Abstract

Quantum cascade lasers and unclad silver halide fibers were used to assemble mid-infrared fiber-optics evanescent-wave sensors suitable to measure the chemical composition of liquid droplets. The laser wavelengths were chosen to be in the regions which offer the largest absorption contrast between constituents inside the mixture droplets. A pseudo-Beer-Lambert law fits well with the experimental data. Using a 300μm diameter fiber with a 25 mm immersion length, the signal to noise ratios correspond to 1 vol.% for α-tocophenol in squalane and 2 vol.% for acetone in aqueous solution for laser wavenumbers of 1208 cm-1 and 1363 cm-1, respectively.

© 2005 Optical Society of America

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References

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Anal. Chem.

B. Lendl, J. Frank, R. Schindler, A. Muller, M. Beck, and J. Faist, �??Mid-infrared quantum cascade lasers for flow injection analysis,�?? Anal. Chem. 72, 1645 (2000).
[CrossRef] [PubMed]

Appl. Opt.

Appl. Phys. B

R. Jimenez, M. Taslakov, V. Simeonov, B. Calpini, F. Jeanneret, D. Hofstetter, M. Beck, J. Faist, and H. van den Bergh, �??Ozone detection by differential absorption spectroscopy at ambient pressure with a 9.6 µm pulsed quantum-cascade laser,�?? Appl. Phys. B 78, 249 (2004).
[CrossRef]

Appl. Spectrosc.

IEEE Circ. Dev.

C. Gmachl, F. Capasso, R. Köhler, A. Tredicucci, A. L. Hutchinson, D. L. Sivco, J. N. Baillargeon, and A. Y. Cho, �??The sense-ability of semiconductor lasers �?? mid-infrared tunable quantum cascade lasers for gas-sensing applications,�?? IEEE Circ. Dev. 16, 10 (2000).
[CrossRef]

IEEE J. Quantum Electron.

A. A. Kosterev and F. K. Tittel, �??Chemical sensors based on quantum cascade lasers,�?? IEEE J. Quantum Electron. 38, 582 (2002).
[CrossRef]

IEEE Proc. Optoelectron.

C. Charlton, F. de Melas, A. Inberg, N. Croitoru, and B. Mizaikoff, �??Hollow-waveguide gas sensing with room-temperature quantumcascade lasers,�?? IEEE Proc. Optoelectron. 150, 306 (2003).
[CrossRef]

J. Appl. Phys.

J. Z. Chen, S. M. Troian, A. A. Darhuber, and S. Wagner, �??Effect of contact angle hysteresis on thermocapillary droplet actuation,�?? J. Appl. Phys. 97, 014906 (2005).
[CrossRef]

J. Chromatogr.

A. Edelmann, C. Ruzicka, J. Frank, B. Lendl, W. Schrenk, E. Gornik, and G. Strasser, �??Toward functional group-specific detection in high-performance liquid chromatography using mid-infrared quantum cascade lasers,�?? J. Chromatogr. 934, 123 (2001).
[CrossRef]

J. Microelectromech. Sys.

A. A. Darhuber, J. P. Valentino, S. M. Troian, and S. Wagner, �??Thermocapillary actuation of droplets on chemically patterned surfaces by programmable microheater arrays,�?? J. Microelectromech. Sys. 12, 873 (2003).
[CrossRef]

Lab Chip

J. Z. Chen, A. A. Darhuber, S. M. Troian, and S. Wagner, �??Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation,�?? Lab Chip 4, 473 (2004).
[CrossRef] [PubMed]

Langmuir

S. S. Datwani, R. A. Vijayendran, E. Johnson, and S. A. Biondi, �??Mixed alkanethiol self-assembled monolayers as substrates for microarraying applications,�?? Langmuir 20, 4970 (2004).
[CrossRef]

Opt. Lett.

Rep. Prog. Phys.

C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, �??Recent progress in quantum cascade lasers and applications,�?? Rep. Prog. Phys. 64, 1533 (2001).
[CrossRef]

Vib. Spectrosc.

M. Kolhed, M. Haberkorn, V. Pustogov, B. Mizaikoff, J. Frank, B. Karlberg, and B. Lendl, �??Assessment of quantum cascade lasers a mid infrared light sources for measurement of aqueous samples,�?? Vib. Spectrosc. 29, 283 (2002).
[CrossRef]

Other

M. G. Pollack, R. B. Fair, and A. D. Shenderov, �??Electrowetting-based actuation of liquid droplets for microfluidic applications,�?? 77, 1725 (2000).

P. G. de Gennes, �??Wetting: statics and dynamics,�?? 57, 827 (1985).

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Figures (5)

Fig. 1.
Fig. 1.

(a) Drawing of experimental setup. (b) Photograph of experimental setup.

Fig. 2.
Fig. 2.

(a) Comparison of FTIR spectra for the bare laser, and the laser coupled to an unclad fiber and clad fiber, respectively. (b) L-I-V measurements comparing the fiber coupled and uncoupled laser.

Fig. 3.
Fig. 3.

Normalized transmitted intensity (I/I0) as a function of immersion length for various liquids, where I0 is the transmitted intensity in the absence of liquid. The symbols are the measured data. The lines are least square fits using an exponential decay fit function of I/I0 = exp(-α∙L), where α and L stand for the absorption coefficient and immersion length, respectively. Absorption coefficients (α’s) of 0.64 cm-1 (α–tocophenol acetate at 1363 cm-1), 0.15 cm-1 (squalane at 1363 cm-1), 0.03 cm-1 (squalane at 1208 cm-1), 0.68 cm-1 (glycerol at 1384 cm-1), 0.55 cm-1 (water at 1363 cm-1), and 0.53 cm-1 (water at 1384 cm-1), are obtained from the exponential fits to the data..

Fig. 4.
Fig. 4.

(a) FTIR thin film transmission spectra of water, ethanol and acetone near the operating laser wavelength. (b) Normalized transmitted intensity (I/I0) as a function of concentrations for ethanol/water and acetone/water mixtures at a laser wavelength of 1363 cm-1, where I0 is the transmitted intensity without liquid. The symbols are measured data; the lines are fitting functions. Fit functions: y=0.253+0.415∙exp(-x/196.1) for ethanol/water on HDT/Au/glass surface; y=-0.0307+0.718∙exp(-x/98.58) for acetone/water on HDT/Au/glass surface, and y=0.613∙exp(-x/35.41) for acetone/water on PFOT/glass surface, where y is normalized intensity (I/I0) and x is the volumetric concentration in percentage.

Fig. 5.
Fig. 5.

(a) FTIR thin film spectra of squalane, n-dodecane and α–tocophenol acetate near the operating laser wavenumber. (b) Normalized transmitted intensity (I/I0) as a function of the concentrations for n-dodecane/squalane and α–tocophenol acetate/squalane mixtures at a laser wavelength of 1208 cm-1, where I0 is the transmitted intensity without immersed liquid. Fit functions: y=0.97-0.048∙exp(-x/173.80) for n-dodecane/squalane on HDT/Au/glass surface; y= -0.049+exp(-x/28.79) for α–tocophenol acetate/squalane on HDT/Au/glass surface. In these fitting functions, x stands for the vol.%, y represents the normalized transmitted intensity. Inset: Normalized transmitted intensity before and after dispensing droplets of pure squalane and squalane with 4% α-tocophenol acetate in it. In the presence of droplet, the normalized intensity decreases due to the absorbance of liquids.

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