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ELECTRICAL INSTABILITY IN RUBY UNDER
OPTICAL PUMPING
S. Basun, A. Kaplyanskii, S. Feofilov
To cite this version:
J O U R N A L DE PHYSIQUE
Colloque C7, suppidment au nOiO, Tome 46, octobre 1985 page C7-267
ELECTRICAL INSTABILITY IN RUBY UNDER OPTICAL PUMPING
S.A. Basun, A.A. Kaplyanskii and S.P. Feofilov
I o f f e PhysicaZ-TechnicaZ I n s t i t u t e , Academy of Sciences o f t h e U. S. S. R., 194021 Leningrad, U. S. S.R.
Abstract
-
The new phenomenon consists in a spontaneous appea- rance in strongly excited concentrated ruby of domains with strong (wMV/cm) electric fields of opposite sign which per- sist for a long time after the termination of pumping, The photoinduced formation of domains was found to depend criti- cally on external conditions (temperature, external electric field, pumping rate and wavelength of optical excitation), as well as on the chromium concentration. Photocurrent measure- ments support the phenomenological interpretation of the phe- nomenon as a result of an electrical instability caused by the negative, absolute and differential, conductivity of concent- rated ruby under optical excitation.The electric fields in ruby, A120j:cr3+, were studied using the
linearin-field pseudo-Stark splitting of the
R
luminescence linesof ruby /I/. It was found /2/ that the symmetric doublet~pseudo- Stark splitting of the lines can be observed also without applicati-
on of external field, by pumping concentrated ruby sample~l with
an
Ar laser at helium temperatures. In the course of excitation the lines undergo spontaneous doublet splitting whose magnitude tends to
saturate at a level
AJ,
corresponding to fields Es on the order of6
10 V/cm. The electric field produced in the crystal persists after
the termination of irradiation both at Low temperatures and with the
sWie warmed up to room temperature.
Our experiments were perfomed on ruby single crystals with c=
C7-268 J O U R N A L D E PHYSIQUE
0.3 wt% Cr203, mainly at T=77
K,
using a cw Ar laser (ordinary ath
~514.5 nm). The samples were thin plates with transparent electrodes deposited on both faces which were perpendicular to the trigonal axis C.
In experiments / 3 , 4 / , the sample was preliminarily irradiated to produce a photoinduced field Es (resulting in a splitting of the
R
line into a doublet with A % ) , after which an external field Eo was applied (Eofb). The resulting quartet structure of the R1 line(Pig.1) indicates unambiguously that the photoinduced field Es is ac- tually spatially nonuniform, namely, that the sample contains domains in which the field parallel to C is equal in magnitude while being
opposite in sign (+ES). When
an
external field Eo is applied, the to-tal field in the domains will become different (EsE,). It is this
effect that accounts for the quartet in the spectrum (for EO=Es, the field in the domains with the sign opposite to that of Eo will beco- me cancelled making the spectrum a triplet), The conclusion on the existence of domains with electric field +Es was confirmed by expe- riments /4/ on the selective excitation of luminescence in domains of the same sign.
-4
04
FREQUENCY (cm"
0 100 200
TEMPERATURE ( K )
Pig. 1
-
Photoinduced RI doublet Fig. 2-
E vs. irradiationin the luminescence spectrum and t emperatur%.
its splitting in
an
external electric field EOI IC.The magnitude of the photoinduced field Es was observed /4,5/ to depend critically on temperature at which the pumping is carried
2
out (%I00 W/cm )(see ~ig.2). At T <Tc=150
K,
the steady-state value E, behaves in a peculiar way withT,
decreasing asT
+
Tc, while for T>,
Tc,
Es=O. The time in which at T<
Tc,
the steady-s tate value Esis achieved depends critically on T, increasing as T + Tc. Hence for
mains +Es(T) i s s t a b l e , w h i l e f o r T a l c , t h e uniform s t a t e without f i e l d i s s t a b l e (Es=O).
A s i m i l a r c r i t i c a l behavior was observed / 4 , 5 / under s t r o n g op- t i c a l pumping of ruby i n an e x t e r n a l e l e c t r i c f i e l d Eo ( f i g . 3 ) , whe- r e t h e c r i t i c a l e x t e r n a l f i e l d EO=E,. For EO<Es, t h e spectrum ( t h e R l i n e s ) t e n d s t o t h e doublet w i t h Eo-independent s p l i t t i n g
aLs
which i n d i c a t e s nonuniform s t e a d y s t a t e . A s Eo i n c r e a s e s , r e d i s t r i b u - t i o n of domains o c c u r s f a v o r i n g t h e domains w i t h t h e f i e l d p a r a l l e l t o Eo. When i r r a d i a t e d i n t h e f i e l d EO=Es, t h e volume of t h e domains w i t h t h e f i e l d o p p o s i t e i n d i r e c t i o n t o Eo becomes z e r o , w h i l e f o r Eo>Es t h e o r d i n a r y uniform s t a t e of t h e sample w i t h t h e f i e l d e q u a l t o Eo w i l l now be s t a b l e .EXTERNAL FIELD (kV/cm)
Pig.
3
-
Dependence on e x t e r n a l Fig. 4-
Photoinduced e l e c t r i cf i e l d EO of measured steady- f i e l d Es v s , o p t i c a l pumping
s t a t e e l e c t r i c f i e l d E i n t h e r a t e p l o t d e r i v e d from z e r o cur-
sample (P=lOO~/cm2). r e n t p o i n t s i n t h e I - V c h a r a c t e -
r i s t i c (o ) and from t h e s p l i t -
t i n g of R1 f o r a sample pum ed
i n a z e r o e x t e r n a l f i e l d ( 0 7 .
The dependence of Es on t h e o p t i c a l pumping r a t e P w a s s t u d i e d i n /2,4/. The s t e a d y s t a t e v a l u e Es d e c r e a s e s w i t h i n c r e a s i n g P o v e r
t h e range 0.1
-
3
kw/cm2 /4/ (Fig.4) and becomes z e r o at P=10 kW/cm 2(j\ =514.5 nm) /2/. When pumping i n a n e x t e r n a l f i e l d , t h e r e l a t i o n
Es(P) a c q u i r e s a c l e a r l y pronounced c r i t i c a l shape /4/ implying t h a t
t h e nonuniform (domain) s t a t e can e x i s t o n l y f o r P 4 P c (PC depends on Eo).
Thus t h e nonuniform s t a t e of ruby w i t h s t r o n g e l e c t r i c f i e l d
domains is produced under i n t e n s e o p t i c a l pumping of c o n c e n t r a t e d ru-
by and i s i n t h e s e c o n d i t i o n s s t e a d y - s t a t e ( s t a b l e ) w i t h i n a l i m i t e d range of e x t e r n a l parameters: T<Tc, Eo<Es, P < P C , A 7
A c ,
c > c c( a c c o r d i n g t o o u r measurements Ac=400 nm, cc=0.15 w t % Cr203
at
P S2
C7-270 JOURNAL DE PHYSIQUE
of t h e s e p a r a m e t e r s a r e of a c r i t i c a l n a t u r e and o c c u r under s t r i c t - l y d e f i n i t e v a l u e s of Tc, Eo, PC. It s h o u l d be emphasized t h a t t h e s e "phase t r a n s i t i o n s w occur o n l y under s u f f i c i e n t l y s t r o n g o p t i c a l ex- c i t a t i o n of ruby. Under weak o p t i c a l e x c i t a t i o n o r i n t h e dark t h e memory of t h e s t e a d y - s t a t e (uniform o r nonuniform) reached i n s t r o n g pumping c o n d i t i o n s may p e r s i s t i n d e f i n i t e l y long.
The photoinduced appearance of s t r o n g f i e l d domains i m p l i e s a spontaneous s p a t i a l s e p a r a t i o n of charge i n t h e sample under t h e ac- t i o n of l i g h t w i t h t h e f o r m a t i o n of s p a c e charge r e g i o n s producing
t h e observed f i e l d s
+Es.
The p e r s i s t e n c e of long-time memory a f t e rt h e t e r m i n a t i o n of pumping shows t h e s p a c e charge t o be l o c a l i z e d at
deep t r a p s ( c r 2 + ? ) which a t T s 3 0 0 K can be i o n i z e d o n l y o p t i c a l l y , and n o t thermally.
EXTERNAL FIELD ( kV/cm )
2
Fig.
5
-
Current through sample (P=lOO W/cm ) vs. e x t e r n a l f i e l dEgo I n s e t : t h e o r e t i c a l j ( E ) dependence.
The phenomenological i n t e r p r e t a t i o n of t h e phenomenon /6,7/ i s
1 based on assuming t h e a b s o l u t e and d i f f e r e n t i a l p h o t o c o n d u c t i v i t y ( ) of c o n c e n t r a t e d ruby i n weak f i e l d s t o be n e g a t i v e , r e s u l t i n g i n N-
shaped dependence of p h o t o c u r r e n t on f i e l d , j ( E ) (Fig.5a). With s u c h
a behavior of j ( E ) , t h e s t a t e of o p t i c a l l y e x c i t e d ruby w i t h E=O i s
u n s t a b l e and, c o n v e r s e l y , t h e s t a b l e s t a t e s are s t a t e s w i t h f i e l d s (+Es) i n which t h e a b s o l u t e c o n d u c t i v i t y ( j / E ) i s z e r o , w h i l e t h e d i f f e r e n t i a l one ( a j / a ~ ) i s p o s i t i v e . Note t h a t i n a s h o r t c i r c u i t sample domains w i t h o p p o s i t e f i e l d s , +Es, should form. The observed c r i t i c a l behavior of t h e e f f e c t f i n d s a n a t u r a l e x p l a n a t i o n i n t h e dependence of t h e r u n of j ( E ) i n o p t i c a l l y e x c i t e d ruby on %emPera-
t u r e T, pumping r a t e P, e x c i t a t i o n wavelength
h ,
chromium concent-r a t i o n c and i n a s h a r p boundary c o n d i t i o n f o r t h e e l e c t r i c i n s t a b i - l i t y of a s t a t e w i t h E=O: h j ( ~ , ~ , c , h
...
) / a ~ = 0 i n weak f i e l d s .1
As seen from Fig.5a, the j ( E ) curve has sections (O<E<ES) where photocurrent flows apainst the field. This unusual situation
was observed experimentally when obtaining
I-V
characteristics of op-tically excited ruby /4,5/(Fig.5). One has revealed also hysteresis
in the formation of the photoelectric domain structure /4,5/. These results attest to the validity of the phenomenological interpretati- on /6,7/.
At the same time the microprocesses responsible for the photo- electric properties of ruby and generating the photocurrent which flows against the electric field remain unclear. In principle, such a current may be due to the presence in the centrosymmetric lattice of ruby of noncentrosynmetric polar sites differing in inversion
and to the photovoltaic current associated with these sites /5,6,7/.
/ I / Kaiser, W., Sugmo, S., Wood, D.L., Phys. Rev. Lett.
6
(1961)605.
/2/ Liao, P.F., Glass, A.M., Humphrey, L.M., Phys. Rev.
B22
(1980) 2276,/3/
Basun, S.A., Kaplyanskii, A.A., Feofilov, S.P., Pis1ma 2%. Eks- per. leor. Fiz.22
(1983) 492./4/ Fsun, S.A., Kaplyanskii, A. A., Feof ilov, S. P., Zh. Eksper. Teor. Flz.
87
(1984)
2047./5/
Brzsun, S.A., Kaplyanskii, A.A., Feofilov, S.P., Furman, A.S., Pis1ma Zh. Eksper. Teor. Fiz.2
(1984)161.
/6/ Dyakonov, M.I., Pis1ma Zh. Eksper. Teor. Fiz. 2p (1984) 158,
