Photo-Induced Effects in Mn 4+ :YAG. Observation of Unusually Efficient Excited State Absorption and a Long-Lived Metast
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INTRODUCTION The potential threat of laser weapons has been understood, 1,2 and the ideal protection (optical limiters) postulated for many years. In the quest for an effective limiter, a class of
4
Photoionization
Excited State Absorption
materials, inorganic oxide crystals, has gone 2 largely unexplored. Yet these materials posses very desirable physical properties. They are Trap Ground State Absorption the same host materials used in solid state laser applications and so exhibit very high optical damage thresholds. They are also hard materials that can be ground and polished into Figure I. Optical limiting mechanisms in inorganic crystals. optical elements. Transition metal doped crystalline materials can undergo excited state absorption (ESA) as well as photoionization, both of which can lead to optical limiting via reverse saturable absorption (Figure 1.). Manganese, in several ionization states, and in several hosts, has exhibited a photo-induced aborption (PIA) spectrum as measured with a monochromator and phase sensitive detection. 4 This work reports on initial investigations of photo-induced absorption in this class of materials.
EXPERIMENT Optical limiting via reverse saturable absorption (RSA) requires that an initial photon absorbed by the material prepares a transient species with higher absorptivity than the ground state. The combination of these two linear processes then results in non-linear absorptivity. In 419 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
order for RSA to work for optical limiting: a transient species or state must be prepared; the absorption of that state must overlap the ground state absorption; and the absorptivity of the transient state must exceed that of the ground state at the wavelength of interest. These properties can be measured by acquiring PIA spectra. All spectra presented in this paper, with the exception of Figure 2, were acquired using a 50W quartz tungsten halogen lamp for the broadband, continuous source and Bruker IFS 66 or 88 interferometers with silicon diode detectors. For both continuous and modulated pump experiments a Coherent Sabre Ar ion laser was used. Modulation was performed with a mechanical chopper wheel and phase sensitive detection with an SRS 510 lock-in amplifier. Pulsed experiments were performed using a Coherent Infinity Nd:YAG at 532 nm. Both interferometers used were capable of operating in step scan mode. In this operating mode, instead of moving the mirror in a continuous fashion, the mirror is stepped to a position, held still while data is acquired and digitized, and stepped to the next position (usually the next zero crossing of the HeNe reference laser).5'6 This permits acquisition of fully time resolved
transient absorption spectra. In these experiments, white light is passed through the sample into the interferometer and the same volume of sample is overlapped with a pulsed laser beam. At each mirror position, the mirror is held fixed while the laser is pulsed and the changing white light signal is d
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