Flexible and stretchable guided-mode resonant optical sensor: single-step fabrication on a surface engineered polydimethylsiloxane substrate

Swagato Sarkar; Sruthy Poulose; Pankaj K. Sahoo; Joby Joseph

doi:10.1364/OSAC.1.001277

1. Introduction

In Guided Mode Resonance (GMR) structures [1,2], the coupling between sub-wavelength grating and thin-film waveguide forms resonant dip (/peak) in the transmission (/reflection) spectra. Due to the narrow linewidth [3], high efficiency [4], and high sensitivity towards the physical parametric changes, many applications such as polarizers, electro-optic switches [2] tunable optical filters [3,4], & bio-sensors [5,6] are reported in the past. In contrast to wavelength and angular-resolved [5] sensing methods, recent works also highlight the impact of phase measurements [7,8] to enhance sensitivity, which is a decisive factor in the field of biosensing. However, the associated resonant thin-film waveguide is mostly supported by a rigid dielectric substrate (quartz or glass), which constrains the flexibility of the device. Thus, the sensor’s response to the variation in physical parameters (post-fabrication) gets restricted.

Polydimethylsiloxane (PDMS), a polymer with high transmission in the visible wavelength range and tunable elasticity has profound applications as a flexible substrate for developing various pressure & stress/strain sensors [9–11]. Combining the flexibility of PDMS with sensitivity of a GMR sensor can redefine the limit of detection of deformable sensors. Recently, Privorotskaya et al. [12] developed a strain sensor with stretchable photonic crystal that has flexible PDMS substrate. Foland et al. [13] opened a new gateway towards the fabrication of highly sensitive, flexible, & compact GMR pressure sensors which can have applications in microfluidics. Nevertheless, fabrications of these flexible resonant subwavelength structures require a series of processes [12] as well as a set of expensive systems [13]. To opt for a much easier fabrication technique, photoresist can be directly spin-coated on PDMS substrate and patterned using Interference Lithography (IL). However, due to the hydrophobic nature of PDMS surface, spin coating on bare PDMS substrate is of extreme challenge. Although physical means such as exposure to oxygen plasma, ozone, and UV light can modify the surface to be hydrophilic [14–17], it can stay only for a shorter period due to the hydrophobic recovery and is thus ineffective for the purposed device.

Thus, we present an effective chemical method of PDMS surface modification to render prolonged hydrophilicity compared to above methods. The modified surface is characterized in terms of contact angle measurement as well as Fourier transform infrared (FTIR) spectroscopy to ensure surface re-engineering. Consequently, a positive photoresist is successfully coated and GMR sensor is fabricated by patterning grating onto it through IL. As a proof of concept, we performed sensing experiments with this flexible GMR structure and further validated the results using FDTD simulations. For higher sensitivity this method also opens the possibility of choosing high indexed waveguiding medium such as TiO2 that can be spin coated through sol-gel technique [18,19].

2. Theory

The basic model of a GMR structure consists of a linear grating of periodicity ‘Λ’ and depth ‘t’ placed on top of a planar waveguide of thickness ‘d’ on a substrate as shown in Fig. 1(a)

Fig. 1 (a) A guided-mode resonant structure in operational mode. Selective incident wavelength on diffraction propagates and couples-out along reflection/ transmission regime to form resonant peak/ dip as seen in the corresponding spectra. (b) Surface engineering of PDMS substrate to attain hydrophilicity. PDMS backbone in the surface is altered using chemically produced atomic oxygen. Final structure shows several branched polyethyleneimine on the activated surface (identical groups distinguished by color) to enhance hydrophilicity.

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. The refractive index (RI) of cover, waveguide, and substrate are given as ‘n_c’, ‘n_w’ and ‘n_s’ respectively. Since the grating is comprised of both high (n_w) and low index (n_c), the average RI of the grating film becomes n_g = n_w (f) + n_c (1-f), where ‘f’ is the fill factor given by f = w/Λ (‘w’, ridge width). On applying a broadband source (I₀), a portion of the light gets reflected as zeroth order (R₀) and the rest gets diffracted as well as transmitted (T₀). Theparameters of the GMR structure are so chosen that the grating only diffracts first order light in to the single mode waveguide at a specific resonant wavelength (λ_R). This λ_R while propagating inside the waveguide encounters the grating again and diffracts in the directions of R₀ and T₀. The coupled-out waves interfere constructively (R₁, R₂ … with R₀)/destructively (T₁, T₂ … with T₀) to form a resonant peak/dip as shown in the corresponding reflection/transmission spectra. Since grating-waveguide associated parameters are directly related to the resonant wavelength [1] of the GMR structure, any physical change causing their variation results in the resonant peak/dip shift. By incorporating the GMR structures into flexible PDMS substrate [13], the change in grating periodicity and hence the resonance wavelength can be directly related to the amount of pressure applied to operate as a precise sensor. We take this concept further into a simpler mode of fabrication through a cost-effective chemical approach of surface modification of the PDMS substrate and IL as discussed further.

The wetting properties of a liquid on solid surface originating from in-between intermolecular forces can be characterized via contact angle (CA), defined as the angle between air-liquid interface and liquid-solid interface. Based on wettability and contact angle, the surfaces are classified into four categories namely; super hydrophilic (~0°), hydrophilic (<90°), hydrophobic (>90°) and super hydrophobic (>150°). Typically, the water contact angle of PDMS is around 110° which proves its hydrophobic nature [15]. The abundant methyl (-CH3) groups present in PDMS makes it more inactive to chemicals. As a common practice, surface activation through oxygen-plasma exposure produces high energy atoms & ions that can effectively break the PDMS backbone allowing it to undergo hydroxylation. Additionally, the carbon-containing components leave the surface as volatile organic species with left-over radicals. Free silicon and oxygen radicals form oxygen-rich Si-O-Si silica like surface that succeeds with the formation of silanol group (Si-OH) causing hydrophilicity [20]. However, beneath the silica-like layer, there are unaffected low molecular weight silicone polymers in the bulk that pierce into the upper layer and replace the hydrophilic groups [21] with time. This produces short-term hydrophilicity and hence unfavorable for spin-coating. Thanks to the chemical means of prolonging this hydrophobic recovery which interested us towards a cost-effective and easily modifiable chemical route. Koh et al. have reported on piranha mediated surface treatment to generate similar silanol groups resulting in a reduction of contact angle [22]. However, highly concentrated piranha solution causes damage to the PDMS surface which is undesirable for fabrication of GMR structures and a suitable chemical process is indeed required. In this letter, we report a facile, efficient and non-destructive method of surface activation through the production of singlet oxygen [23]. A chemical reaction between hydrogen peroxide (H₂O₂) and sodium hypochlorite (NaOCl) generates water, sodium chloride (NaCl), and atomic oxygen species as follows:

_{H_{2} O_{2} + N a O C l \to O_{2} ({}^{1}Δ_{g}) + N a C l + H_{2} O}

This generation of the atomic oxygen plays the key role in hydroxylation as it attacks the Si-CH₃ bonds in the PDMS backbone to generate Si-OH groups, readily making the surface hydrophilic [22]. To avoid hydrophobic recovery, the hydrophilized surface must undergo irreversible covalent interaction with a hydrophilic reagent that can maintain the hydrophilicity of the surface for a longer time. Modification of hydroxylated PDMS surface by coating a hydrophilic polymer [24], integration of distinct coupling reagent as surface initiators for graft polymerization [16] as well as silanization [17] has been reported as means of extending the hydrophobic recovery time. As the silanol activated PDMS surface offers a negatively charged surface area, we choose a stable coating of a cationic and hydrophilic [25,26] polyethyleneimine (PEI) that can neutralize the charge, enhance the hydrophilicity of the surface and prolong the hydrophobic recovery time of PDMS.

3. Results and discussions

3.1 Hydrophilization of PDMS

PDMS substrate is prepared with elastomer kit from Sylgard, Dow Corning 184 by standard mixing of polymer base and curing agent at 10: 1(w/w) ratio. After sufficient stirring and degassing in a vacuum desiccator for 20 minutes, the bubble-free mixture is spread over a Petri dish with a thickness of about 2 mm and kept on a hot plate at 100° C for 1 hour to crosslink. The solidified PDMS thus obtained is cut into 10 mm by 15 mm rectangular blocks and cleaned in a bath sonicator for 20 minutes followed by isopropyl alcohol rinsing. For the hydrophilization, the PDMS blocks are dipped in 10 ml of sodium hypochlorite (NaOCl) taken in a beaker into which 8µl hydrogen peroxide (H₂O₂) is added using micropipette as shown in Fig. 1(b). This process is repeated four times to achieve the maximum silanol group formation. The treated samples are then dipped in 5% polyethyleneimine (PEI) and kept under thermal evaporation for covalent attachment of this PEI to the oxidized PDMS substrate. Branched PEI of molecular weight 25 kDa is selected due to its rich cationic charge and availability in different molecular weight grades. A preformulation assessment has been carried out with smaller molecular weights of PEI (600 Da, 2 kDa, and 10 kDa). It is noticed that the hydrophobic recovery time of the PDMS surface is significantly prolonged when coated with 25 kDa PEI, in comparison to that of lower molecular weights. Higher concentrations (> 25 kDa) may lead to more efficient results but at the cost of losing the transparency which is a prerequisite to study transmissive properties of the fabricated GMR sensor.

Figure 2(a)

Fig. 2 (a) Contact angle measurement PDMS substrate before and after hydrophilization showing clear change from 113° to 53.7° after surface modification. (b) FTIR spectra of treated and untreated samples to detect the presence of PEI

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shows contact angle measurements (KRUSS Drop Shape Analyzer–DSA2), on our surface engineered PDMS (bottom) compared to a bare one (top). Reduction in contact angle from 113° to 53.7° clearly reveals the success of the hydrophilization process achieved through this two-step chemical approach. This decrease in contact angle can be directly attributed to the increase in ‘work of adhesion’ and ‘surface free-energy’, two key factors that decide the wettability [27,28] of the surface. Moreover, to ensure the presence of PEI coating over the PDMS substrate, Fourier transform infrared (FTIR) spectra are compared in Fig. 2(b). Untreated PDMS depicts characteristic absorption bands at 2950 - 2960 cm⁻¹ (asymmetric CH₃ stretching), 1261 cm⁻¹ (CH₃ bending), 1032–1127 cm⁻¹ (Si-O-Si stretching) and 780 cm⁻¹ (C-Si-C stretching) [20]. For the PEI treated PDMS, bands at 1573 and 776 cm⁻¹ describe N-H deformation and wagging pattern of primary amine groups of PEI respectively [29]. Since PEI has abundant secondary carbon units, comparatively stronger and broader absorption band ranging from 2839 - 2934 cm⁻¹ is also observed.

3.2 GMR device fabrication

Once the substrate attains hydrophilicity, a positive photoresist (AZ 1518 resist: AZ EBR 70/30 thinner = 1:2) is spin coated on it at 10000 rpm for 30 secs and a thickness of around 500 nm is obtained. Two beam Lloyd mirror IL setup is used to form a large area surface grating pattern on the coated photoresist. As shown in Fig. 3(a)

Fig. 3 (a) Experimental setup for Lloyd’s mirror based laser interference lithography (b) AFM image of IL induced grating on resist-coated PDMS substrate along with height profile. (c) SEM image with 45° tilt stage showing grating-waveguide structure in cross-section. (d) Actual image of the fabricated device displaying diffracted colors at different angles in reflection.

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, a 355nm semiconductor laser (Genesis CX355-100) of 100 mW power is spatially filtered using a 20x microscope objective (M.O.) along with 15 µm pin-hole and further collimated using a UV lens. The collimated beam passes through a rectangular aperture and illuminates both the mirror and the coated PDMS substrate which are positioned on a rotating mount rotated at θ with their faces perpendicular to each other. The part of the light reflected from the mirror surface interferes with the beam that illuminates the sample directly, (making interference angle, θ = 20.5°) to produce 1D periodic structure with periodicity (Λ~500 nm) given by Λ = λ/(2sinθ), where λ is the incident laser wavelength. The exposed sample is developed in AZ 726 MIF developer for 2 seconds for partial removal of the exposed region (of area 6mm x 8 mm) followed by cleaning in DI water and drying using nitrogen blower. Figure 3(b) shows the grating height profile to be around 200 nm and a periodicity of 522 nm as obtained from AFM data. The tilted cross-sectional view (Fig. 3(c)) along a crack of the sample is obtained from a table top SEM (JCM-6000, JEOL, Japan) that shows a 217 nm grating depth and 234 nm waveguide thickness. These cracks in the photoresist layer on PDMS surface may have mostly formed during post-development nitrogen blowing and sample-handling prior to SEM observation as seen from the large-scale macroscopic image provided in Appendix A. Inset shows zoomed view of crack-free grating region. Figure 3(d) shows the original image of the fabricated GMR samples on the flexible PDMS substrate.

3.3 Refractive index sensing

The experimental set up (schematic) for the GMR sensor is shown in Fig. 4(a)

Fig. 4 (a) Experimental setup to study transmission characteristics of the fabricated device as a RI sensor. (b) XZ view of 2D FDTD simulation model for studying similar transmission response. (c) Straight curves show experimentally obtained normalized GMR dips for air (black) and water (red). Dashed curves show similar dips for simulation model on change of background RI from 1.00 (black) to 1.33 (red).

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. A white light source (Avalight-DH-S-BAL) is spatially filtered and collimated to produce a uniform beam of 4 mm diameter. The fabricated GMR sample on PDMS substrate is adhered to a cleaned glass slide by thick double-sided tape on three sides with the patterned surface facing inward. The finite thickness of the double-sided tape creates a three-sided enclosed cavity of air. The whole cavity along with the substrate is positioned along the beam path such that only the patterned area gets illuminated on normal incidence (θ = 0°). A linear polarizer is used to incident transverse magnetic (TM) polarized light with electric field perpendicular to grating lines. To change the RI of the sample cover region, de-ionized (DI) water is inserted using a syringe into the cavity from the top unclosed portion. The transmitted light for both the cases of air and water is converged by a high NA lens and transferred to a spectrometer (AvaSpec-ULS3648TEC) through a multimode optical fiber cable. The obtained spectrum for air shows a GMR transmittance dip at 772 nm with an insertion loss of 0.78 dB for the fabricated sample. The resonant dip shifts to 783 nm (straight curves in Fig. 4(c)) on changing of the cover region from air to water. Repeated measurements in spectrometer scope mode (photon count) over days have shown consistency of the GMR transmission dip around 772 nm with air as cover medium and are plotted in Fig. 5(a)

Fig. 5 (a) Plotted spectrometer scope-mode data of the fabricated GMR sample with air as cover medium, measured over days. (b) Spectrometer scope-mode data of the fabricated GMR sample in air showing the splitting of the resonant dip for a slight tilt of θ ~2° (bottom) from the case of normal incidence θ = 0°(above).

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. Thus it is confirmed that hydrophobic recovery is legitimately avoided in order to maintain the intactness of the grating-waveguide structure on the surface-modified PDMS substrate. Transmittance is calculated by normalizing the scope mode GMR data for air and water with a non GMR reference recorded for a non-patterned area on the PDMS substrate. Since GMR transmittance dip is sensitive tothe angle of incidence, proper care is given to the orientation of the sample mount to avoid non-degenerate mode splitting in θ

\neq

0° case. Figure 5(b) contains original scope mode data with air as cover medium for both normal (θ = 0°) and oblique incidence (θ

\sim

2°).

3.4 Numerical evaluation

We have validated our sensing results numerically through finite difference in time domain (FDTD) method with proper material properties using Lumerical software package [30]. The optical constants for the photoresist AZ 1518 and PDMS over the entire spectra of 500-1000 nm are fed into the simulation model for appropriate accounting of the indices at significant wavelengths. Figure 6

Fig. 6 Real and imaginary part of refractive indices of (a) AZ 1518 positive photoresist extrapolated from known values at certain wavelengths, (b) PDMS, fitted within Lumerical FDTD solver for scan range 500-1000 nm from data sheet obtained from ref [31].

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contains the respective dispersion curves for both real and imaginaryvalues of the refractive indices. The cross-section of the simulation model is shown in Fig. 4(b) with grating periodicity (Λ) = 522 nm, grating thickness (t) = 200 nm, waveguide thickness (d) = 300 nm, and fill factor (f) = 0.75 along with tooth angle of 60°. This non-vertical sidewall of the grating in the simulation is deliberately incorporated to imitate the fabricated structure as confirmed from the SEM image in Fig. 3(c). The transmission spectrum for TM polarized light for an air background (n_c = 1.00) shows a sharp GMR dip at 770.54 nm that shifts to 784.57nm on change to water (n_c = 1.33). These are shown in Fig. 4(c) in a dotted curve which agrees well with our experimentally observed spectra. The electric field profile corresponding to the GMR dip wavelength 770.54 nm (n_c = 1.00) is shown in Fig. 7(a)

Fig. 7 (a) Simulative studies of the modeled photoresist based GMR structure on PDMS substrate (with Λ = 522 nm and d = 300 nm) showing (a) Electric field profile on resonance at 770.54 nm and (b) Off-resonance at 760 nm. (c) Variation of λ_R with change in Λ for a constant d = 300 nm. (d) Variation of λ_R with change in d for a constant Λ = 522 nm. Increase in waveguide thickness allows multiple resonant modes within the structure. In all the studies, n_c = 1.00.

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. It shows intensified electric fields within the waveguide region as guided modes. As evident from the figure, the leaky modes excited by the grating interface can be collected as the GMR peak in the reflection spectra. On the other hand, Fig. 7(b) depicts the case of an off-resonance excitation at 760 nm, where the modes are less confined within the waveguide due to their non-resonant nature. The color bar in Fig. 7(b) shows a relatively lesser value of the electric field compared to that in Fig. 7(a), which confirms the non-guiding of the modes even for a smaller deviation (∼10 nm) from the resonant wavelength. Thus, despite of a low-index contrast between photoresist and PDMS substrate, guiding of resonantwavelengths remain possible. Although not performed experimentally, a simulative analysis is carried out to realize the effect of variation of grating period and waveguide thickness of our flexible and stretchable GMR sensor. Figure 7(c) shows a linear relation between the grating period and resonant wavelength for a constant waveguide thickness of 300 nm whereas Fig. 7(d) shows the effect of waveguide thickness variation for a period of 522 nm. Multiple resonant modes are supported within the waveguide model for higher waveguide thickness. It should be noted that the simulation reflects only ideal conditions for the defined geometry. It can be the reason behind the sharp transmission dips unlike the experimentally obtained results, where the full-width half maximum (FWHM) of the resonant dip is relatively large. This broadening in the experimental GMR dip as compared to that of simulation (FWHM of 17 nm vs. 3nm) can be attributed to two reasons: non-uniformity in the thickness of waveguide and grating throughout the whole area of illumination (possibly due to the uneven developing rate) and defects within the structure in form of cracks (as seen from the large scale image in Fig. 8

Fig. 8 A large area macroscopic SEM image of the fabricated sample with formation of cracks over the photoresist surface. Inset shows zoomed crack-free region with uniform grating formation.

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in Appendix A). Both factors can be avoided by rigorous optimization and cautious sample preparation. Anyway, the sole purpose of this paper is achieved in demonstrating the new possibilities of surface activation of PDMS substrate via our established chemical method which in turn has led to the successful fabrication of photoresist based flexible GMR device.

4. Conclusion

In summary, the concept of fabricating a flexible GMR device through a simple, cost-effective and easily obtainable method is explored experimentally. The novel chemical route of surface engineering has indeed been the key factor for prolonged hydrophilization of the PDMS surface that resulted in uniform and wrinkle-free coating of the photoresist to achieve this goal. Although we used AZ1518 photoresist to demonstrate and validate the process, one can explore other liquids with similar wetting properties for spin coating PDMS surface hydrophilized by this technique. The response of the fabricated device on varying the RI of the cover region from air to water is experimentally observed under transmission geometry and confirmed through similar FDTD-based optical simulations to provide a proof of concept of this flexible RI sensor. Flexible GMR structures have already made a profound impact through their applications as strain and pressure sensors. Our method opens up newer possibilities of large-scale, cheap and easily procurable device fabrication to motivate further research towards flexible and compact lab-on-chip platforms.

Appendix A Large area SEM image of the fabricated sample

Funding

DST-INSPIRE; IRDE Dehradun (DRDO unit); IIT Delhi

Acknowledgment

We thank Prof. J. P. Singh, IIT Delhi, for the use of contact angle measurement setup.

Disclosures

The authors declare that there are no conflicts of interest related to this article.

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Flexible and stretchable guided-mode resonant optical sensor: single-step fabrication on a surface engineered polydimethylsiloxane substrate

Abstract

1. Introduction

2. Theory

3. Results and discussions

3.1 Hydrophilization of PDMS

3.2 GMR device fabrication

3.3 Refractive index sensing

3.4 Numerical evaluation

4. Conclusion

Appendix A Large area SEM image of the fabricated sample

Funding

Acknowledgment

Disclosures

References

Cited By

Figures (8)

Equations (1)

OSA Continuum