FOUR-WAVE MIXING TECHNIQUE AND COHERENCE EFFECT ON ELECTRONIC STATES OF DYE MOLECULE

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FOUR-WAVE MIXING TECHNIQUE AND

COHERENCE EFFECT ON ELECTRONIC STATES OF DYE MOLECULE

M. Portella, P. Montelmacher, A. Bourdon, P. Evesque, J. Duran

To cite this version:

M. Portella, P. Montelmacher, A. Bourdon, P. Evesque, J. Duran. FOUR-WAVE MIXING TECH-

NIQUE AND COHERENCE EFFECT ON ELECTRONIC STATES OF DYE MOLECULE. Journal

de Physique Colloques, 1987, 48 (C7), pp.C7-521-C7-523. �10.1051/jphyscol:19877124�. �jpa-00226942�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n 0 1 2 , Tome 48, d6cembre 1987

FOUR-WAVE MIXING TECHNIQUE AND COHERENCE EFFECT ON ELECTRONIC STATES OF DYE MOLECULE

M.T. PORTELLA, P. MONTELMACHER, A. BOURDON, P. EVESQUE and J. DURAN

Laboratoire dlOptique de la Matiere Condensee, Universite Pierre et Marie Curie, T-13, 4, Place Jussieu, F-75252 Paris Cedex 05, France

Recent papers 11-31 report on t h e measure of a T2 dephasing t i m e when transient grating experiments a r e conducted on dye molecules (cresyl-violet) embedded in polymer films.

On t h e other hand i t is well known t h a t dye molecules show up a broad continuum of s t a t e s which is induced through a strong coupling between t h e electronic and vibrational s t a t e s of t h e dye molecule. Therefore such a system c a n be hardly considered a s a two level system and t h e proposed interpretation remains doubtfull.

We undertook this work in order t o bring a more detailed information about this problem.

We applied aegenerate-four-wave-mixing technique on polymer films doped with cresyl- violet dye a t various temperatures and at different excitation wavelengths. We obtain results similar to, but more complete than, those which have been published in Ref.3. We will discuss our results on t h e basis of t h e t w o level model proposed in this reference.

The degenerate 4-wave mixing experiment consists in spatially splitting a laser beam

*

^->

of coherence t i m e in two equal intensity beams of wavevector k l and k2, in delaying one of these t w o beams by a t i m e

y2.

One makes them interfere in t h e studied sample

-> -> -> -2.

and measures t h e t o t a l energy diffracted along t h e 2 k l - k2 o r 2 k2 - k l directions a s function of

q2.

Both curves exhibit t h e s a m e width A and a maximum. These t w o maxima a r e arranged symmetrically compared t o

q2

^{= 0} and a r e a t a distance from each o t h e r labelled by definition t.

Considering then a two level scheme characterized by a T2 dephasing time, one can theoretically compute $t and b a s function of T2 and

9,.

For instance, one g e t s t h e curve of Fig.1 when one uses t h e theoretical background of Ref.3.

The value of Ta can be determined by t h e knowledge of and t h e measure of k t by using this simple model. So t h e experimental measures of

st

and Aallow t o plot & t / ~ experimental ratios a s function of T Z a c a t different temperatures. We have performed these experiments a t different excitation wavelengths. We have got :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877124

JOURNAL DE PHYSIQUE

Figure 1

A &'A

Ratio 6 t / d of t h e peak t i m e shift t o width r a t i o versus T2/Cc ratio.

T2 : dephasing t i m e of a n inhomogeneous two level model.

cc

: coherence t i m e of t h e laser.

-

theoretical curve

+ experimental d a t a

1.5..

1

- When exciting t h e sample on t h e high energy side or on t h e top of t h e absorption band of cresyl-violet, our experimental d a t a can be interpreted with a T2 t i m e equal t o 0 (<20 fs).

-

--

- When exciting t h e sample on t h e low energy side of t h e absorption band, where t h e absorption coefficient steeply varies with t h e wavelength, one g e t s large variations of

st

a s function of temperature. The l t experimental values lead t o Tq values varying from 0 at high temperature (300 K) t o 200 f s a t low temperature (<4 K). Plotting s t / & experimental values a s function of these Tq leads t o t h e experimental point of Fig.1. Strong deviation from t h e theoretical curve is obtained f o r t h e experimental points obtained a t low temperature. I t results then t h a t t h e two level scheme of Ref. 3 is not able t o explain our experimental results.

I , . , . , . . . .

0 1 5 10

^{I . .}

T2 /G

^{. .}

Moreover w e have studied t h e correlations which exists between t h e experimental fluctuations on t h e values of

6t

and of

A.

So w e c a n have proved t h a t

f t

depends linearly on

4,

which in turn is not predicted by Ref. 3 theory. We have also concluded t h a t A r e f l e c t s mostly t h e coherence t i m e

gc

of t h e laser beam. We give t h e

s t / '

experimental values

a s function of temperature. This dependance exhibits a critical temperature value ranging around 20-50 K. No interpretation of this critical temperature is y e t given.

References

11

I

^{S. Asaka, H.} Nakatsuka, M. Fujiwara, M. Matsuoka, Phys. Rev. A 29(4), 2286 (1984) 121 S. De Silvestri, A.M. Weiner, F.G. Fujimoto, E.P. Ippen, Chem. Phys. Lett. 112, 195

(1984)

(31 H. Nakatsuka, M. Fujiwara, R. Kuroda, J. d e Physique 46, C7, 511 (1985)