Nonlinear optics, chirality, magneto-optics:

a serendipitous road

Andre Persoons1,2,3,*

1Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium

2College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA 3apersoons@optics.arizona.edu

*andre.persoons@fys.kuleuven.be

Abstract: This paper reviews the work of my group over the last 20 years

on the role of chirality in second-order nonlinear optics of molecular

systems and new developments linking my research to magneto-optics and

organic magnetism.

©2011 Optical Society of America

OCIS codes: (160.4330) Nonlinear optical materials; (160.1585) Chiral media; (190.4350)

Nonlinear optics at surfaces.

References and links

1. F. Fukuyama, The End of History and the Last Man (The Free Press, 1992). 2. M. Kauranen, T. Verbiest, and A. Persoons, “Chiral materials in second-order nonlinear optics,” J. Nonlinear

Opt. Phys. Mater. 8(2), 171–189 (1999).

3. S. Sioncke, T. Verbiest, and A. Persoons, “Second-order nonlinear optical properties of chiral materials,” Mater. Sci. Eng. R42, 115–155 (2003).

4. All serious students of nonlinear optics should have copies of the following books, in order of mathematical

depth first editions given, but see subsequent editions also: G. C. Baldwin, An Introduction to Nonlinear Optics (Plenum Press, 1969); T. Verbiest, K. Clays, and V. Rodriguez, Second-Order Nonlinear Optical

Characterization Techniques (CRC Press, 2009); P. N. Prasad and D. J. Williams, Introduction to Nonlinear

Optical Effects in Molecules and Polymers (Wiley, 1991); D. S. Chemla and J. Zyss, eds., Nonlinear Optical Properties of Organic Molecules and Crystals (Academic Press, 1987); R. W. Boyd, Nonlinear Optics

(Academic Press, 1992); A. Yariv, Quantum Electronics (Wiley and Sons, 1967); Y. R. Shen, The Principles of

Nonlinear Optics (Wiley, 1984); N. Bloembergen, Nonlinear Optics (W. A. Benjamin, 1965). 5. P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich, “Generation of optical harmonics,” Phys. Rev. Lett.

7(4), 118–119 (1961).

6. T. H. Maiman, “Stimulated optical radiation in ruby,” Nature 187(4736), 493–494 (1960). 7. A. Persoons and L. Hellemans, “New electric field methods in chemical relaxation spectrometry,” Biophys. J.

24(1), 119–134 (1978).

8. A. D. Buckingham, “Theory of the dielectric constant at high field strengths,” J. Chem. Phys. 25(3), 428–434 (1956).

9. M. G. Kuzyk and C. W. Dirk, eds., Characterisation Techniques and Tabulations for Organic Nonlinear Optical Materials (Marcel Dekker, 1998).

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13. L. R. Dalton, W. H. Steier, B. H. Robinson, C. Zhang, A. Ren, S. Garner, A. Chen, T. Londergan, L. Irwin, B. Carlson, L. Fifield, G. Phelan, C. Kincaid, J. Amend, and A. Jen, “From molecules to opto-chips: organic electro-

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15. L. D. Barron, Molecular Light Scattering and Optical Activity (Cambridge University Press, 1982).

16. A. Lakhtakia, Selected Papers on Natural Optical Activity (SPIE, 1990). 17. J. A. Giordmaine, “Nonlinear optical properties of liquids,” Phys. Rev. 138(6A), A1599–A1606 (1965).

18. P. M. Rentzepis, J. A. Giordmaine, and K. W. Wecht, “Coherent optical mixing in optically active liquids,” Phys.

Rev. Lett. 16(18), 792–794 (1966). 19. T. Petralli-Mallow, T. M. Wong, J. D. Byers, H. I. Yee, and J. M. Hicks, “Circular dichroism spectroscopy at

interfaces: a surface second harmonic generation study,” J. Phys. Chem. 97(7), 1383–1388 (1993).

20. T. Verbiest, M. Kauranen, A. Persoons, M. Ikonen, J. Kurkela, and H. Lemmetyinen, “Nonlinear optical activity

and biomolecular chirality,” J. Am. Chem. Soc. 116(20), 9203–9205 (1994). 21. M. M. Kauranen, T. Verbiest, A. Persoons, E. W. Meijer, M. N. Teerenstra, A. J. Schouten, R. J. M. Nolte, and

E. E. Havinga, “Chiral effects in the second-order optical nonlinearity of a poly(isocyanide) monolayer,” Adv.

Mater. (Deerfield Beach Fla.) 7(7), 641–644 (1995). 22. M. Kauranen, J. J. Maki, T. Verbiest, S. Van Elshocht, and A. Persoons, “Quantitative determination of electric

and magnetic second-order susceptibility tensors of chiral surfaces,” Phys. Rev. B 55(4), R1985–R1988 (1997).

23. M. Kauranen, T. Verbiest, J. J. Maki, and A. Persoons, “Nonlinear optical properties of chiral polymers,” Synth. Met. 81(2-3), 117–120 (1996).

24. J. J. Maki and A. Persoons, “One-electron second-order optical activity of a helix,” J. Chem. Phys. 104(23),

9340–9348 (1996). 25. M. Kauranen, C. Boutton, T. Verbiest, M. N. Teerenstra, K. Clays, A. J. Schouten, R. J. M. Nolte, and A.

Persoons, “Supramolecular second-order nonlinearity of polymers with orientationally correlated chromophores,”

Science 270(5238), 966–969 (1995). 26. S. Van Elshocht, T. Verbiest, M. Kauranen, A. Persoons, B. M. W. Langeveld-Voss, and E. W. Meijer, “Direct

evidence of the failure of electric-dipole approximation in second harmonic generation from a chiral polymer

film,” J. Chem. Phys. 107(19), 8201–8203 (1997). 27. A. Persoons, M. Kauranen, S. Van Elshocht, T. Verbiest, L. Ma, L. Pu, B. M. W. Langeveld-Voss, and E. W.

Meijer, “Chiral effects in second-order nonlinear optics,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 315(1), 93–98

(1998). 28. T. Verbiest, S. Sioncke, G. Koeckelberghs, C. Samyn, A. Persoons, E. Botek, J. M. André, and B. Champagne,

““Nonlinear optical properties of spincoated films of chiral polythiophenes,” Chem. Phys. Lett. 404(1-3), 112–

115 (2005). 29. C. Nuckolls, T. J. Katz, T. Verbiest, S. V. Elshocht, H. G. Kuball, S. Kiesewalter, A. J. Lovinger, and A.

Persoons, “Circular dichroism and UV-Visible absorption spectra of the Langmuir-Blodgett films of an

aggregating helicene,” J. Am. Chem. Soc. 120(34), 8656–8660 (1998). 30. T. Verbiest, S. V. Elshocht, M. Kauranen, L. Hellemans, J. Snauwaert, C. Nuckolls, T. J. Katz, and A. Persoons,

“Strong enhancement of nonlinear optical properties through supramolecular chirality,” Science 282(5390), 913–915 (1998).

31. T. Verbiest, S. Sioncke, A. Persoons, L. Vyklický, and T. J. Katz, “Electric-field-modulated circular-difference

effects in second-harmonic generation from a chiral liquid crystal,” Angew. Chem. Int. Ed. Engl. 41(20), 3882–3884 (2002).

32. B. Busson, M. Kauranen, C. Nuckolls, T. J. Katz, and A. Persoons, “Quasi-phase-matching in chiral materials,”

Phys. Rev. Lett. 84(1), 79–82 (2000). 33. D. Beljonne, Z. Shuai, J. L. Bredas, M. Kauranen, T. Verbiest, and A. Persoons, “Electro-optic response of chiral

helicenes in isotropic media,” J. Chem. Phys. 108(4), 1301–1304 (1998).

34. F. Araoka, T. Verbiest, K. Clays, and A. Persoons, “Interactions of twisted light with chiral molecules: an experimental investigation,” Phys. Rev. A 71(5), 055401 (2005).

35. E. S. Barr, “Men and milestones in optics. V: Michael faraday,” Appl. Opt. 6(4), 631–637 (1967).

36. M. Mansuripur, “Classical Optics and Its Applications” (Cambridge University Press, 2002), Chap. 10. 37. J. P. van der Ziel, P. S. Pershan, and L. D. Malmstrom, “Optically-induced magnetization resulting from the

inverse Faraday effect,” Phys. Rev. Lett. 15(5), 190–193 (1965).

38. E. Botek, B. Champagne, T. Verbiest, P. Gangopadhyay, and A. Persoons, “A joint theoretical-experimental investigation of the Faraday effect in benzene, toluene, and p-xylene,” ChemPhysChem 7(8), 1654–1656 (2006).

39. P. Gangopadhyay, S. Foerier, G. Koeckelberghs, M. Vangheluwe, A. Persoons, T. Verbiest, “Efficient Faraday

rotation in conjugated polymers,” Proc. SPIE 6331, 63310Z (2006). 40. G. Koeckelberghs, M. Vangheluwe, K. V. Doorsselaere, E. Robijns, A. Persoons, and T. Verbiest,

“Regioregularity in Poly(3-alkoxythiophene)s: effects on the Faraday Rotation and polymerization mechanism,”

Macromol. Rapid Commun. 27(22), 1920–1925 (2006). 41. P. Gangopadhyay, R. Voorakaranam, A. Lopez-Santiago, S. Foerier, J. Thomas, R. A. Norwood, A. Persoons,

and N. Peyghambarian, “Faraday rotation measurements on thin films of regioregular alkyl-substituted

polythiophene derivatives,” J. Phys. Chem. C 112(21), 8032–8037 (2008).

42. P. Gangopadhyay, G. Koeckelberghs, and A. Persoons, “Magneto-optic properties of regioregular

polyalkylthiophenes,” Chem. Mater. 23(3), 516–521 (2011).

43. G. Martin-Gassin, E. Benichou, G. Bachelier, I. Russier-Antoine, Ch. Jonin, and P. F. Brevet, “Compression induced chirality in dense molecular films at the air-water interface probed by second harmonic generation,” J.

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1826–1829 (2010).

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46. M. Kuwata-Gonokami, N. Saito, Y. Ino, M. Kauranen, K. Jefimovs, T. Vallius, J. Turunen, and Y. Svirko, “Giant

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1. Introduction

Ever since Francis Fukuyama published in 1992 the now famous book “The End of History

and the Last Man” - based upon his 1989 essay in National Interest - tumultuous historical

events have changed the modern world unrecognizably [1]. This reminds me of those

scientists in late 19th century who saw the end of science approaching, with only some small

unresolved problems, such as black body radiation. Then in the first decades of the 20th

century the new ideas about space and time and the rise of quantum physics overthrew all

these feelings of the end of science and changed our scientific understanding of nature beyond

recognition. It now appears that the Theory of Everything has to wait for - at least - another

century…

Looking to the field of optics, the science of light, such an emergence of revolutionary

changes is again striking. Mid 20th century the science of optics was well developed but was

considered by many as not very exciting - few academic institutions worldwide had programs

in optical sciences. Optics was often seen as an applied science of lenses and mirrors, of

photography. But, 50 years ago, the laser came - a new source of light, coherent and intense,

created by scientists. Over an extremely short period the properties of this laser light were

used to develop new methods, techniques, devices… and companies, while young and bright

students were flocking together in “laser-labs”. Central in these new developments was the

recognition of the important role of materials, henceforth called optical materials, inviting

many chemists to enter new innovative research on synthesis of new materials. Many of these

new materials – and the research is now going on at an unabated pace - offer the stage for the

photons to realize their full possibilities, and are basic to the new field of photonics.

I would preferably call this review a memoir since this paper will not give an exhaustive

enumeration of all papers and relevant references but rather an indication of the important

papers, previous papers inspiring us, the ideas generated, and results, some of them still

poorly understood. It will be a memoir for me, looking back to all these bright associates,

many of them now involved as professors, researchers, and business people, in the thriving

field of photonics. Moreover, an extensive review has already been given elsewhere [2,3].

And rather than giving lists of references to papers and journals I prefer to indicate the names

of the scientists involved. Indeed these days it is easy and straightforward to go to the Web of

Science with their names – Author Finder – to obtain all information (sometimes surprising!)

about the work/author(s) mentioned.

2. Non-linear optics – second order [4]

Here I give a short overview of second-order nonlinear optics as developed since the seminal,

spotless paper of Peter Franken [5] and associates at the University of Michigan in 1961,

hardly a year after the pulse ruby laser – then still called the optical maser – developed by

Maiman [6]. Peter Franken realized early the potential for generation of optical harmonics

with this new intense light source which produces very large fields with focused pulses. At

such field strengths, approaching interatomic field strengths, the (dielectric) polarization of a

material is no longer linear with the electric field but we have to consider for the polarization

a series expansion in the electric field - I entered the field of nonlinear optics from nonlinear

dielectrics [7], my first encounter with David Buckingham [8]:

(1) (2) 2P = E + E + ... (1)

where P is the polarization, χ(1)

the linear susceptibility, χ(2)

the second-order susceptibility….

(for the sake of argument we make abstraction of vectorial, tensorial properties). From this

simple equation it is immediately clear that with E sinusoidal in time the quadratic term will

result in a polarization containing the second harmonic of the fundamental frequency.

However, taking into account the vectorial character of P and E we also see that the second-

order susceptibility χ(2)

must vanish necessarily in a centrosymmetric medium. We therefore

should take only crystals without a center of inversion, e.g. the quartz used by Peter Franken,

to generate the second harmonic of laser pulses; we need pulses to obtain the required electric

field strengths since χ(2)

is always very small. This is a first and very basic requirement.

However, to obtain significant intensity for the second harmonic a second requirement should

be met. Indeed we must avoid that the second harmonic signal generated at one point along

the propagation of the fundamental beam should be out of phase with the second harmonic

generated at an earlier point, resulting in a destructive interference and resulting in no, or

extremely weak, second harmonic intensity. This necessary phase matching imposes some

further restrictions on crystals for second harmonic generation since we require the indices of

refraction equal for second harmonic and fundamental beam for birefringent crystals.

Everyone aware of modern laser-technology will know that material scientists were able to

meet these requirements in a superb way.

The stringent requirement of noncentrosymmetry attracted the attention of chemists since

they have easy access to noncentrosymmetric molecules and the polarizabilities of molecules

were very well known. Since the polarizability of molecules is due to electrons molecules may

present some advantages over the ionic polarizability of crystalline systems. So we saw in the

70’s of the previous century the emergence of a new field, partially grown out from

dielectrics, investigating the nonlinear properties of molecules, polymers and molecular

systems, the still thriving field of organic nonlinear optics [9].

On a microscopic, molecular level we write Eq. (1) as:

ind = E + E2 + (2)

with μind the induced dipole moment, α the molecular polarizability and β, the first

hyperpolarizability, the molecular property investigated in organic nonlinear optics. Research

was, and still mainly is, most focused on molecules with donor and acceptor groups (electron

donating, resp. attracting) connected by a π-conjugated bridge giving efficient electron

mobility in the molecule, the fruitfly of this research being the molecule para-nitroaniline.

Such molecules are noncentrosymmetric with a nonvanishing first hyperpolarizability.

To develop nonlinear optical materials from these molecules the challenge was to organize

the molecules in a noncentrosymmetric macroscopic arrangement to obtain a second-order

susceptibility. The method of choice became the attachment of the molecules to a polymer

chain – the nonlinear optical polymers – and the use of electric field poling of spin-coated

films of these polymers on a solid substrate [10–12]. In this poling technique the film is

heated above the glass transition temperature and a strong electric field orients the polar

molecules in the polymer matrix. Subsequent cooling below the glass transition temperature,

in the field, will freeze in this orientation and films are obtained with the required

noncentrosymmetry (Cv). These films have the drawback of being thermodynamically

unstable and they tend to return to their isotropic state over time. A tremendous amount of

research in this field has culminated in the development of stable, very fast electro-optical

modulators by Larry Dalton and Alex Jen, who were able to develop synthetic methods for

optical polymers with exceptional stability [13,14].

To summarize the problems encountered in organic nonlinear optics, we need materials,

preferentially films for integrated optics, which should have a stable noncentrosymmetric

structure, and if we are looking for materials for second harmonic generation then issues of

phase matching become important.

These were the problems we discussed in our group beginning of the 90’s and in, often

heated, discussions with the groups of Jean-Luc Bredas (then at Mons University), Bert

Meijer (TUE), Roeland Nolte (University of Nijmegen), François Kajzar (then CEA), David

Reinhoudt (TUT), Jo Zyss (then at CNET, Paris) – we even discussed spelling of words - Seth

Marder (then at Caltech)… and out of these discussions slowly emerged the need of a

thorough investigation of the basic requirements for nonlinear optical materials.

3. Chirality

Strongly influenced by our Dutch colleagues we started focusing on the symmetry of chiral

molecules. Such molecules comes in two mirror forms and show optical activity, e.g.

polarization rotation and circular dichroism arising from the different interaction with left- and

right-handed circular polarized light for the two enantiomers, the signature of their lack of

reflection symmetry. Chiral molecules and chirality have been a central topic in chemistry

research ever since Pasteur in 1849 linked the optical rotation to the presence of two mirror

forms of tartaric acid, a molecule he isolated from wine lees – Pasteur was French. He showed

that the chemically synthesized tartaric acid, an equal mixture of the two enantiomers, the

racemic mixture, has no optical activity. The book of Laurence Barron is a beautiful

introduction, and more, to the field of chirality [15] while Aklhlesh Lakhtakia gave a

marvelous overview on the field of natural optical activity [16]. Most important for us was the

fact that these properties are observed in isotropic chiral materials. And since chiral molecules

are noncentrosymmetric by necessity they should show second-order nonlinear optical

response. Symmetry properties of molecules were described by Giordmaine where we could

find that the susceptibility of chiral molecules with their lack of reflection symmetry had

nonvanishing components [17]. Most interesting to us was the experimental confirmation by

Peter Rentzepis of sum-frequency generation (second harmonic is prohibited by permutation

symmetry) in a solution – isotropic - of enantiomers of arabinose, a chiral sugar –

unfortunately we could not reproduce the experiment [18].

Of great importance to motivate us further into research of chirality was the paper

published by Janice Hicks and associates and showing convincingly the existence of optical

acitivity in nonlinear optics [19]. In a first approach we investigated biological systems [20],

with their inherent chirality. However, after some discussion, we decided to investigate

second-harmonic generation from thin molecular films of well-defined chiral polymers,

abundantly available from Bert Meijer’s group in Eindhoven. Very thin films of such

polymers could easily be made by Langmuir-Blodgett (LB) techniques, a technique which

was well established in our group. Moreover, the thinness of the films would avoid problems

with polarization effects due to the linear light propagation and the nonlinear response could

be directly related to the components of the nonlinear susceptibility tensor. Also, considering

the very thin film as a merely chiral surface we were adding the required noncentrosymmetry

to have a second harmonic signal and we could obtain, in principle, the value of all

components.

As our first polymer film we investigated a LB-film of a chiral polyisocyanide provided by

Roeland Nolte from Nijmegen [21–23]. For the experiments a relatively simple set-up used a

pulsed Nd-YAG laser, the workhorse of nonlinear optics in those days. The results obtained

confronted us quickly with the fact that the basic noncentrosymmetry requirement actually

should be stated, in a correct reading, as a requirement in the “electric-dipole approximation”

where the interaction Hamiltonian between matter and electromagnetic field is described by:

H = - .E (3)

However, taking into account other interactions we should consider:

H = - .E - m.B - Q: E + ... (4)

μ, m and Q are the electric dipole, magnetic dipole and electric quadrupole operators. Usually

the magnetic dipole interaction is much weaker than the electric dipole interaction (by a factor

~1/137, the hyperfine constant) and it is perfectly safe to neglect this interaction unless the

electric dipole interaction is forbidden by symmetry and the magnetic dipole survives in the

material considered, despite the question whether the effect might be so weak that no

nonlinear response would be observed. But here we have the challenge to develop new optical

materials. Since magnetic interaction in chiral media is well established we set out to look for

these effects from the beginning, and since the quadrupole interaction is somewhat smaller

and symmetry-wise not fitting to chiral structures we incorporated quadrupole effects into the

magnetic dipole term – they have both the same symmetry [15].

For the nonlinear polarization we wrote now:

PNL = eee EE + eem EB (5)

and not forgetting that we had to introduce also a nonlinear magnetization:

mee

NLM = EE (6)

Instead of the usual symbol for the nonlinear susceptibility, χ(2)

we wrote χeee

, χeem

, or χmee

where the last two superscripts indicated the interaction annihilating two input photons and

the first the interaction creating the output photon at the second harmonic, at either the electric

dipole interaction (e), or the magnetic dipole interaction (m). The beauty of these simple

equations has always been very pleasing: even in centrosymmetric materials the terms χeem

and χmee

survives the inversion operation since B and M are axial vectors and do not change

sign upon inversion.

The results obtained from our investigation of SHG of the polyisocyanide LB-films clearly

showed circular difference effects for the four measured, s- and p-polarized, reflected and

transmitted (very thin films) second harmonic fields. Even more important was the fact that

the SHG intensity pattern as function of the polarization of the fundamental beam could not be

explained without invoking the xyz term of χeee

, the chirality component in the electric dipole

interaction, ànd the absolute need to invoke χeem

components, being about 10% of the largest

electric-dipole allowed term. These were very pleasing days when we saw chirality expressed

in full in our measurements and magneto-optics entering our studies of organic nonlinear

optics.

When discussing these exciting results we used extensively, and intuitively, the model of a

left- or right-handed screw describing a chiral molecule spurred by the screwish, helical

conformation of the polyisocyanide polymer. After all a box of, e.g. left-handed screws

remains left-handed, whatever the distribution is. This helical model focused our thinking on

the physical aspects of optical activity seeing the role of electric and magnetic dipoles, their

phase relations, the importance of reflection symmetries… Amusingly, our simple figure of

such a helical model was a great success at presentations and several colleagues asked me to

send this illustration, see Fig. 1.

An intuitive view of (the optical activity in) a helical molecule:

Fig. 1. An intuitive view of (the optical activity in) a helical molecule.

The polarization and magnetization induced by the incoming field, forcing the electron to

move in a helical path, are both the source of radiation with a field parallel with the electric

dipole and a field perpendicular to the magnetic dipole - the beauty of Maxwell’s equations!

Simply summing these vectors we obtain a rotation of the plane of polarization. This model

indicates very clearly the essential role of magnetic effects in optical activity, the change of

handedness changing only the sign of optical rotation, the phase relations between electric and

magnetic effects… Later we analyzed with Jeff Maki this model more in depth and calculated

the one-electron second-order optical activity of a helix [24].

Later we analyzed an 18 layer-film (thickness 35 nm) of a nonlinear polyisocyanide mixed

with poly(-S-l-acetoxymethylethylisocyanide) [22]. The results showed that the nonlinear

response was determined largely by the zzz, zxx, and xxz components of the electric-dipole

tensor. These achiral components are due to polar ordering along the substrate normal.

However, certain components of the magnetic tensors χeem

and χmee

were also large with zzz

components of the order of 25% of the largest zzz component of χeee

, even larger than the

chiral xyz component of χeee

. The largest components of the magnetic tensors are thus

associated with chirality, a result in agreement with the interpretation that large magnetic

effects in chiral molecules are directly related to the structural asymmetry of the molecules.

However, the chirality and nonlinear response in polyisocyanide LB-films is mainly

associated with the helical backbone of this polymer while the nonlinear response resides in

the NLO-active chromophore attached to the backbone. Our observations on these NLO

properties indicated a strong coupling between backbone and chromophore, presumably due

to the spatial phase differences the helical backbone imposes on the chromophore positions

[25]. However, it could be argued that the nonlinear chirality effects in polyisocyanide were of

a secondary nature.

In order to possibly observe stronger chiral and magneto-optical effects we turned to chiral

polythiophenes, abundantly available from Bert Meijer, these polymers being one of his

important research topics - out of many. These conjugated polymers show extremely strong

optical rotation in poor solvents and in the solid (film) state, with the displacement of the

electrons along the helical conjugated backbone. Intuitively we felt, again using our pet-model

that we should see strong magnetic effects - it would later prove that our intuition was even

more correct than we ever expected. We indeed found for a 10-layer LB-film of a

polythiophene that the electric and magnetic contributions were of the same order of

magnitude, perhaps somewhat helped by the centrosymmetric arrangement of the monomer

units suppressing the electric dipole interaction [26–28]

By the “diplomatic talents” of Seth Marder (now at GeorgiaTech) we obtained samples of

the chiral helicenebisquinones from Tom Katz (Columbia) - which turned out to be a

goldmine for research in nonlinear chiral optics. These helicenes in the nonracemic,

enantiomeric pure state spontaneously aggregates in the appropriate solvents, e.g. dodecane,

showing an enormous optical activity - we even measured the CD-spectrum from a LB-

monolayer. Indeed these helicenes amazingly formed, rather easily, very stable LB-films. To

our surprise SHG from LB-films made from the nonracemic helicenes was particularly strong,

about 1000 times as intense as the SHG-signal from a racemic helicene film, hence a 30-fold

increase in the susceptibility. Based upon a comparison of the polarization dependence for

SHG from racemic and nonracemic LB-films we could conclude that the dominant part of the

susceptibility in the nonracemic form was due to chirality. Oddly, in the analysis of the SHG-

response no evidence was found of the presence of magnetic dipole interactions. This told us

clearly that chirality and magneto-optical effects are different phenomena - why we see

magnetic effects playing a role in the highly chiral polythiophene films while they are absent

in the highly chiral helicene systems is still puzzling and may be an important issue for future

development in optical materials. Other helicene films, spun-coat, were also investigated. The

films from the nonracemic helicenes showed a SHG response in agreement with a D

symmetry and as the nonvanishing tensor component only χxyz present, with a magnitude of

about 2 pm/V - too small for practical applications but illustrating convincingly the potential

of chirality in nonlinear optical applications. Indeed these films will never relax to a more

stable structure, they are the most stable structure showing excellent thermal stability - above

the melting point of the film the SHG disappears, only to reappear, with the same intensity

upon cooling, a definite proof of stability [29,30].

A beautiful experiment was carried out by Sonja Sioncke and clarified the role of tensor

components, polar and apolar, in these helicene films. She investigated SHG from a binary

mixture of two helicenes in dodecane forming a chiral discotic nematic phase. The

polarization analysis of SHG signals from this sample in a LC cell showed that here again the

symmetry was D symmetry with χxyz the only tensor component present. But when an

electric field was applied to the LC cell the symmetry changed to C and several polar

components appeared which could all be determined. It was also nice to see that upon reversal

of the electric field the values of the polar susceptibility components changed sign - as

expected - while the apolar component remained unchanged [31]. This ability to reverse the

sign of susceptibility components may have an application for quasi-phase matching.

But we could use chirality, with the susceptibility components of opposite sign for the two

enantiomers in a much simpler way to realize quasi-phase matching. As said before phase

matching is of great importance in SHG to build up a sufficient intensity for the SHG signal.

Indeed, we see that in the NLO material the source polarization and the harmonic field will be

out of phase after a given distance, the coherence length, - unless the indices of refraction for

fundamental and harmonic are equal, not easy to realize. An alternative solution, quasi-phase

matching, consists of reversing the sign of the nonlinear optical coefficients after every (or

odd multiple) coherence length. In this way the nonlinear signal will grow continuously. This

is just what we could realize with enantiomeric molecules and it would be sufficient to have a

structure with alternate stacks of enantiomers, the stacks with a thickness of the (odd multiple)

of the coherence length. This is what Bertrand Busson did by using the LB-technique to

deposit stacks of molecular layers of the two enantiomers of the helicenebisquinone. For

practical reasons the experiments were carried out in a reflection set-up since the coherence

length is much shorter. This work beautifully illustrated the potential of chirality to realize

quasi-phase matching [32].

Another interesting possibility of chiral materials we discussed extensively is their use, as

isotropic materials, in electro-optical modulation (Pockels effect). Indeed, for the polarization

we always can write:

ee eee ee eeeP = ( ; ) E( ) + ( ; , 0)E( ) E(0) = [ ( ; ) + ( ; , 0)E( )]E(0) (7)

or,

P = eff E() (8)

We see that we now have an effective susceptibility which is a function of an applied intense

low frequency electric field E(0). We explicitly wrote here the frequency dependence to stress

that the susceptibilities are not the same as for SHG and we should properly take dispersion

into account. Using the well-known relation between susceptibility and index of refraction we

obtain a field-dependent index of refraction - the requirement for optical modulation.

Normally it is expected that the susceptibility χeee

(ω; ω, 0) vanishes in a chiral isotropic

material but this is only in a lossless material; if we include damping this susceptibility may

become nonvanishing opening the possibility of electro-optical modulation [33]. Up to now

this prediction, which is also challenged, is waiting for experimental confirmation, or

refutation.

I want to mention a last item on the research on chirality. In our discussions in the group a

question surfaced if it would be possible to put the chirality in the light beam and not in the

molecule. Could this be realized by using Laguerre-Gaussian or so called twisted light beams?

I was fortunate to have at the right time Fumito Araoka as a post-doc who surprised me, and

every group member, by setting up such an experiment with LG-beams in a few days – I

programmed 6 – 8 weeks! Fumito Araoka did a beautiful piece of work but we were all

disappointed to see that the twisted light, although carrying orbital angular momentum, is not

specific at all in its interaction with chiral matter- an important result in itself [34]. But this

story is not yet ended.

4. Faraday rotation

Looking back at the Pockels effect, the polarization described by Eqs (7) and (8), we started to

think about the possibility of a “magnetic Pockels effect”. Indeed, writing the optical

polarization taking now into account magnetic effects we have, in a complete analogy with the

Pockels effect:

ee eem ee eem

P = ( ; ) E( ) + ( ; , 0)E( ) B(0) = [ ( ; ) + ( ; , 0)B(0)]E( ) (9)

or, again:

P = E( )eff B (10)

But now with an effective susceptibility which is dependent upon a low frequency

magnetic field. How we should relate χeem

(ω; ω, 0) to χeem

(2ω; ω, ω), how we should take

dispersion into account here is still a somewhat open question. This expected “magnetic

Pockels” we quickly identified as the Faraday effect, the rotation of the plane of polarization

when an linearly polarized light beam passes through a material in a magnetic field oriented

parallel to the light beam. This effect was first observed by Michael Faraday in 1845 -

September 13th, see his Diary) in a piece of glass [35,36]. This tells us that actually the

Faraday effect is the oldest nonlinear effect and September 13th the birthday of nonlinear

optics - few scientific fields can claim such an exact birthday.

The Faraday effect, or Faraday rotation, is described by the simple relation:

= V L B (11)

θ is the angle of rotation of the polarization, L the sample length, B the (component parallel to

the light beam) magnetic field and V is a phenomenological constant, the Verdet constant.

Using this equation and considering circular polarized light we could easily deduce a relation

between χeem

and the Verdet constant ([37], using different definition for χ):

V = 42 i eem /n0 (12)

Therefore the imaginary part of χeem

defines Faraday rotation - and in lossless media χeem

is

imaginary. Also, since all materials show Faraday rotation we measured V for some simple

molecules, e.g. aromatics, and obtained the Verdet constant from a theoretical value for χeem

in

good agreement with these experimental values [38]. The fact that all (isotropic) materials

show Faraday rotation indicates also the presence of χeem

(ω; ω, 0) in all materials – as said

before. And since isotropic materials have only magnetic susceptibility components xyz only

magnetic fields in the direction of light propagation will show an effect.

When we applied Eq. (12) to the values measured for the proper χeem

components obtained

in the investigation of SHG in polythiophene films, and neglecting dispersion, the

corresponding Verdet constants seemed unreasonably large as compared with the known

values for materials in current use for Faraday rotators. We therefore set out to measure the

Faraday rotation in the polythiophene films we were also investigating for SHG. One

polythiophene was highly regioregular poly(3-octyloxythiophene). Amazingly, when Palash

Gangopadhyay measured the Verdet constant for this polymer it turned out to be orders of

magnitude larger than the Verdet constant for a commercial material such as TGG (Terbium

Gallium garnet) having a V = 0.077 105 deg T

1 m

1 at 633 nm while the poly(3-

octyloxythiophene) film came with a Verdet constant of V = 2.5 106 deg T

1 m

1 at 800 nm,

still V = 106 deg T

1 m

1 at 1549 nm, far out of resonance. Although we did not understand

the underlying reason or mechanism of this giant Faraday rotation” - we still don’t - there was

a lot of enthousiasm in the group and these effects were soon confirmed for many other

polythiophenes, although - serendipity! - poly(3-octyloxythiophene) still hold the record

value. Since these conjugated polymers are often prone to spurious oxidation effects it was

gratifying to see that the Verdet constants measured were stable over months, even for films

kept in air at room temperature. This way we entered the field of conjugated polymers by

discovering rather unexpected optical properties. Suddenly our polythiophenes became basic

materials for entirely new types of applications in compact, integrated optical devices. We

were able to build magnetometers based upon these materials with a sensitivity approaching

SQUID sensitivity. Motivated by these findings Guy Koeckelberghs, our synthetic wizard in

the group, started a whole program synthesizing polythiophenes and measuring their Verdet

constants in films. Soon we discovered the important role of the polymer structure and their

behavior in films. The really large Verdet constant were only observed for highly regioregular

polythiophenes while the supramolecular structure in the film seemed to play also an

important role since the Faraday rotation is completely lost by heating the films - effects

mirrored in the film structure as determined by X-rays and AFM [39–42].

Maybe we should stress here that the Faraday effect, observed in all materials, not

necessarily chiral, is fundamentally different from the optical rotation in chiral molecules.

Optical rotation is a linear optical effect while Faraday rotation is a second-order nonlinear

effect described by a nonlinear magnetic susceptibility. Moreover due to the spatial

orientation of the magnetic field the Faraday is a nonreciprocal effect – upon reflecting the

light beam the rotation is doubled – as opposed to optical rotation. As a matter of fact this

nonreciprocal behavior is the basis for using Faraday rotation in optical rotators.

Some difficult questions remain in the field. Why is the Faraday rotation in conjugated

polymers, specifically the polythiophenes so large or, the same question, why polythiophene

films show such remarkable strong magneto-optical effects, as opposed to films of other

strongly chiral molecules, e.g. the helicenes? What are the requirements for a material to show

magnetic susceptibilities – other groups observe also these magneto-optical effects but the

relation with the molecular (super)structure remains unclear [43]. This is one of the research

topics of current research, also by former team members distributed over the globe – the group

went global.

It was quite evident that there is now a vigorous research investigating the magnetic

properties of polythiophenes, and other conjugated polymers. Here I just want to describe a

property measured and, although confirmed by other groups now, still controversial, hotly

debated and not yet published. We - the people at NIST - measured a hysteresis behavior in

the magnetization of a sample of poly(3-dodecylthiophene) below 20 K. Such a ferromagnetic

(?) transition is very hard to understand, especially since these polymers are free of spins as

measured by ESR. Even more puzzling is the fact that these phenomenon could only be

observed with a highly regioregular polymer – the regiorandom polymer showed, under

identical experimental conditions, only a paramagnetic behavior. However, I am not any

longer in the review/memoir format and just looking to the excitement of future research

carried out by enthusiastic young scientists.

5. Where next?

Although a paragraph entitled “where next” should not be expected in a review/memoir I want

to refrain from using the more conventional “Conclusion” with its inherent finality because

the work presented here is far from finished. Indeed the research initiated in our group on

nonlinear optics from chiral films still is a continuing journey and takes many unexpected new

directions.

Experiments on SHG from chiral surfaces are still refined in the work of Martti Kauranen

[44] now with his own active group in Finland. And it is a pleasure to see how Pierre-François

Brevet in Lyon is also carrying out magneto-optic research on thin films [43]. Departing from

the more molecular approach Martti Kauranen started also looking at the intricate chirality

aspects of nanoparticles and nanostructures using the new possibilities offered by

nanotechnological techniques to make a whole array of these, also entering the field of

nonlinear chiral imaging [45,46]. This is a road followed also by the group of Thierry Verbiest

and Ventsi Valev – and I feel proud that this group is the successor of my former group –

using very intricate “chiralized” nanostructured surfaces of gold or magnetic materials

contributing to the important field of plasmonics [47–49]. I see that the links between

nanophotonics, nanoelectronics and nanomagnetics are slowly emerging from these

investigations. They try in this way to realize a long-standing goal of the group to realize

metamaterials by making very special chiral nanostructures, still strongly inspired by the

seminal paper of John Pendry on “a chiral route to negative refraction” [50].

We see also new and established researchers in the field of chirality. Ron Shen using his

marvelous sum frequency generation techniques to probe chirality of many systems [51,52]

while Franz Geiger [53,54] use these techniques to obtain new insights in the chirality of

DNA. Ken Singer [55] analyzing the hyperpolarizabilities of chiral molecules, Marie-Claire

Schanne-Klein [56,57] in Paris - a birthplace of chirality - investigating nonlinear circular

dichroism, Gary Simpson [58] studying the chiral sensitivity of nonlinear optics… and a great

pleasure to see David Buckingham still very active, also in the field of nonlinear chirality

working on electro-optics [59,60]. It really was hilarious to see Ron Shen, David Buckingham

and this author at the ACS symposium on the Physical Chemistry of Chirality, San Francsico,

2000, organized by Janice Hicks, all coming with the same talk (at least in title) on SHG in

chiral media and chaired by… Peter Rentzepis who originally this experiment in 1966 [18,61].

Very new developments are also seen in the field of photonic materials where Martin

Wegener is introducing optical activity and chirality in photonic crystals [62–64].

Chirality, that “simple” lack of mirror symmetry” is still a fascinating and absorbing

concept with many questions and possibilities. Surprisingly we do not yet see effects of

chirality, more generally magneto-optics, appearing in the now thriving field of organics

photorefractivity, exemplified by the work of the group of Nasser Peyghambarian in Tucson

[65].

I want to end with a word of thanks to all my co-travelers on the challenging and

serendipitous road leading us from organic nonlinear optics, to chirality, to magneto-optics, to

organic magnetism… from synthetic chemistry, to chemical physics, optics, and, sometimes,

difficult physics. A road giving us also a preview to new aspects of nanoscience, plasmonics,

molecular electronics, metamaterials…

Thanks to Thierry Verbiest, Martti Kauranen and Jeff Maki, travelers from the first hour.

To Bertrand Busson, Sonja Sioncke, Palash Gangopadhyay, Fumito Araoka, Guy

Koeckelberghs… and all post-docs and Ph.D. students who made the “chirality group” in

Leuven such a place-to-be.

Acknowledgements

Most of our work discussed has been supported by the Katholieke Universiteit Leuven, FWO-

Vlaanderen, the Belgian Government through their IUAP programs and the Flemish

Government through their GOA and IWT programs. Support from European programs and

EOARD is also acknowledged.