SEEC Imaging principles
Surface-Enhanced Ellipsometric Contrast Microscopy (SEEC) is a label-free wide-field optical imaging technique that lies in the combined use of a new generation of optical slides - called SEEC slides - displaying contrast-enhancing properties and refleted optical systems (upright or inverted).
This innovative optical technique was developed by D.Ausserré & MP.Valignat [Optics Express, Vol.15, Issue 13, pp. 8329-8339 (2007)].
The underlying idea is that, for infinitesimal amounts of material assuming the shape of nano-objects or nanofilms to become optically detectable, one should get rid of the light reflected off of the substrate surface that, otherwise, would mask what little reflected light comes from the nanometre scale sample.
Now in sketchy optical terms, let the contrast be defined as C = (I-I0)/(I+I0), with I and I0 representing the luminance of the features and the background luminance, respectively. One way to maximise C is to make I0 as small as possible, which is another way to express what has been mentioned above.
The four figures below are schematics describing what happens when one shines a light beam onto a flat surface and collects its reflection after passing through crossed polarisers.
|Figure 1 : Cross polarized reflection off of an ordinary substrate||Figure 2 : Cross polarized reflection off of an ordinary substrate covered in nanomaterials|
Figure 1 shows that the change in polarisation occurring upon reflection off of an ordinary, bare substrate gives rise to some light not being blocked by the analyser so that the detector provides a signal. In Figure 2, the presence of nanomaterials on top of the ordinary substrate does not amplify the light intensity at the detector and cannot be sensed.
|Figure 3 : Cross polarized reflection off of a SEEC slide||Figure 4 : Cross polarized reflection off of a SEEC slide covered in nanomaterials|
In Figure 3, the regular substrate has been substituted for a SEEC slide, the effect it has is that there is no change in polarisation upon reflection so that all the reflected light is being blocked by the analyser. However, when nanomaterials stand on a SEEC slide, they cause such a disruption of the system's polarisation properties as to let light through the analyser which, unlike in the situations described in Figures 1 & 2, originates solely from the nanomaterials allowing for their effective imaging.
The increase in contrast brings about by SEEC slides as compared to a silicon substrate is represented in Figure 5. Contrast is plotted against sample (film) thickness in nanometres. The thinner the film the greater the contrast enhancement, for instance a 1nm thick film sitting on a SEEC slide gives rise to a contrast 200 times as good as that of the same film that would coat a piece of silicon. As a consequence, the resulting optical images are strikingly different. While the multiple bilayers (in growing number from left to right) deposited by the Langmuir-Blodgett technique are virtually indistinguishable from one another and from the silicon substrate (see Figure 6), on the contrary they stand out with tremendous clarity when the substrate is a SEEC slide (here an air SiO2 SEEC slide, as shown in Figure 7).
SEEC slides do bring about a substantial gain in contrast even when the setup is far away from the ideal optical configuration, for instance in bright field (BF) as is depicted in Figure 5 (dashed lines). This is of considerable importance as BF is the simplest illumination technique.
|Figure 5 : Calculated contrast of SEEC slide compared to a silicon substrate. BF: bright field, POL : cross polarized|
|Figure 6 : Bilayers (5.4 nm thick) deposited on silicon substrate by Langmuir-Blodgett technique||Figure 7 : Bilayers (5.4 nm thick) deposited on a SEEC slide by Langmuir-Blodgett technique|
SEEC slide features
SEEC slides consist of a stack of optical layers piled up on a thick base of silicon (for upright OM) or glass (for inverted OM). The astounding optical properties of SEEC slides result from a rigorous control of the conditions and parameters under which specific optical oxide layers are being deposited onto supporting plates. Theoretically, there is no limit to the kind of materials that one can choose as a supporting plate or uppermost layer, which makes it very flexible and convenient for customers whose needs may vary widely in terms of surface chemistry or environment (work in air, water, dilute aqueous solutions, buffers, oil).
|Figure 8 : Sketch of SEEC slide physical make-up|
Note that SEEC slides can be used with any reflected optical systems such as light microscopes but also video cameras.