Dye lasers

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 S0 to the first excited singlet state S1. Fluorescence to the ground state also has a high quantum efficiency, ΦF.Possible processes:1,2: Absorption from S0 to S1, S2

3,4: Rapid collisional relaxation to ground vibrational level of S1

5: Fluorescence to various vibrational states of S0 (basis for laser emission)6: Vibrational relaxation to S0 ground vibrational level7: ISC to triplet state T1

8: Absorption between triplet states9: Phosphorescence to S0

This is essentially a 4-level laser: pumping at 1 or 2; laser emission at 5.

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?

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 (N2, 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, T1

The wide tuneability range, high output power, and pulsed or CW operation make the dye laser particularly useful in many chemical studies.

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.

Nd:YAG laser

The Neodymium:YAG laser

“YAG” = yttrium aluminium garnet (Y3Al5O12)Lasing can be induced between energy levels of Nd3+ embedded in

YAG. Other matrices are yttrium lithium fluoride (YLF), YVO4 (yttrium orthovanadate), and glass

Nd:YAG is a 4-level laser:

Nonlinear processes are used to produce higher-order harmonics:

ν0 (at 1064 nm) is termed the “fundamental”ν1 = 2ν0 (at 532 nm) is the first harmonicν2 = 3ν0 (at 355 nm) is the second harmonicν3 = 4ν0 (at 266 nm) is the third harmonic

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!

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 (Al2O3) doped with 0.1% Ti2O3.

The Ti:sapphire is a 4-level laser with emission between different vibronic energy levels of the Ti3+ 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.