Polarization-Controllable Plasmonic Enhancement on the Optical

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Surfaces, Interfaces, and Applications

Polarization controllable plasmonic-enhancement on the optical response of 2D GaSe layers Wei Wan, Jun Yin, Yaping Wu, Xuanli Zheng, Weihuang Yang, Hao Wang, Jiangpeng Zhou, Jiajun Chen, Zhiming Wu, Xu Li, and Junyong Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03880 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Materials & Interfaces

Polarization controllable plasmonic-enhancement on the optical response of 2D GaSe layers Wei Wan‡1, Jun Yin‡2, Yaping Wu1*, Xuanli Zheng1*, Weihuang Yang3, Hao Wang1, Jiangpeng Zhou1, Jiajun Chen1, Zhiming Wu1, Xu Li1 and Junyong Kang1*

1Department

of Physics, OSED, Fujian Provincial Key Laboratory of Semiconductor

Materials and Applications, Jiujiang Research Institute, Xiamen University, Xiamen, 361005, China 2Pen-Tung

Sah Institute of Micro-Nano Science and Technology, Xiamen University,

Xiamen, 361005, China 3Key

Laboratory of RF Circuits and System of Ministry of Education, Hangzhou Dianzi

University, Hangzhou, 310018, P. R. China.

KEYWORDS: GaSe, plasmonic, SERS, photoluminescence, gratings, polarization control

1

ACS Paragon Plus Environment

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ABSTRACT: Resonant plasmonic coupling has been considered as a promising strategy to enhance the optical response and manipulate the polarization of twodimensional (2D) layer materials while towards the practical applications. Here, a hybrid structure with periodic Ag nanoprisms arrays were designed and fabricated on 2D GaSe layers to enhance these optical properties. By using the optimized hybrid structure with well-matched resonance, significant enhanced Raman scattering and band edge emission were successfully realized, and which is also interestingly found that the higher enhancement would be achieved while decreasing the thickness of GaSe layers. Theoretical simulation indicated that the strongly enhanced local-field and the modified charge densities are the main reasons. By further introducing the patterned gratings on the plasmonic hybrid structure, selective excitation with controllable polarization was readily realized, besides of the strongly enhanced photoluminescence intensity. This work provides a strategy for the plasmonic engineering of polarization controllable 2D optoelectronic devices.

1. INTRODUCTION

2


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Two-dimensional (2D) materials possess appreciable controllability due to the reduced symmetry, which can be a promising system for future optoelectronics, nanoelectronics, and energy harvesting. Recently, a new kind of layered semiconductor materials, 2D Group-IIIA metal-monochalcogenides MX (M= Ga, In; X = S, Se, Te), have attracted considerable attentions with the unique electronic and optical properties 1-8.

However, their weak light absorption and low quantum efficiency inevitably restricted

their efficient applications in photonic devices 9. Specifically, as the thickness of the 2D MX materials is decreased, their optical response, such as Raman scattering and photoluminescence (PL) emission are both reduced radically that even difficult to detect 10, 11.

Therefore, efficient enhancement and flexible control of the optical response of the

2D MX materials are considerable important to satisfy the requirements of practical applications, and which has never been reported so far. Integrating plasmonic metallic nanomaterials to form a hybrid structure that supports the plasmon resonance and coupling effect has been proven to be a promising strategy to enhance the optical response of 2D materials

12-15.

Through the surface

plasmonic response (SPR) of the metallic nanomaterials, the incident light can be 3



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efficiently coupled into the 2D materials, and thus would induce a strong spectroscopy enhancement phenomenon, such as surface-enhanced Raman scattering (SERS) and photoluminescence, etc. 16-19. Since the SPR effect is dependent intensively on both the morphology of individual metallic nanostructure and the arrangement of the array, designing and optimizing the architecture of plasmonic nanostructure will play an important role in maximizing signal enhancement. Moreover, asymmetric plasmonic nanostructures generally exhibit different spatial distribution of optical near field enhancements under different polarization incident lights

20-22.

Special nanostructure

arrays with broken symmetry will give rise to the anisotropic characteristics to the 2D materials, which will endow them with great prospect in polarization-sensitive applications, such as polarized photodetectors, biosensors and nanoantenna 20-22. In this work, we designed a periodic array of Ag metallic nanostructures on typical 2D MX material, GaSe layers, as our plasmonic hybrid structure. The structural parameters were optimized by performing the finite difference time domain (FDTD) simulations. Optimized hybrid structure was then fabricated by nanosphere lithography. Thickness dependent SERS effect and enhanced band edge photoluminescence 4


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emission were realized. Electric filed distribution as well as deformation charge densities were calculated to study the enhancement mechanism. A patterned grating based on the plasmonic hybrid structure was further designed and fabricated to achieve the polarization controllable selective excitation in the photoluminescence response, which shows a well manipulation of the anisotropic optical performance.

2. THEORETICAL AND EXPERIMENTAL SECTION 2.1 FDTD and the first-principle simulations. Commercial FDTD software package was used to perform the full-field electromagnetic wave calculations

23-27.

The unit cells

were simulated using periodic boundary conditions along the x and y axes, and using perfectly matched layers along the z axis. Plane wave was launched incident along the +z direction, while reflection and transmission were monitored with a power monitor placed behind the radiation source and in the interface between GaSe and Ag nanoprisms, respectively. Electric and magnetic fields were detected within the frequency profile monitors. The refractive indexes and extinction coefficients of Ag and

5



SiO2 were obtained from Palik’s Handbook of Optical Constants

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28,

and the dielectric

functions of GaSe were calculated from by the first-principles simulations. The first-principles calculations were performed by using the Vienna ab initio simulation package (VASP) code under the projector augmented wave (PAW) basis sets

29-31.

The dielectric functions of GaSe were obtained based on a 1×1unit cell, and

deformation charge densities of GaSe/Ag were simulated based on a slab model with a 3×3 supercell and a 20 Å vacuum layer along +z direction. An 11×11×1 Monkhorst-Pack grid of k points was used to sample the Brillouin zone, and the cutoff energy for the plane waves was set to 350 eV. The lattice parameters and all of the atoms were fully relaxed with self-consistent convergence criteria of 0.01 eV/Å and 10-6 eV for the atomic forces and total energy, respectively. The preferred adsorption site was determined by calculating the adsorption energy from the equation, ΔEad=EGaSe+EAg-EGaSe/Ag, where EGaSe, EAg, and EGaSe/Ag are the energies of the pristine GaSe, the Ag atom, and the GaSe/Ag system, respectively.

6


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2.2 Fabrication of GaSe/Ag hybrid structure and patterned gratings. For constructing the GaSe/Ag hybrid structure, thin GaSe layers with various thickness were mechanically exfoliated onto SiO2/Si substrate firstly

32.

Then, Polystyrene nanospheres

with a diameter of 360 nm was dip-coated on the GaSe surface and self-assemble to form single layer hexagonal-closed packages

33-35.

The nanospheres were further

etched to adjust the scale of the triangle-gaps. Thereafter, Ag atoms were evaporated onto the triangle-gaps of the nanospheres to form nanoprisms with different side lengths and thickness. By dissolving the nanospheres away, triangular Ag nanoprisms arrays were successfully achieved. Based on GaSe/Ag hybrid structure, the patterned gratings were fabricated through a direct-write laser photolithography machine equipped with a ML® microwriter.

2.3 Experimental characterizations. Raman and photoluminescence signals were pumped by a 532 nm excitation laser and detected using a ×100 objective in Horiba LabRam HR Evolution confocal spectrometer at room temperature. The laser beam size was around 1 μm, and a 1 μm step size was applied in the mapping to cover the whole

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scanning region. The thickness of GaSe layers was measured using a SPA400Nanonavi atomic force microscope (AFM). The extinction spectra of GaSe/Ag hybrid structures were measured by a Cary 5000 uv-visible-near infrared photometer with a sampling spot diameter of about 3 mm and a measurement step of 1 nm.

3. RESULTS AND DISCUSSION 3.1 Theoretical design of GaSe/Ag hybrid structure. As the material selection and geometrical parameters of the metallic nanostructures have significant influence on the plasmon resonance peak, the hybrid plasmonic nanostructures need to be designed to realize the ideal match between the resonance peak and the typical absorption edge of GaSe layers which ranges from 580 to 630 nm

36.

Ag was selected as the metallic

material in this work, which has been well acknowledged as the preferred plasmonic material due to its broad excitation spectra ranged from 400 to 1200 nm

37.

To optimize

the hybrid structure, three-dimensional (3D) FDTD simulations were firstly performed. The schematic of the hybrid system used in this work is shown in Figure 1(a). An 8 nm thick GaSe layer was set on a SiO2/Si wafer. Since numerous investigations have

8


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clarified that the enhancement coefficient for the absorption of the first layer was far more than that of the second layer

38-40,

the thickness of 8 nm was considered thick

enough to simulate the actual situation. Then, Ag nanoprisms array was decorated on GaSe, of which the side length and the thickness are denoted as a and t, respectively. A plane wave was perpendicularly incident into the surface of the Ag nanoprisms.

By altering the geometric parameters of Ag nanoprisms, the resonance wavelength was tuned to match the absorption edge of GaSe. Figure 1(b) shows the extinction spectra of Ag nanoprisms obtained at various side lengths with a fixed thickness of 60 nm. For each curve, there is a significant extinction in a wide band range from 550 to 850 nm, covering the absorption edge of GaSe. Focus on the absorption edge of GaSe, the extinction intensity significantly varies from 27% to 44%, as the side length of the Ag nanoprisms increases from 80 to 120 nm. When the side length exceeds 120 nm, further intensity enhancement at the absorption edge is unnoticeable, while the FWHM of the extinction band increases and an additional shoulder peak presents at the wavelength around 800 nm. Simultaneously, the extinction tends to decrease as the

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side length of the Ag nanoprisms further increases from 140 to 160 nm. Thus, we choose 120 nm as the optimized side length of Ag nanoprisms. Based on the fixed side length of 120 nm, the thickness of the Ag nanoprisms was adjusted to examine the influence on the extinction, as displayed in Figure 1(c). It is found that the extinction around the absorption edge of GaSe is enhanced while the thickness increases. When the thickness varies from 20 to 60 nm, the extinction increases from 31% to 44%, with a 13% increment. Once the thickness is over 60 nm, further enhancement of the extinction becomes ignorable, showing only a 3% increment when the thickness increases from 60 to 100 nm. Therefore, considering the material usage and the simplification of the production process, 60 nm is selected as the optimum thickness of the Ag nanoprisms. Corresponding extinction spectra for the structures with and without Ag nanoprisms on GaSe surface are shown in Figure 1(d). Our optimized hybrid structure is demonstrated that can greatly enhance the extinction at the absorption edge of GaSe (about 588 nm in our experiments). Above results reveal the vital roles of the geometric parameters of Ag nanoprisms in the enhanced optical response, which provided theoretical instruction in designing the metallic nanostructures experimentally. 10


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Figure 1. (a) Schematic representation (3D view) of simulated hybrid structure with a top Ag nanoprisms layer (white), a middle GaSe layer (blue) and a bottom SiO2 layer (purple). (b, c) Calculated extinction spectra with different side lengths and thickness of the Ag nanoprisms, respectively. (d) Comparison of the extinction spectra for the pristine GaSe layer and the optimal hybrid structure. The dash lines are the Gaussian fitting peaks resolved from the dominant extinction spectrum of GaSe/Ag hybrid structure within the wavelength range from 500 nm to 900 nm.

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3.2 Surface plasmon enhanced optical response. Large area Ag nanoprisms arrays, with the optimal dimensional parameters for each Ag prism unit (120 nm in the edge length and 60 nm in the thickness), were then fabricated on mechanically exfoliated GaSe layers through a nanosphere lithography method

33-35.

Hexagonal-

close-packed polystyrene (PS) nanosphere monolayer was employed as the mask on GaSe surface. Figures 2(a, b) show the scanning electron microscopy (SEM) images of the PS nanospheres mask and the prepared trigonal Ag nanoprisms array, respectively. The extinction spectrum of the hybrid structure is shown in Figure 2(c). Remarkably good agreement between the theoretical and experimental results can be well resolved in terms of predicting the resonant peak.

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Figure 2. (a, b) SEM images of hexagonal-close-packed PS nanospheres and the fabricated Ag nanoprisms array on the GaSe layer. (c) Comparison of calculated and measured extinction spectra for the optimized GaSe/Ag hybrid structure.

In order to evaluate the plasmonic enhanced optical properties on GaSe, Raman and photoluminescence spectra of pristine GaSe and GaSe/Ag hybrid structure were measured, in which GaSe layers with different thickness were adopted to investigate the thickness

dependent

SERS

effect

and

enhanced

optical

pumping

in

the

photoluminescence. As plotted in Figures 3 (a-d), the Raman spectra of pristine GaSe layer shows four typical resolved Raman vibration modes, A11g, E12g, E21g and A21g, locating at around 144, 220, 240 and 320 cm-1, agreed well with previously reported results

41.

As for the GaSe/Ag hybrid structure, the positions of Raman characteristic

peaks remain unchanged, while the intensities are all greatly enhanced. The enhancement factor (EF) can be calculated from the expression, EF = ISERS/I, (1), where I and ISERS represent the peak intensities for the pristine GaSe and the GaSe/Ag hybrid structure, respectively. Taking the A21g mode at 320 cm-1 for example, the calculated

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enhancement factor is about 300%, 230%, 150% and 130%, respectively, when the GaSe thickness are 25, 95, 145 and 170 nm. This result suggests that the SERS effect on GaSe/Ag hybrid structure is thickness dependent, which is getting stronger as the GaSe layers tuned thinner. To further verify the uniformity and reproducibility of the SERS effect, Raman mapping for the A21g peak was performed within a 32×32 μm2 area (Figure 3(e)). Figures 3(f, g) show the mapping images for the pristine GaSe and GaSe/Ag hybrid structure with the GaSe thickness of 145 nm. Both images exhibit uniform intensity except the edge, and the brightness for the case of GaSe/Ag hybrid structure is markedly enhanced, which manifests a relatively homogeneous SERS activity on the substrate.

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Figure 3. (a-d) Raman spectra of the pristine and GaSe/Ag hybrid structure with respect to the thickness of GaSe layers of 25, 95, 145 and 170 nm, respectively. (e) Optical microscope image of GaSe layer on SiO2 substrate. (f, g) 2D Raman mapping of the GaSe before and after decorating the Ag nanoprisms array, respectively, in which the brighter contrast corresponds to the stronger signal.

The obtained photoluminescence spectra of pristine GaSe and GaSe/Ag hybrid structure are shown in Figures 4 (a-d). Pristine GaSe layers have a single band edge emission peak around 588 nm. After decorating with arranged Ag nanoprisms, the emission intensities are significantly enhanced, demonstrating an increased optical absorption and photogeneration of electron-hole pairs through the plasmonic mediated pumping. The position of the photoluminescence peak redshifts for about 4 nm, which can be attributed to two main reasons, one is the surface doping from the Ag atoms, the other is the laser-induced heating brings by the plasmonic enhanced optical absorption of the GaSe layer. Similar to that of the SERS effect, the enhanced photoluminescence signal is thickness dependent also. For the GaSe thickness d of 25, 95, 145 and 170 15



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nm, the enhancement factor of the photoluminescence peak is around 2000%, 530%, 250% and 130%, respectively. Since the enhancement effect is found exponentially increased as the thickness of GaSe is down to ~25 nm, it is of essentially crucial for the optoelectronic applications for ultrathin 2D GaSe films. The uniformity and reproducibility of the emission enhancement were further exemplified by the photoluminescence mapping with the GaSe thickness of 95 nm (Figures 4 (e)). Comparing the mapping intensity before and after decorating the Ag nanoprisms, the enhancement on the whole scanning area exhibits relatively uniform, as shown in Figures 4 (f, g).

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Figure 4. (a-d) Photoluminescence spectra of the pristine and GaSe/Ag hybrid structure with respect to the thickness of GaSe layers of 25, 95, 145 and 170 nm, respectively. (e) Optical

microscope

image

of

GaSe

layer

on

SiO2

substrate.

(f,

g)

2D

photoluminescence mapping of the GaSe before and after decorating the Ag nanoprisms array, in which the brighter contrast corresponds to the stronger signal.

3.3 Optical enhancement mechanism. The outstanding SERS effect and enhanced optical pumping in the photoluminescence for the GaSe/Ag hybrid structure suggest a significant plasmonic coupling effect on the 2D material. To further reveal the deep enhancement mechanism, theoretical simulations were carried out to analyze the optical and electronic structure changes after decorating the Ag nanoprisms. The extinction spectrum of GaSe/Ag hybrid structure in Figure 1(d) can be resolved by Gaussian fitting into three dominant peaks located at about 466, 600, 722 nm, where the 600 nm is mostly closed to the absorption edge of GaSe. The near-field distributions of the GaSe/Ag hybrid structure under the three dominant light excitations were simulated, as shown in Figures 5(a, d), (b, e) and (c, f), respectively. It is found that the

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unique patterns of near-field enhancement are strongly affected by the dielectric environment and the shape of the nanostructure. The electric field distribution under 466 nm is notably concentrated on the edge, especially the tips of Ag nanoprisms, suggesting that the extinction peak at 466 nm mainly results from the plasmonic coupling effect between the Ag nanoprisms and the dielectric environment. While under 588 nm (588 nm is the absorption edge of GaSe and closed to a dominant peak of 600 nm, so we study the near-field distribution at 588 nm as an approximation), the filed mostly localizes on the interface between Ag nanoprisms and underlying GaSe, so the coupling effect between Ag and GaSe is responsible for the extinction peak at 588 nm. Through the electronic field coupling, light absorption by the plasmonic nanoprisms can be effectively converted into the electron-hole pair excitations of the GaSe layer. As for 722 nm, the electric field is distributing in the bottom tip of the Ag nanoprisms forming a “hot spots”, indicating a plasmonic coupling between two adjacent Ag nanoprisms. Near-field distribution of these three resonance modes well support the shape dependent localized surface plasmon resonance on this nanoprism stricture. Obviously, compared with that of 466 and 722 nm, the electric field for 588 nm distributes relatively 18


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evenly, which would be beneficial for overall SERS effect and enhanced band edge emission.

For exploring the chemical enhancement process, the deformation charge densities Δρ (r) for the hybrid structure were also analyzed through the first-principles calculations, by employing the following expression: Δρ (r)= ρ(GaSe/Ag, r)-ρ(GaSe, r)ρ(Ag, r), where ρ(GaSe/Ag, r), ρ(GaSe, r) and ρ(Ag, r) are the charge densities of the GaSe/Ag system, pristine GaSe, and Ag atom, respectively. A model with a single Ag atom adsorbed GaSe monolayer was adopted to simplify the simulation. As the results shown in Figures 5 (g, h), most of the positive charge densities are accumulating in the interface, between Ag and Se atoms, which infers the formation of Ag-Se bonds. Bader charge analysis verifies a 0.17e total charge transfer from absorbed Ag atom to the underlying GaSe layer. Interfacial charge accumulation will thus increase the local optical density of states that enhances the interaction between 2D GaSe material and the incident light, promoting the enhancement of optical resonance. Above theoretical calculations can further explain the GaSe thickness dependent

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optical resonance of the hybrid structure. From the near-field distribution image at 588 nm, the localized plasmon resonance is found to concentrate significantly at a surface thickness of about 4 nm from the Ag-GaSe interface, and exhibit an exponential decay as the distance from the surface increases. In the deformation charge densities and charge transfer diagram, the charges of adsorbed Ag atom mainly transfer to the top atomic layer of the GaSe film, indicating that the electronic coupling process occurs primarily in the immediate vicinity of the plasma carrier Ag, and rapidly decreases into the GaSe film. Therefore, the plasmonic enhancement is especially prominent for the thin layer GaSe, just as seen in the SERS effect and enhanced band edge emission in Figure 3 (a) and 4 (a). While as the GaSe layer become thick that is over the spatially limitation of the resonant plasmonic coupling effect, the exceeding thickness hardly bring about additional enhancement. As a result, the ratio of enhanced response signal compared with that of the original optical signal is higher for the thinner GaSe layers than that of the thicker layers. This demonstrates that, the resonant plasmonic coupling strategy is much more effective for the 2D materials than for the traditional 3D bulks. Consider the remarkable effect for above mentioned GaSe thickness (25, 95, 145, and 20


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170 nm), the optical enhancement should be even more dramatical as the thickness is further down to few-layer or monolayer.

Figure 5. Near-field distribution of the GaSe/Ag hybrid structure for the three dominant light illuminations at (a, d) 466, (b, e) 588 and (c, f) 722 nm, respectively. (a), (b), and (c) correspond to the cross-section along x-y plane, while (d), (e), and (f) correspond to the cross-section along x-z plane. The intensity in the maps is the logarithm of the original intensity value with respect to base 10. (g) and (h) are the top view and side view of deformation charge densities of Ag adsorbed 2D GaSe surface, respectively. The blue and yellow colors correspond to charge lost and accumulated, respectively.

3.4 Polarization controllable optical response. Based on the plasmonic-enhanced optical response, the polarization optical properties on GaSe/Ag hybrid structure also 21



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can be manipulated by the controlled plasmonic effect. To realize the anisotropic optical performance, the structural symmetry in the x-y plane should be broken that its transverse and longitudinal surface plasmons can be selectively excited by the choice of the optical polarization. Hence, patterned gratings based on the Ag nanoprisms were designed and fabricated on GaSe layer. As seen in the SEM image in Figure 6(a), a top layer consisting of patterned Ag nanoprisms gratings that were arranged along the x axis with a periodicity of 1176 nm. The grating period was designed approach an integral multiple (twice) of the band edge wavelength of GaSe layer, for achieving the resonant response. In each period, the grating width is equal to the air area. Polarization-dependent photoluminescence spectra were characterized at normal incidence with two different polarizations, whose E-field was perpendicular (TE) and parallel (TM) to the x axis, respectively, as shown in Figure 6. In pristine GaSe layer and normal GaSe/Ag hybrid structure, an unconspicuous polarization of less than 1% is observed in the photoluminescence spectra for TE (E⊥x) and TM (E//x) excitation, confirming an almost polarization-independent optical response due to their low anisotropy in geometric configuration. As for the GaSe/Ag gratings structure, we fixed 22


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the polarization of TE (E⊥x) and TM (E⊥x) excitation respectively, and measured the photoluminescence along two different detection polarizations, as the results shown in Figure 6(d). For the excitation polarization of TM, the photoluminescence at TM and TE polarized detections shows a 9.3% polarization, while as for the excitation polarization of TE, the photoluminescence of TM and TE modes shows an 8.2% polarization. The detected photoluminescence signals are both higher along the TM polarization direction under the TE and TM excitations, indicating an anisotropic optical response. In addition, the TE-polarized incidence excites obviously stronger band edge emission than the TMpolarized incidence does, yielding 14.3% and 15.2% polarizations along two different detection polarization directions, TM and TE, respectively. This result demonstrates polarization selective excitation and emission generated by the plasmonic hybrid structure, confirming that the Ag nanoprisms gratings we designed allow for an anisotropy plasmonic pumping of GaSe luminescence, which will provide a feasible strategy for the design and manipulation of anisotropic 2D nanodevices.

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Moreover, we suggest three possible methods for further improvement of the polarization: (i) Due to the asymmetry in the x-y plane, the optimal dimensional parameters for polarization in the grating structure may not be exactly the same as that of the normal hexagonal periodic array. The dimensional parameters of the triangular prism units can be further optimized for polarization improvement

42-44.

(ii) According to

the grating diffraction principle of the Bragg formula, the period of the grating can be reduced to improve the polarization 45,46, but a rigorous lithography technique is needed. (iii) Alternatively, the polarization may also be further improved by placing the GaSe layers above the Ag nanoprism array instead

47,

while for this method, the GaSe layers

should not be too thick. All the above three strategies will be investigated in our future work.

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Figure 6. (a) SEM image of the Ag nanoprisms gratings on GaSe layers. (b)-(c) measured photoluminescence spectra with TM and TE polarizations for the pristine GaSe layer, the normal GaSe/Ag hybrid structure, respectively. (d) measured photoluminescence spectra for the GaSe/Ag gratings structure, along two detection polarization directions, TM and TE, with TM and TE excitation polarizations, respectively. The “TE-TE” (“TM-TM”, “TE-TM”, “TM-TE”) indicates the spectrum pumped with the TE (TM, TE, TM) polarized incidence and detected along the TE (TM, TM, TE) polarization direction.

4. CONCLUSIONS. In conclusion, we designed a hybrid structure with periodic Ag nanoprisms array on 2D GaSe layer for the investigation of surface plasmonic-enhanced optical response and

25



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polarization control. The geometric parameters were optimized by performing the FDTD simulation to maximize the extinction of GaSe absorption edge. Directed by the theoretical design, large area Ag nanoprisms arrays were fabricated on mechanically exfoliated GaSe layers. Raman and photoluminescence spectra of the hybrid structure exhibit significantly and uniformly enhanced signals which are increased with the decreasing thickness of GaSe layer. Near-field distribution indicates three resonance modes originated from the dramatically different dielectric functions between GaSe, Ag nanoprisms and the dielectric environment, and evidences a forming of localized surface plasmon resonance. Deformation charge densities and charge transfer further reveals an interfacial charge accumulation that enhances the interaction between 2D GaSe and incident light, leading to the enhancement of optical resonance. Based on the surface plasmon-enhanced optical response and the understanding of the mechanism, patterned gratings on the hybrid structure was further fabricated for the polarization control. A 15.2% maximal polarized response was successfully realized in polarizationdependent photoluminescence spectra on the band edge emission of GaSe. These results suggest a significant route for enhancing and detecting the optical involved 26


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signals for extremely thin 2D GaSe, and shows a well manipulation of the anisotropic optical performance for the potential plasmonic-based optoelectronic applications.

■ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (Y. W.).

*E-mail: [email protected] (X. Z.).

*E-mail: [email protected] (J. K.).

ORCID

Wei Wan: 0000-0003-0232-460X

Yaping Wu: 0000-0001-9325-2212

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. 27



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Notes The authors declare no competing ﬁnancial interest.

■ ACKNOWLEDGMENTS We gratefully acknowledge Penggang Li, Jiawei Liao, Bingjie Mo for their helps with the FDTD simulations and data analysis. The work was supported by National Natural Science Foundations of China (Grant Nos. 61674124, 61774128, 61874092, 11604275 and 61704040), Natural Science Foundation of Fujian Province (Grant Nos. 2018I0017, 2017J01012, and 2018J01102), Outstanding Youth Foundation Project of Jiujiang (Grant Nos. 2018042), and Fundamental Research Funds for the Central Universities (Grant Nos. 20720170012 and 20720170018).

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Polarization-Controllable Plasmonic Enhancement on the Optical

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