Nonlinear Optical Probes and Processes in Polymers and Liquid ...

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14 chromophores, the second order nonlinearity (electro-optic effect) can be produced by electric field poling which breaks the symmetry and therefore allows for the second order effects to occur. This is in contrast to crystals where, as it was mentioned in Chapter 1, there is a limited number of symmetry groups that do not possess in- version symmetry, and so only these noncentrosymmetric crystals are considered as candidates for PR materials. The first polymer composite that exhibited PR effect was made in 1990 and con- sisted of an optically nonlinear epoxy polymer bisphenol-A-diglycidylether-4-nitro- 1,2-phenylenediamine (bisA-NDPA) doped with hole transport agent diethylamino- benzaldehyde-diphenylhydrazone (DEH) [8]. Since then, the photorefraction in polymers has developed into an active field. Numerous PR polymer composites with improved performance were subsequently studied . However, there are still many unrevealed mysteries in the PR performance of materials and polymers in particular. These are related to the complexity of both the PR effect itself, and to polymers as a disordered medium. All of the work I am presenting here in Chapters 2 and 3 was directed towards obtaining some understanding of these processes in PR polymer composites. 2.1.2 Origin of photorefractivity in polymer composites The basic phenomena contributing to the PR effect are photoconductivity and the electro-optic effect. The specific processes required for the PR effect include: photo-
generation of charge carriers, transport of mobile carriers, trapping of these carriers in regions of destructive interference and a change of the refractive index in response to the space charge field. In terms of material design, these properties that are necessary to create a PR grating, are generally provided by a photoinduced charge generator, transport medium and nonlinear optical (NLO) chromophores introduced in the composite. In polymer composites, traps are plentiful due to the structure of polymers themselves and also, some NLO chromophores can serve as traps as it will be shown in Section 3.5. Thus, no special agents for creating traps in such a material are needed. However, the simple presence of all of the “necessary” elements in the composite does not guarantee the presence of the PR effect in this composite. Even if a diffraction grating is produced, the two-beam coupling experiment should be performed to prove that the grating is photorefractive. In the basic experimental configuration, two coherent beams of wavelength λ are incident on the PR slab symmetrically at angles ±θ as shown schematically in Fig- ure 2.1. The electric fields of the beams 1 and 2 can be represented as two plane waves E1 = ˜ E10e iϕ1 e −ik1·r , E2 = ˜ E20e iϕ2 e −ik2·r 15 (2.1) where ˜ E10 and ˜ E20 are the amplitudes of the electric fields of beams 1 and 2, ϕ1 and ϕ2 are the respective phases, r is a radius vector and k1 and k2 are the wave vectors defining the propagation directions of beams 1 and 2. The physical quantity that
Page 1 and 2: Nonlinear Optical Probes and Proces
Page 3 and 4: Table of Contents Table of Contents
Page 5 and 6: References . . . . . . . . . . . .
Page 7 and 8: List of Figures 2.1 Schematic exper
Page 9 and 10: 3.31 Time evolution of the diffract
Page 11 and 12: Nonlinear Optical Probes and Proces
Page 13 and 14: Chapter 1 Introduction From ancient
Page 15 and 16: of the polarization density is [2]
Page 17 and 18: degenerate four-wave mixing and the
Page 19 and 20: (Section 3.3) as well as photorefra
Page 21 and 22: References [1] R. Boyd. Nonlinear o
Page 23 and 24: the optics of nonlinear devices bas
Page 25: materials. The figure-of-merit may
Page 29 and 30: after the interference pattern is c
Page 31 and 32: transport is mostly due to drift ra
Page 33 and 34: 2 1' 2' 1 1 Detector 2 d 2 E a 2
Page 35 and 36: coincide. However, due to refractio
Page 37 and 38: The equation to be solved is the wa
Page 39 and 40: It is obvious that the differential
Page 41 and 42: the symmetry group of the crystal a
Page 43 and 44: much weaker than the writing beams
Page 45 and 46: where d is the thickness of the sam
Page 47 and 48: ability to reorient in the space ch
Page 49 and 50: energy levels (ionization potential
Page 51 and 52: ficiency is the external applied fi
Page 53 and 54: photogeneration efficiency φ then
Page 55 and 56: where α is the absorption coeffici
Page 57 and 58: Energy 0 g( ) i Figure 2.6: Schem
Page 59 and 60: charge carrier mobility arises from
Page 61 and 62: 2.2.3 Charge trapping and detrappin
Page 63 and 64: Eqs. 2.29 and so, we will assume be
Page 65 and 66: However, Eq. 2.44 was derived under
Page 67 and 68: So, in this case the form of the so
Page 69 and 70: independent ζ0 [17]. We substitute
Page 71 and 72: on the both photoconductivity and p
Page 73 and 74: Trap-unlimited regime In this secti
Page 75 and 76: m xa 1 0.8 0.6 0.4 0.2 1 -1 TMT1
Page 77 and 78:
simplify these to: ˜ λ1 ≈ 1; ˜
Page 79 and 80:
- 6 0 , 10 40 30 20 10 simulated d
Page 81 and 82:
and ˜γ, the correct estimate of i
Page 83 and 84:
is no longer valid, so the followin
Page 85 and 86:
tion 2.3.2, we show how the ionized
Page 87 and 88:
numerical simulation of the dynamic
Page 89 and 90:
The first order system of equations
Page 91 and 92:
where C, ˜ C are constants determi
Page 93 and 94:
Diffr.eff. , arb.units 1 0.8 0.6 0.
Page 95 and 96:
Figure 2.15: PR speed ν calculated
Page 97 and 98:
a real system the effect from a cha
Page 99 and 100:
ρ0(t) ≈ ˜ρ0 and ionized accept
Page 101 and 102:
the PR performance, in particular d
Page 103 and 104:
2.4.1 “Simple” electro-optic ef
Page 105 and 106:
Eq. 2.71 and compared to the measur
Page 107 and 108:
For one-dimensional molecules whose
Page 109 and 110:
applied electric field Ea, but also
Page 111 and 112:
or, in terms of microscopic quantit
Page 113 and 114:
The photoconductivity part of the P
Page 115 and 116:
[14] Y. Cui, B. Swedek, N. Cheng, J
Page 117 and 118:
[41] P. Gunter and J.-P. Huignard,
Page 119 and 120:
Next, in Section 3.4, we will discu
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a) NC O O C 60 NC O NC O NC PVK c)
Page 123 and 124:
transport is only conducted through
Page 125 and 126:
NA of the C60 molecules (see Figure
Page 127 and 128:
3.3 Photoconductivity experiments I
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photogenerated in the bulk during a
Page 131 and 132:
voltage V , but the voltage V0 at w
Page 133 and 134:
Figure 3.4: Electric field dependen
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320nm Nd:YAG laser with Raman shift
Page 137 and 138:
zero. In these equations, q is the
Page 139 and 140:
non-Gaussian charge transport. In t
Page 141 and 142:
30 V/ m 30 C 0 40 C 0 50 C 0 Figure
Page 143 and 144:
Figure 3.9: Dependence of the mobil
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a) b) Figure 3.10: Current transien
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Conventionally, in the DC photocond
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a) b) 40 V/ m 100 mW/cm 2 Figure 3.
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1 0= 770 s -1 -1 = 4.35 s B = 1.66
Page 153 and 154:
Figure 3.14: Trapping parameter γT
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Figure 3.15: Long time scale photoc
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Figure 3.16: Example of the photocu
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He-Ne laser Beam Splitter Polarizat
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Figure 2.3. Then, the ratio of the
Page 163 and 164:
to explore experimentally which mec
Page 165 and 166:
In most of the PR polymer composite
Page 167 and 168:
disappeared, we opened the other wr
Page 169 and 170:
Now we describe the general trends
Page 171 and 172:
ciency is an increasing function of
Page 173 and 174:
are defined in Eqs. 2.58). Eq. 3.23
Page 175 and 176:
state diffraction efficiency was th
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Figure 3.26: Dependence of the fast
Page 179 and 180:
E a) b) Applied electri
Page 181 and 182:
Figure 3.28: Dependence of the fast
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(i) calculate the PR speed as deter
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h h C 60 (N A ) s T 1 E PVK PDCST
Page 187 and 188:
ferences that affect dissociation r
Page 189 and 190:
serve that the addition of all chro
Page 191 and 192:
is simplified. Knowing the availabl
Page 193 and 194:
the free hole with the ionized acce
Page 195 and 196:
Figure 3.30: Intensity dependence o
Page 197 and 198:
3.5.2 Plasticized composites This c
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the recombination rate γ and the s
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h h h h h h C 60 (N (NA) A) s s s
Page 203 and 204:
Figure 3.33: Concentration dependen
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mophore may contain a large error d
Page 207 and 208:
ather requires knowing trapping rat
Page 209 and 210:
Figure 3.36: Time evolution of diff
Page 211 and 212:
and for low concentrations of AODCS
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concentration x10 %. For the lower
Page 215 and 216:
3.6 Summary of Chapter 3 In Chapter
Page 217 and 218:
References [1] W. E. Moerner and S.
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[27] J. Schildkraut and Y. Cui. Zer
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[51] A. Blythe. Electrical Properti
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E = 0 E 0 Fi
Page 225 and 226:
chromophores are randomly oriented
Page 227 and 228:
where the local field factors are g
Page 229 and 230:
system, and k is Boltzmann’s cons
Page 231 and 232:
Ar Laser + Ti:Sapphire Laser Spectr
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Figure 4.4: Typical data in the EFI
Page 235 and 236:
and the birefringence ∆n(t) is pr
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experiment is the applied field. Ho
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nature. For the transient shown in
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Figure 4.7: Illustration of the HeN
Page 243 and 244:
the screening of the external elect
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is accompanied by the reduction in
Page 247 and 248:
References [1] W. E. Moerner, S. M.
Page 249 and 250:
Chapter 5 Second harmonic generatio
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2 d 2 0 L Figure 5.1: Schematic
Page 253 and 254:
of the nonlinear susceptibilities i
Page 255 and 256:
x z E p E s Figure 5.2:
Page 257 and 258:
(a) (b) (c)
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to βz ′ z ′ z ′ as follows [
Page 261 and 262:
d31 = N 2 〈cos θ sin2 θ〉〈co
Page 263 and 264:
Thus, SHG measurements of the azimu
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a) b) 10 5 -10 -5 5 10 15 -5 -10 c)
Page 267 and 268:
Nd:YAG “Reference” PMT 2 F Quar
Page 269 and 270:
( ) ( ) H3C Si (CH2) 11 O NO2 H 3C
Page 271 and 272:
Figure 5.11: Maker fringes observed
Page 273 and 274:
We observed light-induced alignment
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polarized UV light. The intensity o
Page 277 and 278:
and then d31 = d sub 31 + N 4 cos
Page 279 and 280:
The angular dependence of the secon
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= ⎛ ⎜ ⎝ sin Φ d25cos 2 Φ s
Page 283 and 284:
S-input polarization The case of s-
Page 285 and 286:
pf,y = 0 pf,x = 1 ω nf bf,x = pf,
Page 287 and 288:
References [1] M. B. Feller, W. Che
Page 289 and 290:
Chapter 6 Summary In this, final, C
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anticipated from the theoretical mo
Page 293 and 294:
Bibliography Abkowitz, M., Bassler,
Page 295 and 296:
Daubler, T. K., Bittner, R., Meerho
Page 297 and 298:
Kuzyk, M. G., and Dirk, C. W., Eds.
Page 299 and 300:
Reznikov, Y., Ostroverkhova, O., Si
Page 301:
Zilker, S. J., Grasruck, M., Wolff,
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