**5. Conclusions**

160 Advances in Unconventional Lithography

measurements on a similar sample were conducted. The results show that before any

index variations in the grating between the photochromic polymer (1.509) and the air (1), which is actually what causes diffraction to occur. Thus it is believed that this change plays a negligible role in the measured DE relative changes. Furthermore, the small thickness of

Thus, the reversible DE changes can be attributed exclusively to the light-induced macroscopic deformations of the gratings. Specifically, Figure 12 illustrates the reversible macroscopic changes of the grating before and after UV-green irradiation as taken by AFM microscopy. As shown, the width of the stripes of the grating (α−β) is decreased by ca. 13% after UV irradiation while the distance between the two stripes (β) is increased. A small decrease is also observed in the period of the grating (α) (α and β before UV, 3.971 μm and 2.366 μm respectively; α and β after UV, 3.842 μm and 2.449 μm respectively). After the subsequent irradiation with green light the values recover very close to the initial ones. It is worth noticing that, as shown at Figure 12, there is a dip separating each stripe in two equal parts. As already mentioned at section 2.2, the SM technique which is followed for realizing the grating relies mainly on the capillarity that allows the viscous polymer to spontaneously fill the vertical channels that are made of the recessed features of the elastomeric mold, since the wetting lowers the overall free energy. There is always the possibility that the photochromic polymer may not fill completely such regions, thus may be mostly accumulated in the regions that are adjacent to the protruding areas of the mold, forming thus dips in the central part of the growing capillarity features. This behavior is common to different imprint lithography methods (Zankovych et al 2001, Hong and Lee 2003, Pisignano et al 2004). However, this dip is useful for the AFM morphological analysis of the patterned surfaces, since it makes the volume changes upon UV-green irradiation cycles much clearer. Moreover, it is too narrow to give any contribution to the diffracted light from the grating. In order to compare the experimental result with the existing theory, the basic equation that describes the intensity distribution of monochromatic light passing through a grating, was

=822 nm, the refractive index of the sample is *n*=1.509. After UV irradiation,

*n*=0.029. This difference is very small compared to the periodic refractive

Δ

2 2

<sup>⎡</sup> ⎤ ⎡ <sup>⎤</sup> <sup>=</sup> <sup>⎢</sup> ⎥ ⎢ <sup>⎥</sup> <sup>⎣</sup> ⎦ ⎣ <sup>⎦</sup> (5)

*<sup>0</sup>* is the angle of incidence and

θ

the angle of

 πα λ

 πα λ

sin( / ) sin( / ) / sin( / )

I and I0 are the intensities of the light after the grating at various orders of diffraction and at zero order respectively, β is the distance between two successive stripes, α is the period of the grating, N is the number of stripes, λ is the wavelength of the reading beam, and p =

diffraction. The number of the stripes covered by the reading beam was calculated by dividing the diameter of the spot of the beam by the period of the grating in each case. The angle of incidence of the reading beam was θ0 =20°. In each case, by the AFM images it was measured the different value of α and β before and after UV-green irradiation. Taking into account the measured parameters by the experiment and using the equation 5, it was calculated the ratio I1/I0, and consequently the DE for the gratings before and after the UV– green irradiation. The theoretical calculations confirm that there is an increase of the DE

*I p Np Ip p*

θ

πβ λ

πβ λ *n* even more.

irradiation for

the *n* is higher by

λ

Δ

used (equation 5). (Born and Wolf 1999)

sinθ -sinθ0 =mλ/α (m=0, ±1, ±2 etc) where

0

the gratings (ca. 240 nm) reduces the importance of the

In conclusion, it is demonstrated how soft molding lithography can be employed for the preparation of microstructured photochromic polymeric films, which undergo light controlled photomechanical changes responsible for the control of some functional characteristics of the patterned surfaces, namely the wetting properties and the diffraction

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efficiency. In particular, the light-induced isomerization of the embedded photochromic molecules in the flat surfaces is exclusively responsible for the reversible changes in their wetting properties. When the surface is microstructured by realizing patterns with the SM technique, these wetting properties are greatly enhanced. Moreover the control of the characteristics of the patterns (eg. the period), makes possible to control the light induced alterations in the wetting properties of the structured surface, demonstrating that they are influenced by both the changes in the surface polarity and the volume changes of the patterned structures. Finally, last but not least, it is demonstrated the possibility of fully manipulate the diffraction efficiency of thin photochromic polymer gratings. It is shown, that the produced gratings change their diffraction efficiency in a reversible way upon UVgreen laser irradiation. This effect, which is verified also by a theoretical diffraction model, is attributed to the reversible dimensional changes of the imprinted structures, and not to the refractive index changes as is the case in the majority of previous work. Such findings open a way for the production of optically switchable gratings based on reversible dimensional changes. Moreover, the ability to control the wettability of surfaces by microstructuring and to tune it by using photochromic molecules opens the way to the application of these optimized patterns to various microfluidic devices.
