ULTRAFAST DEPHASING TIME MEASUREMENT IN CRESYL FAST VIOLET DOPED CELLULOSE BY PHOTON ECHOES WITH TEMPORALLY INCOHERENT LIGHT

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ULTRAFAST DEPHASING TIME MEASUREMENT

IN CRESYL FAST VIOLET DOPED CELLULOSE BY

PHOTON ECHOES WITH TEMPORALLY

INCOHERENT LIGHT

H. Nakatsuka, M. Fujiwara, R. Kuroda

To cite this version:

H. Nakatsuka, M. Fujiwara, R. Kuroda.

ULTRAFAST DEPHASING TIME MEASUREMENT

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JOURNAL DE PHYSIQUE

Colloque C7, supplkment a u nOIO, Tome 46, octobre 1985 _pageC7-5 1 1

ULTRAFAST DEPHASING T I M E MEASUREMENT I N CRESYL FAST V I O L E T DOPED CELLULOSE BY PHOTON ECHOES W I T H TEMPORALLY INCOHERENT L I G H T

H. Nakatsuka, M. ~ujiwara' and R. ~uroda'

I n s t i t u t e o f Applied Physics, University o f Tsukuba, Sakura, Ibaraki 305,

Japan

+~epartment of Physics, PacuZty of Science, Kyoto University, Kyoto 606,

Japan

Abstract

-

Photon echoes with broad spectrum nanosecond laser pulses have been used to measure a femtosecond dephasing time

(homogeneous transverse relaxation time) T2 of the SO*S1

transition of Cresyl Fast Violet molecules in a cellulose film. The measured T2 was much shorter at a shorter wavelength 5940A than that at a longer wavelength 6250A near the band edge.

Photon echoes have long been known as a useful technique for measuring dephasing times (homogeneous transverse relaxation times) T2 of

various materials /l/. The relaxation times of condensed matter like liquids and solids are very short and often shorter than a picosecond. For high time resolution ultrashort excitation pulses are necessary. Until now ultrashort pulses with pulse widths of 16 £sec have been generated by using a pulse compression technique /2/. But the practical application of such ultrashort pulses to spectroscopy is still difficult.

Recently it has been proved that photon echoes with temporally incoherent (broad spectrum) laser pulses have a very high time resolution determined by the correlation time T, of the electric

Fig.1

-

Absorption spectra of Cresyl Fast Violet in cellulose. The solid curve is at 10 K , and the dotted curve is at 3 0 0 K. 5600 5800 6000 6200 6400 6600

W A V E L E N G T H ( A )

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C7-5 12 JOURNAL DE'PHYSIQUE

f i e l d o f t h e e x c i t a t i o n p u l s e s /3-7/. I n t h e p r e s e n t p a p e r t h e photon echo w i t h t e m p o r a l l y i n c o h e r e n t l i g h t was a p p l i e d t o measure an

u l t r a f a s t d e p h a s i n g t i m e T2 o f a l a r g e molecule which was doped i n a polymer f i l m . The sample was C r e s y l F a s t V i o l e t (BDH Chemicals Ltd. England) i n a c e l l u l o s e f i l m . F i g u r e 1 shows t h e a b s o r p t i o n s p e c t r a o f t h e SO++Sl t r a n s i t i o n o f C r e s y l F a s t V i o l e t i n a c e l l u l o s e f i l m . The f i l m was 0.2 mm t h i c k , and t h e a b s o r p t i o n a t t h e peak was a b o u t 90 %. The measurement o f T 2 by photon echoes was made a t two wavelengths 6250Aand 5940Ashown by a r r o w s i n F i g . 1.

Variable Delay

-4

Gated

I

-+ -+

F i g . 2

-

Schematic diagram o f t h e experiment. nl and n a r e t h e u n i t v e c t o r s r e p r e s e n t i n g t h e d i r e c t i o n s o f t h e f i r s t E 1 an2 t h e second E 2 beams, r e s p e c t i v e l y . I n t h e i n s e r t t h e p o l a r i z a t i o n s o f t h e two e x c i t a t i o n beams and t h e echoes a r e i n d i c a t e d f o r o r t h o g o n a l p o l a r i z a - t i o n o f t h e e x c i t a t i o n beams.

The e x p e r i m e n t a l s e t u p i s i n d i c a t e d i n F i g . 2. The dye l a s e r was pumped by second harmonics o f a Q-switched Nd:YAG l a s e r . The p u l s e w i d t h , peak power, and s p e c t r a l w i d t h o f t h e dye l a s e r were l o n s e c , 10 kW, and 4 0 A , r e s p e c t i v e l y . The photon echoes were o b s e r v e d a t t h r e e t e m p e r a t u r e s 10 K , 80 K , and 300 K. The d e l a y t i m e T~~ between t h e two e x c i t a t i o n beams El and E 2 was changed by a s t e p p i n g motor, and t h e two beams were f o c u s s e d on t h e sample i n a c r y o s t a t by a 70 cm f o c a l - l e n g t h l e n s . A s

Ss

shown i n F i g . 2, photon echoes a r e e m i t t e d i n two d i r e c t i o n s 2ft2

-

nl and 2s1

-

f t 2 ,

where

ftl

and

s2

a r e t h e u n i t v e c t o r s r e p r e s e n t i n g t h e d i r e c t i o n s o f t h e f i r s t El and t h e second E2 beams, r e s p e c t i v e l y . I n t h e echoes i n t h e two d i r e c t i o n s t h e r o l e s o f t h e two e x c i t a t i o n beams a r e i n t e r c h a n g e d , t h e r e f o r e t h e d i r e c t i o n o f t h e d e l a y t i m e f o r t h e e c h o i n 2S1-A2 i s r e v e r s e d w i t h r e s p e c t t o t h a t f o r t h e e c h o i n 2?i2-sl. From t h e 40 A s p e c t r a l w i d t h s of t h e e x c i t a t i o n p u l s e s , t h e c o r r e l a t i o n t i m e T~ was c a l c u l a t e d t o b e 300 f s e c . T h i s is t h e r e s o l u t i o n t i m e o f t h e photon echo w i t h t e m p o r a l l y * i n c o h e r e n t l i g h t /3-7/. But i n t h e p r e s e n t e x p e r i m e n t by s i m u l t a n e - o u s l y d e t e c t i n g photon echoes i n t h e two d i r e c t i o n s and measuring t h e peak s h i f t between t h e two echo decay c u r v e s , we o b t a i n e d a r e s o l u t i o n t i m e o f s e v e r a l t e n s of femtoseconds i n t h e measurement o f T 2 . I n a l l t h e echo decay c u r v e s

$hewn

below t h e s o l i d l i n e s r e p r e s e n t t h e e c h o e s i n t h e d i r e c t i o n 22 and t h e dashed l i n e s r e p r e s e n t t h e o n e s i n t h e d i r e c t i o n 2?i1

-

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-500 0 500

D E L A Y T I M E r,, ( f s e c ) Fig.3

-

Echo decay curves at 10 K for

( a ) the excitation wavelength of 6250 A.

(a) is for parallel polarization and

I (b) is for orthogonal polarization of

the two excitation beams. The solid curves reprpeqt the echoes in the direction 2n2-nl, and the dashed curves rearesent those in the direction 2z1-n2. In (a) the dashed-

-,' dotted curve is a theoretic21 qne for

- - ... ,_-- the echo in the direction 2n2-nl with

r =300 fsec and T2=0.7 psec.

- 500 0 500 C

DELAY T I M E TIZ ( f s e c ) ( b )

When the polarizations of the two excitation beams are parallel, scattered light by a thermal grating may be superimposed on echoes /4/. In order to avoid the thermal grating effect, as is shown in the insert of Fig. 2 orthogonal polarization of the two excitation beams was used to obtain the echo decay curves / 4 / . The polarizations of the echoes are also shown in the insert of Fig. 2. The result is shown in Fig. 3(b). Although the echo intensities decreased to some extent compared with those by parallel polarization, there was no significant difference in the shape of the echo decay curves between Figs. 3(a) and (b). From this fact we see that the thermal grating effect is negligible in the present experiment. The dashed-dotted curve in Fig. 3(a) is a theoretical one by eq. 7 of ref. 7 withrc=300 £sec and T2=0.7 psec. It agrees quite well with the experiment near the peak, but the discrepancy becomes substantial at the wing where r12 is long. This result implies that T2 of the main component of the dye molecules in the sample is 0.7 psec, but there is some component with long T2 up to several picoseconds. Such a distribution of T2 in a sample is a typical feature of impurities doped in amorphous

material /8/.

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JOURNAL DE PHYSIQUE

Fig. 4

-

Echo decay curves. (a) is for parallel polarization and (b) is for orthogonal polarization of the two excitatizn beams. The solid curves represent the echoes in the direction 21f2-nl, and the dashed curves represent those in the direction 2fi1-i3Z. The abscissa is the delay time T ~ ~ .

By longer wavelength excitation at 6250 A, the dye molecules are excited to the states in the vicinity of the bottom of the S level as is shown in Fig. 1, but by shorter wavelength excitation at k g 4 0 A, they are excited to the states 800 cm-l higher than the bottom. When the dye molecules are excited to higher vibrational states, the intra- molecular relaxation becomes faster than the inter-molecular relaxation to the host material. The photon echo experiment shows that T2 is much shorter at 5940 A than that at 6250 A. In large molecules like Cresyl Fast Violet the number of normal vibrational modes is very large. Therefore, when one mode is highly excited, the energy will be redistributed to other modes very rapidly to establish thermal equilibrium in the vibrational modes. Scherer et al. have shown that in dye solutions the above processes take place in the sub-picosecond region, and that the inter-molecular energy transfer processes to the. solvent are slower and take place in the picosecond region /10/. Our measurement of T2 by the photon echo qualitatively supports their result.

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resonances, but it is the inverse of the total absorption width /g/. The relaxation processes in the multi-level systems are very

complicated. But in the future, the photon echoes and the relaxation processes in the multi-level or continuous-level systems will be avery important problem especially when ultrafast phenomena are considered. In conclusion we found that when the excitation wavelength was 6250 A at 10 K there was some component with long T2 up to several picoseconds besides the main component with T2 of 0.7 psec and that T2 was much shorter at 5940 A than that at 6250 A. The fact that the measured T2 was shorter at 5940 A than that at 6250 A can be attributed to the intra-molecular relaxation and/or the multi-level structure of the molecule. As a method to observe such ultrafast phenomena the photon echo with temporally incoherent light has been proved to be very useful. We believe this simple technique will be a very promissing one for ultra-high time resolution coherent transient spectroscopy.

REFERENCES

/l/ Kurnit, N. A., Abella, I. D. and Hartmann, S. R., Phys. Rev. Lett. 13 (1964) 567.

n/

Weiner, A. M., Fujimoto, J. G. and Ippen, E. P., in Ultrafast Phenomena IV, Auston, D. H. and Eisenthal, K. B., eds. (Springer- Verlaq Berlin, 1984) p.11.

/3/ ~ s a k a , S., ~akats;ka,H., Fujiwara, M. and Matsuoka, M., Phys. Rev. A29 (1984) 2286.

/v

Weiner, A. M. and Ippen, E. P., Opt. Lett. 9 (1984) 53.

/5/ Beach, R. and Hartmann, S. R., Phys. Rev. ~ g t t . 53 (1984) 663. /6/ Morita, N. and Yajima, T., Phys. Rev. A s (198417525.

/7/ Nakatsuka, H., Tomita, M., Fujiwara, M. and Asaka, S., Opt. Commun 52 (1984) 150.

n/

Small G. J., in Spectroscopy and Excitation Dynamics of Condensed Molecular Systems, Agronovich, V. M. and Hochstrasser, R. M - , eds.

(North-Holland Publishing Company, 1983) p.515.

/9/ De Silvestri, S., Weiner, A. M., Fujimoto, J. G. and Ippen, E. P - , Chem. Phys. Letters 112 (1984) 195.

/10/ Scherer, P. 0. Seilmeier, A., Wondrazek, F. and Keiser, W., in Ultrafast Phenomena IV, Auston, D. H. and Eisenthal, K. B., eds.