Femtosecond lasers, principles

For studies in molecular physics, several characteristics of ultrafast laser pulses are of crucial importance. A fundamental consequence of the short duration of femtosecond laser pulses is that they are not truly monochromatic. This is usually considered one of the defining characteristics of laser radiation, but it is only true for laser radiation with pulse durations of a nanosecond (0.000 000 001s, or a million femtoseconds) or longer. Because the duration of a femtosecond pulse is so precisely known, the time-energy uncertainty principle of quantum mechanics imposes an inherent imprecision in its frequency, or colour. Femtosecond pulses must also be coherent, that is the peaks of the waves at different frequencies must come into periodic alignment to construct the overall pulse shape and intensity. The result is that femtosecond laser pulses are built from a range of frequencies the shorter the pulse, the greater the number of frequencies that it supports, and vice versa. 

Rulliere, C. (ed.) 1998 Femtosecond laser pulses principles and experiments. Berlin Springer. 

L. Sarger and J. Oberle, in Femtosecond Laser Pulses Principles and Experiments,... 

The resolution of photoion laser microscopy is limited by two fundamental factors [7] the Heisenberg principle of uncertainty and the presence of the nonzero tangential component of the velocity of the ejected photoion (photoelectron). The same factors restrict the spatial resolution of the field-ion microscopy. It must be emphasized again that the key difference lies in the fact that for photoion microscopy there is no need for a strong (ionizing) electric field that distorts and desorbs the molecules. And also, the femtosecond laser radiation allows the photoion to be photoselectively extracted from certain parts of a molecule. 

C. Rulliere (Ed.). Femtosecond Laser Pulses. Principles and Experiments. Berlin, Heidelberg, New York Springer-Verlag (1998). 

Luntz AC, Persson M, Wagner S, Frischkom C, Wolf M (2006) Femtosecond laser induced associative desorption of H from Ru(OOOl) Comparison of first principles theory with experiment. J Chem Phys 124 244702... 

In this type of reaction, reactant molecule A can be activated coherently by photochemical irradiation using a femtosecond laser pulse, and activated A undergoes a subsequent reaction without collision with other molecules. It is very difficult, in principle, to adjust collisions of two molecules coherently. Therefore, it is rather difficult to apply femtosecond laser pulse technology to conduct a bimolecular reaction, shown in Equation (2.2), like a unimolecular reaction. It is difficult to achieve the collision of two molecules A and B coherently. 

Laser-scanning microscopes can be classified by the way they excite and detect fluorescence in the sample. One-photon microscopes use a NUV or visible CW laser to excite the sample. Two-photon, or Multiphoton , microscopes use a femtosecond laser of high repetition rate. The fluorescence light can be detected by feeding it back through the scanner and through a confocal pinhole. The principle is termed confocal or descanned detection. A second detection method is to divert the fluorescence directly behind the microscope objective. The principle is termed direct or nondescaimed detection. 

Transient grating spectroscopy is relatively easily handled compared with the transient absorption spectroscopy, and is often used to study carrier dynamics at semiconductor electrodes [32]. Figure 14 schematically shows the principle of transient grating spectroscopy. A femtosecond laser pulse for sample excitation is split into two beams, which are crossed again at the semiconductor surface to produce an optical striped interference pattern. The interference pattern produces a striped pattern of the densities of photo-generated electrons and holes near the semiconductor surface. The latter striped pattern gives rise to a striped pattern of optical refractive index near the semiconductor surface, which is monitored by measuring a diffraction pattern of a second probe laser... 

Owing to the short pulse width of a femtosecond laser the energy/firequency spread is large, as required by the uncertainty principle, i.e. 

In principle, the theory of nonlinear spectroscopy with femtosecond laser pulses is well developed. A comprehensive and up-to-date exposition of nonlinear optical spectroscopy in the femtosecond time domain is provided by the monograph of Mukamel. ° For additional reviews, see Refs. 7 and 11-14. While many theoretical papers have dealt with the analysis or prediction of femtosecond time-resolved spectra, very few of these studies have explicitly addressed the dynamics associated with conical intersections. In the majority of theoretical studies, the description of the chemical dynamics is based on rather simple models of the system that couples to the laser fields, usually a few-level system or a set of harmonic oscillators. In the case of condensed-phase spectroscopy, dissipation is additionally introduced by coupling the system to a thermal bath, either at a phenomenological level or in a more microscopic maimer via reduced density-matrix theory. 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

The flash lamp teclmology first used to photolyse samples has since been superseded by successive generations of increasingly faster pulsed laser teclmologies, leading to a time resolution for optical perturbation metliods tliat now extends to femtoseconds. This time scale approaches tlie ultimate limit on time resolution (At) available to flash photolysis studies, tlie limit imposed by chemical bond energies (AA) tlirough tlie uncertainty principle, AAAt > 2/j. 

For a time there was concern over just how the uncertainty principle would limit what could be achieved with ultrafast laser pulses. The potential problem turned out to be chimerical, not a problem, once coherence came to be appreciated. This conceptual advance was essential to the successful development of experimental femtosecond chemistry it did not progress through better lasers and nonlinear optical tricks and faster computers alone. 

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