List of contents 1. Discussion and analysis about fringe formation 2. Field-distribution calculation confirming TM0 waveguide modes 3. Polariton dispersion of the 110-nm-thick MoSe2 sample 4. Effects of exciton linewidth & temperature on polariton dispersion 5. Propagation length of the waveguide exciton polaritons

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide (MoSe 2 ), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides.

We report on nano-optical imaging study of WSe2 thin flakes with the scanning near-field optical microscopy (NSOM). The NSOM technique allows us to visualize in real space various waveguide photon modes inside WSe2. By tuning the excitation laser energy, we are able to map the entire dispersion of these waveguide modes both above and below the A exciton energy of WSe2. We found that all the modes interact strongly with WSe2 excitons. The outcome of the interaction is that the observed waveguide modes shift to higher momenta right below the A exciton energy. At higher energies, on the other hand, these modes are strongly damped due to adjacent B excitons or band edge absorptions. The mode-shifting phenomena are consistent with polariton formation in WSe2. Main TextGroup VI transition-metal dichalcogenides (TMDCs) with chemical formula MX2 (M = Mo, W; X = S, Se, Te) are novel semiconductors with layered structures and remarkable (opto)electronic properties [1][2][3][4]. Light-matter interactions in this class of materials have been widely explored in the far-field regime where many intriguing optical phenomena due to excitons were observed by emission and reflection/absorption spectroscopy [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Recently, a few near-field emission studies of TMDCs were reported where nanoscale

Monolayer transition metal dichalcogenides, a new class of atomically thin semiconductors, possess optically coupled 2D valley excitons. The nature of exciton relaxation in these systems is currently poorly understood. Here, we investigate exciton relaxation in monolayer MoSe_{2} using polarization-resolved coherent nonlinear optical spectroscopy with high spectral resolution. We report strikingly narrow population pulsation resonances with two different characteristic linewidths of 1 and <0.2  μeV at low temperature. These linewidths are more than 3 orders of magnitude narrower than the photoluminescence and absorption linewidth, and indicate that a component of the exciton relaxation dynamics occurs on time scales longer than 1 ns. The ultranarrow resonance (<0.2  μeV) emerges with increasing excitation intensity, and implies the existence of a long-lived state whose lifetime exceeds 6 ns.