Titanium sapphire

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.

Figure B2.1.3 Output of a self-mode-locked titanium-sapphire oscillator (a) non-collinear intensity autocorrelation signal, obtained with a 100 pm p-barium borate nonlinear crystal (b) intensity spectrum.

Historically, the first type of laser to be tunable over an appreciable wavelength range was the dye laser, to be described in Section 9.2.10. The alexandrite laser (Section 9.2.1), a tunable solid state laser, was first demonstrated in 1978 and then, in 1982, the titanium-sapphire laser. This is also a solid state laser but tunable over a larger wavelength range, 670-1100 nm, than the alexandrite laser, which has a range of 720-800 nm. 

The lasing medium in the titanium-sapphire laser is crystalline sapphire (AI2O3) with about 0.1 per cent by weight of Ti203. The titanium is present as Ti and it is between energy levels of this ion that lasing occurs. 

AI2O3 (aluminium oxide) in ruby laser, 346 in titanium-sapphire laser, 348 3142 (cyclic) interstellar, 120 3142 (linear) interstellar, 120... 

The development of lasers has continued in the past few years and 1 have included discussions of two more in this edition. These are the alexandrite and titanium-sapphire lasers. Both are solid state and, unusually, tunable over quite wide wavelength ranges. The titanium-sapphire laser is probably the most promising for general use because of its wider range of tunability and the fact that it can be operated in a CW or pulsed mode. 

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-... 

Fischer A, Cremer C, StelzerEHK (1995) Fluorescence of coumarins and xanthenes after two-photon absorption with a pulsed titanium-sapphire laser. Appl Opt 34 1989-2003... 

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. 

The probability of two-photon absorption depends on both spatial and temporal overlap of the incident photons (the photons must arrive within 10 18 s). The cross-sections for two-photon absorption are small, typically 10 so cm4 s photon-1 molecule 1 for rhodamine B. Consequently, only fluorophores located in a region of very large photon flux can be excited. Mode-locked, high-peak power lasers like titanium-sapphire lasers can provide enough intensity for two-photon excitation in microscopy. 

Figure 6.4. Wavelength distribution of output from a picosecond pulsed titanium sapphire laser. Output at...

Two-photon fluorescence microscopy has also been used with good effect in the near-IR. For example, Ferguson et al.r24> at the University of Strathclyde have used 270 fsec pulses from a titanium sapphire (Ti sapphire) laser at 790 nm to observe visible fluorescence from dyes in zebra fish larvae and erythrocytes. The high depth and lateral definition afforded by the two-photon process and confocal microscopy are useful here. Also, the use of near-IR excitation minimizes photobleaching. 

Bems, M. W., Aist, J. R., Wright, W. H., and Liang, H. (1992) Optical trapping in animal and fungal cells using a tunable, near-infrared titanium-sapphire laser. Exp. Cell Res. 198,375-378. 

Yelin, D., Oron, D., Korkotian, E., Segal, M., and SUbergerg, Y. 2002. Third-harmonic microscopy with a titanium-sapphire laser. Appl. Phys. B-Lasers Opt. 74 S97-S101. 

Fig. 1. Experimental layout of the pump-probe electron diffraction experiment. 2a> = 2 harmonic, and 3oj = 3ri harmonic of the Titanium-Sapphire laser operating at 800 nm.

Experiments were performed using a titanium sapphire laser oscillator capable of producing pulses with bandwidths up to 80 nm FWHM. The output of the oscillator was evaluated to make sure there were no changes in the spectrum across the beam and was compressed with a double prism pair arrangement. The pulse shaper uses prisms as the dispersive elements, two cylindrical concave mirrors, and a spatial light modulator (CRI Inc. SLM-256), composed of two 128-pixel liquid crystal masks in series. The SLM was placed at the Fourier plane [5]. After compression and pulse shaping, 200 pJ pulses were used to interrogate the samples. 

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. 

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 first NeNePo experiments dealt with silver clusters, Ag3, Ags, Ag7, and Ag9, particularly with the first of these. The photodetachment and photoionization were done with a single titanium-sapphire laser producing pulses of approximately 60 fs duration. Doubled in frequency, these could be tuned over a wavelength span from above 420 to below 390 nm. As with the dimer, photodetachment was a one-photon process and photoionization a two-pho-ton process. (The clusters of odd numbers of atoms could be studied this way the even-numbered clusters require at least three photons in the available energy range for photoionization). The interval between pulses could be varied from zero (simultaneous pulses) to 100 ps the two pulses were made to differ in intensity by about a factor of 2, and either could be the leading pulse. 

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