Reverse saturable absorbers

Perry, J. W. Organic and Metal-containing Reverse Saturable Absorbers for Optical Limiters. In Nonlinear Optics of Organic Molecules and Polymers, Nalwa, H. S. Miyata, S., Eds CRC Boca Raton FL 1997, pp 813-840. 

Reverse saturable absorption is another important nonlinear optical effect. Reverse saturable absorbers (RSAs) act as optical limiters by absorbing laser radiation. Therefore, they are particularly useful for laser protection, both civilian and military. Colorless infrared-absorbing RSAs which can absorb green laser radiation are especially important as they safeguard the eyes of pilots, tank commanders, etc. from enemy lasers [61],... 

J.W. Perry, Organic and Metal-Containing Reverse Saturable Absorbers for Optical Limiters, Chapter 13 in [lOj. 

OPTICAL PROPERTIES AND APPLICATIONS OF REVERSE SATURABLE ABSORBERS... 

A reverse saturable absorber CRSA) is a material whose excited state absorption cross section at wavelength > is larger than the... 

The criteria for molecules(crystals, solids or liquids) to be successful reverse saturable absorbers are (1) Ogj (X) should be larger than Ogj.(X) at the desired wavelength. (2) Ogj .(A) should be sufficiently large to insure that the incident pulse saturates the ground state transition (at least somewhat). Otherwise excited state absorption can not occur. The pulse may be focused into the RSA sample so this is not too severe a criteria. (3) Neither the first nor the second excited states should decay to other levels thereby trapping the excitation since this would diminish the presence of molecules which can exhibit excited state absorption,... 

Consider a plane wave pulse with time dependence of the incident intensity I Ct) and with central wavelength X. Let us describe the dynamics of the propagation of the pulse through a reverse saturable absorbing medium of width L. The rate equation for the excited state population density of the RSA at position x and time t, N(x, t), and the wave equation for the intensity of the pulse of light within the RSA at position X at time t, I(x, t) take the form... 

Y.B. Band, D.J. Harter, and R. Bavli, Chem.Phys.Lett. "Optical Compressor Composed of Saturable and REverse Saturable Absorbers", to be published. 

W, Koechner, Solid-State Laser Engineering. (Springer-Verlag,N.Y.,) Y.B. Band and B. Scharf, "Engineering Reverse Saturable Absorbers for Desired Wavelengths", to published. 

Equation 6 which is the general solution to a variety of exitation schemes, can be solved graphically for ( ij (L). This is shown here for two cases. The solutions for a medium exhibiting Sg — transition (Fig. 2) incorporated in a matched ring resonator. The results for a coherent field pumping are depicted in Fig. The results for are used to calculate the output intensity as a function of lin (Fig. 4). The bistable behavior of this system (lower branch due to almost linear absorption and upper branch due to saturated absorption) can be also demonstrated in many other cases. The solutions for reverse saturable absorber - a medium exhibiting Sg — S. Ay T- —Tjj ( -jn on transitions - in the same optical resonator... 

Reverse saturable absorption is an increase in the absorption coefficient of a material that is proportional to pump intensity. This phenomenon typically involves the population of a strongly absorbing excited state and is the basis of optical limiters or sensor protection elements. A variety of electronic and molecular reorientation processes can give rise to reverse saturable absorption many materials exhibit this phenomenon, including fuUerenes, phthalocyanine compounds (qv), and organometaUic complexes. 

The nonlinear material is critical to the operation of a limiter. Several different nonlinear effects can be used in the nonlinear element to provide a limiting function. Nonlinear absorption, where the material absorbance increases with the intensity of the incident light, is the most useful mechanism to remove light from the transmitted beam. A large number of organic and organometallic materials have been reported to exhibit an intensity dependent increase in absorption, sometimes called a reverse saturable absorption. The most effective are phthalocyanines (2, 3,4, 5) and porphyrins. (6,7)... 

A liquid solution may be separated into its constituents by crystallising out either pure solvent or pure solute, the latter process occurring only with saturated solutions. (At one special temperature, called the cryohydric temperature, both solvent and solute crystallise out side by side in unchanging proportions.) We now consider what happens when a small quantity of solute is separated from or taken up by the saturated solution by reversible processes. Let the saturated solution, with excess of solute, be placed in a cylinder closed below by a semipermeable septum, and the w7hole immersed in pure solvent. The system is in equilibrium if a pressure P, equal to the osmotic pressure of the saturated solution when the free surface of the pure solvent is under atmospheric pressure, is applied to the solution. Dissolution or precipitation of solute can now be brought about by an infinitesimal decrease or increase of the external pressure, and the processes are therefore reversible. If the infinitesimal pressure difference is maintained, and the process conducted so slowly that all changes are isothermal, the heat absorbed when a mol of solute passes into a solution kept always infinitely... 

The hydrate formed by photolysis of this substance is one of the few such products (the others are uracil hydrate, 5-fluorouracil hydrate, and uridine hydrate) that have actually been isolated and compared with authentic material of known structure.7 It is nearly the only product formed in the photolysis, is definitely stable at room temperature and neutral pH, and the thermal reversal to dimethyluracil is nearly quantitative. These properties, as Moore observed, make the reaction ideal for mechanistic investigation. Burr and Park have investigated the reaction mechanism by measuring the photolysis rate of dimethyluracil in mixtures of water with several nonaqueous, nonreactive solvents as a function of water concentration.64 The photolysis rate for 10" iM DMU was found to be the same in water-saturated cyclohexane (about 5 x 10-3M in water) as in dry cyclohexane. The photolysis rate in dry, highly purified dioxane was quite insensitive to water, and it was observed that hydrate formation (measured by thin-layer chromatography and by thermal absorbance reversal) became appreciable only at water concentrations above 40%. 

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