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liquid-cladding integrated silicon ARROW waveguides
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Optics Communications 281 (2008) 2062–2066 www.elsevier.com/locate/optcom
Liquid-core/liquid-cladding integrated silicon ARROW waveguides R. Bernini a,*, E. De Nuccio b, A. Minardo b, L. Zeni b, P.M. Sarro c b
a IREA-CNR, Via Diocleziano 328, 80124 Napoli, Italy DII, Seconda Universita` di Napoli, Via Roma 29, 81031 Aversa, Italy c ECTM-DIMES, TUDelft. NL-2600 GB Delft, The Netherlands
Received 17 July 2006; received in revised form 20 November 2007; accepted 27 November 2007
Abstract The fabrication and characterization of a liquid-core/liquid-cladding integrated antiresonant reflecting optical waveguide (L2-ARROW) is presented. In this waveguide, the light is confined vertically by the ARROW mechanism, whereas the lateral confinement is obtained by using liquid-core/liquid-cladding (L2 waveguides) with different refractive indexes. This approach permits to realize L2 waveguides with very low refractive index core (n 1.333) and represents a new solution to solve the difficulty to reduce the optical losses in 2D-ARROWs due to the TM polarization in lateral direction. The device has been fabricated with standard silicon technology. The results show that the optical properties can be tuned by changing the type and the flow velocity of the core and the cladding liquids. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Liquid-core waveguides are of considerable interest in many fields, ranging from sensors to telecommunications. In particular, in sensing applications, this interest is motivated by the increased interaction efficiency offered by these waveguides. In fact, liquid-core waveguides permit to simultaneously confine the light and the substance to be probed in the waveguide core. By means of integrated planar technology, several types of liquid-core/solid-cladding have been fabricated [1–8]. Recently, liquid-core/ liquid-cladding (L2) waveguides have been proposed and fabricated using both glass capillary technology [9], and planar geometry by replica molding polymeric techniques [10,11]. These waveguides are microfluidic devices in which the light is confined inside a high refractive index liquid (the core) by a low refractive index liquid (the cladding), both liquids flowing laminarly [10]. Because L2 waveguides are dynamic, many optical properties (refractive index contrast, geometry, etc.) can be easily changed.
*
Corresponding author. Tel.: +39 0815707979; fax: +39 0815705734. E-mail address: [email protected] (R. Bernini).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.11.070
In this paper, we present the fabrication and the characterization of an integrated silicon liquid-core/liquid-cladding antiresonant reflecting optical waveguide (L2ARROW). Differently from other L2 waveguides, where the radiation is guided by total internal reflection (TIR), in the proposed waveguide the light is vertically confined inside the core by realizing a Fabry–Perot mirror with high reflectivity (ARROW mechanism) [1]. The proposed solution integrates the capability offered by L2 waveguides and ARROW waveguides. In fact, L2 waveguides permits to perform a dynamic optical confinement by tuning the optical properties of the liquids and/or by varying the flow rates, obtaining waveguides with different core dimensions and consequently different modal responses. At the same time the ARROW behavior adds to the L2 waveguides the ability to tailor the frequency or wavelength response of the device. Moreover, differently from conventional L2 waveguides, in which the refractive index of the liquid-core must be higher than the refractive index of the substrate in order to obtain the vertical confinement by TIR, the ARROW mechanism permits us to consider liquids with low refractive index (e.g. water solution n 1.333), getting rid of the limits arising when substrates with high refractive index are used. Beside to
R. Bernini et al. / Optics Communications 281 (2008) 2062–2066
these advantages we have some disadvantages introduced by the L2 waveguides like the increased fluidic complexity, a loss of waveguide index contrast, and a possible sensitivity to the vibrations [12]. In the following, we describe the device and the principal fabrication steps. Afterwards, we show the results of the optical characterization of the L2-ARROW, comparing the experimental result to the numerical analysis of the structure. We also report the results of the L2-ARROW optical losses investigation, carried out by measuring the output optical power values for both the L2-ARROW and the simple ARROW waveguide. The experimental results shows that the L2-ARROW approach permits to reduce the optical losses of a 2D-ARROW arising from the bad confinement of TM polarization in lateral direction and can be a good alternative to the rather complicated ARROW structures previously proposed for a good lateral confinement [13]. 2. Design and fabrication Fig. 1 is a schematic drawing of the L2-ARROW waveguide device. The structure is composed of a central channel where the liquid-core is injected and two lateral arms, in which the cladding liquids flow. Both the liquid-core and the liquid-claddings are injected into the channels by a syringe pump system. By adequately tuning the flow rates of the liquids, the flowing core liquid section is reduced between the flowing slabs of the cladding fluid, due to the hydrodynamic focusing effect. At low and moderate Reynolds numbers, the flow is laminar, and the liquid/liquid interfaces are optically smooth [10]. Manipulating the flow rate and the composition of the liquids, the characteristics of the optical system can be tuned. Seeing the figure, the central channel is designed to have an additional inlet for an optical fiber to easily couple the optical radiation into the L2-ARROW. Fig. 2 shows the A–A’ cross section of the waveguide. The device is based on the ARROW waveguide, in which the light is confined inside the core region with refractive index nc, which is lower than the refractive indices of the surrounding media, by the two cladding layers designed to form a high reflectivity Fabry–Perot antiresonant cavity.
Input optical fiber
Cladding Liquid Inlets
Microscope Objective 20X
A CCD Camera
Core Liquid Inlet
A’
Glass Slide
Fig. 1. Schematic drawing of the L2-ARROW waveguide device.
2063
Substrate
Si SiO2 SiO2
Upper half
d1 n 1
Si3N4 Si3N4
Core
d2 n 2 d2 n 2 d1 n1
Lower half y
x
Section A-A’
Substrate
Fig. 2. Transverse section A–A0 of the (hollow core) ARROW waveguide.
For a fixed core width dc and refractive index nc, the antiresonant condition for the equivalent 1D vertical structure can be written, for the cladding layers thicknesses, as [1] " ! 2 #1=2 k n2c k d 1;2 ¼ 1 ð2N þ 1Þ; þ 4n1;2 2n1;2 d c n21;2 N ¼ 0; 1; 2; . . .
ð1Þ
where k is the operating wavelength. The waveguide is composed by two halves of a silicon wafer (ns = 3.87 i0.02) joined together. The channels were realized by deep silicon dry etching (resulting in a 200 150 lm2 rectangular-core), followed by an LPCVD deposition of silicon dioxide (TEOS) and silicon nitride at the temperature of 850 °C. The two dielectric layers have refractive index n1 = 2.227 and n2 = 1.457, respectively, where the values for the refractive indexes n1, n2 and ns are measured at a wavelength k = 633 nm. The cladding layers thicknesses are: d1 = 266 nm (N = 1) and d2 = 266 nm (N = 0), according to the antiresonant condition (see Eq. (1)) at k = 633 nm and nc = 1.330. Before the LPCVD depositions, the inlet holes were realized by deep silicon dry etching from the bottom side. Then, the two halves were joined by silicon nitride wafer bonding. The picture of the bottom half wafer is shown in Fig. 3, where the microfluidic path, the inlet holes and the groove for the excitation fiber are clearly identifiable. The overall size of the die is 2 1 cm2. The final step is represented by the tubing system assembling: tygon tubes with internal diameter of 0.7 mm were connected to the inlet holes using epoxy and successively are interconnected to a syringe pumping system. The choice of a large (200 150 lm2) rectangular-core dimension permits a simple optical input coupling by the insertion of a multimode optical fiber (100 lm core diameter and 130 lm cladding diameter) directly inside the core of the hollow waveguide. This permits a self-alignment of the waveguide core to the fiber. The fiber was sealed to the waveguide by optical adhesive and the terminations of both the two lateral channels and the orthogonal channels were sealed by optical adhesive.
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R. Bernini et al. / Optics Communications 281 (2008) 2062–2066 Inlets for the liquids 1
Measured intensity profile Simulated intensity profile
Ethylene glycol
200 μm
0.8
Intensity [a.u.]
1 cm 0.6
0.4
Fiber groove
2 cm 0.2
Fig. 3. Picture of the realized lower half wafer. 0 -100
3. Optical characterization
0
50
100
x [μm]
Fig. 4. Measured and simulated intensity profiles of the output light versus the width of the channel. The liquid-core is ethylene glycol. The insets show the CCD camera image of the light intensity distribution at the waveguide end facet.
1
Measured intensity profile Simulated intensity profile
0.9 0.8 Glycerol:water mixture
200 μm 0.7
Intensity [a.u.]
In this section, we report the experimental results carried out in order to investigate the L2-ARROW optical properties. The measurements were performed by using an LED at k = 635 nm, a CCD camera and a syringe pump system to control the flow rate at the inlets. A thin glass slide (130 lm) was placed near to the output channel in order to avoid lens effects due to the liquid surface tension. The light exiting the structure, 1.5 cm away from the cladding liquid inlets point, was imaged on a CCD camera through a 20 microscope objective. Deionized water with a refractive index ncl = 1.333 was used as the focusing (cladding) liquid, whereas the core fluid was either ethylene glycol or a glycerol:water mixture (10% by volume), presenting refractive indexes nc = 1.431 and nc = 1.347, respectively. We chose these solutions for two reasons. First, the chosen fluids are miscible and, therefore, will mix diffusively. Second, many organic dyes are soluble in ethylene glycol. It is important to underline that the choice of the liquids is made only to assure the total internal reflection mechanism for horizontal confinement, whereas the ARROW mechanism guarantees the vertical confinement of the light. The measurements were carried out for different cladding/core flow rate ratio (FRR), ranging in a range from 1 to 4, with a core liquid flow rate fixed to 5 ml/h. Note that the flow rates of the liquids are referred to the inlets. Figs. 4 and 5 show the output light intensity profile as a function of the width of the channel, by setting FRR = 1 and glycerol:water mixture and ethylene glycol as the liquid-core, respectively. In the insets of the figures, it is possible to observe the light confinement in the L2-ARROW under the same conditions. The dotted lines in the insets represent the plot scan lines. In the figures we also report the simulated intensity profiles. The validity of the experimental results was verified by a numerical analysis. The simulations were performed first by solving the 2D coupled Navier–Stokes and diffusion/convection fluid equations using the finite elements method (FEM), so as to obtain the liquid concentration profile along the x-axis.
-50
0.6 0.5 0.4 0.3 0.2 0.1 0 -100
-80
-60
-40
-20
0
20
40
60
80
100
x [μm]
Fig. 5. Measured and simulated intensity profile of the output light versus the width of the channel. The liquid-core is glycerol:water mixture. The insets show the CCD camera image of the light intensity distributions at the waveguide end facet.
Then, assuming that refractive index profile of the L2ARROW waveguide is directly proportional to the concentration of the components of the liquids [16], the optical simulations were carried out by exciting the liquid waveguide with an uniformly distributed input optical field. The different shape of the liquid-core in the two cases (Figs. 4 and 5, inset) is due to a 3D microfluidic effect. In a fact, in dependence of the liquid properties, in pressure-driven microfluidic flow has been shown both
R. Bernini et al. / Optics Communications 281 (2008) 2062–2066
theoretically and experimentally [14,15] that the extent of the transverse diffusive zone near the top and the bottom walls can be larger respect to middle of the channel, as we observed for the glycerol:water mixture (Fig. 5, inset). This effect is due the no-slip nature of the top and the bottom walls of the channel affect the flow profile, and so the diffusive mixing is more extensive (i.e. the core region is broader) in the slower-moving fluid near the wall of the channel than in the middle of the channel. By increasing the FRR under laminar flow conditions, the focusing phenomenon increases and consequently the core spot size is expected to decrease. This is confirmed by the results shown in Fig. 6, where the full width at half maximum (FWHM) value of the measured and simulated output light intensity profiles for both core liquids, is reported as a function of the flow rate ratio. In the glycerol–water mixture case, we measured a minimum core spot size d equal to 22 lm with a FRR = 4. In the ethylene glycol case, the minimum value of d was 33 lm with the same FRR. The difference is due to the diffusion coefficient D that is greater in the ethylene glycol case. A good agreement between numerical and experimental results was found, as it is clear from Fig. 6. Furthermore, for the chosen liquids, the numerical simulations indicate that the minimum refractive index contrast between the core and the cladding fluids (Dn = nmax nmin) is 0.011 with nmax = 1.344 and nmin = 1.333, calculated in the glycerol–water mixture case at FRR = 4. On the other hand, in ethylene glycol case at FRR = 4 we calculated a minimum contrast Dn = 0.057, with nmax = 1.390 and nmin = 1.333. It is worth to notice that the minimum refractive index contrast occurs at the end of the channel, due to the broadening effects of the core/cladding interface, caused by the diffusion. In order to inspect the guiding properties of the proposed structure, a second kind of measurements was performed. As discussed before in L2-ARROW waveguides, the light is confined horizontally by TIR of the formed
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L2 waveguide and vertically by the ARROW effect. So, what we expect is that the transmitted spectrum of the L2-ARROW is similar to the spectrum of a simple slab ARROW. In fact, the transmission properties are tied up to the losses of the waveguide and in a conventional 2DARROW waveguide the losses can be regarded as TE for transverse confinement and TM for lateral confinement [3]. As the formed L2 waveguide suppresses the TM contribution, we can consider in the transmitted spectrum only the TE transverse contribution, due to ARROW mechanism. Rearranging the experimental set-up described above, the transmitted spectrum of the L2-ARROW was measured by using a white lamp to illuminate the waveguide, a polarizer set to 0° along x-axis (see Fig. 2) and a CCD spectrometer. The FRR was set to 1. The measurements were carried out for both ethylene glycol and glycerol:water mixture. In Figs. 7 and 8 the transmitted spectra of the L2-ARROWs, obtained by taking into account the spectrum of the white light source, are reported. From these figures, we recognize the typical interferometric behavior of an antiresonant reflecting optical waveguide with broad antiresonant peaks and narrow resonant valleys. According to the ARROW behavior, the spectral minimum shifts consistently towards lower wavelengths with an increase of the core refractive index [17], offering the possibility to tune the spectral properties of the L2-ARROW. In Figs. 7 and 8 the theoretical normalized transmittance calculated using the results of Ref. [18] are also reported. As it can be observed there is a good agreement with the measured data. As a result of the above analysis, the overall losses of the L2-ARROW waveguide are expected to be lower than the losses of the simple 2D-ARROW, in which the hollow core is uniformly filled with glycerol:water mixture or ethylene glycol. Actually, the formed L2 waveguide which suppress the TM loss contribution, which is significantly higher than the TE loss for the same core thickness [3]. In order to ver-
Normalized transmitted intensity [a.u.]
1
0.8
0.6
0.4
0.2
0
Fig. 6. Full width at half maximum (FWHM) of the measured and simulated values of the intensity profile for the different core liquids, as a function of the flow rate ratio.
400
500
600
700
800
Wavelength [nm] Fig. 7. Measured and simulated transmission spectra, at FRR = 1, for the L2-ARROW waveguide with glycerol:water mixture liquid-core.
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R. Bernini et al. / Optics Communications 281 (2008) 2062–2066
dynamically vary the optical characteristics of the waveguide such as shape and beam spot size, with the advantages offered by the ARROW mechanism, in particular, the possibility to realize L2 waveguides with very low refractive index core. Moreover, ARROW structures can be easily realized with standard silicon technology and offer the possibility to exploit their filtering capabilities in order to tune the wavelength response of the device. L2-ARROW waveguides may be used in biosensing applications for fluorescence detection and single molecule analysis [19] in which the liquids employed have typically a low refractive index.
Normalized transmitted intensity [a.u.]
1
0.8
0.6
0.4
0.2
0
400
500
600
700
800
Acknowledgement
Wavelength [nm] Fig. 8. Measured and simulated transmission spectra, at FRR = 1, for the L2-ARROW waveguide with ethylene glycol liquid-core.
ify this point, the following measurement was carried out. By substituting in the experimental set-up the CCD spectrometer with a powermeter, the value of the optical power at the end of the channel, for both the L2-ARROW and the simple ARROW, was measured. The L2 was formed by setting the flow rates in such a way to realize the focusing effect, eliminating the TM losses, without perturbing excessively the light path in the focused section, reducing the losses phenomena in that zone. The input power (i.e. the output power of the fiber) was PIN = 9.5 lW. The L2 -ARROW ¼ 1:03 lW measured output power values were P OUT 2 ARROW ¼ 0:640 lW for the for the L -ARROW and P OUT ARROW, showing that the L2 lateral confinement effectively reduces the overall losses. Although the difference in measured output power between L2-ARROW and the simple 2D-ARROW is in the order of 60%, we expect that the real difference in terms of waveguide losses is expected to be much higher. As matter of fact, we must also consider that the input power for the L2-ARROW is lower than the input power for the simply 2D-ARROW, due to the losses introduced by the optical taper formed by the hydrodynamic focusing. These losses strongly depends on the taper geometry, which is influenced by the flow rate ratios, the viscosity and the diffusivity of the fluids. An optimization of the taper shape is possible as demonstrated in Ref. [11]. This result together with the measured transmitted spectrum of the L2-ARROW, assures the correctness of the exposed analysis. 4. Conclusions L2-ARROW integrates the advantages offered by liquidcore/liquid-cladding waveguides,that is the possibility to
The authors would like to thank the IC Process Group of DIMES for technical assistance. References [1] R. Bernini, S. Campopiano, L. Zeni, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 106. [2] A. Datta, I.-Y. Eom, A. Dhar, P. Kuban, R. Manor, I. Ahmad, S. Gangopadhyay, T. Dallas, M. Holtz, H. Temkin, P.K. Dasgupta, IEEE Sens. J. 3 (2003) 788. [3] H. Schmidt, D. Yin, J.P. Barber, A.R. Hawkins, IEEE J. Sel. Top. Quantum Electron. 11 (2) (2005) 519. [4] D. Yin, D.W. Deamer, H. Schmidt, J.P. Barber, A.R. Hawkins, Appl. Phys. Lett. 85 (2004) 3477. [5] W. Risk, H. Kim, R. Miller, H. Temkin, S. Gangopadhyay, Opt. Exp. 12 (2004) 6446. [6] T. Dallas, P.K. Dasgupta, Trends Anal. Chem. 23 (2004) 385. [7] D. Yin, D.W. Deamer, H. Schmidt, J.P. Barber, A.R. Hawkins, Opt. Lett. 31 (2006) 2136. [8] O.J.A. Schueller, X.-M. Zhao, G.M. Whitesides, S.P. Smith, M. Prentiss, Adv. Mater. 11 (1) (1999) 37. [9] H. Takiguchi, T. Odake, M. Ozaki, T. Umemura, K.-I. Tsunoda, Appl. Spectrosc. 57 (2003) 1039. [10] D.B. Wolfe, R.S. Conroy, P. Garstecki, M.A. Fischbach, K.E. Paul, M. Prentiss, G.M. Whitesides, Proc. Natl. Acad. Sci. USA 101 (2004) 12434. [11] M. Brown, T. Vestad, J. Oakey, D.W.M. Marr, Appl. Phys. Lett. 88 (2006) 134109. [12] Y. Ito, S. Komori, AIChE J. 52 (2006) 3011. [13] H.P. Uranus, H.J.W.M. Hoekstra, E. van Groesen, Opt. Commun. 260 (2006) 577. [14] R.F. Ismagilov, A.D. Stroock, P.J.A. Kenis, G. Whitesides, H.A. Stone, Appl. Phys. Lett. 76 (2000) 2376. [15] J.B. Salmon, A. Ajdari, J. Appl. Phys. 101 (2007) 074902. [16] D.B. Wolfe, D.V. Vezenov, B.T. Mayers, G.M. Whitesides, R.S. Conroy, M.G. Prentiss, Appl. Phys. Lett. 87 (2005) 181105. [17] S. Campopiano, R. Bernini, L. Zeni, P.M. Sarro, Opt. Lett. 29 (16) (2004) 1894. [18] J.L. Archambault, R.J. Black, S. Lacroix, J. Bures, J. Lightwave Thechnol. 11 (3) (1993) 416. [19] P.M. Goodwin, M.E. Johnson, J.C. Martin, W.P. Ambrose, B.L. Marrone, J.H. Jett, R.A. Keller, Nucleic Acids Res. 21 (1993) 803.
Optics Communications 281 (2008) 2062–2066 www.elsevier.com/locate/optcom
Liquid-core/liquid-cladding integrated silicon ARROW waveguides R. Bernini a,*, E. De Nuccio b, A. Minardo b, L. Zeni b, P.M. Sarro c b
a IREA-CNR, Via Diocleziano 328, 80124 Napoli, Italy DII, Seconda Universita` di Napoli, Via Roma 29, 81031 Aversa, Italy c ECTM-DIMES, TUDelft. NL-2600 GB Delft, The Netherlands
Received 17 July 2006; received in revised form 20 November 2007; accepted 27 November 2007
Abstract The fabrication and characterization of a liquid-core/liquid-cladding integrated antiresonant reflecting optical waveguide (L2-ARROW) is presented. In this waveguide, the light is confined vertically by the ARROW mechanism, whereas the lateral confinement is obtained by using liquid-core/liquid-cladding (L2 waveguides) with different refractive indexes. This approach permits to realize L2 waveguides with very low refractive index core (n 1.333) and represents a new solution to solve the difficulty to reduce the optical losses in 2D-ARROWs due to the TM polarization in lateral direction. The device has been fabricated with standard silicon technology. The results show that the optical properties can be tuned by changing the type and the flow velocity of the core and the cladding liquids. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Liquid-core waveguides are of considerable interest in many fields, ranging from sensors to telecommunications. In particular, in sensing applications, this interest is motivated by the increased interaction efficiency offered by these waveguides. In fact, liquid-core waveguides permit to simultaneously confine the light and the substance to be probed in the waveguide core. By means of integrated planar technology, several types of liquid-core/solid-cladding have been fabricated [1–8]. Recently, liquid-core/ liquid-cladding (L2) waveguides have been proposed and fabricated using both glass capillary technology [9], and planar geometry by replica molding polymeric techniques [10,11]. These waveguides are microfluidic devices in which the light is confined inside a high refractive index liquid (the core) by a low refractive index liquid (the cladding), both liquids flowing laminarly [10]. Because L2 waveguides are dynamic, many optical properties (refractive index contrast, geometry, etc.) can be easily changed.
*
Corresponding author. Tel.: +39 0815707979; fax: +39 0815705734. E-mail address: [email protected] (R. Bernini).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.11.070
In this paper, we present the fabrication and the characterization of an integrated silicon liquid-core/liquid-cladding antiresonant reflecting optical waveguide (L2ARROW). Differently from other L2 waveguides, where the radiation is guided by total internal reflection (TIR), in the proposed waveguide the light is vertically confined inside the core by realizing a Fabry–Perot mirror with high reflectivity (ARROW mechanism) [1]. The proposed solution integrates the capability offered by L2 waveguides and ARROW waveguides. In fact, L2 waveguides permits to perform a dynamic optical confinement by tuning the optical properties of the liquids and/or by varying the flow rates, obtaining waveguides with different core dimensions and consequently different modal responses. At the same time the ARROW behavior adds to the L2 waveguides the ability to tailor the frequency or wavelength response of the device. Moreover, differently from conventional L2 waveguides, in which the refractive index of the liquid-core must be higher than the refractive index of the substrate in order to obtain the vertical confinement by TIR, the ARROW mechanism permits us to consider liquids with low refractive index (e.g. water solution n 1.333), getting rid of the limits arising when substrates with high refractive index are used. Beside to
R. Bernini et al. / Optics Communications 281 (2008) 2062–2066
these advantages we have some disadvantages introduced by the L2 waveguides like the increased fluidic complexity, a loss of waveguide index contrast, and a possible sensitivity to the vibrations [12]. In the following, we describe the device and the principal fabrication steps. Afterwards, we show the results of the optical characterization of the L2-ARROW, comparing the experimental result to the numerical analysis of the structure. We also report the results of the L2-ARROW optical losses investigation, carried out by measuring the output optical power values for both the L2-ARROW and the simple ARROW waveguide. The experimental results shows that the L2-ARROW approach permits to reduce the optical losses of a 2D-ARROW arising from the bad confinement of TM polarization in lateral direction and can be a good alternative to the rather complicated ARROW structures previously proposed for a good lateral confinement [13]. 2. Design and fabrication Fig. 1 is a schematic drawing of the L2-ARROW waveguide device. The structure is composed of a central channel where the liquid-core is injected and two lateral arms, in which the cladding liquids flow. Both the liquid-core and the liquid-claddings are injected into the channels by a syringe pump system. By adequately tuning the flow rates of the liquids, the flowing core liquid section is reduced between the flowing slabs of the cladding fluid, due to the hydrodynamic focusing effect. At low and moderate Reynolds numbers, the flow is laminar, and the liquid/liquid interfaces are optically smooth [10]. Manipulating the flow rate and the composition of the liquids, the characteristics of the optical system can be tuned. Seeing the figure, the central channel is designed to have an additional inlet for an optical fiber to easily couple the optical radiation into the L2-ARROW. Fig. 2 shows the A–A’ cross section of the waveguide. The device is based on the ARROW waveguide, in which the light is confined inside the core region with refractive index nc, which is lower than the refractive indices of the surrounding media, by the two cladding layers designed to form a high reflectivity Fabry–Perot antiresonant cavity.
Input optical fiber
Cladding Liquid Inlets
Microscope Objective 20X
A CCD Camera
Core Liquid Inlet
A’
Glass Slide
Fig. 1. Schematic drawing of the L2-ARROW waveguide device.
2063
Substrate
Si SiO2 SiO2
Upper half
d1 n 1
Si3N4 Si3N4
Core
d2 n 2 d2 n 2 d1 n1
Lower half y
x
Section A-A’
Substrate
Fig. 2. Transverse section A–A0 of the (hollow core) ARROW waveguide.
For a fixed core width dc and refractive index nc, the antiresonant condition for the equivalent 1D vertical structure can be written, for the cladding layers thicknesses, as [1] " ! 2 #1=2 k n2c k d 1;2 ¼ 1 ð2N þ 1Þ; þ 4n1;2 2n1;2 d c n21;2 N ¼ 0; 1; 2; . . .
ð1Þ
where k is the operating wavelength. The waveguide is composed by two halves of a silicon wafer (ns = 3.87 i0.02) joined together. The channels were realized by deep silicon dry etching (resulting in a 200 150 lm2 rectangular-core), followed by an LPCVD deposition of silicon dioxide (TEOS) and silicon nitride at the temperature of 850 °C. The two dielectric layers have refractive index n1 = 2.227 and n2 = 1.457, respectively, where the values for the refractive indexes n1, n2 and ns are measured at a wavelength k = 633 nm. The cladding layers thicknesses are: d1 = 266 nm (N = 1) and d2 = 266 nm (N = 0), according to the antiresonant condition (see Eq. (1)) at k = 633 nm and nc = 1.330. Before the LPCVD depositions, the inlet holes were realized by deep silicon dry etching from the bottom side. Then, the two halves were joined by silicon nitride wafer bonding. The picture of the bottom half wafer is shown in Fig. 3, where the microfluidic path, the inlet holes and the groove for the excitation fiber are clearly identifiable. The overall size of the die is 2 1 cm2. The final step is represented by the tubing system assembling: tygon tubes with internal diameter of 0.7 mm were connected to the inlet holes using epoxy and successively are interconnected to a syringe pumping system. The choice of a large (200 150 lm2) rectangular-core dimension permits a simple optical input coupling by the insertion of a multimode optical fiber (100 lm core diameter and 130 lm cladding diameter) directly inside the core of the hollow waveguide. This permits a self-alignment of the waveguide core to the fiber. The fiber was sealed to the waveguide by optical adhesive and the terminations of both the two lateral channels and the orthogonal channels were sealed by optical adhesive.
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R. Bernini et al. / Optics Communications 281 (2008) 2062–2066 Inlets for the liquids 1
Measured intensity profile Simulated intensity profile
Ethylene glycol
200 μm
0.8
Intensity [a.u.]
1 cm 0.6
0.4
Fiber groove
2 cm 0.2
Fig. 3. Picture of the realized lower half wafer. 0 -100
3. Optical characterization
0
50
100
x [μm]
Fig. 4. Measured and simulated intensity profiles of the output light versus the width of the channel. The liquid-core is ethylene glycol. The insets show the CCD camera image of the light intensity distribution at the waveguide end facet.
1
Measured intensity profile Simulated intensity profile
0.9 0.8 Glycerol:water mixture
200 μm 0.7
Intensity [a.u.]
In this section, we report the experimental results carried out in order to investigate the L2-ARROW optical properties. The measurements were performed by using an LED at k = 635 nm, a CCD camera and a syringe pump system to control the flow rate at the inlets. A thin glass slide (130 lm) was placed near to the output channel in order to avoid lens effects due to the liquid surface tension. The light exiting the structure, 1.5 cm away from the cladding liquid inlets point, was imaged on a CCD camera through a 20 microscope objective. Deionized water with a refractive index ncl = 1.333 was used as the focusing (cladding) liquid, whereas the core fluid was either ethylene glycol or a glycerol:water mixture (10% by volume), presenting refractive indexes nc = 1.431 and nc = 1.347, respectively. We chose these solutions for two reasons. First, the chosen fluids are miscible and, therefore, will mix diffusively. Second, many organic dyes are soluble in ethylene glycol. It is important to underline that the choice of the liquids is made only to assure the total internal reflection mechanism for horizontal confinement, whereas the ARROW mechanism guarantees the vertical confinement of the light. The measurements were carried out for different cladding/core flow rate ratio (FRR), ranging in a range from 1 to 4, with a core liquid flow rate fixed to 5 ml/h. Note that the flow rates of the liquids are referred to the inlets. Figs. 4 and 5 show the output light intensity profile as a function of the width of the channel, by setting FRR = 1 and glycerol:water mixture and ethylene glycol as the liquid-core, respectively. In the insets of the figures, it is possible to observe the light confinement in the L2-ARROW under the same conditions. The dotted lines in the insets represent the plot scan lines. In the figures we also report the simulated intensity profiles. The validity of the experimental results was verified by a numerical analysis. The simulations were performed first by solving the 2D coupled Navier–Stokes and diffusion/convection fluid equations using the finite elements method (FEM), so as to obtain the liquid concentration profile along the x-axis.
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Fig. 5. Measured and simulated intensity profile of the output light versus the width of the channel. The liquid-core is glycerol:water mixture. The insets show the CCD camera image of the light intensity distributions at the waveguide end facet.
Then, assuming that refractive index profile of the L2ARROW waveguide is directly proportional to the concentration of the components of the liquids [16], the optical simulations were carried out by exciting the liquid waveguide with an uniformly distributed input optical field. The different shape of the liquid-core in the two cases (Figs. 4 and 5, inset) is due to a 3D microfluidic effect. In a fact, in dependence of the liquid properties, in pressure-driven microfluidic flow has been shown both
R. Bernini et al. / Optics Communications 281 (2008) 2062–2066
theoretically and experimentally [14,15] that the extent of the transverse diffusive zone near the top and the bottom walls can be larger respect to middle of the channel, as we observed for the glycerol:water mixture (Fig. 5, inset). This effect is due the no-slip nature of the top and the bottom walls of the channel affect the flow profile, and so the diffusive mixing is more extensive (i.e. the core region is broader) in the slower-moving fluid near the wall of the channel than in the middle of the channel. By increasing the FRR under laminar flow conditions, the focusing phenomenon increases and consequently the core spot size is expected to decrease. This is confirmed by the results shown in Fig. 6, where the full width at half maximum (FWHM) value of the measured and simulated output light intensity profiles for both core liquids, is reported as a function of the flow rate ratio. In the glycerol–water mixture case, we measured a minimum core spot size d equal to 22 lm with a FRR = 4. In the ethylene glycol case, the minimum value of d was 33 lm with the same FRR. The difference is due to the diffusion coefficient D that is greater in the ethylene glycol case. A good agreement between numerical and experimental results was found, as it is clear from Fig. 6. Furthermore, for the chosen liquids, the numerical simulations indicate that the minimum refractive index contrast between the core and the cladding fluids (Dn = nmax nmin) is 0.011 with nmax = 1.344 and nmin = 1.333, calculated in the glycerol–water mixture case at FRR = 4. On the other hand, in ethylene glycol case at FRR = 4 we calculated a minimum contrast Dn = 0.057, with nmax = 1.390 and nmin = 1.333. It is worth to notice that the minimum refractive index contrast occurs at the end of the channel, due to the broadening effects of the core/cladding interface, caused by the diffusion. In order to inspect the guiding properties of the proposed structure, a second kind of measurements was performed. As discussed before in L2-ARROW waveguides, the light is confined horizontally by TIR of the formed
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L2 waveguide and vertically by the ARROW effect. So, what we expect is that the transmitted spectrum of the L2-ARROW is similar to the spectrum of a simple slab ARROW. In fact, the transmission properties are tied up to the losses of the waveguide and in a conventional 2DARROW waveguide the losses can be regarded as TE for transverse confinement and TM for lateral confinement [3]. As the formed L2 waveguide suppresses the TM contribution, we can consider in the transmitted spectrum only the TE transverse contribution, due to ARROW mechanism. Rearranging the experimental set-up described above, the transmitted spectrum of the L2-ARROW was measured by using a white lamp to illuminate the waveguide, a polarizer set to 0° along x-axis (see Fig. 2) and a CCD spectrometer. The FRR was set to 1. The measurements were carried out for both ethylene glycol and glycerol:water mixture. In Figs. 7 and 8 the transmitted spectra of the L2-ARROWs, obtained by taking into account the spectrum of the white light source, are reported. From these figures, we recognize the typical interferometric behavior of an antiresonant reflecting optical waveguide with broad antiresonant peaks and narrow resonant valleys. According to the ARROW behavior, the spectral minimum shifts consistently towards lower wavelengths with an increase of the core refractive index [17], offering the possibility to tune the spectral properties of the L2-ARROW. In Figs. 7 and 8 the theoretical normalized transmittance calculated using the results of Ref. [18] are also reported. As it can be observed there is a good agreement with the measured data. As a result of the above analysis, the overall losses of the L2-ARROW waveguide are expected to be lower than the losses of the simple 2D-ARROW, in which the hollow core is uniformly filled with glycerol:water mixture or ethylene glycol. Actually, the formed L2 waveguide which suppress the TM loss contribution, which is significantly higher than the TE loss for the same core thickness [3]. In order to ver-
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dynamically vary the optical characteristics of the waveguide such as shape and beam spot size, with the advantages offered by the ARROW mechanism, in particular, the possibility to realize L2 waveguides with very low refractive index core. Moreover, ARROW structures can be easily realized with standard silicon technology and offer the possibility to exploit their filtering capabilities in order to tune the wavelength response of the device. L2-ARROW waveguides may be used in biosensing applications for fluorescence detection and single molecule analysis [19] in which the liquids employed have typically a low refractive index.
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Wavelength [nm] Fig. 8. Measured and simulated transmission spectra, at FRR = 1, for the L2-ARROW waveguide with ethylene glycol liquid-core.
ify this point, the following measurement was carried out. By substituting in the experimental set-up the CCD spectrometer with a powermeter, the value of the optical power at the end of the channel, for both the L2-ARROW and the simple ARROW, was measured. The L2 was formed by setting the flow rates in such a way to realize the focusing effect, eliminating the TM losses, without perturbing excessively the light path in the focused section, reducing the losses phenomena in that zone. The input power (i.e. the output power of the fiber) was PIN = 9.5 lW. The L2 -ARROW ¼ 1:03 lW measured output power values were P OUT 2 ARROW ¼ 0:640 lW for the for the L -ARROW and P OUT ARROW, showing that the L2 lateral confinement effectively reduces the overall losses. Although the difference in measured output power between L2-ARROW and the simple 2D-ARROW is in the order of 60%, we expect that the real difference in terms of waveguide losses is expected to be much higher. As matter of fact, we must also consider that the input power for the L2-ARROW is lower than the input power for the simply 2D-ARROW, due to the losses introduced by the optical taper formed by the hydrodynamic focusing. These losses strongly depends on the taper geometry, which is influenced by the flow rate ratios, the viscosity and the diffusivity of the fluids. An optimization of the taper shape is possible as demonstrated in Ref. [11]. This result together with the measured transmitted spectrum of the L2-ARROW, assures the correctness of the exposed analysis. 4. Conclusions L2-ARROW integrates the advantages offered by liquidcore/liquid-cladding waveguides,that is the possibility to
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