Optical Properties and Applications of Reverse Saturable Absorbers

Abstract

A reverse saturable absorber (RSA) is a material whose excited state absorption cross section at wavelength λ, σex(λ), is larger than the ground state absorption cross section, σgr (λ) and which possesses four other necessary properties detailed below. Recently, applications of reverse saturable absorbers (RSA’s) to a number of areas in optical pulse processing and laser physics have been described.RSA’s can be used as optical pulse shorteners and optical energy limiters for laser pulses whose temporal widths are small compared with the decay time of the first excited state of the reverse saturable absorber.1 RSA’s in conjunction with saturable absorbers can be used to mode-lock lasers whose gain media have high saturation energies (the saturable absorber cuts the leading edge of the pulse and the RSA cuts the trailing edge of the pulse) and stabilize high gain mode-locked CW lasers against the onset of relaxation oscillations.2 A combination of RSA and saturable absorber can also be used as an extracavity optical pulse compressor to eliminate the wings of short optical pulses which may have been produced by the unwanted process of superfluorescence.5 Such a pulse compressor can be inserted into a laser amplifier chain. RSA’s can also be used as power limiters and pulse smoothers for pulses whose rate of change of intensity is slow compared with the decay time of the first excited state of the RSA.1 There are many examples of molecules and crystals which behave as RSA’s at specific wavelengths (e.g. the molecular dye rhodamine 6G behaves as a RSA at 435 nm wavelength). However, a suitable RSA may not be readily available, at least not readily known, for a given wavelength of interest. This is due in part to the paucity of information available regarding excited state absorption cross sections as compared with ground state absorption cross sections. Thus, choosing appropriate candidates for RSA’s at a given wavelength can prove difficult.