Femtosecond Titanium Sapphire Lasers

A favorable feature of the Ti sapphire lasers is that they are self-mode-locking. If one a H apphiie las op- 

An advantage of the n sapphire laser is that it is a sedid-state device. There are no flowing dyes to be re- 

Ti sap d)ire laseis are bang widdy used for two-photon and muldphoton excitadon. In this case, one can use their intense fundamental oudxit to exdie fluorophores by simultaneous absoipdon of two or more phomns. The use of muldphoton excitation has been found k be particulariy valuable in microscopy, where localized excitation occurs only at the focal poiat of the excitation beam. 

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This is just what has been done with Ag2. A sputtering source followed by a phase-space compressor chamber provided a beam of cooled negative cluster ions of many sizes. From these, the dimers were selected, accumulated in a quadrupole trap, and photodetached with a femtosecond, titanium-sapphire laser. After photodetachment by a 60-fs pulse, the neutral dimers oscillate, causing corresponding oscillations in the ionization cross section, in turn generating the oscillations that dominate the intensity pattern in Fig. 11. This is a simple phenomenon, yielding in a simple way the... 

The typical shape of the fluorescence signals in scattering media is shown in Fig. 5.55 and Fig. 5.56. The fluorescence of beads stained with IRD38 (Li-Cor, Inc.) embedded in agarose phantoms was recorded by TCSPC. A femtosecond titanium-sapphire laser was used for excitation, and a H7422-50 PMT module for detection. The IRF width of the system is about 300 ps. Figure 5.55 shows the variation in the recorded fluorescence decay data with the pH for beads at the surface of the phantom. 

Fig. 2.7. Autocorrelation trace (a) and spectrum at A = 800 nm (b) of the femtosecond titanium sapphire laser. Assuming a sech pulse shape, a pulse width At — 71 fs (FWHM) is measured. The spectrum reveals a bandwidth Ai = (1/A )Z A = 181 cm- With these data the pulse is 1.2 times band width-limited (see Table 2.2)...

Fig. 2.8. Interferometric autocorrelation trace of the femtosecond titanium sapphire laser (taken from [203]). The pulse width is 70 fs. The trace was recorded with a step width of 1 fs...

Figure B2.1.1 Femtosecond light source based on an amplified titanium-sapphire laser and an optical parametric amplifier. Symbols used P, Brewster dispersing prism X, titanium-sapphire crystal OC, output coupler B, acousto-optic pulse selector (Bragg cell) FR, Faraday rotator and polarizer assembly DG, diffraction grating BBO, p-barium borate nonlinear crystal.

Titanium sapphire lasers typically deliver pulses with durations between 4.5 and 100 fs, and can achieve a peak power of some 0.8watts, but this is not high enough to obtain adequate signal-to-noise ratio in experiments where the number of molecules that absorb light is low. To overcome this limitation, the peak power of a femtosecond laser can be dra-... 

The introduction and diversification of genetically encoded fluorescent proteins (FPs) [1] and the expansion of available biological fluorophores have propelled biomedical fluorescent imaging forward into new era of development [2], Particular excitement surrounds the advances in microscopy, for example, inexpensive time-correlated single photon counting (TCSPC) cards for desktop computers that do away with the need for expensive and complex racks of equipment and compact infrared femtosecond pulse length semiconductor lasers, like the Mai Tai, mode locked titanium sapphire laser from Spectra physics, or the similar Chameleon manufactured by Coherent, Inc., that enable multiphoton excitation. 

Here, the principal features and characteristics of the ultrafast laser systems used are briefly summarized. Besides the titanium sapphire laser which acts as the workhorse in nearly all of the discussed experiments, a synchronously pumped dye laser is employed to study the ultrafast dynamics of Nas on a picosecond timescale (see Sect. 3.2.2). For measurements with femtosecond time resolution and wavelengths located between 600 and 625 nm a synchronously titanium sapphire pumped optical parametric oscillator followed by frequency doubling is used. To investigate the Nas C state, two mode-locked titanium sapphire lasers have been synchronized. In all cases the essential parameter of the generated laser pulses, the pulse width, has to be determined. This problem is solved by an autocorrelation technique. Hence, the principles of an autocorrelator are briefly described at the end of this section. 

Fig. 2.6. Femtosecond configuration of the titanium sapphire laser cavity. AOM acusto-optic modulator Pi, P2 pump mirrors Mi to Mio cavity mirrors PRi to PR4 prisms, OC output coupler (by courtesy of Spectra Physics)...

Synchronization of Two Mode-Locked Titanium Sapphire Lasers. Besides utilizing OPOs - described above - the use of two independently tunable ultrafast laser sources is possible. This is of special interest while using picosecond laser sources, since the efficiency of nonlinear optical processes is, due to the peak power, much lower than for equivalent femtosecond processes. To achieve the conditions for the pumpfeprobe technique the pulse trains of the two independent lasers have to be synchronized. A successful approach to this problem is described below. For further details on the design of the appropriate stabilization see [50]. 

The details of the femtosecond RIKES setups used for the studies described in this chapter have been reported elsewhere (Wiewior et al, 2002 Shirota, 2005 Shirota et al., 2009). In short, the light sources for the setups were titanium sapphire lasers pumped by approximately 3.5 W of 532-nm light from a neodymium vanadate laser (Spectra Physics). The center wavelength of the titanium sapphire lasers was 800-810 ran, with a full width at half maximum value of ca. 60 or 75 nm and a repetition frequency of ca. 85 MHz. The output... 

Since the development of titanium-sapphire (Ti Sa) femtosecond laser source, the domain of research fields or development covered by the use of ultrafast lasers is in continuous expansion. In femtochemistry, it was realized that, when in a photochemical reaction different pathways lead to a given final state, the presence of a well-controlled frequency pulse chirp might greatly enhance the probability with which this final state is reached [1,2]. Chirp control is needed and mastering the phase of the laser pulse is the key point. 

Multiphoton microscopy (MPM) utilization has grown rapidly and continuously since then, with more than 200 refereed publications per year citing the use of MPM or two-photon microscopy. Commercial sources for MPM instruments did not become available until several years later, but adequate titanium sapphire (Ti sapphire) 100-femtosecond lasers were (and are) available, albeit at exorbitant costs. Many MPM instruments were and still are assembled by the scientists using them, a point that may become relevant in future specialized chemical imaging opportunities. Convenient laboratory instruments for MPM imaging are now available from Zeiss Microscopy. 

An almost ideal, though expensive, light source is the Titanium-Sapphire (Ti Sapphire) laser. The benefit of the Ti Sapphire laser are its short pulse width and its tunability. Depending on the version of the laser and the pump power, tunability from 700 to 980 nm can be achieved. The pulse width is a few pieosee-onds for picosecond versions and about 100 fs for femtosecond versions. Lasers with less than 20 fs pulse width are available. 

V. Petrov, D. Georgiev, and U. Stamm, Improved Mode Locking of a Femtosecond Titanium-Doped Sapphire Laser by Intracavity Second Harmonic Generation , Appl. Phys. Lett. 60, 1550 (1992). 

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