Session W17: Optics and Photonics in Polymers and Soft Matter II

Computational optical microscopy by PSF engineering - or – how and why to ruin a perfectly good microscope

In localization microscopy, the positions of individual nanoscale point emitters (e.g. fluorescent molecules) are determined at high precision from their point-spread functions (PSFs). This enables highly precise single/multiple-particle-tracking, as well as super-resolution microscopy, namely single molecule localization microscopy (SMLM). To obtain 3D localization, we employ PSF engineering – namely, we physically modify the standard PSF of the microscope, to encode the depth position of the emitter.  In this talk I will describe how this method enables unprecedented capabilities in localization microscopy; specific applications include dense emitter fitting for super-resolution microscopy, multicolor imaging from grayscale data, volumetric multi-particle tracking/imaging, dynamic surface profiling, and high-throughput in-flow colocalization in live cells. We often combine the optical encoding method with neural nets (deep-learning) for decoding, i.e. image reconstruction; however, our use of neural nets is not limited to image processing - we use nets to design the optimal optical acquisition system in a task-specific manner. Finally, I will describe a new method we have developed to fabricate high quality diffractive optical elements (DOEs) based on 3D-printing.   

Quadratic Electro-optic Effect in the Nonconjugated Conductive Polymer Iodine-doped Polyethylene Terephthalate

Quadratic electro-optic effect has been measured in the nonconjugated conductive polymer iodine-doped polyethylene terephthalate (PET). The measurement has been made using field-induced birefringence at 633 nm with lock-in detection. The magnitude of the Kerr coefficient as measured is about 5x10^-11 m/V² at high doping level. The Kerr coefficient was observed to increase with the doping level. This is another example of a nanometallic system with sub-nanometer particle sizes. More detailed work on this is in progress.   

Templating Polymer/Chromophore Co-Crystallization in Block Copolymer Domains

Future advances in optical information processing are dependent on new materials that can manipulate light, similar to electronic materials that control electrons. Although advances in design parameters have led to significant progress in organic nonlinear optical (NLO) materials, long-term stability and material processing have prevented widespread implementation. Recently we reported an alternative approach in which an NLO chromophore is co-crystallized with a polymer, forming a non-centrosymmetric hybrid host-guest complex. Although the materials demonstrate promise, the orientation of co-crystals in the films randomize at elevated temperature, resulting in a loss of second harmonic generation (SHG) activity that is not reversible on cooling. To improve the thermal stability and harness reversible SHG activity, polymer/chromophore films were prepared using a poly(styrene)-poly(ethylene oxide) (PS-PEO) diblock copolymer as the template to confine the PEO/chromophore co-crystals between rigid PS domains. Interestingly, the  PS-PEO/chromophore samples self-assemble into a gyroid morphology, which has the potential to exhibit chiral SHG. The work presented here opens new avenues for creating materials exhibiting responsive and chiral-dependent SHG properties.   

Two-photon spectroscopy reveals protonation-induced symmetry-breaking in the ground electronic state of nominally centrosymmetric organic fluorophores

One-photon absorption (1PA) and two-photon absorption (2PA) of inversion-symmetric systems display alternative parity rule, i.e. transition to 1PA-allowed excited electronic states are 2PA-forbidden. Therefore, comparison of 1PA and 2PA spectra of nominally centrosymmetric organic chromophores offers valuable insights into symmetry-breaking due to conformational changes, vibronic interactions, solvent-chromophore interactions. etc.

Here, we use femtosecond 2PA spectroscopy to show, for the first time, that a nominally centrosymmetric organic fluorophore can be switched from a centrosymmetric- to non-centrosymmetric state and back by  variation of pH of the solution.  We synthesize novel fluorophores comprising pyrrollopyrrole core with symmetrically-attached pair of moieties with affinity to protonation.  In pH-neutral solvent such as neat methanol, there are no protons attached, and the 1PA and 2PA spectra show hallmark alternative behavior. Adding a small amount of diluted triflic acid causes protonation of one of the two moieties, which breaks the ground state inversion symmetry as evidenced by simultaneously 1PA- and 2PA-allowed lowest-energy electronic transition. Upon further decrease of the pH, both moieties become protonated, thus restoring nominal inversion symmetry.   

Prediction of Effective Optical Properties of Anisotropic Composites Beyond the Quasistatic Limit

The preponderance of previous treatments to predict the effective dielectric constant of composites apply to the quasistatic (long-wavelength) regime. Recently, Torquato and Kim [PRX11, 021002 (2021)] derived the exact nonlocal "strong-contrast" expansion formalism for the effective dynamic dielectric constant tensor for general two-phase microstructures that is applicable from very long to intermediate wavelengths. While accurate approximation formulas were obtained for isotropic media previously, here we extract for the first time corresponding formulas for microstructures of various symmetries in three dimensions. Similar to the previous approximation, these approximations are rational functions that depend on certain integrals involving the two-point correlation function or its Fourier counterpart (i.e., spectral density), and hence, account for the multiple scattering to all orders. We apply our new formulas to a variety of hyperuniform and nonhyperuniform disordered anisotropic models. Our theoretical results compare favorably to full-waveform simulations.   

Low-threshold exciton-polariton condensate via fast polariton relaxation in organic microcavities

Exciton-polaritons, in which the electronic state of an excited organic molecule and a photonic state are strongly coupled, can form a Bose-Einstein condensate (BEC) at room temperature. However, so far, the reported thresholds of organic polariton BECs under optical excitation are as high as P_th ~ 11 - 500 μJ cm^-2, which is still too high for many optoelectronic applications. Therefore, achieving a significant decrease of the BEC threshold continues to be a major challenge in the field.

One route towards lowering the condensation threshold is to increase the polariton relaxation rate (W_ep). However, the relationship of W_ep with a condensation threshold has not been fully explored. In this study, we demonstrated a room-temperature polariton condensate at a threshold pump fluence of 9.7 ± 0.1 μJ cm⁻², in a microcavity containing 4,4'-bis((E)-4-(3,6-bis(2-ethylhexyl)-(9H-carbazol-9-yl))styryl)-1,1'-biphenyl (BSBCz-EH). By using a semiclassical model to describe the polariton kinetics, we revealed that these low threshold results from the rapid relaxation rate from the dark exciton reservoir to the set of the lower polariton states forming the condensate, indicating that accelerating polariton relaxation is an important factor for realizing low-threshold polariton condensates.   

Solution-processed hybrid organic/inorganic microcavities for exciton-polaritonics

We present here a polymer-based organic/inorganic hybrid material with unique and versatile properties, including a tunable refractive index, low optical losses, and solution-processability, applied for the fabrication of active microcavities. An active microcavity consists of mirrors confining photon modes and enclosing a molecular species. Such structures are of great interest because if strong coupling between the photon modes and the molecular optical transitions is achieved, exciton-polaritons arise modifying the active layer’s molecular dynamics, charge transport, light emission, and beyond. To date, inorganic microcavities have been the preferred choice to accomplish photon mode confinement – but their fabrication can be very complex and, in many cases, compatible with limited material options. Most attempts using organic materials have led to microcavities with insufficient confinement for polaritonics. In contrast, here we demonstrate fully solution-processed microcavities produced from Bragg mirrors with alternating layers of a titanium oxide hydrate/poly(vinyl alcohol) hybrid and a fluorinated polymer exhibiting exciton-polariton formation. Photon mode confinement can be tuned via thermal annealing as it allows in-situ modulation of the hybrid refractive index and, thus, assists in attaining strong light-matter coupling with a perylene diimide-derivative. The measured energy dispersion and transmitted group delay of the microcavity show anti-crossing between two polariton modes. The tunability of the hybrid refractive index via its composition and post-processing treatments should allow these microcavities to attain a wide spectral range of photon modes, rendering them ideal for the future harnessing of exciton-polaritons in a broad variety of materials.   

Magnetic dipole emission in profile-modulated metal-dielectric-metal structure

Plasmonic and cavity modes in metal-dielectric-metal structures can provide strong control on electric and magnetic emission of the emitters placed inside the structures.  Modulation of the profile allows one to excite plasmons with direct illumination. The excitation of surface plasmons, including gap plasmons was predicted with COMSOL simulations in dependence on the thickness of intermediate layer.  A clear conformity between experimental observations and theoretical predictions was observed. In our experiment, we study the electric and magnetic dipole emission from Eu3+ ions in the profile modulated three-layer Ag-Emitter layer-Ag structures.   

Holographic Immunoassays: Direct Detection of Antibodies Binding to Colloidal Spheres

The size of a probe bead reported by holographic particle characterization depends on the proportion of the surface area covered by bound target molecules and so can be used as an assay for molecular binding. We validate this technique by measuring the binding kinetics for the antibodies immunoglobulin G (IgG) and immunoglobulin M (IgM) as they attach to individual micrometer-diameter colloidal beads coated with protein A. These measurements yield the antibodies’ binding rates and can be inverted to obtain the concentration of antibodies in solution. Holographic molecular binding assays therefore can be used to perform fast quantitative immunoassays that are complementary to conventional serological tests.   

Topological supermodes in photonic crystal fibre

A challenge in photonics is to create a scalable platform in which topologically protected light can be transmitted over large distances. Here we design, model, and fabricate photonic crystal fibre (PCF) characterised by a topological invariant. The fibre is made using a stack-and-draw technique in which glass capillaries are stacked, molten, and drawn to desired size. We directly measure the bulk winding-number invariant and image the associated boundary modes predicted to exist by bulk-boundary correspondence. In contrast to length limitations of both planar waveguides and resonance-based metamaterials, topological photonic crystal fibre (TopoPCF) guides visible light over metre length scales. Due to optomechanical coupling, bending these long waveguides allows us to explore the non-monotonic effects of on-site disorder for topological states. Based on the robustness of quantum and nonlinear states within topological waveguides, we envision future technologies exploiting our scalable fibre platform.   

Silicon Inverse Colloidal Diamond

Recently, our group has developed a method for self-assembling colloidal scale diamond crystals. The inverse of this lattice is predicted to have a photonic band gap at ~1.5 micron wavelength, a regime commonly used for IR communication. In this session, I will be discussing how we can grow larger, more functional crystals, and invert those crystals with Silicon to make a high-index inverse diamond.   

Dexterous holographic trapping of dark-seeking particles with Zernike holograms

Optical traps projected with computer-generated holograms can be optimized for compatibility with the physical properties of individual target particles. Dynamic optimization is especially desirable for manipulating dark-seeking particles that are repelled by conventional optical tweezers, and even more so when dark-seeking particles coexist in the same system as light-seeking particles. We address the need for dexterous manipulation of dark-seeking particles by introducing a class of “dark” traps created from the superposition of two focused Gaussian modes. The constituent modes have different waist diameters and opposite phases and therefore create a dark central core that is completely surrounded by light. These difference-of-Gaussian (DoG) traps confine dark-seeking particles rigidly in three dimensions and can be combined with conventional optical tweezers and other types of traps for use in heterogeneous samples. The ideal hologram for a DoG trap being purely real-valued, we introduce a method based on the Zernike phase-contrast principle to project real-valued holograms with the phase-only diffractive optical elements used in standard holographic optical trapping systems.   

Simulations of the dynamics of colloidal sphere clusters in Laguerre-Gaussian beams

The orbital and spin angular momentum carried by light in elliptically-polarized Laguerre-Gaussian (LG) beams can couple to particles trapped in them, resulting in optically-driven particle motion. We discuss recent Brownian dynamics simulations of the motion of wavelength-sized sphere clusters in LG beams. We use T-matrix light scattering computations to efficiently model the optical forces acting on the clusters even when the clusters are far from positional or orientational trapping equilibria. We also incorporate the full diffusion tensor for the clusters to model the hydrodynamic resistance and thermal fluctuations they experience. For clusters of two identical spheres, we show that we can control the spin of the clusters about their long axis via the the beam polarization and can control the clusters’ orbital motion around the beam axis via the LG mode index. We perform additional simulations to show that the full three-dimensional dynamics of non-axisymmetric clusters can be measured through an interferometric technique, digital holographic microscopy. Our results may lead to new insights into the optical manipulation of colloidal particles for studying self-assembly or performing microrheology.