The Ruby Laser

As we have mentioned, laser action was first observed in ruby [8.14], Ruby is a red crystal of AI2O3 with an addition of about 0.05% Gr20s. Only the ions are of interest here since the other ions do not participate m the process. The chromium ion has three d electrons hi its unfilled shell and has a term as the gromid-state term. The next higher state is a term. The ruby crystal has a weakly rhombic structure. Because of the action of the crystal field, the term will be split into the levels iid A.2, where the designations 

---

Neodymium and YAG Lasers. The principle of neodymium and YAG lasers is very similar to that of the ruby laser. Neodymium ions (Nd +) are used in place of Cr + and are often distributed in glass rather than in alumina. The light from the neodymium laser has a wavelength of 1060 nm (1.06 xm) it emits in the infrared region of the electromagnetic spectrum. Yttrium (Y) ions in alumina (A) compose a form of the naturally occurring garnet (G), hence the name, YAG laser. Like the ruby laser, the Nd and YAG lasers operate from three- and four-level excited-state processes. 

Pumping is with a flashlamp, as in the case of the ruby laser, and a pulse energy of the order 1 J may be achieved. Frequency doubling (second harmonic generation) can provide tunable radiation in the 360-400 nm region. 

Despite the fact that the first laser to be produced (the ruby laser. Section 9.2.1) has the remarkable property of having all its power concentrated into one or two wavelengths, a property possessed by most lasers, it was soon realized that the inability to change these wavelengths appreciably, that is to tune the laser, is a serious drawback which limits the range of possible applications. 

Laser action involves mainly the 3/2 hi/i transition at about 1.06 pm. Since is not the ground state, the laser operates on a four-level system (see Figure 9.2c) and consequently is much more efficient than the ruby laser. 

Lasers produce spatially narrow and very intense beams of radiation, and lately have become very important sources for use in the UV/VIS and IR regions of the spectrum. Dye lasers (with a fluorescent organic dye as the active substance) can be tuned over a wavelength range of, for instance, 20-50 nm. Typical solid-state lasers are the ruby laser (0.05% Cr/Al203 694.3 nm) and the Nd YAG laser (Nd3+ in an yttrium aluminium garnet host 1.06 pm). 

But the next year, in 1960, Ted Maiman built the first laser, the ruby laser. He did that work at Hughes, and I ll ask you to note, now, as we moved from the ideas to its being a hot field with everybody interested, that... 

Lasers are devices for producing coherent light by way of stimulated emission. (Laser is an acronym for light amplification by stimulated emission of radiation.) In order to impose stimulated emission upon the system, it is necessary to bypass the equilibrium state, characterized by the Boltzmann law (Section 9.6.2), and arrange for more atoms to be in the excited-state E than there are in the ground-state E0. This state of affairs is called a population inversion and it is a necessary precursor to laser action. In addition, it must be possible to overcome the limitation upon the relative rate of spontaneous emission to stimulated emission, given above. Ways in which this can be achieved are described below, using the ruby laser and the neodymium laser as examples. 

The first laser produced was the ruby laser, invented in 1960. Rubies are crystals of aluminum oxide (corundum, AI2O3), containing about 0.5% chromium ions Cr3+, as substitution impurities, CrA, and laser action, as well as color, is entirely due to these... 

Solid-state lasers, such as the ruby laser, neodymium doped yttrium aluminium garnet (Nd-YAG) laser and the titanium doped sapphire laser. 

Figure 1.16 Transitions between energy levels in the ruby laser...

There are many solid state lasers. One of the most commonly treated types in laser textbooks is the ruby laser (Al203 Cr +), which was the first laser system demonstrated by T. H. Maiman at the Hughes Research Laboratory early in 1960 (Maiman, 1960). Figure 6.9 in Chapter 6 will show the quantum energy levels associated with the unfilled 3d inner shell of the Cr + ion when it substitutes for the AP+ ion in the AI2O3 lattice crystal. By using a ruby rod placed inside a spiral flashlamp filled with a hundreds of torrs of xenon, it is possible to optically pump Cr + ions from the " A2g ground state into the broad " T2 and " Ti bands of the excited levels. After a rapid relaxation down to the very sharp Eg level, laser emission can be produced at 694 nm via the Eg " A2g transition. 

The optical features of a center depend on the type of dopant, as well as on the lattice in which it is incorporated. For instance, Cr + ions in AI2O3 crystals (the ruby laser) lead to sharp emission lines at 694.3 nm and 692.8 nm. However, the incorporation of the same ions into BeAl204 (the alexandrite laser) produces a broad emission band centered around 700 nm, which is used to generate tunable laser radiation in a broad red-infrared spectral range. 

Cr + ions in aluminum oxide (the ruby laser) show a sharp emission (the so-called Ri emission line) at 694.3 nm. To a good approximation, the shape of this emission is Lorentzian, with Av = 330 GHz at room temperature, (a) Provided that the measured peak transition cross section is c = 2.5 x 10 ° cm and the refractive index is = 1.76, use the formula demonstrated in the previous exercise to estimate the radiative lifetime, (b) Since the measured room temperature fluorescence lifetime is 3 ms, determine the quantum efficiency for this laser material. 

The experiment also yields deactivation cross-sections in addition to results on the photochemical reaction, and it sets an upper limit for continuous absorption in Br2 at the ruby laser wavelength. 

Polymerization of styrene and p-isopropylstyrene could be photo-initiated with radiation from the ruby laser in the absence of photosensitizers and oxygen Since ordinarily no unsensitized photoinitiation of styrene is detected for wavelengths longer than 4000 A, the results of this experiments must be due to two-photon processes. 

Very broadly speaking, two situations have to be considered in explaining devices such as those we have mentioned. In the first, which is relevant to the ruby laser and to phosphors for fluorescent lights, the light is emitted by an impurity ion in a host lattice. We are concerned here with what is essentially an atomic spectrum modified by the lattice. In the second case, which applies to LEDs and the gallium arsenide laser, the optical properties of the delocalised electrons in the bulk solid are important. 

We shall see now the role played by a similar forbidden transition in the operation of the ruby laser. 

In phosphors and in the ruby laser, light was absorbed and emitted by electrons localised on an impurity site, but in other optical devices, delocalised electrons emit the radiation. In the next section, therefore, we shall consider the absorption and emission of radiation in solids with delocalised electrons, particularly in semiconductors. 

An essentially incomplete list of other dyes suitable for operation at the ruby laser wavelength can be found in 1>. Some merocyanine dyes were investigated as Q-switches in 26>. 

Sometimes 3(d — n ) and k ) states are said to be derived from delocalized orbitals and d—d) state from localized orbitals. The shift of the chelate emission from that of the free ligand increases in the sequence Rh(III) < Ir(III) < Ru(II) and reflects increasing cf-orbital participation in the emission orbital. The decrease in the chelate emission lifetime from the free ligand values also reflect the contamination of the molecular orbitals with d-character. The role of metal complexes as quenchers of excited states of it-electrons in organic compounds can be rationalized from such considerations. Emission from Cr8+ is the basis of one of the most important solid state laser system, the Ruby laser (Figure 10.14). 

A continuous laser operates by continually pumping atoms or moie-cules into the excited state from which induced decay produces a continuous beam of coherent radiation. The He—Ne laser is an example of continuous system. Another mode of operation is to apply an energy pulse to the system, exciting a considerable fraction into the excited state. When all these molecules or atoms are induced to decay simultaneously, intense but exteremely short pulse of coherent radiation is emitted. The ruby laser falls in this category. 

The first laser produced was the ruby laser. Ruby is a crystal of A1203 in which a fraction of 1 percent of the Al3+ ions is replaced by Cr3+ ions. A diagram of the lowest Cr3+ levels in the ruby crystal is given in Fig. 3.3. The symbols A2, E, T2 are the symmetry-species symbols (Section 1.19). (These are derived using ligand-field theory.) The ruby laser consists of a... 

A major advance in the investigation of the intramolecular dynamics of spin equilibria was the development of the Raman laser temperature-jump technique (43). This uses the power of a laser to heat a solution within the time of the laser pulse width. If the relaxation time of the spin equilibrium is longer than this pulse width the dynamics of the equilibrium can be observed spectroscopically. At the time of its development only two lasers had sufficient power to cause an adequate temperature rise, the ruby laser at 694 nm and the neodymium laser at 1060 nm. Neither of these wavelengths is absorbed by solvents. Various methods were used in attempts to absorb the laser power, with partial success for microsecond relaxation times. 

These are some of the most important lasers used in photochemical research. As they are rather similar only the ruby laser is described here in detail. The active material of the ruby laser is a dispersion of Cr3+ ions in alumina, A1203, in the form of a glass rod. This is in fact a synthetic ruby , not the natural half-precious stone which would not have the required degree of purity the details of the synthetic process are outside the scope of this book. 

---