Session H50: Advances in Optical Microscopy and Photopolymerization

Imaging and Writing 3D Polymer Nanostructures with Visible Light

Developments in far-field microscopy have enabled imaging at nanoscale resolutions using visible light. This circumvention of the diffraction limit opens the benefits of optical microscopy to polymer systems at the relevant nanometer length scales. Three dimensional images of spatial crosslink heterogeneities in colloidal and bulk hydrogels using a model system of poly(N-isopropylacrylamide) containing a fluorophore tagged crosslinker will be used to exemplify the potential for super resolution microscopy to advance the polymer sciences. Progress in the application of photochromic gels towards achieving super resolution interference lithography will also be presented.   

A polymerizable photoswitchable fluorophore for super-resolution imaging of polymer self-assembly and dynamics

While single-molecule super-resolution microscopy has been routinely used for visualizing nanostructures in life sciences, the application of this technique in polymer science is much more rare, especially for visualizing polymer dynamics in situ. A major constraint is from the lack of suitable fluorophore chemistry and simple strategies to label polymer chains. In this talk, we will discuss a functional diarylethene-based photoswitchable fluorophore, which can be directly copolymerized with standard monomers such as styrene and methyl methacrylate with no further post-coupling reactions or purifications needed. The attachment of fluorophores onto polymer chains enables super-resolution imaging of model polymer blend systems (PS/PMMA) with different nanostructures. As each individual fluorophore can switch between its bright and dark state many times, multiple time-lapse images can be acquired to observe the dynamic nanostructural evolution of polymer blends during solvent vapor annealing. With the advantages of a universal, simplified labelling strategy and the ability to visualize polymer assembly in situ, this fluorophore may promote the widespread use of super-resolution microscopy in the polymer community.
  

Heterogeneous photoswitching kinetics of diarylethenes and their effect on localization microscopy resolution

Diarylethenes (DAE) have recently been used as effective fluorescent probes for single-molecule super-resolution imaging of soft materials owing to their ability to switch between an emissive and dark state multiple times via an intramolecular photocylization reaction. However, these dyes display a broad distribution in the number of times each molecule switches and in the lifetime of each state, which can negatively impact the ultimate resolution of an image. In this work, we characterize the heterogenous switching behavior of a DAE derivative and examine its effect on image resolution. Complementary cumulative distribution functions reveal broad lifetime distributions of the emissive and dark states, spanning multiple decades for both individual molecules and large ensembles. The emissive state lifetime distributions can be described by a power law, while the dark state lifetime distributions display two distinct regions, which can be fit to a weighted sum of a stretched exponential function and a power law, suggesting there is more than one dark state. Fits of these distributions were used to simulate super resolution experiments in various polymers to investigate the impact on image resolution due to heterogenous photoswitching kinetics.   

Direct nanoscale patterning of in-plane-aligned polymer via split-tip NSOM

Polymer electronic devices have been developed for a wide array of applications, including LEDs, organic transistors, solar cells, and more. These devices display ordered regions, where the conducting carbon-carbon backbones all line up in the same direction, and disordered regions, where the carbon-carbon backbones are tangled. Here, we demonstrate the ability to fabricate the ordered regions of a polymer. When placed in a high humidity environment, the polymers in a water-soluble poly(phenylene vinylene) (PPV) precursor film become reorientable. To control this orientation, we use a split-tip, near-field scanning optical microscope (NSOM) system. The split-tip applies a high, localized electric field in the plane of the surface, and subjects that region to UV light. When wet, the electric field forces the polymers to align beneath the tip, and the UV light engenders insoluble PPV, preventing loss of alignment. We align a large rectangular region by scanning a probe using this technique, and use cross-polarization measurements to demonstrate alignment. The ability to fabricate ordered layers removes material variations from studies of conduction and interfacial phenomena, thus represents a significant step towards measuring the intrinsic properties of conducting polymers.   

Exploring Textures of a Nematic Liquid Crystal for Fourier Phase Contrast Microscopy

Phase contrast microscope is used in teaching and research labs to view transparent specimens such as live cell cultures. It enables to study dynamic biological processes. In the past we have developed Fourier phase contrast microscopy technique using photoinduced birefringence in liquid crystals that constitute a fascinating class of matter characterized by the counterintuitive combination of fluidity and long-range order. A low power laser is passed through a commercial inverted microscope to facilitate the Fourier plane at the output of the video port. When the liquid crystal cell is placed at the Fourier plane, low spatial frequencies at the center of the Fourier spectrum are intense to transform the liquid crystal molecules into isotropic phase whereas high spatial frequencies are not intense enough and remain in the anisotropic phase. This results π/2 phase difference between high and low spatial frequencies, a basic requirement for phase contrast imaging. Liquid crystal materials are known for their exceptionally successful applications in displays, smart windows, and biosensing applications. Here we exploit different textures of a nematic liquid crystal 5CB: planar, perpendicular, hybrid and twisted, and investigate their abilities to improve the contrast of phase images.   

Tractor beams for colloidal particles are not self-accelerating modes

We previously have demonstrated that structured laser beam can act as a tractor beam for colloidal particles. In addition to their ability to pull objects upstream, these modes of light have other remarkable properties. Due to the homology of the paraxial wave equation with Schrödinger’s equation, the wave function for a quantum mechanical particle in a circular box can be prepared in shape-preserving wave packets that rotate at constant angular speed around the center of the box, similar to a tractor beam. This apparent violation of Ehrenfest’s theorem is resolved by considering the force exerted on the particle’s wave packet by the enclosing wall. Remarkably, the wave packet continues to rotate even after the wall potential is removed. We show that this force-free finite-energy rotating state actually corresponds to classical motion with constant velocity, again in agreement with Ehrenfest’s theorem. Even so, the classical angular momentum carried by the rotating states poses a conceptual challenge because it differs from its quantum mechanical angular momentum, and indeed can have the opposite sign.   

High efficiency Fresnel lens design and fabrication in a two-stage photopolymer

Diffractive optical elements (DOEs) assimilate optical functionality within thin (≤100 µm), lightweight films. With the recent advent of high dynamic range two-stage photopolymers, gradient-index volume DOEs can now achieve diffraction efficiencies competitive with conventional surface-relief DOEs, while also offering the advantages of contact-free, self-processing optical recording into a flat film that can be laminated between protective sheets. Here we design and fabricate Fresnel lenses with what we believe to be the highest reported diffraction efficiencies achieved to date using this gradient-index DOE approach. Our analysis shows that these high diffraction efficiencies are crucially enabled by the high index modulation of the photopolymer (n₁ > 0.01) and the high pixel count of the single-shot recording exposure (6400 × 6400 pixel chrome mask). The recorded lenses are 50 µm in thickness and up to 16 mm in diameter, with f-numbers ranging from 16 – 24 and diffraction efficiencies up to 85%. This high performance represents an important step toward practical applications, ranging through solar energy concentrators, customized vision optics, integrated photonics, heads-up displays, and hybrid lenses.   

Quantitative index metrology for 3D voxelated structures in photopolymer

Thanks to recent advances in two-stage photopolymers, it is now possible to dictate arbitrary 3D voxelated refractive index structures within a continuous volume of polymer, with dynamic range Δn in excess of 0.01 and with micron-scale spatial resolution. However, quantitative metrology for such structures remains an under-explored challenge. Here we demonstrate that TIE-based phase imaging, in conjunction with scanning confocal reflection microscopy, can achieve unambiguous quantitative characterization of arbitrary voxelated index structures. This characterization then guides the design, fabrication, and validation of novel optical components, including flexible gradient-index lenses for in situ medical imaging.

  

Bio-inspired design of Mechanochromisms via surface engineering

A bilayer structure composed of polyvinyl alcohol composite thin film atop thick polydimethylsiloxane substrate was prepared. The bilayer structure shows dynamic strain-responsive optical properties. The transition between a transparent state to an opaque state can be easily achieved by uniaxially stretching and releasing the device. Also, a series of derivative mechanochromisms with capabilities of switch “on/off” fluorescence, change fluorescent color, reveal/hide information upon mechanical stimuli are prepared. These devices feature virtually no changes in optical/mechanical properties after being repeatedly stretched and released thousands of times, promising for widespread applications.   

Reversible Self-Focusing in Light-Responsive Spiropyran Functionalized Gels

Self-focusing, where interactions between a light beam and its medium creates a refractive index gradient that acts as a waveguide for the beam, has been observed at low powers (nW-mW) in a variety of soft-matter systems. This intensity dependent refractive index change is often created by the irreversible photopolymerization of monomers in solution. However, a reversible system would enable potential applications in soft all-optical computing based on dynamic interactions between self-focused beams. Here we show a fully reversible soft self-focusing medium based on stimuli-responsive hydrogels (poly(acrylamide-co-acrylic acid) and poly(N-isopropylacrylamide)) doped with photoresponsive spiropyran pendant groups. In this system the intensity-dependent contraction of the gel leads to a local increase in the refractive index, resulting in self-focusing of the incident beam. In additition to being reversible this system shows long-range interactions between beams separated by over 10 beam widths. To explain these experiments we have developed a numerical model coupling the spiropyran isomerization, gel dynamics, and the propagation of light. This model allows us to design more complex beam interactions and thus to create systems where light is dynamically controlled by light.