Abstract-An ultrafast terahertz probe of the transient evolution of the charged and neutral phase of photo-excited electron-hole gas in a monolayer semiconductor

Xuefeng Liu1,2, Hongyi Yu3, Qingqing Ji4, Zhihan Gao1,2, Shaofeng Ge1,2, Jun Qiu1,2, Zhongfan Liu4,Yanfeng Zhang4,5 and Dong Sun1,2


http://iopscience.iop.org/article/10.1088/2053-1583/3/1/014001/meta

We investigate the dynamical formation of an exciton from photo-excited electron-hole plasma and its subsequent decay dynamics in monolayer MoS2 grown by chemical vapor deposition (CVD) using ultrafast pump and terahertz probe spectroscopy. Different photo-excited electron-hole states are resolved based on their distinct responses to THz photon and decay lifetimes. The observed transient THz transmission can be fitted with two decay components: a fast component with a decay lifetime of 20 ps, which is attributed to the exciton lifetime, including its formation and subsequent intra-exciton relaxation; a slow component with an extremely long decay lifetime of several ns, possibly due to a long-lived dark exciton state. The relaxation dynamics are further supported by temperature and pump-fluence-dependent studies of the decay time constants. The sign of the transient THz observed in this experiment is the opposite of that measured in a recent parallel transient THz work on MoS2 [1]. The observed decay dynamics are also different, and the possible reasons for these discrepancies are discussed.

Exciton–photon dynamics in graphene

Exciton–photon dynamics in graphene [Print to PDF] [Print to RTF]

An EU-funded project studied the physics underpinning light–matter interaction in two-layered graphene with a band gap. Project findings will pave the way to developing pioneering optoelectronic devices.

http://cordis.europa.eu/result/rcn/159578_en.html

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Excitons — neutral quasiparticles that exist in semiconductors — demonstrate strong coupling with light. Embedding bilayer graphene with a band gap in optical microcavities allows controlling interaction that can lead to a strong coupling regime. Such an interaction results in the formation of a new kind of quasiparticle known as exciton-polariton that is a half-light and half-matter bosonic quasiparticle.

With EU funding of the project 'Bilayer graphene exciton polariton' (BIGEXPO), scientists sought to enhance understanding of the bilayer graphene coupling to the photonic field of a microcavity. Based on a non-perturbation approach, BIGEXPO focused on studying the phenomena taking place when a dipole layer such as a graphene sheet interacts with an electromagnetic field.

Study findings demonstrated that the Purcell effect breaks down — counterintuitively, the spontaneous emission rate plummets in a strong coupling regime. Furthermore, scientists concluded that current approximations to photonic emissions have to be modified.

Another task was to develop a microscopic theory describing the coupling between excitons in bilayer graphene and photons. Once completely developed, this theory should provide a comprehensive description of the underlying physics of light–matter interaction. The coupling non-perturbative nature should account for extraordinary physical effects.

BIGEXPO sought to enhance understanding of the physical processes governing the exciton–photon dynamics in microcavities. Considering its large excitonic dipole moment, the graphene microcavity system could push back the frontiers of research into solid-state cavity quantum electrodynamics. Not only will it allow observing a novel, strongly correlated light–matter coupling regime, but also lead to a new generation of terahertz and mid-infrared super-efficient optoelectronic devices.