Sculptured-thin-®lm spectral holes for optical sensing of ¯uids Akhlesh Lakhtakia a , Martin W. McCall b, * , Joseph A. Sherwin a , Qi Hong Wu c , Ian J. Hodgkinson c a Computational and Theoretical Materials Sciences Group CATMAS), Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802-6812, USA b Department of Physics, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BZ, UK c Department of Physics, University of Otago, Private Bag 56, Dunedin, New Zealand Received 14 November 2000; received in revised form 13 March 2001; accepted 21 March 2001 Abstract A novel class of ¯uid sensors is proposed based on monitoring the optical properties of multi-section chiral sculptured thin ®lms STFs) that function as spectral re¯ection holes. Using a nominal model that treats a chiral STF as a two-phase composite material with locally biaxial dielectric properties, we predict that the presence of a ¯uid in the porous ®lm results in a red-shift of the spectral holes. Several device operation modes are proposed, and their relative merits are compared. Proof-of-concept experiments with both circularly polarized and unpolarized incident light con®rm the red-shift of the spectral holes, and demonstrate operation in a practical situation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluid concentration sensors; Sculptured thin ®lms; Spectral re¯ection holes 1. Introduction The porosity of sculptured thin ®lms STFs) makes them attractive for ¯uid concentration sensing ap- plications [1±3], because their optical response properties must change in accordance with the number density of molecules intruding into the void regions. In particular, theoretical research has shown that the Bragg wavelength regime of a chiral STF ± which is periodically nonhomogeneous along the so-called helicoidal axis of its microstructure ± must shift accordingly, thereby providing a measure of the ¯uid concentration [2]. The sensitivity of a chiral STF-based ¯uid concentration sensor can be improved if the shift in a spectral feature narrower than the Bragg regime can be exploited. Very recently, we have shown experimentally as well as theoretically that, by cascading two identical chiral STF sections with one section twisted with respect to the other by 90° about their common helicoidal axis, a spectral re¯ection hole can be realized [4]. 1 July 2001 Optics Communications 194 2001) 33±46 www.elsevier.com/locate/optcom * Corresponding author. Tel.: +44-171-594-7749; fax: +44-171-594-7714. E-mail address: m.mccall@ic.ac.uk M.W. McCall). 0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0030-401801)01225-1