Induced absorption in “pure” KCl and CaF2

Solid State Communications Vol. 4, pp.645-647, 1966.

INDUCED ABSORPTION

Pergamon Press Ltd. Printed in Great Britain

IN " P U R E " KCI A N D CaF2 *

E. Panizza and P.J. Regensburger Institute of Optics, University of Rochester, Rochester, N.Y. (Received 28 October 1966 by J.A. Krumhansl)

The experimental difficuRy in distinguishingbetween double absorption and induced absorption is discussed. R is shown that KCI and CaF~ crystals with a diminutive amount of impurities can give rise to apparent double absorption signals even though double absorption is impossible due to energy considerations. The threshold for laser induced damage was found correlated with impurities.

T H E H I G H INTESITY luminous flux produced by solid state lasers has been successfully used during recent years in the study of two-photon absorption in insulating crystals, t-e This new kind of spectroscopy allows one to investigate electronic states "invisible" in ordinary absorption spectroscopy. For example, in systems with a center of inversion, states with the same parry as the ground states are connected in the dipole approximation only by a two-photon process. The simultaneous absorption of two photons in crystals can be accomplished by means of a real intermediate state (induced absorption) or via a virtual intermediate state (double absorption). A crude form for the absorption coefficient ~ f~)ofa light b e a m (probe) tn the presence of a second intense b e a m (laser), valid in both cases, is:

In order to show that a diminutive amount of impurities can give rise to signals resembling double absorption we have analysed crystals in which intrinsic double absorption was aprtori forbidden in the range of wavelengths employed. Namely, we have examined crystals where the s u m of the laser photon energy (I. 78 eV) and the b e a m probe energy (2.0 - 4.0 eV) is less than the band gap energy.

We r e p o r t here the observation of induced absorption in KC1 and CaFe c r y s t a l s . In the f i r s t case the induced absorption is a t t r i b uted to the p r e s e n c e of a s m a l l concentration of F c e n t e r s intentionally introduced in the samples. In the second case the effect is attributed to the p r e s e n c e of impurities in an amount undetectable, in s a m p l e s of r e a s o n a b l e size, by means of ~(~) ~ c~ a~ FN ~ (1) o r d i n a r y absorption s p e c t r o s c o p y . The experimental a r r a n g e m e n t for m e a s u r i n g absorption o~ is the c r o s s - s e c t i o n for a one photon t r a n s i t induced by the l a s e r beam is shown in Fig. 1. ion between the ground state and the intermediate The sample, a cube of 1 cm edge Was exposed state; a~ is the c r o s s - s e c t i o n for a one photon to radiation f r o m a xenon flash lamp ( EG & G, t r a n s i t i o n between the intermediate state and the FX-33) through a Corning CS-0-52 filter. The final state; F is the flux of the l a s e r beam; N is xenon flash lamp produced a light pulse of 5 x the concentration of absorbing centers; • can be 10 - 4 s e c duration. At the time when the maxidefined as the " l i f e t i m e " of the i n t e r m e d i a t e state. m u m intensity of xenon flash lamp was reached, v ls of the o r d e r of 1 0 - e - 1 0 - a s e c f o r a r e a l light f r o m a Q-switched ruby l a s e r (20 nsec, 30 intermediate state and of the o r d e r of 10 - ~6sec MW) was imposed on the sample n o r m a l to the for a virtual state. Equation (1) shows that a very radiation f r o m the xenon flash lamp, t r a n s m i t t e d low concentration of i m p u r i t i e s (N = 10 ~~ - 10 ~s/ by the sample and f l R e r e d by a m o n o c h r o m a t o r , c m s ) can give r i s e to an induced absorption c o m was detected by a high c u r r e n t photodiode. The parable with an intrinsic double absorption in c r y signal produced by the flash lamp was introduced stals where N = 1 0 ~ a t o m s / c m . 3 into one channel of a differentialamplifier. A • Supported in part by the Air F o r c e Office of Scientific Research. 645

646

INDUCED ABSORPTION IN KCI AND CaF~

PHOTOOIODE It, XENON FLASHLAMP

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GRATING MOEK)O4ROMATOR

Vol. 4,No. 12

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FIG. 2 Absorption s p e c t r u m induced by the l a s e r pulse in KC1 at RT. F center a b s o r p t i o n band is indicated by dashed curve.

d . c . r e f e r e n c e signal of the same height as the p e a k of the flash l a m p pulse was introduced into

the other channel. Thus it w a s possible to obtain on the s c o p e only the v a r i a t i o n in intensity of the f l a s h l a m p pulse induced by the l a s e r pulse, cf. Fig. 2. This s y s t e m enabled us to detect an a b s o r b a n e e induced by the l a s e r b e a m of l e s s than 10-3 • Nominally pure KC1 c r y s t a l s w e r e obtained f r o m Harshaw Chemical Company and Optovac. A s m a l l concentration of F c e n t e r s (N = 10 ~ ' / c m 3) was introduced by b r i e f l y

i r r a d i a t i n g the c r y s t a l in the O H - b a n d (2030]~) always p r e s e n t in t h e s e c r y s t a l s . ~ The F c e n t e r s w e r e c r e a t e d out of OH- c e n t e r s as proved by a b s o r p t i o n s p e c t r o p h o t o m e t r y of the s a m p l e bef o r e and a f t e r irradiation. Induced absorption was detected only in colored s a m p l e s . The s p e c t r u m is shown in Fig. 3. The positive portion inducates an a b s o r p t i o n due to m e t a s t a b l e s t a t e s excited by the l a s e r b u r s t . The lifetime of t h e s e m e t a s t a b l e s t a t e s , deduced f r o m the decay t i m e of the induced a b s o r p t i o n pulse, was found to be 2 - 3 x 10 -8 s e c (Fig.2). The negative portion of the s p e c t r u m coincides with the position of the F center a b s o r p t i o n band and indicates that the l a s e r b u r s t excites s o m e of the F c e n t e r s even though the wave length of the l a s e r (6943]~) is f a r in the tail of the F a b s o r p t ion band. The negative signal, induced t r a n s mission, is caused by depopulation of the ground state by the l a s e r . At r o o m t e m p e r a t u r e e l e c t r o n s excited by the l a s e r light f r o m the F c e n t e r s a r e t h e r m a l l y f r e e d into the conduction band and fall into a m e t a s t a b l e s t a t e of unknown origin, givir~ r i s e to the o b s e r v e d a b s o r p t i o n peak at 4 3 0 0 L At low t e m p e r a t u r e the effect was quenched probably b e c a u s e of the drift and narrowing of the F a b s o r p t i o n band. It m u s t be pointed out that in our c a s e the induced a b s o r p t ion cannot be i n t e r p r e t e d in the s a m e way as in r e c e n t e x p e r i m e n t s by F r o l i c h and Mahr ~ c a r r ied out in colored KI at LNT. At RT the excited s t a t e of the F center in KC1 is unstable (T <10 "~ sec), t h e r e f o r e , it cannot be identified with the m e t a s t a b l e s t a t e in question (~ = 2 - 3 x 10 "e sec). P u r e CaF2 c r y s t a l s f r o m I-Iarshaw and Optovac w e r e a l s o tested, tn this c a s e without any previous coloration. Both w e r e found to have an undetectable tmpuritT content active in the visible and ultraviolet region, when analysed

Vol. 4, No. 12

INDUCED

ABSORPTION

IN KCI A N D

CaF2

647 PHOTON WAVELENGTH IN m ~ 350 400 450 8 4 3 0 550 60( i i

with an ordinary s p e c t r o p h o t o m e t e r (Cary 14). Only CaF~ c r y s t a l s f r o m Harshaw w e r e found

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p r e s e n t in the Harshaw CaF~ c r y s t a l s and not in Another interesting observat-

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ion about the c r y s t a l s of CaF~ used in this experiment was that the threshold for damage produced by the l a s e r b u r s t is c o r r e l a t e d with the

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Optovac crystals.

presence of induced absorption; w e noticed in fact d a m a g e at increased laser flux only in the c r y s t a l s which showed induced absorption. This suggests that induced absorption measurements can represent a useful, non-destructive test for optical laser components. D a m a g e of materials in fact limits the performance of current pulsed lasers and the choice of good components is therefore critical. CaF~ represents also a fairly c o m m o n host crystal for rare earth laser rods. l ~ o m the results reported above w e conclude that caution must be observed in the interpretation of two photon absorption experiments in insulating crystals since even a content of impurities undetectable with ordinary absorption spectroscopy can give rise to an appreciable induced absorption. W e notice in addition that the dependence of both induced and double absorption are linear with the laser power; so that in the case in which the lifetime of the metastable state of the impurity responsible for induced

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Absorption spectrum induced by the laser pulse in C a F ~ at RT. absorption were of the s a m e order of magnitude as the duration of the laser pulse, it would be very difficuR to distinguish between an intrinsic double absorption process and an induced absorption process due to impurities.

Acknowledgments - The authors are indebted to Prof. K.J. Teegarden, Prof. M. Hercher, Prof. A. Gold, and Dr. F.A. Collins for interesting and helpful discussions.

References I.

KAISER W. and GARRETT C . G . B . , Phys. Rev. L e t t e r s 7__ 229 (1961).

2.

BRAUNSTEIN R. and OCKMAN N., Ph),s. Rev. 134, A499 (1964).

3.

HOPFIELD J . J . and WORLOCK J . M . , Phys. Rev. 137, A1455 (1965).

4.

FROHLICH D. and MAHR H., Phys. Rev. L e t t e r s 16, 895 (1966).

5.

FROHLICH D and MAHR J. Phys. Rev. L e t t e r s 14__, 494 (1965).

6.

PARK K . , Phys. Rev. 140, A1735 (1965).

7. ROLFE J . , Phys. Rev. L e t t e r s I_, 56 (1958). Es wird dargelegt, class Doppelabsorption und induzierte Absorption e x p e r i m e n t e l l schwierig zu unterscheinden sind. Nach u n s e r e n Messungen kS"nnen ~'usserst geringe Verum'einigungen Doppelabsorption in KC1 - und CaF2 - K r i s t a l l e n vort~uschen, obwohl die Energie der Photonen dazu nicht a u s r e i c h t . Die Intensit~t des L a s e r Strahles, die z u e i n e r Z e r s t 6 r u n g der Kristalle fflhrt, ist abh~ngig yon der Konzentration der Verunreinig-ungen.