Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers

Abstract

We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS2, WS2, and WSe2, can be substantially tuned by > 60% in the imaginary part and > 20% in the real part around exciton resonances using CMOS-compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of refractive index. This work may pave the way towards the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits.

Measurements of electrically tunable refractive index of MoS2 monolayer and its usage in optical modulators

“…This modulation depth observed in our device is among the best for TMDs based modulators. [4][5][6]37 The changes observed in Fig. 4c, d are attributed to the V g induced refractive index changes (Fig.…”

“…We found that the peak positions of E 1 2g and A 1g modes were largely unaffected by the V g within the experimental error of ±0.2 cm −1 while Raman intensities of both E 1 2g and A 1g modes for Au/Si 3 N 4 / MoS 2 hybrid nanostructure changed significantly with the applied gate voltage. According to previous studies, 5 the intensity ratio of the Raman peaks could be used to explore exciton properties (bandgap renormalisation, damping factors, oscillator strengths associated with the excitons) of a MoS 2 monolayer. For example, the ratio of intensities of the E 1 2g and A 1g peaks (I E2g /I A1g ) of MoS 2 could be used to identify the number of layers in MoS 2 ( Fig.…”

“…[1][2][3] As a result, 2D atomic semiconductor crystals emerged as a potential alternative to bulk semiconductors for optoelectronic applications compatible with CMOS technology. [4][5][6] The most investigated 2D semiconductor-a molybdenum disulphide (MoS 2 ) monolayer-deserves special attention as it hosts a high quality electronic system with a unique band structure and a technologically favourable band gap. [7][8][9] A MoS 2 monolayer is a direct band semiconductor with E g ≈ 1.9 eV and is composed of hexagonal planes of S and Mo atoms glued together by ionic-covalent interactions in a trigonal prismatic arrangement.…”

Composite material anti-resonant optical fiber electromodulator with a 3.5 dB depth

“…To this end, we note that electrostatic gate tunable electro-absorption has recently been observed in silicon backgated TMDC planar (normal incidence, non-waveguide) device geometries. Evidence of such are reported by Yu et al [5], or ITO backgated Hafnia/MoS2 structures fabricated by Vella et al [6]. In this Letter, we offer an alternative method of electro-optic modulation through the use of a functionalized hollow core fiber waveguide.…”

Composite material hollow core fibers: functionalization with silicon and 2D materials

“…This was confirmed by replacing the film with WS2. The refractive index was changed to 4.80, as discussed 11 Figure 2b when the fiber has no deposition inside. As can be seen, a single resonance occurs at around the 600nm mark.…”