(Institute of Physics and Astronomy, U Potsdam)
chair of experimental physics
Activities in photo-sensitive materials
Reversible structuring of photosensitive polymer films by surface plasmon near field radiation: azobenzene-polymer thin films with integrated optically active elements supposed to support and steer the response of polymer films to external illumination by acting as nano-scale antennas. During irradiation surface plasmon polarizations (SPP) are generated on a metallic mask. The interaction of the SPP with azo polymers results in printing of near field intensity distributions into topography with the pattern size below the diffraction limit. The topography can be driven reversible, this allows us to analyze different patterns by changing polarization or wavelength at the same position.
Recent selected publications
Polarized 3D Raman and nanoscale near-field optical microscopy of optically inscribed surface relief gratings: chromophore orientation in azo-doped polymer films
Giuseppe Di Florio, Erik Brundermann, Nataraja Sekhar Yadavalli, Svetlana Santer, and Martina Havenith
Soft Matter 10 (2014) 1544--54
07 Jul 14 -- kept for reference. Check for experimental status on alignment in complex light field, compared to French group around [LagugneLarbarthet07] and earlier.
received 30 Jun 2013 --
We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.
Mapping a plasmonic hologram with photosensitive polymer films: standing versus propagating waves
Thomas Papke, Nataraja Sekhar Yadavalli, Carsten Henkel, and Svetlana Santer
ACS Appl. Mater. Interfaces 6 (2014) 14 174--80
30 Apr 2014 -- first joint paper on relief formation in polymer films, here with a structured plasmon-enhanced light field. Funny aspect: "hologram" from interference between laser light and scattered plasmon wave.
We use a photosensitive layer containing azobenzene moieties to map near-field intensity patterns in the vicinity of nanogrids fabricated within a thin silver layer. It is known that azobenzene containing films deform permanently during irradiation, following the pattern of the field intensity. The photosensitive material reacts only to stationary waves whose intensity patterns do not change in time. In this study, we have found a periodic deformation above the silver film outside the nanostructure, even if the latter consists of just one groove. This is in contradiction to the widely accepted viewpoint that propagating surface plasmon modes dominate outside nanogrids. We explain our observation based on an electromagnetic hologram formed by the constructive interference between a propagating surface plasmon wave and the incident light. This hologram contains a stationary intensity and polarization grating that even appears in the absence of the polymer layer.
In-situ atomic force microscopy study of the mechanism of surface relief grating formation in photosensitive polymer films
Nataraja Sekhar Yadavalli and Svetlana Santer
J. Appl. Phys. 113 (2013) 224304
15 Jan 14 -- very similar is [Fabbri11] where intensity and polarization gradients are also studied. Also a "real time" scan as the polarization is turned. -- 22 Nov 13 -- more details on which optical parameters (intensity, polarization) are relevant for the "polymer flow" under irradiation. This is also quite complicated chemistry, with alignments of chains that persist from one irradiation cycle (50min!) to another.
When photosensitive azobenzene-containing polymer films are irradiated with light interference patterns topographic variations in the film develop that follow the local distribution of the electric field vector. The exact correspondence of e.g. the vector orientation in relation to the presence of local topographic minima or maxima is in general difficult to determine. Here we report on a systematic procedure how this can be accomplished. For this we devise a new set-up combining an atomic force microscope and two-beam interferometry. With this set-up it is possible to track the topography change in-situ while at the same time changing polarization and phase of the impinging interference pattern. This is the first time that an absolute correspondence between the local distribution of electric field vectors and the local topography of the relief grating could be established exhaustively. Our setup does not require a complex mathematical post-processing and its simplicity renders it interesting for characterizing photosensitive polymer films in general.
Structuring of photosensitive material below diffraction limit using far field irradiation
NatarajaSekhar Yadavalli, Marina Saphiannikova, Nino Lomadze, LeonidM. Goldenberg, and Svetlana Santer
Appl. Phys. A 113 (2013) 263--72
04 Mar 14 -- similar experiments in [Yadavalli13a]. See also [Kang_2011] where not a film, but an array of stripes is irradiated and analyzed with AFM. They also assign maxima to polarization features.
In this paper, we report on in-situ atomic force microscopy (AFM) studies of topographical changes in azobenzene-containing photosensitive polymer films that are irradiated with light interference patterns. We have developed an experimental setup consisting of an AFM combined with two-beam interferometry that permits us to switch between different polarization states of the two interfering beams while scanning the illuminated area of the polymer film, acquiring corresponding changes in topography in-situ. This way, we are able to analyze how the change in topography is related to the variation of the electrical field vector within the interference pattern. It is for the first time that with a rather simple experimental approach a rigorous assignment can be achieved. By performing in-situ measurements we found that for a certain polarization combination of two interfering beams [namely for the SP (vertical, horizontal) polarization pattern] the topography forms surface relief grating with only half the period of the interference patterns. Exploiting this phenomenon we are able to fabricate surface relief structures with characteristic features measuring only 140 nm, by using far field optics with a wavelength of 491 nm. We believe that this relatively simple method could be extremely valuable to, for instance, produce structural features below the diffraction limit at high-throughput, and this could significantly contribute to the search of new fabrication strategies in electronics and photonics industry.
Selective mass transport of azobenzene-containing photosensitive films towards or away from the light intensity
Nataraja Sekhar Yadavalli, Tobias König, and Svetlana Santer
J. Soc. Inf. Display 23 (2015) 154--62
Here, we report on two photosensitive amorphous polymers showing opposite behavior upon exposure to illumination. The first polymer (PAZO) consists of linear backbone to which azobenzene-containing side chains are covalently attached, while in the second polymer (azo-PEI), the azobenzene side chains are attached ionically to a polyelectrolyte backbone. When irradiated through a mask, the PAZO goes away from the intensity maxima, leaving behind topography trenches, while the direction of the mass transport of the azo-PEI polymer points towards the intensity maxima. This kind of behavior has been reported only for certain liquid crystalline polymers that exhibit in-phase reaction on illumination, that is, topography maxima coincides with the intensity maxima. Furthermore, flat nanocrystals placed on top of azo-PEI film was found to be moved together with the mass transport of the underlying polymer film as visualized using in situ atomic force microscopy (AFM) measurements. It was also demonstrated that the two polymer films respond differently on irradiation with the polarization and intensity interference patterns (IPs). To record the kinetic of the surface relief grating formation within two polymers during irradiation with different IPs, we utilized a homemade setup combining the optical part for the generation of IP and AFM. A possible mechanism explaining different responses on the irradiation of amorphous polymers is discussed in the frame of a theoretical model proposed by Saphiannikova et al. [Toshchevikov_2009].