Open Access
Add to BibSonomy
Abstract
Layered two-dimensional (2D) materials provide a wide range of unique properties as compared to their bulk counterpart, making them ideal for heterogeneous integration for on-chip interconnects. Hence, a detailed understanding of the loss and index change on Si integrated platform is a prerequisite for advances in opto-electronic devices impacting optical communication technology, signal processing, and possibly photonic-based computing. Here, we present an experimental guide to characterize transition metal dichalcogenides (TMDCs), once monolithically integrated into the silicon photonic platform at 1.55 μm wavelength. We describe the passive tunable coupling effect of the resonator in terms of loss induced as a function of 2D material layer coverage length and thickness. Further, we demonstrate a TMDC-ring based hybrid platform where resonance shift has been mapped out as a function of flake thickness, which correlates well with our simulated data. These experimental findings on passive TMDC-Si hybrid platform open up a new dimension by controlling the effective change in loss and index, which may lead to the potential application of 2D material based active on chip photonics.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Atomically thin two-dimensional (2D) materials research has progressed rapidly after the isolation of graphene in 2004 [1–4]. While graphene shows many exceptional properties, its lack of an electronic bandgap has stimulated the search for other graphene-like 2D materials [5–8]. Transition metal dichalcogenides (TMDCs), a family of materials with a general formula of MX2, where M is a transition metal (Mo, W, Re) and X is a chalcogen (S, Se or Te), provide a promising alternative to integrate them into atomically precise heterostructures combining conducting (graphene) and insulating (hBN) 2D materials [9,10]. A stable member of the TMDCs family, molybdenum disulfide (MoS2), has attracted widespread attention for a variety of next-generation electrical and opto-electronic properties such as high room temperature mobility, high switching characteristics, bandgap tunability, and high exciton binding energies [11–13]. Bulk MoS2 has an indirect bandgap of ~1.2 eV which, due to quantum confinement, crosses over to a direct bandgap of ∼1.8 eV, when the material is a monolayer [12]. Due to this tunable electrical and optical properties, MoS2 could be a prospect for future advances in the field of nano-optics and photonics [13,14]. This material is studied here as monolithically integrated with Silicon photonics as just one example, where we are interested in the respective impact of optical absorption and index-shift whence heterogeneously added to a Silicon photonic waveguide and microring resonator (MRR). Beyond MoS2, other TMDCs could be studied as well in follow-up work on the same or similar integrated photonics platform, which is interesting, since the spectral distance of each TMDC’s exciton relative to the waveguide probing wavelength (here, λ = 1550 nm) is different; thus, the real vs. imaginary part index decay away from the exciton resonance has a different impact on the telecom-operating photonic structures.
Silicon photonics is becoming an integration platform of large interest for optical datacom and telecom applications [15]. However, Silicon’s weak electro-optic properties and indirect bandgap severely limit opto-electronic device functionality. In contrast, hybrid or heterogeneous photonic integration solutions offer an appealing approach, when combined with an optical low-loss, yet commercially accessible large volume and low-cost CMOS fabrication technology such as Si/SiN photonics [16–19]. Other active opto-electronic materials such as transparent conductive oxides, while showing high switching performance, usually introduce relatively high optical losses [20]. Whereas, because of the advent of sufficiently strong van der Waals (vdW) force, 2D materials can (in principle) be easily integrated with photonic chip, offering a rich variety of electronic and optical properties that enable light generation, modulation, and detection could be a promising platform for next-generation PIC [21–24]. In reality, the state-of-the-art of TMDCs transfer techniques is not benign with taped-out chip technology due to the inability to place a single 2D material flake on the pre-fabricated photonics chip without incurring significant cross-contamination (e.g. transfer of undesired flakes). We recently provided a solution for this challenge developing a 2D material printer enabling cross-contamination-free transfers without impacting the underlying photonic waveguide structures reported in ref [25].
Here, we demonstrate a novel heterogeneous platform to study the physical properties of TMDCs by newly developed 2D printer transfer technique, enabling rapid and precise transfer of 2D atomic layers on the integrated photonic chip without any cross contamination. Using the TMDC-Silicon heterogeneous integrated platform, we perform a comparative study to determine the optical loss and refractive index change, respectively on Silicon waveguide and microring resonator (MRR) by varying the coverage length and thickness of MoS2 at a telecommunication wavelength. The effect of MRR-to-waveguide coupling has been mapped out in terms of resonance shift as a function of monolayer coverage analyzing the loss induced by monolayer MoS2 which is found to be about 0.012 dB/μm. We obtain a resonance shift per unit waveguide coverage length of 0.064 nm/μm as a function of thickness which matches our numerical results well. Together these experimental studies of integrating MoS2 with Silicon photonics shows an induced negligible loss, but relatively strong index-tuning potential thus paving the way for future studies of active opto-electronic device technology.
2. Methods
Here, we demonstrate a heterogeneous platform to study the effect of ultrathin TMDCs towards building on-chip active device component. The study is performed on taped-out Si photonic chips (APPLIED NANOTOOLS INC.). It is important to keep all the physical parameters unchanged before and after the transfer of the 2D materials to single out the influence solely from atomic layered materials, thus; first, we coat a uniform layer of polymethyl methacrylate (PMMA) (~300 nm) as a cladding for the improvement of coupling efficiencies of the grating couplers (GC). Then, in order to keep a similar coupling efficiency and to eliminate the variation of light coupling, it is important to make an opening (box) by electron beam lithography and transfer the 2D materials over the targeted areas of the devices (Figs. 1(a)-1(d)).
Fig. 1 Optical microscope image of (a) Bare SOI chip with micro ring resonators, (b) EBL patterned opening to transfer 2D materials on top of the ring resonator, (c) & (d) MoS2 flakes have been transferred on targeted device area by using our micro stamper based 2D printer setup.
In order to understand the potential of 2D TMDCs at telecommunication wavelength range, here we study MoS2 integrated Si photonic platform as a function of layer thickness. Few-layer MoS2 flakes are obtained using scotch tape exfoliation technique, whereas the monolayers are triangular flakes grown on Si/SiO2 substrate by CVD process [26,27]. First, we transfer those flakes onto an intermediate PDMS substrate via a KOH assisted wet chemical etching step. Then, the precise transfers of TMDCs are performed by using our developed 2D printer method very efficiently [28]. Briefly, this method comprises of a micro stamper to transfer the material from intermediate PDMS which is transparent and can be aligned under a microscope precisely at any desired device location via micro-positioners. We need to scan over a PDMS (Gel pack) substrate to find a flake of proper dimensions so that it could be transferred at a targeted location without having any cross contamination. Thereafter, the micro-stamper guides a flake to the target location and transfers it onto the substrate provided the effective contact area of the stamper is greater than the flakes area. After the transfer is complete, the devices are tested for their optical transmission and spectral shift again to determine the effect caused by the TMDCs layer as a function of coverage length and thickness on the waveguide and MRR, respectively.
The experimental setup for measuring the hybrid TMDC-Si devices consists of a tunable laser source (Agilent 8164B) and a broadband source (AEDFA-PA-30-B-FA) from where light is injected into the grating coupler optimized for the TM mode propagation in the waveguide. The light output from the MRR is coupled to the output fiber by a similar grating coupler and detected by a detector or an optical spectral analyzer (OSA202). The fundamental modes of the waveguide which integrate the MoS2 layer of different thickness are found from the finite element method (FEM) simulations using the mode analysis tool in Comsol Multiphysics.
3. Results and discussion
In order to realize MoS2 as an active material at telecommunication wavelength (here, 1.55 μm), it is imperative to understand the interaction of monolayer and few layers system on integrated Si photonic devices (Figs. 2(b) and 2(d)). We study the loss and coupling in detail as a function of monolayer coverage and thickness of the MoS2. It is well-known that the optical properties of TMDCs materials are dominated by excitons: bound electron-hole pairs with strong binding energy due to quantum confinement and weak screening of the Coulomb interaction [12]. For any active integrated device structures, one needs to characterize the individual material system systematically such as loss or effective index change at the targeted device operation wavelength range. Here, we investigate the loss impact and impact on the effective mode index of the Si waveguide hundreds of nanometers away from the excitonic transition of MoS2 (A exciton ~1.88 eV & B exciton ~2.06 eV) [13]. We anticipate a minimal loss impact but meaningful index change upon heterogeneous integration. While we are not modulating the TMDC here electrically, the passive impact of the modes index and impact on an MRR provide fundamental insights in the potential of TMDC-Si hybrid devices such as phase modulators [29–31]. We, therefore, study the waveguide bus-to-ring coupling change by shifting the MRR’s phase upon adding MoS2. We measure the corresponding transmission spectra which show a coupling change as compared to bare ring indicating MRR mode index tuning (Figs. 2(c) and 2(f)). The improvement of coupling can be attributed as shifting the MRR from the over coupled regime towards the critically coupled regime by inducing loss which are evident since the quality factor decreases as a function of the flake thickness, suggesting gradual increase of loss mainly due to scattering effect, caused by small impedance mismatch between bare and MoS2 covered sections of the ring.
Fig. 2 MoS2 loaded micro-ring resonator (MRR). (a) Schematic (b) optical microscope image of an MRR (R = 40 μm & W = 500 nm) covered by two monolayer MoS2 flakes with coverage lengths (l1 and l2) precisely transferred using our developed 2D printer technique [25]. (c) Transmission output before and after the transfer of monolayer MoS2 showing improvement of coupling efficiency. (d) & (e) optical microscope and AFM image of a MRR showing a multi-layer MoS2 flake with coverage length 22 μm and thickness of 30 nm, respectively. AFM cross section (inset) (f) Transmission output before and after the transfer of multilayer MoS2 flakes which display a gradual increase of visibility suggesting the improvement of coupling efficiency as it brings the device close to critically coupled regime after the transfer of the TMDCs layer.
Quantitative modeling and analyzing the MRR’s resonance change, the fringe-visibility can be optimized (shifting towards critical coupling) in two ways: either by increasing coverage length or by increasing the thickness of flakes. So, to understand the coupling effect, it is important to extract coupling coefficients, especially round-trip transmission coefficients (a) as a function of coverage. The transmission, T, from an all-pass MRR is given by,
where is the round-trip phase shift, r is the self-coupling coefficient and a is round-trip transmission coefficient related to the power attenuation coefficients by,where l = TMDCs coverage length, R is the radius of MRR, ⍺Si and ⍺TMDC-Si are the linear propagation losses for Si waveguide and the TMDC-transferred portion of the Si waveguide in the ring, respectively. To obtain the propagation loss more quantitatively, we deploy the cutback method and integrate for mono-few layers of MoS2 on linear waveguides of different lengths and measure the relative transmission (Figs. 3(b) and 3(c)). The propagation losses obtained from the linear fit are 0.0009 dB/μm, 0.012 dB/μm (Fig. 3(d)), and 0.037 dB/μm, respectively for Si,monolayer and multi-layer flakes (Table 1). Hence, in order to determine the extra loss induced by MoS2, we subtract the loss of bare Si device from the loss obtained in hybrid structure, resulting in 0.011 dB/µm and 0.036 dB/µm, respectively for monolayer and multilayer flakes.Fig. 3 MoS2 loaded Linear waveguides. (a) optical microscope image of the waveguide with an opening (400 μm x 100 μm) for TMDC transfer to keep all physical parameters unchanged during the optical measurement. (b) & (c) optical microscope image of waveguide covered by a monolayer CVD grown and a multilayer exfoliated flake with coverage length 10 μm and 50 μm, respectively. (d) Optical loss output as a function of monolayer coverage length originates mainly due to the edge scattering effect. The propagation loss (⍺TMDC-Si) for a TMDC-covered portion of the ring is found to be 0.012 dB/μm using cutback measurement. (e) Tunability of round-trip transmission coefficients explains the coupling improvement as a function of coverage length for four different thickness values.
Table 1. Table of Losses associated with 2D materials with heterogeneously integrated Si Platform
The loss can be attributed to a combination of absorption and edge scattering effect of the flakes. Since the popagation wavelength through the waveguide is well below the bandgap of MoS2(1.2-1.8 eV) the losses due to absorption are negligible. Hence, to get more insights, we plot round trip transmission loss for four different thickness values, inserting the propagation loss in Eq. (2) (Fig. 3(e)). The plot displays the variation of a from 0.88 to 0.52 as a function of coverage length for multilayer flakes (~100 nm) and from 0.96 to 0.90 for monolayer flakes, respectively. The result explains the improvement of visibility inducing the transition from over-coupled to towards critically-coupled regime since a = 1 is the zero-loss condition of the ring. Hence, the loss of tunability in MRRs can be manipulated accordingly by controlling the coverage length and thickness. We conclude that for a given device, one can determine the coverage length and thickness by configuring the device at a critically coupled condition for optimized light-matter-interaction [32]. It is evident from the Fig. 3(e) that the loss increases as function of thickness due to an increase of surface roughness. The root-mean-square (rms) roughness of Si waveguide is 1.3 ± 0.6 nm. As a result, the monolayer MoS2 flake on waveguide (rms roughness: 0.8 ± 0.2 nm) have a much rougher surface compared with the one on SiO2 (rms roughness: 0.20 ± 0.07 nm). For the multilayer flakes, the surface roughness of underlying substrate is not playing the significant role since the rms roughness for 40 nm MoS2 flake is 3.5 ± 0.9 and 3.7 ± 1.3 nm on both SiO2 and Si waveguide, respectively.
Si-based MRRs provide a compact and ultra-sensitive platform to find sensitive detection of an unknown analyte for various applications [33,34]. Here, the detection mechanism is mostly based on the change of the refractive index in the top-cladding of the MRR. This change can be sensed by the evanescent tail of the propagating optical mode governed by the effective mode index(neff) and can be translated into resonance shift (). We observe the resonance shift of 1.5 nm upon increase of monolayer coverage length from 10 to 60 μm (Fig. 4(c)) showing monotonic red shift which can be explained as follows: the resonant wavelength () of a MRR is proportional to the effective refractive index of the propagating mode in the circular waveguide [34]. Therefore, the change in effective mode index () after transfer of MoS2 flakes as compared to bare ring is related to a change in resonance shift () by following Eq. (3)
where, is the effective mode index for bare waveguide. The effective index for the bare waveguide can be found from FEM Eigenmode analysis choosing the TM-like mode in correspondence with our TM-grating designs used in measurements.Fig. 4 Mode profile (electric |E|-field), obtained through Eigen mode analysis, for the portion of the ring with MoS2 transferred flakes (a) 20 nm and (b) 50 nm thick (Scale bar 200 nm). Variation of Resonance shift () and effective index change () extracted from (c) as a function of MoS2 coverage length and (d) thickness, which is fully corroborated by a numerical simulation study.
Since, here the MRR is partially covered by MoS2 flakes the effective refractive index of the ring can be formulated as an effective length-fraction index via [32],
where R is the radius of the ring and l is the MoS2 coverage length. The effective index for monolayer covered waveguide is found to be 1.723 for TM mode. The effective index (neff) for flakes with different thickness can be found using FEM Eigenmode analysis. The refractive index of the MoS2 layer was taken from [35] and Si refractive index from [36]. Figures 4(a) and 4(b) represent the normalized in-plane (|Ex|) electric field distribution for TM mode along the device which integrates a layer of MoS2 of 20 and 50 nm thickness, respectively. It is possible to observe that the higher intensity of the electric field in correspondence of the MoS2 layer and the consequently decreased leakages in air and substrate suggest a higher confinement for multi-layer MoS2 (50 nm). Our measurement is designed for TM mode, which means that the electric field component parallel to the side walls (y-direction) is negligible, thus there is no significant interaction nor coupling between the evanescent wave and the flake leaning on the lateral side of the photonic waveguide. Hence, using Eq. (4), we can obtain and hence the resonance shift (), which is showing an unequivocal correlation with our experimental data (Fig. 4(d)). We map out the resonance shift () as a function of MoS2 flake thickness (Fig. 4(d), (i)) upto 50 nm and observe a resonance shift per coverage length of 0.064 nm/μm, which is beyond the resolution limit of our spectrometer (~0.03 nm). The results indicate mono-multilayer MoS2 integrated on Si photonic platform could be an interesting choice for future active modulator device at telecommunication wavelength.4. Conclusion
We have demonstrated the interaction between mono to multi-layers of MoS2 heterogeneously integrated onto a Silicon photonic waveguides and microring cavities. The coupling regime of the ring can be tunable from over coupled regime to under coupled regime. The underlying physical mechanism of tunable coupling can be explained by extracting different coupling and loss coefficients as a function of coverage length and thickness. This study demonstrates a method to determine critical coverage for a given ring resonator which is an important parameter for obtaining maximum extinction ratio for the active modulator. We have mapped out resonance shift as a function of monolayer coverage and thickness of MoS2 flakes which shows a resonance shift of 0.064 nm/μm correlates well with our simulated result. These findings on passive Si photonic platform could be useful tools in future heterogeneous integrated photonic and opto-electronic devices.
Funding
Air Force Office of Scientific Research (AFOSR) (FA9550-17-1-0377); National Science Foundation (NSF) (DMREF 14363300/1455050); (EFRI 2-DARE 1542879); Army Research Office (ARO) (W911NF-16-2-0194).
Acknowledgment
Authors acknowledge Mengqiang Zhao for providing useful informations about the samples.
References
1. A. K. Geim and K. S. Novoselov, “The rise of graphene,” Nat. Mater. 6(3), 183–191 (2007). [CrossRef] [PubMed]
2. S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak, and A. K. Geim, “Giant intrinsic carrier mobilities in graphene and its bilayer,” Phys. Rev. Lett. 100(1), 016602 (2008). [CrossRef] [PubMed]
3. M. Freitag, “Graphene: nanoelectronics goes flat out,” Nat. Nanotechnol. 3(8), 455–457 (2008). [CrossRef] [PubMed]
4. R. Amin, Z. Ma, R. Maiti, S. Khan, J. B. Khurgin, H. Dalir, and V. J. Sorger, “Attojoule-efficient graphene optical modulators,” Appl. Opt. 57(18), D130–D140 (2018). [CrossRef] [PubMed]
5. S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S. S. Hong, J. Huang, A. F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones, W. Windl, and J. E. Goldberger, “Progress, challenges, and opportunities in two-dimensional materials beyond graphene,” ACS Nano 7(4), 2898–2926 (2013). [CrossRef] [PubMed]
6. M. Xu, T. Liang, M. Shi, and H. Chen, “Graphene-like two-dimensional materials,” Chem. Rev. 113(5), 3766–3798 (2013). [CrossRef] [PubMed]
7. Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides,” Nat. Nanotechnol. 7(11), 699–712 (2012). [CrossRef] [PubMed]
8. R. Mas-Ballesté, C. Gómez-Navarro, J. Gómez-Herrero, and F. Zamora, “2D materials: to graphene and beyond,” Nanoscale 3(1), 20–30 (2011). [CrossRef] [PubMed]
9. S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V. Yazyev, and A. Kis, “2D transition metal dichalcogenides,” Nat. Rev. Mater. 2(8), 17033 (2017). [CrossRef]
10. K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically Thin MoS2: A New Direct-Gap Semiconductor,” Phys. Rev. Lett. 105(13), 136805 (2010). [CrossRef] [PubMed]
11. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-Layer MoS2 Transistors,” Nat. Nanotechnol. 6(3), 147–150 (2011). [CrossRef] [PubMed]
12. M. M. Ugeda, A. J. Bradley, S. F. Shi, F. H. da Jornada, Y. Zhang, D. Y. Qiu, W. Ruan, S. K. Mo, Z. Hussain, Z. X. Shen, F. Wang, S. G. Louie, and M. F. Crommie, “Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor,” Nat. Mater. 13(12), 1091–1095 (2014). [CrossRef] [PubMed]
13. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence in monolayer MoS2,” Nano Lett. 10(4), 1271–1275 (2010). [CrossRef] [PubMed]
14. S. Mukherjee, R. Maiti, A. Midya, S. Das, and S. K. Ray, “Tunable direct bandgap optical transitions in MoS2 nanocrystals for photonic devices,” ACS Photonics 2(6), 760–768 (2015). [CrossRef]
15. D. A. Miller, “Attojoule optoelectronics for low-energy information processing and communications,” J. Lightwave Technol. 35(3), 346–396 (2017). [CrossRef]
16. S. Wu, S. Buckley, A. M. Jones, J. S. Ross, N. J. Ghimire, J. Yan, D. G. Mandrus, W. Yao, F. Hatami, J. Vučković, A. Majumdar, and X. Xu, “Control of Two-Dimensional Excitonic Light Emission via Photonic Crystal,” 2D Mater 1(1), 011001 (2014). [CrossRef]
17. X. Gan, Y. Gao, K. Fai Mak, X. Yao, R. J. Shiue, A. van der Zande, M. E. Trusheim, F. Hatami, T. F. Heinz, J. Hone, and D. Englund, “Controlling the spontaneous emission rate of monolayer MoS2 in a photonic crystal nanocavity,” Appl. Phys. Lett. 103(18), 181119 (2013). [CrossRef] [PubMed]
18. T. K. Fryett, K. L. Seyler, J. Zheng, C.-H. Liu, X. Xu, and A. Majumdar, “Silicon Photonic Crystal Cavity Enhanced Second-Harmonic Generation from Monolayer Wse2,” 2D Mater 4(1), 015031 (2016). [CrossRef]
19. G. Wei, T. K. Stanev, D. A. Czaplewski, I. W. Jung, and N. P. Stern, “Silicon-nitride photonic circuits interfaced with monolayer MoS2,” Appl. Phys. Lett. 107(9), 091112 (2015). [CrossRef]
20. R. Amin, R. Maiti, C. Carfano, Z. Ma, M. H. Tahersima, Y. Lilach, D. Ratnayake, H. Dalir, and V. J. Sorger, “0.52 V-mm ITO-based Mach-Zehnder Modulator in Silicon Photonics,” APL Photon. 3(12), 126104 (2018). [CrossRef]
21. Y. Q. Bie, G. Grosso, M. Heuck, M. M. Furchi, Y. Cao, J. Zheng, D. Bunandar, E. Navarro-Moratalla, L. Zhou, D. K. Efetov, T. Taniguchi, K. Watanabe, J. Kong, D. Englund, and P. Jarillo-Herrero, “A MoTe2-based light-emitting diode and photodetector for silicon photonic integrated circuits,” Nat. Nanotechnol. 12(12), 1124–1129 (2017). [CrossRef] [PubMed]
22. N. Youngblood, C. Chen, S. J. Koester, and M. Li, “Waveguide-integrated black phosphorus photodetector with high responsivity and low dark current,” Nat. Photon 9, 247-252 (2015).
23. Y. Ye, Z. J. Wong, X. Lu, X. Ni, H. Zhu, X. Chen, Y. Wang, and X. Zhang, “Monolayer excitonic laser,” Nat. Photonics 9(11), 733–737 (2015). [CrossRef]
24. B. Lee, W. Liu, C. H. Naylor, J. Park, S. C. Malek, J. S. Berger, A. T. C. Johnson, and R. Agarwal, “Electrical Tuning of Exciton-Plasmon Polariton Coupling in Monolayer MoS2 Integrated with Plasmonic Nanoantenna Lattice,” Nano Lett. 17(7), 4541–4547 (2017). [CrossRef] [PubMed]
25. R. A. Hemnani, C. Carfano, J. P. Tischler, M. H. Tahersima, R. Maiti, L. Bartels, R. Agarwal, and V. J. Sorger, “Towards a 2D Printer: A Deterministic Cross Contamination-free Transfer Method for Atomically Layered Materials,” 2D Materials 6(1), 015006 (2018). [CrossRef]
26. D. Kim, D. Sun, W. Lu, Z. Cheng, Y. Zhu, D. Le, T. S. Rahman, and L. Bartels, “Toward the growth of an aligned single-layer MoS2 film,” Langmuir 27(18), 11650–11653 (2011). [CrossRef] [PubMed]
27. G. H. Han, N. J. Kybert, C. H. Naylor, B. S. Lee, J. Ping, J. H. Park, J. Kang, S. Y. Lee, Y. H. Lee, R. Agarwal, and A. T. Johnson, “Seeded growth of highly crystalline molybdenum disulphide monolayers at controlled locations,” Nat. Commun. 6(1), 6128 (2015). [CrossRef] [PubMed]
28. R. Hemnani, “2D material printer video”, https://sorger.seas.gwu.edu/2d-material-printer.html
29. C. T. Phare, Y. H. Lee, J. Cardenas, and M. Lipson, “Graphene electro-optic modulator with 30 GHz bandwidth,” Nat. Photonics 9(8), 511–514 (2015). [CrossRef]
30. I. Datta, S. H. Chae, G. R. Bhatt, B. Li, Y. Yu, L. Cao, J. Hone, and M. Lipson, “Giant electro-refractive modulation of monolayer WS2 embedded in photonic structures,” Conference on Lasers and Electro-Optics, OSA technical Digest (Optical Society of America, 2018), paper Stu4N.7. [CrossRef]
31. Y. Ding, X. Zhu, S. Xiao, H. Hu, L. H. Frandsen, N. A. Mortensen, and K. Yvind, “Effective electro-optical modulation with high extinction ratio by a graphene–silicon microring resonator,” Nano Lett. 15(7), 4393–4400 (2015). [CrossRef] [PubMed]
32. R. Maiti, R. Hemnani, R. Amin, Z. Ma, M. Tahersima, T. A. Empante, H. Dalir, R. Agarwal, L. Bartels, and V. J. Sorger, “Microring Resonators Coupling Tunability by Heterogeneous 2D Material Integration,” arXiv:1807.03945 (2018).
33. I. M. White and X. Fan, “On the performance quantification of resonant refractive index sensors,” Opt. Express 16(2), 1020–1028 (2008). [CrossRef] [PubMed]
34. G. N. Tsigaridas, “A study on refractive index sensors based on optical micro-ring resonators,” Photonic Sens. 7(3), 217–225 (2017). [CrossRef]
35. C. Yim, M. O’Brien, N. McEvoy, S. Winters, I. Mirza, J. G. Lunney, and G. S. Duesberg, “Investigation of the optical properties of MoS2 thin films using spectroscopic ellipsometry,” Appl. Phys. Lett. 104(10), 103114 (2014). [CrossRef]
36. D. T. Pierce and W. E. Spicer, “Electronic structure of amorphous Si from photoemission and optical studies,” Phys. Rev. B 5(8), 3017–3029 (1972). [CrossRef]
