Ultrashort laser sources

To introduce the application of ultrashort laser sources in microscopy, we want to review some properties of femtosecond pulses first for a comprehensive introduction the reader may refer to one of the established textbooks on femtosecond lasers (Diels and Rudolph 2006). The most important notion is the Fourier transform relation between the temporal shape of a pulse and the spectrum necessary to create it. This leads to the well-known time-bandwidth product for the pulse temporal width (measured as full width at half maximum, FWHM) At and the pulse spectral width Av. 

Infrared ultrashort laser sources with high pulse energy are most commonly used with pulse length ranging from about 200 fs to some picoseconds and a power up to few hundreds of milliwatts. These types of laser are in safety class 3B. The emitted radiation is potentially dangerous for eyes and skin. Specular reflections... 

The ultrashort laser sources used to generate the CARS are also efficient in exciting other multiphoton processes, such as TPF and SHG. As the signals are spectrally separated, it is possible to integrate a multimodal nonlinear microscope, able to acquire simultaneously CARS, TPF, and SHG. Figure 14.4 shows an example of the signals that can be acquired in a multimodal CARS system, where the SHG is produced by the pump beam of an erbium fiber laser emitting at 780 nm. 

Besides these noteworthy features for the acceleration of electrons and ions, the use of powerful, ultrashort laser pulses has also opened the possibility of creating new X/q-ray sources of unprecedented brightness and shortness. 

Ultrashort laser-plasma X-ray sources based upon the Ka emission process have been exploited since about 10 years ago to study some dynamical effects in simple systems, typically involving atomic motion, occurring on the timescale of 100fs. A complete review of such effects, in particular in... 

The conventional narrowband CARS process probes one particular vibrational mode selectively. Conversely, so-called broadband CARS measurements, using ultrashort pulsed laser sources, can probe multiple RS-active vibrational modes simultaneously [19, 29-31]. In the case of two-beam broadband CARS method, one of the two beams has a narrow bandwidth and the other a broad bandwidth. Therefore, the technical issue is how to generate these beams from a single laser source. Typically, subpicosecond pulses from a conventional solid-state femtosecond laser... 

When an ultrashort laser pulse is used as the light source for the SHG spectroscopy, the signal contains dynamical information of the system. In time-resolved SHG spectroscopy, two ultrashort pulses with tunable relative time-delay irradiate the sample. The first pulse (pump pulse) excites the system, and the SHG intensity or its spectra induced by the second pulse (probe pulse) are measured as a function of the time delay. 

Fig. 7.15. Typical transient absorption set-up using a subpicosecond laser source as the pump and a continuum of white light as the probe. For ultrashort time resolution additional stages for compensating the group velocity dispersion in both light pulses are needed. P pump, F filter.

In these equations and are the excited state populations of the donor and acceptor molecules- and and are the lifetimes of the donor and acceptor molecules in the excited state the notation x° is used to distinguish it from the radiative constant x° (in other words x° = Xg, for the donor) is given by (C3.4.5I and A the corresponding rate constant for the backward energy transfer from acceptors to donors can be foimd by me same means. Finally, and represent external sources of excitation, for example the absorption of laser light by the donor and acceptor molecules. Commonly, for example in the case of 8-pulse excitation (in practice an ultrashort laser pulse), (C3.4.6) yields exponential decay kinetics for p(f) and n t). The opposite case of steady excitation (CW light), yields the equilibrium ratio... 

The effective resolution of a TCSPC experiment is characterised by its instrument response function (IRF). The IRF contains the pulse shape of the light source used, the temporal dispersion in the optical system, the transit time spread in the detector, and the timing jitter in the recording electronics. With ultrashort laser pulses, the IRF width at half-maximum for TCSPC is typically 25 to 60 ps for microchannel-plate (MCP) PMTs [4, 211, 547], and 150 to 250 ps for conventional short-time PMTs. The IRF width of inexpensive standard PMTs is normally... 

Ultrashort pulses may be also used for vibrational spectroscopy with high frequency resolution. As a first example we have demonstrated FT-CARS of a supersonic expansion. Several advantages of the technique should be noted. The effect of transit time broadening can be eliminated. Artifacts via the nonresonant part of the third order susceptibility are negligible. A possible dynamic Stark effect during the excitation process does not influence the ns signal transient. Precise spectroscopic information is provided without narrow-band laser sources. 

In this section the ultrafast structural redistribution of Aga, initiated by a 100fs laser pulse, is presented. The experiment makes use of a high-intensity cluster anion source, an ion trap, a mass-analyzing detector for cluster cations, and a laser system which produces pairs of ultrashort laser pulses with an adjustable time delay between the pump and the probe laser pulse (for details see Sect. 2.1.2). 

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