CARS Microscopy of Colloidal Gels Evangelos Gatzogiannis

CARS (Coherent Anti-Stokes Raman)

CARS Microscopy  M. D. Duncan, J. Reintjes, and T. J. Manuccia Optics Letters, Vol. 7, Issue 8, pp (Deuterated Onion Cells) Revived by Zubmusch, Xie in 1999.

Chemical selectivity without labeling. High sensitivity. Signal photon at higher frequency - no spectral overlap with one-photon fluorescence background. Small excitation volume for microscopy. Unlike fluorescence, CARS is a coherent process and signal is proportion to ~N 2 where N is the number density of scatterers. CARS Advantages

 pump  probe  Stokes Sample  CARS Lens ~610nm~650nm ~1000cm -1 ~610nm ~575 nm Elimination of Non Resonant FWM Background Strong vibrational resonance at ~1000 cm -1. My Previous Work With CARS

DPA Molecule, i E pump, E probe E Stokes SPORE (~1µm) STAND-OFF CARS SPECTROSCOPY Detector x y z Phaseonium (Goal) Coherent Radiating Dipoles CARS Signal

Experimental Setup 1kHz/10Hz Regen OPA Tsunami THG UV Shaper Stokes OPA Pump/Probe OPA Stokes UV CARS CARS Microscope Millenia Evolution Quanta Ray Cost: $700,000+

Fs/Ps Laser 1 Fs/Ps Laser 2 Laser 1 repetition rate control 100 MHz 50 ps SHG BBO SFG SFG Cross- Correlation Phase shifter 14 GHz Phase shifter 14 GHz Loop gain 76 MHz Loop gain Fast Sampling Oscillosc ope Delay Experimental Setup for RF Locking Essential for CARS, Many Uses in Metrology, Frequency Standards

Stokes Laser (Master) Pump/Probe Laser (Slave) Feedback Loop To CARS microscope 76 MHz 14 GHz At the CARS Microscope, Forward vs. Epi Detection

Forward CARS Epi-CARS Good for sub-wavelength structures, Less background

BBO SFG/Cross-Correlation 14 GHz Dichroic mirror  as p,s p,s 3-D scanner APD/PMT WP/PC 80 MHz 14 GHz Stokes Laser (Master) Pump/Probe Laser (Slave) Phase Shifter Phase Shifter 14 GHz Loop gain 76Mhz Loop Gain DBM Filter APD/PMT Synchrolock-Based Setup

Simplified Setups (Improved Performance)

Several CARS Images

Maximum packing φ RCP ≈0.63 φ xtal ≈0.54φ liquid ≈0.48φ HCP =0.74 Maximum packing

J. Chem. Phys. 125, Direct Imaging of Attractive and Repulsive Colloidal Glasses Attractive Glass: Significant Motion Repulsive Glass: Less Motion, Coop.

Cluster Formations Is a Precursor To Colloidal Gelation – NOT Well Understood

This is an SEM picture of the ASM204 ~1micron colloids I am working with. Current Experimental System

Colloidal Gel Basics  A gel will not form at low volume fraction unless it is buoyancy matched.  For U/k B T << 1, hard sphere like behavior, monodisperse particles jam at Φ=0.63.  For U/k B T >> 1, irreversible aggregation, fractal clusters are formed.  Can bear stresses, have interesting mechanical properties.  Physics of formation, aging, and other question remain unresolved.

Most groups use fluorescence (downsides include): rapid bleaching, photo physics, alters system (in some cases, cell fixing) can’t do in vivo studies CARS: Can image for longer times (hours) depending on laser stability (without long delays frame-to-frame), Chemical specificity Resonant coherent process (better signal/background) In vivo studies, intrinsic 3d sectioning with improved spatial resolution in some cases. Noninvasive. Label-Free High Speed Imaging. No perturbation of system.

Colloid-Polymer Mixtures Provide Rich Phase Behavior Blue: Gel Red: Fluid of Clusters Green: Fluids

Topology and Structure Fromd 3D Images Shortest path between two particles (red stripes) along the gel, yellow, red, second shortest path.

Dinsmore, PRL (2006) Radial distribution function provides direct measure of fractal dimension. Consistent with Diffusion Limited Cluster Aggregation Length of chains related to fractal dimension.

Low Interaction Energies, U ≤ 2.6k B T No Structures U≥2.9k B T space filling networks with Changing Morphology Static over 30 min observation time, no signs of aging.

3D Colloidal Gel

Three Days Later, After Stirring

Zooming In: Colloids Still Quite Small

Trajectories of Colloids In 20% Gel

Van Hove Function G(r,t)dr is the number of particles j in a region dr around a point r at time t, given a particle i at the origin at time t=0. It separates into two terms: Distinct part: Self part: G s (r,0) = δ(r), G d (r,0) = ρg(r)

Van Hove II

Signature of particles moving into positions occupied by other particles.

Measures Heterogeneity and Indicative of Cage Escape