Broadband shaped pulses

There are many tricks to get around the problem, such as sandwich 180° pulses (e.g., 90 -180 -90 ) and broadband shaped pulses. Figure 8.4 (top) shows the inversion profile for a simple 180° pulse at the highest available power (fp = 28.4 p,s, yB l2it — 17.6 kHz). The profile is obtained using an inversion-recovery sequence (180°x — r — 90° ) with recovery time r = 0. The final 90° pulse frequency and the 13C peak (13CH3l) are both at the center of the spectral window, but the frequency of the 180° pulse is moved in 10 ppm (1500 Hz)... 

We will see that the major application of shaped pulses is to select a narrow region of the spectrum, thus displaying a narrow bandwidth. But there are also shaped pulses designed to do just the opposite — to give even excitation over a very wide range of frequencies. These broadband shaped pulses are specialized for inversion (Sz — S-) or refocusing... 

T Advanced Multiphoton and CARS Microspectroscopy with Broadband-Shaped Femtosecond Laser Pulses... 

Benefits of Ultrashort Broadband Laser Pulses and Pulse Shaping for Nonlinear Microspectroscopy Comparing Different Techniques... 

To demonstrate the versatility of nonlinear microspectroscopy with shaped broadband laser pulses, TPF is chosen as a second example. Remember that TPF can be implemented only by programming the pulse shaper differently and sampling a different wavelength range of the signal spectrum in the same experimental setup. TPF is so far probably the most widely applied nonlinear optical spectroscopy... 

Written by an international panel of experts, this volume begins with a comparison of nonlinear optical spectroscopy and x-ray crystallography. The text examines the use of multiphoton fluorescence to study chemical phenomena in the skin, the use of nonlinear optics to enhance traditional optical spectroscopy, and the multimodal approach, which incorporates several spectroscopic techniques in one instrument. Later chapters explore Raman microscopy, third-harmonic generation microscopy, and nonlinear Raman microspectroscopy. The text explores the promise of beam shaping and the use of a broadband laser pulse generated through continuum generation and an optical pulse shaper. 

Figure 6.6 Two-state quantum system driven on resonance by an intense ultrashort (broadband) laser pulse. The power spectral density (PSD) is plotted on the left-hand side. The ground state 11) is assumed to have s-symmetry as indicated by the spherically symmetric spatial electron distribution on the right-hand side. The excited state 12) is ap-state allowing for electric dipole transitions. Both states are coupled by the dipole matrix element. The dipole coupling between the shaped laser field and the system is described by the Rabi frequency Qji (6 = f 2i mod(6Iti-...

Shaped pulses are created from text files that have a line-by-line description of the amplitude and phase of each of the component rectangular pulses. These files are created by software that calculates from a mathematical shape and a frequency shift (to create the phase ramp). There are hundreds of shapes available, with names like Wurst , Sneeze , Iburp , and so on, specialized for all sorts of applications (inversion, excitation, broadband, selective, decoupling, peak suppression, band selective, etc.). The software sets the maximum RF power level of the shape at the top of the curve, so that the area under the curve will correspond to the approximately correct pulse rotation desired (90°, 180°, etc.). When an experiment is started, this list is loaded into the memory of the waveform generator (Varian) or amplitude setting unit (Bruker), and when a shaped pulse is called for in the pulse sequence, the amplitudes and phases are set in real time as the individual rectangular pulses are executed. 

Multiple-pulse sequences are indispensable tools for the practical implementation of Hartmann-Hahn experiments in liquid state NMR. They often consist of thousands of defined rf pulses with or without intermediate delays and allow one to create a desired form of the effective Hamiltonian for a given class of spin systems. In the field of high-resolu-tion NMR, multiple-pulse sequences are also used for broadband het-eronuclear decoupling (Levitt et al., 1983 Shaka and Keeler, 1986). Composite pulses (Levitt, 1986) and shaped pulses (Warren and Silver, 1988 Freeman, 1991 Kessler et al., 1991) may be considered as special classes of multiple-pulse sequences. 

JuuAN, R. K. Cox, K. Cooks, R. G. Broadband excitation in the quadrupole ion trap mass spectrometer using shaped pulses (inverse Fourier transform). 

The potential of broadband laser excitation and fs-pulse shaping for different microspectroscopy techniques ranges from pure dispersion compensation (in the case of SHG, THG), to highly functional pulse shaping (in the case of CARS), as summarized in Table 7.2. It is worth mentioning again that all techniques can be implemented in the same approach—broadband laser and pulse shaper. The detection technique of choice is just selected by the corresponding pulse shapes. 

The added benefit of intrinsic interferometric detection is only a further example of the great flexibility of using the pulse shaper in single-beam nonlinear microspectroscopy. The setup used here for CARS in its variants is, of course, also capable of immediately performing all the other nonlinear microspectroscopies simply by changing the shape of the excitation pulses with computer control. This is shown in the next section, where we discuss a broadband TPF application. 

A rather different technique was recently demonstrated to transfer PA-produced Cs2 molecules to the lowest vibrational level of theX E+ state [37]. Ultrafast (100 fs) pulses were used for broadband pumping of the initial w = 1-7 molecules back to the excited state. These excited molecules would decay back into X E+. The specttum of this pumping light was shaped to eliminate any components tihat could excite from w = 0. As a result, if a molecule decayed into w = 0, it would stay there. After a sequence of many of these shaped broadband pulses, a large fraction, 70%, of the initial V = 1-7 population would thus accumulate in w = 0. The various X vibrational level populations were monitored by vibrationally state-selective pulsed ionization. 

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