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BROADBAND ENHANCEMENT OF SPONTANEOUS EMISSION IN 2D SEMICONDUCTORS USING PHOTONIC HYPERCRYSTALS Tal Galfsky, Zheng Sun, Cristopher R. Considine, Cheng-Tse Chou, Wei-Chun Ko, Yi-Hsien Lee, Evgenii Narimanov, and Vinod Menon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01558 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016
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Nano Letters
BROADBAND ENHANCEMENT OF SPONTANEOUS EMISSION IN 2D SEMICONDUCTORS USING PHOTONIC HYPERCRYSTALS Tal Galfsky1,2,†, Zheng Sun1,2,†, Christopher R. Considine1, Cheng-Tse Chou3, Wei-Chun Ko3, YiHsien Lee3,*, Evgenii E. Narimanov4, and Vinod M. Menon1,2,* 1
Department of Physics, City College, City University of New York (CUNY), New York, USA
2
Department of Physics, Grad. Center, City University of New York (CUNY), New York, USA
3
Department of Material Science, National Tsing-Hua University, Hsinchu 20013, Taiwan
4
Birck Nanotechnology Center, School of Computer and Electrical Engineering, Purdue
University, Indiana, USA †
Both authors made equal contributions to this work
KEYWORDS: Hyperbolic Metamaterials, Plasmonics, Photonic Hyper Crystal, Purcell Enhancement, 2D Materials, Transition-Metal Dichalcogenides
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ABSTRACT. The low quantum yield observed in two-dimensional semiconductors of transition metal dichalcogenides (TMDs) has motivated the quest for approaches that can enhance the light emission from these systems. Here, we demonstrate broadband enhancement of spontaneous emission and increase in Raman signature from archetype two-dimensional semiconductors: molybdenum disulfide (MoS2) and tungsten disulfide (WS2) by placing the monolayers in the near field of a photonic hypercrystal (PHC) having hyperbolic dispersion. The hypercrystals are characterized by a large broadband photonic density of states (PDOS) due to this hyperbolic dispersion while having enhanced light in/out coupling by a sub-wavelength photonic crystal lattice. This dual advantage is exploited here to enhance the light emission from the 2D TMDs and can be utilized for light emitters and absorbers using two-dimensional semiconductors.
Two-dimensional (2D) materials are touted as the next big material revolution in electronics and optoelectronics thanks to their exceptional electrical, optical, mechanical and thermal properties. Among the range of 2D materials that have emerged in recent years, transition-metaldichalcogenides (TMDs) have been pursued for opto-electronic applications because in the 2D limit (monolayer) these materials possess a direct bandgap1 and also show novel light emission properties2–5. However a very low quantum yield of ~ 10-2 -10-3 at 300K6 is a major hindrance in developing practical light emitting devices using these materials. Various approaches using photonic cavities7–9, plasmonic structures10,11 as well as chemical methods12 have been pursued to enhance the light emission from TMDs. Both photonic and plasmonic approaches rely on frequency resonances and hence are often bandwidth limited while the chemical approach is still in its infancy. Here we report on broadband enhancement of spontaneous emission from two
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archetypical TMDs, Molybdenum-disulfide (MoS2) and Tungsten-disulfide (WS2) monolayers using photonic hypercrystals (PHC) that have hyperbolic dispersion. PHCs are a new class of photonic media which combines control of the effective dielectric environment by a hyperbolic metamaterial (HMM) component together with the Bragg diffraction properties of a photonic crystal component13,14. Previously, enhancement of spontaneous emission (also known as Purcell enhancement) for quantum emitters such as dyes, nitrogen vacancy centers, and quantum dots have been shown in the near field of HMM substrates
15–20
. This enhancement stems from the presence of unbound wave-vector states
known as high-k modes and hence an increase in PDOS when the topology of the iso-frequency surface transitions from elliptical to hyperbolic dispersion20 (see sketch in Fig. 1a). In the monolayer limit WS2 and MoS2 can be considered as quantum emitters with an in-plane dipole moment as the materials transition to direct bandgap semiconductors with a large exciton binding energy (0.3 - 0.5 eV)21. By placing the monolayer in the near field of a HMM or PHC (see Fig. 1a and 1b), spontaneous emission from the in-plane dipoles can be enhanced by the PDOS near the hyperbolic substrate. Due to its infinitesimal thickness, the monolayer can be positioned in very precise distance to the substrate to achieve maximal enhancement effect of the PDOS. Fig. 1c shows the expected decay rate enhancement for WS2 and MOS2 obtained from 3D FDTD simulations where the fluorescent semiconductor emitter is simulated by an in-plane dipole with the material’s typical emission spectrum, placed on a HMM or a PHC (see Methods for details on the simulation). In the figure, the Purcell factor, calculated as the ratio between the decay rate of the emitter on top of the metamaterials and the decay rate of the emitter in freespace, is plotted as a function of wavelength. Notice that for the spectral range between 600700nm the Purcell factor of both HMM and PHC is very similar. This is because the mechanism
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responsible for decay rate enhancement is the same in both HMM and PHC, namely, coupling of radiation into plasmonic modes carrying high momentum (high-k modes). In addition, the emission spectra of WS2 and MoS2 are also plotted below the curve. The main distinction between PHC and HMM is that since Purcell enhancement in HMM is tied to emission into high-k modes which do not propagate to free space, a part of dipole’s radiation is essentially trapped inside the HMM and eventually dissipates through ohmic losses (which is not the case for PHC). The situation is illustrated in Fig. 1d where the electric field radiated by a dipole emitter with WS2 center of emission wavelength (618nm) on top of a HMM is plotted from a side view of the HMM (cross-section). It is easy to discern that most of the radiation is directed into the HMM (this is due to the metamaterial’s high PDOS). Even though the total decay is enhanced by the HMM ~83% of the emitted power is trapped in high-k modes propagating inside the metamaterial and will eventually decay as ohmic loss. To solve this problem we have recently demonstrated the concept of photonic hypercrystal (PHC) which combines the hyperbolic dispersion of HMMs with a periodic photonic crystal that allow the scattering of high-k states into free space 13,14. A schematic of the PHC is shown in Fig. 1b. The PHC is achieved by patterning a periodic lattice of holes in the HMM extending down through the first couple of periods and the period is optimized through simulations to out-couple the light trapped in high-k modes. Fig. 1e shows a simulation of the electric field emitted by the same dipole configuration on a PHC where it is seen that most of the dipole radiation is out-coupled in the upwards direction into free-space. In the PHC only ~24% of the total radiation is trapped inside the metamaterial and the remaining 76% is emitted into the far-field. Moreover, the farfield radiation is concentrated in a narrower cone. Of the portion of the emitter’s power reaching the far-field 70% is radiated in a 50 degrees half-angle cone for PHC in comparison to only 40%
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for HMM. As mentioned previously, for both structures Purcell enhancement is nearly identical since it stems from the PDOS of the hyperbolic substrate in the extreme near-field limit where the periodic modulation of the PHC mostly acts as a weak perturbation. This is demonstrated in Fig. 1c. We use WS2 flakes grown by chemical vapor deposition (CVD) and MoS2 exfoliated from bulk crystal (see Fig. 1f). CVD grown WS2 flakes typically form a triangular shape with a multilayer crystal seed at the center and the monolayer surrounding it22. Exfoliated MoS2 forms irregular shaped flakes consisting of a few layers down to a monolayer. These materials are transferred from the growth substrates onto HMM and PHC substrates (see methods section). The HMM substrates are fabricated by electron beam evaporation of alternating layers of Al2O3 of ~20nm thickness and Ag of ~10nm thickness. For each Ag layer an ultra-thin germanium seed layer (
