Dye laser, also known as organic dye laser is a tunable laser that uses a dye (usually in liquid form) as the active medium. Most of the laser dyes used are organic molecules mixed in solvents like alcohol or water. The wavelength emission of a dye laser depends on the type of laser dye used as the active medium which usually ranges from the ultraviolet region to the near-infrared region. Some of the laser dyes used are rhodamine 6G, fluorescein, coumarin, stilbene, umbelliferone, tetracene, malachite green, etc. The use of different dyes can cover large wavelength ranges and thus dye lasers exhibit a broad spectrum bandwidth. This also allows for selective emission of wavelengths and can also generate ultra-short pulses using techniques like q-switching and mode-locking.

Dye lasers are usually pumped using another laser whose wavelength depends on the emission range of the laser dye used. Dye lasers can be either continuous wave or pulsed wave lasers. This depends on the type of pump laser used for the excitation of the dye molecules.

The laser resonator of a dye laser consists of a dye jet or dye cuvette as the gain medium. The pump laser is directly fed to the active medium containing the laser dye which then emits the required wavelength. This is then amplified using the cavity mirrors and we get the required laser output. There are additional wavelength-selective elements in the resonator that further helps in the generation of shorter pulses.

Working

A dye laser is a four-level laser and the transitions occur between the electronic energy levels of the dye molecule. A typical dye molecule consists of two singlet energy states (S₀ & S₁) and two triplet states (T₁ & T₂) that take part in the transition process. Each electronic state consists of several vibrational and rotational states that result due to the vibrational rotational motion of the molecule.

When the dye molecules are mixed with the solvents, their interaction causes the broadening of these sublevels in the molecule and the energy levels form a continuum.

When this dye solution is pumped by another source, the molecules in the ground singlet state S₀ get excited to the higher vibrational levels of S₁. Due to collision with other molecules, these get deexcited to the lower vibrational levels of S₁. Thus a population inversion condition is established between the ground state S₀ and the lower vibrational states of S₁. This causes the lasing action to begin. This is usually termed fluorescence. The molecules in the S₁ level can also make a non-radiative relaxation to the triplet state T₁. This process is known as an intersystem crossing. This results in the reduction of population in the S₁ level, which can inhibit laser oscillation. These molecules can then make transitions to upper triplet levels. To avoid this, triplet quenching additives like oxygen are added to the dye solution to reduce the lifetime of the triplet state. Thus laser emission occurs only between the singlet states.

Commonly used dyes in dye lasers with their wavelength maximum

Applications

A dye-mediated tunable laser system uses a complex organic dye as the laser-active medium, usually a liquid concentrated solution of Rh6G. Rhodamine dyes are widely used in bioscience, e.g., fluorescence, flow cytometry, etc. where ultrafast optical pulses are desired. A well-known application of dye lasers is isotope separation spectroscopy (e.g., copper vapor laser pumps tunable dye laser to excite specific uranium isotopes, giving rich uranium species. Dye lasers are used to etch birthmarks, break kidney stones, etc. A pulsed dye laser selectively destroys small blood vessels in the underlayer (dermis) of the skin without damaging the surrounding tissue or outer layer of the skin. A long-pulsed dye laser is used to treat fine veins, telangiectasia, and blushing. A pulse dye laser is used to treat vascular lesions like nevus flammeus, hemangiomas, keloids, and hypertrophic scars. A pulse dye laser is also used to treat pigmented nevi.