Colorant, Nonlinear Optical
Light is composed of an electromagnetic field that can interact with the elementary charges in matter, whose response can in turn influence the behavior of the other light waves. When light passes through any material, its electric field induces changes in the polarization of the material’s molecules. In “linear” materials the degree of electron displacement, characterized by the linear polarizability α, is proportional to the strength of the applied electric field.
The distinguishing characteristic of nonlinear optical colorants is that their polarization response to optical waves depends nonlinearly on the applied electric field strength. This can result in the emission of new radiation fields which are altered in phase, frequency, polarization, or amplitude relative to the incident optical radiation. Many of these effects are sensitive to specific characteristics of the local optical properties and interfaces. Multi-photonic absorption can also result in electronic excitations that for a given incident light beam are more strongly localized in space than those resulting from linear absorption processes.
Nonlinear optical materials continue to attract the interest of both industrial and academic researchers due to their many versatile applications in the domain of optoelectronics and photonics.
Over the past decades, a variety of materials have been investigated for their nonlinear optical properties, including inorganic materials, coordination and organometallic compounds, liquid crystals, organic molecules, and polymers [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Nonlinear optics was born in 1875 when John Kerr discovered that the refractive index of glass and organic liquids could be altered by an applied electric field with the induced changes being proportional to the square of the applied field strength. Kerr measured, for the first time, the third-order nonlinear susceptibility, or what is today called the Kerr effect or the electro-optical effect. This discovery was shortly followed in 1883 by the observation of a similar but linear electric field effect in quartz; this latter process now referred to the Pockels effect. These nonlinear effects had limited use until the invention of the laser in 1960 by Maiman and, in the following year, the observation by Franken et al. of second harmonic generation in quartz. Following these events, the field of nonlinear optics developed explosively throughout the 1960s, highlighted by the work of Bloembergen and coworkers in exploring the full range of nonlinear optical responses of material systems and that of Buckingham and colleagues in exploring nonlinear processes in atoms and molecules of interest to chemists .
Initially, optical communications technology used components almost exclusively based on inorganic NLO materials. At the time it was thought that these compounds were more suitable than organic compounds due to their greater chemical, photochemical, and physical stability and ease of processing. The two main classes of materials investigated were inorganic crystals of lithium niobate (LiNbO3) and barium titanate (BaTiO3) or semiconductors such as gallium arsenide (GaAs) or zinc selenide (ZnSe). Electro-optical devices that use lithium niobate have been on the market for several decades. However, these crystals have several drawbacks: high-quality single crystals are difficult to grow, are expensive, and are not easy to incorporate into electronic devices. During the 1980s it became clear that organic materials might be a better choice for use in nonlinear optical applications. A lot of organic compounds exhibit extremely high and fast nonlinearities, much better than those observed in inorganic crystals. In addition, due to the versatility of organic synthesis, their nonlinear optical properties can be modified depending on the desired application. Furthermore, organic chromophores can be incorporated into a variety of macroscopic structures such as crystals, Langmuir-Blodgett (LB) films, self-assembled films, poled polymers [1, 2, 8, 9], zeolites , and nanofibers .
At present, several organic systems (molecular as well macroscopic) with sufficiently high nonlinearities have been developed. The focus of the research in this area seems to be shifting to the optimization of a variety of other parameters, i.e., thermal and chemical stability and optical loss. Also, new approaches to design efficient nonlinear optical materials have lately emerged [7, 9].
Principles and Origin of Nonlinear Optical (NLO) Effects
For practical applications, organic second-order NLO materials can be considered to be dipolar molecules organized into noncentrosymmetric lattices. Therefore, high β values are a necessary but not sufficient condition in order to obtain efficient NLO second-order materials [5, 8, 9].
Nonlinear Optical Effects and Their Practical Applications
First-order susceptibility χ (1) : Refraction is the change in direction of a light wave due to a change in its phase velocity. This phenomenon is the foundation of most geometric optical effects and has applications in optical fibers and lenses.
- Second-order susceptibility χ (2) :
Optical second harmonic generation (SHG) is the nonlinear conversion of two photons of frequency ω into a single photon of frequency 2ω (ω + ω → 2ω) which, in the electric dipole approximation, requires a noncentrosymmetric medium. This effect was first demonstrated by Franken et al.  in 1961 and has applications in the frequency doubling of lasers. More recently, SHG-imaging has developed through the last decade as a progressively popular analytical technique especially for high-resolution optical microscopy for biological imaging. In fact nonlinear optical imaging has revolutionized microscopy for the life sciences due to the capacity of this technique to produce high-resolution images from deep inside biological tissues .
Frequency mixing: an NLO material may be used to add frequencies of two input waves to produce a single wave, whose frequency is their sum: ω1 + ω2 → ω3 – or difference: ω1 − ω2 → ω3. This effect has application in the frequency conversion of lasers and various nonlinear spectroscopic techniques.
Parametric amplification: an NLO material may transmit two light waves of different frequencies, with one light wave leaving the material amplified at the expense of the other. This effect allows the production coherent light at wavelengths for which no lasers are available and is frequently used in high-resolution spectroscopy.
Linear electro-optical effect (Pockels effect) is the change of refractive index of an NLO material which occurs on the application of an electric field, the extent of the change being related to the strength of the field (ω + 0 → ω). This effect has application in waveguides and electro-optical modulators.
- Third-order susceptibility χ (3) :
Kerr effect is a change in the refractive index of a material in response to an applied electric field. This effect has applications in optical transistors and image processing.
Third harmonic generation: light of frequency ω enters the material and leaves with frequency 3ω, (ω + ω + ω → 3ω). This effect has applications in optical image processing and scanning microscopy .
Two-photon absorption is the induced electronic transition provoked by the near simultaneous absorption of two incident photons. This effect was first studied theoretically by Maria Goeppert-Mayer already in the 1931 and is often used in nonlinear microscopy to allow deeper penetration of the incident light, avoid photobleaching, and increase spatial resolution .
Nonlinear Optical Materials: Classes, Advantages, and Limitations
The search for novel chromophores for optoelectronics and nonlinear optics (NLO) is one of the main goals of modern materials science and physics. Their practical applications require not only an appropriate design but also the relevant macroscopic properties of newly established materials. The even susceptibilities χ2, χ4, etc., are only nonzero in materials lacking a center of symmetry. Therefore, the arrangements of the molecules on a macroscopic level are vitally important to NLO activity. For example, if a crystalline material with a large value of β crystallizes in a centrosymmetric structure, the nonlinear responses of the individual molecules will cancel each other out, and there will be no resultant NLO activity. The same applies to an organic compound with an amorphous structure. If there is no order in the molecule at all, the statistical average of the NLO responses of the molecules will be zero or nearly zero, and again, the material will be not be an active NLO material. Thus to use the strong hyperpolarizabilities often found in organic molecules in a bulk phase, the constituent molecules must be somehow noncentrosymmetrically ordered. The main types of ordered assemblies that have been investigated for use in NLO materials are the following [5, 8, 9, 10, 11, 12, 13].
These materials were the first to be studied since the observation by Franken et al.  of second harmonic generation in quartz. Several ionic crystals such as potassium dihydrogen phosphate (KDF=KH2PO4), lithium niobate LiNbO3, barium sodium niobate (Ba2NaNb5O15), and β-barium borate (BBO=BaB2O4) were developed and are currently used for several optical applications such as frequency converters, electro-optical modulators, and optical switches. The main advantages of inorganic materials are their high stability, high mechanical strength, and high nonlinear optical susceptibilities. However, the growth of these crystals is time consuming (frequently requiring 1–8 weeks) and the crystals tend to be hygroscopic, and expensive, somewhat difficult to integrate into electronic devices and are often limited by the low response speeds.
Coordination and Organometallic Complexes
Octopolar NLO chromophores are not so commonly investigated compared to dipolar ones. Nevertheless, their advantage is that they show the same optical transparency as their linear analogues but their second-order response is higher. Due to the fact that they possess zero overall dipole moment, they can often be oriented in the bulk phase in a manner that their molecular polarizability is additive. On the other hand, not possessing a permanent dipole moment invalidates their use in some electro-optical applications. The usual way to design SHG octopolar molecules consists in the synthesis of substituted trigonal or tetrahedral π-conjugated systems that allows an efficient charge transfer from periphery to the center of the molecule (Fig. 2b).
Oriented Guest-Host Polymers
Oriented Side-Chain and Main-Chain Polymers
Nonlinear optical chromophores can be also incorporated into a polymer by covalently attaching the chromophores to a polymeric backbone as part of the side chain (side-chain polymers) or by incorporation of the chromophores into the backbone of the polymer (main-chain polymers).
Another approach to organize molecules has been to incorporate organic NLO chromophores into noncentrosymmetric Langmuir-Blodgett films. The Langmuir-Blodgett film technique is used to build up ordered assemblies of molecules onto a substrate from a floating monolayer on a liquid, usually water. The molecule must be amphiphilic, that is, hydrophilic at one end and hydrophobic at the other, so that the molecules in the monolayer at the water’s surface have uniform orientation. This approach offers the advantage of much greater chromophore alignment and chromophore density. However, these films often have poor optical quality and poor temporal stability and are often very fragile.
Very recently, nanofibers of poly(L-lactic) acid (PLLA) produced by the electrospinning technique, in which donor-acceptor organic compounds such as 2-methyl-4-nitroaniline, urea, and β-glycine were imbedded, exhibit a permanent nonvanishing quadratic nonlinear susceptibility. The nonlinear optical properties displayed indicate that a noncentrosymmetric polar state was achieved and maintained a long time, allowing the use of otherwise centrosymmetric organic materials. Moreover, it was proved that the SHG efficiency of these fibers strongly depends on the diameter of the nanofibers and can be increased up to an order of magnitude by controlling the electrospinning processing parameters .
As most of the donor-acceptor organic molecules with large hyperpolarizabilities tend to crystallize in centrosymmetric structures which invalidate their use in quadratic nonlinear optical applications, the inclusion of these molecules in electrospun nanofibers may provide a means of overcoming this limitation for any donor-acceptor organic molecules with delocalized π electrons. The results of this work could have a direct impact on the design of novel nanodevices for a variety of nanophotonic applications (e.g., electro-optical transducers, pyroelectric sensors, optical frequency converters).
Experimental Methods for the Determination of Second-Order Nonlinear Effects
Future and Perspectives
From the proceeding description, it is clear that the molecular design of molecules with strong nonlinear optical responses has reached a high level of sophistication. Important advances are likely to come from breakthroughs in methods that are able to make use of these strong individual molecular responses by incorporating in restricted environments that break their tendency to aggregate in centrosymmetric forms. The future of organic nonlinear optical materials is undoubtedly a bright one.
- 1.Chemla, D.S., Zyss, J.: Nonlinear Optical Properties of Organic Molecules and Crystals, vol. 1 and 2. Academic Press, New York (1987)Google Scholar
- 2.Prasad, P.N., Williams, D.J.: Introduction to Nonlinear Optical Effects in Molecules and Polymers, pp. 132–174. Wiley, New York (1991)Google Scholar
- 3.Zyss, J.: Molecular Nonlinear Optics: Materials, Physics and Devices. Academic Press, Boston (1994)Google Scholar
- 4.Nalwa, H.S., Miyata, S. (eds.): Nonlinear Optics of Organic Molecules and Polymers. CRC Press, New York (1997)Google Scholar
- 6.Meyers, F., Marder, S.R., Perry, J.W.: Advanced polymeric materials - high performance polymers. In: Interrante, L.V., Hampden-Smith, M.J. (eds.) Chemistry of Advanced Materials: An Overview, pp. 207–269. Wiley-VCH, New York (1998)Google Scholar
- 11.Di Bella, Dragonetti, C., Pizzotti, M., Roberto, D., Tessore, F., Ugo, R.: Coordination and organometallic complexes as second-order nonlinear optical molecular materials. Top. Organomet. Chem. 28, 1–55 (2010)Google Scholar
- 14.Franken, P.A., Hill, A.E., Peters, C.W., Weinreich: Generation of optical harmonics. Phys. Rev. Lett. 7, 118–119 (1961)Google Scholar
- 17.Varanasi, P.R., Jen, A.K.-Y., Chandrasekhar, J., Namboothiri, I.N.N., Rathna, A.J.: The important role of heteroaromatics in the design of efficient second-order nonlinear optical molecules: theoretical investigation on push − pull heteroaromatic stilbenes. J. Am. Chem. Soc. 118, 12443–12448 (1996)CrossRefGoogle Scholar
- 20.Raposo, M.M.M., Fonseca, A.M.C., Castro, M.C.R., Belsley, M., Cardoso, M.F.S., Carvalho, L.M., Coelho, P.J.: Synthesis and characterization of novel diazenes bearing pyrrole, thiophene and thiazole heterocycles as efficient photochromic and nonlinear optical (NLO) materials. Dyes Pigments 91, 62–73 (2011)CrossRefGoogle Scholar