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Towards nanoscale efficient light sources

Unveiling the luminescence mechanisms in TMD – silicon nanoantenna hybrid systems

by Guy Molénat - published on

The light emission from transition metal dichalcogenides (TMD) can be enhanced by coupling to optical nanoresonators, but the underlying mechanisms are not yet understood. By using specific nanoresonators and different TMD, the different channels of the light emission (optical resonance, antenna effect, exciton type, strain) are addressed separately. These findings are essential for designing efficient nanosources of light compatible with CMOS technologies.

The development of quantum technologies needs miniaturized light sources, based on quantum emitters such as monolayers (ML) of transition metal dichalcogenides (TMD) transferred on optically resonant dielectric nanostructures acting as nanoantennas. Such hybrid systems, aimed at controlling both the optical emission rate and the emission pattern, allow tuning parameters such as the geometry, size and environment of the nanostructures, and measuring their impact on the TMD ML optoelectronic properties.

Researchers from Toulouse (CEMESLPCNOLAAS), in collaboration with other French and Japanese groups (LETI, NIMS, LIMMS), have used different TMD (WSe2, MoSe2) transferred on silicon nanostructures (Si-NS) produced by patterning techniques. The Si-NS have the shape of two pillars of submicron size separated by a gap (30-300 nm). The TMD directly transferred on the samples adapts to the shape of the nanopillars. For comparison, a second type of Si-NS was fabricated, with the gap between the Si-NS is filled with SiO2, yielding to a flat ML lying at the sample surface.

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TMD layer placed on Si nanoantennas (ACS Photonics cover, left). Spatially resolved emission in WSe2 layer on nonplanarized sample (right)

For both configurations, referred to as nonplanarized and planarized respectively, the photoluminescence (PL) mapping resulted in enhanced photon emission of the TMD ML above the nanopillars. They also allowed distinguishing the different mechanisms governing the light emission of the TMD/NS systems:

  • For planarized samples, the neutral exciton characterized by a dipole oscillating in the TMD plane is observed. Both its emission rate and emission pattern are modified by the Si-NS characteristics. This allows extracting the optical influence of the nanoantenna on the emission of WSe2 and MoSe2.
  • For nonplanarized samples, both antenna and strain effects can be investigated. At the edges and in the gap of the Si-NS, either WSe2 or MoSe2 experiences large tensile strain, at the origin of a decrease of the band gap energy of the TMD, thus of the local PL enhancement.
  • For WSe2 on nonplanarized Si-NS, an important additional contribution to emission arises attributed to the dark exciton, corresponding to an out-of-plane oscillating dipole. For flat WSe2 ML (including the planarized sample case), the contribution of this exciton is negligible (hence its name “dark”). In present case, the vertical parts of the WSe2 ML along the Si-NS edges offer the best geometry for dark exciton detection, further improved by the antenna effect.

These results yield to a better understanding of the mechanisms governing light emission in TMD/Si-NS systems. The light emission can be tailored by optimizing independently the nanoantenna design, the TMD material and its strain level.

This work has been funded by ANR HiLight (ANR-19- CE24-0020-01), NanoX 2DLight (ANR-17-EURE- 0009), Calmip P12167

 

Contacts* : Pr. Vincent Paillard (CEMES) and Dr Bernhard Urbaszek (LPCNO)

Publication : Unveiling the Optical Emission Channels of Monolayer Semiconductors Coupled to Silicon Nanoantennas. J. M. Poumirol et al., ACS Photonics 7 (11), 3106-3115 (2020)