Sensitivity enhancement for evanescent-wave ... - OSA Publishing

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Dimitris Sofikitis,1,2 Katerina Stamataki,1,3 Michael A. Everest,4 Vassilis Papadakis,1 Jean-Louis Stehle,5. Benoit Loppinet,1,6 and T. Peter Rakitzis1,2,7.
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OPTICS LETTERS / Vol. 38, No. 8 / April 15, 2013

Sensitivity enhancement for evanescent-wave sensing using cavity-ring-down ellipsometry Dimitris Sofikitis,1,2 Katerina Stamataki,1,3 Michael A. Everest,4 Vassilis Papadakis,1 Jean-Louis Stehle,5 Benoit Loppinet,1,6 and T. Peter Rakitzis1,2,7 1

Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, Heraklion-Crete 71110, Greece 2

3 4

Department of Physics, University of Crete, Heraklion-Crete 71003, Greece Department of Chemistry, University of Crete, Heraklion-Crete 71003, Greece

Department of Chemistry, Westmont College, Santa Barbara, California 93108, USA 5

Sopralab, 7 rue du Moulin des Bruyeres, Courbevoie 92400, France 6 e-mail: [email protected] 7

e-mail: [email protected]

Received February 6, 2013; revised March 5, 2013; accepted March 7, 2013; posted March 8, 2013 (Doc. ID 185029); published April 1, 2013 We demonstrate a method to increase the sensitivity of the s-p phase shift under total internal reflection (TIR) for optical sensing. This is achieved by the introduction of two simple dielectric layers to the TIR surface of a fused silica prism. The enhanced sensitivity is demonstrated using evanescent-wave cavity-ring-down-ellipsometry by measuring the refractive index of liquid mixtures and by studying the adsorption of polymers to the TIR surface of the fused silica prism. © 2013 Optical Society of America OCIS codes: 120.2130, 280.4788, 240.2130, 260.6970.

Total internal reflection offers a practical configuration for optical sensing applications, which have been explored over the last 40 years for a variety of techniques, such as surface plasmon resonances (SPRs), optical waveguide spectroscopy (OWS), ellipsometry, and others [1–5]. The observables in these experiments are modulations in either the light intensity or in the phase of the reflected light, which is often measured through the polarization dependence of the total internal reflection (TIR)-induced phase shift. Recently an approach for the measurement of the phase shift between the s and p polarization components, Δ  δp − δs , was demonstrated using evanescentwave cavity-ring-down ellipsometry (EW-CRDE) [6–8]. However, evanescent-wave ellipsometry suffers from limited sensitivity. The s-p phase shift Δ upon TIR (R > 0.995) in a simple Fresnel interface is not very sensitive to changes on the low refractive index side, since the s and p waves are affected in similar ways. Several methods have been proposed to enhance internal reflection sensitivity [9,10]. Here we propose a way to improve it by introducing a thin layer system at the TIR surface. The enhanced sensitivity is demonstrated by measurements of the refractive indices of liquid mixtures and adsorption of polymers at the prism TIR surface. A thin layer of a high refractive index material (TiO2 ) is added to the TIR surface. This layer acts as a partial polarization-dependent beam splitter, and results in a decoupling of the s and p light. Finally, a thin SiO2 layer is added on top. This simple design increases the phase-shift sensitivity while preserving the properties of the substrate. Figure 1(a) displays the phase shifts for s and p polarized light calculated using the transfer matrix [11] for the simple fused-silica/water Fresnel interface and for the layered system, as a function of the incidence angle θ under TIR conditions. For the calculations, the refractive index of the fused-silica substrate was set to 1.46, the TiO2 highrefractive index layer had n ≈ 2.5 with thickness of 50 nm, and the top layer of SiO2 had n  1.46 with thickness 0146-9592/13/081224-03$15.00/0

20 nm. The phase shifts of the s and p waves are very similar for the simple fused silica/water Fresnel interface. However, they are very different in the layered system, for which δs is almost independent of θ, δp varies strongly with θ, and thus Δ varies much more compared to that of the simple Fresnel interface [see Fig. 1(a)]. This leads to the increased sensitivity discussed bellow. The phase shift Δ was measured with the EW-CRDE technique. A laser pulse linearly polarized at 45° enters a cavity consisting of two highly reflecting mirrors and a prism in which light undergoes TIR, shown in

Fig. 1. (Color online) (a) Dependence of δp (red dashed lines), δs (blue dotted lines), and Δ  δp − δs (black solid lines) on the angle of incidence for the case of the simple fused silica/water Fresnel interface (left) and for the layered system (right). (b) Experimental setup (see text for details). (c) EW-CRDE traces acquired via the uncoated (top) and coated (bottom) part of the TIR prism surface, in contact with water. © 2013 Optical Society of America

April 15, 2013 / Vol. 38, No. 8 / OPTICS LETTERS

Fig. 1(b). A linear polarizer oriented at 45° to the plane of incidence is placed at the output of the cavity. The final output (ideally) has the simple form It  Ae−t∕τ Cos2 ωt∕2 [6], where A is a normalization constant, τ is the photon lifetime in the cavity, and ω is the polarization beating frequency. The ring-down time τ is determined from an exponential fit, and ω from the fast Fourier transform of the data. The beating frequency ω is related to Δ, the single pass s-p phase shift as ω  cΔ∕d, where d is the cavity length and c the speed of light. We use a microchip Nd:YAG laser (Horus HLX-V-F-100), which delivers 0.6 ns pulses of 0.6 μJ at 532 nm at 80 kHz repetition rate. The near concentric cavity, formed by two mirrors (R  0.996) with radius of curvature 10 cm placed at d  20 cm, is chosen to be longer than the laser pulse length. Fused silica prisms were custom made (Laser 2000) with only half of their TIR surface coated with the TiO2 (50 nm nominal thickness) and SiO2 (20 nm nominal thickness) layers. This allowed us to switch the TIR spot from the coated to the uncoated region by translating the prism only a few mm, which did not significantly degrade the cavity alignment. The prism (with dimensions 7.5 × 2.5 × 2.5 cm) is placed at the center of the cavity, and the partial reflections from the input and output surfaces, which are not antireflection coated, are kept in the cavity. The incidence angle of 70° satisfies the TIR condition for a range of sample refractive indices between 1 and 1.365, which includes water (n  1.33) and most protein and polymers aqueous solutions. The prism has been characterized by spectroscopic ellipsometry (GES5E Semilab) both in the internal and in the external reflection configuration, which revealed birefringence as high as 20°, depending on the position of the ray in the prism. In order to assess the role of prism birefringence in our measurements, EW-CRDE was simulated by the Jones matrix method. It showed that prism birefringence adds to the final result of Δ, thus preventing the measurement of the absolute values of the TIR s-p phase shift, but did not affect the dependence of the measured Δ on external refractive index. Therefore, relative measurements are expected to be accurate. A glass flow cell with a gap of 0.1 mm (Hellma) was placed on the TIR surface and connected to syringes to allow introduction of liquids. Two characteristic CRDE traces, acquired via the uncoated and the coated part of the prism, with pure water inserted in the flow cell are shown in Fig. 1(c). The polarization beating frequency ω∕2π is ∼50 MHz and ∼300 MHz for the uncoated and coated surface, respectively; however, there is no significant difference in the ring-down time. The measured phase shift Δ for various water/ethanol and water/methanol mixtures are shown in Fig. 2, as a function of the refractive index measured by an Abbe refractometer, for both the uncoated (black circles) and the coated (red squares) part of the prism. The dependence of Δ on refractive index is significantly increased by the coating with a change of more than 35° over the covered range of refractive index, compared to less than 10° for the uncoated prism. The black line in Fig. 2 corresponds to the Δ value expected for the simple fusedsilica/water interface, with the addition of a constant value introduced to account for prism birefringence.

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Fig. 2. (Color online) Variation of phase shift Δ as a function of refractive index of liquid solutions, for the uncoated (black points) and coated (red points) part of the prism surface. Black and red lines correspond to simulations for the two situations, respectively. Statistical error bars are of order 10−2 (not shown here).

The red dashed line is the expected dependence of Δ on the refractive index when the nominal thicknesses and refractive indices are used. A better fit is obtained when a smaller thickness of 30 nm of the TiO2 layer is assumed, as shown by the solid red line. The difference in resolution between Δ measurements using the coated and uncoated prism is illustrated in the inset of Fig. 2, showing two typical measurements of Δ over a period of 100 s for the uncoated (left) and coated (right) part of the prism in contact with water. A longtime fluctuation of characteristic time ∼15 s was apparent in the phase measurements, independent on the nature of the TIR surface (coated/uncoated). The solid lines denote the change of Δ that corresponds to a change of 2 × 10−4 refractive index units (RIUs). This value appears to be within the detection limit for the case of the dielectric layers, while it is not resolvable in the absence of the dielectric layers. The predicted sensitivity of Δ (dΔ∕dn) for the coated region was calculated to be a factor of ∼4 times larger than the uncoated region. However, the enhanced sensitivity resulted in an amplification of the longtime fluctuation, therefore reducing the net gain in resolution and determining the detection limit δΔmin of the measurement. This fluctuation could relate to the sample conditions such as temperature fluctuations and mechanical instabilities. The resolution improvement is found to be significantly better when we consider interface phenomena sensing, like adsorption, rather than bulk refractometry (∼10−4 RIU). The resolution δΔmin , including the longtime fluctuation, was found in the absence of dielectric coatings to be on the order of δΔmin  0.034°. Such a change in Δ would correspond to the formation of a 1.2 nm layer with refractive index n  1.4 at the water/ fused silica interface, i.e., an adsorbed amount on the order of 120 ng∕cm2 for standard organic materials. In the presence of the dielectric coatings, the resolution was found to be δΔmin  0.07°, corresponding to a thickness of 0.21 nm, or 21 ng∕cm2 . The total resolution to adsorption sensing was therefore found to be improved ∼5.5 times.

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Fig. 3. (Color online) Study of multilayer creation on the coated prism. (Top) Phase shift Δ modification caused by successively alternating the electrolyte and the polymer solution in the flow cell. The red line is a simulation of the average phase shift modification due to the multilayer creation. (Bottom) Corresponding ring-down times have been simultaneously recorded, showing no significant changes.

The increased sensitivity allows the monitoring of the creation of multilayers with thicknesses of less than a nanometer. We demonstrated the sensitivity by measuring in situ the formation of polyelectrolyte multilayers produced by the standard layer-by-layer deposition technique [12], the successive introduction of dilute positively/negatively charged poly-electrolyte solutions and water to the TIR face of the prism. Sodium polystyrene sulfonate (PSS) and polydiallyldimethyl-ammonium chloride (PDDA) solutions at 1% by weight in 1M NaCl solution, were used. Sequences of water/PDDA/water/ PSS were repeated where each liquid was left ∼50 s in contact with the interface. Δ was measured in real time with the EW-CRDE setup and results are displayed in Fig. 3. Δ was found to decrease with increasing layer formation. Individual steps correspond to the change of refractive index between the different solutions. The change of Δ after each sequence was attributed to the variation of the thickness of the composite PDDA-PSS layer. Thickness was best estimated when in contact with water. The thickness of each layer could be estimated using a sensitivity of 0.32°∕nm calculated assuming the 30 nm TiO2 layer calibration obtained from the refractometry study. The step in Δ between two brine solutions is typically of the order of 0.11°, corresponding to a thickness of 0.34 nm (or 34 ng∕cm2 ). We see that the formation of every individual layer could be resolved with the exception of the first PDDA layer. Notably the step in Δ is approximately the same for each layer, showing a linear dependence of Δ on layer number, as expected for the particular system [12]. The inset of Fig. 3 shows the time-dependence of Δ for one sequence. Given the subsecond time resolution of the technique, the observed changes of Δ on timescales

of few seconds were attributed to the actual evolution of the sample. Additionally, as shown in the lower part of the figure, the simultaneously recorded ring-down time was found to be insensitive to this multilayer deposition process, as expected for transparent dielectric materials. In conclusion, we have demonstrated a significant enhancement in the sensitivity of s-p phase shift upon TIR, accomplished with a simple dielectric coating. Our sensing resolution is increased by more than a factor of 5 to a level of approximately 20 ng∕cm2 , on the second timescale, which is convenient for monitoring most adsorption processes. Established techniques like SPR and OWS achieve detection limits lower than 1 ng∕cm2 , down to 0.1 ng∕cm2 [13] or 10−6 RIU for SPR and silicon prism TIR measurements [9]. It is worth noting that use of resonant dielectric layer structures should lead to sensitivities up to ∼ng∕cm2 , imposing, however, extra constraints on the probe wavelength and/or alignment. The enhancement achieved allows using the simple TIR geometry to reach the area of ng∕cm2 in a subsecond time scale, which is attractive for sensing applications. In principle, the CRDE technique can detect phase shifts on the microsecond timescale. In our current setup the acquisition time is limited by the repetition rate of our data acquisition system (digital oscilloscope). However, in future implementations, a fast FPGA based detection scheme will be limited only by the laser’s 80 kHz repetition rate, permitting either an increase in sensitivity by reducing the noise via accumulation, or operation with the current sensitivity on sub millisecond timescales. We thank the EU for partial support through the European Research Council grant TRICEPS (GA no. 207542), and the FP7 IAPP SOFORT (PIAP-GA-2009-251598) and ESMI-Infrastructure CS&CSA-2010-262348. References 1. E. Kretschmann, Zeitschrift fur physik 241, 313 (1971). 2. M. Maisonneuve, I.-H. Song, S. Patskovsky, and M. Meunier, Opt. Express 19, 7410 (2011). 3. J. D. Swalen, J. Phys. Chem. 83, 1438 (1979). 4. W. Chen, L. J. Martinezmiranda, H. Hsiung, and Y. R. Shen, Phys. Rev. Lett. 62, 1860 (1989). 5. R. Konradi, M. Textor, and E. Reimhult, Biosensors 2, 341 (2012). 6. A. Karaiskou, V. Papadakis, B. Loppinet, and T. P. Rakitzis, J. Chem. Phys. 131, 121101 (2009). 7. M. A. Everest, V. M. Papadakis, K. Stamataki, S. Tzortzakis, B. Loppinet, and T. P. Rakitzis, J. Phys. Chem. Lett. 2, 1324 (2011). 8. K. Stamataki, V. Papadakis, M. A. Everest, S. Tzortzakis, B. Loppinet, and T. P. Rakitzis, Appl. Opt. 52, 1086 (2013). 9. S. Patskovsky, I. H. Song, M. Meunier, and A. V. Kabashin, Opt. Express 17, 20847 (2009). 10. S. Otsuki and M. Ishikawa, Opt. Lett. 35, 24 (2010). 11. R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light (North-Holland, 1987). 12. G. Decher and J. Schmitt, Progress in Colloid Polymer Science (Steinkopff, 1992), Vol 89, p. 160. 13. P. Kozma, A. Hámori, S. Kurunczi, K. Cottier, and R. Horvath, Sens. Actuators B 155 446 (2011).