Low Temperature Photon Echo Measurements of Organic Dyes in Thin Polymer Films

1
Low Temperature Photon Echo Measurements of
Organic Dyes in Thin Polymer Films

• Richard Metzler 06, Eliza Blair 07, and Carl
  Grossman, Department of Physics and Astronomy,
  Swarthmore College

Temperature Dependence
Photon Echo Theory
Abstract The relaxation of optically excited
molecules to their ground states is characterized
by two quantities the optical relaxation time T1
and the optical dephasing time T2. While T1 is
characteristic of the molecule and tells us
something about the transition time to its ground
state, T2 is the time constant corresponding to
the decay of the coherent superposition of ground
and excited states after optical excitation.
Thus, T2 can be thought of as a measure of the
influence of the local environment on a
molecules optical state and should have a strong
dependence on temperature. Measurements at
varying temperatures are performed using photon
echoes on on bulk samples are used to extract
this dependence. Comparison of this data with
previous data taken using scanning confocal
microscopy should provide insight in ultra-fast
molecular processes.
The photon echo results from the 3rd order
perturbation expansion of the interaction term HI
of the total molecular Hamiltonian. HI represents
the first incident beam which then interacts with
the delayed beam to produce the correlation
signal that we measure. Classically, we have
induced an oscillating dipole in the sample and
are measuring its phase information through use
of nonlinear optical processes with a second
delayed beam. We approximate the total
wavefuction as a sum of two states the ground
state 1gt, and a total excited state 2gt, which
in actuality is grouping of all distinct excited
states. From the Schrödinger we obtain the effect
on the probability amplitudes of the
wavefunction Through some manipulation we find
the inversion and the density of the
mixed-state terms where the damped terms are
added from phenomenological observation. Solution
to these coupled equations to third order and
correlation with the delayed beam give us the
total intensity with respect to the delay time
tau
Nile Blues Dephasing Time vs. temperature
Dye Selection and Sample Fabrication Dyes were
chosen for their known bulk absorption and
emission wavelength ranges and molecular
structure. The following dyes were studied in
the SMF experiment Rhodamine-640, Nile Blue,
Disperse Red 1, Disperse Red 11, Nile Red,
Rhodamine-101, Styryl-7, Bodipy, and Cyanine.
Dyes used as samples for the echo experiment were
dissolved in thin PMMA (polymer) films. Other
dyes in with peak emissions on the red side a
samples peak absorption were pumped with a YAG
laser to produce large dephasing times.
Of interest are two of the resulting terms from
the total intensity. These beams the form of two
identically shaped beams of different trajectory
delayed by a time 2T2 - hence the term photon
echo. It is these which we measure in the lab.
Sample Fabrication Procedure Dissolve polymer
and dye in chlorobenzene, filter and drop on
sapphire slide. Air dry over night in a clean
room as annealing can produce ripples in the film
causing unwanted optical artifacts. Absorption
spectra of various dyes (right).
Echo Data
Analysis Conclusions
Photon Echo Experiment
The experimental technique uses two-beam,
time-delayed degenerate four wave mixing (DFWM)
with incoherent light. Dephasing lifetimes were
measured directly as a function of temperature. A
plot of Scattered Intensity vs. Time Delay for
Rhodamine-640 at 40 K is shown at left below.
Peak shift is twice the dephasing time (T.
Kobayashi et al, Applied Physics B, 47, 107,
1988). This peak shift of 46.5 fs gives a
dephasing time of 23 fs. Dephasing times
approach 10 fs for extrapolation to room
temperature for R-640 and Nile Blue (middle and
right below, respectively).
Acknowledgements
Thank you to our awesome advisor Carl Grossman,
Ed from the Vietnamese Place, the Howard Hughes
Medical Institute, and Swarthmore College.
12 Sept. 2005