Fermi polaron-polaritons in charge-tunable atomically thin semiconductors

Abstract

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 1-5 . Optical creation of an exciton or a polariton in a twodimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation 6 and exciton-electron interactions leading to polaron or trion formation 7,8 . Here, we present cavity spectroscopy of gatetunable monolayer MoSe 2 (ref. 9) exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode 10,11 . 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. Transition metal dichalcogenide (TMD) monolayers represent a new class of two-dimensional (2D) semiconductors exhibiting features such as strong Coulomb interactions 14 , locking of spin and valley degrees of freedom due to large spin-orbit coupling 9 and finite electron/exciton Berry curvature with novel transport and optical signatures 15,16 . Unlike quantum wells or 2D electron systems (2DES) in III-V semiconductors, TMD monolayers exhibit an ultralarge exciton binding energy E exc of order 0.5 eV (ref. 14) and strong trion peaks in photoluminescence (PL) that are redshifted from the exciton line by E T ∼ 30 meV (refs 9,17). These features provide a unique opportunity to investigate many-body physics associated with trion 18 formation as well as coupling of excitons to a 2DES 19 and to cavity photons 20,21 , provided that the experimental set-up allows for varying the electron density n e and light-matter coupling strength g c .Here, we carry out an investigation of Fermi polarons 1 in a charge-tunable MoSe 2 monolayer embedded in an open microcavity structure (Fig. 1a,b). Since E exc is much larger than all other relevant energy scales, such as the normal mode splitting (2g c ), E T and the Fermi energy (E F ), an optically generated exciton in a TMD monolayer can be considered as a robust mobile bosonic impurity embedded in a fermionic reservoir (Fig. 1c). The Hamiltonian describing the system iswhere the first line describes the coupling of 2D excitons, described by the exciton annihilation operator x k to a 0D cavity mode c 0 whose resonance frequency ω c can be tuned by applying a voltage (u p ) to a piezoelectric actuator that changes the cavity length. This part of the Hamiltonian corresponds to the elementary building block of the recent ground-breaking experiments based on coupled 0D-polariton systems 22 . The second line of the Hamiltonian describes the Feshbach-like physics associated with the bound-molecular (trion) channel and the corresponding effective interactions between the excitons and the electrons 1 . Thi...

Realization of an Electrically Tunable Narrow-Bandwidth Atomically Thin Mirror Using MonolayerMoSe2

“…We find that the broadened line shapes are reasonably well fit by dispersive Lorentzians, allowing us to compare the dependence of maximal extinction on total exciton linewidth γ tot to the theoretical value 1 − ðγ tot − ΓÞ 2 =γ 2 tot . This comparison in turn allows us to determine the radiative decay rate to be in the range 1.3 < Γ < 1.8 meV, which is in good agreement with the values previously determined from normal-mode splitting of exciton polaritons [11,12]. We remark that, at the locations yielding the highest observed extinction factor of ∼0.9, we extract consistently higher values of Γ.…”

“…Because of strong exciton-electron interactions, the nature of elementary optical excitations can be drastically modified by applying V g , which modifies the free electron density (n e ) in the TMD monolayer [12]. A typical V g -dependent transmission spectrum is shown in Fig.…”

“…Encapsulation of TMD monolayers with hexagonal boron nitride (h-BN) [7,8] dramatically improves the optical quality, yielding an exciton linewidth ∼2 meV [9,10]. These narrow linewidths have a dominant contribution from radiative decay of ∼1.5 meV for MoSe 2 [11,12]. Motivated by these developments, we previously analyzed the optical response of a monolayer TMD theoretically [13] and showed that it realizes an atomically thin mirror [13][14][15].…”

“…The heterostructure is placed on top of a transparent fused-silica substrate and capped with bilayer graphene. A gate voltage V g applied between bilayer graphene and the MoSe 2 layer allows for tuning the electron density n e [16,17], thereby modifying the resonance frequency as well as the nature of the elementary optical excitations [12].…”

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

“…The first member of the transition metal dichalcogenides (TMDC) to be established as a direct gap semiconductor in monolayer (ML) form was MoS 2 [1,2], which has resulted in a global research effort exploring this promising 2D semiconductor family [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. First prototype device applications, such as transistors [17][18][19] and light emitters [20][21][22], have shown the promise of this atomically thin material for electronics and optoelectronics.…”