Beam waist

Fig. 7. Schematic diagram of forces exerted on a cell when using an inverted microscope with (A) epi-illumination (i.e., laser focused through the objective) or (B) transillumination (i.e., laser focused through the condenser). is the axial force, and Fl is the lateral trapping force. Curved arrows represent the laser beam waist and point in the direction of light propagation.

In practical applications, it should be kept in mind that the beam waist itself usually depends on the wavelength (at the diffraction limit, uiq is proportional to >.) and, therefore, that the comparison between IPA and 2PA excitation rates may not be as straightforward as that shown in Fig. 2. In general, the 2PA excitation volume depends on the focusing conditions and beam parameters used for the material excitation. 

Fig. 4. Comparison of closed aperture Z-scans for pure thf and solutions of Au(4,4 -C=CC6H4C=CC6H4N02)(PPh3) , thf 1.56 weight % O, 3.08 weight %. Wavelength = 800 nm, pulse duration = 100 fs, beam waist = 50 /u,m, maximum light intensity = 93 GW cm 1. The increasing asymmetry of the Z-scan indicates a strong two-photon absorption due to the imaginary part of the molecule nonlinearity. The real part of the nonlinearity of the solute is in this case positive (the same sign as the nonlinearity of the solvent).

The optical trapping method uses a highly focused laser beam to trap and manipulate particles of interest in a medium (illustrated in Figure 3). The laser is focused on a dielectric particle (e.g., a silica microscopic bead), the refractive index of which is higher than the suspension medium. This produces a light pressure (or gradient force), which moves the particle towards the focal point of the beam, that is, the beam waist (Lim et al., 2006). 

Open-aperture z-scan experiments with 7 and 20 p,m beam radii are given in Fig. 25. The sample 2 in Fig. 24a with 68% transmittance with 7 p,m beam waist radius shows very low NL absorption. The sample has no NL absorption, while the transmittance is 82%. On the other hand, open-aperture z-scan experiments with 20 Jim beam waist radius have high NL absorption for both transmittances, Fig. 25. 

Any thermal effect due to heat accumulation was not observed in these experiments. The NL absorption coefficients obtained from 7 xm beam waist radius were... 

Laser two-focus velocimetiy is applied in the BIRAL L2F. The measurement volume is formed by focusing two laser beams to two small waists of about 10 pm diameter. The result is a light gate operating with concentrations orders of magnitude greater than possible with the LDV system. Particle velocity is determined by the time it takes the particle to pass from one beam waist to the other. 

It is common to call tvp the beam-waist radius. In the literature one often finds the phrase at the beam waist, which refers to that value of z for which the function u has its minimum radial extent. For u defined by (8), this occurs at z = 0. The distance Zp is called the confocal distance. When z < Zp we say that the Gaussian beam is in the near field. When z > Zp, the Gaussian beam is in the far field. The majority of this chapter is concerned with the behavior of u in the range 0 < z < Zp, the near-field region. The phase and amplitude of m is a complicated function of position in the near field. When z Zp and p z or when we are in the far field and the paraxial approximation is valid, it is straightforward to show that the asymptotic behavior of u approaches a diverging spherical wave from a point source at z = 0. 

We have used the much simpler conical horn on our detector and Fabry-Perot resonator. We may estimate the ratio of the beam waists for a scalar and conical horn by calculating the ratio of their grains =... 

---