Integration of atomically thin layers of transition metal dichalcogenides into high-Q, monolithic Bragg-cavities - an experimental platform for the enhancement of optical interaction in 2D-materials

We demonstrate a new approach to integrate single layer MoSeR2R and WSeR2R flakes into monolithic all-dielectric planar high-quality micro-cavities. These distributed-Bragg-reflector (DBR) cavities may e.g. be tuned to match the exciton resonance of the 2D-materials. They are highly robust and compatible with cryogenic and room-temperature operation. The integration is achieved by a customized ion-assisted physical vapor deposition technique, which does not degrade the optical properties of the 2D-materials. The monolithic 2D resonator is shown to have a high Q-factor in excess of 4500. We use photoluminescence (PL) experiments to demonstrate that the coating procedure with an SiO2 coating on a prepared surface does not significantly alter the electrooptical properties of the 2D-materials. Moreover, we observe a resonance induced modification of the PL-spectrum for the DBR embedded flake. Our system thus represents a versatile platform to resonantly enhance and tailor light-matter-interaction in 2D-materials. The gentle processing conditions would also allow the integration of other sensitive materials into these highly resonant structures.

Transition metal dichalcogenides (TMDCs) are semiconducting 2D-materials with direct bandgaps in the visible range from 1.0 to 2.5 eV. These consist of a layer of transition metals such as W or Mo sandwiched between two chalcogen layers, i.e. S, Se, or Te layers.
Monolayer TMDCs exhibit peculiar optical effects, which are related to the confinement of electronic motion in a 2D plane and the absence of dielectric screening, as well as to their crystal symmetry. The absorption of photons with energy above the bandgap in TMDCs causes the generation of hot electrons [<cui2014>], which swiftly form bound electron-hole pairs, termed excitons. Excitons in TMDCs are highly stable with binding energies in the range of hundreds of meV [<chernikov2014>]. Both the linear and nonlinear electronic [<zhu2015>, <zhang2017>] and optical [<saleh2018>, <tonndorf2015>] properties of TMDCs are strongly affected by these excitons. Due to their stability and robustness [<palummo2015>, <ruppert2017>], TMDCs are ideal candidates for exciton experiments. They exhibit non-linear properties [<saleh2018>, <nie2016>], making them interesting for experiments such as sumfrequency generation [<wang2015>-<janisch2014><janisch2014a>], but also for the generation of entangled photon pairs [<he2016>].
However, due to their single-layer nature, they are also highly susceptible to environmental parameters [<liu2015>], process conditions [<mcdonnell2016>], properties of the substrate material [<akinwande2017>, <lippert2017>], and substrate geometry [<kim2018>]. This makes experiments difficult to reproduce and highly dependent on laboratory conditions, which may be hard to control. The integration of TMDC layers in well-defined optical coatings and materials, such as glasses, would eliminate some of these issues and help establish TMDCs as a reproducible experimental platform.
An application with highly challenging requirements for the integration of 2D-materials in optical systems comes in the form of strong coupling experiments [<lundt2016a>-<luo2018><shahnazaryan2017>]. Strong coupling refers to an exciton being coupled resonantly to an optical cavity of high quality and small modal volume. These excitons hybridize with the cavity mode and form so-called exciton-polaritons, the branches of which are separated by Rabi-splitting. Strong coupling can be observed if the dipole coupling strength, i.e. the product of the dipole moment of the exciton and the electric field at the position of the exciton, exceeds radiative and dissipative losses, e.g. photon leakage out of the cavity, represented by the cavities' quality factor (q-factor), and/or emitter dephasing [<savona1995>]. The q-factor is typically measured from spectral data as the ratio of the resonance wavelength and the line width of the cavity. As strong-coupling has already been demonstrated, is well understood and yet technically highly challenging, we find it to be a superb test-case to demonstrate the capability of our method to fabricate systems with a bandwidth and q-factor that is unpreceded for monolithic cavities.
For MoSe2 it was shown [<lundt2016>] that monolithic distributed-Bragg-reflector-cavities (DBR-cavities) exhibit strict distinguishability [<liu2013>] of the Rabi-peaks for both cryogenic and room-temperature operation if a q-factor of ≫ 1300 [<lundt2016>] can be achieved. Although strong coupling was observed for lower q-factors [<liu2014a>], we use the predictions from [<lundt2016>] as a benchmark as it guarantees the strict distinguishability of the Rabi-peaks. It also opens a new path to high-quality, room-temperature polaritonic device architectures. Moreover, ion assisted PVD (IAD), employed here, generally imposes lower thermal loads than plasma-enhanced CVD (PECVD) [<liu2014a>], thus maximizing the selection of embeddable materials. IAD also has a larger set of materials to choose from, which can, for example, be used to implement higher refractive index contrasts. This also leads to a higher degree of flexibility and a broader range of applications for our method.
Recent results [<dhara2018>] underline the capabilities of PVD-techniques to fabricate systems for fundamental investigations in many-body polaritonics, which are only accessible to platforms with increased q-factors. Although the authors demonstrate a q-factor of = 600, several questions remain open. These may be answered in systems with further increased qfactors; in accordance with the distinguishability-related benchmark ≫ 1300 derived above. Such systems can be attained with the IAD technique presented here.
Beyond , <ge2018>] exhibited a higher qfactor, but the 2D-material cannot be placed at the position of the peak field enhancement, thus the high q-factor cannot be exploited to the fullest.
In this work, we report on an ion-assisted physical vapor deposition process (IAD) with a temperature below 350 K used to embed TMDCs into a planar Fabry-Perot microcavity based on SiOR 2R /TiOR 2R layerstacks (see Fig. 1). By using TiO2 as a high refractive material and a high number of high-index-low-index-pairs (HL-pairs), we achieve access to increased q-factors and larger bandwidths. The process allows us to integrate exfoliated MoSeR 2R and WSeR 2R flakes with high-quality optical materials into monolithic, solid state layer systems with a high level of control on the material composition and thickness.

Methods
First, we determined the maximal q-factor of an unloaded Bragg cavity that can be achieved in our process. It is limited by the absorbance and the scattering of the coatings produced in the IAD process and by the maximal thickness of the layer stack, which can be fabricated without delamination. Both absorptive and scattering losses have been characterized for the IAD in prior works [<bennett1989>, <thielsch2002>]. The real wavelength dependent material parameters have been used for analytical calculation via OptiLayer, which we used to predict and optimize our structures [<tikhonravov2014-2018>].
The q-factor of the cavity depends on the transition bandwidth at the resonance position and hence increases with the number of high-index-low-index-layer-pairs used for both mirrors [<garmire2003>, <reichman2000>]. This can be seen in Fig. 2. Numerical calculations show that a q-factor of > 1300 can be achieved with 7 HL-pairs on both sides of the cavity. For later ease of observation of strong coupling, we thus chose to pursue a design with 8 HL-pairs  Both mirrors have been optimized for high reflectivity between 630 nm and 850 nm. The TiOR 2R layer had a refractive index of TiO 2 = 2.284 at 750 nm. The SiOR 2R had a refractive index of SiO 2 = 1.455 at 750 nm. The layers of both materials were tuned to 4 ⁄ -thickness resulting in 129.3 nm thick SiOR 2R-layers and 79.3 nm thick TiOR 2R -layers. Note that the design is limited to 300 nm for single TiOR 2R layers to avoid detrimental influences from oversized polycrystalline growth, thus retaining smooth surfaces with low scattering, low absorbance, and high optical quality [<bennett1989>, <leprince-wang2000>]. The combined central SiOR 2R-spacer has an optical thickness of 375 nm to tune the resonance wavelength to = 750 nm, as confirmed by a pronounced dip in the reflection spectrum shown in Fig. 3(a). The calculated bandwidth of the resonance peak was 0.063 nm (full width at half maximum), equating into a q-factor of = 11900.   Fig.4(a)) and off-resonant excitation at = 752 nm (see Fig.4(b)) each at normal incidence. Because of the 8/10-HL-stack-design, the cavity is not symmetrical to the center. Nevertheless, the antinode position of the electromagnetic wave is at the middle of the spacer-area, coinciding with the position of the 2D-material, such that we can fully utilize the high q-factor. The fabrication of the TMDC-loaded cavity was carried out in an ion-assisted deposition (IAD) process. Mechanically exfoliated MoSeR 2R and WSeR 2R monolayer flakes [<dean2010>] were placed on an optical base substrate, in our case a sputtered DBR made from 10 pairs of SiOR 2R and TiOR 2R layers, deposited on a quartz substrate. A few-layer boron nitride flake (hBN) with a thickness of about 10 nm was deposited on the 2D-flakes to protect the TMDC-flakes from influences caused by the subsequent coating process [<dean2010>]. The second cavity mirror consisting of eight SiOR 2R-TiOR 2R -pairs was deposited directly on the top via IAD preserving gentle coating conditions to comply with the weak van-der-Waals adhesion of the TMDC flakes. The deposition procedure was performed in a physical vapor deposition plant Buhler SyrusPro1100 using a background pressure of about 10P -5 P mbar with a maximal process temperature of about 80 °C. An overview of the coating process is depicted in Fig. 5. The SiOR 2R surface of the bottom DBR with the van-der-Waals-bound TMDC and hBN flakes on top was pretreated with an Ar-Ion plasma using 60 V Bias and 30 A discharge current with 10 sccm Ar for 7 seconds. This enhances the surface energy of the SiOR 2R -top layer by cracking OH-bonds, creating chemically active sites, to which subsequently deposited material may crosslink [<meyer1974>-<martinu2000><terpilowski2015><bhattacharya2005>]. The pretreatment provides an additional cleaning effect for the surface. Plasma exposure time and plasma energy have been determined from prior experiments to be sufficient to create a significant adhesion effect while maintaining a low dose to prevent delamination of the TMDC flake from the bottom mirror. No resulting increase in surface roughness was observed.
Next, we deactivated the plasma and coated the activated surface with SiOR 2R evaporated by an electron beam using a low deposition rate of about 0.4 nm/s. This rate was chosen as the lowest reproducible deposition rate as its typical fluctuation is in the order of 0.2 nm/s. The first 10 nanometers of the SiO2-layer were deposited without plasma assistance to prevent an overexposure of the surface while the coating is still thin and may not yet be fully coalesced.
The SiOR 2R builds amorphous layers covering both the TMDC islands as well as the surrounding dielectric surface. An important parameter is the densification with argon and oxygen ions. The plasma leads to highly densified layers and increased refractive indices. It also induces compressive stress and reduces the tendency of the material to delaminate caused by tensile stress-induced cracking [<teixeira2001>]. This enhances the capability of the system to withstand large temperature differences and allows us to deposit more layers in a more reproducible manner. In a next step, the densification of the layer material was slowly increased by raising the Ar-flux and ion energy values typical for IAD [<schulz1996>]. For the outer layers of the DBR, a densification with up to 150 V bias voltage was used. Because of stoichiometry considerations, OR 2R was added to the active plasma gas with 10 sccm gas flux for SiOR 2R -layers and 30 sccm for TiOR 2R .
For the fabrication of the cavity, it must be considered that both the TMDC as well as the hBN-flake contribute to the optical path in the spacer layer, and thus to the resonance wavelength. Their respective refractive indices have been taken from literature values The SiOR 2R thickness of the spacer layer was reduced accordingly. The gradient refractive index of the less densified SiOR 2R-layer, as well as the thickness of the higher refractive SiOR 2R -sheath, was taken into account as well. A microscopic top-view of a TMDC-loaded part before and after the coating process of the cavity is depicted in Fig. 6(a). While the contrast is greatly reduced due to the reflected light from the top-mirror, it still can be seen that the process does apparently not damage the TMDCflakes. An SEM of the cavity cross section is depicted in Fig. 6(b). Due to the limited resolution of the SEM, the TMDC-layer cannot be observed directly. The fabricated cavities have been subjected to cryostatic conditions and undergone multiple cooling-heating cycles between 5 K and 300 K without any signs of delamination or damage to the TMDCs.

Results
First, we verified in two steps if it is possible to coat the TMDCs with SiO2 layers. A prior experiment on the adhesion properties was performed. TMDCs placed on the bottom mirror and covered with approximately 10 nm thick hBN sheets were treated with Ar-plasma. Contactangle measurements showed sufficient increase of the surface energy providing the required precondition for adherent coating on these surfaces without delamination of the 2D-materials.
We then focused on the question of whether it is possible to preserve the structural and electrooptical properties of the 2D-material. We therefore coated 20 nm SiO2 without plasma assistance directly on the 2D-flake. This second experiment was conducted to yield more specific information on the influence of embedding the TMDC on its photoluminescence (PL) properties. Room-temperature PL experiments were carried out with a 532 nm excitation laser providing 400 µW energy. The influence of the SiO2 on the PL of the hBN covered WSe2flakes is shown in Fig. 7(a). The increase of the layer thickness to 120 nm caused no shift of the PL peak positioned at 745 nm. A linewidth of roughly 40 meV could be achieved.
In Fig. 7(b), we present PL measurements of MoSe2 with hBN-cover, both with 20 nm and 120 nm SiO2 coating at room temperature and at 5 K. The linewidth is about 40 meV at room temperature and about 8 meV for excitonic and trionic resonances at 5 K. These results are comparable to former experiments by Lundt et.al. [<lundt2018>]. The prominent splitting in two peaks at cryogenic conditions indicates, that both excitonic and trionic oscillations are essentially unaffected by the application of the SiO2 coating. Fig. 7. PL intensity of SiO2 covered layers. a) WSe2-PL hBN covered flakes for 20 nm and 120 nm SiO2 thickness b) MoSe2-PL for 20 nm and 120 nm SiO2 thickness at 300 K and 5 K Next, we measured the optical reflectance of the deposited layer stacks using a standard UV/VIS spectrometer (Lambda 900 by Perkin Elmer), as well as a UV-NIR Micro-Spectrometer (USPM by Olympus). The subsequent morphological investigation of the encapsulation and the material distribution in the cavity was carried out with an optical stereo microscope by Leica systems and with an SEM-system Sigma by Carl Zeiss (Fig. 6(b)).
Prior to the complete embedding of the TMDC, we analyzed the optical performance on a bare DBR-mirror coated on a plane substrate to verify the validity of our DBR coating process. The reflection spectrum of the mirror and calculated design are depicted in Fig. 8(a). Both are in accordance, which proves that our coating process is operating as predicted.

K 5 K
Next, we fabricated a TMDC loaded DBR-cavity as discussed in the methods section. A measured reflection spectrum is depicted in Fig. 8(b). A reflectance spectrum of the DBR cavity at the resonance frequency is provided in Fig. 8(c). It shows the observed resonance at = 749.3 nm, which is roughly 0.1 % off the target value. This is consistent with typical variations of the coating process. A resonance bandwidth of Δ = 0.16 nm was determined, which equates into a quality factor of = 4683. This value is lower but quite in the magnitude of the design value of 11900. The difference between measurement and calculations may be attributed to defect spots in the layers, slight surface roughness and inhomogeneity's of refractive indices. Then we verified, that the PL properties of the TMDC are unaffected by the DBR stack and will not produce any kind of background fluorescence, which would later negatively affect possible experiments at exciton wavelength.
To perform cross-sectional PL measurements of the DBR stack, a cross-sectional lamella was prepared from the cavity by means of Focused Ion Beam (FIB) milling using a FEI Helios NanoLab G3 UC. The lamella was attached to a TEM lift-out grid and thinned down to a final thickness of 200 nm with 30 kV Ga ions. No further low energy cleaning to remove amorphous layers or Ga ion contamination was performed. An SEM image of the lamella is depicted in Fig. 9(a).
Following, the lamella was transferred to a confocal laser-scanning microscope (PicoQuant MicroTime200). The microscope was used with a 40x/0.65NA objective corresponding to lateral resolution of about 1 µm with an excitation laser working at 532 nm with 80 MHz rep. rate and about 100 ps pulse length. The PL light was filtered with a 715 nm long pass filter. A measurement area of 40x40 µm was scanned with piezo positioning. The dataset of the measurement area was integrated along the lateral axis to receive a linescan, orientated perpendicular to the cavity system. The ensuing PL signal is superimposed on the SEM-image in Fig. 9(a). It shows two fluorescence peaks, one emanating from the expected location of the TDMC-flake, the other one from the substrate material at the bottom. To further clarify the nature of these two PL peaks, we measured their spectra at the peak locations marked with the colored circles in Fig. 9(a). We used a Horiba spectrometer iHR320 with an integration time of 600 s operating at room temperature. The two spectra are shown in Fig. 9(b). The PL in the substrate exhibits a flat spectrum and is therefore not related to the MoSeR 2R but rather to residual defect mediated autofluorescence of the substrate with a high contamination of Ga due to the lamella cutting process [<vaskin2018>]. The PL signal in the spacer area shows two spectral peaks. The first spectral peak occurs close to the resonance wavelength of the cavity at 755 nm. The small difference to the cavity wavelength may either be caused by self-bending of the membrane, or due to cracking and subsequent extension of the spacer-layer, which both start to occur during FIB-milling at the layer thickness of 200 nm. The second spectral peak occurs at 780-795 nm. While the latter peak clearly represents the characteristic A-exciton PL wavelength of MoSeR 2R [<tonndorf2013>], we attribute the former to the action of the cavity. The PL peak is close to the position reported in the literature. Differences may be caused by the influence of the doped embedding material and from the strain induced by the FIB treatment. At this wavelength, the PL of the exciton is indeed enhanced by the cavity. Note that in this cross-sectional membrane, we cannot expect a high qfactor as it is only 200 nm thin and has a highly scattering surface. These results show the presence of pristine, high-quality MoSeR 2R in the cavity, the electronic properties of which have not been affected in a detrimental manner by the coating process. It also shows that its excitons do indeed couple to the cavity mode. WSe2 would show similar results as suggested by the previous experiments.

Conclusion
We demonstrated a new approach to integrating single layer MoSeR 2R and WSeR 2R flakes into dielectric optical coatings and layer stacks. Our approach is based on a modified Ion Assisted Physical Vapor Deposition process. The gentle processing conditions allow us to integrate 2Dmaterials into optical coatings and layer stacks.
We selected the integration into monolithic, all-dielectric, high-q planar DBR-cavities as a benchmark for our process. This was selected for possible experiments on strong coupling and polaritronics, which require both high-quality 2D-materials as well as high-quality, smallvolume resonators. The monolithic cavity could be realized without cracks, without damage to the TMDC, and with accurate reproduction of the theoretical layer-design. The ratio of the resonance wavelength and the line width of the resonance (Q-factor) of the cavity was higher than 4500 at 749.3 nm.
The presence of TMDC-material in the resonator, as well as its being unaffected by the coating process, was proven with photoluminescence measurements. For DBR-cavities, we observed photoluminescence from the MoSeR 2R exciton at its fundamental wavelength and an enhancement of the PL emission at the slightly detuned cavity resonance. Our results suggest that the process presented in this work provides a viable platform for the study of strong coupling, polaritronics, and the enhancement of nonlinear-optical effects in 2D TMDC.

Acknowledgements
This work has been supported by the Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.. We gratefully acknowledge the financial support by the German Federal Ministry of Education and Research via the funding "2D Nanomaterialien für die Nanoskopie der Zukunft" FKZ: 13XP5053A. Financial support from the Thuringian State Government within its Pro-Excellence initiative (ACPP 2020 P ) is gratefully acknowledged. The Würzburg group gratefully acknowledges financial support by the state of Bavaria. C.S. acknowledges support by the European Research Council within the project UnLiMIt-2D.