Two-level laser

It has been said tliat anytliing will lase if pumped witli enough energy, but tire efficiency of tire pumping process is important for practical, economical devices. In tliis regard two-level lasers are of little interest because, except under extraordinary pumping conditions, one can only equalize tire populations of tire upper and lower levels. A... 

Note that operation of a two-level laser system requires inversion of the population of the upper laser level with respect to the ground state—a theoretical impossibility under steady-state conditions if optical excitation is employed. 

The mechanism consists of using a conventional energy source (flash-lamp or other) to excite atoms or molecules from the ground state to some excited state, so that an inversion of population occurs in the Arrhenius55 sense this is usually best understood in a three-level laser, although two-level lasers are also discussed (see Problem 10.10.1), and many are four-level lasers (see Fig. 10.14). 

PROBLEM 10.10.1. By using Einstein s theory of optical absorption and spontaneous and stimulated emission (Section 3.33), show that an isolated two-level laser system at thermodyamic equilibrium is impossible. (People talk about two-level lasers as possible if and only if there is some way of getting rid of a direct decay path from the excited state to the ground state, for example, by using an external magnetic field.)... 

The relaxation step is needed so that our emitting state will have an appropriate lifetime before stimulated emission takes place. In other words, we cannot use a two-level laser, i.e.- excited state and ground state, because the excited state will not possess a lifetime consistent with that required to achieve an Inverted population. Thus, a two-level system is fine for spontaneous emission (fluorescence) but not for a laser. 

In the three-body reaction (Reaction [13.5]), the Ne acts as a buffer. Given that the krypton fluoride so produced is electronically excited and has a short lifetime (about 2.5 ns), it rapidly decays by photon emission to the lower energy state as shown in Fig. 13.5. Because this is an unbound state in which the force between the atoms is always repulsive, the exciplex molecule then immediately dissociates into its constituent atoms. As a result, this state never attains a large population, and a population inversion therefore exists between it and the higher energy bound exciplex state. The decay transition can therefore be efficiently stimulated to produce laser emission. One noteworthy characteristic of this particular laser system is that it represents a rare example of a truly two-level laser. 

FIGURE 2 The conduction and valence bands of a direct band-gap semiconductor crystal act as the excited and ground state energy levels in a quasi two-level laser system, (a) Thermal equilibrium. (b) Pumping via injected current. 

In the previous section we discussed light and matter at equilibrium in a two-level quantum system. For the remainder of this section we will be interested in light and matter which are not at equilibrium. In particular, laser light is completely different from the thennal radiation described at the end of the previous section. In the first place, only one, or a small number of states of the field are occupied, in contrast with the Planck distribution of occupation numbers in thennal radiation. Second, the field state can have a precise phase-, in thennal radiation this phase is assumed to be random. If multiple field states are occupied in a laser they can have a precise phase relationship, something which is achieved in lasers by a teclmique called mode-locking Multiple frequencies with a precise phase relation give rise to laser pulses in time. Nanosecond experiments... 

Ulness D J and Albrecht A C 1996 Four-wave mixing in a Bloch two-level system with incoherent laser light having a Lorentzian spectral density analytic solution and a diagrammatic approach Rhys. Rev. A 53 1081-95... 

It was shown above that the normal two-level system (ground to excited state) will not produce lasing but that a three-level system (ground to excited state to second excited state) can enable lasing. Some laser systems utilize four- or even five-level systems, but all need at least one of the excited-state energy levels to have a relatively long lifetime to build up an inverted population. 

Figure 12.9 The excitation-power dependence of the emission count rate of single DMPBI nanocrystals (dots), and a saturation curve calculated from a two-level model (solid line). One count rate value to one laser power was calculated as an average of 30 nanocrystals. S. Masuo, A. Masuhara, T. Akashi, M. Muranushi,...

Dye lasers are usually pumped by another laser, and a selection of around 25 dyes typically provides coverage of a wide region from 350 to 900 nm. Organic dye lasers are four-level lasers, even though only two electronic levels may be used (Figure 1.19). 

The active medium also determines the pumping scheme. Commonly, two types of operational schemes are used to describe laser operation /oMr-/cvc/ and three-level laser systems ... 

Toschek and coworkers 345) used a technique called tuned laser differential spectrometry which is based on simultaneous interaction of gas atoms with two different laser beams, one of these being a weak probe beam the tuning of which scans the saturation on the common level of the two transitions induced by the other beam 346) The experiment employed the two He-Ne laser lines at X = 1.15 ju and X = 0.6328 which share the common lower level. 

As already introduced in section I of this chapter, in a CARS process (Figures 7.9a-c see also Figure 7.1c), a Raman transition between two vibrational energy levels of a molecule is coherently driven by two optical laser fields (frequencies co and co) and subsequently probed by interaction with a third field at frequency co, . This generates the anti-Stokes signal at the blue-shifted frequency cars = p- The... 

Accuracy of the radiofrequency measurements of the classic 2S — 2P Lamb shift [15, 16, 23, 24, 25] is limited by the large (about 100 MHz) natural width of the 2P state, and cannot be significantly improved. New perspectives in reducing the experimental error bars of the classic 2S — 2P Lamb shift were opened with the development of the Doppler-free two-photon laser spectroscopy for measurements of the transitions between the energy levels with different principal quantum numbers. Narrow linewidth of such transi-... 

Figures 6a-c show the population dynamics encountered in a three-level system (see Fig. 4) interacting resonantly with two Fourier-transform-limited laser pulses with three different delay times between the two pulses. The calculation was done assuming that the chosen Rabi frequencies fulfill the relation > 1/pulse duration) in all three cases. This relation ensures that the typical time for a Rabi oscillation of the population in an isolated two-level system is shorter than the pulse duration. Ionization from level 2 was introduced as a fast laser intensity-dependent decay of level 2 [6, 60], and resonant laser frequencies were assumed.

General Principles Two-, Three- and Four-level Lasers... 

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