All-optical AZO-based modulator topped with Si metasurfaces

All-optical communication systems are under continuous development to address different core elements of inconvenience. Here, we numerically investigate an all-optical modulator, realizing a highly efficient modulation depth of 22 dB and a low insertion loss of 0.32 dB. The tunable optical element of the proposed modulator is a layer of Al-doped Zinc Oxide (AZO), also known as an epsilon-near-zero transparent conductive oxide. Sandwiching the AZO layer between a carefully designed distributed Bragg reflector and a dielectric metasurface—i.e., composed of a two-dimensional periodic array of cubic Si—provides a guided-mode resonance at the OFF state of the modulator, preventing the incident signal reflection at λ = 1310 nm. We demonstrate the required pump fluence for switching between the ON/OFF states of the designed modulator is about a few milli-Joules per cm2. The unique properties of the AZO layer, along with the engineered dielectric metasurface above it, change the reflection from 1 to 93%, helping design better experimental configurations for the next-generation all-optical communication systems.

www.nature.com/scientificreports/ Generally, zinc oxide (ZnO) is an attractive TCO due to its wide bandgap of about 3.5 eV, efficiently high conductivity, easy doping, thermal stability in the presence of III group elements, abundance in nature, and nontoxic nature 31,32 . The conductivity of this material can significantly increase by high levels of doping 33 .
One may deposit Al-and Ga-doped ZnO using methods like sputtering 34 , MBE 35 , PLD 26 , CVD 36 , and ALD 37 . Thin films of aluminum-doped zinc oxide (AZO) exhibit electrical and optical properties that vary with deposition conditions (doping level, temperature, and oxygen adsorption during deposition) and post-deposition treatment conditions (oxygen desorption during thermal annealing) 31 .
Due to AZO's non-stoichiometric nature, the deposition conditions largely influence its physical properties. We can tune the ENZ wavelength by controlling the deposition conditions (like changing the Al content, altering the deposition temperature, or varying the film thickness) or the post-deposition thermal annealing conditions 38 . Moreover, adding ZnO buffer layers and protecting layers (e.g., HfO 2 or Al 2 O 3 ) also prevents Zn evaporation upon high-temperature treatment 39 .
Low-loss AZO layers with very high carrier concentration are required to obtain ENZ properties associated with the telecommunication wavelength range (1260-1625 nm) or to secure the transparent biological spectrum (600-1350 nm) 40 . The ALD-grown AZO often holds poor optical properties and deficient carrier concentrations due to low efficiencies in heavily doped cases (> 10 20 cm −3 ) 39 . For other techniques such as MBE, PLD, CVD, and sputtering, synchronous application of Zn and Al species with accurately controlled flux ratios for a broad range gives rise to uniformly distributed Al or Ga and efficiently highly doped species 41 .
In optoelectronic modulators or switches, by exerting voltage and electrical field on the TCO, a few nanometers accumulation layer appears temporarily at the interface of TCO and dielectric materials 42 . In this mechanism, the energy consumption will be about a few fJ/bits, but RC delay is the limiting factor for modulation speed. Energy consumption and modulation speed are both the primary figures of merits 43 . However, all-optical modulators are excellent options for reaching ultrafast modulation 30 . There is always a trade-off between the modulation speed, insertion loss (IL), and modulation depth (MD) 44,45 . Therefore, scholars continuously try to design the structure with maximum MD and modulation speed. Although the all-optical modulators have absorption/recombination time limitations that influence modulation speed, by low-temperature fabrication techniques, one can control the bulk and surface recombination centers and nanoparticle trapping to reduce the recombination time 46 .
Depositing AZO with extreme oxygen deficiency leaves a large density of oxygen vacancies in the film, providing additional intrinsic carriers and generating deep-level defects 42 . These deep-level defects reduce the carriers' recombination time drastically.
In this work, we propose an all-optical modulator, realizing a highly efficient MD, very low IL, and great modulation speed. Here, we demonstrate the application of an AZO thin film as an optically tunable layer for the modulation of C-band telecommunication window wavelength. Sandwiching AZO film between a distributed Bragg reflector (DBR) and a top patterned metasurface provides the desirable optical properties for the proposed modulator. The fascinating modulation capability of AZO along with its piezoelectric property 47 can make it an appropriate candidate for applications like birefringence control 48 and optical polarizer 49,50 .
The rest of the paper is as follows: first, we elaborate on the proposed structure of the modulator, containing its geometry and functionality. Then, we study the optical tunability of the AZO and its dependent properties on the illuminated light, followed by a brief suggested experimental setup to consider its feasibility besides simulation results. The next step includes the results and verification of the ability of the all-optical AZO-based modulator, topped with a metasurface, to earn a promising figure of merit (FOM). The results show superb potential for the designed AZO-based modulator for integration into next-generation all-optical telecommunication systems.
The proposed structure. The light employed as the controlling signal in the proposed all-optical modulator has photons of energy higher than the bandgap of the active material to excite the valance band electrons to the conduction band through interband excitation. These excess carrier concentrations penetrate the bulk to a certain depth and alter the optical permittivity and refractive index accordingly, shifting the reflectance and absorbance spectra. Figure 1 illustrates a three-dimensional perspective schematic of the proposed all-optical reflective modulator structure. As depicted in this figure, the proposed modulator consists of a distributed Bragg reflector (DBR) formed by a twenty-pair stack of quarter-wavelength SiO 2 /Si 3 N 4 deposited on a sapphire substrate. We designed the DBR to reflect the range of incident light almost entirely, assumed as the structure loss. The 120-nm thick oxygen-deprived AZO layer deposited on top of the DBR has an essential role in the tunability of the optical modulator. The metasurface consists of a 2D array of cubic Si gratings patterned on top of the structure, coupling the incident light as a guided-mode resonance (GMR). We have optimized its geometry that highly affects the GMR wavelength according to the procedure we have already described elsewhere 43 to achieve the critical coupling condition for maximum absorption and minimum reflection at λ = 1310 nm in the modulator OFF state. In other words, when the pump pulse is OFF, the reflectivity at 1310 nm drops to near zero, but in the presence of a pump pulse, it becomes more than 0.9. Table 1 tabulates the physical and geometrical parameters of the structure shown in Fig 1. To show how one may employ the proposed modulator in practice, we schematically depicted a suggested experimental setup in Supplementary (Fig. S1).
Modulation principles. The SiO 2 /Si 3 N 4 DBR is a perfect mirror for 1305 < λ < 1315 nm, with negligible reflectance. In this regard, the AZO layer and the grating topped this layer bear the principal modulation role. www.nature.com/scientificreports/ AZO has a metal-like behavior in the infrared spectrum and is highly reflective. On the other hand, in the visible spectrum, AZO is highly transparent and has dielectric-like properties. The intersection of these two regimes occurs at the ENZ.
The optical pump with λ = 327.5 nm is incident to the structure. At this wavelength, the photons have enough energy to excite the valence band electrons, changing the carrier concentration of the AZO layer and subsequently modifying the permittivity and refractive index of the AZO 31 . One can calculate the plasma frequency ω p via 9 , Here, ε opt is the optical permittivity, n opt is the optically generated carrier density, and m eff is the electros' effective mass. Equation (1) modifies the complex permittivity of the AZO through the Drude-Lorentz model at a near-IR frequency ω 9,51,52 , wherein γ L and ω τ are the damping coefficient and frequency, and ω L represents the Lorentz resonance frequency.
The wavelength-dependent modulation depth of the modulator can be determined from its reflection spectra in the OFF (R OFF (λ)) and ON (R ON (λ)) states obtained via numerical simulation by COMSOL Multiphysics 53 Moreover, the modulator insertion loss, is also of practical importance because it directly impacts the system's efficiency 54 . As mentioned before, it is difficult to further improve the modulation speed in electro-optical modulators due to the functional response time limitations of the electrical section [55][56][57] . Nevertheless, all-optical modulation can overcome the modulation rate limitations by using one light beam to control the transmission/reflection of another light beam 30,58 . Several papers have reported all-optical modulation schemes with high speeds of 200 GHz based on graphene devices, in which a graphene sheet covers each structure. As a result of the required low insertion loss, these schemes have relatively low modulation depth or modulation efficiency [59][60][61] .
One of the essential characteristics of TCOs is the maximum achievable change in the optical properties, which in this case, is the result of excess carrier generation.
By gating the active layer in electro-optical modulation, only a few nanometers thick accumulation layer forms at the TCO/dielectric interface. Nonetheless, in all-optical modulation, the optical excitation is more than ten times throughout the bulk 26 .

Results and discussion
We employed finite element methods (using COMSOL Multiphysics) to simulate the optical behavior of the proposed AZO layer under pump pulse and also analyze the entire device under optically ON and OFF conditions, along with the application of MATLAB for MD and IL calculations.
In this virtual experiment, we illuminate the metasurface simultaneously by the light signal of wavelength λ s = 1310 nm and a quarter wavelength optical pump pulse (λ p = 327.5 nm). The signal's photons energy is less than the AZO layer bandgap. Hence, in agreement with the experimental studies [18][19][20] , reporting AZO with ~ 80% transparency for a bandwidth including 1310 nm, it enjoys the same transparency for the incident signal photons. Notice that the fit parameters used for as-deposited AZO film with Aluminum content of 1.9% 9,51,52 took the actual AZO layer transparency into account. On the other hand, the AZO layer bandgap is smaller than the pump photons' energy, absorbing them. This absorption can result in the generation of excess carriers of ~ 4×10 20 cm −3 , which is essential for the modulation operation, using the total pulse fluence of only a few milli-Joules (2.41 mJ•cm −2 ). Figure 3a depicts a 3D profile of the excess carrier concentration generated within the volume of an AZO unit cell of dimensions 900 × 900 × 120 nm, optically pumped at λ = 327.5 nm with the fluence of 2.41 mJ•cm −2 . Figure 3b shows the penetration depth of the maximum carrier concentration carriers generated within the 120-nm thick AZO layer. Our calculations show the estimated average carrier concentration within the top 60 nm of the AZO layer under optical pump pulse irradiation is ~ 1.38×10 21 cm −3 (with a maximum of ~ 1.54×10 21 cm −3 ). The carrier concentration within the bottom 60 nm almost equals the initial carrier concentration. The difference in the carrier concentrations changes the plasma frequency of the top segment of the AZO layer, Eq. (1), hence changing its refractive index (Eq. 2).
Here, we demonstrate how the pump fluence affects the mean carrier concentration of the AZO layer, thereby altering the AZO layer's permittivity through Eqs. (1) and (2). The solid blue circles in Fig. 4 (the left axis) depict the resulting mean carrier concentration versus the pump fluence. On this curve, we can see the mean carrier concentration increases in a nonlinear manner as the pump fluence increases, shifting the GMR of the device to its ON state. As a result, the reflected signal varies at each optical pump fluence, as shown by the open pink circles (the right axis).  www.nature.com/scientificreports/   www.nature.com/scientificreports/ Figure 5 shows the numerical results for the spectral response of the proposed modulator in the OFF and ON states, obtained via the COMSOL Multiphysics simulations. The solid magenta line demonstrates that when the pump pulse is OFF, the modulator structure reflects the incident signal in a vast part of the wavelength range of 1305 < λ < 1315), except in a narrow bandwidth (FWHM = 0.13 nm) about λ = 1310 nm, where R OFF, min ≈ 0.8%. It is worth mentioning that this nearly perfect (99.2%) absorption is due to the appropriately engineered Si metasurfaces satisfying the critical coupling condition at λ = 1310 nm. Moreover, any deviation in the metasurface geometry from the dimensions given in Table 1, alters the narrow linewidth at 1310 nm 43 . The linewidth of the reflectance in the OFF state indicates the device quality factor Q > 10,000. Such a narrow bandwidth profile with a sharp peak agrees with the result of an experimental study, showing the feasibility of obtaining narrow linewidth and high-peak response for the guided mode resonance reflection filters 62 . For demonstrating the effects of DBR and the AZO layer on the modulator's total reflectance, we assessed the optical response-i.e., reflectance, transmittance, and absorbance-of the DBR and the DBR topped with the AZO layer as plotted in Fig. S2 of Supplementary. Turning on the pump pulse with photons energy higher than the AZO layer bandgap generates excess carriers, modifying the AZO layer plasma frequency and permittivity. According to the data shown in Fig. 4b and Eq. (1), the excess carrier concentration generated within the top 60 nm of the AZO layer (ON state) changes the permittivity of this segment from ε OFF ≈ 0.831+0.025i to ε ON ≈ − 0.213 + 0.055i. The resulting decrease in the Re [ε(ω)] causes a blue shift in the resonant frequency of the designed Si metasurfaces. On the other hand, the increase in the Im [ε(ω)] makes the AZO layer more absorptive. These can be observed from the blue dots in Fig. 5, indicating a 3-nm blue shift in the reflection's minimum of the ON state (R ON, min ≈ 0.719). The solid magenta line in Fig. 5 represents the spectra for the OFF (R OFF ) state.
Employing Eq. (3) and using the data shown in Fig. 5, we have calculated the modulator MD spectrum (Fig. 6). The results show the proposed modulator enjoys the modulation depth of MD ≈ 22 dB at the resonant wavelength of λ = 1310 nm, with an acceptable insertion loss as low as IL = 0.32. In most cases, like in high-speed interconnections or high-sensitivity sensors, a high modulation depth of > 7 dB is desirable. Hence, the simultaneous high MD and low IL values demonstrated by the proposed all-optical AZO-based modulator topped with Si metasurfaces make it unique among other all-optical counterparts. According to the composition of the AZO layer specified in this study, a modulation speed greater than 1 THz with a relaxation time of fewer than 500 fs is achievable 26 .

Conclusion
Conquering the RC delay of electro-optic modulators should be mentioned as one of the main priorities alloptical modulators possess. In this numerical study, by recruiting a 120 nm AZO layer over a 20-pair Si 3 N 4 /SiO 2 DBR, appropriately designed for near-perfect reflection within a vast part of the wavelength range including 1305 nm < λ < 1315 nm except for a narrow band around λ = 1310 nm, that is covered with 700 × 700 × 43 nm Si metasurfaces, we obtained a highly efficient modulator. An optical pump pulse at λ = 327.5 nm is irradiated to the AZO layer to change the intrinsic carrier concentration of the top 60 nm of this layer from 9.8 × 10 20 to 1.38 ×10 21 on average. Carrier concentration variation leads to tuning the resonant wavelength of the Si metasurfaces and consequently shifts the reflection spectrum of the modulator. The simulation results show the proposed alloptical modulator enjoys modulation depth and insertion loss of MD = 22 dB and IL = 0.32 dB, with a pump fluence level of 2.41 mJ•cm −2 .

Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding author upon reasonable request. www.nature.com/scientificreports/