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Abstract
Spatial light modulators (SLMs) are important for various applications in photonics, such as near-infrared imaging, beam steering and optical communication. After decades of advances, current commercial devices are typically limited to kilohertz modulating speeds. To realize higher operating speeds, an electro-optic (EO) polymer and silicon nitride hybrid SLM has been demonstrated in this work. We utilize a specially designed metasurface to support a relatively high quality resonance and simultaneously confine most of the incident light in the active EO polymer layer. Combing with the high EO coefficient of the polymer, a clear modulation at 10 MHz with a driving voltage of Vp-p=±10 V has been observed in the proof-of-concept device. Our first-generation device leaves vast room for further improvement and may open an attractive route towards compact SLM with an RF modulation higher than 100 GHz.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Spatial light modulators (SLMs) are a kind of devices that manipulate the intensity or phase of a light beam propagating in free space [1] . Driven by the demand of active and tunable photonic devices in the market, SLMs have attracted more and more research interest during the last decade. Current techniques for SLMs mainly employed liquid crystals (LC) [2,3], micro-electromechanical systems (MEMS) [4,5] and digital micro-mirror devices (DMD) [6]. Despite the exciting progress, they suffer from intrinsic material or device structure properties that block them for high-speed applications. The LC SLMs have the advantage of low modulating voltage, but the response time of LC materials is in the millisecond regime [7,8] . MEMS and DMD SLMs also exhibit low voltage operation but the modulation speeds are generally limited to a few kHz due to the slow mechanical response [1,9].
In contrast to the aforementioned techniques, electro-optic (EO) modulations based on carrier plasma dispersion, electro-absorption or Pockels effect are intrinsically ultrafast processes with the ability to provide high-speed operation [10–13]. State-of-art EO modulators utilizing optical waveguide structures have demonstrated amplitude and phase modulation at speeds exceeding 100 Gbps [14]. If, the EO modulations could be employed, high-speed SLMs should find attractive applications in optical communication modules, fast programmable optical tweezers and so on [15] . Recently, 2D material EO SLMs have been exploited by embedding graphene as EO materials inside slickly designed photonic structures [16–18]. In most of these demonstrations, however, the operating wavelength was limited to the mid-infrared or longer range [18–20]. For various applications, such as optical communication and imaging, SLMs working at near-infrared wavelength with a low driving voltage are highly demanded.
Organic electro-optic (EO) polymers have shown their potential in recent years owing to their high EO coefficient (r33) >100 pm/V [21], ultrafast EO response time (less than 10 femtoseconds) [22], and straightforward spin-coating fabrication process. These attractive features of EO polymers have empowered diverse and outstanding waveguide modulators [23,24]. Previously, we have demonstrated various EO polymer waveguide modulators with high EO coefficients, low driving voltages and high-speeds up to 200 Gbps [25]. As a result, if EO polymer is applied in SLMs, it may promote a high-speed modulation, a low driving voltage and a relatively simple fabrication.
In this work, we design and fabricate an EO polymer SLM based on a photonic metasurface. The metasurface is constructed by periodic Si3N4 arrays covered by 10 nm-thick Au and 1.8 μm-thick EO polymer. The specially designed metasurface can not only support a resonance with a high quality (Q) factor but also confine most of the light in the active EO polymer layer. In the SLM, the EO polymer is sandwiched directly between the top and bottom electrodes, thus significantly increasing the electric poling and modulating field applied on the EO polymer layer. Combining all of these merits together, the resulting fabricated SLM exhibits an in-device EO coefficient r33 as high as 108 pm/V and a clear modulation at 10 MHz with a low voltage Vp-p=±10 V. Such device is also ideal for RF photonics application, and the high speed response of the SLM is possible to be beyond 100 GHz with an improved electrode structure.
2. EO polymer SLM design
The schematic of the EO polymer SLM is depicted in Fig. 1. It is composed of an EO polymer with a thickness of 1.8 μm sandwiched between the top Au electrode (80 nm–thick) and the bottom ITO. Consequently, a z-direction electric field can be loaded on the EO polymer under applied bias by taking advantage of the ITO as the bottom electrode and the Au as the top electrode. Since the EO polymer exhibit relatively modest refractive index change under applied bias, a metasurface is designed on the ITO surface to support high-Q resonant mode to exhibit significant EO modulation. The metasurface consists of periodical silicon nitride (SiN) arrays, on top of which a 10 nm ultra-thin Au layer is deposited. The thickness h and width w of the SiN is 0.3 μm and 0.5 μm, respectively. The lattice spacing of SiN array is set as p in order to optimize the resonant performance.
As a plane-wave light is normally incident to the device as shown in Fig. 1, it will excite resonant mode in the SLM due to the metasurface. Therefore, we can obtain a resonant dip in the reflected spectrum. Our modulation scheme is to tune the in-device refractive index via the Pockels effect of the EO polymer and thus to change the resonance wavelength. In the SLM, the in-device refractive index change Δn can be expressed as, [26]
where n is the effective refractive index, r33 denotes the EO coefficient of the EO polymer and Γ represents overlap factor between the optical field and the electrical field E. As aforementioned, E is along z direction due to the vertical electrode structure, so Γ can be considered as the portion of the optical mode confined in the EO polymer. To achieve a high electric-field dependent refractive index change Δn/E, majority of the optical mode would need to reside within the EO polymer layer. Figure 2(a) shows the simulated resonant mode distribution in the SLM when p = 0.8 μm. It can be observed that most of the optical mode is confined in the EO polymer. For comparison, we also simulate the resonant mode in the same structure but without the ultra-thin Au layer as shown in Fig. 2(b). Obviously, the 10 nm Au deposited on the SiN results in more light confinement in the EO polymer. According to our simulations, Γ is 90% in Fig. 2(a), but only 70% in Fig. 2(b). As a result, a more effective EO modulation can be expected in our designed SLM.
Fig. 1. 3D illustration of the proposed structure: it consists of an Au back plane, EO polymer (the structure of the used chromophore is shown inside), and a thin ITO film on which we pattern SiN grating arrays. Unit cell dimensions are chosen as follows: width of the grating is w, thickness of the SiN grating is h, lattice spacing is p. The thickness of ITO, Au back plane and EO polymer are 0.1 μm, 0.08 μm and 1.8 μm respectively.
Fig. 2. Simulated electric field distribution at the the resonance wavelength (a) Au film deposited on the SiN arrays and (b) bare SiN arrays.
To create strong EO activity, EO polymer must be poled with an enough high DC voltage near its glass transmission temperature [21] . However, it remains a challenge to obtain a sufficiently high EO activity in traditional devices with inorganic claddings. Such a difficulty is attributed to the EO polymer’s much lower electrical resistivity compared with inorganic materials’ (such as SiO2, SiN and so on) [27] . In our design as shown in Fig. 1, the ultra-thin Au on the SiN serves as a “conductive medium” between the EO polymer and ITO, so all the DC poling or driving voltage can drop on the EO polymer layer. This will overcome the traditional difficulty, enabling a high poling efficiency and a low driving voltage. Figure 3(a) exhibits the simulated reflected spectra with different applied DC voltages. In the calculation, the EO coefficient of the EO polymer is set as 100 pm/V. From the spectra, it can be seen that the SLM hold an extinction ratio of 11 dB and a Q-factor of 448 under different biases. The tunability of the device is determined by calculating reflection as a function of resonance peak wavelength with the device biased at several DC voltages. The resonant wavelength versus the different applied voltage is plotted in Fig. 3(b), which shows the tunability of the modulator as 0.029 nm/V.
Fig. 3. (a) Simulated results of the reflected spectra under different voltages, and (b) resonance wavelength shifts under different applied DC voltages.
To identify the optimal structure parameters of the SLM, we firstly investigate the SiN lattice spacing p on the reflected spectra. The influence of p on the reflected spectra is shown in Fig. 4(a). With the increase of p from 0.76 to 0.82 μm, the Q-factor also increases, but the extinction ratio reaches the maximum value around p = 0.8 μm and then starts decreasing rapidly. In order to gain a high-Q resonance and a large extinction ratio, we choose p = 0.8 μm in this work. In our design, another critical technological parameter with the largest contribution to the SLM performance should be the nano-scale Au on the SiN array. Figure 4(b) shows relationship between the Au thickness and the reflected spectra. We find that the variation of the thickness from 5 to 15 nm has little influence on the reflected spectra.
Fig. 4. Simulated results of the reflected spectra and the Q-factor with (a) different lattice spacing and (b) different thickness of Au.
3. Experimental demonstration and characterization
We carried out a proof-of-concept experiment to demonstrate the ability of the designed SLM. A scanning electron microscopy (SEM) image of the fabricated device before spin-coating the EO polymer is shown in the inset of Fig. 5(a). The fabrication procedure is described in the fabrication section. The reflected spectra of the SLM were measured by using a home-built optical system as schematically shown in Fig. 5(a). A super continuum laser was used as the light source. The light was firstly collimated by a lens (L) and iris (I). A polarizer (P) was inserted after the iris to control the input light polarization. After passing a 50:50 beam splitter (BS), the light refracted by a mirror was finally focused by another lens onto the SLM. The reflected light from the device under test (DUT) was diverted by the BS and sent to an optical spectrum analyzer (OSA) (AQ6370D, Yokogawa).
Fig. 5. (a) Custom-build optical system of spectrum measurement: L is lens, P is polarizer, I is iris, M is mirror and BS is beam splitter, DUT is the device under test. The SEM image is the fabricated SiN arrays. (b) Experimental reflected spectra of the SLM with the absence of voltage and under modulate voltage of 70 V and (c) linear fitting of the resonance wavelength under different DC voltages.
Figure 5(b) shows the obtained reflected spectra that have been normalized by that measured in the un-patterned region. The reflected spectrum without applied bias exhibits a clear resonant dip at 1244 nm with a Q-factor of 145. The difference of the Q-factor between the experiment and simulation may be consequences of the deviation of the fabricated SiN lattice spacing and the sidewall roughness of the etched SiN [28]. The Q-factor could be enlarged by optimizing fabrication processes in future. To characterize the EO properties of the SLM, we applied different DC biases and observed the change in the reflection spectra. Under the voltage of 70 V as shown in Fig. 5(b), the resonance has an obvious wavelength shift of 2.2 nm. By the linear fitting of resonance wavelengths at different voltages in Fig. 5(c), the tunability of the modulator is extracted as 0.031 nm/V. To confirm the stability of the modulation, we applied the voltage up to 70 V dozens of times continuously and found that the tunability has no changes. In this study, the value of the tunability was obtained from the linear fitting of the data, involving OSA resolution, the applied biases, and the data points taken into account for linear fitting. These outer factors may have some influence on the value of the tunability, so the estimated error of the tunability is ±10%. Based on Eq. (1) and the tunability, we used FDTD simulations to deduce the actual in-device r33 of the EO polymer. The deduced in-device r33 is 108 ±11 pm/V. The obtained tunability and the in-device r33 of our SLM are comparable to those realized in the state-of-art waveguide-based EO modulators [29,30].
For verification of the SLM as the light intensity modulator, we fixed the input laser at 1241.8 nm to obtain the largest extinction ratio. The insertion loss is around 6 dB. The Vp-p=±10 V sine wave signal from a signal generator is applied to the electrodes via an electrical probe. Frequency of the applied voltage was 10 MHz. The measurement system was similar to that shown in Fig. 5(a), but the OSA was replaced with a photodetector which was then connected to an oscilloscope (Tektronix TDS2024C). The input electric signal and the output optical response monitored by the oscilloscope are shown in Fig. 6. It is clear that the fabricated SLM has an ability of fast modulation. The change of the output light intensity by the modulation depth is around 1 dB at this voltage. This modulation depth may be greatly increased by optimizing the fabrication processes to enlarge the Q-factor as aforementioned. If the designed Q-factor of 448 could be realized, the modulation depth will be increased by at least factor of 3.25, such that values of higher than 3.25 dB can be expected.
Fig. 6. (a) Temporal response of the device at the frequency 10 MHz with Vp-p=±10 V the black curve represents the electric signal, and the red curve is the detected optical signal.
In this work, we only measured the device at a frequency of 10 MHz. The high frequency modulation of the current SLM is not limited by the device concept but is a consequence of the non-optimum electrical design of our first-generation structure. In an EO resonator modulator, the 3 dB modulation bandwidth (f3dB) is determined by the photon lifetime 1/fτ and the RC time 1/fRC, as expressed by [31]
Here, τ=λQ/(2πc) is the cavity photon lifetime (c is the light speed in vacuum and Q is the quality factor), R is the contact resistance and C is the device capacitance. By using the measured Q-factor of 145 and wavelength of 1241.8 nm, fτ can be calculated to be around 1.66 THz. In contrast, in order to facilitate the electrical probe loaded on the SLM, the top-electrode pad of 4×4 mm2 was used in this work. This leads to significant RC delay [32], which increases strongly with frequency. According to our calculations, fRC is estimated to be 10.6 MHz under such a large electrode. Since the incident light spot diameter is around 50 μm2, the top-electrode with a size of smaller than 100 μm2 is reasonable and hence the low (femtosecond) RC time readily permits fRC up to several hundred gigahertz in future work [10,12,33]. As a result, we would expect the EO response of the SLM could be operated > 100 GHz with an improved electrode structure.
4. Conclusion
In conclusion, we have demonstrated an EO polymer and SiN hybrid SLM based on a photonic metasurface. The modulation originates from the variation of the resonance wavelength due to the Pockels effect of the EO polymer. The hybridization of the SiN metasurface with the EO polymer enables strong field confinement and achieves extinction ratio of 3.6 dB under the DC voltage of 70 V. A fast modulation speed of 10 MHz is realized under the modulate voltage of Vp-p=±10 V. If we further minish the electrode, we believe that the combined advantages will improve the frequency up to hundred gigahertz. The proposed concept of SLM may open new opportunities for future dynamic holograms, nano projectors, and high-speed special communication devices.
5. Methods
Device Fabrication: The proposed device was fabricated via standard thin film deposition processes and E-beam lithography techniques. Each layer was stacked in order from bottom (ITO) to top (Au). First, we deposited a 100 nm ITO layer on a quartz substrate using radio frequency (RF) sputtering in Ar-O2 plasma. The sample was annealed to modify the permittivity of ITO, which was verified by using spectroscopic ellipsometry. The pattern was fabricated on a 0.3 μm-thick SiN thin film which was prepared by using plasma-enhanced chemical vapor deposition (PECVD). Afterwards, the SiN pattern was performed by E-beam lithography (EBL) and development (ma-N 2405 as the photosensitive film and ma-D 525 as the developing agent). Etching was carried out via inductively coupled plasma (ICP). Subsequently, the device was cleaned with an O2 plasma treatment. Thin Au film was deposited on the SiN pattern by using thermal evaporation. Next, we spin-coated EO polymer on the device and baked it over 12 hours. A guest-host polymer was used in this work, which was composited of PMMA and chromophore (as shown in Fig. 1). Finally, the Au electrode layer was using thermal evaporation as the deposition mechanism (the rate was 0.5A S-1). To induce the EO effect, the polymer was poled at the temperature of 96℃ with a poling voltage of 85 V. The poling temperature was increased to 96℃ at a rate of 10℃/min. The sample was kept at 96 ℃ for 5 min, and then was cooled to room temperature rapidly.
Simulations: We used the commercial software Lumerical to simulate and optimize the reflected spectra of the different structures. In the simulations, a plane wave excitation was used. The boundary conditions are perfectly matched layer along z direction and periodic boundary along x and y directions. In the simulations, all of the refractive indices based on the measured results by the spectroscopic ellipsometry, as SiN 1.92, ITO 1.63 and EO polymer 1.62.
Funding
National Natural Science Foundation of China (62075184).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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