Nonlinear multiphoton-absorption

This article introduces the field of nonlinear optics and the electronic nonlinear optical (NLO) response of polymers and pol5mier composites. Both second- and third-order NLO phenomena are included, with primary emphasis on harmonic generation, the intensity-dependent refractive index, and nonlinear (multiphoton) absorption effects. The beginning sections introduce the phenomena and explain how the order of the nonlinearity can be understood from a series expansion of the polarization in powers of the electric-field. In addition to listing the variety of nonlinear optical phenomena and some applications, some of the advantages of polymeric materials for NLO applications are also surveyed. 

The process of nonlinear multiphoton absorption enables spectroscopic examination of unoccupied states of solids and solid surfaces. Like its linear counterpart, multiphoton photoemission probes the initial states below the Fermi level, E, within an energy range defined by nfia — O = Eq, where n is the order of the nonlinear process, O is the work function of the sample and Eq is the positive binding energy of the initial state with respect to The MPP... 

The use of lanthanides are common for optical purposes because of their narrow and sharp bands, and distinguishable long lifetimes, accomparied by low transition probabilities due to the forbidden nature of the transitions [10-13]. Thus chromophoric sensitization of ligand to metal has been subjected to numerous theoretical and experimental investigations [14—16]. However, only limited classes of organic-lanthanide complexes have been developed and shown to display nonlinear processes [17-19]. Common nonlinear processes from lanthanide complexes include harmonic generation, photon up-conversion and multiphoton absorption induced emission. 

This chapter reviews the important aspects of multiphoton absorption sensitization of lanthanide complexes and their nonlinear behavior some typical nonlinear processes from the conversion of long-wavelength excitations to give short-wave-length emissions will be presented in Section 7.2. Their basic features and their differences will be described. 

Examples of studies on multiphoton absorption processes and nonlinear second-and third-harmonic generation processes will be discussed along with some possible radiative and nonradiative processes. The selection rules for multiphoton absorption will be mentioned in Section 7.3, and molecular examples will be shown along with their correlating photophysical properties in Section 7.4. The effect of some parameters relating to second-order activity along the lanthanide... 

The structure-property relationships are affected by three main structural elements-coordination and hence packing, which determines some of the nonlinear harmonics the degree of conjugation along the backbone and the effects of the substituents-all of which have previously been reported to affect two-photon absorption processes thus it is proposed that the same applies for other higher-order systems. However, it is rare for reports of these high-order multiphoton absorption processes displayed by organic metal complexes to be accompanied with detailed molecular structure to show the correlation between the photoproperties. 

Another example of a different type of correlation of structural to photophysical properties is shown in a study of a unique terbium compound [63]. This compound will be briefly discussed and is depicted in Figure 7.9 with its nonlinear emission properties with excitation at 800 nm. The photophysical properties are atypical and rather extraordinary due to the unusual molecular structure of the co-crystallization compound (4) of the organic chromophore and the terbium salt This compound shows both multiphoton absorption induced green f-f emission from the terbium ion as well as second-harmonic generation. However, unlike previously... 

LeBozec and co-workers have reported nonlinear behavior in a series ofterpyri-dyl and dipicolinic acid complexes, with further studies on these complexes by Maury and co-workers [83, 84]. Their research was on new molecular materials for optoelectronics, with studies based on octupolar nonlinear optical molecules showing that molecular quadratic hyperpolarizability values were strongly influenced by the symmetry of the complexes [85]. Other studies on organic-lanthanide complexes with nonlinear optics have also reported second- and third-harmonic generation behavior with simultaneous multiphoton absorption properties [50]. Such studies have shown the importance of coordination chemistry as a versatile tool in the design of nonlinear materials. 

Although the peak power of the pump laser must be high, the power of the probe laser should be kept low to avoid nonlinear effects (multiphoton absorption, stimulated Raman scattering see Section 3.9.2) and dielectric breakdown (ionization of molecules) that damage the sample. Thus, signal averaging of many pulses (high repetition rate) is made to obtain acceptable S/N ratios. Multichannel detectors such as an intensified silicon photoiode... 

Nonlinear optical responses of molecules near fractal metal clusters are expected to be enhanced by many orders of magnitude. They are proportional to a high-order function of the local field [361], Strong enhancement of multiphoton absorption potentially exists because simultaneous absorption of n photons scales the intensity (/) to the power of n (/"). Thus, net multiphoton absorption is proportional to the average value of /" over a given volume. It can therefore approach a value that is orders of magnitude larger. 

We have seen how the molecular properties in nonlinear optics are defined by the expansion of the molecular polarization in orders of the external electric field, see Eq. (5) beyond the linear polarization this definition introduces the so-called nonlinear hyperpolarizabilities as coupling coefficients between the two quantities. The same equation also expresses an expansion in terms of the number of photons involved in simultaneous quantum-mechanical processes a, j3, y, and so on involve emission or absorption of two, three, four, etc. photons. The cross section for multiphoton absorption or emission, which takes place in nonlinear optical processes, is in typical cases relatively small and a high density of photons is required for these to occur. 

Nonlinear Absorption. The nonlinear absorption contains the saturation of strongly allowed one-photon transitions and multiphoton absorption. These nonlinear optical processes were detected as an anharmonic thermal grating [116, 117]. 

The multiphoton absorption cycle was confirmed by a comparison of the temporal profile of the fluorescence of anthracene-doped polystyrene films with computational results based on the cyclic process [97]. In the computational studies, the ground state, first excited singlet state, and lowest triplet state have been included. The calculated temperature rise during the laser pulse depends nonlinearly on the laser intensity. Rapid internal conversion within the triplet manifold is the most effective mechanism for depositing heat at the irradiated surface. 

One-photon spectroscopy is due to the linear term, whereas the nonlinear terms lead to the simultaneous absorption of two or more photons. Although the theory was worked out almost 50 years ago, observation of multiphoton absorption was made feasible only after the development of lasers. This chapter deals with the application of two-photon excitation (TPE) to kinetic studies in low-pressure gas-phase samples. For a systematic, extensive discussion of spectroscopic applications, one of the excellent reviews available should be consulted. ... 

The few examples shown above illustrate that nonlinear spectroscopy represents an important branch of laser spectroscopy of atoms and molecules. Its advantages are the Doppler-free spectral resolution if narrow-band lasers are used and the possibilities to reach high-lying states by multiphoton absorption with pulsed or cw lasers. Because of its relevance for molecular physics, numerous books and reviews cover this field. The references [203, 205, 281-289] represent only a small selection. 

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