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Monitoring adsorption and sedimentation using evanescent-wave cavity ringdown ellipsometry Katerina Stamataki,1,2 Vassilis Papadakis,1 Michael A. Everest,1,3 Stelios Tzortzakis,1 Benoit Loppinet,1 and T. Peter Rakitzis1,4,* 1

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

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

3

Department of Chemistry, Westmont College, Santa Barbara, California 93108, USA 4

Department of Physics, University of Crete, Heraklion-Crete 71003, Greece *Corresponding author: [email protected]

Received 8 November 2012; revised 21 December 2012; accepted 23 December 2012; posted 7 January 2013 (Doc. ID 179550); published 8 February 2013

We monitor the adsorption of Rhodamine 800, and the sedimentation of a polytetrafluoroethylene (PTFE) suspension at the surface of a fused-silica prism, by measuring both the absorption and s-p phase shift Δ of a 740 nm probe laser beam, using evanescent-wave cavity ringdown ellipsometry (EW-CRDE). The two systems demonstrate the complementary strengths of EW-CRDE, as the progress of adsorption of the Rhodamine 800 dye can only be observed sensitively via the measurement of absorption, whereas the progress of sedimentation of PTFE can only be observed sensitively via the measurement of Δ. We show that EW-CRDE provides a sensitive method for the measurement of Δ and demonstrates precision in Δ of about 10−4 deg. © 2013 Optical Society of America OCIS codes: 120.2130, 260.6970, 120.5050.

1. Introduction

Cavity ringdown spectroscopy (CRDS) [1–4], evanescent wave (EW) spectroscopy [5], and ellipsometry (E) [6], are three widely used techniques with distinct advantages for the spectroscopic interrogation of samples. CRDS has been introduced as an efficient means to measure low absorbance. It does so by measuring the time dependence of the intensity decay of a pulse of light in a high-finesse optical cavity. Advantages include an extremely large path length (which can be in the range of 10 km, for 104 passes through a 1 m cavity length), and insensitivity to laser shot-to-shot intensity fluctuations (as the measurement of the ringdown time depends only on the cavity losses, and not on the intensity of the laser 1559-128X/13/051086-08$15.00/0 © 2013 Optical Society of America 1086

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pulse). EWs, which are produced at the surface of total internal reflection (TIR) of the probe beam, can be used to probe the interface approximately within one wavelength of the TIR surface. Ellipsometry, which is the measurement of the change in the polarization state of the reflected light of a probe light beam from a surface, is used to obtain information on the interfacial optical properties (such as the refractive index and absorption, which often is used to determine the composition and thickness of thin films). Of these three techniques, the three ways of combining two of them have all been achieved: EW-E [7], EW-CRDS [8–17], and CRD-E [18]. EW-E is an established technique, and allows the ellipsometric probing of liquid, solid, and thin film samples placed in the EW of light formed by TIR at the surface of a prism. The low optical losses (from scattering and absorption) of the TIR geometry allow the combination of the EW-CRDS techniques to enhance absorption

signals. Recently, Karaiskou et al. demonstrated CRD-E [18], showing how the s-p phase shift, φsp , can be enhanced by the number of mirror reflections N, in a ring cavity, to give a total phase shift of Nφsp . This large, time-accumulated phase shift produces a polarization beating in the ringdown signal with frequency ω, from which φsp can be determined accurately (with about 0.01° precision) from a single ringdown trace (with better than microsecond time resolution). Very recently, Everest et al. [19] have shown how to combine all three techniques, to introduce evanescent-wave cavity-ringdown ellipsometry (EWCRD-E), in which the advantages of all three techniques are able to be combined: sensitive, cavity-enhanced, and fast ellipsometric measurements of samples in the microvolumes of EWs. In this paper, we describe in more detail how the experimental signals are analyzed to obtain the ellipsometric/ polarimetric angles, sensitively, and in a single laser shot. We also illustrate the power of the technique by demonstrating a few distinct cases of changes at the solution-prism interface using a liquid cell: the adsorption of a dye and of polymer chains, and the sedimentation of a suspension of polymeric particles. 2. Theory and Data Analysis

When a laser pulse is introduced into a two-mirror optical cavity, with mirror reflectivities Rm ≈ 1 and a laser pulse duration shorter than the cavity roundtrip time, the time dependence of the light intensity exiting the cavity, It, is approximated well by an exponential decay: −t∕τ

It I 0 e

;

d∕c ; 1 − Rm

Es N E0s

p2N p2N Rm Rs e2iNφs ;

p E0s E0 1 − Rm Rs cos θi cos θo eiφs :

where d is the mirror separation. Our experimental setup is somewhat more complicated (see Fig. 1), with the addition of a third reflective surface in the cavity, that of the TIR inside the prism, as well as the addition of an input polarizer (with the axis at angle θi to the axis perpendicular to the cavity plane) and an output polarizer (at angle θo ).

(4)

The number of round-trip cavity passes N is related to time t by N ct∕2d, where 2d is the round-trip cavity length. For reflectivities Rm and Rs close to unity, Rm Rs N is well-approximated by e−t∕2τs . Substituting these two expressions into Eq. (3) allows it to be written as Es t E0s e−t∕2τs eictφs ∕d ;

(5)

where τs

d∕c : 1 − Rm Rs

Similarly, the analogous expressions polarized light can be written: (2)

(3)

where each round-trip pass within the cavity requires two mirror reflections and two TIR-surface reflections, the phase shift for the two TIR-surface reflections is 2φs, and E0s is the electric field amplitude for s-polarized light that passes directly through to the detector (i.e., N 0), given by

(1)

where τ is the ringdown time, given by τ

The reflectivity of the TIR surface of the prism is Rs and Rp for s- and p-polarized light, respectively. We follow a similar derivation as that for Eq. (1), at first considering separately the electric field amplitudes for the s- and p-polarization modes of the cavity. After N round-trip passes through the cavity, the electric field amplitude for s-polarized light is given by

Ep N E0p

p2N q2N e2iNφp ; Rm Rp

q E0p E0 1 − Rm Rp sin θi sin θo eiφp ; Ep t E0p e−t∕2τp eictφp ∕d ; τp

d∕c : 1 − Rm Rp

(6) for

p-

(7)

(8) (9) (10)

The total time-dependent light intensity exiting the cavity is then given by the square of the sum of s- and p-polarization electric field amplitudes given by Eqs. (5) and (9): It jEs t Ep tj2 ; Fig. 1. EW-CRDE experimental apparatus for the study of gas or liquid samples at the interface of a prism, similar to that used in [19], but the addition of a flow cell.

(11)

which can be expressed explicitly in terms of the ringdown times τs and τp , and the polarization beating ω, as 10 February 2013 / Vol. 52, No. 5 / APPLIED OPTICS

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It I 0 1 − Rm 2 Rs cos2 θi cos2 θo e−t∕τs Rp sin2 θi sin2 θo e−t∕τp q 2 Rs Rp sin θi cos θi sin θo × cos θo e−1∕τs 1∕τp t∕2 cosωt Δ;

(12)

where the polarization beating ω is given by ω cΔ∕d

(13)

and where Δ φp − φs . Note that the phase shift Δ, in the term cosωt Δ, is derived from the fact that the first output pulse (which defines t 0) has undergone a single TIR reflection, with s-p phase shift Δ. When absorption losses at the TIR surface are small, then Rs ≈ Rp ≈ 1, for which τs ≈ τp τ. Using these approximations, and setting θi θo 45°, Eq. (12) can be simplified to −t∕τ

It Ae

2

cos

ωt Δ ; 2

(14)

where A is a normalization constant. In practice, we find that a more general fit function is more convenient for fitting experimental signals: It Ae−t∕τ cos2 ωt∕2 ϕ B:

(15)

Ideally, the constants B and ϕ should be equal to 0 and Δ∕2, respectively. However, experimental imperfections, in particular linear birefringence of the prism, cause B and ϕ to depart from their ideal values, and to be best treated as fit parameters. The important result of Eq. (15) is that the experimental signals are fundamentally simple, taking the form of an exponential decay multiplied by an oscillation, from which the frequency ω can be measured, allowing the determination of Δ from Eq. (13). Figure 2 shows a typical experimental EW-CRDE signal, where the exponential decay τ and the oscillation with frequency ω (with period of about 85 ns) can be observed. We discuss here how to invert τ and ω from the data. The faster oscillation that is also visible in the data, with a period of about 6 ns, is related to the round-trip time of the fs laser pulse through the cavity, is discussed in more detail in the Experimental section. We employ a method to invert τ and ω from the data using the fast Fourier transform (FFT). The analysis of the ringdown traces is performed on a shot-to-basis in real time, and the steps of the analysis procedure are shown in Fig. 3. The raw data, shown in Fig. 3(A), is first fit to an exponential decay [20], which is then subtracted from the data, yielding a decaying oscillation signal, shown in Fig. 3(B) [note that the oscillation in Fig. 3(B) now occurs symmetrically about the x axis]. The Fourier transforms of the signals in Figs. 3(A) and 3(B) are shown in Figs. 3(C) and 3(D), respectively. In Fig. 3(C), we see the large 1088


Fig. 2. Typical experimental EW-CRDE traces for a sample of air. Two periodic signals are observed: the slow polarization beating (period of about 84 ns) from the s-p phase shift, and the fast oscillation (period of about 6 ns) from the round trip time of the 800 nm light pulses from the Ti-sapphire femtosecond laser.

signal at ∼74 MHz, which is the polarization beating frequency ω, and also the peak at ∼1 GHz, which is related to the laser-pulse round-trip frequency. In fact, the peaks from the individual laser pulses are twice as frequent, every 6 ns instead of the expected 12 ns. The extra peak is likely caused by electronic ringing, and also by backscattering at the prism TIR surface, creating a pulse with opposite directionality to the original pulse. In addition, the sum and differences of the frequencies at ∼74 MHz and ∼1 GHz can also be seen. The Fourier transform of an exponential decay gives a Lorentzian centered at ω 0. This large Lorentzian can potentially obscure signals close to it.

Fig. 3. Analysis of the EW-CRDE data: (A) The ringdown trace is fit to an exponential decay; (B) the exponential is subtracted from the data, leaving only the oscillating part of the signal; (C) the Fourier transform of (A), yielding the frequency components of the signal, in particular the polarization beating at 0.074 GHz, the cavity round-trip frequency at about 1 GHz, and a large Lorentzian peak at 0, which partially overlaps with the 0.074 GHz peak; and (D) the Fourier transform of (B) showing that the subtraction of the exponential decay removes the large Lorentzian peak at 0, so that the peak at 0.074 GHz is now baseline resolved.

The tail of this Lorentzian overlaps somewhat with the signal of interest at about 74 MHz. The overlap problem becomes worse for the measurement of smaller beat frequencies. The subtraction of the exponential from the raw data [Fig. 3(B)] minimizes this problem. The Fourier transform of the data, after the exponential has been subtracted, is shown in Fig. 3(D); we see that the Lorentzian at ω 0 has been largely removed, that the polarization beating peak at 74 MHz can be observed more clearly, and that smaller beating frequencies can be observed without interference by the Lorentzian at ω 0. The center of the polarization-beating Lorentzian peak, ωmax [such as those shown in Figs. 3(C) and 3(D), at ∼74 MHz], are determined by considering the three points that are closest to the peak: x1 ; y1 , x2 ; y2 , and x3 ; y3 . From these 6 coordinates, the value of ωmax can be determined simply and quickly by the function ωmax

x21 y1 y3 − y2 x22 y2 y1 − y3 x23 y3 y2 − y1 ; 2x1 y1 y3 − y2 x2 y2 y1 − y3 x3 y3 y2 − y1 (16)

which is determined by simultaneously solving the three equations, yi a∕xi − ωmax 2 b (i 1, 2, 3), for a, b, and ωmax . This extrapolation method of determining ωmax is expected to work best for narrow peaks in the FFT trace (where the side points are about at the half-maximum of the middle point), and allows the optimum sampling rate to be determined. 3. Experimental

A schematic for the experimental setup is shown in Fig. 1, which is similar to the one used by Everest et al. [19], with the addition of a flow cell attached to the prism, which allows for the application of liquid or gas samples. Briefly, a linearly polarized, pulsed laser beam passes through the input polarizer, and is then coupled into a high-finesse cavity. Two different laser beams were used for the experiments. One was a 740 nm pulsed laser beam generated from an excimer-pumped dye laser (Lambda Physik LPX300 and LPD3000), operating at 10 Hz, with pulse width of about 30 ns and a pulse energy of about 5 mJ∕ pulse. The second was a Ti:sapphire amplified pulsed laser system, operating at 1 kHz repetition rate, with a central wavelength at 800 nm, a pulse width of about 35 fs, and a pulse energy up to 2 mJ∕pulse. The cavity mirrors were Newport SuperMirrors, with reflectivity R > 0.9997 between 761–867 nm (though the reflectivity was also R > 0.999 at 740 nm) and a radius of curvature of 1 m. The entrance and exit surfaces of the prism were antireflection (AR) coated for light between 740–850 nm, with reflection losses of about 0.1%. Thus, significantly longer ringdown times could have been achieved if the quality of AR coating were reduced to about 0.01% (alternatively, similar ringdown times, with larger signals, could have been achieved using mirrors with R ≈ 0.999). The cavity length was 1.8 m, with the prism placed

approximately in the center. Ideally, the cavity length should be greater than the pulse length (cΔt, where c is the speed of light, and Δt is the pulse width), to avoid interference effects of the laser pulse within the cavity, which can change the shape of the ringdown trace. This condition was satisfied for only one of our two laser sources; however, no significant problems were observed with the longer laser pulse. Note that the input polarization and detection polarization directions are both set to 45° (with equal projections on the s and p axes). A half-wave plate is used to rotate the laser polarization at the input to 45° (in our case, this involves setting the half-wave plate to 22.5°). The trapezoidal prisms were made of fused silica (n 1.453 at 800 nm) with a 1 cm2 square face used for TIR and two AR coated faces with 70.0° 0.1° angle with the TIR face. Fused silica chosen for the prism material to ensure low absorption and scattering losses, necessary to achieve about 1000 cavity passes. The prism angle of 70° was chosen so that the TIR condition is satisfied by aqueous samples (with refractive index n ≈ 1.33, and critical angle θC ≈ 66°). The laser beam entered normal to the entry faces with an incidence angle of 70° with the face of interest. The size of the region probed by the EW is set by the laser spot on the TIR face for the lateral dimensions and by the penetration depth of the EW intensity for the normal dimension. For 1 m focal length mirror and laser beam size of 2 mm, the lateral dimension of the laser-focus spot size at the TIR surface is typically of the order of 300 × 825 μm as the oblique 70° incidence imposes a 2.75 times larger extent along the prism surface. The extent of the beam normal to the surface is set by the penetration depth of the EW intensity, which depends on the refractive index ratio between the two medium and the incidence angle, given by dp λ∕2πn1 sin2 θ − n2 ∕n1 2 1∕2 . For 70° incidence, dp 0.124λ (∼100 nm at 800 nm) for silica–air interface, and dp 0.374λ (∼300 nm at 800 nm) for silica–water interface. Signals were measured with a photomultiplier tube (PMT, Hamamatsu H7732-10), and the photocurrent was analyzed by a fast oscilloscope (either a LeCroy 104MXi-A, 1 GHz, 10 GSamples∕s, as used for data in Figs. 2, 3, 5, or a LeCroy 9310A, 400 MHz, 100 MSamples∕s, as used for data in Figs. 4 and 6). The averaged ringdown traces (typically between 10–100 laser shots) were analyzed (as described in Section 2) in real time, returning the ringdown time τ and the polarization beating frequency ω. Typical precision in changes in τ and ω is 1% and 0.1%, respectively, which was sufficient for the timedependent systems studied here (better precision was obtained with signal averaging on time-invariant signals). A home-made stainless steel flow cell (volume of about 5 ml) was used to contain liquids and gases at the TIR interface of the prism. A rubber o-ring 10 February 2013 / Vol. 52, No. 5 / APPLIED OPTICS

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Fig. 4. (Color online) Phase angle variation of a water sample, showing long-term stability over several minutes of δΔ ∼ 10−3 deg, corresponding to a refractive-index variation of δn ∼ 10−5 . Shortterm variations can be at least an order of magnitude smaller. Sources of variation include temperature and laser-beam-profile fluctuations.

contacted the prism and ensured a tight seal. At t 0, the fluid was injected (in about 1 s), and subsequent measurements were performed under quiescent conditions (stop flow). A typical ringdown trace using the fs laser is shown in Fig. 2 for an air sample, showing the polarization beating. Measurement of ω, described in Section 2, gives the s-p phase shift Δ from Eq. (13). Subsequently, the refractive index of a sample in the EW, nsample , is given by q nsample sin θ 1 − tan2 Δ∕2tan2 θ; (17) nprism where θ is the incidence angle between the laser beam and the surface normal; in our experiments, θ 70° and nprism 1.456. Typical phase angles are Δ ≈ 27° and Δ ≈ 9°, for samples of air (n ≈ 1.00) and water (n ≈ 1.33), respectively. In Fig. 4, we show a real-time measurement of Δ and the refractive index of a water sample placed in the EW. Without stabilizing the temperature of the sample or prism, and using our laser with a repetition rate of 10 Hz (each measurement represents an average over 300 laser shots), we observe drifts in Δ of order 10−3 deg, corresponding to drifts in refractive index of order 10−4. Not shown here, using our laser with a repetition rate of 1 kHz, averaging 500 laser shots, we were able to measure Δ with a precision of about 10−4 deg. We investigated the effects of gas pressure in the flow cell attached to the prism. Changing the gas pressure in the flow cell causes stress-induced birefringence in the prism, which changes the value of the s-p phase shift, Δ, and therefore, the beat frequency ω. To investigate the effect of gas pressure in the flow cell on ω, the flow cell was evacuated and xenon gas was introduced, at pressures between 0 and 800 mbar. We found that ω depends sensitively and reproducibly on the pressure of xenon in the cell. In Fig. 5(A), the polarization beating frequency ω is shown to vary linearly with the gas pressure, changing about 500 kHz over the 800 mbar pressure range, corresponding to maximum effective change in the φsp phase shift of 1090


Fig. 5. (Color online) (A) Stress-induced birefringence caused by the pressure of Xe gas, inducing a linear change in the beating frequency of about 0.6 MHz atm−1 . Error bars are the 95% confidence interval. (B) Data traces for 0 and 100 mbar Xe gas, at long times of about 11 μs, showing the differences in the beat frequency as visible differences in signal (the two signals are overlapped at short times). (C) Fourier transform of (B). Note slight changes in the points adjacent to the maximum, from which the line centers are determined (see Eq. 2).

0.5°. In Fig. 5(B), the experimental signals from xenon pressures of 0 and 100 mbar are shown at ringdown times of about 11 μs, where the small differences in their polarization beating frequencies ω can be seen clearly. The Fourier transform of the data in Fig. 5(B) is shown in Fig. 5(C), where the individual points from the discrete Fourier transform can be seen. The observed change of about 0.6° atm−1 is much too large to be attributed to the change of refractive index due the introduction of the Xe gas. In particular, for Xe (with n 1.0007), we expect a phase change Δ 0.02° atm−1 , which is about 30 times smaller than what we observe. Therefore, the change in Δ is largely attributed to stress-induced birefringence in the fused silica prism. This first example demonstrates the sensitivity of the technique, and points out challenges in the determination of absolute s-p phase shift angles, due to the presence of stressinduced birefringence in the prisms. 4. Results

The ability of EW-CRDE to detect both reflectivity and the s-p phase shift at the TIR surface is illustrated in the following three examples.

A.

Rhodamine 800 Adsorption

The changes in reflectivity induced by the adsorption of Rhodamine dye molecules, onto the fused silica TIR surface, was monitored with the EW-CRDE setup. A solution of Rhodamine 800 dye was introduced using the flow cell. Figure 6(A) shows the change in the ringdown time as a function of time after adding the Rhodamine solution, showing how the adsorption of the dye molecules causes light absorption and decrease of the ringdown time. At 1000 s, the ringdown time has fallen to half the initial value, at which point the solution was rinsed with pure solvent, and the ringdown time increases again, showing that the dye molecules begin to desorb; however, the ringdown time does return to the original value (before dye application), indicating the presence of strongly adsorbed dye molecules at the prism surface. The polarization beating frequency is measured simultaneously and shown in Fig. 6(B). No significant change in the polarization beating frequency, ω, due to adsorption or desorption of the dye molecules is observed. These data can be interpreted as in a typical EW-CRD experiment, and a characteristic time for the formation of an adsorbed layer (∼100 s) is observed. The adsorption of dye on SiO2 surface has received some attention [21]. It is analyzed in term of Langmuir adsorption (competition between adsorbtion and desorption leading to a monolayer formation).

The absorbance is simply related to the decay time of the cavity: 1 d 1 1 A − ; ln 10 c τ τ0

(18)

and it can be used to estimate an adsorbed amount. The ringdown time evolution measures a local increase of Rhodamine concentration in the evanescent field, i.e., near the silica surface as a result of the Rhodamine adsorption. From Fig. 6(A), we see that the introduction of the dye solution rapidly increases the absorbance (and decreases the ringdown time from τ0 1.4 to τ 1.2 μs), which corresponds to an absorbance of 3 × 10−4 in the presence of the dye solution. Subsequently, dye molecules adsorb to the TIR silica surface, and over 900 s the ringdown time decreases from τ0 1.2 to τ 0.8 μs, corresponding to an absorbance of Aads 7 × 10−3 for the adsorbed layer of dye molecules. The molar absorptivity for Rh800 in methanol at 740 nm was measured from a solution UV-vis transmission measurement to be ε 10.7 × 103 mol−1 l cm−1 , or 10.7 × 106 mol−1 cm2. The Rh800 adsorbed amount can be deduced as Aads ∕ε 7 × 10−3 ∕10.7 × 106 0.65 × 10−9 mol∕cm2 or 3.2 mg∕m2.

Fig. 6. (A) Ringdown and (B) beating frequency measurements for the time-dependent adsorption of a solution of Rhodamine 800 dye to the prism surface. (C) Ringdown and (D) beating frequency measurements for the time-dependent sedimentation of the PTFE polymer suspension. 10 February 2013 / Vol. 52, No. 5 / APPLIED OPTICS

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B.

Particle Sedimentation

For materials that absorb too weakly for the absorption to be measured sensitively, an alternate method of detection is offered by the measurement of the refractive index of the material. In the case of material adsorption in the EW at a TIR surface, the change in the refractive index at the interface causes a change in the s-p phase shift Δ of the reflected light. The ability to sense Δ as a result of adsorption, particularly in the case of weakly absorbing materials, is given in the two following examples. A gravity-driven sedimentation of micrometric polytetrafluoroethylene (PTFE) dielectric particles onto the fused silica TIR surface leads to a measurable change in Δ with no large corresponding variation of the ringdown time. The sedimentation is observed by introducing a suspension of PTFE using the flow cell. PTFE is used as it is almost index match with water, and therefore reduces the scattering losses; also, the high density of PTFE (∼2 g∕cm3 ) leads to fast sedimentation. Here, the observed results are in strong contrast to that observed for the Rhodamine adsorption: in Fig. 6(C), no strong changes in the ringdown time is observed, whereas in Fig. 6(D), the polarization beating frequency changes visibly. The evolution of Δ reflects a local change of the refractive index profile within the evanescent field, which we attribute to the enrichment in PTFE particles within the EW as sedimentation proceeds. The size of the PTFE particles are of the same order as the penetration depth of the EW (∼1 μm), so that only the first layer of particles is probed. The sedimentation velocity determined from Stokes law is about 1 μm∕s, without taking into account the slow down due to the vicinity of the wall. The long timescale of the observed variation in Δ (∼1000 s) indicates the presence of structural rearrangement of the interfacial layer. Further analysis is hindered by large size of the particles (similar or larger than the penetration depth of the EW), and the lack of precise information on the distribution of the size of the particles. 5. Discussion and Conclusion

We presented, in detail, the EW-CRDE technique, an extension of EW-CRDS, which includes measurement of the time-dependent polarization properties of the light, in particular measurement of the s-p phase shift, Δ, by introducing light into the cavity with large components of both s- and p-polarized light. We demonstrated the ability of EW-CRDE to monitor adsorption, particularly in the case where the adsorbing moiety does not absorb light measurably, and would not otherwise be able to be detected using EW-CRDS. Therefore, measurement of the polarization properties of the light yields complementary information to the more standard ringdown time measurements of the sample absorption. There are a number of techniques that utilize singlepass polarization spectroscopy as a detection method for adsorption, such as surface plasmon resonance (SPR), dual-beam polarization interferometry, and 1092


conventional ellipsometry. The advantage of the cavity-enhanced EW-CRDE technique is the observables, the absorption and the s-p phase shift, can be measured in a single ringdown trace from a single laser pulse, on the microsecond timescale, and even observe changed within a single ringdown trace, on the nanosecond timescale. These timescales are orders of magnitude faster than the other techniques. This time resolution should allow the observation of surface responses on the nanosecond and microsecond timescales, such as laser-induced deformation and ablation of solid surfaces and thin films. A well-known limitation of the use of the s-p phase shift Δ is the relatively low sensitivity to the change of refractive index at the surface. Several methods have been applied to increase the sensitivity of Δ to the refractive index profile at the interface, such as (a) using substrates with a large refractive index, such as sapphire and silicon [22], (b) applying multilayer dielectric thin films to the TIR surface [23], and (c) using SPR and plasmonic nanostructures at the TIR surface [24]. These improved phase-sensitivity schemes can be applied to EW-CRDE to improve the sensitivity of the s-p phase shift to changes in the refractive index at the TIR interface, with no major changes of the overall setup. In conclusion, EW-CRDE is a label-free sensor, with a good temporal resolution. Future improvements include improving the ellipsometric sensitivity by modifying the substrate as mentioned above, and adding spectroscopic resolution, using a white light laser and multichannel detector. References 1. A. O’Keefe and D. A. G. Deacon, “Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources,” Rev. Sci. Instrum. 59, 2544–2551 (1988). 2. M. D. Wheeler, S. M. Newman, A. J. Orr-Ewing, and M. N. R. Ashfold, “Cavity ring-down spectroscopy,” J. Chem. Soc., Faraday Trans. 94, 337–351 (1998). 3. G. Berden, R. Peeters, and G. Meijer, “Cavity ring-down spectroscopy: experimental schemes and applications,” Int. Rev. Phys. Chem. 19, 565–607 (2000). 4. B. A. Paldus and A. A. Kachanov, “An historical overview of cavity-enhanced methods,” Can. J. Phys. 83, 975–999 (2005). 5. F. de Fornel, Evanescent Waves: From Newtonian Optics to Atomic Optics (Springer, Berlin, 2001). 6. R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light, North-Holland (Amsterdam, 1987). 7. W. Chen, L. J. Martinez-Miranda, H. Hsiung, and Y. R. Shen, “Orientational wetting behavior of a liquid-crystal homologous series,” Phys. Rev. Lett. 62, 1860–1863 (1989). 8. C. Vallance, “Innovations in cavity ring-down spectroscopy,” New J. Chem. 29, 867–874 (2005). 9. L. van der Sneppen, F. Ariese, C. Gooijer, and W. Ubachs, “Liquid-phase and evanescent-wave cavity ring-down spectroscopy in analytical chemistry,” Annu. Rev. Anal. Chem. 2, 13–35 (2009). 10. M. Schnippering, S.R.T. Neil, S. R. Mackenzie, and P. R. Unwin, “Evanescent wave cavitybased spectroscopic techniques as probes of interfacial processes,” Chem. Soc. Rev. 40, 207–220 (2011). 11. H. Waechter, J. Litman, A. H. Cheung, J. A. Barnes, and H.-P. Loock, “Chemical sensing using fiber cavity ring-down spectroscopy,” Sensors 10, 1716–1742 (2010). 12. L. van der Sneppen, C. Gooijer, W. Ubachs, and F. Ariese, “Evanescent-wave cavity ringdown detection of cytochrome c

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