Creating an Inverted Population

Creating an Inverted Population

The excitation of the atoms in a laser can be done in several ways to produce the FIGURE 28–19 Energy levels of necessary inverted population. In a ruby laser, the lasing material is a ruby rod chromium in a ruby crystal. Photons

consisting of Al 2 O 3 with a small percentage of aluminum (Al) atoms replaced by of energy 2.2 eV “pump” atoms from chromium (Cr) atoms. The Cr atoms are the ones involved in lasing. In a process E 0 to E 2 , which then decay to called optical pumping, the atoms are excited by strong flashes of light of wave- metastable state E 1 . Lasing action length 550 nm, which corresponds to a photon energy of 2.2 eV. As shown in occurs by stimulated emission of photons in transition from E 1 to E 0 Fig. 28–19, the atoms are excited from state . E

0 to state E 2 . The atoms quickly decay either back to E 0 or to the intermediate state E 1 , which is metastable with a

lifetime of about 3 * 10 –3 s (compared to 10 –8 s for ordinary levels). With strong

0.4 eV E 1 (metastable)

pumping action, more atoms can be found in the E 1 state than are in the E 0 state.

Thus we have the inverted population needed for lasing. As soon as a few atoms

in the E 1 state jump down to E 0 , they emit photons that produce stimulated

1.8 eV emission of the other atoms, and the lasing action begins. A ruby laser thus emits a

2.2 eV

beam whose photons have energy 1.8 eV and a wavelength of 694.3 nm (or “ruby-red” light).

E 0 An excited atom may land in such a state and can jump to a lower state only by a so-called forbidden

transition (Section 28–6), which is why its lifetime is longer than normal.

SECTION 28–11 Lasers 821

Collision In a helium;neon laser (He–Ne), the lasing material is a gas, a mixture of Helium E Neon

E 1 about 85% He and 15% Ne. The atoms are excited by applying a high voltage to

1.96 eV

E 2 the tube so that an electric discharge takes place within the gas. In the process, ⬘ some of the He atoms are raised to the metastable state E 1 shown in Fig. 28–20,

which corresponds to a jump of 20.61 eV, almost exactly equal to an excited state 20.61 eV

20.66 eV

18.70 eV

in neon, 20.66 eV. The He atoms do not quickly return to the ground state by spontaneous emission, but instead often give their excess energy to a Ne atom when they collide—see Fig. 28–20. In such a collision, the He drops to the ground œ

E 0 E ⬘ 0 state and the Ne atom is excited to the state E 3 (the prime refers to neon states). The slight difference in energy (0.05 eV) is supplied by the kinetic energy of the

FIGURE 28–20 Energy levels for

3 state in Ne—which is metastable—becomes œ œ electric discharge to the E more populated than the E level. This inverted population between E and 1 E state. 2 3 œ 2

moving atoms. In this manner, the œ E

He and Ne. He is excited in the

This energy is transferred to the œ E 3 is what is needed for lasing.

level of the Ne by collision. œ E 3 is

Very common now are semiconductor diode lasers, also called pn junction

lasers, which utilize an inverted population of electrons between the conduction stimulated emission.

metastable and decays to E 2 by

band and the lower-energy valence band (Section 29–9). When an electron jumps down, a photon can be emitted, which in turn can stimulate another electron to make the transition and emit another photon, in phase. The needed mirrors (as in Fig. 28–18) are made by the polished ends of the pn crystal. Semiconductor lasers are used in CD and DVD players (see below), and in many other applications.

Other types of laser include: chemical lasers, in which the energy input comes from the chemical reaction of highly reactive gases; dye lasers, whose frequency is tunable; CO 2 gas lasers , capable of high power output in the infrared; and rare-earth solid-state lasers such as the high-power Nd:YAG laser. The excitation of the atoms in a laser can be done continuously or in pulses. In a pulsed laser, the atoms are excited by periodic inputs of energy. In a continuous laser, the energy input is continuous: as atoms are stimulated to jump down to the lower level, they are soon excited back up to the upper level so the output is a continuous laser beam.

CAUTION

No laser is a source of energy. Energy must be put in, and the laser converts Laser is not an energy source

a part of it into an intense narrow beam output.