Femtosecond pulsed lasers

Valdmanis J A and Fork R L 1986 Design considerations for a femtosecond pulse laser balancing self phase modulation, group velocity dispersion, saturable absorption, and saturable gain IEEE J. Quantum. Electron. 22 112-18... 

It has been possible to determine transition structures computationally for many years, although not always easy. Experimentally, it has only recently become possible to examine reaction mechanisms directly using femtosecond pulsed laser spectroscopy. It will be some time before these techniques can be applied to all the compounds that are accessible computationally. Furthermore, these experimental techniques yield vibrational information rather than an actual geometry for the transition structure. 

Venkatakrishnan K, Tan B, Sivakumar NR. 2002. Sub-micron ablation of metallic thin film by femtosecond pulse laser. Opt Laser Tech 34 575-578. 

Figure 4.16. Experimental setting of the combined femtosecond pulsed laser and step scan IR spectrometer (left) and modifications of the infrared interferogram after the laser pulse (right) [187].

Dantus, M., Lozovoy, V. V., and Pastirk, 1. 2007. MBPS characterizes and corrects femtosecond pulses. Laser Focus IForZd 43(5) 101. ... 

While much has been learned about S near the geometry of tS in solution, there have been no reports on the spectroscopic detection of the twisted excited singlet state of stUbene. Information about the twisted excited singlet state of tetrapheny-lethene will be described in this chapter in Section 2.3. Interesting photochemistry also occurs following the excitation of cw-stilbene however, because of the much shorter lifetime of c -stilbene, femtosecond-pulsed lasers must be used." ... 

Ohno K, Tanabe T, Kannari F (2002) Adaptive pulse shaping of phase and amplitude of an amplified femtosecond pulse laser by direct reference to frequency-resolved optical gating traces. J. Opt. Soc. Amer. B19 2781-2790... 

The two-temperature equation is used to characterize mutual interactions among lattice temperature, and number density and temperature of carriers during pico- to femtosecond pulse laser processing [12]. In this study, a new parameter related with non- equilibrium durability is introduced and its characteristics for various laser pulses and fluences are discussed. 

Lee, S.H., Lee, J.S., Park, S., Choi, Y.K. (2003) Numerical Analysis on Heat transfer Characteristics of a Silicon Film Irradiated by Pico-to-Femtosecond Pulse Laser, Numerical Heat Transfer, Part A, Vol. 44, pp. 833-850. 

Brandbyge M, Hedegard P, Heinz TF, Misewich JA, Newns DM (1995) Electronically driven adsorbate excitation mechanism in femtosecond-pulse laser desorption. Phys Rev B 52 6042... 

W. Kautek, N. Sorg, and J. Kruger, Femtosecond pulse laser second harmonic generation on semiconductor electrodes, Electrochim. Acta 39(8/9), 1245, 1994. 

Giridhar MS, Seong K, Schulzgen A, Khulbe P, Peyghambarian N, Mansuripur M. Femtosecond pulsed laser micromachining of glass substrates with application to microfluidic devices. Appl Opt 2004 43 4584-9. 

Fig. 10 Femtosecond-pulse laser treatment of polyimide. r=150 fs, 1=800 nm. Squared diameters of the modified areas of polyimide in dependence on the maximum laser flu-ence for circularly polarized light. Solid symbols focusing with/= 25 mm lens, right axis. Hollow symbols focusing with/=60 mm lens, left axis [18]...

Fig. 22 Scanning electron microscope pictures of femtosecond pulse laser-ablated poly-imide. r= 150 fs, 1=800 nm, F0=L3 J cm-2, N= 50, linear polarization, (a) Total aspect, (b) detail [18]...

Compared to hand-made perforations by means of a specially grinded needle (Fig. 27a,b), the use of a femtosecond pulse laser resulted in well-defined perforations of the polyethylene membrane (Fig. 27c,d). Obviously, the reproducibility of the laser-generated structures is higher (Fig. 27b,d). It was impossible to reach diameters smaller than 50 pm with the mechanical technique (Fig. 27a). The fs-laser structuring yields the opportunity to tune the... 

When experiments are performed in the intense I > 1011 W/cm2 electromagnetic fields typically provided by picosecond or femtosecond pulsed lasers, it is necessary to solve the time-dependent Schrodinger equation in a non-perturbative approach (see Section 9.1.2). The total Hamiltonian is... 

S.-M. Wang, S.-L. Cong, K.-J. Yuan, Y.-Y. Niu, Photoionization of NO molecule in two-color femtosecond pulse laser fields, Chem. Phys. Lett. 417 (2006) 164. 

Femtosecond spectroscopy. See Section 24.9 for a more detailed discussion. Until recently, because of their exceedingly short lifetimes, there have been no direct observations of the activated complexes postulated to exist in the transition state of chemical reactions. But. after the development of femtosecond pulsed lasers, species resembling activated complexes can now be studied spectroscopically. Transitions to and from the activated complex have been observed and such experiments have greatly extended our knowledge of the dynamics of chemical reactions. 

The experimental arrangement for such femtosecond experiments is exhibited in Fig. 10.14. The output pulses from a femtosecond pulse laser (Sect. 6.1.5) are focused by the same lens into the molecular beam. The probe pulses are sent through a variable optical-delay line and the absorption a (At) of the probe pulse as a function of the delay time At is monitored via the laser-induced fluorescence. Cutoff Alters suppress scattered laser light. 

The physics behind laser ablation is much more complicated than as explained above. Three characteristic timescales are involved to define the nature of laser interaction with a metallic material [1], They are the electron cooling time lattice heating time Tj, and laser pulse duration tl. For a nanosecond pulse laser, tL 3> Xi. the process is predominantly laser heating. For a picosecond pulse laser, x femtosecond pulse laser, the process is exclusively a laser ablation process. Laser ablation of semiconductor and dielectric materials involves different mechanisms [2]. 

Wolfe DB, Ashcom JB, Hwang JC, Schaffer CB, Mazur E, Whitesides GM (2003) Customization of poly(dimethylsiloxane) stamps by micromachining using a femtosecond-pulsed laser. Adv Mater 15(l) 62-65... 

Two-photon molecular excitation was performed by very high local intensity provided by tight focusing in a laser scanning microscopy (LSM) [37]. This technique was combined with the temporal concentration of femtosecond pulsed lasers... 

Finally, in contrast to the conventional anti-Stokes probes, such as two-phonon absorption or second harmonic generation, upconversion nanophosphors exhibit higher emission efficiency and can be excited by continuous-wave laser rather than the costly femtosecond pulsed laser. 

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