Subpicosecond pulses

PTFE is not amenable to clean ablation in the quartz-UV unless subpicosecond pulses are used. This lack of clean ablation is due to insufficient photon-PTFE interaction. Although PTFE is highly intractable, a method of incorporating... 

The reduction of obtainable light-pulse durations down to subpicosecond pulses (halfwidth about 10 sec) allows fast transient phenomena which were not accessible before to be studied in the interaction of light with matter. One example is the extension of spin echoe-techniques, well known in nuclear-magnetic-resonance spectroscopy, to the photon echoes in the optical region. 

PICOSECOND AND SUBPICOSECOND PULSE-RADIOLYSIS STUDIES OF GEMINATE ION RECOMBINATION IN LIQUID HYDROCARBONS... 

Here the progress in the picosecond and subpicosecond pulse radiolysis is described first and then the experimental studies on the kinetics of the geminate ion recombination is explained in connection with their application to advanced technology such as the next generation nanolithography and nanotechnology. 

With the development of the picosecond pulse radiolysis, the kinetics data of the geminate ion recombination have been directly obtained. The history of picosecond and subpicosecond pulse radiolysis is shown in Fig. 7. Very recently, the first construction of the femtosecond pulse radiolysis and the improvement of the subpicosecond pulse radiolysis started in Osaka University. 

Subpicosecond Pulse Radiolysis, Absorption, Osaka Univ, (2000) 800 fs Compact Pulse Radiolysis of about 10 ps time resolution started or under construction. 

Both Construction of the First Femtosecond Pulse Radiolysis and Improvement of the Subpicosecond Pulse Radiolysis started in Osaka University (2002) ... 

Figure 7 History of picosecond and subpicosecond pulse radiolysis and the start of construction of the first femtosecond pulse radiolysis.

Figure 8 The time-dependent behavior of the hydrated electron obtained in the subpicosecond pulse radiolysis of neat water using 2-mm optical path sample cell, monitored at the wavelength of 780 nm.

Figure 10 The geminate decay of the radical cation in irradiated neat -dodecane observed by the subpicosecond pulse radiolysis before (a) and after (b) the improvement of the system by mainly using the double-pulse method.

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

Figure 11 The components of the timing jitter of the laser synchronized subpicosecond pulse radiolysis, crj is the length of the electron pulse (rms), c,- is the length of the probe light (rms), and ctj is the timing fluctuation (rms).

Subpicosecond Pulse-Radiolysis Studies of Geminate Ion Recombination... 

The kinetics data of the geminate ion recombination in irradiated liquid hydrocarbons obtained by the subpicosecond pulse radiolysis was analyzed by Monte Carlo simulation based on the diffusion in an electric field [77,81,82], The simulation data were convoluted by the response function and fitted to the experimental data. By transforming the time-dependent behavior of cation radicals to the distribution function of cation radical-electron distance, the time-dependent distribution was obtained. Subsequently, the relationship between the space resolution and the space distribution of ionic species was discussed. The space distribution of reactive intermediates produced by radiation is very important for advanced science and technology using ionizing radiation such as nanolithography and nanotechnology [77,82]. 

The experiments were carried out using the subpicosecond pulse radiolysis system [77] described in Sec. 10.2.2 ( Subpicosecond Pulse Radiolysis ). Considering the signal intensity and the degradation of the time resolution, a sample cell with the optical length of 2 mm was mostly used. The sample was saturated by Ar gas to eliminate the scavenging effect by the remaining O2 gas. 

Further detailed kinetics of the geminate recombination of electrons and positive ions and their application to the advanced technology will be studied by higher time resolution of the femtosecond pulse radiolysis and both by the higher S/N ratio and the wider wavelength monitoring light of the improved subpicosecond pulse radiolysis shown in Fig. 7. 

Absorption due to main intermediates such as polymer cation radicals and excited states, electrons, and alkyl radicals of saturated hydrocarbon polymers had not been observed for a long time by pulse radiolysis [39]. In 1989, absorption due to the main intermediates was observed clearly in pulse radiolysis of saturated hydrocarbon polymer model compounds except for electrons [39,48]. In 1989, the broad absorption bands due to polymer excited states in the visible region and the tail parts of radical cation and electrons were observed in pulse radiolysis of ethylene-propylene copolymers and the decay of the polymer radical cations were clearly observed [49]. Recently, absorption band due to electrons in saturated hydrocarbon polymer model compounds was observed clearly by pulse radiolysis [49] as shown in Fig. 2. In addition, very broad absorption bands in the infrared region were observed clearly in the pulse radiolysis of ethylene-propylene copolymers [50] as shown in Fig. 3. Radiation protection effects [51] and detailed geminate ion recombination processes [52] of model compounds were studied by nano-, pico-, and subpicosecond pulse radiolyses. 

Figure 1. Diagram of the intensity / (W/cm2) vs. duration of laser pulse tp(s) with various regimes of interaction of the laser pulse with a condensed medium being indicated very qualitatively. At high-intensity and high-energy fluence 4> = rpI optical damage of the medium occurs. Coherent interaction takes place for subpicosecond pulses with tp < Ti, tivr. For low-eneigy fluence (4> < 0.001 J/cm2) the efficiency of laser excitation of molecules is very low (linear interaction range). As a result the experimental window for coherent control occupies the restricted area of this approximate diagram with flexible border lines.

Saeki, A., Kobawa, T., Yoshida, Y., Tagawa, S. 2001. Study on geminate ion recombination in liquid dodecane using pico- and subpicosecond pulse radiolysis. Radiat. Phys. Chem. 60 319-322. 

Pt(l 11) [6-8], Cu(l 1 1) [9] and Ag(l 1 1) [9], and CO fromPt(00 1) [10] andPt(l 1 1) [11,12]. On the other hand, these molecules are not desorbed from Ni and Pd metal surfaces in spite of the isoelectronic character of the metals Ni, Pd and Pt [13,14]. Desorption induced by subpicosecond-pulsed laser takes place via multiple correlated (and partially coherent) electronic transitions DIMET. DIMET is a very different mechanism from DIET [15-17] and in DIMET the vibrational excitation during the multiple electronic transitions leads to the desorption. Desorption via multiple vibrational transitions has also been observed using an infrared laser [18]. However, these topics are not described in this review. 

The c.w. dye laser can also be passively mode-locked and two different arrangements have been used. The first employed two free flowing dye streams, one for the laser dye and the other for the absorber (see Fig. 4) [18, 19]. In the alternative arrangement, the saturable absorber dye flows in a narrow channel of variable thickness (0.2—0.5mm) and in contact with a 100% broadband reflectivity mirror. With an absorber thickness of 0.5 mm, output pulses of 1 ps duration have been obtained [20]. Pulses as short as 0.3ps were produced when the DODCI cell length was shortened to 0.2 mm. The subpicosecond pulses produced in this arrangement were transform-limited in bandwidth. 

Muroya Y, Watanabe T, Wu G, Li X, Kobayashi T, Sugahara J, Ueda T, Yoshii K, Uesaka M, Katsumura Y. (2001) Design and development of a subpicosecond pulse radiolysis system. Radiat Phys Chem 60 307-312. 

Kozawa T, Saeki A, Yoshida Y, Tagawa S. (2002) Study on radiation-induced reaction in microscopic region for basic understanding of electron beam patterning in lithographic process (1) development of subpicosecond pulse radiolysis and relation between space resolution and radiation-induced reactions of onium salt. Jpn J Appl Phys 41 4208-4212. 

Kozawa T, Mizutani Y, Miki M, Yamamoto T, Suemine S, Yoshida Y, Tagawa S. (2000) Development of subpicosecond pulse radiolysis system. Nucl Inst Meth A 440 251-253. 

The conventional narrowband CARS process probes one particular vibrational mode selectively. Conversely, so-called broadband CARS measurements, using ultrashort pulsed laser sources, can probe multiple RS-active vibrational modes simultaneously [19, 29-31]. In the case of two-beam broadband CARS method, one of the two beams has a narrow bandwidth and the other a broad bandwidth. Therefore, the technical issue is how to generate these beams from a single laser source. Typically, subpicosecond pulses from a conventional solid-state femtosecond laser... 

The new accelerator at Brookhaven is based on an RF photocathode gun with one or more resonant cavities in which microwaves create transient electric fields up to 1 MeV cm [104], A pulse of laser light is used for generating photoelectrons which are accelerated to 9 MeV in a distance of 30 cm. The laser pulse can also be used as the analyzing light source this means it is closely synchronized with the electron pulse. The time resolution of the electron pulse is therefore that of the laser pulse, so that subpicosecond pulse radiolysis is possible. A similar system is planned at Argonne National Laboratory [146],... 

With optical detection, the overall time resolution is limited by the different velocities of fast electrons and photons in condensed media this results in loss of synchronization as the two beams pass through the sample cell. This desynchronization is approximately 10 ps cm in water [145], so the optical path length has to be reduced proportionally to achieve the improved time resolution provided by subpicosecond pulses. There is thus a compromise between having short time resolution (short optical path) and high absorbance signals (long optical path). 

The earliest subpicosecond systems incorporated dye laser technology. Shank, Ippen, and their colleagues at the Bell Laboratories [34] were the first to develop mode-locked subpicosecond lasers and to show how to compress pulses to very short values. With the colliding-pulse mode-locked (CPM) laser they achieved reduced pulse widths well into a subpicosecond range. Two approaches based on synchronously pumped dye lasers and colliding pulse dye lasers are commonly employed to produce subpicosecond pulses. These are briefly discussed below. 

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