**1. Introduction**

148 Advances in Unconventional Lithography

In summary, we designed and fabricated two types of binary diffractive convex lenses using EBL on a PET film. In the case of the binary diffractive convex lens with 2-mm focal length, it is possible to control the luminous intensity distribution. To improve the diffraction efficiency and realize a thin LED light source, we designed a binary diffractive lens with 140-μm focal length. This type of lens with focal wavelengths in the micrometer range can

To realize the binary diffractive lens with the 100-μm-order focal lengths, we characterize the relationship between the diffractive lens structure and its light intensity. It is clear that the intensities of the zero- and first-order diffractions are controlled by the structure of the binary diffractive lens. By using this lens, wide luminous intensity distribution can be obtained.

This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18360008 and 21360007), Mie University COE Projects, a Start-up Grant Program for Mie Venture Business from Mie Industry Enterprise Support Center, the Kinki Invention Center, and the Knowledge Cluster Initiative from the

The authors thank Mr. T. Sato (JA Woollam Japan Co., Inc.) for ellipsometry measurement. The authors also thank Dr. H. Miyake, Mr. K. Manabe, Mr. N. Machida, Mr. Y. Nakayama, Mr. K. Arakawa, and Mr. Y. Seriguchi for their help in the experiments and valuable discussions.

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**4. Conclusion** 

**5. Acknowledgment** 

**6. References** 

Stimuli-responsive polymeric materials are able to change their chemistry and their conformation upon an external signal. The external signal may be derived from a change in temperature, chemical composition or applied mechanical force of the specific material, or can be triggered externally with exposure to an electric or magnetic field or to light irradiation. In this respect, a photochromic substance is a stimuli responsive material which is characterized by its ability to alternate between two different chemical forms having different absorption spectra, in response to light irradiation of appropriate wavelengths (Brown 1971). Due to this important property, a significant amount of effort has been devoted to the formation of polymeric materials functionalized with photochromic molecules for the creation of photosensitive "smart material" systems, that change reversibly their physical and chemical properties by the use of light. The corresponding reversible effects of the molecules such as dipole moment, surface energy, refractive index, and volume are preserved in the polymer matrix, and have numerous promising applications in devices for three-dimensional (3D) optical memories, (S. Kawata & Y. Kawata 2000), in actuators (Yu et al 2003, Athanassiou et al 2005), in holographic or diffractive optics, (Fu et al 2005, Tong et al 2005) or in microfluidics, (Caprioli et al 2007, Walsh et al 2010) etc. Concerning microfluidic devices using photochromic plastic films, the transportation of fluids happens without the need for their molecules to be charged, as done in other studies (Mitchel 2001). This is achieved by gradually modifying the surface tension, and thus the wettability, by irradiating with increasing time along the direction of the fluid movement (Ichimura et al 2000). The gradual wettability changes are exclusively based on the photochemical modification of the embedded photochromic molecules caused by the photoisomerization process. In addition, in the case of the diffraction gratings the development was generally done by interference of different polarized laser beams, or by electric-field application, and the modification of their diffraction efficiency is connected with the changes of the refractive index of the photochromic molecules during this procedure (Yamamoto et al 2001, Fu et al 2005).

Here we present how the volume changes induced to the photochromic polymers by the photoisomerization of their embedded photochromic molecules, can improve significantly

Photocontrolled Reversible Dimensional Changes of Microstructured Photochromic Polymers 151

Briefly, UV and visible irradiation causes the reversible transformation of these chemical species, between two states (isomers) that have light absorption bands in characteristic spectral regions. This property is retained when the photochromic molecules are incorporated in polymer matrices, where they are homogeneously dispersed forming miscible systems. Specifically, the absorption properties of the photochromic polymer films prepared as described above change reversely upon UV-visible irradiation as shown at Figure 2. Initially the system is transparent at the visible range of the spectrum. Upon pulsed UV laser irradiation the SP is slowly converting to the MC isomer, fact indicated by the new absorption band in the visible region of the spectrum (ca. 565 nm). The intensity of the peak increases with the number of UV pulses until a plateau is reached, which suggests that the photoisomerization is completed and that the system has reached the equilibrium. The subsequent irradiation with green laser light, causes the decrease of the intensity of the previously formed MC peak, while after a certain number of pulses the spectrum reaches its initial form, indicating that MC reverts fully to the SP isomer. These data confirm that under the irradiation conditions mentioned in the figure caption of Figure 2, the reversible properties of SP are retained in the host polymer matrix. Depending on the irradiation conditions and the weight percentage of the photochromic molecules in the polymer matrix (usually ≤10%), it has been shown that typically about 4-10 irradiation cycles can be performed, while further irradiation causes the degradative photooxidation of the photochromic molecules, restricting thus the lifetime of the system (Athanassiou et al 2006c). Additionally, the degradative phenomena start to be evident usually after the third cycle. In order to exclude this parameter from the following study, results derived by the first three

Fig. 2. Absorption spectra of the PEMMA/SP 10% wt upon UV and visible irradiation. For the specific study, the irradiation conditions used are: λUV=355 nm, fluence FUV=20 mJ cm-2,

irradiation cycles are presented.

λvis=532 nm, Fvis=35 mJ cm-2.

the performance of these two different type of applications, namely the microfluidic devices and the diffraction gratings. In both cases the lithographic technique used for the microstructuring of the photochromic doped plastic films is the soft molding. Concerning the microfluidics applications, the presented microstructured photochromic plastic films exhibit a significant improvement on the reversible wetting characteristics compared to those on the flat surfaces. This improvement is due to the combination of the changes in the surface polarity and thus in the wetting properties with the modified surface conformation, both provoked by the light induced changes of the photochromic molecules. Moreover, regarding the optical gratings, we present a different approach in where the control of their diffraction efficiency relies on the dimensional variations of the gratings upon laser irradiation. Following this approach, the efficiency of the gratings is significantly improved with respect to previous works.

Such findings open the way for the production of optically switchable gratings based on reversible dimensional changes, and can be of great importance in all-optical signal processing systems. Moreover, the ability to control the wettability of surfaces by microstructuring and to tune it by using photochromic molecules, permits the application of these lithographically formed structures to all-optically controlled switches capable of operating with tunable speed, and to microfluidic actuation.
