Nonlinear transmission

Figure 21.3 Nonlinear transmission of silver -dendrimer nanocomposites Ag(0) E at 532 nm. Pulse repetition rate- 10 Hz. Inset nonlinear transmission results for Ag(0) E in a concentration of 2.95x 10 4 mole/kg at 532 nm, with the variation of the pulse repetition rate t indicates the specific repetition periods.

Figure 21.4 Nonlinear transmission for the internal type [70, 77] samples Ag(0) i when probed at 532 nm.

In a separate study, the cross section of diphenylbutadiene in chloroform has been measured at 532 nm by two different methods and reported to be 40 8GM by degenerate four-wave mixing, and 34 12GM by nonlinear transmission [66]. It should be pointed out that both results were obtained using ns excitation pulses, but that the cross sections obtained by the two methods are comparable one to the other and on the same order of the cross sections listed earher (although those were obtained in a different wavelength range). 

The shape of the Z-scan curve can be modified if a nonlinear absorption or nonlinear transmission (absorption bleaching) takes place in the sample, e.g., due to the presence of an imaginary part of 3) of the material. The curves then become asymmetrical due to increased absorption or transmission when the sample is close to the focal plane. By analyzing the shape of such a modified Z-scan curve, one can determine the nonlinear absorption coefficient f32 or the related imaginary part of 3). Alternatively, to determine the nonlinear absorption properties of a sample, the total transmission through the sample can be monitored, i.e., the total intensity of the transmitted beam can be measured without an aperture, as a function of the sample position with respect to the focal plane. Such an experiment is usually referred to as an open aperture Z-scan. It is often used for the investiga-... 

Two photon absorption was observed in two materials - Yb(cp)3 and phenyl dithiolene. Figure 2 shows the nonlinear transmission through the dithiolene in a collimated beam. The solid curve is the best fit two photon absorption (18) obtained from -... 

Figure 2. Nonlinear Transmission through bis [1,2-diphenyl-1,2-ethenedithiolato(2-)-S,S ] Nickel.

Nonlinear transmission (NLT) measurements have frequently been used for single wavelength measurements (although some studies have exploited frequency-dependent NLT, for example, see Ref. [20]) here a transmitted light intensity, It, is simply measured as a function of incident light intensity, I0. In the case... 

If intrinsic two-photon absorption cross-sections are the subject of investigation short pulses (ps or fs) should be used and two-photon fluorescence (see below) should be measured to verify the Independence of the TPA process. Most often, two-photon absorption is measured either with the Z-scan technique (see above) or with the nonlinear transmission method [38]. 

In nonlinear transmission experiments the transmission of a sample is measured as a function of the input intensity. At small intensities the transmission increases linearly with increasing input intensity. For large intensities the nonlinearity sets in and a deviation from the linear behavior is visible (see Fig. 12a). A theoretical analysis of such curves based on... 

Two-photon excited fluorescence (TPEF) is one of the most useful ways to quantify TPA in optical materials competing with nonlinear absorption. While nonlinear transmission yields absolute quantities describing TPA, TPEF requires knowledge of the collection efficiency if absolute data must be acquired. Thus, the use of standards is the method of choice to determine 5 in case of TPEF. 

TOL, toluene THF, tetrahydrofurane DMSO, dimethylsulfoxide CH3CN, acetonitrile CHC13, chloroform. TPEF, two-photon excited fluorescence WLC, white light continuum NLT, nonlinear transmission. cfs, femtosecond ps, picosencond ns, nanosecond. 

DMSO, dimethylsulfoxide TOL, toluene EtOH, ethanol THF, tetrahydrofiirane CH2C12, methylene chloride. TPEF, two-photon excited fluorescence NLT, nonlinear transmission fs, femtosecond ns, nanosecond. Wavelength used for excitation maximum is localized at other wavelength. 

One representative example of a covalently bound chromophore is the polymer 124. This material was subjected to nonlinear absorption measurements using the neat film. The TPA coefficient was 7 cm/GW [524], which can be considered large. Furthermore, this polymer shows a reduction of the photon-number noise of 0.1 dB (4.6%) during nonlinear transmission. This is important in order to minimize the optical loss at the laser wavelength. 

Because of this wide range of applications, much effort was dedicated to the design and synthesis of new molecules with optimized TPA efficiency in this context, the characteristics of the designed molecules (linear absorption, solubility, substituents...) will depend on the targeted application. The TPA response of moleciUes can be imderstood in the context of its TPA cross-section ajpA. which can be measmed using different techniques, such as nonlinear transmission, two-photon induced fluorescence and the Z-scan method although in a pure TPA process ajpA does not depend on the laser pulse duration, the nonlinear absorption can be more efficient in the nanosecond regime than in the femtosecond one, due to excited state reabsorption phenomena [34]. 

The of thin films of C60 and H2(TPP) composites were measured by a research team at Naval Research Laboratory using DFWM and nonlinear transmission at 590.5 nm. - Kajzar and associates measured x values... 

S.V. Serak, N.V. Tabiryan, T.J. Bunning, Nonlinear transmission of photosensitive cholesteric liquid crystals due to spectral bandwidth auto-tuning or restoration. J. Nonlinear Opt. Phys. Mater 16, 471-483 (2007)... 

A new type of nonlinear nano-layered polymeric optical material is described. The material comprises alternating polymer layers with thousands of layers and a layer thickness down to 30 nm or less. A nonlinear response is introduced by including an appropriate dye in alternate polymer layers. Such materials have several uses, including optical limiting. Several nano-layered materials using optical limiter dyes in alternate layers were fabricated. Some of the linear optical properties and a verification of nonlinear transmission and reflectivity at high intensity for example materials are reported. 

Two-photon absorption can be measured by several techniques. Two of them are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Pulsed lasers are most often used because TPA is a third-order nonlinear optical process and therefore is most efficient at very high intensities. In the nonresonant TPA, two photons combine to bridge an energy gap larger than the energies of each photon individually, and the transition occurs without the presence of the intermediate state. This can be viewed as being due to a virtual state created by the interaction of the photons with the molecule. 

The evaluation of TPA activity of the materials can be made by various techniques such as up-converted fluorescence emission, nonlinear transmission, transient absorption, Z-scan, and four-wave mixing [122]. Among them, the up-converted fluorescence emission method is a simpler technique and the setup is shown in Fig. 49.10. In this method, the TPA cross-section value, 0-2 can be estimated according to the following equation [123] ... 

TPA cross-section (ajPA) values were determined from experimentally measured two-photon absorption coefficient, p, which is obtained by measuring the nonlinear transmissivity (Ti) of chromophores in solution for a given input intensity, / , and a given thickness of a sample solution, lo, from the following relationships (23,24) ... 

The shape of the Z-scan curve is additionally modified if nonlinear transmission (absorption bleaching) or nonlinear absorption occur. Due to the presence of the intensity-dependent absorption coefficient, which can often be expressed as a — ocq + the curves become unsymmetrical because of increased transmis-... 

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