A Femtosecond Laser System

An example of a femtosecond laser system employing chirped pulse amplification is the one housed at the Laser Support Facility at the Rutherford Appleton Laboratory a schematic layout is shown in Fig. 2. The system is essentially two-stage, with short, low energy pulses generated in the first stage and amplified in the second. To generate the pulses, a multi-line argon-ion laser with principal lines at 488 and 514 nm and 5 W continuous wave output is used to 

LEGEND S mirror beam splitter L lens P prism 

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The advent of ultrafast pump-probe laser techniques62 and their marriage with the TOF method also enables study of internal ion-molecule reactions in clus-ters.21,63-69 The apparatus used in our experiments is a reflectron TOF mass spectrometer coupled with a femtosecond laser system. An overview of the laser system is shown in Figure 4. Femtosecond laser pulses are generated by a colliding pulse mode-locked (CPM) ring dye laser. The cavity consists of a gain jet, a... 

A brief outline of a time-of-ffight mass spectrometer (TOF mass) and a femtosecond laser system are described. Figure 2.2 shows a TOF system with a mass resolution of to/Ato = 2000. Vaporized samples are introduced at a pressure of 10 5 Pa under a background pressure of 10 7 Pa. The aperture size of the slit of extraction plate was 1 mm or 0.5 mm. 

The experimental setup is divided in two parts a supersonic molecular beam and a femtosecond laser system (Fig. 1). 

Fig. 2.2. Experimental setup (as used for the investigations on NasB). An argon ion laser (ps mode-locked fs all lines, visible) pumps either a femtosecond laser system (a) (OPO synchronously pumped optical parametric oscillator SHG second-harmonic generator) or a picosecond laser system (b) (taken from [178]). The pulse duration and spectral width of the laser pulses are measured by an autocorrelator (A) and a spectrometer (S) respectively. A Michelson arrangement allows the probe pulses to be delayed At) with respect to the pump pulses. A quadrupole mass filter (QMS) enables the selection of the ensemble of investigated molecules ionized by a pump probe cycle. A secondary electron multiplier (SEM) detects the intensity I of the ions as a function of the delay time At. A Langmuir-Taylor detector (LTD) measures the total intensity /o of the cluster beam. The ratio I/Iq as a function of the delay time At is called the real-time spectrum...

Figure 4. A schematic of the colliding pulse mode-locked femtosecond laser system. Taken with permission from ref. 65.

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

The above experiment is only one example in which both ON and OFF processes were measured in the same molecular system. However, the pulse duration used was longer than the switching rates. To know switching rates precisely, it is necessary to use a shorter pulse. Using a femtosecond laser pulse, the photocoloration rate of 18a1261 and the photodecoloration rate of 19b were measured. 

The technique of up-conversion photoluminescence allows one to record the transient PL of a system at the temporal resolution of the laser pulse. It is used to study very fast processes below the picosecond time domain. A typical set-up for this experiment is shown in Fig. 3. The sample is excited at frequency uq by a femtosecond laser pulse and its PL at ujj- is mixed with that of an optically... 

In the presence of Y(OTf>3 (1.0 x lO M), photoexcitation of the Fc AQ system using a femtosecond laser also results in efficient ET from Fc to AQ within 500 fs (53). However, the transient absorption band is observed at 700 nm in the presence of Y(OTf>3 and is significantly red-shifted as compared with that observed at 600 nm in the absence of Y(OTf>3, as shown in Fig. 15(a) (53). Such a red-shift was reported for the complex formation of semiquinone radical anions with metal ions (54). The decay of absorbance at 420 and 700 nm due to the Fc + —AQ" Y(OTf)3 complex obeys first-order kinetics to afford... 

Muroya Y, Lin M, Watanabe T, Wu G, Kobayashi T, Yoshii K, Ueda T, Uesaka M, Katsumura Y. (2002) Ultra-fast pulse radiolysis system combined with a laser photocathode RF gun and a femtosecond laser. Nucl Instrum Meth A 489 554-562. 

In a second example the discrete time-reversible propagation scheme for mixed quantum-classical dynamics is applied to simulate the photoexcitation process of I2 immersed in a solid Ar matrix initiated by a femtosecond laser puls. This system serves as a prototypical model in experiment and theory for the understanding of photoinduced condensed phase chemical reactions and the accompanied phenomena like the cage effect and vibrational energy relaxation. It turns out that the energy transfer between the quantum manifolds as well as the transfer from the quantum system to the classical one (and back) can be very well described within the mixed mode frame outlined above. 

Figure 3.61. Photocycloaddition products formed by TP excitation of 5,7-dimethox-ycoumarin (149) in anisole using a Ti sapphire femtosecond laser system [487].

In 1997, the old dye laser record with respect to the shortest pulses was improved by a Ti sapphire laser. Pulses with a duration of 5 fs could be generated [9]. Despite the many advantages of solid-state femtosecond laser systems compared to dye lasers (power, compactness, no toxic dyes and solvents etc.), one drawback should be noted. The passive mode-locking process is not self-starting in many cases. 

Circular light polarization generated by means of a zero-order quarter-wave plate from the linearly polarized output of the femtosecond laser system, on the other hand, caused a radial orientation of the ripples with a period of 0.8 pm (Fig. 23a). Additionally, the morphology changed to an ar-... 

Nevertheless, during recent years, an increasing number of potential commercial applications of femtosecond laser treatment have been demonstrated, e.g., the fabrication of injection nozzles and medical implants (stents) [72]. Some femtosecond laser systems have already been installed particularly for mask repair in industry [73, 74]. In particular, biological tissue (cornea, tooth, bone) can be treated successfully [11, 75, 76]. Even transparent human chromosomes were laser-cut (170 fs, 800 nm, minimum FWHM cut size -100 nm) by a multiphoton-mediated process [77]. But, a killer application is incalculable. 

It can be suggested that the application of short-pulse (femtosecond) lasers as contactless drillers in dentistry can be an alternative to the classical mechanical technique to improve the quality of the tooth treatment. A critical point for the application of the laser in practice is the time a dentist would need for the treatment of a patient. Machining volumes of about 40 mm3 per minute are necessary for practical applications. Recently, an efficiency of nearly 1 mm3 per minute was demonstrated with a Tksapphire femtosecond laser system at 3 kHz repetition rate [82]. That means that repetition rates of the order of 100 kHz would be needed to reach the conventional drilling productivity. 

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