Higher order multiphoton excitation

Development of a Near-Infrared 35 fs Laser Microscope and its Application to Higher Order Multiphoton Excitation... 

All of the cases treated in this section have been for the simplest three-level upconversion systems, under both GSA/ETU and GSA/ESA two-photon excitation conditions. For higher order multiphoton excitation processes, a number of... 

The NIR femtosecond laser microscope realized higher order multi photon excitation for aromatic compounds interferometric autocorrelation detection of the fluorescence from the microcrystals of the aromatic molecules confirmed that their excited states were produced not via stepwise multiphoton absorption but by simultaneous absorption of several photons. The microscope enabled us to obtain three-dimensional multiphoton fluorescence images with higher spatial resolution than that limited by the diffraction theory for one-photon excitation. 

With a Ti Sapphire laser or another high-repetition rate femtosecond laser, the sample can be excited by simultaneous multiphoton absorption [132, 164, 278, 282, 343, 471, 472]. For biological specimens, three-photon or higher-order excitation is rarely used. Nevertheless, such microscopes are normally called Multiphoton microscopes. 

It should be noted that noble metal nanostructures (e.g. gold nanorods) can be easily tuned for monophotonic applications in the wavelength range from 600 to 1000 nm by adjusting their aspect ratio R = A/B. Monophotonic excitation is advantageous, because its absorption cross-section is several orders of magnitude higher than for multiphotonic excitation ... 

Therefore, a nonresonant third-order process can be overcome by a resonantly enhanced higher order process. Strong two-photon excitation or absorption saturation at 2ct), would generate strong fifth or higher order nonlinearities. Therefore, a careful characterization of a nonlinear optical response necessitates the investigation of the possible roles of higher order nonlinearities enhanced by multiphoton resonances or saturation processes. 

The full width (FWHM) is 2V, sometimes called the power broadening width . A few other simple one and multiphoton excitation problems can be solved by equally simple back-of-the-envelope analytical expressions in the quasiresonant approximation. An interesting generalization of the Rabi formula has been given by Shirley for higher odd order resonances with n = 2p + 1 (integer p > 0)... 

As a general point of view, the 2PA process corresponds to the simultaneous interaction of two photons with matter, leading to the absorption transition from a lower level to a higher excited one, through a virtual level. It is worth noting that multi-higher order (>2) absorption processes can be also observed (multiphoton absorption, MPA, e.g. 3PA, 4PA, etc.) 2PA and 3PA processes are displayed in Pig. 5.1, as examples. 

The recent availability of ultrafast and intense mid-IR laser pulses has opened the way of controlling nuclear motion in the electronic ground state by multiphoton vibrational excitation. In order to access higher vibrational levels, anharmoni-cities have to be taken into account. This is commonly done by using chirped laser pulses, which change the instantaneous frequency during their duration,... 

A technique which combines the high sensitivity of resonant laser ionization methods with the advantages of nonlinear coherent Raman spectroscopy is called IDSRS (ionization detected stimulated Raman spectroscopy). The excitation process, illustrated in Figure 5, can be briefly described as a two-step photoexcitation process followed by ion/electron detection. In the first step two intense narrow-band lasers (ct L, 0) ) are used to vibrationally excite the molecule via the stimulated Raman process. The excited molecules are then selectively ionized in a second step via a two- or multiphoton process. If there are intermediate resonant states involved (as state c in Figure 5), the method is called REMPI (resonance enhanced multi-photon ionization)-detected stimulated Raman spectroscopy. The technique allows an increase in sensitivity of over three orders of magnitude because ions can be detected with much higher sensitivity than photons. 

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