1. Introduction

Manipulation of the materials properties by changing their geometrical organization at nano and microscale levels with the aim to create new properties, untypical for the raw material attracted great attention of the scientific community [1–4]. This trend is closely related to the latest advances in material engineering and science on metamaterials or transformation optics [5]. Precise geometry, shape, size, orientation and arrangement of such “artificial” materials can affect light waves in a manner not observed in natural materials [6–8]. However, the formation of “artificial” optical properties is very technologically pretentious. Desired effects can be achieved by incorporating into metamaterials structural elements with sub-wavelength dimensions [9].

Chiral periodic structures and photonic crystals have attracted a great deal of interest due to their applications in advanced photonics. Chirality is optical functionality of the “soft” matter systems that may be divided into two different levels: (i) molecular (determined by chemical bonding) and (ii) supramolecular. In the latter case a chiral supramolecular structure of the molecules may be created even if the chirality at the molecular level does not exist [10,11]. Controlling of chirality at supramolecular level is more interesting because of its greater potential for new approaches in artificial plasmonic materials (including metamaterials), smart materials, and asymmetric synthesis [12].

A number of strategies that make possible to create architectures at different length scales including supramolecular chemistry, self-assembly and external stimuli, provide access to increasingly complex functionalities [13–15]. Self-assembling of non-chiral monomers into chiral structures is realized through hydrogen bonding [16,17] or complexation with metal [18] of different achiral molecules. It has also been shown that irradiation with polarized light is another way to induce and control the chirality in special materials such as liquid crystal, azo-containing polymers or their combination [19,20]. Azobenzene moieties attached to polymer chain undergo photo-isomerization and re-orientation perpendicularly to the light polarization direction [21–23]. When circularly or elliptically polarized light was used, photo-induced optical activity was induced in thin films of achiral polymers with azo-benzene chromophores [24–26]. So, photo-induced optical chirality of polymers is known, but it is strongly restricted to the class of azo-containing polymer. In the case of the other materials several research were carried out to demonstrate the creation of light-induced chirality, but most of them dealt with inorganic materials [27,28].

In this paper we propose a new approach for chirality creation, which can be extended too much wider range of polymer materials, independently on the presence of azo-benzene chromophores. It is well known, that linearly polarized excimer laser beam induces formation of the grating structure on the exposed polymer surface and this grating structure can linearly polarize transmitted or reflected light. Likewise, it was proposed that the chiral polarization of modifying excimer beam can introduce the formation of artificial structures with chiral optical response. We tried to verify experimentally this possibility, i.e. if the polymer film can undergo a surface modification under circularly polarized laser beam and if the created structures will show chiral response.

In this study application of large-area chiral excimer laser patterning for the creation of artificial chirality is described. Polymethylmethacrylate was doped with chromophore (Fast Red ITR) and this film was irradiated with polarized KrF excimer laser light to different fluencies and laser pulse number. Surface morphology of pristine and treated samples was studied by confocal and AFM microscopy, optical absorption of these samples was study by FTIR spectroscopy.

2. Experimental

Polymethylmethacrylate (PMMA) of optical purity was supplied by Goodfellow Inc. (Llangollen, UK) and Fast Red ITR of 96% grade (λ_{abs max} = 226 nm) was purchased from Sigma-Aldrich Corp. (St. Louis, MO, USA). Both materials were used without additional purification. The Fast Red-doped PMMA films were prepared by separate dissolving the PMMA (M_w ~1,500 K) and the Fast Red ITR in 1,2-dichloroethane. Then, 7.0 wt. % PMMA and 2.8 wt. % FR solutions were mixed and spin-coated onto freshly cleaned glass substrates [29].

The samples (schematic representation of experimental set-up see in Fig. 1) were irradiated with KrF excimer laser (COMPexPro 50F, Coherent, Inc., wavelength 248 nm, pulse duration 20-40 ns, repetition rate 10 Hz). The laser beam was polarized linearly with a cube of UV-grade fused silica with an active polarization layer. Then the laser beam passes through the proper quarter-wavelength plate to convert linear into circular polarization. The proper quarter-wavelength plate was supplied by Photop Technologies, Inc. (New Port Richey, USA). The samples were irradiated by 50 to 550 laser pulses with laser fluencies from 12 mJ cm⁻² [29]. The angle of laser beam incidence with respect to the sample surface normal was 0° and the aperture with the area of 5 × 10 mm² was used.

 

Fig. 1 Schematic representation of experimental set-up and results to be expected: (i) and (ii) samples prepared by different experimental conditions (see the text). (i) and (ii) are the typical pictures of chiral structures.

Download Full Size | PDF

The surface morphology was examined by AFM technique using a VEECO CP II device (‘tapping’ mode, probe RTES PACP, Veeco Instruments). Optical images were taken using upright laser confocal microscope Olympus Lext working with a 405 nm laser light. Vibration Circular Dichroism (VCD) spectra were recorded in the region 1800-1000 cm⁻¹ at room temperature with a resolution of 8 cm⁻¹ using an IFS-66/S Fourier-transform infrared spectrometer (Bruker, Germany) equipped with a VCD/IRRAS module PMA 37 (Bruker) by a procedure that has been described in our previous work [18]. The samples were prepared on CaF₂ windows directly. The samples were rotated around the light propagation axis at different angles and the spectra were obtained from front and back sides of the window. In all the cases, the spectral pattern was the same. Demonstrating controlled induction of chirality by any means is difficult because there are many possible sources of experimental artifact. To avoid appearance of possible artifacts related to sample preparation controlled VCD measurements were performed on glass substrate and on glass substrate with PMMA films.

3. Results and discussion

Many achievements in the fabrication of artificial chiral structures have been made over the last decade, but most of them are related to light coupling with polarization sensitive materials, in particular with azo-polymer [30]. Here, we exposed a thin PMMA film to coherent, large area excimer beam with 248 nm wavelength and circular polarization. According to our previous studies irradiation with linearly polarized light lead to the creation of periodical surface structures with ridges oriented along polarization of light [29]. Likewise, it was proposed, that irradiation to circularly polarized light can induce creation of periodical supramolecular structures with artificial chirality.

Figure 1 represents experimental set-up of our experiments. In particular, excimer laser beam with 248 nm wavelength was linearly polarized and transmitted through the quarter-wave plate onto polymer thin film to induce creation of nanopattern on the polymer surface [30]. It was expected, that illumination with circularly polarized light will induce the appearance of surface structures with one of the typical shape, reported for supramolecular “artificial” chiral structures: (i) open rings or (ii) more sophisticated, hierarchical geometries [30].

The effects of the laser irradiation at the fluence of 12 mJ cm⁻² and with 150 and 450 laser pulse numbers is illustrated on confocal morphology maps shown in Fig. 2. For lower laser pulse number a system of microstructures with spiral shape is well visible (Fig. 2(a)). In the case of higher pulse number a quasi-ordered surface structure is formed (Fig. 2(b)).

 

Fig. 2 Confocal images of surface structures on the doped polymer samples created at fluence 12 mJ cm⁻² by 150 pulses (a) and 450 pulses (b).

Download Full Size | PDF

Further details of the observed spiral relief patterns are visible from AFM morphology and phase scans, taken in two scales - 12 and 3 µm and shown in Fig. 3 (samples with homogeneous patterning were taken for AFM analysis - see Fig. 2(b)). One can see that from the morphology point of view the irradiated surface represents system of quasi-ordered dots. More detailed morphology scan (Fig. 3(b)) shows that the dots are inhomogeneous, with daisy shaped symmetry, where one deeper spot is surrounded by greater. Phase scans indicate even more complex structure of each dot. From the point of material distribution each dot seems to consist of three discs with a regularly displaced center of symmetry. This material arrangement is believed to result from the excimer-induced polymer redistribution, associated with the creation of twisted structures.

 

Fig. 3 AFM images of surface morphology and phase scans of doped PMMA treated by circularly polarized excimer laser (fluence 12 mJ cm⁻², 450 pulses) taken in two scales - 12 (a) and 3 µm (b).

Download Full Size | PDF

Polymer surface patterning can take place by two mechanisms: redistribution of material or ablation. Our previous work was carried out with linearly polarized excimer beam under the similar experimental conditions (laser fluency, used materials) and the apparent ablation was observed [31]. Similarly, we can propose that application of circularly polarized laser beam will also lead to the surface patterning due to ablation of materials.

The illumination with circularly polarized light of the doped PMMA was initially expected to give rise to a photo-induced circular dichroism (CD) response. Sample chirality was tested directly by VCD technique and obtained spectra are shown in the Fig. 4. In particular, infra-red absorption spectra are shown for pristine films, films irradiated by circularly polarized laser at two inverse position of quarter wave plate, and films dissolved after irradiation. Control measurements were performed on glass substrate and on glass substrate with PMMA film. The optical activity of these samples was not detected. Figure 4(a) shows “classical” IR absorbance spectra. Well visible peaks in the Fig. 4(a) are related to the vibration and deformation oscillation of PMMA bonds. FR manifests itself as weakly distinguishable peak between characteristic PMMA signals. Figure 4(b) gives the difference between absorption of left-handed and right-handed light transmitted through the samples. It is evident, that pristine film is non-chiral (the difference is zero). Modification under “fewer” pulses at higher fluence leads to appearance of chirality, which manifest itself as a nonzero double signal at 1730 cm⁻¹. So, structures presented in the Fig. 2(a) introduce slight “artificial” chirality into previously non-chiral polymer film. Modification by larger number of pulses leads to more significant chiral response. This is evident from the pronounced doublet carbonyl C = O located at 1730 cm⁻¹ and strong singlet peak carbon-oxygen bond C-O at 1150 cm⁻¹. Rotation of the quarter wave plate at 180 degrees will lead to “opposite” circular light polarization and CD spectrum of such samples is also presented in the Fig. 4B. It is evident, that IR absorption remains constant but the sample chirality becomes inverse. So, irradiation with circularly polarized light introduces chiral properties into thin polymer film through creation of chiral structures on polymer surface (Fig. 2). By careful choice of the experimental conditions it is possible to achieve significant chirality (see Fig. 4).

 

Fig. 4 Absorption spectra of pristine sample and PMMA doped and irradiated by KrF laser (fluence 12 mJ cm⁻², 150 and 450 pulses): (a) - “normal” absorption spectra, (b) - difference between absorbance of left-handed and right-handed light transmitted through the samples.

Download Full Size | PDF

In the present case a large area of the polymer can be modified, which is great advantage, in comparison with the creation of chiral structure on azo-polymer by focused laser beam (spot creation) [32]. The present technique opens new possibilities in the creation of “artificial” chirality at large scale in non-azo polymers, which may lead to design of novel materials, in which optical response is driven by material construction ether than chemical composition.

4. Conclusion

The preparation of surface patterns with chiral properties on the doped PMMA film by the illumination with circularly polarized light from excimer laser is described. Artificial chirality of the prepared samples was proved by measurement of absorption of left-handed and right-handed light transmitted through the samples. Present experiments indicate that circularly polarized light induces the formation of many microscopic spirals or more complex structures on the polymer surface. Surface structures were analyzed by confocal microscopy and AFM techniques. It was shown that the shape and density of surface structures can be controlled by experimental conditions. In all cases the structures with well pronounced chirality, equivalent to the classical optically active media, are created on initially non-chiral doped PMMA. In particular, created structures gave rise to a photoinduced circular dichroism response, with response determined by the experimental conditions. The resulting chiral structures exhibit interesting possibility to manipulate the light polarization and show long-time stability.