Monochromatic radiation coherent

We now add die field back into the Hamiltonian, and examine the simplest case of a two-level system coupled to coherent, monochromatic radiation. This material is included in many textbooks (e.g. [6, 7, 8, 9, 10 and 11]). The system is described by a Hamiltonian having only two eigenstates, i and with energies = and Define coq = - co. The most general wavefunction for this system may be written as... 

Both processes involve the generation of radiation which is unidirectional, highly monochromatic and coherent. The latter feature describes the in-phase nature of the wavelets and, hence the high intensities that can be achieved with solid lasers... 

Conversely, we may observe an exceedingly narrow spectral line, so that o(x ) is approximated by <5(x ). Now the data i(x) represent the response function. This principle can, in fact, be used to determine the response function of a spectrometer. The laser, for example, is a tempting source of monochromatic radiation for measuring the response function of an optical spectrometer. Coherence effects, however, complicate the issue. We present further detail in Section II of Chapter 2. 

Another revolutionary application of electronically excited molecular systems is in laser technology. Lasers are intense sources of monochromatic and coherent radiation. From their early development in 1960 they have found wide fields of application. They have provided powerful tools for the study of diverse phenomena ranging from moonquakes to picosecond processes of nonradiative decay of excitational energy in molecules. The intense and powerful beam of coherent radiation capable of concentra-... 

Laser Doppler velocimetry is a powerful technique for the in situ measurement of fluid velocities. The basic optical configuration for the measurement is shown in Figure 6.1. The velocity measurement is made at the intersection of two laser beams that are focused to a point in the flow. The use of laser radiation is essential since the light must be monochromatic and coherent. This is required since the intersection of the two beams must create an interference pattern within the fluid. Such a pattern is shown in Figure 6.2, where two plane waves intersect at an angle 2(J). The two waves will have the following form [55] ... 

The basis of NLO-effects arising from susceptibilities of second order, is the interaction of three electric fields with a material. The practical implementation of optical devices requires strong, coherent and monochromatic radiation and hence, laser technology. Not all of the interacting fields need to be optical fields, however. In devices that make use of the Pockels effect, an externally applied electric field is used to alter reversibly the refractive index of a material. In a second harmonic generation (SHG) process two photons of circular frequency w can be transformed into one photon of frequency Iw. SHG is the NLO effect used most for the evaluation of /3-tensor elements in solution. 

A Raman spectrum is excited by irradiating a sample with coherent or non-coherent monochromatic radiation in the ultraviolet, the visible, or the near-infrared range. By an elementary process described in Sec. 2.4, the sample produces usually non-coherent radiation the strong Rayleigh line at the frequency of the exciting radiation and weak lines at frequencies. shifted from the frequency of the exciting radiation by definite quantities, the Raman spectrum. 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

Lasers with short pulses are not used in Raman spectrometers, mainly because the detectors in Raman spectrometers are tuned to high sensitivity. Such detectors are very easy to saturate and this is a case where short and intense laser pulses are employed for excitation of Raman scattering. It must be noted, that gas lasers are not perfect sources of monochromatic radiation. Together with intense coherent radiation such lasers produce weak incoherent radiation, caused by a different transition between electronic energy levels of the gas. The intensity of this incoherent and noncollimated radiation can be suppressed by increasing the distance between the laser and the sample, by placing a spatial filter (consisting of two lenses and a pinhole) or a narrow-band filter (usually an interference filter) into the laser beam. 

The term laser is an acronym (light amplification by stimulated emission of radiation) that denotes a technical device operating on the basis of the stimulated emission of light. A laser emits monochromatic, spatially coherent, and strongly polarized light. The essential parts of a laser device are an active material and a resonator, i.e. an optical feedback (see Fig. 6.10). 

Several different laser techniques for generating VUV and XUV radiation are now available, and these have been discussed briefly, along with the relevant theory. Of these, harmonic generation and four-wave frequency mixing have been shown to provide coherent and monochromatic radiation which is tunable over broad regions of the spectrum, from 200 nm to 70 nm, and with limited tunability to 50 nm. While these laser-driven sources are not presently available commercially, they have been developed in several laboratories around the world, and have been used in various fields of research. 

General Analytical and Industrial Applications Lasers are used both for analytical and industrial purposes. Table 3.3 summarises the main analytical fields of application. The most obvious reason to involve lasers in analytical chemistry is the directionality of the radiation (beam divergence <0.5 mrad), which implies high irradiances at remote objects (up to 10 W cm ) and compatibility with miniaturised systems. Characteristics as monochromaticity and coherence are still of less importance. The monochromaticity of the laser lines is of major importance in techniques such as RS and those based on multiphoton processes. Some important analytical applications of lasers are ... 

Figure 2.4 shows some types of refractive microlenses that can be fabricated utilizing the standard microfabrication procedures in materials convenient for the MWIR and LWIR ranges. Most of them are loosely based on the solutions for microlenses used in fiber optics to improve coupling between laser sources and fibers [98]. These immersion lenses were thus intended for the operation with coherent and monochromatic radiation, while most of the microlenses in the field of IR detector technology are intended for incoherent, mono- or polychromatic Lambertian sources and, of course, they operate in different atmospheric windows. 

---