Large Excitonic Reflectivity of MonolayerMoSe2Encapsulated in Hexagonal Boron Nitride

Abstract

scattering has emerged as a method for overcoming these limitations and for controlling light at the atomic scale (3-11). For instance, highly reflective mirrors based on individual quantum emitters have been demonstrated by coupling them to optical cavities and nanophotonic waveguides (3-8). Such resonant mirrors feature very unusual properties due to their extraordinary nonlinearity down to the single-photon level (3-8). A two-dimensional (2D) layer of emitters, such as atomic lattices or excitons (9-11), has also been predicted to act as an efficient mirror when the incident light is resonant with the resonance frequency of the system. Such atomically thin mirrors represent the ultimate miniaturization limit of a reflective surface, and could enable unique applications ranging from quantum nonlinear optics (9-11) to topological photonics (12,13).This Report demonstrates that transition metal dichalcogenide (TMD) monolayers can act as atomically thin, electrically switchable, resonant mirrors. These materials are direct-bandgap semiconductors that support tightly bound excitons. Excitonic transitions in TMD monolayers exhibit large oscillator strengths (14-16), resulting in large radiative linewidths compared to excitons in other semiconductor systems. In addition, the excitonic response in monolayers can be controlled electrically via gate-induced doping and by shifting the chemical potential (17)(18)(19).Importantly, these monolayers can be easily integrated with other 2D materials via Van der Waals stacking to improve their quality or add new functionalities. One of the most studied amongst such heterostructures is a TMD monolayer encapsulated by two hexagonal boron nitride 4 (hBN) flakes: this "passivated" monolayer exhibits enhanced carrier mobility (19,20) and reduced photoluminescence linewidth (21,22).Our experiments make use of a device that consists of an hBN-passivated molybdenum diselenide (MoSe 2 ) monolayer placed on an oxide-covered silicon (Si) substrate, and we measure its reflectivity with a normally incident laser beam (Figs. 1A and 1B). The doped Si substrate is used as a gate electrode: by applying a gate voltage (V g ), MoSe 2 monolayers can be made intrinsic or n-doped. When a monochromatic laser beam is tuned to the exciton resonance, we observe substantial reflection from a monolayer device (M1) at V g < 10 V at T = 4 K (Fig. 1C).The reflection contrast between the monolayer region and the substrate disappears at V g > 20 V ( Fig. 1D), indicating that the reflection can be turned off electrically. When we illuminate another monolayer device (M2) with a supercontinuum laser and spectrally resolve the reflection, we find that both the magnitudes and wavelength positions of the reflectance peaks change with V g (Fig. 1E). When the monolayer is intrinsic (V g < 10 V), the reflection is dominated by a peak at the wavelength of the neutral exciton transition. When MoSe 2 is n-doped (V g > 20 V), however, the reflection by the neutral exciton disappears, and a new, weaker, reflectance peak ap...

Near-Unity Light Absorption in a Monolayer WS2 Van der Waals Heterostructure Cavity

“…For very small γ T , γ r,0 dominates the linewidth and the absorption decreases as the TMD becomes highly reflective (corresponding to negative absorption, as seen in (Fig. 2b)) [43][44][45] . For very large γ T , γ nr and γ d dominates the linewidth and the absorption is decreased as the TMD becomes transparent.…”

“…We introduce an analytical approach to describe the light-exciton-cavity interaction, which is based on the semiconductor Bloch equations combined with a quantum transfer matrix method. The model takes into account the contribution of both the exciton radiative and non-radiative decay rates, γ r and γ nr , which have already been shown to affect the excitonic coherence and high reflection of monolayer TMDs [43][44][45] . In addition, we include the existence of a pure dephasing rate, γ d , to account for quantum coherence effects as part of the multiple interferences within the cavity.…”

“…This allows us to calculate the absorption, via the reflection operator's expectation value, taking into account the contributions of both γ r,0 (and its Purcell enhancement), γ nr , and γ d . The latter is a quantum coherent effect that is important to consider due to the multiple interferences in the cavity, making the absorption sensitive to exciton dephasing 43 .…”

Filtering the photoluminescence spectra of atomically thin semiconductors with graphene

“…As a result, X 0 and X emission dominate the PL spectrum of Mo-based TMDs [9], whereas the emission spectra of W-based TMDs display a complex series of lines stemming from X 0 , bi-excitons (XX 0 )[12-15], charged excitonic states (including X [10,16] and charged biexcitons (XX ) [12][13][14][15]), spin-dark excitons [17][18][19], defectinduced emission and exciton-phonon sidebands [20].Considerable progress has been made to determin-istically observe intrinsic TMD emission features. In particular, encapsulation of TMDs in hexagonal boron nitride (BN) films results in narrower neutral exciton linewidth [21,22], approaching the radiative limit [6,23,24], without however, getting rid of the other emission features mentioned above. Even in electrostatically gated devices tuned near the charge neutrality point, sizeable emission sidebands remain observable at energies close to the X feature, suggesting residual charge inhomogeneity [9,10] or intrinsic contributions from longer-lived exciton-phonon replicas [20].The complex emission spectra of TMD stimulate lively scientific debates.…”

Nonlinear optical response of a two-dimensional quantum-dot supercrystal: Emerging multistability, periodic and aperiodic self-oscillations, chaos, and transient chaos

“…Thus, in this region of frequencies, the SQD supercrystal operates as a perfect mirror. It has been reported recently that an atomically thin mirror can be realized based on a monolayer of MoSe 2 [71,72]. SQD supercrystals represent yet another class of nanoscopically thin reflectors.…”