CARS Microscopy of Colloidal Gels Evangelos Gatzogiannis.

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. 350-

(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 ~N2 where N is the number density of scatterers.

CARS Advantages

pump

probe

Stokes

Sample

CARS

Lens

Lens

~610nm ~650nm

~1000cm-1

~610nm

~575 nm

Elimination of Non Resonant FWM Background

Strong vibrational resonanceat ~1000 cm-1.

My Previous Work With CARS

DPA Molecule, i

Epump, Eprobe

EStokesSPORE (~1µm)

STAND-OFF CARS SPECTROSCOPYSTAND-OFF CARS SPECTROSCOPY

Detector

x

yz

Phaseonium (Goal)Coherent Radiating Dipoles

CARS Signal

Experimental Setup

1kHz/10Hz RegenOPA

Tsunami

THG

UV Shaper

Stokes OPA

Pump/ProbeOPA

Stokes

UV CARS

CARSMicroscope

Millenia Evolution

Quanta Ray

Cost: $700,000+

Fs/Ps Laser 1

Fs/Ps Laser 2

Laser 1repetitionrate control

100 MHz

50 ps

SHG

SHGBBO

SFG

SFG Cross-Correlation

Phase shifter

14 GHz14 GHz

Phase shifter

14 GHz Loop gain

76 MHz Loop gain

Fast Sampling Oscilloscope

Delay

Experimental Setup for RF LockingEssential for CARS, Many Uses in Metrology, Frequency Standards

Stokes Laser (Master)

Pump/Probe Laser (Slave)

Feedback Loop

To CARSmicroscope

76 MHz14 GHz

At the CARS Microscope,Forward vs. Epi Detection

Forward CARS

Epi-CARS

Good for sub-wavelengthstructures,Less background

BBO

SFG/Cross-Correlation

14 GHz

Dichroic mirror

as

p,s

3-D scanner

APD/PMT

WP/PC

WP/PC

80 MHz14 GHz

Stokes Laser (Master)

Pump/Probe Laser (Slave)

PhaseShifter Phase

Shifter

14 GHz Loop gain

76Mhz Loop Gain DBM

DBM

Filter

APD/PMT

asω

Synchrolock-Based Setup

Simplified Setups (Improved Performance)

Several CARS Images

Maximum packingφRCP≈0.63φxtal≈0.54φliquid≈0.48 φHCP=0.74

Maximum packing

J. Chem. Phys. 125, 074716 2006

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 amworking with.

Current Experimental System

Colloidal Gel Basics

A gel will not form at low volume fraction unless it is buoyancy matched.

For U/kBT << 1, hard sphere like behavior, monodisperse particles jam at Φ=0.63.

For U/kBT >> 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 improvedspatial 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.

3~)( fdrrg

Dinsmore, PRL 96 185502 (2006)

Radial distribution functionprovides direct measure of fractal dimension.

Consistent with DiffusionLimited Cluster Aggregation

Length of chains related to fractal dimension.

Low Interaction Energies, U ≤ 2.6kBT No Structures

U≥2.9kBT 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

)0()([1

),(1 1

N

i

N

j

ij rtrrN

trG

Van Hove Function

N

iiis rtrr

NG

1

)]0()([1

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:

N

i

N

ijijd rtrr

NG

1

)]0()([1

Distinct part:

Self part:

Gs(r,0) = δ(r), Gd(r,0) = ρg(r)

Van Hove II

Signature of particles movinginto positions occupied by other particles.

Measures Heterogeneity andIndicative of Cage Escape