Excited state enhancements, nonlinear optics

A novel pump-probe degenerate four-wave mixing technique was introduced to study the excited state enhanced nonlinear optical response in polymers [7]. In this technique (see Fig, 3) a picosecond laser pulse at w, is divided into three beams... 

On the basis of the molecular orbital energies, all the non-carbon clusters considered seem to be better electron donors and better electron acceptors than Ceo- They also possess lower electronically excited states and therefore should display enhanced nonlinear optical properties. Interestingly, the energy gap, which plays a direct role in the determination of the metallic behavior of the system, changes negligibly when one considers the HF level data for the semiconducting Si and Ge or the metal Sn type clusters. 

Figure 6.1 Nonlinear optical responses, (a) Second-order SF generation, the transition probability is enhanced when the IR light is resonant to the transition from the ground state g to a vibrational excited state V. CO is the angular frequency of the vibration, (b) Third-order coherent Raman scheme, the vibrational coherence is generated via impulsive stimulated...

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

A high intensity of the incident radiation enhances the probability for simultaneous multi-photon interactions with a single molecule, i.e. two or more photons are annihilated and absorbed by the molecule in a single quantum mechanical process. The frequency of the scattered photon does in such cases not have to be equal that of the absorbed photons, e.g. two quanta with frequency m may be annihilated, creating a third photon with frequency 2a). As indicated in Fig. 2, the system returns to its ground state 0> after the interaction has taken place, and the intermediate virtual state is separated from the first excited state by an energy A . This is an example of a nonlinear optical process known as second-harmonic generation, which can... 

In the above discussion, we have only considered the effects due to the CTE-CTE repulsion, which contribute to the resonant nonlinear absorption (as well as to other resonant nonlinearities) by the CTE themselves. Here, however, we want to mention a more general mechanism by which the nonlinear optical properties of media containing CTEs in the excited state can be enhanced. This influence is due to the strong static electric field arising in the vicinity of an excited CTE, If, for example, the CTE (or CT complex) static electric dipole moment is 20 Debye, at a distance of 0.5 nm it creates a field Ecte of order 107 V/cm. Such strong electric fields have to be taken into account in the calculation of the nonlinear susceptibilities, because they change the hyperpolarizabilities a, / , 7, etc. of all molecules close to the CTE. For instance, in the presence of these CTE induced static fields, the microscopic molecular hyperpolarizabilities are modified as follows... 

One subject that attracted much attention is the nonlinear optical properties of these semiconductor nanoclusters [17], The primary objective is to find materials with exceptional nonlinear optical response for possible applications such as optical switching and frequency conversion elements. When semiconductors such as GaAs are confined in two dimensions as ultrathin films (commonly referred to as multiple quantum well structures), their optical nonlinearities are enhanced and novel prototype devices can be built [18], The enhancement is attributed mostly to the presence of a sharp exciton absorption band at room temperature due to the quantum confinement effect. Naturally, this raises the expectation on three-dimensionally confined semiconductor nanoclusters. The nonlinearity of interest here is the resonant nonlinearity, which means that light is absorbed by the sample and the magnitude of the nonlinearity is determined by the excited state... 

Fullerene-Doped Polyvinylcarbazole. Fullerenes are known to be good electron acceptors. In the presence of electron donors such as aromatic amines, weakly bonded charge-transfer complexes can be formed [115]. Through virtual excitation, the existence of charge-transfer interaction can enhance the second-order optical nonlinearity of fullerenes [116], With direct excitation, excited state electron transfer between fullerenes and various electron donors such as aromatic amines [115,117], semiconductor colloids [118], porphyrin [119], and polymers [101, 103, 120] can occur. This electron-accepting property led to the development of fullerene-doped polymeric photoconductors [101,103]. 

To fulfill the need for understanding what structures will allow enhancement of optical nonlinearity, we have coupled ab-initio theoretical calculations of optical nonlinearity with synthesis of sequentially built and systematically derivatized model compounds, and the measurement of their optical nonlinearities. Now I would like to discuss very briefly our efforts to compare microscopic optical nonlinearities. An expression, similar to the expansion of the bulk polarization as a function of the applied field, can be written for the induced dipole moment. Naturally, the nonlinear term Y, for example, is the third derivative of the induced dipole moment with respect to the applied field. Also, using the Stark energy analysis, one can write the nonlinear terms 3 (and Y) as a sum over all excited states terms involving transition-dipoles and permanent dipoles, similar to what one does for polarizability. Consequently, the two theoretical approaches are (i) the derivative method and (ii) the sum-over-s1j tes method. We have used the derivative method at the ab-initio level. We correlate the predictions of these calculations with measurements on systematically derivatized and sequentially built model compounds. Some conclusions of our theoretical computations are as follows ... 

A series of new ( )-4,4 -bis(diphenylammo)stilbene derivatives have been synthesized to investigate nonlinear absorptivities with attention paid to the peripheral substituent effect and multibranched modification effect by the open aperture femtosecond Z-scan technique and the nanosecond nonlinear optical transmission (NET), respectively [36]. Two-photon fluorescence for ( )-4,4 -bis(di-phenylamino)stilbene in THE was detected. It was found that substituent group attached to the periphery of BDPAS has no obvious contribution to TPA enhancement and that the dramatic increase in effective TPA cross sections of multibranched samples in nanosecond regime strongly suggested their larger excited-state absorption. 

We first take a brief look at the recent progress of experimental methods in a conceptual manner, and then consider what and how we can do theoretically to enhance the progress. Before starting though we note that the present chapter is not dedicated to an introduction to the vast field of photochemistry or excited-state chemistry, in which usually rather weak optical source is applied in a level of weak perturbation and create an excited state with a timed wavelength resonant to the relevant energy gap. Also, the recent extensive progress in nonlinear optics and its applications to molecular science will be mentioned here. 

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