STFs offer many possibilities as passive optical devices. The ability to engineer the birefringent properties of STFs and fabricate very compact devices with multiple layers allow for an extremely wide range of design possibilities not available with traditional crystal optics. The design of such devices, mainly with layers of CTFs is extensively explored in a book by Hodkinson and Wu (1997). Chiral STFs can be used to create highly reflective circular polarisation filters only a few helical cycles thick. Hodgkinson et al. (2002) have proposed a method to enhance the performance of these circular polarisation filters with the addition of anti-reflection coatings. Their calculations have demonstrated that the cross-polarised reflectance of chiral STFs within the Bragg regime can be almost entirely eliminated with proper design of the anti-reflection coatings, producing an extremely pure circularly polarised reflected beam. In applications requiring mirrors, which reflect both polarisations, two solutions have been found. Sequential layers of chiral STFs with opposite handedness have been shown to reflect both RCP and LCP light equally well independent of the order of the two cascaded layers (Lakhtakia and Venugopal 1998). Calculations on chiral STFs with i modulated as a function of depth (Polo and Lakhtakia 2004) have shown that, with sufficient modulation amplitude, the film acts as a conventional mirror over a range of wavelengths. Furthermore, the width of the reflection regime can be controlled by changing the amplitude of modulation of %.
Because of finite bandwidths and sharp roll-offs, the Bragg regimes of both chiral STFs and SNTF may be used, for circularly polarised and linearly polarised light respectively, as bandpass, bandstop and notch filters. Chiadini and Lakhtakia (2004) have theoretically explored creating circular polarisation filters with even wider bandwidths than available in a simple chiral STF by cascading several chiral STFs with a range of half-pitch values, Q. In slanted chiral STFs, which because of their structure have a surface grating, circular polarisation beam splitters may be possible (Wang and Lakhtakia 2002). Rugate filters have been produced using both vertical columns with undulating density (Kaminska et al. 2004) as well as SNTFs with a modulated % (McPhun et al. 1998). For extremely narrow bandwidth filters, the introduction of defects of various types into the periodic structure of STFs has been investigated theoretically (Hodgkinson et al. 2000; Polo and Lakhtakia in press) and experimentally (Hodgkinson et al. 2000) for both SNTFs and chiral STFs. The defect produces a feature in the Bragg regime known as a spectral hole. Over a very narrow range of wavelengths, X0<0.1 nm, the reflective Bragg effect is destroyed leading to a very sharp dip to nearly zero reflectance in the spectrum. Equivalently, a very sharp spike in the transmission spectrum arises with essentially 100% transmission.
The use of STFs as active optical elements has also been suggested and investigated. One possibility is to use the STF as a chemical sensor (Lakhtakia 1998; Hodgkinson et al. 1999). Due to high porosity, STFs are easily infiltrated with fluids which may alter the overall constitutive relations of the STF. Changes in constitutive relations then affect various optical properties of the STF, which can be monitored. An extremely sensitive sensor makes use of the shift in the spectral hole of a spectral hole filter. The effect has been demonstrated experimentally (Lakhtakia et al. 2001) with the redshift of a spectral hole in a STF exposed to moisture. The possibility of using a Solc filer made with a STF as a sensor of gas concentration has been investigated theoretically (Ertekin and Lakhtakia 1999). The effect of fluid infiltration, as well as quasi-static electric and magnetic fields, on STFs have been proposed as means of creating dynamically tunable laser mirrors (Lakhtakia and Venugopal 1998). Mechanical control of chiral STFs using the piezoelectric effect has also been investigated in this regard ( Wang et al. 2002, 2003). Kopp et al. (2003) have looked at the use of chiral STFs as the lasing medium of the laser. The states at the edge of the Bragg regime as well as the defect state created in a spectral hole filter were shown to have desirable properties. Infiltrating a STF with a liquid crystal may make a display device with the best properties of both worlds. The STF offers mechanical and thermal stability with engineered conformation which may be imposed on the infiltrated LC. The conformation in the STF is controllable over larger thickness than normally obtainable with LCs. LCs on the other hand offer electrical addressability. Both the imposition of order (Robbie et al. 1999) on the LC as well as electrical switching (Sit et al. 2000) have been demonstrated experimentally for chiral STFs infiltrated with a LC. The infiltration of a LC into a STF may also be useful in increasing the optical activity of a chiral STF and reducing degrading, environmental effects on the optical properties of the STF (Kennedy et al. 2001). Infiltrates may affect optical properties in another way. Lakhtakia and Horn (2003) have calculated the effects of columnar thinning in chiral STFs. They show that post deposition thinning of the STF by etching or bioreduction can change the optical characteristics of the film. It also allow sensing certain chemicals which react with the film.
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