Applications of lasers M. Rudan

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2 ELECTROMAGNETIC SPECTRUM LASER (Zanichelli, 1999), p. 17.

3 HETEROSTRUCTURE LASER LASER (Zanichelli, 1999), p. 103.

4 FUNDAMENTAL SCIENCE (I) The laser emission is monochromatic and has a high brightness. These properties may be exploited, e.g., to investigate phenomena related to the interaction of radiation with matter. Second-harmonic generation. If a material does not exhibit a transition at the laser frequency, the most important phenomenon is polarization. In the linear case, P = c E with P the polarization vector, E the electric field, and c the dielectric susceptivity. In the high-field, scalar case it is P = c 1 E + c 2 E 2 + c 3 E 3

5 FUNDAMENTAL SCIENCE (II) For isotropic materials the even terms are missing because it must be P (E) = P( E). For non-isotropic materials, assuming that E has the form E = E 0 sin ( ωt ), the second-order summand contributes the term P 2 = (1/2) c 2 E 2 0 [ 1 - cos ( 2ωt ) ] to the series, which contains the second harmonic. The emitted second harmonic has the same direction as the incoming radiation.

6 APPLIED SCIENCE - BRIGHTNESS (I) The small diameter of the laser beam, absence of contamination, and fine control of the released energy, can be exploited for treating materials. This concept is applicable to different fields, e.g., Microelectronics technology: precision etching, annealing. Mechanical technology: shaping of hard crystals in watch industry, precise formation of holes in hard metals, balancing of rotating masses by evaporation of small amounts of material (vibration measurement performed during rotation), soldering of refractory materials, cleaning of surfaces (e.g., by shallow UV absorption).

7 SURFACE CLEANING LASER (Zanichelli, 1999), p In many materials, the absorption of UV radiation happens in a very shallow layer. The surface dirt absorbs the UV radiation and vaporizes.

8 APPLIED SCIENCE - BRIGHTNESS (II) Biology: excitation of living cells, killing of cells to monitor the behavior of the neighboring cells. Medicine: the laser beam may be used as a scalpel. If the emission is in the blue region (e.g., Ar + laser), there is a strong absorption by the blood cells, which helps the blood to coagulate. The laser is also used to fix the retina by thermal effect. The laser beam crosses harmlessly the transparent parts of the eye. In all medicine application, the functioning of the laser is in the pulse mode.

9 FIXING OF THE RETINA LASER (Zanichelli, 1999), p. 35. This type of operation exploits the transparency of the external layer and of the interior of the eye. The retina interacts with radiation and is heated up.

10 APPLIED SCIENCE - COHERENCE (I) Laser gyroscope: it is based on the Doppler effect. Long-distance interferometry: precision mechanical tools (e.g., equipments for electron-beam lithography), geological survey (shift of earth platforms, low-frequency vibrations? terrestrial tides ), radar detection (e.g., missiles), extra-terrestrial survey (measurement of the earth-moon distance), long-term deformations and buildings (e.g., hysteresis of dams may lead to destruction), analysis of the internal strain in mechanical structures or buildings.

11 LASER GYROSCOPE LASER (Zanichelli, 1999), p. 61. LASER (Zanichelli, 1999), p. 60. When the equipment rotates, the Doppler effect produces interferences that are detected at the mirrors.

12 LASER INTERFEROMETRY (I) LASER (Zanichelli, 1999), p. 57.

13 LASER INTERFEROMETRY (II) LASER (Zanichelli, 1999), p. 47. Remote control of machinery

14 LASER INTERFEROMETRY (III) LASER (Zanichelli, 1999), p. X. Measuring changes in the atmospheric composition due to pollution.

15 LASER INTERFEROMETRY (IV) LASER (Zanichelli, 1999), p. 2. LASER (Zanichelli, 1999), p. 5. Measuring the Earth-Moon distance. Pulse duration < 1 ns, error < 2.5 cm.

16 APPLIED SCIENCE - COHERENCE (II) Telecommunications: very large band (the optical range provides an available band that is about 10 4 times larger than the microwave band). The diffraction divergence of an optical beam is about 10 4 times smaller (same reason). The difficulty arising from the strong attenuation due to the atmosphere is solved by using the optical fibers. Modulation is achieved, e.g., by solid-state lasers. Holography: both the intensity and phase information are stored in a film. Optical data processing: based on the far-field properties.

17 HOLOGRAPHY (I) LASER (Zanichelli, 1999), p. 62. Picture using a stenopeic pinhole. The effect of the pinhole is letting each point in the film be illuminated by a single point of the original object. The film detects the intensity of each color component.

18 HOLOGRAPHY (II) Picture using holography. The film detects the intensity, however, the latter is due to the interference between the radiation reflected by the object and the reference beam. The light source is monochromatic. After the interference pattern is stored in the film, illumination by the reference beam interacts with the interference pattern. This produces the three-dimensional image of the original object. From the mathematical point of view, the procedure is equivalent to replacing the data of a PDE with a set of boundary conditions, such that the solution inside a given volume is left unchanged. LASER (Zanichelli, 1999), p. 64.