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Abstract
Here we demonstrate light blocking and phase modulation in nano-plasmonic donate shape rings based on the thermo-plasmonic effect in these structures. For this purpose, we use a laser writing system to fabricate nanohole arrays and cover them with plasmonic gold thin film via a sputtering machine. The chemical vapor deposition method is also used to produce a WS2 layer, which is suitable for light blocking and phase modulation due to the nonlinearity of this two-dimensional material. After theoretically and experimentally evaluating the plasmonic donate-shaped substrate, we use the ellipsometric method to characterize the optical modes of the samples and record the switching manner and light-blocking phenomena under the probe laser excitation set to 980 nm. Our results show phase modulation based on the thermo-plasmonic effect of nano gap in donated double rings and light blocking by thermal expansion of the WS2 layer, which can open new insight into plasmon and two-dimensional material-based devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
I. Introduction:
There is currently an electronic bottleneck issue due to the growing demand for more bandwidth and ultra-fast operation speed from various internet-based applications. Light offers a number of advantages to electrons, including greater capacity, faster speed, and fewer interactions. Despite the fact that optical fiber cables are employed in communication systems, the capacity of optical systems cannot be completely utilized since the conversion of light to electricity slows down operations. As a result, entire optical networks, all-optical devices, and all-optical signal processing have been recognized as prospective replacements for traditional electronic-integrated circuits [1–3]. All processes, including signal production, processing, encoding, modulation, transmission, demodulation, decoding, filtering, etc., must be carried out entirely by optical means in order to create an optical network. Among all of these techniques, phase modulation (PM) and light blocking (LB) are two techniques used in all-optical signal processing for a variety of applications, including ultrafast optical communication systems such as sensing and signal processing [4]. PM and LB can modulate the phase of an optical signal and block or attenuate certain wavelengths of light in an optical signal respectively.
One of the main methods proposed to achieve the ability of PM and LB in all optical processing was based on the nonlinear phenomena in two-dimensional (2D) layered materials [5]. Until now, there have been a lot of reports which focused on the enhanced third-order nonlinear interactions, including self-phase modulation and four-wave-mixing based on Graphene, Graphene oxide, and also MoS2 for integration with silicon photonic waveguides [6,7]. In addition, Tungsten disulfide (WS2) is one of the Transition Metal Dichalcogenides (TMDs) materials which has an intriguing combination of optical properties, including good transparency and a strong nonlinearity in the near-infrared wavelength range, that sets it apart from the aforementioned 2D materials. This makes it a potential candidate for on-chip nonlinear optics, particularly in the traditional telecom windows at 1.5 µm [8,9]. TMDs are a great way to study nonlinear optics because they break inversion symmetry and interact with light and matter effectively [5,10]. Recently, researchers have been exploring the use of WS2 as a material for PM to transmit information by varying the phase of a carrier signal [11]. WS2 as a 2D material has unique electronic and optical properties with high electron mobility and can be easily integrated into electronic devices [12].
On the other hand, strongly enhanced light-matter interactions are made possible by localized surface plasmon resonance (LSPR), which is widely known for its capacity to confine the electromagnetic (EM) field at the nanoscale and thus can enhance light matter interaction and nonlinearity to get benefit from the PM [13]. The term “plasmon-induced temperature” refers to the rise in temperature of a substance brought on by the absorption of plasmons, quasiparticles produced by the collective oscillations of electrons in a substance [14].
On the dependency of optical properties of 2D materials on the temperature is one of the important physical phenomena reported in some lectures as follows. By increasing the temperature from 83 to 583 K, Gaur et al. investigated the resonant Raman scattering of 1 layer and bulk WS2 [15]. Under resonance conditions, monolayer WS2 showed more intense second-order Raman bands with B exciton than bulk. Additionally, the 2LA(M) Raman peak's intensity is seen to be at its highest at room temperature (RT) due to a greater disparity between incoming laser energy and band gap energy at temperatures higher or lower than RT [15]. Using a supported monolayer WS2 sample on a silicon substrate, Huang et al. reported the same result [16]. They note that the maximal laser intensity appears at a temperature close to 223 K and that the energy of the B exciton resonates with the energy of the 514.5 nm wavelength laser. In addition, Fan et al. used various laser energy to conduct a resonance Raman scattering investigation of bulk and monolayer MoS2 and bulk TMDs, by tuning the resonance by adjusting the sample temperature [17]. The resonance energies for WS2 and MoS2 match A and B exciton, however for WSe2 resonance Raman was detected over a wider energy range and could not be explained by its excitonic energies. Along with sample temperature, which influences the electronic band structure and therefore the intensity of the Raman mode, a material's optical characteristics and temperature dependencies. Cherroret et al. studied the link between the band structure and the refractive index of semiconductors as a function of temperature [18]. In nm-thick WS2 samples, Zobeiri et al. investigated the temperature-dependent Raman intensity. They were able to see the resonance Raman of the E2g and A1g Raman modes by changing the temperature from extremely low (77 K) to very high values (757 K). The four WS2 samples’ excitonic transition energy and broadening parameter are investigated using the Raman intensity of the A1g mode at various temperatures and resonance Raman conditions. The lattice and therefore the phonon population dynamics, which are critical in the energy dissipation following optical excitation, have not yet been directly examined in spectroscopic investigations on monolayer TMDs, which have mostly concentrated on the electronic excitation features [12].
Jinwei Shi et al. [19], reported enhancement of the second-harmonic generation (SHG) was achieved from monolayer tungsten disulfide (WS2) incorporated onto a 2D silver (Ag) nanogroove grating with subwavelength pitch. By tuning surface plasmon mode and second-harmonic frequency in resonance with the C exciton in WS2. While Arindam Dasgupta et al. [20], explained how to manipulate the local phase of the nonlinear polarization from the rotated WS2 monolayer crystals, they are also experimentally demonstrating the nonlinear generation of Hermite−Gaussian beams at second-harmonic frequencies via the binary phase manipulation on the patterned WS2 monolayer crystals. Moreover, the deterministic control over the polarization state of the generated nonlinear beam is demonstrated due to the crystal symmetry properties of the TMD monolayer. In a monolayer molybdenum disulfide (MoS2) and plasmonic hybrid structure, Hyuntae Kim et al. [21], looked into the interconnected mechanisms of each plasmon-exciton connection. Their absorption, modeling, electrostatics, and emission spectrum data demonstrate that the interaction between photoexcited carrier and exciton modes is successfully connected by exciton recombination and energy transfer mechanisms. By developing the plasmonic hybrid platform, it is possible to specifically improve the neutral exciton, trion, and biexciton. All of these findings suggest that there is more latitude to manage the specific augmentation of each exciton mode during the creation of nano optoelectronic devices. All of these findings suggest that there is more latitude to manage the specific augmentation of each exciton mode during the creation of nano optoelectronic devices. Depending on these facts, we propose a substance with the ability of plasmon-induced temperature enhancement is WS2 to modulate the phase and get switching manner also in these 2D substances.
II. Experimental setup:
A 2D micro cavity array was prepared via direct laser writing experimental setup using 405 nm laser beam. The laser beam was expanded and transferred to the silica substrate by objective lens with numerical aperture of 0.8. The substrate, covered with SU8 2002 polymer, was placed onto the motorized x-y stage and the array of microcavities was wrote onto the polymer. The laser power was set to 0.3 mW with the arrays consisting of 50 in 20 rings with width and radius set to 4.58 µm and 871 nm respectively. The gap between two rings was selected as 416 nm and the periodicity of the structure was assigned to 20 µm as displayed in Fig. 1(a).
Fig. 1. The schematic diagram of the experimental setup to excite plasmon (a) via near field experimental setup and (b) Motorized Krestchmann setup, (c) On resonance field localization and (d) off resonance field distribution of the donut twin particles.
The WS2 nanosheets were prepared by CVD (chemical vapor deposition) method. The Tungsten three oxide (WO3) and sulfur (S) powders were used as raw materials. Also, Potassium Chloride (KCl) was used as a catalyst and Ar/H2 gas mixture was injected during the reaction. The furnace was first kept at a temperature of 250 °C for 10 minutes and then the temperature increased to 820 °C and kept for 30 minutes. The main modes and confirmation of WS2 production was done by Raman spectroscopy before any production onto the main sample. After the synthesis of WS2 layer, the sputtering machine was used to deposit gold thin film with the thickness of 35 nm.
To plasmon excitation and use of thermo-plasmonic phenomena, we used the near-field measurement system in transmission mode by using high numerical aperture objective lens as shown schematically in Fig. 1(a). The halogen lamp via the Glan-Taylor prim reach to the objective lens and thus the sample by two different polarizations. After plasmon excitation in this near field setup, the light gathered by another normal objective lens and focuses onto the spectrometer or charge coupled device (CCD) camera. In addition, we must investigate the sample in reflection mode and under the angular modulation which carry by motorized surface plasmon resonance recording system (PLASENS-VIS) in all of visible region. For this purpose, we keep the sample in Krestchmann experimental configuration as shown in Fig. 1(b) and use the plasmon excitation in this setup by halogen lamp in all of the visible region and all of the incidence angles from 10 to 70 degree. In both of plasmon excitation setups, we use probe laser beam set to 980 nm to excite the absorption in WS2 sample also and record the switching manner, LB and PM in off and on state of the laser.
Furthermore, to get more sense about the light localization in the donate twin particles as a motif of our fabricated lattice, we simulated the structure consists of the micro-rings array based on SU-8 material by a thin gold coating, which was arranged on a glass substrate as shown schematically in Fig. 1(a) by the aid of Lumerical software. The mesh size in the x, y, and z directions was considered to be 100 nm and the refractive index of the glass (SiO2), SU-8 2000, and Au materials were considered from the data presented by Palik et al., Microchem, and Johnson and Christy, respectively (which are comprehensively available online at the website [22,23].
The periodic boundary condition was applied in the x and y direction, and the perfectly matched layer (PML) boundary condition was applied in the z-direction.
Finally, we used our ellipsometry parameters to gather the psi and delta parameters as $ta{n^{ - 1}}\left( {\frac{{Rp}}{{Rs}}} \right)$ and also $\Delta = {\delta _p} - {\delta _s} $ respectively as completely derive and explain in our previous reports [24] to define our LB and also PM percentage.
III. Results and discussion
The Scanning electron microscopy (SEM) of the fabricated samples are shown in Figs. 2(a) and (b) for five lines of the sample and also one motif which we use for our simulation process also. As shown in Figs. 1(c) and (d), we have light localization in each motif onto on and off resonance field distribution of the donut twin particles respectively. It can confirm that in on resonance$,\; \lambda = 568{\; }nm$, distribution, we have dipole moment distribution in the middle of each donate with circular symmetry and in the off resonance, only in the corners we have localization due to surface plasmon resonance of the gold nanostructure with ${\sigma _x} $ symmetry in the donate based unit cell. Based on this fact that our proposed structure has c4v group symmetry over the simple cubic structure.
Fig. 2. The SEM image of (a) five lines of the proposed array, (b) one motif in one-unit cell, (c) electric field distribution recorded by CCD camera, (d) enlarged excited couple of donuts, and (e) the Raman spectrum of the WS2 sample.
The Raman spectra were utilized to determine whether the WS2 on Si substrate had a single or multilayer structure. As shown in Fig. 2(f), typical Raman spectra of monolayer WS2 regions using 514.5 nm, shows three peaks (E2g, A1g and 520 cm−1), the first mode is for WS2 monolayer at 352 cm−1 while the A1g mode is for the frequency of the phonon mode monotonically, finally, 520 cm−1 phonons from the silicon substrate were used for calibration. The previously explained mode are almost matched with Ref. [25].
Furthermore, the concept of thermo-plasmonic effect in our plasmonic donates arrays is that we must have the boost of electric field localization in each motif of the periodic structure via near field excitation process which is confirms in Figs. 2(c-d) by CCD camera. It is obvious that exactly in each motif and between two donates (around 500 nm distances), we have efficient electric field localization to benefit from the thermo-plasmonic effect.
As shown in the Raman spectra, WS2 has several phonon modes, including Raman active modes A1g and E2g, and acoustic two phonon modes. The E2 mode involves vibrations that are symmetric with respect to a plane, while the A1g mode involves vibrations that are antisymmetric. These optical modes are sensitive to changes in electronic structure and can be used for characterizing defects or doping in WS2 [26]. Based on this fact that WS2 layer has main phonon modes which appear in the Raman spectrum, A1g, it can get affect from the temperature enhancement by the following formula [27].
This temperature enhancement comes from two important factors like as thermo-plasmonic effect onto the donate array structure and also laser irradiations. The thermo-plasmonic enhancement factor of this plasmonic microstructure get confirmation from the CCD imaging setup as shown in Figs. 2(c-d). The above-mentioned temperature affected phenomena can directly change the optical properties like as the amplitude and phase of the reflection against temperature. In addition, there is another important fact which can be considered in these situations as the thermal expansion of the WS2 sample in different incidence angles of the probe light irradiances. We measured the temperature of the sample using thermal camera and thermometer and the results are illustrated in Figs. 3(a and b). The sample was irradiant by 980 nm laser and the temperature was measured for different duration of irradiance time.
Fig. 3. (a) A schematic array of the simulated donate-shaped microstructure and (b) the simulated transmission spectra of the proposed structure consisting of the single- and double-microring periodic array.
The WS2 monolayer absorbs energy and warms up when it is excited by a laser. The substrate subsequently receives this heat and begins to heat up. The temperature of the WS2 monolayer remains high even after the excitation source is turned off because heat transfer from the WS2 monolayer to the substrate continues to occur, as Ref. [28] confirmed. Additionally, it was observed that WS2 monolayers made from a potassium-based precursor emit peculiar defect-related room-temperature emission. Following the removal of the excitation source, these flaws may also contribute to the heating of the WS2 monolayer [29].
We go step by step and investigated the effect of temperature enhancement factor in the optical transmission of the sample with and without laser irradiations. It means at first, we have only the thermo-plasmonic effect by near field excitation and in second step, add the thermal expansion of the WS2 by probe laser irradiation also in-spite of the thermo-plasmonic.
The transmission spectrum of the sample as donuts couple arrays/ Gold/ WS2 sample were simulated and recorded by near field excitation as shown in Figs. 3 and 4 respectively.
Fig. 4. (a) The Transmission spectrum of the sample for P and S polarizations, (b) psi and delta spectrum of the sample, (c) The Transmission spectrum of the sample for P and S polarizations under probe laser excitation and (d) psi and delta spectrum of the sample under probe laser excitation
The simulated transmission spectrum of the plasmonic structure and the schematic of simulated structure, were shown in Fig. 3 consists of a periodic array of single microrings (instead of pairs of donate) and the couple of donates. As can be seen, the resonance dip appeared clearly for the double-donate structure, which is due to the coupled resonance properties of the double-donate array. While the resonance dip for the structure consisting of the single-microring periodic array is so weaker than the double-donate structure (Fig. 3(b)).
Measured spectra confirming that we have modes changes for different polarizations which is logic and we have thinner transmission window for S polarization (Fig. 4(a)). In addition, plasmon induced transparency appear in 560 nm for P polarizations as shown in the inset of Fig. 4(a). Direct effect of this plasmon induced transparency for one polarization appear in ψ graph also in this wavelength exactly as shown in Fig. 4(b) by red symbols and right y-axis of this graph. Accordingly, Fano-shape response in the Δ parameters which is the sign of phase difference between two polarizations by fast changes between progress and late of each polarization's phase.
It means we have progress of p polarization phases at first and suddenly, late of this phase takes happens. This Fano shaped in delta spectrum and thus plasmon induced transparency takes place in 670 nm also as shown in two graphs of Transmission and psi and also delta in Figs. 4(a) and (b).
By excitation the sample via 980 nm laser to enhance the absorption of the WS2 medium and also thermal expansion of this 2D material, we have at the first glance, LB phenomena for S polarization by 90% as shown in Fig. 4(c); in spite of this fact that, we have plasmon induced transparency like as previous case in Transmission and Psi graphs (Fig. 4(d)). In addition, the phase difference between these two cases with and without laser excitation show deeply changes in the comparison between Figs. 4(b) and (d). Furthermore, due to the anisotropic deformation of WS2, which can be altered by altering the temperature induced phenomena by the aid of thermo-plasmonic and also thermal expansion, LB phenomena take happens with a specific polarization. In general, photoconductivity means that the imaginary portion of the dielectric function, which is connected to the real portion of the complex photoconductivity, can reflect the quantity of free carriers in the material when it is exposed to different light wavelengths. The photoconductivity of WS2 is zero when they are exposed to the light diffusing over the infrared area. The value of photoconductivity becomes clear when light energy rises to 3.12 eV for the WS2 single layer. WS2 monolayers can therefore be utilized as a light-activated switch [30].
Nevertheless, we investigate the effect of laser induced thermal expansion in the WS2 substance by switching manner recording as shown in Fig. 5(a) which confirms the shift in the main modes and also intensity changes by the sample in two different states with and without laser excitation. Switch manner of the device records in the main resonance modes by time dependence mode of the spectrometer for 60 seconds as shown in Figs. 5(b) and fall time sweeps which fitted by exponential delay as shown in Fig. 5(c). This indicated that the proposed switch represented the second decay rate of 11 s in 60 s and it means 0.18 as the switching time for the one second of excitation by laser light.
Fig. 5. (a) The Transmission spectrum of the sample for P polarization with and without laser excitation, (b) time dependence of intensity in the main mode and (c) The fitted switching manner.
Finally, to record the PM and LB in reflection mode of the sample under the thermo-plasmonic and also thermal expansion by laser excitation were recorded by Krestchmann setup as explained before in experimental part. For this purpose, we record the reflection in both polarizations and extract the amplitude and phase difference as psi and delta as explained before with and without laser excitation as shown in Fig. 6.
Fig. 6. (a) The phase difference, Δ, and psi, ψ, spectrum for the sample with and without laser excitation.
The PM as π-phase change in the sample under laser excitation appear in all of the visible region and mainly in the vicinity of the region which shows in Fig. 6(a). Because the material warms up due to optical absorption, the index of refraction changes over time, causing the phase or frequency shift that is of concern in this case.
The phase shift at the output through a sample for a Gaussian beam abruptly applied at t = 0 starts to increase linearly over time as a function of time and radius [31]
In which, ω and c are the angular frequency and velocity of optical wave; I0 is the intensity at beam center and w0 shows the radius of the beam to (1/e) point in amplitude; α and ρ correspond to the absorption coefficient and density of material; dn/dT shows the rate of change in the refractive index with temperature and cp is the specific heat at constant pressure of length of sample, L. For times longer than the thermal time constant t, defined as (K/ρcp), where K denotes thermal conductivity, thermal conduction becomes significant. When the temperature profile approaches steady state, the refractive index changes stop changing over time [31].
The Kerr material may experience a change in refractive index as a result of the incoming laser. On the other hand, the medium may experience a change in refractive index as a result of the light's phase shift as it propagates through the medium. The phase shift that occurs as a laser beam travel through the nonlinear 2D material dispersion is ($\Delta \psi $) [32].
In addition, temperature-related changes in the refractive index are adjusted as [33].
LB by thermal expansion and also thermo-plasmonic of the sample shows in Fig. 6(b). Because of its distinct electrical band structure, which includes a direct bandgap that effectively absorbs and emits light, WS2 is able to block light. Excitons, which are the bound states of an electron and a hole, are produced when light interacts with the WS2 material. The amount of light that is transmitted through the material decreases as a result of these excitons relaxing and emitting light at a lower energy. As a result, WS2's significant absorption of incoming photons with energies over its bandgap serves as the physical basis for LB. Furthermore, the LB occurs in the sample as shown in Fig. 6(b) along this fact that with sample temperature, which influences the electrical band structure and therefore the intensity of the Raman mode, a material's optical characteristics and how they change with temperature may also have an impact. In reality, resonance Raman occurs as the temperature rises because the B-exciton's energy is becoming close to that of the incident laser. Additionally, when the temperature rises, the transition energy of the B-exciton varies from the laser light's photon energy and the intensity drops which means we have LB phenomena [12].
In addition, in this sample, the electronic transition is closely related to the absorption coefficient, which is analogous to the imaginary portion of dielectric functions in the low energy area (0.00 eV - 3.50 eV). As a result, it has been discovered that the WS2 monolayer has an absorption coefficient in the infrared and visible area of around 0.2 eV. However, in the high energy area, a high run chart in absorption coefficient appears in comparison to the imaginary portion of dielectric functions due to the obvious impact of incoming light (ω) on absorption coefficient (α) as $\alpha = 2K\omega /c$ . This explains why these single layers have a high near-ultraviolet absorption coefficient [30].
IV. Conclusion
A miniaturized light blocker and phase modulator was produced based on plasmonic nano gap in donate shaped ring arrays which covered by two- dimensional WS2 monolayer. Plasmonic substrate was produced experimentally and investigate theoretically by laser writing system and 3D finite-difference time-domain method. The main optical modes of the plasmonic substrate were investigated by ellipsometric parameters and we have mode coupling via analogy to bonding and anti-bonding coupling in the structure. The thermo-palsmonic effect by near field excitation and the thermal expansion of the WS2 by probe laser irradiation can introduce phase modulation and light blocking in the system due to nonlinear properties of WS2 monolayer.
Acknowledgments
Author Contributions H. M. Hamodi design the main sample, measure all the processes and also write the main text, R. S. Fyath and S. M. Hamidi supervised the work and edit the main text.
Disclosures
There is no any conflict of interest.
Data Availability
Data in this manuscript are available by request from the corresponding author by email to m_hamidi@sbu.ac.ir.
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