Laser power

At low laser powers, the fluorescence signal is Imearly proportional to the power. Flowever, the power available from most tunable laser systems is suflFicient to cause partial saturation of the transition, with the result that the fluorescence intensity is no longer linearly proportional to the probe laser power. While more... 

The fluorescence signal is linearly proportional to the fraction/of molecules excited. The absorption rate and the stimulated emission rate 1 2 are proportional to the laser power. In the limit of low laser power,/is proportional to the laser power, while this is no longer true at high powers 1 2 <42 j). Care must thus be taken in a laser fluorescence experiment to be sure that one is operating in the linear regime, or that proper account of saturation effects is taken, since transitions with different strengdis reach saturation at different laser powers. 

Figure B2.3.13. Model 2-level system describing molecular optical excitation, with first-order excitation rate constant W 2 proportional to the laser power, and spontaneous (first-order rate constant 21) stimulated (first-order rate constant 1 2 proportional to the laser power) emission pathways.

Laser desorption to produce ions for mass spectrometric analysis is discussed in Chapter 2. As heating devices, lasers are convenient when much energy is needed in a very small space. A typical laser power is 10 ° W/cm. When applied to a solid, the power of a typical laser beam — a few tens of micrometers in diameter — can lead to very strong localized heating that is sufficient to vaporize the solid (ablation). Some of the factors controlling heating with lasers and laser ablation are covered in Figure 17.2. 

CW dye lasers are usually pumped wifh an argon ion laser, up to abouf f W of continuous dye laser power being produced, compared wifh abouf f MW peak power which may be produced in a pulsed dye laser. 

From the write and read process sketched so far, some requirements for MO media can be derived (/) a high perpendicular, uniaxial magnetic anisotropy K in order to enable readout with the polar Kerr effect (2) a magnetoopticady active layer with a sufficient figure of merit R 0- where R is the reflectivity and the Kerr angle (T) a Curie temperature between 400 and 600 K, the lower limit to enable stable domains at room temperature and the upper limit because of the limited laser power for writing. 

Fig. 16. Maximum achievable signal-to-noise ratio (SNR) on read-out of different writable optical data storage systems as a function of the writing energy (laser power) (121). SQS = Organic dye system (WORM) PC = phase change system (TeSeSb) MO = magnetooptical system (GbTbFe). See text.

Raman spectroscopy of graphite can be an experimental challenge, because the material is a strong blackbody absorber. Generally, low (1—10-mW) laser power is used to minimise heating, which causes the band positions to change. In addition, the expansion of the graphite causes the material to go out of the focus of the optical system, an effect which can be more pronounced in microprobe work. 

Multiphoton Absorption and Ionization. High laser powers can induce the simultaneous absorption of two or more photons that together provide the energy necessary to excite a transition this transition may be one that is forbidden as a single-photon process (8,297). Such absorption can be made Doppler-free by propagating two laser beams of frequency V in opposite directions, so the Doppler shifts cancel and a two-photon transition occurs at 2v for any absorber velocity. The signal is strong because aU absorbers contribute, and peak ampHtudes are enhanced by, which may... 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

For the detection of weak Raman lines, high laser power, high signal amplification, long pen period, and very slow scanning speed should be... 

Figure 5. Dependence of photon Figure 6. The Zeeman sub-levels of the ground state return per Watt of laser power on second excited state, and the effects of optical...

For a typical spot size of 1 arcsec the laser intensity is nominally 2 mW/cm, so the laser power must be < 2.4 W to avoid saturation for a single frequency... 

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