Themed issue: nonlinear optics. The evolving field of nonlinear optics—a personal perspective

Seth R. Marder

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The field of organic nonlinear optics (NLO) started shortly after the invention of the laser with the observation that crystals of some organic compounds could exhibit second-harmonic generation. Interest in the field increased significantly in the late 1970s when various groups developed techniques to accurately measure hyperpolarizabilities, elaborated theoretical models and quantum-chemical calculation tools to both predict and understand the origin of NLO phenomena, and implemented strategies to obtain non-centrosymmetric materials by electric-field-induced poling. Over the past twenty years or so the field has had its ups and downs, as competing technologies have advanced, as described below. As new discoveries were made in the discipline of organic nonlinear optics, inevitably the research focus has evolved. Some areas that at one point were “hot”, such as frequency doubling of 800 nm light to 400 nm, have been rendered almost inconsequential when inexpensive diode lasers became available. Other applications that were once considered impossible have become practicable due to subsequent discoveries and importantly, the systematic improvement of not only molecular properties, but materials properties as well. In other cases processes that were at first deemed “parasitic”, in particular, two-photon absorption (which is a potential loss mechanism in optical waveguides), have found potential applications and have now become areas of intense research activity. Indeed most industrial firms had abandoned their efforts in nonlinear optical polymers, only to see the field press forward, predominantly in academic institutions. While the field has evolved in many ways it continues to advance, and, in fact, has led to commercialized products—for example, for multiphoton imaging and microfabrication and for THz generation—leading to renewed interest by both large and small companies.

Given the evolution of the field and some of the recent advances, now seems like a good time to take a snapshot of the field: the papers in this themed issue of the Journal of Materials Chemistry, will, I hope, provide readers with a glimpse of research that defines the state-of-the-art in organic nonlinear optics. The reader will see that over the past fifteen years very significant advances have been made in the area of electro-optic polymers. Materials that combine very large electro-optic coefficients, low loss, and high temperature stability are now a reality. The figures-of-merit for these polymeric materials have reached the point that these materials are now being carefully examined for a variety of defense and civilian applications including broadband THz generation and on-chip communication. In the late 90s few people would have expected such performances would be realized, but through a combination of systematic discoveries and serendipity, aided by significant progress in computational chemistry, the Dalton (see for example DOI: 10.1039/b905368a in this issue) and Jen (see for example: DOI: 10.1039/b907173c) groups, in particular, have made advances that, in my view, constitute breakthroughs in materials performance (Fig. 1).

Fig. 1 Example of a high performance electro-optic chromophore described in DOI: 10.1039/b905368a from the Dalton group.

Another area that has progressed significantly is the design and application of molecules with large two-photon absorption cross-sections (and, to a lesser extent, multi-photon absorption cross-sections). My colleagues and I have been fortunate to have made some early contributions to this field; in 1998 we suggested that quadrupolar π-conjugated molecules were promising candidates to examine in this regard.¹

This work, as well as studies on donor–acceptor molecules by the Prasad and Reinhardt groups, stimulated the community to flesh out these design guidelines carefully and encouraged people to see two-photon absorption as a useful property, rather merely a parasitic loss mechanism (as noted above). Now dipolar, quadrupolar (see for example article by Belfield et al. DOI: 10.1039/b820950b), and octupolar (see for example article by Cho et al. DOI: 10.1039/b906361g), materials have been extensively examined and materials with very large cross-sections compared to those previously known are routinely available (Fig. 2).

Fig. 2 Generic structures of dipolar, quadrupolar and octupolar structural motifs.

As a consequence, multiple companies are marketing materials, services, and instruments for multi-photon imaging and microfabrication and, while it is not yet a routine technique, various demonstrations by groups all over the world point to the utility of two-photon microfabrication for fabrication a diverse set of structures ranging from photonic crystals to MEMS devices in various materials. Multi-photon-absorbing materials are also finding use in optical pulse suppression systems (see for example Padhilla et al. DOI: 10.1039/b907344b), particularly when the initial multi-photon absorption process provides a gateway into a subsequent strong one-photon transition which can arise due to excited state absorption (from either singlet or triplet states) or from radical ions generated by excited-state electron transfer.

A third area that had been considered “a bridge too far” is the use of conjugated organic materials for all-optical switching applications. In the 1990s the consensus was that the optical nonlinearities of organic materials were too small and the linear and nonlinear losses were too high for these materials to be of interest for any practical device. To all intents and purposes, this area of research lay fallow for many years, with the exception of limited activity by a few groups. Even now this area of research is not being pursued actively by many groups, but I am of the opinion that the time is now right for studies based upon using the real part of the third-order susceptibility of materials to be given increased attention. In this issue we see some reports on materials that may have some promise for all-optical switching, [see for example Bubeck et al. (DOI: 10.1039/b908033c) and Ohira and Bredas (DOI: 10.1039/b906337d)] and I expect that over the next two to three years further reports on promising materials will appear. In addition, new concepts for device fabrication based upon confinement of optical fields in nanoscale optical structures offer the possibility of exploiting large enhancements of effective nonlinearities that could be observed in such structures. The combination of advances in low-loss, third-order nonlinear optical materials and novel devices design could finally lead to the development of practical all-optical switching elements.

All of these advances create significant opportunities, but I would be remiss if I failed to point out challenges that must be met if we, as a community, are to fully realize the potential of organic nonlinear optical materials. Some of these significant challenges have persisted for years; while advances have been made, further progress is needed. For some applications, such as those based upon electro-optic polymers, two-photon absorbing materials, and all-optical switching materials, optical loss and optical damage remain significant issues. Loss can arise not only from absorption, but also from scattering due, for example, to inhomogeneities in the materials. Optical damage has been to some extent been ameliorated in certain systems by separating molecular components of the materials through dendrimer encapsulation and by addition of singlet oxygen scavengers. Even taking these advances into account, my view is that significant work over the next several years will need to be dedicated to addressing these issues, along with developing processing techniques to utilize nanofabricated devices. This will require a sustained effort with adequate funding if the field is to move forward. However, in contrast to the 1980s, there have recently been demonstrations of unprecedented materials and device performances (not simply the promise of such performance), indicating that such investments should now be much less risky.

In conclusion, the field of organic nonlinear optics has matured significantly since the late 1980s and early 1990s. As is the case in many fields, the initial enthusiasm and hype gave way to a more realistic viewpoint. The field contracted quite a bit, but those who remained active and those who entered the field recently have helped to redefine it, and have several significant successes to show for their efforts. The field of organic nonlinear optics remains healthy and opportunities still exist for new researchers with varied expertise to make significant contributions for the foreseeable future.

Acknowledgements

I would like to thank my current and former colleagues and in particular Drs Stephen Barlow and Mariacristina Rumi for their many contributions to the development of this editorial and to the many authors who have contributed to this themed issue.

Reference

1. M. Albota et al., Science, 1998, 281, 1653–1656.

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