Pulses ultrashort

Figure Al.6.8. Wavepacket interferometry. The interference contribution to the exeited-state fluoreseenee of I2 as a fiinotion of the time delay between a pair of ultrashort pulses. The interferenee eontribution is isolated by heterodyne deteetion. Note that the stnieture in the interferogram oeeurs only at multiples of 300 fs, the exeited-state vibrational period of f. it is only at these times that the wavepaeket promoted by the first pulse is baek in the Franek-Condon region. For a phase shift of 0 between the pulses the returning wavepaeket and the newly promoted wavepaeket are in phase, leading to eonstnietive interferenee (upper traee), while for a phase shift of n the two wavepaekets are out of phase, and interfere destnietively (lower traee). Reprinted from Seherer N F et 0/1991 J. Chem. Phys. 95 1487.

For CW applieations of optieal-heterodyne eonversion, two laser fields are applied to the optoeleetronie material. The non-linear nature of the eleetro-optie effeet strongly suppresses eontimious emission relative to ultrashort pulse exeitation, and so most of the CW researeh earried out to date has used photoeonduetive anteimae. The CW mixing proeess is eharaeterized by the average drift veloeity t and earrier lifetime Xq of the mixing material, typieally... 

Agrawal G P 1989 Ultrashort pulse propagation in nonlinear dispersive fibers The Supercontinuum... 

Kane D J and Trebino R 1993 Single-shot measurement of the intensity and phase of an arbitrary ultrashort pulse by using frequency-resolved optical gating Opt. Lett. 18 823-5... 

Most molecular vibrations are well described as hannonic oscillators with small anlrannonic perturbations [5]. Por an hannonic oscillator, all single-quantum transitions have the same frequency, and the intensity of single-quantum transitions increases linearly with quantum number v. Por the usual anhannonic oscillator, the single-quantum transition frequency decreases as v increases. Ultrashort pulses have a non-negligible frequency bandwidth. Por a 1... 

In this method the creation of defects is achieved by the application of ultrashort (10 ns) voltage pulses to the tip of an electrochemical STM arrangement. The electrochemical cell composed of the tip and the sample within a nanometer distance is small enough that the double layers may be polarized within nanoseconds. On applying positive pulses to the tip, the electrochemical oxidation reaction of the surface is driven far from equilibrium. This leads to local confinement of the reactions and to the formation of nanostructures. For every pufse applied, just one hole is created directly under the tip. This overcomes the restrictions of conventional electrochemistry (without the ultrashort pulses), where the formation of nanostructures is not possible. The holes generated in this way can then be filled with a metal such as Cu by... 

Using a perturbative analysis of the time-dependent signal, and focusing on the interference term between the one- and two-photon processes in Fig. 14, we consider first the limit of ultrashort pulses (in practice, short with respect to all time scales of the system). Approximating the laser pulse as a delta function of time, we have... 

Equation (65) illustrates that in the limit of ultrashort pulses the two-pathway method loses its value as a coherence spectroscopy 8s is fixed at it/2 irrespective of the system parameters. From the physical perspective, when the excitation is much shorter than the system time scales, the channel phase carries no imprint of the system dynamics since the interaction time does not suffice to observe dynamical processes. 

Kruger, J. and Kautek, W. Ultrashort Pulse Laser Interaction with Dielectrics and Polymers, Vol. 168, pp. 247-290. 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

With the ultrashort pulse excitation, the initial condition of Eq. (III. 15) is given by [90]... 

The identification of excited states in strong field interactions with molecules has lead to some novel forms of molecular spectroscopy, allowing previously inaccessible states to be studied. For the most part, this comes from the ability to do transient spectroscopy in the time domain with ultrashort pulses. But, the strong field interaction also allows for new population mechanisms. 

With development of ultrashort pulsed lasers, coherently generated lattice dynamics was found, first as the periodic modulation in the transient grating signal from perylene in 1985 by De Silvestri and coworkers [1], Shortly later, similar modulation was observed in the reflectivity of Bi and Sb [2] and of GaAs [3], as well as in the transmissivity of YBCO [4] by different groups. Since then, the coherent optical phonon spectroscopy has been a simple and powerful tool to probe femtosecond lattice dynamics in a wide range of solid... 

Figure 5.1 depicts filamentation that has been imaged during the propagation of ultrashort pulses through a crystal of BaF2. The spectrum associated with the filaments that are seen shows a predominantly blue emission [39] that is centered on 330 nm, one of the two luminescence bands (the other one... 

Figure 8.2 shows an example of the electron density map obtained by numerical simulation, in this case 1.5 ps after the entrance of the ultrashort pulse in the gas. The laser is coming from the right hand side. The picosecond precursor was included in the code, while the ASE was not. 

In this section, we will briefly describe the two main techniques devoted to detecting ultrashort pulses the streak camera and the auto correlator. 

L). Our recently published theoretical model defines, for the first time, the optimum focusing conditions for SHG using focused beams in the ultrashort-pulse regime, where GVM is significant (i.e. where L > ). 

The parabolic equations derived in a slowly varying envelope approximation that describe the second harmonic generation (SHG) of ultrashort pulses in media with locally inhomogeneous wave-vector mismatch, have the form ... 

---