Ultrafast real-time observation of double Fano resonances in discrete excitons and single plasmon-continuum

R. K. Chowdhury, S. Mukherjee, S. N. B. Bhaktha, and S. K. Ray

Phys. Rev. B 101, 245442 – Published 29 June 2020

Abstract

Ultrafast observation of multiple asymmetric Fano line shapes in energetically overlapped single continuum and multiple discrete states is of great theoretical interest in different domains of physics, though their real-time experimental demonstrations are yet to be achieved. Here, we present an experimental observation on time-domain response of the double Fano asymmetries in a metal–two-dimensional semiconductor hybrid prototype at room temperature. The ultrafast interactions in two discrete spin-resolved excitons of ${MoS}_{2}$ and a single metal plasmon-continuum allow us to observe the generation and evolution of the double Fano line profiles in real time. Apart from the subpicosecond development of double Fano line shapes, we calculate all the time-dependent nonlinear double Fano parameters. These results suggest a better understanding of ultrafast Fano physics in condensed-matter systems and have potential prospects in ultrafast photonic devices using double Fano resonances even at room temperature.

Received 14 November 2019
Revised 3 June 2020
Accepted 9 June 2020

DOI:https://doi.org/10.1103/PhysRevB.101.245442

Physics Subject Headings (PhySH)

Excitons Plasmons Quantum description of light-matter interaction Ultrafast optics Ultrafast phenomena

Nanostructures Transition metal dichalcogenides

Femtosecond laser spectroscopy Time-resolved reflection spectroscopy

Atomic, Molecular & OpticalNonlinear DynamicsCondensed Matter, Materials & Applied Physics

Authors & Affiliations

R. K. Chowdhury¹, S. Mukherjee¹, S. N. B. Bhaktha¹, and S. K. Ray ^1,2

¹Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
²S. N. Bose National Centre for Basic Sciences, Kolkata 700106, India

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Issue

Vol. 101, Iss. 24 — 15 June 2020

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Images

Figure 1
Structural and optical characterizations for Au- ${MoS}_{2}$ hybrids. (a) Typical AFM image of Au nanoislands and the corresponding histogram (inset) showing the height distribution with an average value $\sim 7.0$ nm. (b) Typical FESEM micrograph and corresponding inset show the lateral size distribution of Au nanoislands revealing the formation of Au nanodisks on top of ${MoS}_{2}$ . The average diameter of the Au nanoislands is $\sim 14.0$ nm. (c) Steady-state normalized absorption spectra of Au nanodisks and ${MoS}_{2}$ layers. (d) Experimentally measured and simulated steady-state reflectivity ( $R$ ) spectra of Au- ${MoS}_{2}$ hybrid along with substrate.
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Figure 2
E-field distribution for Au- ${MoS}_{2}$ hybrid structure. (a) The schematic representation of Au nanodisks on layered ${MoS}_{2}$ . (b) In-plane electric-field coupling (675 nm) at the interface of Au nanodisks and ${MoS}_{2}$ . Panels (c), (d), and (e) show the out-of-plane E-field distribution at off-resonant and resonant conditions (rectangle with white-colored dashed line) suggesting that the metal (Au)–semiconductor ( ${MoS}_{2}$ ) coupling is activated near the excitonic absorption (within 600–700 nm) of ${MoS}_{2}$ . The input wave is propagating along the $k$ direction.
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Figure 3
Real-time mapping of double Fano resonances: scheme and results. (a) Schematic of the sample structure for noncollinear pump-probe measurements to investigate the evolution of double Fano line shapes in the time domain (i.e., as a function of time delay, $Δ τ$ ) using self-assembled Au nanodisks/layered ${MoS}_{2}$ system. (b) A graphical representation of the Fano interference scheme between double discrete spin-orbit (SO) coupled excitons ( $X_{A}^{0}$ and $X_{B}^{0}$ ) of ${MoS}_{2}$ and a single plasmon-continuum (P) due to self-assembled Au nanodisks. (c) Transient spectra for Au nanostructures, pristine ${MoS}_{2}$ , and Au- ${MoS}_{2}$ hybrid at 5.0 ps probe delay for 1.0 GW/ ${cm}^{2}$ pump power, depicting clear double Fano line shape for both of the exciton-plasmon overlaps for Au- ${MoS}_{2}$ hybrid (blue curve).
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Figure 4
Typical ultrafast transient spectra (pump power density, $P_{pump} \sim 10^{9} W / {cm}^{2}$ ) of Au- ${MoS}_{2}$ hybrid nanostructures in 600– 700 nm wavelength range. The $Δ OD$ spectra with red points at 300 and 500 fs probe delay represent the excitonic peaks ( $X_{A}^{0}$ and $X_{B}^{0}$ ) of ${MoS}_{2}$ within the first 500 fs, suggesting that discrete excitons and continuum of plasmons are not coupled together. On the contrary, the curve with green points is at an intermediate-coupling limit for 1.0 ps probe delay. Thereafter, the coupling between excitons and plasmons becomes robust displaying Fano line shapes up to a delay of 5.0 ns.
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Figure 5
Nonlinear response of double Fano resonances. (a) Transient spectra of as-grown pristine ${MoS}_{2}$ on 300 nm ${SiO}_{2}$ substrates revealing both of the discrete excitonic peaks ( $X_{A}^{0}$ and $X_{B}^{0}$ ). Panels (b)–(d) display the pump-power-dependent (1.0, 2.0, and 4.0 GW/ ${cm}^{2}$ ) ultrafast observation of asymmetrical line shapes as a result of coupling between discrete spin-resolved excitons from TMDs and continuum of plasmons from metal nanostructures. The power-dependent transient mapping supports the evolution of nonlinear Fano resonance in Au- ${MoS}_{2}$ hybrid systems.
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