1. Introduction

Conjugated polymers are under intense study from both the academic and industrial research groups due to their promising optoelectronic properties and solution processability. The use of conjugated polymers in different applications, such as multicolored displays [1], integrated plastic circuits [2] and non-conventional light sources [3] requires patterned thin film technology [4]. To match the needs of recent developments in nano-electronics and photonics, lithography techniques have been pushed down to the nanometric scale [5]. However, the spatial resolution achievable by optical lithography by classical methods is diffraction limited and non-conventional methods are required for sub-micron scale resolution. In this context, scanning near-field optical microscopy (SNOM) is a valuable and inexpensive nanolithography tool to prepare nano-structures below 100nm [6,7]. Especially, SNOM is a non-invasive technique that with Poly(methyl methacrylate) (PMMA) resist containing azobenzene mesogenic units, provides nano-structuring [8] for data recording. The working principle of these materials consists of the trans-cis isomerization of the azobenzene groups, which can orient the trans-moieties orthogonally with respect to the radiation polarization direction. The light induced isomerization is responsible for both photomechanical and optical modifications. The first process is due to mass migration inducing a surface relief formation (SRF) [9]. Recently, this intriguing phenomenon has been thoroughly investigated [10] and explained [11]. The second process is due to alignment of the azo-side chains inducing optical birefringence. By using a SNOM equipped with a polarization-modulation system sensitive to local birefringence variations, this pure optical nano-writing process was demonstrated [12] and recently it has been optimized on specifically synthesized azobenzene containing block copolymer [13]. Although successful, this procedure requires a complex and delicate near-field set-up to detect the optical polarization induced on the polymer surface.

We have explored the possibility of including in PMMA copolymers molecules with an optical response faster than the azo-molecules. In particular, we have been attracted by the spiropyran derivatives. The spiropyran molecules are photochromic compounds [14]. Upon UV light irradiation, the spiropyran colorless isomer undergoes heterolytic cleavage of the N-O bond to form the colored isomer (merocyanine) changing the refractive index. Interestingly the process is reversible: the merocyanine came back to the spiropyran form when it is illuminated with green light. For several years they have been employed as fluorescent probes [15] and found application also in the photodynamic therapy, in optical photolithography and high density optical storage [16]. At present, there is a renew interest for spiropyran-based materials because of their potential applications in optical recording media, photo switching devices, nonlinear optics, etc [17,18]. Indeed, this molecule is able to switch from the weak- to ultra strong-coupling regime reversibly, by using an all-optical control and a simple low-Q metallic microcavity [19], opening new possibilities to the observation of ultrastrong coupling phenomena in the visible window. In addition the possibility of optical tuning the refractive index of the material opens up a new way to fabricate re-writable sub-wavelength nanostructures for high-density optical storage devices. Here, we report on a simple, efficient and highly repeatable pure optical writing/reading process at the nanometer scale, performed by SNOM onto a PMMA-spiropyran (relative wt concentration 3:2) thin film. A resolution as high as 160 nm and writing speed as fast as 400nm/s was achieved We also demonstrate that suitable optical power injected into the SNOM fiber leads to both optic and topographic writing of the thin film surface. These results open a new way to build, just using light, planar photonic structures such as bi-dimensional photonic crystals and high density optical storage devices, with dimension below the diffraction limit.

2. Experimental setup

The experiments are performed on 130 nm thin films deposited by spin coating Toluene solution of PMMA-spiropyran on a quartz substrate. Figure 1 shows the absorption spectrum of the samples (black line) and the chemical structures of the spiropyran/merocyanine. Since the optical response of the spiropyran has a broad absorption band centered around 355 nm, we have used for the writing procedure a low-cost solid state laser at 404 nm that was available in our laboratory. The dashed line in Fig. 1 shows the absorption spectra of the film upon irradiation at 404 nm for 120 seconds with an optical density of 5 mW/cm². Although the exciting wavelength is not peaked at the spiropyran absorption, the heterolytic cleavage takes place, as evidenced by the broad absorption band at 560 nm that is a fingerprint of the merocyanine state [19]. For the reading procedure, we use a diode pumped solid state laser source at 532 nm that falls in the absorption range of the colored isomer.

 

Fig. 1 Chemical structures of Spiropyran and Merocyanine molecules (top of the figure); the far-field absorption spectrum of Spiropyran (black line) and Merocyanine upon irradiation with violet light (dashed line).

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The experimental procedure consists of two steps: first the sample undergoes a writing procedure by the SNOM probe and second the surface is read by the same apparatus working in transmission mode [20], as sketched in Fig. 2. To this purpose, we use a tapered optical fiber with a nominal aperture of 50 nm (Cr/Al coated Nanonics LTD fiber). Both lasers are coupled to the SNOM probe, but the temporal sequence writing/reading is simply performed by on/off switching of one laser or the other, hence making it possible to perform the reading after the writing with minimal drift. The tip-sample distance is controlled by a shear force feedback system based on a tuning fork mechanism [20]. The optical signal is collected in the transmission mode by a 20x objective installed inside the piezoelectric tube that works simultaneously as a nano-actuator and as a sample holder. The light collected by the objective is detected by a Hamamatsu miniaturized photomultiplier tube (PMT). To improve the S/N ratio during the reading scan, the lasers are modulated at 2 KHz by an optical chopper (Signal Recovery, Model 197) and detected by a lock-in amplifier (7625 EG&G Instruments). An optical power meter (Newport model 840) measures the power of the laser beam at the laser-to-fiber coupler. Both the storage and the reading processes are performed at room temperature.

 

Fig. 2 Schematic diagram of the experimental setup for nano-writing/reading in near-field.

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3. Results and discussion

One of the unique features of the SNOM consists of its ability to collect both the optical and the topographic data at the same time. This allows detection of morphological effects, if any, of the nano-machining. Figure 3 shows the pure optical nano-writing experiment.

 

Fig. 3 2µm × 2µm (a) topographic and (b) optical image of the pattern obtained by illuminating the sample at 404 nm and line profile (c) of the optical image. From (c) we evaluate an optical resolution of the writing process of 80 nm by accounting for the probe aperture size and the effects of the optical convolution.

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The image is inscribed in a 2µm × 2µm wide area and it consists of an array of 9 single dots obtained by illuminating the sample surface with the violet laser line for 40 sec/dot. The laser power coupled to the SNOM fiber is about 5mW. The writing process in near-field is repeated dot after dot following the rows and moving along the column until the pattern is completed. The distance between each dot is 200 nm. Figures 3(a) and 3(b) show the SNOM topographic and optical image, respectively, collected after the writing process. The morphology of the sample is almost flat and does not change upon interaction with violet photons, the sole effect of which is the photoisomerization of the spiropyran to merocyanine with no SRF. The locally induced photochemical switching is clearly seen in the optical image that exhibits a matrix of 9 dots placed at the left lower quarter of the inspected area. The line profile of the optical image, marked by the blue line in Fig. 3(b), reveals that each dot has a full width at half maximum (FWHM) of about 160 nm. The actual resolution depends on the aperture size of the fiber tip, which in working conditions may be different from the nominal value of 50 nm. An important point is that the recorded optical image consists of the convolution between sample features and probe aperture size and the dots are written with the same probe used for the reading. Therefore, although from Fig. 3(b) it might appear that the resolution of the writing process is 160 nm, the true achieved optical structure is 80 nm, which resemble the dimension of the tip aperture.

A fast pure optical nano-writing on the same sample and under the same conditions, performed by driving the SNOM probe over the sample surface along a defined path at a speed of 0.4 µm/s exploits the rapidity of the photoisomerization process as shown in Fig. 4.

 

Fig. 4 2µm × 2µm (a) topographic and (b) optical image of the pattern at the nanoscale obtained by driving the SNOM probe over the sample surface along a squared path at a speed of 0.4 µm/sec.

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A much higher speed may be achieved using a shorter wavelength at which the absorption of the Spiropyran is higher. Increasing the laser power, also a shorter writing time is expected. Nevertheless, it is well known that SNOM tapered fibers suffer of a low optical throughput (10⁻⁶ for pulled fibers) causing some photon energy absorption in the metal coating [21]. Coupling higher power causes the probe to heat to several hundred degrees, especially in the proximity of the apex. That could lead to unwanted thermolithographic effects [22]. The flatness of the thin film surface assures that no SRF effect occurs and the power launched into the fiber is not able to increase the temperature of the apex over the glass transition temperature (T_g) of PMMA (about 90-110°C) [23]. Figure 5 shows an optical and topographic nano-machining obtained by increasing the power coupled to the probe up to 13 mW. The printing process has been repeated three times with exposure times of 2.5, 5 and 10 seconds respectively to produce three dots placed along a vertical line. The line profile taken along the blue line of Fig. 5(a) reveals that with 10 seconds of exposure time a conical indent with a FWHM of about 220 nm and a mean depth of about 60 nm is produced. These features are in good agreement with the typical shape of the pulled SNOM probes. Because of the higher laser power, the tip of the optical fiber of SNOM warms up and it melts the underlying polymer. The photoconversion also takes place and the formation of dots in the optical image is observed [Fig. 5(c)]. The line profile shows a FWHM of about 120 nm, in good agreement with the resolution obtained with lower optical density.

 

Fig. 5 2µm × 2µm (a) topographic and (c) optical image of the pattern obtained by illuminating the sample at 404 nm with 2.5, 5 and 10 seconds of exposure (from the bottom upwards); line profile of the topographic (b) and optical (d) image.

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4. Conclusion

In conclusion, we demonstrate a nanowriting process on PMMA/spiropyran films. Optical processes are activated by photons at 404 nm, a wavelength which is not even optimized on the peak absorption of the spiropyran molecule. Moreover, pure optical nano-writing is achieved without need for polarized excitation, as is required for nano-writing on PMMA-azo samples. The present writing resolution is well below the diffraction limit and the writing speed is fast as 0.4 µm/sec. The nano-machining has been obtained with a simple SNOM set-up and detected by measuring the optical contrast in transmission mode. The written nanostructures can be reversed with visible radiation and therefore this material is well suited for building high density optical write/erase memories. Plasmon resonance and second harmonic generation in apertureless experiments may improve both resolution and the writing efficiency. Increasing the laser power coupled to the apex of the SNOM probe increases the temperature over the Tg of the PMMA matrix resulting in the controlled nano-indentation of the surface. Remarkably, no SRF effect is observed. Thus, spiropyran has very special optical features and its nano-structuring opens the door to further engineering of the electromagnetic field and more generally to design nano-optics devices where lithography at the nanoscale is a keyword.