RICERCA ITALIANA     

Photochromic polymers as active materials for innovative reference surfaces for optical interferometry

Politecnico di Milano

Abstract

Diarylethenes showing peculiar photochromic properties will be produced as active layers in reference surfaces for optical interferometry. The innovation of the project lies in the realization of rewritable reference surfaces (computer generated holograms, CGH) which exploit the reversibility of the photochromic process. As a consequence, only one device will be needed for interferometric measurements of free form optics.
The two research units have already collaborated in developing photochromic devices for astronomical instrumentation and the project will benefit fof the results obtained from previous works.
Concerning the synthesis of the targeted materials, optimization of the contrast, of the quantum yield values and of the refractive index modulation will be the guidelines for the design of the photochromic molecules. The materials which fulfill the requirements will be processed into films through solution techniques. For each film the contrast in specific wavelength ranges, the optical quality of the surface and the wavefront distortion will be determined. A suitable set-up for interferometric measurements will be implemented starting from the equipment (Fizeau interferometer)already available. Moreover, a setup for the writing process of the photochromic layers will be built. It will have to be characterized by high resolution and good repeatability. As preliminary measurement of the functionality of the proposed system, CHGs will be written on the photochromic substrates to test optics with well-known shape and quality. The final step of the project will consist of the development of an integrated metrology set-up for free form optics which will exploit the rewritability of the photochromic CGH device. <<<

Principal Investigator

Giuseppe Zerbi Politecnico di MILANO

Research Objectives

The development of very large telescopes with mirror diameters larger than 10-20 meters, known as Extremely Large Telescopes (ELTs), is a challenge for the design of new optical instrumentations in the astromonomical field.
The design of such instrumentations will find an important breakthrough with the availability of non traditional optics, such as variable curvature surfaces or even more challenging free-form optics. Even if actual optical manufacturing technologies allow to work very complex shapes, it is not always possible to verify the quality of such surfaces. Consequently, the availability of suitable metrology set-ups for such complex optics is very important.

The objectives of this project are:
- the design and synthesis of new organic materials with peculiar optical properties;
- the use of such materials as optical substrates suited for rewritable reference surfaces for interferometric tests;
The final goal of this research project is to develop an integrated metrology system for free-form optics.

To achieve these aims, two different research groups with complementary know-hows will work together onto this project. The group of Politecnico of Milan (RU1) has a wide experience in the research and development of functional organic materials and in their characterization/trasformation; the other group at the Osservatorio Astronomico di Brera (RU2) involved in the our joint project, currently carries on the design and development of optical instrumentations and optical components for astronomy.
The project will benefit of the results obtained by these two groups in the past: rewritable focal plane masks for multi-object spectroscopy were realized and, more recently, volume-phase holographic gratings are being studied.

Photochromic materials belonging to the class of diarlyethenes offer us a new possibility: to develop rewritable computer-generated holograms (CGHs from now on) to interferometrically test complex optical surfaces. This innovative idea will exploit the reversible change of transmittance and refractive index in the photochromic materials by means of a light stimulus with a chemical structure optimized for this aim.

As starting materials, diarylethene molecules and backbone photochromic polymers available (and fully carachterized) at the Politecnico di Milano will be used. On the basis on the first results, new photochromic materials will be synthetized. The materials will be processed into thin films that will be written later by RU2 to make the final working device.
A key point to reach the goals of the project is the realization of thin films with good optical properties: film deposition techniques which start from a polymer solution (casting, casting assisted by a control coater, spin coating) will be used; another approach will be used to make films: the polymerization in situ of the photochromic monomers during the film formation.

The optical quality will be verified by RU2,which will take care of the developing of the final measurement set-up.
It will consist of:
- a rewritable reference film of photochromic material
- a laser based writing set-up suited for CGHs
- an CGH interferometer.

The holograms will be written by using a laser device able to plot onto a photochromic substrate a synthetic fringe pattern (hologram). The set-up will also carry the erasing part of the system which consists of a UV lamp. After the hologram is written, it will be placed in the interferometer where it will generate the reference wavefront for the target optics.

Starting from an optical element with a complex shape, it will be possible to simulate its wavefront, and the hologram obtained with the inteference of that wavefront with a second tilted plane wavefront. Such computer generated hologram will be written and some interferometric measurements will be done to test surface quality.

The multidisciplinarity of the project is well evident and the collaboration between the two groups during last years is a qualifying element to achieve the proposed results. <<<

Timescale

24 months

National and international background

The development of optical technologies has allowed in the last years advances in many scientific and technological fields. For example astrophysics has made great strides exploiting the opportunities coming from the new ground and space telescopes. Photonics has allowed to control efficiently the industrial processes, moreover many diagnostic techniques take advantages from the progress in photonics and they are able to measure, with high accuracy, mechanical, thermal, optical properties.

Many emerging markets, such as digital cameras, did not have an exponential grow if there were not efficient techniques to produce polymeric lenses of good quality. The information technology is so widespread thanks also to the production of more and more miniaturized integrated circuits; it was possible only through the development of very precise aspheric lenses.

Each step forward in the optical manufacturing (more and more complex) has gone on in parallel to an improvement of the optical metrology, that is all the measurement techniques that permit to check the manufacturing process of the optics. At present, the frontier in such field is the "free form" optics. They consist of optical surfaces that depart from the simple spherical surface, showing a more complex design; such optics permit the improvement of the optical systems. Indeed it is possible to reduce the number of optical elements of a system increasing, consequently, the efficiency, reducing the complexity of the optical supports and, finally, increasing the image quality of the optical system.

The free form optics make challenging the metrology to test the surface quality. Some standard techniques, i.e. profilometry and interferometry, have been successfully used, but we are far from a reliable and rapid method.

For example, profilometry, measures one or more surface sections by moving a probe on it or vice versa. In order to get information of the whole surface it is necessary to control precisely its moving; it requires time and a good mechanical stability. Moreover it is a indirect technique based on a digital reconstruction of the surface form.

Interferometry performs a rapid measurement of the entire surface, which prevents some errors due to the surface reconstruction as happen in the case of profilometry. Nevertheless the interferometric techniques can be applied only to very simple surfaces (plane, spherical) and sometimes to ellipsoids, parabola. The new free form optics requires the development of peculiar interferometric techniques. A possibility lies in the Computer Generated Hologram (CGH) [1-4], which generates the reference wavefront that will be compared with the wavefront generated by the optics under measurement. The idea is to simulate the behaviour of the optics by using a model that permits to rebuild the wavefront shape generated by the optical surface. If such wavefront interferes with a plane wavefront, an interference pattern, characteristic of that surface, will be observed. This pattern can be used to measure a real surface if the CGH is printed on a suitable substrate in the interferometer.

The use of CGH for optical testing is not new, but it shows strong limitations: (1) every optical surfaces need a new CGH that has to be computed and printed; (2) the CGH must show good quality both for the substrate and the writing technique; (3) the CGH must be placed with high accuracy in the interferometer to ensure a precise measurement. The time needed for a single measurement makes the use of CGH rare.

Practically, a CGH consists of a transparent substrate, usually glass, where it is written a pattern that, as mentioned before, was generated before by using a PC. The device is a binary structure of transparent/opaque regions, this is the case of "amplitude" type CGH or it is a structure close to that of diffraction gratins (periodic change of the surface form, change in the refractive index), this is the "phase" type CGH. The techniques and materials commonly used to make such devices come from the semiconductor industry; indeed lithographic techniques (laser writer, electron beam writer) are used. Moreover the pattern can be transferred using contact printing. Typical amplitude type CGHs are made of a chrome pattern on a glass substrate.

The possibility to have a system which is able to write quickly CGHs into the interferometer with high accuracy could overcome the limit explained before, making possible the spread of such measuring technique for any complex optical surface.

It is clear from the description of the scenario of CGH, that a material which show a reversible change in transparency and refractive index is appealing for optical testing based on interferometry with CGHs. Photochromic materials meet these requirements as will be described.

Photochromic materials exist in two stable forms that are characterized by very different optical properties (color, refractive index) and physico-chemical properties [5-7]. The two forms can switch one into the other, in a reversible way, by using photon of suitable energy (usually in the UV-visible spectral region). These peculiar features make the photochromic materials attractive both for the academic research and for the industry: in the literature are described many classes of materials, both organic and inorganic and also many applications.

Among the organic photochromic materials, diarylethenes are known as the most efficient photochromic systems and they are a good choice for the technological applications [8]. Indeed they show the following properties:
- High stability of the two photochromic forms;
- High fatigue resistance (number of cycles that the photochromic material can bear without a degradation of its properties);
- High conversion;
- Fast response;

For about ten years, the research group of the Prof. Zerbi (RU1) has been studying the diarylethenes, both for the synthesis on new materials and for the developing of new applications and it has been distinguished in the international scientific community for the introducing of a new class of photochromic materials with a macromolecular structure[9,10].
The availability of photochromic materials with a polymeric nature has a fundamental significance for the realization of devices. Indeed, the diarylethene polymers combine the optical properties of the photochromic materials with the easy processability (in films) of the polymers. The main consequence is that it is not necessary a polymer matrix to disperse the chromophore; this usually brings to a low concentration of the active molecules (less than 10%) to prevent aggregation/segregation phenomena that limit the optical quality of the films. The low content of active molecule causes a decrement of the photochromic response of the layer (mainly in terms of transaprency and wavefront degradation).
The polymers with a diarylethene backbone, synthesized in the laboratory of Politecnico di Milano, showed both the union between the photochromic polymers and the filmability properties and an improvement of the photochromic features if compared to the starting monomers or to the low molecular weight diarylethenes [11,12].

As for the technological applications of photochromic systems, historically, they were proposed as materials for rewritable optical memories. RU1 suggested and realized a readout method of the information stored that did not degrade the same information. It was based on the change of the absorption spectra of the two forms in the mid infrared. The RU1 carried on a theoretical work to determine the chemical structure that enhanced the change in the IR spectrum and it realized a simple working device[13,14].

Some years ago, the two research units involved in the present research project (Politecnico di Milano e Osservatorio Astronomico di Brera) started a collaboration in order to develop innovative optical elements for the astronomical instrumentation based on the molecular response of materials. Particularly, thanks to this collaboration, the two research groups tried to direct some demands of the optical design where specific features, different from the usual, were required with the functionalities of suitable organic materials.
This research, formalized with the DROP project (Double Role of Photochromism, founded by INAF, Istituto Nazionale di Astrofisica) and with the OPTICON project (OPTical Infrared Coordination Network for astronomy, Sixth Framework Programme, EU), brought to the realization of rewritable focal plane masks for multi-object spectroscopy and to the development of volume phase holographic gratings (VPHGs) [15].

The focal plane mask selects, during the observation, the sky objects of interest and it prevents the noise signal of the sky to reach the detector. Usually, such masks are made of a thin sheet of metal with the slits punched by using different techniques. It is evident that each observation needs a new focal plane mask. The idea was to exploit the change in transmittance of the photochromic materials and the reversibility of the photochromic process to realize rewritable focal plane masks. Materials showing a strong change of transparency in the visible (in particular between 500 and 700 nm) were used to make films on a glass substrate. These films were exposed to UV light to convert the photochromic molecules in the colored (opaque) form. The observation pattern was written by using an automatic set-up (ARATRO) consisting of a red laser and a system of two slits that moved the glass slide following the designed pattern. The laser converted the molecules into the transparent form in the slits position. The masks can be erased after the observation, by lighting with UV light and they are ready for another slits pattern. The high fatigue resistance allows to rewrite the device many times with a lifetime of the mask longer than one year. The whole system can be made fully robotic and it will find the final application in the space telescopes.
The photochromic focal plane masks were successfully used in the AFOSC camera of the Asiago telescope[16].

More recently, the possibility of making volume phase holographic gratings (VPHG) based on photochromic materials was explored. This kind of dispersing element attracted the attention of the scientific community in the last ten years, because they show a large peak efficiency (&gt;90%), the possibility to tune in wavelength the peak efficiency by changing the incidence angle. Moreover they can reach large dimensions and a very dense line pattern (up to 6000 l/mm).
By using such elements, Osservatorio Astronomico di Brera (RU2) updated different astronomical spectrographs (AFOSC, Asiago; do.lo.res,Telescopio Nazionale Galileo, La Palma) of Italian telescopes. Consequently RU2 developed the know how for the design of optical instrumentations based on VPHG.
VPHGs exploit the diffraction that takes place in the volume of a film showing a lines pattern with high and low refractive index. A material can be considered a good candidate for making a VPHG if it possible to induce a periodic change of the refractive index.

The common materials to make such gratings are the materials for holography and in particular the dichromated gelatines (DCG). The writing process of the lines pattern requires a light exposure in an interferometer followed by a tricky chemical process (developing) that enhances the modulation of the refractive index.
The change of the refractive index that takes to the photochromic process, makes such materials good candidate as basic materials for VPHGs. Indeed, by using photochromic materials, the chemical process is no more needed, making the process simpler and doing away with some limitations in term of film thickness.

In this contest, a preliminary study concerned of the study of the molecular features that maximized the difference of the refractive index between the two forms. This study required a theoretical analysis based on quantum mechanical calculations.
Diarylethene molecules with low molecular weight and backbone photochromic polymers have been synthesized. The modulation of the refractive index was measured on thin film by using ellipsometry and spectral reflectance. The photochromic polymers, as described before, do not need a polymer matrix and they showed the best performances. Values of 0,03 at 1500 nm were reached and they represent the largest value in the literature for the photochrromic materials[17].
A photochromic polyester has been used to realize the first prototypes of VPHGs. The polymer showed a change of the refractive index of approximately 0.02, a good efficiency of the photochromic process in both directions and a good filmability. The grating was written by transferring a 600 l/mm pattern of a Ronchi ruling glass slide. The photochromic grating was characterized with optical microscopy and collecting the diffraction pattern of a white light.

The collaboration in the last years between the research groups of Politecnico di Milano e Osservatorio Astornomico di Brera allowed the developing of cross skills and a common language, which are essential to carry on a interdisciplinary research.
The results obtained show how the collaboration between the two research units was and it is still fruitful and it represents a good starting point for this research project since the kind of material and the phenomena involved (change in transparency and refractive index) are the same. <<<