1. Dye lasers The gain medium in a dye lasers is a solution made with an organic dye molecule. The solution is intensely coloured owing to the very strong absorption from the ground electronic state S 0 to the first excited singlet state S 1. Fluorescence to the ground state also has a high quantum efficiency, Φ F. Possible processes: 1,2: Absorption from S 0 to S 1, S 2 3,4: Rapid collisional relaxation to ground vibrational level of S 1 5: Fluorescence to various vibrational states of S 0 (basis for laser emission) 6: Vibrational relaxation to S 0 ground vibrational level 7: ISC to triplet state T 1 8: Absorption between triplet states 9: Phosphorescence to S 0 This is essentially a 4-level laser: pumping at 1 or 2; laser emission at 5.

2. The fluorescence is red-shifted relative to the absorption spectrum. Absorption by the triplet state is a significant loss process. The absorption and fluorescence spectra are comprised of a broad continuum of vibrational and rotational states as the following Rhodamine B/methanol spectrum shows: Dye lasers How is wavelength tuning accomplished?

3. Typically, each dye can be tuned over several tens of nm. Many laser dyes are available: Dye lasers Pulsed dye lasers may be pumped by flashlamps or other pulsed lasers (N 2, excimer, Nd:YAG). CW dye lasers are usually pumped by Ar ion lasers. The dye solution must be circulated to prevent overheating and degradation, and to replace molecules in the triplet state, T 1 The wide tuneability range, high output power, and pulsed or CW operation make the dye laser particularly useful in many chemical studies.

4. Usually a tuning element, such as a diffraction grating or prism, is incorporated in the cavity. This allows only light in a very narrow frequency range to resonate in the cavity and be emitted as laser emission. Tuning the wavelength: Dye lasers Rotating a mirror or tuning element selects which wavelengths are resonant in the laser cavity.

5. Nd:YAG laser The Neodymium:YAG laser “YAG” = yttrium aluminium garnet (Y 3 Al 5 O 12 ) Lasing can be induced between energy levels of Nd 3+ embedded in YAG. Other matrices are yttrium lithium fluoride (YLF), YVO 4 (yttrium orthovanadate), and glass Nd:YAG is a 4-level laser: Nonlinear processes are used to produce higher-order harmonics: ν 0 (at 1064 nm) is termed the “fundamental” ν 1 = 2ν 0 (at 532 nm) is the first harmonic ν 2 = 3ν 0 (at 355 nm) is the second harmonic ν 3 = 4ν 0 (at 266 nm) is the third harmonic

6. Nd:YAG laser System is pumped optically with a Kr arc lamp (CW operation) or a flashlamp for higher power pulsed operation. Laser diodes are now commonly used as the pump source owing to their high efficiency and good match to the absorption bands of Nd:YAG. The Nd:YAG is very common as a pump laser (for e.g., dye lasers) and as a source of high power pulses in the visible and ultraviolet. Possible pump configurations: Output power range from mW to 100 W (CW); pulse energies range from 0.1 J to 100 J, giving peak powers of up to 100 MW!

7. Ti:sapphire laser The titanium sapphire (Ti:sapphire) laser Ti:sapphire is solid-state laser tuneable over a very broad spectral range (670 – 1100 nm) The gain medium is a sapphire crystal (Al 2 O 3 ) doped with 0.1% Ti 2 O 3. The Ti:sapphire is a 4-level laser with emission between different vibronic energy levels of the Ti 3+ ion. The system may be either CW or pulsed. Pumping is usually by Ar ion or Nd:YAG laser to match the absorption band at 480 – 540 nm. A major advantage of the Ti:sapphire system is that mode-locking is possible to produce a train of extremely short pulses allowing ultrafast (10 -13 – 10 -14 s) studies of chemical processes.