c.n. ironside, r.a. taylor and j. ryan clarendon...
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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.
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