Fs-laser

Apparent non-RRKM dynamics has also been observed in time-resolved femtosecond (fs) experiments in a collision-free enviromnent [117], An experimental study of acetone illustrates this work. Acetone is dissociated to the CH and CH CO (acetyl) radicals by a fs laser pulse. The latter which dissociates by the channel... 

A commercial fs-laser (CPA-10 Clark-MXR, MI, USA) was used for ablation. The parameters used for the laser output pulses were central wavelength 775 nm pulse energy -0.5 mj pulse duration 170-200 fs and repetition rate from single pulse operation up to 10 Hz. In these experiments the laser with Gaussian beam profile was used because of the lack of commercial beam homogenizers for femtosecond lasers. 

Figure 10-5. Transient transmission changes AV/Po in PPV for different lime delays between the pump and probe pulse. The pump pulse is a 100 fs laser pulse at 325 nm obtained by frequency doubling ol amplified dye laser pulses, (a) and (b) correspond to different sides of a PPV-film. The spectra in (a) were obtained lor the unoxidized side of the sample while the set of spectra in (b) was measured for the oxidized side of the same sample. The main differences observed are a much lower stimulated emission effect for the oxidized side. The two bottom spectra depict the PL-spectra for comparison. The dashed line indicates the optical absorption (according to Kef. (281).

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

Figure 1.6. Quantum theory of IBr-Ar dissociation, showing a snapshot of the wavepacket states at 840 fs after excitation of the I-Br mode by a 100 fs laser pulse. The wavepacket maximum reveals predominant fragmentation of the IBr molecule along the r coordinate at short IBr-Ar distances [R coordinate), whilst a tail of amplitude stretches to longer R coordinates, indicating transfer of energy from the I-Br vibration to the IBr-Ar dimension, which propels the argon atom away from the intact IBr molecule.

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

Matsuda, H., Fujimoto, Y, Ito, S., Nagasawa, Y, Miyasaka, H., Asahi, T. and Masuhara, H. (2006) Development of near-infrared 35 fs laser microscope and its application to the detection of three- and four-photon fluorescence of organic microcrystals. J. Phys. Chem. B, 110, 1091. 

Third-order nonlinear optical properties of CdTe QDs were examined by Z-scan and FWM experiments in the nonresonant wavelength region. We found that the two-photon absorption cross section, a, is as high as 10 GM, although this value decreases with decreasing size. In addition, the nonlinear response is comparable to the pulse width of a fs laser and the figures of merit (FOM = Re Xqd/ Xqd)... 

Quite recently, Scherer and co-workers [39] have used a 10-fs laser pulse to excite B at room temperature and based on their detailed experimental results, they have proposed the following kinetic scheme ... 

Note that the usage of 10-fs laser pulse leads to rich oscillatory components as well as these rapid kinetics in their pump-probe time-resolved profiles. Obviously in this timescale, the temperature T will have no meaning except for the initial condition before the pumping process. In addition, such oscillatory components may be due not only to vibrational coherence but also to electronic coherence. A challenging theoretical question may arise, for such a case, as to how one can describe these ultrafast processes theoretically. 

It should be noted that the calculated anisotropy may not be applied to fs time-resolved anisotropy measurements because fs time-resolved experiments involve pumping and probing conditions and may involve overlapping between the vibronic structures of several electronic states due to the use of fs laser pulses. Nevertheless, we think the calculated anisotropy using Eq. (2.54) can provide a reference in comparing models. 

Recently, Scherer et al. have used the 10-fs laser pulse with A,excitation = 860 nm to study the dynamical behavior of Rb. Sphaeroides R26 at room temperatures. In this case, due to the use of the 10-fs pulse both P band and B band are coherently excited. Thus the quantum beat behaviors are much more complicated. We have used the data given in Table I and Fig. 19 to simulate the quantum beat behaviors (see also Fig. 22). Without including the electronic coherence, the agreement between experiment and theory can not be accomplished. 

Fig. 3 Transient spectra obtained upon the application of a 200-fs laser pulse to a solution of stilbene (S) and chloranil (Q) in dioxane. (a) The fast decay ( 20 ps) of the contact ion-radical pair S+ , Q generated by direct charge-transfer excitation (CT path), (b) The slow growth ( 1.6 ns) of the ion pair S+ Q due to the diffusional quenching of triplet chloranil (A path) as described in Scheme 13. Reproduced with permission from Ref. 55.

Indeed, the (200-fs) laser excitation of the EDA complexes of various benz-pinacols with methyl viologen (MV2+) confirms the formation of all the transient species in equation (59). A careful kinetic analysis of the decay rates of pinacol cation radical and reduced methyl viologen leads to the conclusion that the ultrafast C—C bond cleavage (kc c = 1010 to 1011 s- ) of the various pinacol cation radicals competes effectively with the back electron transfer in the reactive ion pair. 

The use of short (fs) laser pulses allows even highly transient ion-radical pairs with lifetimes of t 10 12 s to be detected, and their subsequent (dark) decay to products is temporally monitored through the sequential spectral changes. As such, time-resolved (ps) spectroscopy provides the technique of choice for establishing the viability of the electron-transfer paradigm. This photochemical (ET) mechanism has been demonstrated for a variety of donor-acceptor interactions, as presented in the foregoing section. 

Summary. Coherent optical phonons are the lattice atoms vibrating in phase with each other over a macroscopic spatial region. With sub-10 fs laser pulses, one can impulsively excite the coherent phonons of a frequency up to 50THz, and detect them optically as a periodic modulation of electric susceptibility. The generation and relaxation processes depend critically on the coupling of the phonon mode to photoexcited electrons. Real-time observation of coherent phonons can thus offer crucial insight into the dynamic nature of the coupling, especially in extremely nonequilibrium conditions under intense photoexcitation. 

The latest advancement in femtosecond (fs)-based micromachining technology has opened a new window of opportunity for fabrication of microdevices. Direct exposure of most solid materials (including fused silica glass) to high power fs laser pulses may lead to the ablation of a thin layer of materials at the laser focal point13. Due to the multiphoton nature of the laser-material interaction, the ablation process can be conducted on the material surface as well as within its... 

Fig. 7.8 Fiber inline FPI device fabrication using an fs laser. Reprinted from Ref. 18 with permission. 2008 Optical Society of America...

Szameit, A. Bloemer, D. Burghoff, J. Pertsch, T. Nolte, S. Lederer, F. Tuennermann, A., Hexagonal waveguide arrays written with fs laser pulses, Appl. Phys. B. 2006, 82, 507 512... 

Fig. 23 (a) Dependence of the hole mobility in a film of poly-spiro-bifluorene-co-benzothiazole (PSF-BT) as function of the time elapsed after charge carrier generation by a 130 fs laser pulse at different applied voltages. The horizontal lines represent the electron and hole mobilities inferred from ToF experiments, (b) Momentary mobility as a function of the averaged distance that a carrier travelled after a given time. The inset depicts the chemical structure of PSF-BT. From [154] with permission. Copyright (2009) by the American Institute of Physics... 

FIGURE 7.5 Supercontinuum generation in a photonic crystal hber (PCF). (a) Schematic drawing of a PCF, showing the solid micrometer sized core surrounded by a periodic array of air-hlled holes, (b) Experimental demonstration of ultrabroad bandwidth generation from a standard 100 fs laser (PCF input, dashed curve) in only 12 mm of PCF. This spectrum (PCF output) has enough bandwidth to be compressed to sub-15 fs pulses (see text and Figure 7.7). 

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