Electrically Switchable Diffraction Grating Based on Electrophoretic Redistribution of Carbon Black Nanoparticles

ABSTRACT

We investigate a novel concept of a switchable diffractor in which the slit spacing can be switched by controlling carbon black (CB) nanoparticles. The slit width varies depending on the driving mode, where the nanoparticle positions are controlled by vertical electric fields generated between the planar electrode on the top substrate and the interdigitated electrodes on the bottom substrate. The diffraction angle of the proposed diffractor can be adjusted by controlling the slit spacing, operating in either wide‐slit or narrow‐slit modes. The diffractor also exhibits bistability, maintaining both slit modes without an applied field when the solution is optimized with a thickening agent. Furthermore, the device demonstrates a wide operating temperature range from −50°C to at least 100°C. In addition, the CB diffractor operates independently of the polarization state of incident light. Accordingly, this tunable diffractor utilizing CB nanoparticles exhibits outstanding potential for application in various advanced photonic devices.

Electrically reconfigurable diffraction gratings based on electrophoretic carbon black (CB) nanoparticles are demonstrated. Voltage‐driven particle redistribution forms periodic absorbing patterns with switchable wide‐ and narrow‐slit diffraction modes. The proposed diffractor exhibits negligible polarization dependence, wide‐temperature operation from −50 to 100°C, and bistable functionality, enabling low‐power tunable photonic and beam‐steering applications.

1. Introduction

Optical diffraction technology has been widely studied for manipulating light propagation and steering optical beams. Optical diffraction gratings redistribute incident light into diffraction orders and when illuminated by polychromatic light spectral dispersion can be achieved. These properties play a pivotal role in contemporary spectroscopy, optoelectronics, and the fabrication of nanomaterials, showcasing a wide range of applications such as precise wavelength division [1], laser beam modulation [2], storage media [3, 4], optical tomography [5], and lithography masks [6]. Recent studies have focused on tunable diffractive systems, capable of dynamic control of optical responses through electrical, thermal, or optical stimuli.

Liquid crystal materials have been extensively investigated for a wide range of photonic and optoelectronic applications, including polymer‐network composites [7, 8, 9, 10, 11], low‐power liquid crystal displays [12, 13, 14], photoluminescence modulation media [15, 16, 17], and electrically tunable diffraction gratings [10, 18, 19, 20, 21]. Among these applications, LC‐based diffraction gratings have attracted considerable attention because their optical properties can be dynamically controlled by an external electric field. However, conventional LC diffractors generally rely on birefringence‐induced phase modulation and therefore exhibit polarization‐dependent diffraction characteristics. Although such behavior is advantageous for polarization‐selective applications, it can limit their use in optical systems requiring polarization‐insensitive operation. To overcome this limitation, diffraction gratings employing optically isotropic LC states have been developed. Representative examples include nanostructured polymer‐dispersed liquid crystals [22, 23] and polymer‐stabilized blue‐phase liquid crystals [7, 9], which exhibit significantly reduced polarization sensitivity compared with conventional nematic liquid crystal diffractors. Nevertheless, these optically isotropic liquid crystal diffractors require continuous voltage application to maintain their diffractive state, leading to an increase in power consumption. In addition, polymer‐based liquid crystal composite devices may be concerned about long‐term stability issues, such as materials degradation or photo‐induced discoloration depending on their chemical structures. Furthermore, liquid crystal‐based diffraction gratings are in general optimized for operation within the visible spectral range.

In this study, we propose a novel strategy for electrically tunable diffraction based on the electrophoretic manipulation of carbon black (CB) nanoparticles dispersed in a dielectric fluid [24, 25]. The key innovation of this work is the realization of an electrically reconfigurable diffraction grating formed by electrophoretically controlled CB nanoparticles, enabling dynamic switching of diffraction angles together with negligible polarization‐dependent optical modulation. Owing to the intrinsic broadband absorption of carbon black, the proposed device is capable of modulating light over a wide spectral range extending from the UV to the near‐infrared (NIR) region [26]. When an electric field is applied across interdigitated electrodes patterned on a substrate, the charged CB nanoparticles migrate through electrophoresis and accumulate near selected electrode branches. This electrically driven particle redistribution generates a spatially periodic absorption profile that predominantly functions as an amplitude diffraction grating, although secondary contributions from scattering and weak refractive‐index modulation may also be present. By dynamically controlling the distribution of CB nanoparticles, the diffraction characteristics of the device can be reversibly tuned in real time, primarily through modulation of the spatial absorption profile.

Consequently, we introduce a strategy for optimization that successfully expands the tunability of diffraction modes by means of selectively applying electric fields to different branches of electrodes, which makes the absorbing pattern can be electrically changed. For example, applying a field to all electrode branches forms a grating with the same periodicity as the interdigitated electrodes, whereas applying a field to alternating branches generates a grating with twice the period. This selective actuation thus enables electrical switching between different diffraction modes without modifying the device physically.

To reduce power consumption by keeping the diffractive state after turning off the applied electric field, we incorporate a thickener into the CB‐dielectric fluid mixture. This additive may provide higher viscosity, which suppresses particle redistribution and slow relaxation, allowing the light‐absorbing pattern to remain relatively stable over time. In contrast to traditional liquid crystal‐based diffractors, the proposed device based on CB nanoparticles offers several advantages. The optically isotropic characteristics of CB result in substantially polarization‐insensitive optical modulation. The broadband absorption of CB allows operation over wide spectral range from UV to NIR. In addition, the possibility of polymer‐related degradation or discoloration are reduced. Also, CB functions across a significantly broader temperature range. These characteristics make the proposed CB nanoparticles‐based diffractor a promising platform for tunable photonic devices and broadband optical modulation systems.

2. Experiments

2.1. Materials

In order for the CB particles to function as a voltage‐driven diffractor, they should be well dispersed into a dielectric liquid without agglomeration during switching between on‐ and off‐states and maintain the nanoparticles’ original charge state. To meet such requirements, the CB particles were coated with silica and silane and dispersed in Isopar M/Halocarbon 0.8 oil (Nspectra Co., Korea) [26] at an optimal ratio as a dielectric liquid. A high concentration of CB particles, such as 10 wt/vol%, was well dispersed in the dielectric liquid without showing any sedimentation.

2.2. Characterization

Characterization of the CB suspension was performed using a nanoparticle analyzer (ELSZneo, Otsuka Electronics). The zeta potential of the as‐received CB suspension was measured to be −30.09 mV, indicating that the particles were negatively charged and exhibited sufficient electrostatic stability in the dielectric solvent. After the addition of the thickener, the zeta potential changed to −35.55 mV, suggesting a slight modification of the electrokinetic properties of the suspension. The average particle size and viscosity values were provided by Nspectra. Upon addition of the thickener, the average particle size increased from 195.0 to 270.5 nm, while the viscosity increased from 4.2 to 24.9 cP. These characterization results are summarized in Figures S1 and S2.

2.3. Cells

A CB diffractor was fabricated with indium‐tin‐oxide (ITO)‐coated plain and interdigitated electrodes. The interdigitated electrodes have a width (w) of 7 µm, a distance between electrodes (l) of 15 µm, and a cell gap (d) of 11 µm. Additionally, an organic insulating layer was coated on both electrode surfaces to prevent electrical shorts.

2.4. Electro‐Optic Measurement

The behavior of CB particles was observed using an optical microscope (Eclipse E600 POL, Nikon) equipped with a CCD camera (DXM 1200, Nikon). The experimental setup used to study the diffraction behavior of the grating is shown in Figure 1. The diffraction efficiency was determined by measuring the transmittance change of the diffracted beam from a He–Ne laser (λ = 632.8nm) in response to a DC voltage applied to the CB diffractor. The distance between the cells and a screen was fixed at 33 cm to realize far‐field diffraction. To regulate the polarization of the light source for the diffraction measurement, a polarizer was placed in front of the laser source, and an aperture was placed in front of the photodetector to accurately measure the intensity of the order of diffracted beams. A photodetector recorded the change in transmittance as diffraction signified by the applied field. Snapshots of diffraction spots on the screen placed behind the cell were also recorded simultaneously using a camera (iPhone 15 Pro, Apple).

FIGURE 1.

Experimental setup used for characterization of diffraction and polarization dependence of the proposed CB nanoparticle electrophoretic diffractor.

3. Results and Discussion

3.1. Switching Principle and Optimal Driving Electric Field Strength

The structure and switching mechanism of the proposed diffractor are shown in Figure 2. The top substrate has a plain transparent ITO electrode, while the bottom substrate has transparent interdigitated ITO electrodes. Both electrode surfaces are coated with polyimide‐type insulation layers. Negatively charged CB nanoparticles are dispersed in the dielectric liquid. CB exhibits broadband optical absorption, and uniformly dispersed CB nanoparticles within the cell at an optimal concentration and cell gap (solution occupying thickness) result in significant absorption of the incident light, producing an opaque dark state, as shown in Figure 2a. To rapidly achieve the dark state from the slit modes, a positive voltage is applied to the top electrode while a negative voltage is applied to the bottom patterned electrodes. In this configuration, a uniform vertical electric field is generated within the cell, driving the negatively charged CB particles toward the top electrode by electrophoretic motion, as illustrated in Figure 2b. The redistribution of CB nanoparticles results in the optically absorbing dark state.

FIGURE 2.

Schematic illustration of the operating principle of the CB nanoparticle diffractor. (a) Initial field‐off dark state and (b) vertically biased dark state obtained under an applied electric field. Diffraction modes with (c) a wide‐slit grating and (d) a narrow‐slit grating realized through electrophoretic redistribution of CB nanoparticles dispersed in a dielectric liquid.

To obtain diffraction states, two different driving methods are employed. First, as shown in Figure 2c, a negative voltage is applied to the upper ITO electrode while a positive voltage is applied to the odd alternating branch of the lower interdigitated electrodes, with the adjacent even alternating electrodes left electrically floating. The negatively charged CB nanoparticles are moving toward the positively biased electrode branches. Thus, the CB nanoparticles stick to the biased electrode line periodically, forming an absorbing stripe pattern with a grating pitch (Λ) of 44 µm. This configuration operates as a wide slit diffraction mode. Second, as shown in Figure 2d, a negative voltage is applied to the top electrode while a positive voltage is applied to all lower interdigitated electrodes. In this case, CB nanoparticles migrate toward all branches of the interdigitated electrodes, generating a narrow absorbing pattern with half of the pitch Λ = 22 µm.

To determine the optimal driving electric field for each diffraction mode, the motion of CB nanoparticles under an applied electric field was first observed using optical microscopy, and the diffraction intensities were subsequently measured as a function of the electric field strength. Figure 3 presents the optical microscopy images and the measured intensities of the 0th, 1st, and 2nd diffraction orders for both the wide‐slit (Figure 3a,c) and narrow‐slit (Figure 3b,d) modes.

FIGURE 3.

(a,b) Optical microscopy images of the CB nanoparticle diffractor under different applied electric fields in the wide‐slit and narrow‐slit modes, respectively. (c,d) Measured diffraction intensities of the 0th, 1st, and 2nd diffraction orders as a function of the applied electric field for the wide‐slit and narrow‐slit modes, respectively. (e) UV–visible transmittance spectra of the initial field‐off dark state and the vertically biased dark state.

In the wide‐slit mode, only a slight change in optical appearance is observed under the microscope at approximately 0.81 V/µm (Figure 3a,c). Although the first‐order diffraction intensity reaches its maximum near 0.4 V/µm, insufficient intensities are reached at this field for 0th and 2nd. At 0.81 V/µm, the intensity distribution among the diffraction orders is most clearly distinguished, as shown in Figure 3c. In the narrow‐slit mode, a similarly small change in optical appearance is observed at 1.27 V/µm (Figure 3b). As shown in Figure 3d, the zeroth‐ and first‐order diffraction intensities approach sufficient value at 1.27 V/µm, while the second‐order diffraction intensity reaches its highest level. Based on these results, the optimal driving electric fields were determined to be 0.81 V/µm for the wide‐slit mode and 1.27 V/µm for the narrow‐slit mode.

The optical densities of two different dark states, namely the initial field‐off dark state (Figure 2a) and the vertically biased dark state (Figure 2b), were quantitatively compared using UV–visible spectroscopy, as shown in Figure 3e and Figure S3. The ratio of the averaged transmittance of the biased dark state to that of the initial field‐off dark state was measured to be 1.19, 2.35, 1.64, and 1.29 for the 400–410, 500–510, 600–610, and 700–710 nm wavelength ranges, respectively. These results indicate that the biased dark state exhibits slightly higher transmittance than the initial field‐off dark state. Nevertheless, both states maintain low transmittance across the measured spectral range and effectively function as dark states.

3.2. Diffraction Characteristics

Figure 4 shows the optical microscopy images and diffraction patterns of the CB nanoparticle diffractor operated at 0.81 V/µm in the wide‐slit mode (Figure 4b,e) and at 1.27 V/µm in the narrow‐slit mode (Figure 4c,f). When voltage is applied to one branch of the interdigitated electrode while the adjacent branch remains floating, CB nanoparticles pile up on the biased electrode branches, resulting in an absorbing pattern of the wide‐slit mode with a larger effective grating period, as shown in Figure 4b. On the other hand, when voltage is applied to both branches of the interdigitated electrodes, CB nanoparticles pile up on both biased branches, resulting in an absorbing pattern with half the period in the narrow‐slit mode, as shown in Figure 4c. As a result, the effective grating periodicity can be electrically controlled by selectively biasing the electrode branches patterned on the bottom substrate.

FIGURE 4.

(a–c) Optical microscopy images and (d–f) corresponding diffraction patterns of the CB diffractor. (a,d) Initial field‐off state, (b,e) wide‐slit mode operated at the optimal driving electric field, and (c,f) narrow‐slit mode operated at the optimal driving electric field.

To verify the change in the diffraction angle (𝜃_m) caused by effective grating periodicity, we compared the experimentally measured diffraction angles with the theoretical values calculated using the equation mλ  =  Λsinθ_m where Λ is the pitch of the grating, m is the diffraction order (0, ±1, ±2, ±3, …), and 𝜆 is the wavelength of the incident light. In the wide‐slit mode, the grating pitch is Λ = 2(w + l), whereas in the narrow‐slit mode the pitch becomes Λ = w + l. As the grating period becomes half, it leads to doubled diffraction angles for the same diffraction order at the same wavelength of incident light.

Experimentally, the diffraction angle θ _m can be determined as, tan θ_m = D _m/L , where D _m is the separation between m _th and zeroth‐order beam on the screen, and L is the distance between the diffractor and the screen. The distance between the diffracted beams and the zeroth‐order beam was measured, and the diffraction angle was calculated.

The diffraction efficiency (𝜂 _m ) of the device was measured by a photodetector in front of the screen in the same optical setup used for the measurement of the diffraction patterns. The diffraction efficiency was v calculated as ${\eta }_{\mathrm{m}}(\%)=\frac{I_{\mathrm{m}}(V)}{I_0}\times 100$, where I _m (V) is the intensity of m _th order at voltage V, and I₀ is the incident light intensity measure in the field‐off state. Based on the measured values, the CB diffractor exhibited a first‐order diffraction efficiency approximately three times higher in the narrow‐slit mode than in the wide‐slit mode, as shown in Table 1. These results confirm that decreasing the grating period increases the diffraction angle and enhances the first‐order diffraction efficiency.

TABLE 1.

The measured and calculated values of diffraction depending on the driving method of CB diffractor.

Diffraction order	Wide‐slit mode				Narrow‐slit mode
	D m (cm)	𝜂m (%)	θm (o)	θ m (o) (theoretical)	D m (cm)	𝜂m (%)	θ m (o)	θ m (o) (theoretical)
1st	0.4	21.2	0.7	0.8	0.9	60.0	1.6	1.7
2nd	0.8	5.7	1.4	1.7	1.8	2.8	3.1	3.3

3.3. Polarization, Response Times, and Temperature Dependency

To investigate the polarization dependence of the proposed CB diffractor, two complementary measurements were performed. First, the intensity of the first‐order diffracted beam was measured using the optical configuration (laser)–(polarizer)–(sample)–(analyzer)–(detector), where the analyzer angle was varied while monitoring the diffracted intensity. As shown in Figure 5a, similar angular dependences were obtained with and without the CB diffractive cell. This result indicates that the device does not exhibit the strong polarization sensitivity typically associated with birefringent liquid‐crystal‐based diffraction gratings.

FIGURE 5.

Polarization‐dependent characterization of the CB diffractor. (a) Normalized optical intensity measured using the optical configuration of (laser)–(polarizer)–(sample)–(analyzer)–(detector) as shown in Figure 1. The analyzer was rotated while the intensity was measured with and without the CB diffractive cell. (b,c) Verification of residual polarization dependence using the optical configuration of (laser)–(polarizer)–(sample 0° or 90°)–(detector) as shown in Figure S4, where the polarization direction of the incident light was either oriented at 0° or 90° relative to the long axis of the patterned electrodes.

To further evaluate possible weak polarization dependence arising from the electrode geometry or particle aggregation, an additional measurement was performed using the optical configuration (laser)–(polarizer)–(sample 0° or 90°)–(detector) as shown in Figure S4. In this experiment, the polarization direction of the incident light was varied directly with respect to the long axis of the patterned electrodes due to the angle at which the sample was placed. The results are shown in Figure 5b,c for the sample electrode long axis 0° and 90°, respectively.

A weak but measurable polarization dependence was observed. The effect is relatively small because the characteristic dimensions of the diffraction structure (electrode width ≈ 7.5 µm and electrode spacing ≈ 15 µm) are significantly larger than the wavelength of the incident light. Under these conditions, optical absorption by the CB nanoparticles remains the dominant mechanism governing diffraction. Nevertheless, weak anisotropy may arise from scattering by aggregated CB particles and electromagnetic boundary effects associated with the patterned ITO electrodes. In particular, when the polarization direction is perpendicular to the electrode long axis, interactions with the electrode edges can slightly modify the diffraction efficiencies of the diffracted orders and the transmitted zeroth‐order beam. The accumulation of CB nanoparticles near the electrode branches may further enhance this weak anisotropy through additional absorption and scattering contrast. Therefore, unlike conventional liquid crystal diffractors that exhibit strong polarization‐dependent diffraction due to refractive‐index anisotropy [10, 18, 19, 20, 21], the proposed CB diffractor exhibits only weak polarization dependence while maintaining largely polarization‐insensitive diffraction characteristics.

Figure 6 shows the response time characteristics of the bistable CB diffractor for both wide‐slit and narrow‐slit modes. The change in transmittance in response to 1.6 V/𝜇m was measured. The rise and decay times were defined as the time required for the transmittance to change from 10% to 90% and from 90% to 10%, respectively, for the wide‐slit (Figure 2c), narrow‐slit (Figure 2d), and dark modes (Figure 2b). For the wide‐slit mode, the measured rise and decay times were approximately 0.34 and 0.36 s, respectively. And, for the narrow‐slit mode, the measured rise and decay times were approximately 0.41 and 0.36 s, respectively. We observed that the rise time of the narrow‐slit mode was slightly slower than that of the wide‐slit mode, which would be due to the larger area over which CB nanoparticles must redistribute. In this mode, CB nanoparticles must be positioned over all interdigitated electrodes on the lower substrate, whereas in the wide‐slit mode they must be positioned only on the selected branches of the interdigitated electrodes. Although the diffractor using the non‐bistable CB solution provides faster response and higher diffraction intensity, it does not provide the reduced power consumption and operational stability associated with bistable operation. The bistable CB diffractor thus offers a trade‐off between diffraction efficiency and power‐saving capability.

FIGURE 6.

Response‐time characteristics of the bistable CB diffractor. (a,b) Time‐dependent responses of the zeroth‐order diffraction intensity in the wide‐slit and narrow‐slit modes, respectively, under an applied electric field of 1.6 V/µm. The rise and decay times were 0.34 s and 0.36 s for the wide‐slit mode, and 0.41 and 0.36 s for the narrow‐slit mode.

The temperature tolerance of the proposed diffractor was evaluated using optical microscopy and diffraction‐pattern measurements on a temperature‐controlled hot stage. As shown in Figure 7, stable diffraction operation was observed over a wide temperature range from −50°C to 100°C based on diffraction‐pattern measurements and from −30°C to 100°C based on optical microscopy observations. Only slight deviations from the room‐temperature (23°C) results were observed throughout the investigated temperature range.

FIGURE 7.

Optical microscopy images and diffraction‐pattern photographs of the CB diffractor measured at various temperatures. Optical microscopy observations were performed from −30°C to 100°C, while diffraction‐pattern photographs were obtained from −50°C to 100°C.

To further evaluate the low‐temperature performance of the bistable CB diffractor, the time‐dependent transmittance was measured under an applied electric field of 1.6 V/µm at 0°C, as shown in Figure S5. The rise and decay times were measured to be approximately 0.8 and 1.8 s, respectively. The slower response compared with room‐temperature operation is attributed to the increased viscosity of the thickener‐containing CB suspension at low temperature, which reduces the electrophoretic mobility of the CB nanoparticles. Nevertheless, the device remained fully operational and maintained its diffraction functionality under low‐temperature conditions.

3.4. Bistability and Driving Endurance

Conventional tunable diffractors based on liquid crystal materials generally require continuous voltage application to maintain their diffraction characteristics. Continuous driving increases power consumption and may affect long‐term device stability. To overcome this limitation, we investigated bistable operation in which the diffraction state is retained after removal of the applied voltage. To achieve bistability, a thickening agent was added to the CB suspension to increase the viscosity of the dielectric liquid. The increased viscosity suppresses particle redistribution after the electric field is removed, allowing the electrically formed absorbing pattern to remain stable for extended periods. Similar to the previously described diffractor, the CB nanoparticles remain well dispersed in the suspension. The fabricated bistable diffractor with a concentration of c = 10 wt/vol% and a cell thickness of d = 11 µm exhibits a high‐contrast initial dark state.

Bistability was evaluated using the narrow‐slit mode. The optimal driving electric field was determined as the field that maximized the first‐order diffraction intensity. As shown in Figure 8a, the optimal field was found to be 1.6 V/µm for an electrode width of w = 7 µm and an electrode spacing of l = 15 µm. The inset shows an optical microscopy image obtained under this driving condition. The maximum first‐order diffraction intensity of the bistable CB diffractor was approximately 24% lower than that of the non‐bistable CB diffractor. This reduction is attributed to the increased viscosity of the suspension, which limits the uniform redistribution of CB nanoparticles over the electrode surface.

FIGURE 8.

Bistable operation and cycling endurance of the CB diffractor. (a) First‐order diffraction intensity as a function of applied electric field. The inset shows an optical microscopy image obtained at the optimal driving electric field 1.6 V/µm. (b) Retention of the first‐order diffraction intensity in the narrow‐slit mode after removal of the applied electric field, demonstrating bistable operation. (c) Optical transmittance measured during 100 repeated on‐off switching cycles. (d) First‐order diffraction intensity as a function of applied electric field after 100 switching cycles. The inset optical microscopy image indicates a slightly less uniform particle distribution after repeated operation, while the overall diffraction functionality is maintained.

Figure 8b further demonstrates bistability by monitoring the first‐order diffraction intensity after application and subsequent removal of an electric field of 1.6 V/µm. The diffraction intensity remained essentially unchanged for approximately 12 h after the electric field was removed, confirming stable retention of the diffractive state.

To evaluate operational stability, repeated on‐off switching tests were performed for 100 cycles. As shown in Figure 8c, the optical transmittance remained stable throughout the cycling test. The voltage‐dependent diffraction characteristics measured after 100 cycles are presented in Figure 8d. Although minor changes were observed near the threshold and saturation electric fields, the overall diffraction performance was preserved. The optical microscopy image shown in the inset of Figure 8d suggests a slightly less uniform particle distribution after repeated switching. In addition, the response times measured after 100 cycles increased to approximately 3.9 and 6.1 s for the rise and decay processes, respectively (Figure S6). Nevertheless, no measurable change in diffraction angle was observed, and the first‐order diffraction intensity remained largely preserved after cycling. These results indicate that repeated operation may introduce minor changes in particle redistribution or electrokinetic behavior; however, the bistable diffraction functionality and overall optical performance remain largely maintained after 100 switching cycles.

Table 2 compares the key performance characteristics of the CB diffractor with representative state‐of‐the‐art liquid crystal diffractors reported in the literature. As shown in the table, liquid crystal diffractors generally exhibit higher diffraction efficiency and significantly faster switching speeds in the millisecond range. In contrast, the proposed CB diffractor exhibits switching times on the order of several hundred milliseconds and lower diffraction efficiency in the wide‐slit operating mode. However, the CB diffractor offers several distinctive advantages. First, the device operates over a substantially wider temperature range (−50°C–100°C) than conventional liquid crystal diffractors, whose performance is inherently limited by liquid‐crystal phase behavior. Second, the CB diffractor operates in a bistable manner, requiring electrical power only during switching, whereas liquid crystal diffractors typically require continuous voltage application to maintain the diffractive state. Furthermore, the narrow‐slit operating mode of the CB diffractor achieves a first‐order diffraction efficiency of approximately 60%, comparable to that of representative liquid crystal diffractors. These results demonstrate that the proposed CB diffractor provides a different performance trade‐off from conventional liquid crystal‐based tunable gratings, prioritizing wide‐temperature operation and low‐power bistable functionality over ultrafast switching speed.

TABLE 2.

Diffraction efficiency, switching time, driving voltage, working temperature range and operating mode for comparison of characteristics between carbon black and liquid crystal diffractors.

Diffractor type	Diffraction efficiency				Switching time		Driving Voltage [V/µm]	Temperature range c [°C]	Power consuming mode
	Switching mode 1 a		Switching mode 2 b		on [ms]	off [ms]
	1st order [%]	2nd order [%]	1st order [%]	2nd order [%]
Carbon black (CB)	21.17	5.70	60.02	2.75	340	410	∼1	−50 to 100	Bistable (Voltage application required only when switching)
Liquid crystal (LC)	60.73 d	19.58 d	64.68 d	21.66 d	0.8 e	1.0 e	∼10	−10 to 86	Monostable (Voltage required continuously)

^a Voltage applied in between the branches of the patterned electrodes at the bottom (wide‐slit mode in CB, in‐plane‐switching mode in LC).

^b voltage applied among patterned and plain electrodes at both top and bottom (narrow‐slit mode in CB, vertical‐in‐plane switching mode in LC).

^c The performances at extreme temperatures may drop for both CB and LC diffractors.

^d Ref. [27].

^e Ref. [22].

4. Conclusions

In this study, we demonstrated the electro‐optical and thermal characteristics of diffraction gratings based on CB nanoparticles. The proposed diffractor enables electrical switching between different diffraction modes, allowing the diffraction pattern and diffraction angle to be tuned through selective driving of interdigitated electrodes. The diffraction angle is further determined by the geometrical parameters of the electrode pattern. Owing to the isotropic optical properties of CB nanoparticles and a diffraction mechanism predominantly governed by periodic absorption modulation, the device exhibits diffraction behavior with negligible polarization dependence over a wide operating temperature range. In addition, a bistable CB nanoparticle diffractor was realized by increasing the viscosity of the suspension, enabling retention of the diffractive state after removal of the applied electric field and thereby reducing power consumption. Unlike conventional liquid‐crystal diffractors that rely on refractive‐index anisotropy, the proposed device utilizes electrophoretic redistribution of isotropically absorbing CB nanoparticles to form electrically reconfigurable diffraction gratings. The combination of negligible polarization dependence, wide operating temperature range, and bistable operation highlights the potential of CB nanoparticle diffractors for applications in low‐power photonic devices, adaptive optical systems, and optical components operating under extreme environmental conditions.

Funding

This work was supported by the Commercialization Promotion Agency for R&D Outcomes (COMPA) grant funded by the Korean Government (Ministry of Science and ICT) [RS‐2023‐00304743]; by the Basic Science Research Program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Science and ICT (MSIT), Korea [2022R1A2C2091671]; and supported by the BK21 FOUR Program, Jeonbuk National University.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll74873‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.