El Presidente resaltó la importancia del correcto funcionamiento de las instituciones de la democracia
Eje 3: Transformaciones en las instituciones, la cultura y la sociedad en la Europa y
We have seen (paragraph 2.3.1) that in isotropic medium, the second order electrical susceptibility has no contribution to the nonlinearity. But if we consider a solution of asymmetric molecules and their individual contributions, then the random alignment of two molecules within a wavelength of the fundamental laser light will be enough to cause a break of symmetry which will be enough to in turn cause the scattering of a photon of the second harmonic. This effect of SHG in isotropic medium was first observed by Terhune, Maker and Savage with a Ruby laser in liquid and fused silica in 1965"^^.
Similarly to equation (2.22) expanding the induced polarisation on the macroscopic level, on the microscopic level the induced dipole moment of a molecule can be expanded in a Taylor series in with an applied electric field
fli = a , j - E ^ + ^ ■ E.
•E.
+ . . . (5.1)where Pijk is the first hyperpolarisability tensor related to the macroscopic
The SHG signal generated by each molecule is incoherently scattered and proportional to (//(2(w)//*(2(w)). This technique known as Hyper Rayleigh Scattering (HRS) makes
C h apter 5 H yp er R ayleigh S cattering species in the solution, the H RS signal is extremely low . The total intensity o f the second- harmonic scattered light is given by**^:
l2„=g-B^-C=g-SN.-A'-C
(5.2)S
W here g depends upon the scattering geometry and the local-field corrections. is the number density o f sp ecies 5 with second-order N L O polarizabilty fin ally /„ is the incident intensity. In our case the solutions studied contains only tw o com pounds, the solute and the solvent, hence:
W hich is linear with h w ref
Varying !„ with the half-wave plate w e can deduce from the calculation o f the slop e the value o f ^ and using ^solvent as a reference w e deduce the value o f Psoiute-
T w o photons fluorescence also depends on the square o f the incident power and cannot be distinguished from the SHG signal. This can lead to an overestimate the P value o f the sample. However, it is possible to separate the tw o signals in time using ultrashort pulses to differentiate between the instantaneous nonlinear response and the slow er fluorescence^®’^'. Care must also be taken to filtrate the solutions in order to prevent
52 scattering from particles that could be in suspension in the solution .
r 2 _ n
R
^ -I- N
B
^
solvent Absolvent so lu te A bsolute (5.3)
2 2
W e measure the signal h a and the reference ' ®uree ' sim ultaneously and
then plot:
Chapter 5 Hyper Rayleigh Scattering
5.2
Experimental set-up
We used a Nd^'^rYAG Q-switched laser emitting 15ns pulses at 1064mn and with a repetition rate of lOHz and an energy of a few millijoules. The intensity was varied using a rotating half-wave plate between two polarisers. The reference is measured from the SHG generated by the NPP powder filtered and detected by a photomultiplier tube. Solutions of the investigated molecule are injected into a 4-cm long fluorescence cell through a 0.4 ^xn
Millipore filter in order to avoid optical noise resulting from the interaction between the focused laser beam and the dust impurities in the liquid. The scattered SHG signal is then filtered and detected by a photomultiplier. Both reference and signal detection systems are in light tight boxes. The stepper motor rotating the half-wave plate is computer controlled and the reference and signal photomultipliers are connected to a boxcar integrator triggered by the laser and read by the computer via an A/D board. The experiment is calibrated using as reference a solution of N- (4-nitrophenyl)-(S)-prolinol (NPP) or Chloroform of known
Chapter 5 H yper Rayleigh Scattering
A
Sample
i i
Nd:YAG
Figure 5-1: Experimental setup for the HRS, P:polari;^rs, BS: beam splitter, L / 2: half-wave plate, L1 and L2 focusing and collecting lenses. 'Reference and signal are both in light-tight boxes.
The experiment consists in measuring the change in SHG intensity when varying the light intensity with the rotating half-wave plate. The SHG signal represented against the laser intensity obeys a power dependence which can makes the fitting procedure more complex and the comparison between samples difficult. In this set-up the reference is taken as the SHG generated from NPP powder. This way the laser output can direcdy be measured in a quadratic way and the representation against the sample SHG signal will be a linear fit. Plotting the SHG signal directly against the SHG reference speeds up the analysis making the comparison between samples easier and more instinctive.
Chapter 5
Hyp«r
SctUt^ing
o Sample vsMPP
Linear fit (slope=2.1) 'CQ
I 1 . 0 - nj E 0.9- o Z 0 . 8 - 0 S 0-": ■ §. 0.6-
1
0 .4 -ccs
O O 0 .3- 0.2 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05SHG NPP Reference Normalised
Figure 5-2: H RS Signal and Reference
Figure 5-2 represents the variation o f the sample SHG signal compared to that o f the NPP reference. There is a large difference in intensity between the two signals; the sample SHG is about 20 times lower than the NPP SHG. The NPP SHG reference is therefore attenuated in order to compare it directly to the sample. The low level signal produced by HRS explains the relatively high noise o f the measurement. The steps visible on the reference signal are due to the limitation o f the A/D board.
The sample nonlinearity cannot directly be compared to NPP powder. In order to get the p
value o f the solute it is necessary to know the value o f the solvent m order to compensate for its contribution to the SHG signal. I f the solvent nonlinearity is known then it can be
Chapter 5 Hyper Rayleigh Scattering known solution and the solvent measured separately before the sample solution. It is also necessary to compensate for the sample concentration in order to obtain the final result.
5.3 Samples
Molecular engineering is used to maximise the NLO properties of materials. This consists in the design, synthesis, and characterisation of molecules fitting the criteria required for high NLO response. The simplest design consists in a molecular dipole created between strong electron donor and acceptors species bind together by a structure rich in 7i-type
I
electron orbital facilitating electron transfer.
D
Figure 5-3: Stilbene Molecule
The investigation of Stilbene and carbinol molecules NLO properties was the aim of the study for which number of samples y^value were measured using HRS.
5.4 Results Molecule Mass g/mol D A Measured p 10 '^“ ESU Concentration lO’^ mol/1 Solvent C-237 303.41 Me2N P(0)(0H )2 5.3 ± 1 3.9 DMSO C-260 379.61 Me2N P(0)(0K )2 6.9 ± 2 2.66 H2O C-261 347.41 Me2N
P(0)(0Na)2
12.9 ± 0 .2 1.48 H2O C-274 290.21 MeO P(0)(0H )2 4.7 ± 0.5 3.9 DMSO C-279 334.21 MeOP(0)(ONa)2
1 5 .0 ± 1 1.57 H2O C-280 366.41 MeO P(0)(0K )2 5.7 ± 1 1.71 H2O C-193 442.46 MeO P(0)(0C6H5)2 55 ± 5 0.1 CHCI3H yper Rayleigh Scattering
Molecule Conjugated transmitter
Mass g/mol Measured P 10'^“ ESU Concentration mol/1 Solvent C H C I3 - - 0.19 - - N P P - - 26 - CHCI3 C-97 437.37 18 ± 1 9.15 1 0'' CHCI3 C-97 D im ethylam ino- stilbenyles 437.37 200 ± 10 6.86 10'^ C2HF2O2 C-168 659.66 10 ± 1.5 9.1 10'" CHCI3 C-168 659.66 260 ± 20 3.94 10'^ C2HF2O2 C-216 516.464 10 ± 0 .5 1.04 10'^ CHCI3 C-216 516.464 320 ± 15 5.81 10'^ C2HF2O2 C-103 360.8 7.5 ± 2.5 1.07 10'^ CHCI3 C-103 Ferrocenylethen -diylphenyl 360.8 150 ± 50 7.76 10'^ C2Hf‘2 0 2 C-104 520.46 5 5 ± 10 5 10" CHCI3 C-104 520.46 400 ± 200 1 10-^ C2HF2O2 C-105 682.48 900 ± 150 3.22 lO'*’ C2HF2O2 C-106 908.55 1700 ± 250 2.64 10*’ C2HF2O2
Table 5-2: Carbinol values
Table 5-1 sum m arise the measured P values for a fam ily o f Stilbei\e with different com bination o f donor and acceptor groups.
Table 5-2 sum m arise the measured P values for tw o fam ilies o f carbinols with different conjugated transmitter. The species are m easure in solutions o f chloroform and in the carbonated form in difluoracetic acid. T he exact form s o f the molecules are represented in Figure 5-4.
C hapter 5 H y p e r R ayleigh Scattering O H ^ NM e^ C -9 7 C -I4 0 o u C - 1 6 8 M o lecu la r W eig h t = A 11X I M o lecu la r Form ula = C^Hj^FeNO
M olecu lar W eigh t = 6 5 9 .6 6 M o lecu la r Formula = C4,H4 2F e N jO OH M o le c u la r W e ig h t = 9 0 8 . 5 5 g /r a o l M o le c u la r F o r m u la = c-216 M o lecu la r W eigh t = 5 1 5 .4 6 4 M o lecu la r Form ula = C „ H ,,F eN O
C - 1 0 6 M o le c u la r W e ig h t = 6 8 2 . 4 8 g /m o l M o J e c u la r F o r m u la = C j j H j j F e , 0 M o le c u la r W e ig l n = 5 2 0 .4 6 g /m o l M o le c u la r F o r m u la = C ,5H ,j F e O c-sos C - U 4 o r C - 1 0 4 M o !e t-a !a r W e ig h t = 3 6 0 .8 g / m o l M o lc c a la r F o r m u la = C j , H j5F e S i C - I 0 3
Figure 5 A : C arbinol molecules
5.5 Conclusion
The theory and experimental details o f hyper R ayleigh scattering were sum m arised. The technique was used to measure the first hyperpolarisability of a group of newly synthesised Stilbenes and Carbinols as part of the European TM R netw ork D ELOS for the m olecular engineering of m aterials with high nonlinear properties.
Chapter 5 Hyper Rayleigh Scattering
This family of molecules was synthesised in order to study the NLO properties of ferrocene derivatives with different electron acceptor groups, and with different molecular geometry. The carbinol molecules need to be in presence of a strong acid in order to form the carbenium ions o f interest. The acid activation of the nonlinear properties of the molecule is clear in the species that were measured both in Chloroform and in Difluoroacetic acid. The NLO response is even multiplied by 32 for the molecule C-216. However, the most promising molecule, the Carbinol C-106 which j3 value was measured
o n
to be (1700 ± 250) 10' ESU could only be measured in a solution of difluoroacetic acid. The full analysis of the chemical meaning of these results is currently pending publication.
C h apter 5
References:
H yp er R ayleigh Scattering
Meredith, G. R. (1982). "Second-order cascading in third-order nonlinear optical processes." Journal of Chemical Physics 77(12): 5863-5871.
Vidakovic, P., J. Zyss, D. Kim, W. Toruellas, G. Stegeman, W. E. M oemer, R. Twieg and G. Bjoklund (1994). "Cascading of second-order processes in quadratic molecular media at the origin of very large cubic effects." Syntheric Metals 67: 303-307.
D.Y.Kam, W.E.Torruellas, J.Kang, C.Bosshard, G.I.Stegeman, P.Vidakovic, J.Zyss, W.E.Moemer, R.Twieg and G.Bjorklund (1994). "Second order cascading as the origin of large third-order effects in organic single-crystal cored fibers." Optics Letters 19(12): 868- 870.
Terhune, R. W ., P. D. M aker and C. M. Savage (1965). "Measurements of nonlinear Hght scattering." Phys. Rev. Lett. 14(17): 681-684.
48
Kaatz, P. and D. P. Shelton (1996). "Spectral measurements of hyper-rayleigh ligth scattering." Rev. Sci. Instrum. 67(4): 1438-1444.
Clays, K. and A. Persoons (1991). "Hyper-Rayleigh Scattering in Solution." Phys. Rev. Lett. 66(23): 2980-2983.
Geert Olbrechts, R. S., Koen Clays, and Andre Persoons (1998). "High-frequency demodulation of multi-photon fluorescence in hyper-rayleigh scattering." Rev. Sci. Instrum. 69(6): 2233-2241.
Geert Olbrechts, K. W., Koen Clays, and Andre Persoons (1999). "High-frequency demodulation of multiphoton fluorescence in l o n g - w a v e l e n g t h hyper-Rayleigh scattering."
Optics Letters 24(6): 403-405.
Morrison, I. D., R. G. Denning, W. M. Laidlaw and M. A. Stammers (1996). "Measurement of first hyperpolarizabilities by hyper-rayleigh scattering." Rev. Sci. Instrum. 67(4): 1445-1453.
Chapter 6 Conclusions
6 Conclusions