High peak power

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

The extremely high peak power densities available ia particle beams and lasers can heat the small amounts of matter ia the fuel capsules to the temperatures required for fusion. In order to attain such temperatures, however, the mass of the fuel capsules must be kept quite low. As a result, the capsules are quite small. Typical dimensions are less than 1 mm. Fuel capsules ia reactors could be larger (up to 1 cm) because of the iacreased driver energies available. 

A second disadvantage is the need to have substantial amounts of power conditioning on the vehicle. This equipment must be sized to match the highest force and speed for the entire system, even if it rarely needs such high peak power. If the air gap is large this power conditioning equipment can be heavy and expensive. 

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. 

The fact that several lasers can generate very short light pulses (down to 10 sec duration) with high peak powers (up to 10 watts) which can be used to investigate short term transitions (e. g. lifetime measurements, flash photolysis, etc.). 

It is mainly three of the qualities discussed in Chapter II which make the laser a powerful tool for studying photochemical reactions the monochromatic output of frequency controlled lasers, the very short duration of laser pulses, and their high peak power. 

Multiphoton processes caused by the high peak power of ultrafast pulses significantly contribute to sample photodamage (Hopt and Neher 2001 Nan et al. 2006) these processes are reduced at longer excitation wavelengths. No sample photodamage was observed at excitation powers of 20-30 mW for the wavelengths provided by this OPO. 

With its high peak power, pulsed light penetrates opaque materials more effectively than does continuous light. 

The most convenient means of making time-resolved SH measurements on metallic surfaces is to use a cw laser as a continuous monitor of the surface during a transient event. Unfortunately, in the absence of optical enhancements, the signal levels are so low for most electrochemical systems that this route is unattractive. A more viable alternative is to use a cw mode-locked laser which offers the necessary high peak powers and the high repetition rate. The experimental time resolution is typically 12 nsec, which is the time between pulses. A Q-switched Nd YAG provides 30 to 100 msec resolution unless the repetition rate is externally controlled. The electrochemical experiments done to date have involved the application of a fast potential step with the surface response to this perturbation followed by SHG [54, 55,116, 117]. Since the optical technique is instantaneous in nature, one has the potential to obtain a clearer picture than that obtained by the current transient. The experiments have also been applied to multistep processes which are difficult to understand by simple current analysis [54, 117]. 

The use of femtosecond pulses in nonlinear optics has one obvious advantage high peak power, necessary to observe the nonlinear effect, can be obtained from low total power in the pulse. This made samples with a low damage threshold amenable for HRS measurements. One example of such sample is amorphous polymer films. These films do not have the optical quality of single crystals. They are more susceptible to optical damage. With the femtosecond pulses, we have been able to perform HRS measurements on solid thin films, to study the orientational correlation between nonlinear optical chromophores in the film.10 13... 

The titanium-sapphire wavelength itself is too short for vibrational excitation in most molecules, and too long for electronic excitation. However, these high peak powers permit efficient frequency conversion. For example, certain crystals can convert two photons with frequency co into a single photon with frequency 2co. In many ways this can be viewed as similar to a second-order chemical reaction, such as the dimerization of NO2 to form N2O4. The rate of that reaction is proportional to the square of the NO2 concentration the rate of this frequency doubling is proportional to the square of the photon concentration (the intensity), so high powers are very useful. It is also possible to combine two photons with different frequencies co and o>i in either sum-... 

Recent developments in ultrashort, high-peak-power laser systems, based on the chirped pulse amplification (CPA) technique, have opened up a new regime of laser-matter interactions [1,2]. The application of such laser pulses can currently yield laser peak intensities well above 1020 W cm 2 at high repetition rates [3]. One of the important features of such interactions is that the duration of the laser pulse is much shorter than the typical time scale of hydrodynamic plasma expansion, which allows isochoric heating of matter, i.e., the generation of hot plasmas at near-solid density [4], The heated region remains in this dense state for 1-2 ps before significant expansion occurs. 

A. Braun, G. Korn, X. Liu, D. Du, J. Squier, G. Mourou, Self-channeling of high-peak-power femtosecond laser pulses in air, Optics Letters 20, 73-75 (1995)... 

A. Iwasaki, N. Akozbek, B. Ferland, Q. Luo, G. Roy, C.M. Bowden, S.L. Chin, A Lidar technique to measure the filament length generated by a high-peak power femtosecond laser pulse in air, Applied Physics B 76, 231 (2003)... 

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