The wavelengths can be selected from 300nm to 1200nm, based on the type of dye, solvent, and pumping source. Various dyes are used in laser but a red dye called rhodamine 6G is the most commonly used due to its high efficiency and wavelengths suitable for spectroscopic experiments.

 

Different dyes oscillate different wavelengths. Rhodamine 6G oscillates within the range of 560-650nm. Using a prism or diffraction grating in the resonator allows selection of wavelengths in the range of 560~650nm, which is very useful in spectroscopic application.

 

Figure 1. Single spin, single-spin transition of rhodamine 6G in ethanol solution.

σ₀ refers to absorption scattering cross-section, σ_e refers to stimulated emission scattering cross-section.
Triple spin, triple spin transition, σ₃ refers to absorption scattering cross-section.

 

Rhodamine 6G

Rhodamine 6G, also known as rhodamine 590 chloride, has the chemical composition of 2-[6(ethylamino)-3- (ethylimino-2,7-dimethyl-3H-xanthen-9-yl]-benzoic acid, methyl ester, chloride with a formula of C₂₇H₂₉CIN₂O_3. The molecular structure consists of hydrocarbon with two joined carbon atoms and a hexagonal carbon ring (benzene ring). The benzene ring and electrons form a relatively loose bond, which affects the energy level of the organic dye.

 

HELIOSⅡ/LOTUSⅡ/HYPERION – Manufacturer: LASEROPTEK(www.laseroptek.com)

 

Here, nitrogen functions as a chromophore. The noncovalent bond allows free movement of electrons. The active medium contains rhodamine 6G dissolved in the solvent of alcohol or ethylene glycol at the concentration of 10^-3mole/L. The peak absorption wavelength of rhodamine 6G is 530nm and absorption region is 130nm. The peak fluorescence emission wavelength is 560nm. The absorption and fluorescence spectrum of rhodamine 6G is shown in Figure 1.

 

To create a dye solution, the right amount of solvent is needed. Generally, methanol, ethanol (dye concentration of 4.5×10^-5mole/L) or methanol/water mixture (dye concentration of 1×10^-4 mole/L) are used as solvents. Rhodamine is delivered continuously to the pump from the storage tank and heat is generated by the pumping source during dye excitation. This has a devastating effect on the system due to the raised temperature of the dye solution.

 

It causes changes in the density, fluorescence pathway and efficiency. To minimize this effect, an appropriate mixture ratio is needed. If the storage tank capacity is 1 liter, the concentration of rhodamine 6G in the solvent of ethanol (0.02g) should be 5×10^-5mole/L.

 

-To be continued