JOURNAL DE PHYSIQUE

Colloque C6, supplement au nOlO, Tome 44, oetobre 1983 page C6- 579

PHOTOTHERMAL DETECTION OF PICOSECOND PHOTOINDUCED DICHROISM

C.N. Ironside, R.A. Taylor and J. Ryan

Clarendon Laboratory, Parks Road, Oxford OX] 3PU, U.K.

Resume.- On decrit une technique qui utilise la spectroscopie par deflex-ion photothermique pour la mesure du dichrolsme photo induit. Elle possedel'avantage d'eliminer tous les effets non lineaires dus aux melanges desfaisceaux sonde et pompe dans les experiences picoseconde.

Abstract.- A technique whiehuses photothermal deflection spectroscopy todetect photoinduced dichroism is described. It has the advantage that ef-fects due to nonlinear mixing of pulse and probe in picosecond experimentsare eliminated.

In picosecond spectroscopy a standard technique for obtaining temporal infor-

mation about light sample interaction is the so-called pulse probe method. The

sample is excited by the bleaching pulse and the induced absorption characteristics

are monitored, as a function of time, by the delayed probe pulse. This method

has been used widely to study various picosecond processes, however, in some cases

it is difficult to separate effects due to nonlinear interactions between the pulse

and probe (such as four wave mixing) and changes in absorption of the sample CIJ .In 1975, Ippen and Shank [2J showed how the pulse-probe method could be

applied to the study of reorientatiQn of dye molecules in various solvents. The

bleaching pulse creates a dichroism in the dye solution by saturating the absorp-

tion of those molecules with their interaction dipole aligned parallel with the

polarisation of the pump beam. The dichroism rotates the probe polarisation through

a small angle, of the order of 1.5 degrees. The recovery from the induced dichro-

ism is recorded by measuring the transmission of the probe pulse between crossed

polarisers as a function of delay. At zero delay there is nonlinear mixing between

the pulse and probe resulting in an anomalously large signal called the "coherence

spike". This signal can be as much as three orders of magnitude larger than that

due to the induced dichroism and obscures the dichroism signal close to zero delay.

Another reported difficulty [3J with this experiment is the effect of any small

birefringence of the optical components this can cause the results to be difficult

to interpret.

These expe~mentalartifacts may be overcome if, instead of detecting the

probe beam after it has propagated through the sample, we observe directly the

energy deposited in the sample by the prObe beam. Photoacoustic and photothermal

C6-580 JOURNAL DE PHYSIQUE

techniques offer a method of achieving this. In particular, in this paper we

investigate the detection of induced dichroism by photo thermal deflection spectr

scopy (PDS) [4J. In this method the proportion of the energy deposited by the

probe which is converted into heat is detected by the change in the refractive

index that it causes. The refractive index change deflects a PDS beam (usually

a He Ne laser). The deflection is measured using a position sensitive detector.

To monitor the induced dichroism, the polarisation of the probe beam is modulated

and the deflection of the PDS beam at the modulation frequency is recorded using

standard phase sensitive detection electronics to process the signal.

The theory of the PDS detection of dichroism is similar to that of conven-

tional PDS. The dichroic absorption coefficient is defined as

- a.1. (I)

where a.1.and all are the absorption coefficients with the probe

perpendicular and parallel to the bleaching pulse polarisation.

a sample of low thermal conductance then the deflection angle is

pulse polarisati

For the case of

given by [5J.

e = dNdT

P-2 2

WPCITa

2

(I - exp (a ,q,))(- 2 ( ~ ) exp (- ~ ))D 2 2a a(2)

hdN.

h h f... h

.1

were dT ~s t e c ange of re ract~ve ~ndex w~t temperature, P ~s the aser pr

incident power, w is the modulation frequency of the laser probe polarisation, ~

is the heat capacity per unit volume, a is the radius of the interaction region

the laser pump with the laser probe. x is the distance between this interactior

region and the photothermal probe. L is the length of the interaction between th

PDS probe and the bleach-probe overlap region.

The theory of how the dichroism recovery is related to the characteristic

reorientational time can be simply derived from the theory of the Ippen and Shar

cross polariser experiment.

Our experiment is sensitive to

(t) - N (t)) (3)

where 0 is the absorption cross section for the ground state to the first excite

state N" ,~(t) is the effective concentration for Beer's law absorption of light

polarised in the parallel or perpendicular direction with respect to the bleachi

pulse polarisation. The reorientational information is contained within, N (t

and N (t). To facilitate comparison with previous work [6J, the following

definition of the polarisation anisotropy is used

(N'I (t) - N (t))/(N 't)+ 2N (t))

.1. ,,\ .1..(4)

The excited state decay has also to be considered

k(t) = N" (t) + 2N1.(t) (5)

Combining equations (3),(4) and (5) aD(t) can be written as

or (t)k (t) (6)

In the case where r(t) and k(t) are single exponentials with time constants tr and

t respectively the equation (6) decays as a single exponential with a measured

time constant

t -1m t -Ir

(7)

OPTICAL

DELAY LINE

:-4 ;, I: I,,,I,

MODE LOCKED

DYE LASER

, "' !

HE-N~LASER

Experimental Details

Fig. (I) Experimental layout for Photothermal detection of induced dichroism

in Dyes

The experimental arrangement for photothermal

is illustrated in Fig (1). A synchronously pumped

was operated with Rhodamine 6G and produced pulses

detection of induced dichroism

mode-locked dye laser (Cr 599.04)

of about 5 ps duration and

1.5 nJ energy at a repetition rate of 228 MHz that is equivalent to 4.2 ns between

pulses. The average power was 300 mw. The bleaching pulse travels through a

variable optical delay line that could scan 1000 ps. The probe beam is directed

through a Pockels cell that modulated its polarisation through 90 degrees at a

variable frequency. The counter-propagating bleach and probe beams were aligned

through a 100 micron pinhole and crossed in the sample where the average power of

bleach and probe beams were around 140 row and 10 row respectively. The sample cell

was 1 mm thick and contained dyes in various solvents in concentrations of 10-4 to

10-5 molar. Photothermal deflection was observed using a 0.5 mW He Ne laser beam

whose position was initially fixed by aligning through the same pinhole as the dye

laser bleach and probe beams and subsequently repositioned to obtain optimum PDS

signal. The deflection of the PDS beam was recorded with a quadrant silicon photo-

diode. The processing electronics to obtain a signal proportional to displacement

on the quadrant photodiode were made from a standard design. Phase sensitive

electronic processing produced a signal proportional to the PDS probe deflection

at modulation frequency. This signal is directly proportional to the induced

C6-582 JOURNAL DE PHYSIQUE

dichroism. The dichroism recovery as a function of probe delay was observed by

scanning the optical delay line.

~-9

L--199

L--159 ~

Fig. (2) Photoinduced dichroism recovery for DODCI in Methanol

Results and Assessment

Figure (2) and Figure (3) show the induced dichroism decay curves for the

DODCI and DQOCI in 10-5 mol. solutions of methanol. The wavelength of the dye

laser was approximately 630 nm. The first observation to make about the two

figures is that there is no coherence spike at zero delay and indeed in all our

results so far we have seen no evidence of a coherence spike.

The time constant, tm of DODCI taken from fig (2) is 389 ! 50 ps. It is

taken from the first part (0-220 ps) of the decay (the sudden drop in signal is

probably due to misalignment of the delay line). For a fluorescence lifetime,

of 1.5 ns then using equation (7) the reorientational lifetime, tr is calculate

to be 266 ! 50 ps. This compares with the Ippen and Shank measurement of tr =

for the same dye in the same solvent. However Fleming et al have demonstrated

a significant build up of photoisomer can approximately double the measured val

of tr. This may account for the discrepanqv between our value of tr and that c

Ippen and Shank as our average power is probably higher and the wavelength at ~

the measurement was taken is near to the photoisomer peak absorption.

ORIENTATIONAL RELAXATION

OoaCI IN METHANOL

BL-L-.l

SBL~

I~I I I I I I I

ISBL-L-L J.

TIME <pS)

Fig. (3) Induced dichroism recovery for DQOCI in Methanol

TheDQOCItm canbe foundfromfigure(3) and is 83 ! 10 ps. The excitedstate lifetime is 3 ps therefore our measurement must be depended on the creation

of a longer lived photoisomer which will alter the ground state recovery time to

around 4 ns; this would make tr 81 ! 10 ps. The reorientational lifetime in

acetone was found to be 52 ! 10 ps. The relationship between tr in methanol,

viscosity 0.6 cp and that in acetone, viscosity 0.4 cp, is in good agreement with

the simple hydrodynamic model of molecular reorientation which predicts that tr

will scale linearly with viscosity.

From the signal to noise ratio in the experiment we estimate that the minimum-6

detected deflectionangle was 3.3 x 10 rad, taking the following typical anddN -4 -6 -3 -3

approximate values dT = 4.1 x 10 , pc = 2 x 10 JM , P = 10 x 10 W, w = 10 Hz

and x = a = 100 x 10-6 rn, then from equation (2) it is calculated that a minimum

aD of 2 x 10-lm-lcould be measured. In conventional PDS, using similar absorbed

Rowers, the minimum deflection angle reported is 10-~rad that is three orders of

magnitude more sensitive than this experiment. The difference is accounted for by

the turbulent convection current present in the sample because of absorption from

the bleaching beam which induces the dichroism. The convection current noise is

at low frequencies around 2 Hz but increasing the modulation frequency to avoid

this noise was limited by the l/w roll off in the signal expressed in equation (2).

C6-584 JOURNAL DE PHYSIQUE

Fig. (4) Experimental arrangement for solids

Solids

The recent observation of orientational gratings in semiconductors [8,9J

suggests that it should be possible to create induced dichroism in solids which

would relax on the picosecond time scale. The orientational grating has been

produced in germanium wafers and due to anisotropic state filling.

In solids there is no turbulent convection current to contend with and there

fore the photothermal deflection detection may be considerably more sensitive th

in liquids. Fig (4) shows a scheme for observing photoinduced dichroism which we

have tried out in GaSe, which has a convenient band gap for Rhodamine 6G operatio

of the mode-locked dye laser. However, induced dichroism was not observed at thes;

peak power as that in the dye solutions suggesting that the dye laser pulses have

to be amplified before induced dichroism can be observed.

Conclusion

The use of photothermal deflection to detect induced dichroism has been demo

strated. The major advantages of the technique are the absence of a coherence

spike and that there appear to be no spurious effects due to birefringence of the

optical components. The signal to noise ratio in the experiment in liquids was

limited by the turbulent convection current from the bleaching beam.

In semiconductors a greater peak power of the bleaching beam

before induced dichroism can be observed.

is required

Acknowledgements

This work was financed by the SERC.

References

( 1 )

(2)(3)

(4)

(5)

(6)(7)

(8)

(9)

"Laser Handbook" ed. Stitch, M.L. p. 753Lessing, H.E. and Von Jena, A.NORTH HOLLAND 1979.

Shank, C.V. and Ippen, E.P. Appl. Phys. Lett. 26 62 1975.Waldeck, D., Cross, A.J., McDonald D.B. and Fleming, G.R. J. Chem. Phys.74 338] 1981.Jackson, W.B., Amer, N.M., Boccara, A.C., Fourier, D. Appl. Optics. 20 13:]98].

Boccara, A.C., Fourier, D., Jackson, W.B. and Amer, N.M. Optics Lett. 5 371980.Tao, T. Biopolymers 8 609 1969.Fleming, G.R., Knight, A.E.W., Morris, J.M., Robbins, R.J. and Robinson, GChem. Phys. Letts. 49] ]977.Smirl, A.L., Boggess, T.F., l~errett, B.S., Perryman, G.P. and Miller, A.Phys. Rev. Letts. 49 933 1982.Boggess, T.F., Smirl, A.L. and Wherrett, B.S. Optics Commun. 43 128 ]982.