This study demonstrated a tunable photonic crystal (PhC) with 70 nm-wide spectral tuning (535 nm to 605 nm) and 3 ms of response time. The tunable PhC is based on reciprocal capillary action of liquid in the nanoscale PhC voids. By wetting the porous silicon PhC with ethanol and water, the PhC can be bistably switched respectively between liquid-filled state (orange color) and vapor-filled state (yellow color). Owing to the energy barrier between the two wetting states, the tunable PhC can remain at either of the two states with no external power consumption.
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Various driving mechanisms for tunable photonic crystal (PhC) at the visible range, such as electrically tunable opals , electrically tunable block copolymer , magnetically tunable colloidal , electrically controlled birefringence of liquid crystal  and electrically tunable ferroelectric multilayer , had been proposed over the past decade. All these tunable PhCs have succeeded in either wide spectral tuning range or fast response time. Some PhCs have more than 10 nm of tuning range by swelling or shrinking of elastomer [1,2] or magnetically changing the periodicity . Nevertheless, these PhCs involved fluid transport mechanisms [6–8] and had response time longer than 1 second. In contrast, those PhCs with response time shorter than 10 ms featured electrical control of anisotropic material [4,5]. The material anisotropicity limits the spectral tuning range to 2 nm or below. A tunable PhC provided with both wide spectral tuning and fast response time has never been achieved.
To overcome the obstacle, a tunable PhC featuring wide spectral tuning, fast response and powerless bistability was proposed. The driving mechanism for the tunable PhC was based on the reciprocal capillary action in the nanoscale PhC voids, which was disclosed in our earlier paper . In this work, the porous silicon photonic crystal (PSiPhC) featuring hydrophobically modified void was tuned by alternately wetting its surface with ethanol and water. The wide spectral tuning results from a large refractive index change of the PSiPhC which is done by switching the PSiPhC between a vapor-filled state and a liquid-filled state via reciprocal capillary action. The fast response time is contributed by the enormous capillary pressure with a capillary radius at nanoscale. The powerless bistability is due to the energy barrier between the two wetting states. Table 1 lists the comparisons of different tunable PhC method in tuning range, response time and powerless bistability.
The color-changing method of the capillary-driven PSiPhC is shown in Fig. 1 . All the tuning processes are carried out in 30% ethanol. Initially, the ambient 30% ethanol is unable to infiltrate the vapor-filled PSiPhC due to the strong hydrophobicity of the void surface (Fig. 1 a). After injecting a 99.5% ethanol jet, the low surface tension ethanol film wetted on the PSiPhC surface induces capillary attraction, which allows the 99.5% ethanol to infiltrate. Subsequently, the 99.5% ethanol gradually dissipates until the ethanol concentration in the voids reaches diffusion equilibrium with the outer 30% ethanol (Fig. 1 e). In contrast, the liquid-filled PSiPhC can be tuned again by injecting a water jet. Water is strongly repelled by FDTS coated silicon surface and has a 105° contact angle . After the water diffused into the PSiPhC and diluted the void liquid, the increased surface tension of the void liquid induces capillary repulsion, which leads to the exfiltration of the void liquid. After that, the PSiPhC returns to its original vapor-filled state.
3. Sample preparation
The PSiPhC fabrication including porous silicon etching and hydrophobic surface modification had been detailed in our previous work . The PSiPhC was fabricated via electro-chemical etching of silicon , which has current-controlled porosity compared to silicon stain etching method . Periodic porous structures were formed by alternating the etching current density, resulting in alternating high and low porosity structure in the PSiPhC. After fabricating the PSiPhC, the void surface of the PSiPhC was modified with self-assembled monolayer of hydrophobic heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (FDTS) via molecular vapor deposition process [12–14].
In Fig. 2 (a) , scanning electron microscopy (SEM) of the PSiPhC reveals the periodic structure in the cross section view (left image) with 1000 nm in thickness and the porous surface in the top view (right image) with 5 to 15 nm pore size distribution. In Fig. 2 (b), the hydrophobically modified PSiPhC shows unaltered yellow color when immersed in water (i.e. vapor-filled state for high surface tension liquid). In contrast, the color of the PSiPhC changed to orange when immersed in 99.5% ethanol (i.e. liquid-filled state for low surface tension liquid). Regarding the tuning range, Fig. 2 (c) shows the normal-incident reflectivity spectra of the PSiPhC immersed in water and ethanol, respectively. The wavelength at maximum intensity is 535 nm and 605 nm when immersed in water and ethanol respectively.
4. Experiments and discussions
Reversible patterning test and dynamic characterizations for the PSiPhC were performed in this work. Reversible pattering test was used to test the powerless bistability of the PSiPhC. The dynamic characterizations for the color-changing of the PSiPhC were performed using CCD camera imaging and response time measurement.
In the reversible pattering test, the PSiPhC was immersed in 30% ethanol. Two sets of needle and syringe were also immersed in the 30% ethanol and separately driven by two air-pulsed fluid dispensers. Each needle was aimed at the PSiPhC surface at approximately 45° incline angle. Figure 3 shows how the color of the PSiPhC was tuned by wetting with ethanol and water. In step 1, the PSiPhC immersed in 30% ethanol appeared to be yellow. Then, an orange tic-tac-toe pattern was formed in step 2 and step 3 by repeatedly injecting and wetting the PSiPhC with ethanol. After the whole PSiPhC surface turned orange (see step 4 and step 5), another yellow tic-tac-toe pattern, shown in step 6 and step 7, was formed by repeatedly injecting and wetting the PSiPhC with water. In step 8, the whole PSiPhC surface returned to yellow again when wetted with water. The tic-tac-toe patterns formed in each step can permanently remain with no external power consumption.
In the CCD camera imaging, color changes of the PSiPhC were monitored with a 30 Hz video camera. More precise dynamic changes were monitored using a monochromatic high speed CCD camera (Motion Pro Y4, Integrated Design Tools, Inc.) operated at 300 Hz. The videos were captured while synchronizing the two dispensers to alternately inject ethanol and water in the same region on the PSiPhC surface. Each injection of ethanol and water separately took the first 20 ms of its 500 ms in a 1 Hz repetition.
Color changes of the PSiPhC monitored by 30 Hz video camera are shown as the sequential frames in Fig. 4 and the multimedia file (Media 1). The color of the PSiPhC was invariable between two alternated injections. The immediate color changes are right next to each injection as shown in frame 03 and frame 33 for the capillary attraction (yellow to orange) and frame 18 for the capillary repulsion (orange to yellow). Dynamic changes of the PSiPhC monitored by 300 Hz video camera are shown as the sequential frames in Fig. 5 . The PSiPhC surface adjacent to the needle outlet was initially yellow (the white area in the center of frame 6). After injecting ethanol from the needle, the color of the PSiPhC gradually turned into orange (the dark area in the center of frame 11). The dynamic changes for both capillary attraction and capillary repulsion are provided in the multimedia file (Media 2).
The response time measurement was carried out with a laser diode and a monitor photodiode. The PSiPhC surface was irradiated by a 50 mW industrial 532 nm green laser with a near right angle incidence and a silicon PIN photodiode received and converted the reflected laser ray into electric current. The relative photocurrent intensity was monitored by a 500 MHz oscilloscope. Figure 6 shows the measured response time via laser irradiance and photodiode monitoring. With reference to the PSiPhC reflectivity at 532 nm in Fig. 2 (c) the higher photocurrent output at 9.9 depicts the higher reflectance of the vapor-filled PSiPhC (yellow color). Whereas the photocurrent output at 6.6 depicts the lower reflectance of the liquid-filled PSiPhC (orange color). The time constant is approximately 3 ms for both the capillary attraction and repulsion.
Table 2 illustrates transition times for the three types of tuning processes described in section 2. For capillary action, the dynamics for both attraction and repulsion in fluidic channels can be resolved by the modified Lucas-Washburn equation  as Eq. (1). The estimated time scale for capillary attraction is 1 × 10−5 to 2 × 10−5 s (e.g. 5 nm in radius and 1000 nm in height) and the time for capillary repulsion is even shorter for the liquid slippage on hydrophobic boundary [16,17]. For liquid diffusion, the estimated time scale is 2 × 10−4 to 7 × 10−4 s based on Fick's second law of diffusion with reference to the mutual diffusion coefficient of water and ethanol  as Eq. (2). For liquid injection, estimated time scale for millimeter traveling of the liquid jet front adjacent to the needle outlet is roughly 2 × 10−3 to 3 × 10−3 s based on the dynamic pressure and Hagen-Poiseuille law as Eq. (3) and Eq. (4), respectively. The injection flow rate in the liquid injection process is the key limit on response time owing to the expansion and deceleration of liquid jet from the needle outlet.
In summary, the wide spectral tuning, fast response, and powerless bistability of the capillary-driven tunable PhC are demonstrated. First, the powerless bistable color states of the PSiPhC have been verified via reversible patterning test. Second, high speed imaging and response time measurement revealed the millisecond response time of the tunable PSiPhC. Future works include shortening the response time by scaling down the dimensions. To expand spectral tuning range, introducing new materials to enhance the refractive index contrast of liquids and PhC solid is necessary.
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