Motivation

It has become increasingly recognized [1] that “photonic integration” is an important next step in the evolution of computing, telecommunications, sensing, transportation, medical, defense, and entertainment technologies. Such integration permits the best features of electronics and photonics to be exploited for information technology applications. There are important technological drivers of photonic/electronic integration, including realization of improved bandwidth and thermal management – transporting and manipulating information using photons avoids the high-frequency resistive losses and heating associated with movement of electrons in metal. Photonic/electronic integration is also evolving to include “chipscale” integration wherein both electronic and photonic circuitries are integrated on to the same chip, analogous to complementary metal-oxide semiconductor (CMOS) electronic integration, which has revolutionized computing. Potential advantages of chipscale integration include far-reaching improvements in size, weight, power consumption, performance, reliability, and cost. While chipscale photonic/electronic integration is not likely to be monolithic, as in CMOS electronic integration, and the problems to be faced will certainly be challenging, there can be little doubt that it will ultimately occur and will have considerable societal and economic impact. Such integration has been greatly advanced by recent developments in the field of silicon photonics, plasmonics, and metamaterial device architectures [2 to 21]. For example, the high index of refraction of silicon has permitted a striking reduction in the size of photonic circuits, making the dimensions of these circuits more compatible with chipscale integration. Further reductions in circuit dimensions appear possible by exploiting plasmonics.

A key component of photonic/electronic integration is the interconversion of signals between the electronic and photonic domains. This is where electro-optics comes into play. An electro-optic (EO) material is one in which the electrical fields associated with photons and electrons can communicate through a highly hyperpolarizable (easily perturbed by electric fields) charge distribution. For an electro-optic material to be optimum for the transduction of electronic signal information into photonic signal information, the charge distribution should be easily perturbed by small electric field potentials (ideally by millivolt electric field potentials to minimize power consumption) and should have very fast (ideally femtosecond) response to time-varying electrical fields. It is these features that have attracted attention to organic EO materials. The fundamental response time of conjugated π-electron systems is the phase relaxation time and is almost universally of the order of tens of femtoseconds.

General conclusions

In the preceding chapters, an introduction has been provided into the fundamentals and state-of-the-art of organic nonlinear optical materials and devices with particular emphasis on electro-optic, second-harmonic generation, difference-frequency generation, optical rectification, and photorefractive materials, devices, and applications related to organic second-order nonlinear optical materials. Organic electro-optic materials exhibit extremely attractive features with respect to temporal response to time-varying electric fields and with respect to the magnitude of second-order optical nonlinearity, which can translate into energy-efficient devices. Organic materials also are attractive in providing a wide variety of processing options including crystal growth (from solution, melt, and vapor phase), sequential synthesis/self-assembly (both Langmuir–Blodgett and Merrifield methods), electric field poling of macromolecular materials near their glass transition temperature, and even laser-assisted electric field poling. Organic materials are amendable to nano-imprint lithography, leading to stamping out complex circuitry, and they are amenable to lift-off techniques for production of conformal and flexible devices. Organic second-order nonlinear optical materials are compatible with a wide variety of materials including semiconductors, metal oxides, and metals; this feature greatly facilitates the production of hybrid devices including those based on silicon photonics, plasmonics, photonic crystals, and metamaterials. They have been integrated into stripline and resonant device structures, cascaded prism device structures, and even structures for slow wave propagation. The low dielectric constants of organic electro-optic materials can also be an advantage for certain applications such as sensing.

Of course, there is a great diversity of organic nonlinear optical materials with a corresponding diversity of properties. However, organic second-order nonlinear optical materials do not typically possess the extremely low optical loss and high optical damage threshold of crystalline inorganic materials. Nevertheless, they frequently exhibit lower optical loss than inorganic electro-absorptive materials and much higher dielectric breakdown properties than semiconductor materials.

The performance properties of organic nonlinear optical materials continue to evolve at a significant rate. However, the commercialization of organic nonlinear optical materials is very limited and immature relative to inorganic materials, and thus the cost of devices based on organic materials does not benefit from large-scale production. Moreover, device engineering utilizing organic nonlinear optical materials is relatively immature compared with the device engineering focused on inorganic materials.

While literally the word photorefraction may describe all kinds of photo-induced changes of the refractive index of a material and therefore any photo-induced phase grating would belong to this category, it has become customary in the literature to consider only a smaller class of materials as being photorefractive. These materials possess two important properties: they are photoconductive and exhibit an electro-optic effect. Photoconductivity ensures charge transport, resulting in the creation of a space-charge distribution under inhomogeneous illumination. The electro-optic effect translates the internal electric fields induced by the inhomogeneous space-charges into a modulation of the material refractive index. This is the main mechanism for photorefraction in inorganic and organic crystals [1]. In polymers and liquid crystals, the concept of photorefraction has been expanded to include refractive index changes governed by a field-assisted molecular reorientation of the chromophores. The photorefractive effect is also distinguished from many other mechanisms leading to optically induced refractive index gratings by the fact that it is an intrinsically non-local effect, in the sense that the maximum refractive index change does not need to occur at the spatial locations where the light intensity is largest.

In the strict sense mentioned above, the photorefractive effect was first observed in the mid-1960s by Ashkin and co-workers [2]. They found that intense laser radiation focused on ferroelectric LiNbO3 and LiTaO3 crystals induced semi-permanent index changes. This phenomenon was unwanted for their purposes, and therefore they referred to it as “optical damage”. However, the potential of this new effect for use in high-density optical storage of data was soon realized by Chen and co-workers [3]. The effect later became known as the photorefractive effect, and it is understood as a modulated refractive index change. Although the photorefractive effect was first discovered in inorganic-ferroelectric materials, in the past few years highly polarizable and photoconductive organic crystals and polymeric materials with extended π-electron systems have presented themselves as possible candidates for photorefractive applications.

The photorefractive process, which culminates with the formation of the phase grating, is described by the mechanisms shown in the schematic diagram of Fig. 11.1. The three most important properties that a material must fulfill are depicted in the figure in the shaded boxes: optical absorption, charge transport, and electro-optic effect or field-assisted molecular reorientation.

Nonlinear optical frequency conversion

As discussed in Chapter 2, there are many different possibilities for converting optical frequencies to other frequencies or even static fields in second-order nonlinear optical materials, such as sum- and difference-frequency generation, including second-harmonic generation and optical rectification (see Fig. 2.2). The big advantage of organic nonlinear optical materials compared with inorganic ones is high nonlinear optical figures-of-merit, reflecting their almost purely electronic response to external fields, as discussed in Section 3.2. Because of this, the best organic materials exhibit considerably higher second-order nonlinear optical susceptibilities compared with the best inorganic materials. This is illustrated in Fig. 10.1, which shows figures-of-merit for second-harmonic generation d2/n3 versus transparency range for various organic and inorganic crystals. One can clearly see that the nonlinear optical figures-of-merit of organic materials can be several orders of magnitude higher than in the best inorganic materials. This makes organic materials extremely attractive for nonlinear optical applications.

In the 1980s, the most attractive frequency conversion applications included frequency doubling because of the above advantages and because of the interest in generating blue or green coherent light by using widely available (near-)infrared laser sources, such as diode lasers [1], Ti:sapphire lasers, and Nd:YAG lasers. The early organic materials considered were transparent in the visible. Later on, organic materials with much higher nonlinear optical susceptibilities were developed, but these are no longer transparent in the visible (see Fig. 10.1) and therefore second-harmonic generation with these materials is of limited applicability. At present, the most attractive frequency-conversion applications with organic materials include infrared and far-infrared light generation, as well as generation of electromagnetic waves in the THz frequency range. In this section we mainly describe infrared frequency conversion possibilities with the best organic crystals, such as DAST, DSTMS, and OH1 (see Chapter 6 for details on these materials), and in the next section we look at THz generation with these and other organic materials.