Ultrastrong Coupling of Band‐Nested Excitons in Few‐Layer Molybdenum Disulphide

Abstract

able in organic systems, with typical Rabi splitting values of ≲150 meV or ≲6% of the exciton energy. In organic systems, Rabi splitting of >10%, which is in the ultrastrong coupling regime, is common. [11][12][13][14][15][16][17][18] In the ultrastrong coupling regime, coupling interactions are expected to be more efficient and emergent quantum coherent phenomena, such as ground-state virtual photons and entangled pairs, are predicted to exist even at room temperature or without resolvable Rabi splitting due to broad resonators. [19,20] As such, it is desirable to achieve this regime in a CMOS-compatible system like the TMDCs. In this paper, we exploit the large oscillator strength of the C exciton in few-layer molybdenum disulphide (FL-MoS 2 ) to achieve ultrastrong coupling at room temperature. The C Exciton of 2D TMDCs has Unique PropertiesThe C exciton of the 2D TMDCs does not arise from the same type of band structure features as the more commonly studied A and B excitons near the band edge, and thus, it has unique behavior. The A and B exciton absorption features in FL-MoS 2 are labeled in Figure 1a. These excitons correspond to transitions between the conduction band minimum and the spinorbit-split valence band maxima at the K point in Figure 1b. The next prominent absorption feature is the C exciton, which has significantly larger oscillator strength than the A or B, as seen by comparing the Lorentzian fits in Figure 1a.Interestingly, the C exciton does not arise from transitions between two opposite concavity parabolic bands, but rather between regions of parallel bands, referred to as nested bands, shown by the shaded region in Figure 1b. [21] The C exciton oscillator strength is large because of this parallel band region of k-space, and thus, the density of states at the C exciton transition energy is large.An unusual consequence of the nested bands is that C exciton carriers are expected to spontaneously self-separate in momentum space [22][23][24] and exhibit slowed hot carrier cooling relative to the A and B excitons. [23] This slowed cooling has been seen by Wang et al. [23] and ourselves [25] in ultrafast spectroscopic studies. Typically, photoexcited hot carriers in semiconductor devices thermalize to the band edges on femtosecond timescales before reaching the contacts. Photovoltage is therefore limited to the bandgap potential. In contrast, for hot carrier devices, The 2D transition-metal dichalcogenides (2D TMDCs) are an intriguing platform for studying strong light-matter interactions because they combine the electronic properties of conventional semiconductors with the optical resonances found in organic systems. However, the coupling strengths demonstrated in strong exciton-polariton coupling in the 2D TMDCs remain much lower than those found in organic systems. In this paper, a new approach is taken by utilizing the large oscillator strength of the above-band gap C exciton in few-layer molybdenum disulphide (FL-MoS 2 ). A k-space Rabi splitting of 293 meV is shown when coupling FL-MoS 2 C...

Ultrastrong Coupling Leads to Slowed Cooling of Hot Excitons in Few-Layer Transition-Metal Dichalcogenides

“…However, by exploiting the enhanced oscillator strength of the C exciton, we achieve Rabi splitting of Ω = 293 meV, which is 11.0% of the uncoupled exciton energy, placing it in the ultrastrong coupling regime. The Rabi splitting value is the result of fitting a semiclassical coupled harmonic oscillator model to the data and agrees within 1% of the experimentally determined minimum energy spacing between the peaks of the two branches (complete steady-state analysis details published elsewhere).…”

Emerging Schemes for Advancing 2D Material Photoconductive-Type Photodetectors

“…Two-dimensional layered materials (2DLMs) are denoted as a class of ultra-thin phases in which atomic-scale planar structural units are bonded by weak van der Waals force, while the intralayer atoms are commonly conjugated by strong covalent bonds [22][23][24][25][26][27][28]. In recent years, 2DLMs have attracted widespread attention from researchers worldwide due to their excellent and abundant physical and chemical properties, and these materials have been widely applied in various industries, such as fundamental physics [29][30][31][32][33][34], electronics [35][36][37][38][39][40][41][42], photonics [43][44][45][46][47][48], piezo-phototronics [49], catalysis [50][51][52], batteries [53][54][55][56], energy storage [57], thermal management [58], etc. Due to the high in-plane carrier mobility, self-passivated surface, excellent flexibility, wide availability, good compatibility with the modern microfabrication platform, and thickness/strain-dependent energy band structures, 2DLMs have shown indisputable application prospects in the next generation of photodetection applications [59][60][61][62]…”