1. 1.5 Types of lasers
Lasers may be classified according to several criteria:
Whether the gain medium is a gas, liquid, or solid
Fixed frequency or tuneable
Emission range (UV to IR)
Excitation source
CW or pulsed
Laser power
We will somewhat arbitrarily look at lasers based on whether the gain medium is a gas, liquid, or solid.
Gas: HeNe laser, excimer lasers, CO2 lasers
Liquid: dye lasers
Solid: Nd:YAG laser, Ti:sapphire laser
Diode (semiconductor) lasers

2. Summary of some common lasers
Types of lasers
Summary of some common lasers
Laser
Pulsed
or CW*
Tuneable
Emission wavelengths
HeNe CW No
632.8 nm (1.15 μm)
Ar ion
351, 455, 458, 466, 477, 488, 497, 502, 515, 529 nm N2 P
337 nm
Excimer
No**
190 – 350 nm
CO2
Both
Yes
10.6, 9.6 μm**
Dye lasers
365 – 930 nm
Nd:YAG
1064 nm (532, 355, & 266 nm)***
Alexandrite
720 – 800 nm
Ti:sapphire
670 – 1100 nm
Diode lasers
UV to mid-IR
* Typical mode of operation
** Tuneable over a narrow range (or depending on particular laser)
*** 2nd, 3rd etc. harmonics

3. The helium-neon (HeNe) laser
The HeNe laser is a common, relatively low power and low cost CW laser. The gain medium is a helium-neon gas mixture. The HeNe is a 4-level laser with the following energy levels:
Lasing occurs between excited energy levels of neon, but helium is necessary to transfer pump energy (from an electric or microwave discharge) to the neon atoms.
Helium atoms are excited by collisions with electrons to various excited states, of which the 21S0 and 23S1 states are metastable and long-lived because radiative transition back to the 11S ground state is forbidden.

4. HeNe laser
Ne has ground configuration 1s22s22p6 and excited states 2p5ns1 and 2p5np1 (with n>2).
The 2p5ns1 levels are much-longed lived than the 2p5np1 levels, so it is feasible to produce a population inversion.
Lasing may occur between various energy levels, but the nm line (red) is most commonly used for HeNe lasers.
The near-IR lines at 3.39 μm are not often used but interfere with lasing at 633 nm by depleting the population of the upper state.
This effect can be minimised using cavity mirrors that only reflect red light, allowing lasing to take place at 633 nm only.
Decay from 2p53p1 to 2p53s1 is rapid, but 2p53s1 is a long-lived state. 2p53s1 Ne atoms can lose this energy to wall collisions – for this reason, HeNe lasers have narrow tubes.

5. Excimer lasers Excimer lasers are pulsed, high power lasers
“Excimer”: excited dimer (E.g., He2)
“Exciplex”: excited complex (dissimilar atoms) (E.g., ArF)
Excimers and exciplexes are molecules characterised by a dissociative ground state, but by a bound potential for an excited electronic state:
Since the lower state is very short-lived, a population inversion can also be achieved relatively easily.

6. Noble gas-halogen exciplexes are useful for many laser applications.
Excimer lasers
Noble gas-halogen exciplexes are useful for many laser applications.
(While technically exciplex lasers, these lasers are usually called excimer lasers.)
E.g., ArF (193 nm), KrF (248 nm), XeF (351 nm), KrCl (222 nm), XeCl (308 nm), XeBr (282 nm)
An electric discharge is used to pump the laser.
Although the ground energy level is short-lived, in some cases the lower level potential may be very slightly bound, allowing some tuneability of the laser.
Note that the excimer laser can be changed by exchanging the gas mixture (along with the HR and OC).

7. The carbon dioxide laser
CO2 laser
The carbon dioxide laser
The CO2 laser is a high power infrared laser of high efficiency that may be pulsed or CW. It lases between vibrational levels of CO2.
CO2 has 3 normal modes of vibration – the symmetric stretch (ν1) at 1354 cm-1, the bending vibration (ν2) at 673 cm-1, and the assymetric stretch (ν3) at 2396 cm-1.
Since the lower state is very short-lived, a population inversion can also be achieved relatively easily.

8. A He:CO2:N2 mixture is used.
CO2 laser
Pumping is achieved by electric discharge – some CO2 molecules are directly excited, together with efficient transfer from excited N2 to CO2.
Transitions between vibrational levels also involve rotational transitions, giving rise to a relatively large number of closely spaced emission lines – the laser can be tuned between these transitions.
A He:CO2:N2 mixture is used.
Very high laser (up to kW) powers can be achieved.