Dynamic photochemical and optoelectronic control of photonic Fano

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Dynamic photochemical and optoelectronic control of photonic Fano resonances via monolayer MoS trions 2

Xingwang Zhang, Nicolas Biekert, Shinhyuk Choi, Carl H. Naylor, Chawina De-Eknamkul, Wenzhuo Huang, Xiaojie Zhang, Xiaorui Zheng, Dake Wang, Alan T. Charlie Johnson, and Ertugrul Cubukcu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04355 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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Dynamic photochemical and optoelectronic control of photonic Fano resonances via monolayer MoS2 trions Xingwang Zhang,§,

#

Nicolas Biekert,

†, #

Shinhyuk Choi,

†

Carl H. Naylor,

ǁ

Chawina De-

Eknamkul, § Wenzhuo Huang, † Xiaojie Zhang, † Xiaorui Zheng, § Dake Wang, §, ‡ A. T. Charlie Johnson, ǁ,⊥ and Ertugrul Cubukcu §, †, * §

Department of Nanoengineering,

†

Department of Electrical and Computer Engineering,

University of California, San Diego, La Jolla, California 92093, United States ǁ

Department of Physics and Astronomy, ⊥ Department of Materials Science and Engineering,

University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ‡

Physics Department, Furman University, Greenville, SC 29613, USA

KEYWORDS: Fano resonance, photonic crystal, 2D materials, dynamic control, trions, excitons

ACS Paragon Plus Environment

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ABSTRACT: Active tunability of photonic resonances is of great interest for various applications such as optical switching and modulation based on optoelectronic materials. Manipulation of charged excitons in atomically thin transition metal dichalcogenides (TMDC) like monolayer MoS2 offer an unexplored route for diverse functionalities in optoelectronic nanodevices. Here, we experimentally demonstrate the dynamic photochemical and optoelectronic control of the photonic crystal Fano resonances by optical and electrical tuning of monolayer MoS2 refractive index via trions without any chemical treatment. The strong spatial and spectral overlap between the photonic Fano mode and the active MoS2 monolayer enables efficient modulation of the Fano resonance. Our approach offers new directions for potential applications in the development of optical modulators based on emerging 2D direct bandgap semiconductors.


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Monolayers of transition metal dichalcogenides (TMDC) such as molybdenum disulphide (MoS2) are atomically thin direct band gap semiconductors.1, 2 In addition to electronic devices such as field-effect transistors (FET),3 the direct band gap property enables a diverse range of optoelectronic devices such as, photodetectors,4 solar cells,5 and light-emitting devices.6 In addition, the optical and electrical properties of TMDC monolayers can be controlled through strain tuning7 and doping,8, 9 which have wide applications in 2D tunable active optoelectronic devices.10, 11 Among them, the doping of TMDC monolayers via charge injection is of great interest.10, 12 Due to the enhanced Coulomb interactions between charge carriers and reduced dielectric screening in TMDC monolayers, the n-type doping (i.e. excess electrons) of TMDC monolayers can generate negatively charged trions, quasiparticles composed of two electrons and one hole.13-16 More interestingly, the formation of trions is accompanied by the suppression of excitons, such that both the photoluminescence (PL) and absorption of TMDC monolayers can be consequently modulated through the conversion between excitons and trions.17-19 Here, by utilizing MoS2 trions we experimentally demonstrate that Fano resonances of a dielectric photonic crystal (PhC) can be dynamically modulated via photochemical and electrical manipulation of the refractive index of a monolayer directly integrated on top of the PhC. As shown in Fig. 1a and 1b, the PhC consists of a 2D square lattice of holes etched through a 100 nm thick freestanding silicon nitride (SiN) membrane (see Methods for the device fabrication). A top metal (Cr/Au: 5/30 nm) electrode and a transparent indium tin oxide (ITO) back gate are incorporated into the MoS2 metal-insulator-semiconductor (MIS) capacitor structure for electrically controlling the monolayer excitons (Fig. 1c). The use of an ITO back gate instead of a metal one is critical to minimize the optical losses. Furthermore, this configuration allows for photochemical tuning of Fano resonances via illumination with a 532 nm laser, which generates


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trions and suppresses excitons via modifications of the surface adsorbents (see Methods for the experiment setup). In MoS2 monolayers, excitonic effects are responsible for the A exciton absorption near 650 nm wavelength. By injecting electrons into the MoS2 monolayer via optical or electrical doping, we can increase the density of negatively charged excitons, i.e. A- trions, and suppress A excitons. This leads to a relatively large concurrent refractive index change. This ability to dynamically manipulate the refractive index is crucial for actively tunable photonic devices such as optical modulators based on the modulation of optical resonances in dielectric micro/nanostructures. To this end, the spatially extended “cavityless” 2D nature of the PhC Fano resonances arising from the hybridization (Fig. S1) of guided in-plane PhC modes and out-of-plane Fabry–Pérot resonances of a dielectric slab offer strong spatial overlap with a MoS2 monolayer.20,

21

Furthermore, the spectral overlap and electromagnetic coupling between the Fano resonances and monolayer excitons are highly tunable by adjusting the lattice constant (Fig. 1d).20 Due to this coupling in the hybrid PhC-MoS2 photonic structure,22-29 we can dynamically control the Fano resonance by both photochemical and electrical tuning of the MoS2 monolayer refractive index. Consequently, the transmittance of the PhC Fano resonance can be modulated with an atomically thin MoS2 monolayer. In order to get an insight on the photochemical control of the PhC Fano resonances with the monolayer MoS2, we first investigate the underlying mechanism for the optical manipulation of the exciton-trion conversion in the monolayer MoS2. Under ambient conditions, both O2 and H2O molecules are inherently adsorbed on the surface of the MoS2 monolayer by trapping


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electrons.17,

30-33

When the monolayer MoS2 is illuminated with a 532 nm laser, the

photogenerated holes recombine with the negatively charged O2 and H2O molecules and release the trapped electrons, leading to the increased n-type doping of the monolayer MoS2 (Fig. 2a).31 Consequently, this increase in the overall electron concentration in the monolayer leads to a larger concentration of trions and a slight suppression of excitons as it becomes more likely for a free electron to bind to an exciton and form a trion. The optically generated trions can manifest themselves in the pump power dependent PL spectrum of the suspended MoS2 monolayer. We use a suspended MoS2 monolayer to study the exciton-trion conversion as this allows us to rule out the substrate induced effects. As can be seen in Fig. 2b, the PL spectra of the suspended MoS2 monolayer can be deconvoluted with the contributions from the A exciton and the A- trion PL emission. While the overall PL intensity increases as the pump intensity goes up, the ratio between the A- trion and the A exciton contributions also increases indicating the suppression of A excitons due to optical n-type doping (see Supporting Information for the details on the excitation laser power dependent exciton-trion conversion). It should be noted that the PL spectrum of the monolayer MoS2 does not show a measurable feature that can be attributed to the B exciton, which allows us to study the photochemical and optoelectronic manipulation of B excitons. On the contrary, the A exciton emission is prominent. Therefore, we only focus on the conversion between A exciton and A- trion in this work. The photogeneration of trions can be further corroborated by the time dependent PL spectrum of the suspended MoS2 monolayer. As shown in Fig. 2c, the emission intensity ratio between the A- trion and the A exciton depends on the illumination duration at constant laser intensity. The trion-exciton PL ratio increases when the illumination duration is increased from 2 s to 300 s indicating trion accumulation with excitation time. Due to the suppression of more luminescent


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A excitons and the build-up of A- trions, the overall PL intensity decays as a function of the laser illumination time (Fig. 2d). Meanwhile, the measured time dependence of the overall PL intensity under constant illumination intensity can be fit with a bi-exponential dependence, which indicates that there are two electron trapping mechanisms involved. Although the detailed mechanism is not clearly understood at the moment, these fast and slow PL decay processes have previously been attributed to the adsorbed O2 and H2O molecules, respectively (see Supporting Information for the details on the mechanism of photochemical doping of monolayer MoS2).30, 32 In addition, to further verify the role of the adsorbed O2 and H2O molecules as trapping centers, we respectively immobilize (3-Aminopropyl)triethoxysilane (APTES) or deposit Al2O3 on monolayer MoS2, which are expected to passivate the adsorbed O2 and H2O molecules. Compared with the uncapped monolayer MoS2, the PL spectrum for the capped MoS2 is almost independent on the laser excitation time indicating no photoinduced exciton-trion conversion. Therefore, we conclude that the photochemical doping of monolayer MoS2 is caused by the photoinduced charge transfer between MoS2 and the adsorbed O2/H2O molecules (see Supporting Information for the details on the mechanism of the photochemical doping of monolayer MoS2). It should be noted that the laser power density we used is below 200 µW/ µm2, which is low enough to avoid the photothermal effect34 (see Supporting Information for the details on the the excitation laser power dependent exciton-trion conversion). To study the effect of optical excitation on the dielectric constants of the MoS2 monolayer, the transmittance spectra of the MoS2 monolayer on a SiN slab (thickness: h=100 nm) is measured as a function of the optical pump intensity as shown in Fig. 2e and Fig. S6. The A exciton strength is visibly weakened by the optical excitation while the A- trion strength is enhanced, which further confirms that the n-type doping is induced by the optical excitation. By using a


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Kramers-Kronig constrained variational analysis, the refractive index of the MoS2 monolayer can be extracted from the transmittance spectra in Fig. 2e (see Methods for the calculation of refractive index).35, 36 Figure 2f shows that the complex refractive index of the MoS2 monolayer can be tuned significantly via optical pumping, establishing the feasibility of photochemical control of photonic resonances by manipulating MoS2 trions. In order to demonstrate this for PhC Fano resonances with the integrated MoS2 monolayer, the device is illuminated with a 532 nm laser for 3 min at an intensity of 120 µW/ µm2. This duration is confirmed to be long enough for the saturation of exciton to trion conversion limited by the initial concentration of adsorbed molecules (Fig. 2d). As shown in Fig. 3a, the Fano resonance blueshifted from the initial peak position when the transmittance is measured immediately after the end of the 3 min illumination period. In order to study the pump power dependence of the Fano resonance, we have systematically measured the differential transmittance spectrum, that is the calculated by subtracting the initial spectrum without illumination from the power dependent spectrum. Figure 3b shows the differential transmittance spectrum (∆T) normalized to the initial spectrum (T0) at different illumination intensities, which reveals that the change of the PhC Fano resonance increases with the optical pump intensity (Fig. 3b). In contrast, no resonance change is observed in the PhC slab without the active MoS2 monolayer at the same optical pump intensities indicating that the observed spectral changes originate from the refractive index tuning of the MoS2 monolayer and not SiN and ITO layers (Fig. S13a). In addition, as can be seen in Fig. 3b, the normalized transmission modulation depth for the PhC Fano resonance with the active MoS2 monolayer is an order of magnitude higher than that of the MoS2 on unpatterned SiN slab (Fig. S6), which is attributed to the strong interaction of MoS2 with the photonic resonance.


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We have also investigated the time dependence of the differential transmittance at a constant intensity of 120 µW/ µm2. In Figure 3c, we plot the spectrally integrated values (Fig. S9) of the normalized differential transmittance for the device at various times without laser illumination and after the end of a 3 min laser illumination cycle at 532 nm. Without the optical pump, the Fano resonance remains unchanged. Shortly after the pump laser is blocked, a large change of the PhC Fano resonance is observed as a result of the refractive index change of the MoS2 monolayer. When the pump laser is blocked for 4 min, the PhC Fano resonance is fully recovered, indicating the optically generated trions have completely turned back to excitons (Fig. 3d). Therefore, the dynamic photochemical control of photonic Fano resonances via monolayer MoS2 trions is reversible. In addition, the time dependent change of the PhC Fano resonance in Fig. 3c reveals two time constants, which are consistent with the material properties of the MoS2 monolayer shown in Fig. 2d (Fig. S10). It should be noted that the photochemical tuning is determined by the photo-induced charge transfer between MoS2 and O2/H2O molecules, which limits the tuning speed on the order of few seconds. Alternatively, the PhC Fano resonances can be electrically controlled via generation of trions in the integrated MoS2 monolayer. To achieve this, we first explore the electrical manipulation of trions in the MoS2 monolayer. The negatively charged trions can be generated by electrical injection of excess electrons in the MoS2 monolayer by applying a positive gate voltage (Vg>0) on the MIS capacitor structure as shown in Fig. 1a. This results in the accumulation of A- trions and suppression of A excitons induced by the electrical n-type doping as evidenced by the gate voltage dependent photoluminescence of the MoS2 monolayer (Fig. S11). With the positive gate voltage applied, the electron injection induces a decrease in the A exciton emission and increase in the A- trion emission, leading to the weakened PL intensity and red shift of PL spectrum.


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Similar to its optical counterpart, the electrically enabled exciton-trion conversion in the MoS2 monolayer can also result in a concurrent refractive index change. To characterize the electrical tuning of the refractive index of the MoS2 monolayer, we measure the differential transmittance spectra of the MoS2 monolayer with different gate voltages applied on the MIS capacitor structure (Fig. S12). The transmittance around 650 nm increases with the increasing gate voltage, which indicates the suppression of A excitons. In contrast, the transmittance around 670 nm decreases with increasing gate voltage, indicating the accumulation of A- trions. Therefore, the A excitons are converted into A- trions by the electrical n-type doping. Finally, the voltage dependent refractive index of the MoS2 monolayer can be extracted from the measured transmittance spectra by the Kramers-Kronig constrained variational analysis (see Methods for the calculation of refractive index). As shown in Fig. 4a, a significant change in the MoS2 refractive index can be achieved with the applied gate voltage. In order to optoelectronically tune the PhC Fano resonance with the trions in the integrated MoS2 monolayer, we apply a positive gate voltage. A large tuning of the Fano resonance peak is observed at a gate voltage of 30V (Fig. 4b) while the peak differential transmittance increases with the increasing the gate voltage. (Fig. 4c). As no resonance change is observed in the PhC slab without the MoS2 monolayer when the same voltage is applied (Fig. S13b), the resonance changes observed in Fig. 4b and 4c are attributed to the gate voltage induced refractive index changes in the MoS2 monolayer. Due to the strong interaction of MoS2 with the photonic Fano mode, the transmission spectrum is efficiently modulated with the active MoS2 monolayer, which is an order of magnitude higher than that of the MoS2 on unpatterned SiN slab (Fig. S12). We have also investigated the time dependence of optoelectronic tuning by monitoring in real time the peak differential transmittance around the Fano resonance. As can been seen in Fig. 4d,


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the transmittance can be modulated by the applied gate voltage in real time and fully recovers when the gate voltage is turned off (Fig. 4d). On the other hand, the measured optoelectronic tuning speed of PhC Fano resonances is limited by the time resolution of our measurement method based on the integration time of the measured spectrum (~0.8 s, see Supporting Information for the details on the PhC Fano resonance tuning speed analysis). Faster tuning speed is expected by further improving the data acquisition method. Since the Fano resonances can be geometrically tuned, the approach presented here can be utilized for a broader range of wavelengths. We note that this further benefits from the fact that the optical constants of monolayer MoS2 are highly tunable in the spectral range from 580 nm to 750 nm (Figs. 5a and 5b). To this end, with the wavelength tunable PhC-MoS2 hybrid system, the photochemical and optoelectronic modulation of peak differential transmittance is expected to be realized in the spectral range from 580 nm to 750 nm. With this in mind, we fabricate four different PhC slabs with the Fano resonances ranging from 640 nm to 690 nm, each of which is integrated with monolayer MoS2 as the active material for the resonance tuning. As expected, transmittance changes are observed for all of these resonant structures around their own photonic resonances, which is consistent with the simulation results (Fig. S14). As can be seen in Figs. 5a and 5b, both the real and imaginary parts of the complex refractive indices of monolayer MoS2 can be tuned up to ±0.2 by both the photochemical and optoelectronic doping method. For the photochemical doping method, magnitude of the refractive index tuning is limited by the initial density of the surface adsorbed O2/H2O molecules. Therefore, by increasing the adsorbent density on the monolayer MoS2, the transmittance modulation depth for the PhC-MoS2 hybrid resonators can be further increased. On the other hand, for the optoelectronic doping method, the


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magnitude of refractive index tuning can be increased by applying a higher gate voltage, which can in turn increase the transmittance modulation depth. In conclusion, we demonstrate that the PhC Fano resonances can be dynamically controlled by both the optical and electrical manipulation of the exciton-trion conversion in the integrated MoS2 monolayer without any additional chemical treatment. The exciton-trion conversion can lead to significant changes in the refractive index of the MoS2 monolayer, which can efficiently tune the PhC Fano resonances through the strong spatial and spectral overlap with the active MoS2 monolayer. We envision that this universal approach based on a monolayer semiconductor will have potential applications in future nano-enabled optical switches and modulators.


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Methods. Device fabrication. The photonic crystal was fabricated on a commercial silicon nitride (SiN) membrane. First a 30 nm thin layer of indium tin oxide (ITO) layer was deposited on the backside of the SiN membrane via RF sputtering. Then, a 30 nm thick gold layer was deposited following a 5 nm thick chromium adhesion layer to form the electrodes on the SiN membrane through a series of electron beam lithography (EBL), e-beam evaporation and lift-off processes. The photonic crystal was patterned on the membrane by EBL and reactive-ion-etching (RIE) processes. Finally, CVD grown MoS2 monolayer flakes were transferred onto the SiN membrane. Experimental setup. A collimated broadband white light source illuminated the sample from the top. The transmitted light was collected by a 40X (NA=0.6) objective and the transmittance spectra was analyzed by a spectrometer equipped with a cooled charge-coupled device (CCD) camera. To measure the PL spectra and characterize the dynamic optical control of the device, the sample was pumped by a continuous wave (CW) 532 nm laser. Calculation of refractive index. From the transmittance spectra, we determine the refractive index using Kramers-Kronig constrained variational analysis,35 using the Fresnel equations for normally incident light on a layered thin film system as a constraint.36 We fit oscillators to the transmittance spectra in order to obtain a complex dielectric function or refractive index in the form of:

= 1 +

− −


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Where N, and are the total number of oscillators, oscillator width and strength, respectively. The functional form of the oscillator automatically satisfies the Kramers-Kronig relation.


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ASSOCIATED CONTENT Supporting Information. The Fano resonance in the photonic crystal slab. The mechanism for photochemical doping of monolayer MoS2. PL spectrum normalization procedure. The excitation laser power dependent exciton-trion conversion. The differential transmission spectrum for the photochemical doped MoS2. The effect of the substrate on the photochemical doping of monolayer MoS2. Verification of the refractive index calculation method. The definition of the spectral integrated differential transmittance. The time constants of the optical dynamic control of the PhC Fano resonance with MoS2 monolayer. The PL spectrum for the optoelectronic doped MoS2. The differential transmission spectrum for the optoelectronic doped MoS2. The control experiments with the PhC without MoS2. The simulation results for the photochemical and optoelectronic control of photonic Fano resonances via monolayer MoS2 trions. Electrostatically deformed monolayer MoS2. PhC Fano resonances tuning speed analysis. Photochemical doping of the monolayer MoS2 with different excitation lasers. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions #

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the National Science Foundation (NSF) under the NSF 2-DARE Program (EFMA-1542879) and partially under ECCS-1632797. REFERENCES 1. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Phys. Rev. Lett. 2010, 105, (13), 136805. 2. Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C. Y.; Galli, G.; Wang, F. Nano Lett. 2010, 10, (4), 1271-5. 3. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nat. Nanotechnol. 2011, 6, (3), 147-50. 4. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Nat. Nanotechnol. 2013, 8, (7), 497-501. 5. Pospischil, A.; Furchi, M. M.; Mueller, T. Nat. Nanotechnol. 2014, 9, (4), 257-61. 6. Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; Cobden, D. H.; Xu, X. Nat. Nanotechnol. 2014, 9, (4), 268-72. 7. He, K.; Poole, C.; Mak, K. F.; Shan, J. Nano Lett. 2013, 13, (6), 2931-6. 8. Mouri, S.; Miyauchi, Y.; Matsuda, K. Nano Lett. 2013, 13, (12), 5944-8. 9. Li, Z.; Xiao, Y.; Gong, Y.; Wang, Z.; Kang, Y.; Zu, S.; Ajayan, P. M.; Nordlander, P.; Fang, Z. ACS Nano 2015, 9, (10), 10158-64. 10. Yang, S.; Yue, Q.; Cai, H.; Wu, K.; Jiang, C.; Tongay, S. J. Mater. Chem. C 2016, 4, (2), 248-253. 11. Yang, S.; Wang, C.; Sahin, H.; Chen, H.; Li, Y.; Li, S. S.; Suslu, A.; Peeters, F. M.; Liu, Q.; Li, J.; Tongay, S. Nano Lett. 2015, 15, (3), 1660-6. 12. Sidler, M.; Back, P.; Cotlet, O.; Srivastava, A.; Fink, T.; Kroner, M.; Demler, E.; Imamoglu, A. Nat. Phys. 2016, 13, (3), 255-261. 13. Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Nat. Mater. 2013, 12, (3), 207-11. 14. Ross, J. S.; Wu, S.; Yu, H.; Ghimire, N. J.; Jones, A. M.; Aivazian, G.; Yan, J.; Mandrus, D. G.; Xiao, D.; Yao, W.; Xu, X. Nat. Commun. 2013, 4, 1474. 15. Lui, C. H.; Frenzel, A. J.; Pilon, D. V.; Lee, Y. H.; Ling, X.; Akselrod, G. M.; Kong, J.; Gedik, N. Phys. Rev. Lett. 2014, 113, (16), 166801. 16. Mai, C.; Barrette, A.; Yu, Y.; Semenov, Y. G.; Kim, K. W.; Cao, L.; Gundogdu, K. Nano Lett. 2014, 14, (1), 202-6. 17. Tongay, S.; Zhou, J.; Ataca, C.; Liu, J.; Kang, J. S.; Matthews, T. S.; You, L.; Li, J.; Grossman, J. C.; Wu, J. Nano Lett. 2013, 13, (6), 2831-6. 18. Nan, H. Y.; Wang, Z. L.; Wang, W. H.; Liang, Z.; Lu, Y.; Chen, Q.; He, D. W.; Tan, P. H.; Miao, F.; Wang, X. R.; Wang, J. L.; Ni, Z. H. ACS Nano 2014, 8, (6), 5738-5745. 19. Lee, B.; Liu, W.; Naylor, C. H.; Park, J.; Malek, S. C.; Berger, J. S.; Johnson, A. T. C.; Agarwal, R. Nano Lett. 2017, 17, (7), 4541-4547. 20. Fan, S.; Joannopoulos, J. D. Phys. Rev. B 2002, 65, (23), 235112. 21. Crozier, K. B.; Lousse, V.; Kilic, O.; Kim, S.; Fan, S.; Solgaard, O. Phys. Rev. B 2006, 73, (11), 115126.


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22. Liu, X.; Galfsky, T.; Sun, Z.; Xia, F.; Lin, E.-c.; Lee, Y.-H.; Kéna-Cohen, S.; Menon, V. M. Nat. Photon. 2014, 9, (1), 30-34. 23. Sobhani, A.; Lauchner, A.; Najmaei, S.; Ayala-Orozco, C.; Wen, F.; Lou, J.; Halas, N. J. Appl. Phys. Lett. 2014, 104, (3), 031112. 24. Reed, J. C.; Zhu, A. Y.; Zhu, H.; Yi, F.; Cubukcu, E. Nano Lett. 2015, 15, (3), 1967-71. 25. Ye, Y.; Wong, Z. J.; Lu, X.; Ni, X.; Zhu, H.; Chen, X.; Wang, Y.; Zhang, X. Nat. Photon. 2015, 9, (11), 733-737. 26. Yi, F.; Ren, M.; Reed, J. C.; Zhu, H.; Hou, J.; Naylor, C. H.; Johnson, A. T.; Agarwal, R.; Cubukcu, E. Nano Lett. 2016, 16, (3), 1631-6. 27. Akselrod, G. M.; Ming, T.; Argyropoulos, C.; Hoang, T. B.; Lin, Y.; Ling, X.; Smith, D. R.; Kong, J.; Mikkelsen, M. H. Nano Lett. 2015, 15, (5), 3578-84. 28. Butun, S.; Tongay, S.; Aydin, K. Nano Lett. 2015, 15, (4), 2700-4. 29. Khurgin, J. B. Optica 2015, 2, (8), 740. 30. Late, D. J.; Liu, B.; Matte, H. S.; Dravid, V. P.; Rao, C. N. ACS Nano 2012, 6, (6), 563541. 31. Pak, J.; Min, M.; Cho, K.; Lien, D. H.; Ahn, G. H.; Jang, J.; Yoo, D.; Chung, S.; Javey, A.; Lee, T. Appl. Phys. Lett. 2016, 109, (18), 183502. 32. Illarionov, Y. Y.; Rzepa, G.; Waltl, M.; Knobloch, T.; Grill, A.; Furchi, M. M.; Mueller, T.; Grasser, T. 2D Mater. 2016, 3, (3), 035004. 33. Gao, J.; Li, B.; Tan, J.; Chow, P.; Lu, T. M.; Koratkar, N. ACS Nano 2016, 10, (2), 262835. 34. Reed, J. C.; Malek, S. C.; Yi, F.; Naylor, C. H.; Charlie Johnson, A. T.; Cubukcu, E. Appl. Phys. Lett. 2016, 109, (19), 193109. 35. Kuzmenko, A. B. Rev. Sci. Instrum. 2005, 76, (8), 083108. 36. Yeh, P., Optical Waves in Layered Media. Wiley & Sons: 2005.


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FIGURES

Figure 1. (a) Schematic of the device. The device consists of a free-standing 100 nm thick SiN membrane with a 30 nm Indium Tin Oxide (ITO) transparent gate electrode on the back side and metal (Au=30 nm, Cr=5 nm) electrodes on the top side. A photonic crystal (PhC) pattern consisting of a periodic square array of air holes is etched into the SiN membrane. The active MoS2 monolayer is transferred on top of the device. The transmission of the hybrid device can be dynamically controlled either all optically by illumination with a 532 nm laser or optoelectronically by applying a gate voltage (Vg) across the Au and ITO electrodes. (b) SEM image (top view) of a representative PhC slab consisting of air holes. (c) Optical micrograph of the device. The boundaries of the PhC slab and the integrated with the MoS2 monolayer are outlined with a black dashed square and blue dotted line. The regions of MoS2 monolayer running over the Au electrode (yellow region) provide the electrical connection. (d) Measured


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transmittance spectra for various PhC slabs with different lattice constants (Λ). The hole diameters (D=220 nm) are the same for all four cases. The large dips in the spectra denote the Fano resonances, which can be tuned by varying the geometric parameters of the PhC.


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Figure 2. Photochemical dynamic control of monolayer MoS2 trions. (a) Schematic of charge transfer from the adsorbed H2O and O2 molecules to the MoS2 monolayer due to 532 nm laser illumination. (b) Normalized measured photoluminescence (PL) spectra for a suspended MoS2 monolayer at different laser intensities (dashed lines). We normalize each PL spectrum with respect to its own PL peak at 658 nm (i.e. the peak wavelength of the A exciton). Individual PL spectra are deconvolved into A exciton and A- trion contributions (solid lines). The A exciton to the A- trion emission intensity ratio decreases with increasing laser intensity (see Supporting Information for the details on the PL spectrum normalization procedure). (c) Normalized PL spectra at constant laser intensity of 105 µW/ µm2 measured 2 s (red curve) and 300 s (blue curve) after the illumination was turned on. Individual PL spectra are deconvolved into A exciton and A- trion contributions (solid lines). The A exciton to the A- trion emission intensity ratio decreases with increasing illumination time. (d) Illumination time dependence of the PL intensity for the suspended MoS2 monolayer at constant optical pump intensity (105 µW/ µm2). The measured PL intensity decay (blue circles) is fit with a bi-exponential function (red curve)


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revealing two time constants of τ1=3 s and τ2=123 s. (e) Measured transmittance spectra of the MoS2 monolayer on an unpatterned region of the SiN slab before (red curve) and after 3 min laser illumination (blue curve). With the laser illumination, the A exciton peak is weakened while the A- trion is enhanced. (f) Laser illumination induced changes in the real (n) and imaginary (κ) parts of the monolayer refractive index extracted from (e).


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Figure 3. Photochemical dynamic tuning of PhC Fano resonances via A- trions. (a) Measured transmittance spectra for the PhC slab with the integrated MoS2 monolayer before (red curve) and after 3 min laser illumination (blue curve). Inset: Transmittance spectra in a broader spectral window. (b) Laser intensity dependence of the normalized differential transmittance (∆T/T0) for the same 3 min illumination intervals. ∆T refers to the difference between the transmittance spectrum acquired before the beginning (T0) and immediately after the end of the illumination interval. The changes in ∆T/T0 near the PhC Fano resonances increases with increasing laser intensity. (c) Time dependence of the recovery of the PhC Fano resonance after the laser illumination was turned off. The integrated ∆T/T0 intensity (see Fig. S3) remains constant (blue region) before the beginning of the 3 min laser illumination (120 µW/ µm2) interval. When the


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laser was turned off, the intensity decay follows a bi-exponential dependence with time constants τ1=6.2 s and τ2=91.2 s. (d) ∆T/T0 for the PhC slab corresponding to different times in (c). ∆T/T0 change around the PhC Fano resonance reverses shortly after the illumination laser is blocked and fully recovers 240 s. The curves in (b) and (d) are vertically shifted for clarity.


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Figure 4. Optoelectronic dynamic tuning of PhC Fano resonances. (a) Gate voltage (Vg) dependence of the real (n) and imaginary (κ) parts of the monolayer refractive index. A larger Vg corresponds to a larger n-type doping (i.e. electron injection) and A- trion density. (b) Measured gate voltage dependence of the transmittance spectra for PhC slab with the integrated MoS2 monolayer The PhC Fano resonance corresponding to the dip in the transmittance spectra blueshifts when Vg changes from 0 V to 30 V. Owing to the electromechanical deformation for the suspended monolayer MoS2, additional refractive index changes can be induced by the electrostatic force. Therefore, the observed transmission change is larger than the theoretically expected value (see Supporting Information for the details on the electrostatically deformed monolayer MoS2 and the theoretical expected transmission changes). (c) The corresponding


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normalized differential transmittance (∆T/T0) modulation at different applied gate voltages. ∆T refers to the difference between the transmittance spectrum acquired with (T1) and without gate voltage applied (T0). The curves are vertically shifted for clarity. (d) The dynamic modulation of the peak value of ∆T/T0 when Vg increases from 0 V (OFF) to 10 V (ON) at a ramp up rate of 2 V/s. ∆T/T0 recovers back to the OFF state when Vg is ramped back down to 0 V.


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Figure 5. Fano resonance detuning within the A- trion peak. The optoelectronic (a) and the photochemical (b) tuning of refractive indices of the monolayer MoS2. The optoelectronic (c) and photochemical (d) control of ∆T/T0 for PhC samples for various periods (Λ) with correspondingly different Fano resonances between 620-700 nm. For the optoelectronic doping case, a gate voltage of 30V is used, while for the photochemical doping case, a laser intensity of 100 µW/µm2 is used. The hole diameters are 220 nm for all 4 samples. Similar levels of intensity modulations can be achieved for all samples over a broad spectral range, where the monolayer refractive index can be modulated. The curves are vertically shifted for clarity.


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Table of contents graphic


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Dynamic photochemical and optoelectronic control of photonic Fano

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