Synergistic Effect of Chiral Metasurface and Hot Carrier Injection Enabling Manipulation of Valley Polarization of WSe2 at Room Temperature

The unique band structure of monolayer transition metal dichalcogenides (TMDC) provides an important platform for spintronic and valleytronic devices. Various approaches, such as in‐plane electric field and out‐of‐plane magnetic field, have been proposed to actively control the valley polarization. In this work, we propose a synergistic effect involving chiral near‐field interactions and hot carrier injection to actively control the valley polarization emission of WSe2 at room temperature (RT). The degree of valley polarization is enhanced from near zero (for pure WSe2) to 20% under non‐resonant optical excitation (532 nm) when monolayer WSe2 is coupled with the chiral near field of plasmonic metasurface. More importantly, the application of near‐infrared light (wavelength of 970 to 1600 nm) illumination further enhances the valley polarization from 20% to 30%, which is attributed to plasmonic‐induced hot carrier injection from the metasurface to WSe2. The synergistic effect of the chiral near field and infrared light pumping offers another strategy to manipulate the valley polarization emission in monolayer TMDs at room temperature, paving the way for future applications of opto‐valleytronic/spintronic devices based on these 2D materials.


Introduction
9][10] Controlling valleyspin light emission is not only an interesting phenomenon of condensed matter physics but also a promising approach for developing quantum information devices.aOne feasible approach to selectively excite the different valleys is circularly polarized light (CPL) pumping, which gives rise to valley-dependent optical selection rules: the left-and righthanded circularly polarized (+ and -) lights coupling to the interband transitions of K and -K valley due to angular momentum conservation. [11,12][15][16] However, at room temperature, the carriers in the excited valley will also be transferred into the unexcited valley by intervalley scattering and intervalley electronhole interaction.As a result, the degree of valley polarization (which is defined as DVP = , where I  + and I  − are the right-and left-hand circular polarization photoluminescence (PL) intensities.) is very weak.Applying additional physical filed, such as gate electric field [16,17] and an out-of-plan magnetic field, [18][19][20] also has been proposed to manipulate the valley polarization emission from TMDCs.[23][24] This approach can enhance the emission rates from one valley through the nearfield interaction of the plasmonic chiral metasurface and valleypolarized excitons.[27][28][29][30][31][32][33][34] Thus, to integrate monolayer TMDCs with chiral plasmonic metasurfaces is supposed to be an efficient approach to manipulating the valley polarization of PL.However, the DVP is fixed by the geometry of the given plasmonic metasurface which is hard to be tuned after fabrication.
In this work, we investigate the enhanced optical response and valley-polarized PL in the hybrid system of monolayer WSe 2 coupled with a chiral metal-dielectric-metal (MDM) metasurface at room temperature.Under non-resonant optical excitation (532 nm), the DVP of WSe 2 is enhanced from near zero (for pure WSe 2 ) to 20% due to the selective interaction between valley spin excitons and the chiral metasurface.More interestingly, we found that the PL intensity and the DVP can be controlled by additional pumping light with a photon energy lower than the exciton energy of WSe 2 .When an infrared light is applied to the device, the intensity of PL is suppressed, while the DVP increases from 20% to 30%, which is attributed to the hot electron injection from the metasurface.The synergistic effect of the chiral near field and light pumping offers another strategy to actively control the valley polarization in monolayer TMDs at room temperature and toward developing valley optoelectronic devices.

Result and Discussion
Figure 1a depicts the schematic diagram of our device, which consists of a chiral plasmonic metasurface, a periodic array of "S"shaped gold antennas on top of a 30 nm boron nitride (BN) spacer with a 50 nm gold backplane.Figures 1a and 1b show the parameters and SEM image, respectively, of the "S"-shaped gold nanoantennas.A monolayer of mechanically exfoliated WSe 2 is sandwiched between the "S"-shaped gold antennas and the BN layer (Figure 1c).The fabrication details are described in the method section.Due to the broken inversion symmetry, the "S"-shaped Au antenna exhibits strong optical chirality with distinct optical responses to right (RCP or +) and left (LCP or -) circularly polarized light [36] at the resonant wavelength.
To investigate the chiral optical response of the device, Finite Difference Time Domain (FDTD) simulations were performed.+ and polarized lights were incident on the front side of the device to measure the reflectance spectra.As shown in Figure 1d, for + light, a high Q resonance peak is observed at a wavelength of 745 nm, which coincides with the neutral A-exciton emission wavelength in the PL spectrum of monolayer WSe2.For light (Figure 1e), two weak peaks are observed at a wavelength ≈750 nm, and a high Q resonance peak is observed at a wavelength of 700 nm, which is off the exciton resonance of WSe 2 .The experimentally measured reflection spectra for + and polarization lights exhibit the same resonance peaks as the simulation results.This chiral selective optical response can be explained by the destructive (LCP) and constructive (RCP) interference of the illumination beams and relies on the fact that the planar metamaterial is lossy, anisotropic, and results in linear polarization conversion. [35]he high Q resonance peak indicates the localized resonance plasmonic mode with strong electromagnetic confinement on the metasurface.Figure 1f shows the electromagnetic field at a wavelength of 745 nm.For + light, the intensity of the hot spot at the "S"-shaped corner is about three times higher than that for light.When WSe 2 is coupled with this chiral plasmonic metasurface, the light-matter interaction will be selectively enhanced through the chiral selective resonant plasmon and can be a promising approach to modify the valley polarization of WSe2.
We obtained the + and polarization components of the valley exciton emission of the monolayer WSe 2 under both + and polarization light excitations through PL spectroscopy with polarization analysis.The excitation laser wavelength was 532 nm (Epump = 2.33 eV), which was much higher than the neutral exciton resonant energy (EX = 1.66 eV, wavelength of 745 nm) of WSe 2 .Circular polarization excitation light was achieved by placing a linear polarizer and a quarter-wave plate in the excitation light path.The polarized PL signal was analyzed by a broadband quarter-wave plate and a polarizer in front of the spectrometer.The experimental results are shown in Figure 2a,b.There is no intensity difference between the + and polarization components for the two excitations, which is consistent with previous studies at room temperature. [8,9,10]ccording to the valley-dependent optical selection rule of TMDs, the valley excitonic transitions in the K (-K) valley will only be excited by + (-) photons, which result in the corresponding light emission with special circular polarization.However, due to the optical phonon-assisted intervalley scattering of excitons be-tween K and -K valley, the spin angular momentum of the electron at the valley will be exchanged at a finite rate. [10,31,32]As a result, when the K (-K) valley was selectively excited by + (-) polarized light, the PL emission not only contains + polarized light due to the excitonic recombination at the K(-K) valley but also contains signals from the recombination at -K(K) valley, as shown in Figure 3a.Here, we define the degree of valley polarization as DVP = , where I  + and I  − are the intensity of right-and left-hand circular polarization emission lights.Obviously, DVP is almost zero for + and excitation, as shown in Figure 2c. Figure 2d and e show the PL intensity spectra from the WSe2 with the metasurface under + and excitation.Comparing with the PL signal from bare WSe 2 , significant enhancement is observed.For the two kinds of excitation, the intensity of + emission is enhanced from ≈1700 to 3000 counts, while the intensity of emission is enhanced from ≈1700 to 2000 counts.Correspondingly, a significantly higher degree of valley polarization is obtained.We found that the DVP is up to 20% (Figure 2f), which is much higher than that of bare WSe2.
There are mainly three possible reasons for this chiral selective emission.One is the chiral selective absorption of excitation light due to the metasurface.However, according to the measured and simulated reflection spectrum, there is no absorption difference between + and light for excitation light at the wavelength of 532 nm (more information in Figure S1, Supporting Information).Chiral selective absorption of the emission light due to the metasurface is another possible reason for this phenomenon.Nevertheless, it contradicts the fact that both + and emission are enhanced.Here, we attribute this phenomenon to the coupling of chiral plasmonic mode and valley exciton of WSe 2 .As shown in Figure 3b, due to the optical phonon-assisted intervalley scattering of excitons between K and -K valley at room temperature, no matter + or excitation, K valleys are filled with + exciton and -K valley is filled with exciton.The excitons then rapidly deliver their energy to plasmons via exciton-plasmon coupling.Finally, the excited plasmons behave like an antenna and undergo radiative and nonradiative damping, resulting in the increase of radiative decay rate and enhanced PL intensity in the far field.
The enhancement of chiral light emission from the metasurface can be described by the chiral Purcell factor, [37,38] , where Q is the quality factor and V c is the chiral mode volume.V c is defined as , where U and u c,max are the total energy of the cavity and the maximum chiral energy density inside the cavity, respectively.They are given by U = , where E(r) and B(r) are the electric and magnetic fields, respectively, and  and  are the permittivity of vacuum and the angular frequency.
As shown in Figures 1e,d, the metasurface exhibits a higher Q factor for + light than light.Through FDTD simulation, we can also obtain the total energy of the cavity U and the optical chirality C distribution under the excitation of + and dipoles.The total electromagnetic energy U in the cavity at the wavelength of 745 nm for + dipole excitation is about two times higher than that of excitation, as shown in Figure S2 (Supporting Information).The near-field distributions of the optical chirality at the resonant wavelength of 745 nm show distinct patterns under the excitation of opposite circular polarizations, as shown in Figures 3c,d.When the chiral metasurface interacts with + excitation, the chiral fields around the gold rods show positive C/C 0 of 5, where C 0 is the value of optical chirality for a circularly polarized light without the metasurface.When the chiral metasurface interacts with excitation, as shown in Figure 3d, the chiral field shows a very weak optical chirality (C/C 0 ∼0.5).Therefore, the metasurface exhibits a much smaller chiral mode volume ( ) for + dipole, resulting in enhanced spontaneous emission.The enhancement of chiral light emission from the metasurface can also be confirmed by the simulated far-field electromagnetic energy, as shown in Figure S3 (Supporting Information).In the far field, the right-hand polarization electrical field ( E R = 1 2 (E x + iE y )) is much higher than the left-hand polarization electrical field ( E L = 1 2 (E x − iE y )), which is attributed to the interaction between + excitons and the chiral near-field.
Next, we investigate the modulation of valley polarization emission using infrared pumping light.As shown in Figure 4a, a near-infrared light (wavelength of 970-1600 nm) is directly illuminated on the device during the measurement of PL spectroscopy with polarization analysis (experimental setup is shown in the Method Section).The PL spectrum excited by + is shown in Figure 4b.When the infrared light illuminates the device, both the intensities of + and emission are reduced (Figure 4c).Moreover, the decrease in the intensity of emission light is more significant than that of + emission light, resulting in an increase in DVP.Compared to the emission infrared light illumination, the DVP is increased from 20% to 30% (Figure 4d).
The reduction in PL intensity under infrared light illumination can be explained by photon-induced hot electron injection from the chiral metasurface.When infrared light is incident on the gold metasurface, the light absorption of gold is enhanced by localized surface plasmon resonances.As a result, hot electrons (holes) are generated upon photon absorption.The hot holes with energy lower than the valence band of WSe 2 can then be injected into WSe 2 near the Schottky junction of WSe 2 /Au, [39][40][41] thereby increasing the hole concentration of WSe 2 in the valence band (the schematic diagram is shown in Figure S4, Supporting Information).The injection of hot holes can reduce the PL intensity through two possible mechanisms.
First, the injected hole may combine with the neutral exciton, forming a charged exciton (positive trion), [42,43] which reduces the emission intensity of the neutral exciton.Another possible outcome of the hole injection is the suppression of the excitation of electrons from the valence band to the conduction band.The interband transition from valence to conduction band is guided by is the Fermi distribution of carriers at the valence band and conductor band.Normally, f(E V ) → 1 and f(E c ) → 0. However, as the hot carriers injecting into the valence band of WSe 2 , f(E V ) will be reduced, and the interband transition is suppressed, thereby reducing the intensity of the PL signal.
To confirm the photo-induced carrier injection in WSe 2 , we measured the infrared absorption spectrum and the corresponding photocurrent response of the WSe 2 /metasurface device under the illumination of infrared light (experimental details are shown in the Method Section).As shown in Figure 4e, the device exhibits an absorption peak at the wavelength of ≈1135 nm, which agrees well with the FDTD simulation result.As the absorption band of WSe 2 is cut off at the wavelength of ≈750 nm, the absorption peak at 1135 nm should be attributed to the plasmonic resonance-induced absorption of the metasurface.Correspondingly, a clear photocurrent is observed, as shown in Figure 4e.As a comparison, we also measured the photoresponse of pure WSe 2 , which shows almost zero photocurrent (Figure S5, Supporting Information).Therefore, the photocurrent of the WSe 2 /metasurface device indicates the increase in hole concentration, which confirms the hot carrier injection into WSe 2 .
The hot carrier injection also suppresses the intervalley relaxation process for the enhancement of valley polarization, which is similar to the electrostatic doping by electrical gate. [26,44]In detail, the intervalley relaxation process of excitons can be manipulated through screening the long-range electron-hole exchange interaction by electrostatic doping.The intervalley scattering rate  −1 v is proportional to the inverse of the Thomas-Fermi wave vector k TF ,  −1 v ∝ k −2 TF , while the Thomas-Fermi wave vector is a function of carrier density, k TF ∝1 − e −n/T , where n is the carrier density, T is temperature, and  is a constant.Therefore, as the hot holes injected from the metasurface into the WSe 2 , the hole density and k TF will increase, resulting in a decrease in the intervalley scattering rate.In other words, the hot carrier injection into WSe 2 can help screen the momentum-dependent long-range electronhole exchange interaction, which reduces the intervalley scattering and leads to the enhancement of valley polarization.

Summary
To summarize, our study has showcased the effectiveness of chiral metasurfaces in fine-tuning the valley-polarized photoluminescence (PL) of monolayer WSe 2 at room temperature.The chiral selective plasmonic localized mode of the metasurface greatly amplifies the exciton emissions in the WSe 2 monolayer with specific chiral emission, resulting from the coupling of WSe 2 excitons with the chiral plasmons.Under nonresonant optical excitation (532 nm), the degree of valley polarization (DVP) of PL emission is enhanced from near zero (for pure WSe2) to 20%, thanks to the selective chiral polarization light confinement of the chiral metasurface.Moreover, by applying near-infrared light illumina-tion (wavelength of 970 to 1600 nm), we found that the valley polarization is further enhanced from 20% to 30%, attributed to the hot carrier injection from the plasmonic mode of the metasurface.This synergistic effect of chiral metasurfaces and infrared light pumping provides an additional approach to manipulate the valley polarization emission in monolayer TMDs at room temperature, opening up new opportunities for future applications of opto-valleytronic/spintronic devices based on these 2D materials.

Experimental Section
Device Fabrication: A 60 nm gold film was deposited on a Si/SiO 2 wafer as a gold mirror.A 40 nm h-BN thin flake was exfoliated from a bulk BN crystal on PDMS and then transferred onto the gold mirror using a PMDS/PPC stamp (see Figure S6, Supporting Information for more details).Monolayer WSe 2 was exfoliated from a bulk WSe 2 crystal and transferred onto the prepared gold/BN structure.An "S"-shaped gold nanoantenna periodic array was fabricated on a Si/SiO 2 wafer using standard electron beam lithography and metal deposition, followed by a PMMA film spin coating on the "S"-shaped gold nano-antenna periodic array as a supporting layer.The gold nano-antenna periodic array was then transferred onto the WSe 2 /BN/gold mirror using a wet transfer process (see Figure S7, Supporting Information for more details).Two electrodes were fabricated onto the WSe 2 before the "S"-shaped gold nano-antenna periodic array transferred onto it for measuring the photoresponse (see Figure S5, Supporting Information for more details).
PL Characterizations: The PL spectra were measured using a Horiba Raman spectrometer system with a 532 nm laser (2.33 eV) as the excitation source with a quarter waveplate for chiral excitation.The emission signal was collected and analyzed using a 100× objective lens and a 600 lines/mm grating.A quarter waveplate and a linear polarizer were used to analyze the chiral polarization of the emission light.Infrared light was generated using a 200 W tungsten lamp, and a long wavelength pass filter (pass edge of 970 nm) was used to filter out short wavelength light (see Figure S8, Supporting Information for more details).
Photoresponse Measurement: For the photoresponse measurements, the 200 W tungsten lamp generated infrared light, which passed through a long wavelength pass filter (pass edge of 970 nm), and was then vertically incident onto the WSe 2 /metasurface device.The photocurrent was measured using a semiconductor analyzer (see Figure S9, Supporting Information for more details).

Figure 1 .
Figure 1.a) Schematic of the device, where WSe 2 is sandwiched in the chiral metamaterial consisting of the "s"-shaped gold antenna, dielectric spacer and gold mirror.The dimensions of "s"-shaped gold antenna are a = 435 nm, b = 142 nm, c = 260 nm, d = 174 nm.The periodic of unit cell is Px = 610 nm, Py = 480 nm.The thicknesses of gold antenna, dielectric spacer and the gold mirror are 40, 40 and 100 nm.The scale bar is 1 μm.b) SEM image of the plasmonic chiral metasurface.c) microscopic photograph of the device.d,e) the simulation and experimental reflection spectra of the device.(f) Electromagnetic field at the wavelength of 745 nm with + (up) and -(down) light excitation.

Figure 2 .
Figure 2. a,b) PL spectra of WSe 2 excited by right-and left-hand circular polarization laser.c) DVP of WSe 2 excited by − and − laser.d,e) the PL spectra of WSe 2 integrated with chiral metasurface excited by right-and left-hand circular polarization laser.f) DVP ( I + −I + I + +I + ) of WSe 2 integrated with

Figure 3 .
Figure 3. a) Schematic diagram of valley exciton emission at room temperature b) Schematic of the mechanism of chiral selective enhancement of valley emission due to the coupling of chiral plasmonic mode and valley exciton of WSe 2 .c) and d) Near-field optical chirality distributions under + and excitation.

Figure 4 .
Figure 4. a) Schematic diagram of controlling the valley polarization emission by an infrared pumping light.b) PL spectrum of WSe 2 excited by + laser and illuminated by infrared light.c) peak intensities of + and emission excited by + laser with and without infrared light illumination.d) DVP of the valley polarization excited by + laser.e) infrared reflection spectrum of WSe 2 integrated with metasurface.f) photocurrent response of WSe 2 integrated with metasurface under illumination of infrared light.