LASE

Fundamentally, the conditions for lasing are detemiined unambiguously once the populations and coherences of the system density matrix are known. Yet, we have been unable to find in the literature any simple criterion for lasing in multilevel systems in temis of the system density matrix alone. Our conjecture is that entropy, as... 

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... 

Laser type Lasing wavelength >. Efficiency r (%) Mode of operation Typical output power... 

The light emitted in the spontaneous recombination process can leave tire semiconductor, be absorbed or cause additional transitions by stimulating electrons in tire CB to make a transition to tire VB. In tliis stimulated recombination process anotlier photon is emitted. The rate of stimulated emission is governed by a detailed balance between absorjDtion, and spontaneous and stimulated emission rates. Stimulated emission occurs when tire probability of a photon causing a transition of an electron from tire CB to VB witli tire emission of anotlier photon is greater tlian that for tire upward transition of an electron from tire VB to tire CB upon absorjDtion of tire photon. These rates are commonly described in tenns of Einstein s H and 5 coefficients [8, 43]. For semiconductors, tliere is a simple condition describing tire carrier density necessary for stimulated emission, or lasing. This carrier density is known as... 

J. Gasteiger, Obtaining the 3D shnacture from inffared spectra of organic compounds lasing neural networks, in SoJiware-EntuHcklung in der Chemie 11, G. Pels, V Schubert (Eds.), Gesellschaft Deutscher Chemiker, Frankfurt/Main, 1997. 

Lasing substances Physical state Laser wavelength (nm) Pulse length or continuous wave Typical maximum power output (watts)... 

If the temperature were raised, more molecules would attain the excited state, but even at 50,000°C there would be only one excited-state atom for every two ground-state atoms, and stimulated emission would not produce a large cascade effect. To reach the excess of stimulated emissions needed to build a large cascade (lasing), the population of excited-state molecules must exceed that of the ground state, preferably at normal ambient temperatures. This situation of an excess of excited-state over ground-state molecules is called a population inversion in order to contrast it with normal ground-state conditions. 

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. 

The timing of the emission is clearly dependent on the system in use. For example, if pumping is relatively slow and stimulated emission is fast, then the emergent beam of laser light will appear as a short pulse (subsequent lasing must await sufficient population inversion). This behavior is... 

If a triplet-state molecule (A ) meets a singlet-state molecule (B ), a short-lived complex can be formed (an exciplex). In the latter, the molecules exchange energy, returning to its singlet state (A ) and B raised to its triplet state (B ). If the new triplet state is relatively long-lived, it can serve to produce the population inversion needed for lasing, as in the He/Ne laser. 

Two typical dye molecules. The europium complex (a) transfers absorbed light to excited-state levels of the complexed Eu , from which lasing occurs. The perylene molecule (b) converts incident radiation into a triplet state, which decays slowly and so allows lasing to occur. 

Photons of energy hcv are generated initially in the cavity through spontaneous emission. Those that strike the cavity mirrors at 90° are retained within the cavity causing the photon flux to reach a level which is sufflciently high to cause stimulated emission to occur, and the active medium is said to lase. 

Before we look at the various methods of pumping we shall consider the types of energy level scheme encountered in lasing materials. 

In the three-level system of Figure 9.2(b) population inversion between levels 2 and 1 is achieved by pumping the 3-1 transition. The 3-2 process must be efficient and fast in order to build up the population of level 2 while that of level 1 is depleted. Lasing occurs in the 2-1 transition. 

Figure 9.2 (a) Two-level (b) three-level and (c) four-level lasing systems... 

Methods of pumping, irrespective of the type of level system and of whether lasing is to be pulsed or CW, fall into two general categories—optical and electrical pumping. 

In some gas lasers it is preferable to use a mixture of the lasing gas M and a second gas N, where N serves only to be excited to N by collisions with electrons and to transfer this energy to M by further collisions ... 

The lasing medium in the titanium-sapphire laser is crystalline sapphire (AI2O3) with about 0.1 per cent by weight of Ti203. The titanium is present as Ti and it is between energy levels of this ion that lasing occurs. 

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