Very few materials are able to absorb visible light without dissipating some of the resulting energy into phonon modes, and these excited modes have the capability to act back on the electronic excitation that is generated. By the same token, very few probes of photophysical processes in materials are able to directly probe the coexistence of both electronic and thermal departures from equilibrium or directly visualize the impact of the spatiotemporal interaction of electronic and thermal excitations. I will nevertheless, describe such a capability that leverages not only the ps time resolution associated with electronic to thermal energy transduction but that also provides direct spatial maps of localized photoinduced electronic and temperature profiles and their coupled evolution. I will how how this approach allows us to investigate thermoelectric effects in few-layer MoS_2 and that it can be more broadly applied to other emerging semiconductors.
Characterizing the intrinsic properties of low-dimensional transition metal dichalcogenides (TMDCs) is necessary for explaining how their novel properties arise and are modified by their local environment. Excitations in few-layer TMDCs and heterostructures are difficult to probe directly because of their low photoluminescence quantum yield. With time-resolved elastic scattering microscopy, we spatiotemporally resolve both in-plane and out-of-plane nanoscale transport in several TMDC species and architectures as a function of layer thickness and pump-induced carrier density. We directly observe interlayer exciton transport in TMDC heterostructures and find that these species diffuse an order of magnitude farther and faster than excitations do in their isolated counterparts.
One of the central challenges for practical applications of single-photon sources is the ability to efficiently extract light from a single quantum emitter. A useful single-photon source must emit into a well-defined direction because in practice one can collect light only in a finite solid angle. Here, we propose to harness the exceptional light molding capabilities of photonic metasurfaces to engineer the emission from quantum emitters and achieve highly directional emission.
We have designed a phase gradient reflectarray metasurface, which efficiently collects spontaneous emission from a quantum emitter, located in the far-field (d~5 wavelengths), and redirects it back to the source. By controlling the phase imprinted by the metasurface on the incident light, we control the emission properties of the emitters. We apply this concept to design a metasurface for use with hexagonal boron nitride (hBN) single photon emitters operating at 620 nm. We have observed experimentally bright single photon emission at 620 nm with a remarkably narrow spectral width of zero-phonon line emission from multilayer hBN films synthesized by chemical vapor deposition. Simulations show that at a wavelength of 620 nm, the reflection efficiency of our metasurface is greater than 85%, and that the emission from these emitters are highly directional with deviation from emission in the surface-normal direction of 2∆θ ~ 20°. We will report on experimental measurements of hBN quantum emitters coupled to metasurfaces and describe metasurface designs for coupling of multiple quantum emitters.
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