The dye is a large molecule with a large number of closely spaced vibrational states – essentially a continuum of states. The pump pulse populates the singlet states S 1 and S 2. Fast internal conversion (radiationless transitions in which energy id disspated thermally) occurs down to the lowest state of S 1 which lases down to an excired vibrational state of the ground state S o. Care needs to be taken maintian population inversion by ensuring internal conversion to the triplet state does not deplete S 1 to quickly SoSo S1S1 S2S2 T1T1 T2T2

Grating is rotated gradually to tune the resonant cavity and so scan across the required frequency range High power fixed-frequency laser pulse is split to pump both dye laser cell ( ~40%) and amplifier cell (~60%) Dye Cell Output coupler Beam expanding telescope Dye amplifier Pump Laser

Rhodamine 6G

Solid-State LasersThe gain medium in a solid-state laser is an impurity center in a crystal or glass. Solid-state lasers made from semiconductors are described below. The first laser was a ruby crystal (Cr3+ in Al2O3) that lased at 694 nm when pumped by a flashlamp. The most commonly used solid-state laser is one with Nd3+ in a Y3Al5O8 (YAG) or YLiF4(YLF) crystal or in a glass. These Nd3+ lasers operate either pulsed or cw and lase at approximately 1064 nm. The high energies of pulsed Nd3+:YAG lasers allow efficient frequency doubling (532 nm), tripling (355 nm), or quadrupling (266 nm), and the 532 nm and 355 nm beams are commonly used to pump tunable dye lasers. Dye Lasersmediumimpurity crystalglassrubycrystalnmmostcrystalglassnmfrequencynm The gain medium in a dye laser is an organic dye molecule that is dissolved in a solvent. The dye and solvent are circulated through a cell or a jet, and the dye molecules are excited by flashlamps or other lasers. Pulsed dye lasers use a cell and cw dye lasers typically use a jet. The organic dye molecules have broad fluorescence bands and dye lasers are typically tunable over 30 to 80 nm. Dyes exist to cover the near-uv to near- infrared spectral region: nm.mediummoleculesolvent cell fluorescencenm

Semiconductor Lasers Semiconductor lasers are light-emitting diodes within a resonator cavity that is formed either on the surfaces of the diode or externally. An electric current passing through the diode produces light emission when electrons and holes recombine at the p-n junction. Because of the small size of the active medium, the laser output is very divergent and requires special optics to produce a good beam shape. These lasers are used in optical-fiber communications, CD players, and in high-resolution molecular spectroscopy in the near-infrared. Diode laser arrays can replace flashlamps to efficiently pump solid-state lasers. Diode lasers are tunable over a narrow range and different semiconductor materials are used to make lasers at 680, 800, 1300, and 1500 nm.diodescavitycurrent junctionopticsshapeCD spectroscopyrangesemiconductornm

In the following description, which ignores triplet states, it is necesary to remember that energy differences are related to frequency differences via Planck's energy equation. In Fig. 1, the ground electronic state is S0 and the first excited state is S1. Molecules are excited from the ground state to either S1 or S2. If excited to S2 the molecules rapidly decay, in a radiationless manner, to S1. From the lower level of S1 transitions can take place to any level at the ground state manifold. It is this energy decay range that gives origin to tunability. This is explained in greater details, with corresponding equations, in [14]. Initially organic lasers used a liquid gain medium were the organic dye molecules are diluted in a solvent such as ethanol or methanol [1-3]. In a solid-state dye laser the organic dye molecules are uniformly distributed in a highly homogenous polymer matrix. In a tunable polymer laser the gain medium is said to be a dye- doped polymer. An example of such polymer is a highly pure form of poly(methyl-methacrylate) (PMMA). In the case of semiconductor gain media, or diode gain media, the tunability range depends approximately on the separation of the conduction and the valence bands, minus the energy band gap, of the semiconductor.