A tunable photonic crystal (PhC) based on the capillary action of liquid is demonstrated in this work. The porous silicon-based photonic crystal (PSiPhC) features periodic porosity and is fabricated by electrochemical etching on 6” silicon wafer followed by hydrophobic modification on the silicon surface. The capillary action is achieved by varying the mixture ratio of liquids with high and low surface tension, yielding either capillary attraction or capillary repulsion in the nanoscale voids of the PSiPhC. By delivering the liquid mixture into and out of the voids of the PSiPhC, the reflective color of the PSiPhC can be dynamically tuned.
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Tunable photonic crystals (PhCs) are PhCs with photonic band gap (PBG) tunability . Tunability in the visible range permits color changes in the PhCs and can potentially lead to applications in imaging, displays, and lighting, among others. During the past decade, reported driving methods for tunable PhCs in the visible range include mechanical deformation , magnetically induced particle motion , chemical swelling , the electro-optic effect of ferroelectrics , temperature controlled dopant solubility in liquid crystal , thermally induced phase transition of liquid crystal , electrically controlled birefringence of liquid crystal , and liquid mixture imbibitions . Table 1 summarizes approaches mentioned above that adjust the lattice constant a, refractive index n, or both a and n for tunable PhCs in the visible range.
In this work, we proposed a tunable PhC method based on the capillary action of liquid. As shown in Fig. 1 , a PhC with hydrophobic voids is immersed in a binary liquid mixture. The surface tension of the liquid mixture can be modulated by varying the mixture ratio of liquids with high and low surface tension. The PhC is tuned using reciprocal capillary action that drives liquid in and out of the voids of the PhC by modulating the surface tension of the liquid mixture.
Porous silicon-based photonic crystal (PSiPhC) is used in this application as the higher-index medium. Porous silicon with parametric pore sizes and porosity (i.e. volume densities of the pores) can be fabricated by electrochemical etching . The periodic refractive index structure of the PSiPhC can be achieved in a one step etching process . The nanoscale voids of the porous silicon also benefit the capillary action for the scale effect of capillary pressure. The lower-index medium is constituted of an ethanol-water mixture and vapor in the voids of the PSiPhC. The surface tension of the ethanol-water mixture can be adjusted from 22.0 dyne/cm to 72.0 dyne/cm over the entire concentration range at room temperature . In addition, the combination of silicon and liquid/vapor has a strong contrast of refractive index over the visible spectrum.
To clarify the significance of the liquid mixture in the PSiPhC, we compare the method of liquid mixture imbibitions and our work. The former has an adjustable refractive index of the liquid mixture, and the liquid mixture is permanently stayed in the PhC voids . In contrast, the liquid mixture in our work has adjustable surface tension while offering no significant change in refractive index. The refractive index change of the PSiPhC primarily relies on the fill ratio of the liquid in the voids.
2. Sample preparation
The PSiPhC was fabricated on a six-inch 0.004 Ω-cm n-type (111) silicon wafer where an area of about three inches at the center defined by a polypropylene (PP) mask permits electric current to flow through the silicon surfaces. The electrochemical etching process was carried out in 1:2.33 mixtures of 49% hydrogen fluoride (HF) and 95% ethanol at 10 °C. Periodic structures were fabricated by alternating the current density between 100 mA/cm2 and 40 mA/cm2, which resulted in alternating high and low porosity structure in the PSiPhC. The PSiPhC was cleaned by an ethanol rinse and air dry. The voids' surfaces of the PSiPhC were deposited with a heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (FDTS) self-assembled monolayer (SAM) via molecular vapor deposition [13,14]. During the deposition process, the PSiPhC was pretreated using oxygen plasma for 10 minutes. Then, the deposition reaction of 15 minutes was carried out. In the reaction, FDTS and de-ionized water acted as a precursor and a catalytic agent respectively. The hydrophobic FDTS SAM is conformal and uniform, reducing fluidic friction in the voids of the PSiPhC and making the voids initially hydrophobic.
The nanostructures of the PSiPhC were observed using a scanning electron microscope (SEM), as shown in Fig. 2 . The SEM micrograph of the cross section reveals five and a half periods of alternating porosity. The PSiPhC is estimated to be 520 nm in thickness. Reflectivity of the PSiPhC over visible spectra (i.e. 400 nm to 700 nm) was measured at normal incidence and at an incident angle of 45° (see Fig. 3 ). The maximum reflection peak at normal incidence, denoted as peak A, approximates 80% at a center wavelength of 470 nm. At an incident angle of 45°, the peak A shifts to a shorter wavelength of about 400 nm. Unlike an ideal quarter wave reflector, more than one peak was observed in the reflection spectra because the low porosity layers are intentionally thinner to facilitate fluidic motion.
Contact angle and reflectivity measurements for the ethanol-water mixtures on top of the PSiPhC were carried out before the color-changing experiment. The ethanol-water mixtures are composed of 99.5% ethanol and de-ionized water with different mixture ratios. As shown in Fig. 4 , the contact angle measurements reveal the affinity between the ethanol-water mixtures and the FTDS coated PSiPhC surface. The contact angle reduced from 105° to 30° with the increase ethanol concentration of the mixture from 0% to 99.5%. This contact angle result demonstrates the significant change of affinity between the mixture and the PSiPhC surface. As shown in Fig. 5 , colorless droplets of 0% (i.e. pure de-ionized water), 50%, and 99.5% ethanol mixtures on top of the PSiPhC were inspected from the top and a 45° angle. The 0% ethanol droplet has the color of the underneath PSiPhC unaltered as shown in the top view and the 45° view. The unaltered color implies no infiltrated liquid in the voids of the PSiPhC. Contrastingly, the PSiPhC surface under the 50% and 99.5% ethanol droplets shows remarkable color changes from cyan blue toward green (as shown in the top view) and from purple toward cyan blue (as shown in the 45° view). A significant increase of the refractive index of the PSiPhC voids can explain the color change toward longer wavelength and indicates that the PSiPhC is infiltrated with liquid. Finally, quantitative reflectivity measurements for the color changes were carried out with individual ethanol-water droplets sandwiched by transparent glasses and the PSiPhC. As shown in Fig. 6 , the center wavelength of peak A shifts from 470 nm to 530 nm with the increase of ethanol concentration, which is consistent with the observed color changes in Fig. 5.
To demonstrate the color-changing experiment, the PSiPhC was initially immersed in a 0% ethanol mixture (i.e. pure de-ionized water). Then the ethanol mixture was gradually exchanged with 99.5% ethanol to obtain specific ethanol concentrations from 0% to 90%. In each step, the drained ethanol mixture and the newly added 99.5% ethanol were measured and precisely controlled prior the exchanging procedure. The time duration between each step was minimized to abate the influence of liquid evaporation. After the concentration of the ethanol mixture in the trough reached 90%, the ethanol concentration was reduced in a similar way by gradually exchanging the ethanol mixture with de-ionized water until the ethanol concentrations dropped to 2.5%.
4. Experimental results
The color evolution of the PSiPhC in the experiment is shown in Fig. 7 . The irregular line patterns on the PSiPhC result from the effect of non-uniform doping of silicon after electrochemical etching. At first, the color of the PSiPhC under 0% ethanol was unaltered cyan blue, which indicates no infiltrated liquid in the voids of the PSiPhC. After gradually increasing the ethanol concentration to 90%, the color of the PSiPhC evolved to green. This color evolution indicates that the voids of the PSiPhC were gradually being infiltrated with ethanol mixture. After gradually decreasing the ethanol concentration to 2.5%, the color of the PSiPhC returned to unaltered cyan blue. Consequently, the color evolution proves the capillary attraction and repulsion of the liquid mixture in the voids of the PSiPhC.
Finally we carried out in situ reflectivity measurement and examined the reversibility of the tunable PhC. The center wavelength of peak A as function of the varying ethanol concentrations is shown in Fig. 8 . Remarkably, a significant hysteresis effect reveals the different energy barriers during the capillary-driven tuning process. This hysteresis imply that the liquid exfiltration requires more energy to restore a higher energy state (i.e. the liquid stayed on top of the vapor filled PSiPhC), whereas the liquid infiltration requires less energy to penetrate the vapor voids in the PSiPhC with liquid .
In this work, the nanoscale voids of a PSiPhC were hydrophobically modified with FTDS SAM. The capillary attraction and repulsion in the nanoscale voids of the PSiPhC were achieved by modulating the surface tension of the liquid mixture. The color of the PSiPhC was tuned by capillary-driven liquid infiltration and exfiltration. Consequently, capillary action is proven to be an applicable driving method for dynamically tuning the PBG of PhCs.
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