy enables direct determination of electron and hole g-factors. Spin relaxation is observed in high fields, enabling unambiguous identification of the trion charge. Importantly, we show that spin flips are completely inhibited for Zeeman splittings below the low-energy bound for confined acoustic phonons. This reveals a characteristic unique to colloidal quantum dots that will promote the use of these versatile materials in challenging quantum technological applications