TY - JOUR A1 - Jelken, Joachim A1 - Henkel, Carsten A1 - Santer, Svetlana T1 - Polarization controlled fine structure of diffraction spots from an optically induced grating JF - Applied physics letters N2 - We report on the remote control of the fine structure of a diffraction spot from optically induced dual gratings within a photosensitive polymer film. The material contains azobenzene in the polymer side chains and develops a surface relief under two-beam holographic irradiation. The diffraction of a polarized probe beam is sensitive to the orientation of the azobenzene groups forming a permanently stored birefringence grating within the film. We demonstrate that the fine structure of the probe diffraction spot switches from a Gaussian to a hollow or a hollow to a "Saturn"-like structure by a change in polarization. This makes it potentially useful in photonic devices because the beam shape can be easily inverted by an external stimulus. Y1 - 2020 U6 - https://doi.org/10.1063/1.5140067 SN - 0003-6951 SN - 1077-3118 VL - 116 IS - 5 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Arya, Pooja A1 - Jelken, Joachim A1 - Feldmann, David A1 - Lomadze, Nino A1 - Santer, Svetlana T1 - Light driven diffusioosmotic repulsion and attraction of colloidal particles JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - In this paper, we introduce the phenomenon of light driven diffusioosmotic long-range attraction and repulsion of porous particles under irradiation with UV light. The change in the inter-particle interaction potential is governed by flow patterns generated around single colloids and results in reversible aggregation or separation of the mesoporous silica particles that are trapped at a solid surface. The range of the interaction potential extends to several times the diameter of the particle and can be adjusted by varying the light intensity. The "fuel" of the process is a photosensitive surfactant undergoing photo-isomerization from a more hydrophobic trans-state to a rather hydrophilic cis-state. The surfactant has different adsorption affinities to the particles depending on the isomerization state. The trans-isomer, for example, tends to accumulate in the negatively charged pores of the particles, while the cis-isomer prefers to remain in the solution. This implies that when under UV irradiation cis-isomers are being formed within the pores, they tend to diffuse out readily and generate an excess concentration near the colloid's outer surface, ultimately resulting in the initiation of diffusioosmotic flow. The direction of the flow depends strongly on the dynamic redistribution of the fraction of trans- and cis-isomers near the colloids due to different kinetics of photo-isomerization within the pores as compared to the bulk. The unique feature of the mechanism discussed in the paper is that the long-range mutual repulsion but also the attraction can be tuned by convenient external optical stimuli such as intensity so that a broad variety of experimental situations for manipulation of a particle ensemble can be realized. Y1 - 2020 U6 - https://doi.org/10.1063/5.0007556 SN - 0021-9606 SN - 1089-7690 VL - 152 IS - 19 PB - American Institute of Physics CY - Melville, NY ER - TY - JOUR A1 - Jelken, Joachim A1 - Henkel, Carsten A1 - Santer, Svetlana T1 - Formation of half-period surface relief gratings in azobenzene containing polymer films JF - Applied physics : B, Lasers and optics N2 - We study the peculiar response of photo-sensitive polymer films irradiated with a certain type of interference pattern where one interfering beam is S-polarized, while the second one is P-polarized. The polymer film, although in a glassy state, deforms following the local polarization distribution of the incident light, and a surface relief grating (SRG) appears whose period is half the optical one. All other types of interference patterns result in the matching of both periods. The topographical response is triggered by the alignment of photo-responsive azobenzene containing polymer side chains orthogonal to the local electrical field, resulting in a bulk birefringence grating (BBG). We investigate the process of dual grating formation (SRG and BBG) in a polymer film utilizing a dedicated set-up that combines probe beam diffraction and atomic force microscopy (AFM) measurements, and permits acquiring in situ and in real-time information about changes in local topography and birefringence. We find that the SRG maxima appear at the positions of linearly polarized light (tilted by 45 degrees relative to the grating vector), causing the formation of the half-period topography. This permits to inscribe symmetric and asymmetric topography gratings with sub-wavelength period, while changing only slightly the polarization of one of the interfering beams. We demonstrate an easy generation of sawtooth profiles (blazed gratings) with adjustable shape. With these results, we have taken a significant step in understanding the photo-induced deformation of azo-polymer films. Y1 - 2020 U6 - https://doi.org/10.1007/s00340-020-07500-w SN - 0946-2171 SN - 1432-0649 VL - 126 IS - 9 PB - Springer CY - Heidelberg ER - TY - JOUR A1 - Arya, Pooja A1 - Jelken, Joachim A1 - Lomadze, Nino A1 - Santer, Svetlana A1 - Bekir, Marek T1 - Kinetics of photo-isomerization of azobenzene containing surfactants JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistry N2 - We report on photoisomerization kinetics of azobenzene containing surfactants in aqueous solution. The surfactant molecule consists of a positively charged trimethylammonium bromide head group, a hydrophobic spacer connecting via 6 to 10 CH2 groups to the azobenzene unit, and the hydrophobic tail of 1 and 3CH(2) groups. Under exposure to light, the azobenzene photoisomerizes from more stable trans- to metastable cis-state, which can be switched back either thermally in dark or by illumination with light of a longer wavelength. The surfactant isomerization is described by a kinetic model of a pseudo first order reaction approaching equilibrium, where the intensity controls the rate of isomerization until the equilibrated state. The rate constants of the trans-cis and cis-trans photoisomerization are calculated as a function of several parameters such as wavelength and intensity of light, the surfactant concentration, and the length of the hydrophobic tail. The thermal relaxation rate from cis- to trans-state is studied as well. The surfactant isomerization shows a different kinetic below and above the critical micellar concentration of the trans isomer due to steric hindrance within the densely packed micelle but does not depend on the spacer length. KW - genomic DNA conformation KW - water-interface KW - light photocontrol KW - driven KW - manipulation KW - photoisomerization KW - molecules Y1 - 2020 U6 - https://doi.org/10.1063/1.5135913 SN - 0021-9606 SN - 1089-7690 VL - 152 IS - 2 PB - American Institute of Physics CY - Melville ER -