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PHOTON GATED HOLEBURNING BY 2-STEP
PHOTOIONIZATION
A. Winnacker, R. Shelby, R. Macfarlane
To cite this version:
A. Winnacker, R. Shelby, R. Macfarlane. PHOTON GATED HOLEBURNING BY
J O U R N A L DE P H Y S I Q U E
Colloque C7, suppldment a u nOIO, Tome 46, o c t o b r e 1985 page C7-543
PHOTON GATED H O L E B U R N I N G BY 2 - S T E P P H O T O I O N I Z A T I O N A. winnacker', R.M. Shelby and R.M. Macfarlane
IBM Research Laboratory, San Jose, CaZifomia 951 93, U . S. A.
Abstract
-
We report the observation of photon-gated spectral holeburning in which persistent holes are burned in an inhomogeneoulsy broadened absorption line only in the presen2f of a second gating light source. The measutpents were made in BaC1F:Sm and involve 2-step photoionization of Sm.
A remarkable and novel feature of this system is that holes can be recovered after temperature cycling to 300 K for several days.Optical spectral holeburning is the selective bleaching of an inhomogeneously broadened absorption line by a narrow band laser. In general this involves the depletion of the groundstate population of a subset of ions or molecules, which can result in either a transient hole ( nsec'secs) or a persistent hole (lifetime
'hrs.). Holeburning provides a spectroscopic probe which is uniquely suited to the measurement of small (zMHz) frequency shifts and splittings and has the potential to become an important method for frequency domain information storage I l l . During the last decade a number of mechanisms leading to persistent holeburning have been discovered. These include proton tautomerism [2,31 , irreversible photochemistry
141, proton transfer 151, photoionization of color centers [ 6 1 and rare earth ions
171 in crystals, and small structural rearrangements in glasses and polymers L81
.
These cases involve l-photon processes, one consequence of which is that a certain amount of bleaching inevitably occurs during the probing or reading of the hole which produces a degradation of the signal. We have observed the first example of a 2-photon "photon-gated" process [g] , i.e. holeburning which occurs at a specific laser trequency a only in the presence of a second gating light source W
.
Persistent spectra! holes burned in this way are very stable tc probing light at in the absence of a. This is very useful in high resolution spectroscopic t ~ u d i e s and of crucial ilhportance for frequency domain optic& data storage. Thematerial we found to exhib t t is effec BaC1F:Sm where persistent
holeburning occurs in the 'F D+' and 'Fo D, absorption liner at 2K by a 2-step photoionization of
h2+.
%imi lar 2-colour gated holeburning has more recently been observed in an organic impurity system-
carbazole in boric acid glass [IOJ.'permanent address : Physikalisches Institut der Universitzt Heidelberg, D-6900 Heidelberg, F.R.G.
JOURNAL DE PHYSIQUE
Energy leveJ+scheme of BaC1F:Sm
Fig. I shows syplified energy level diagram of B ~ C I F : S ~ ~ + '[ill. G h e grqund
tate is 4f Fo, , and the low_est three excited levels D D
'D are metastable, wlth lifetimes = msecs. At higher energies aP; brodd
ab$orption bands arising from transitions to the 4 f 5d states. In BaCIF, Sm ccupies ntn-centrosymmetric C sites, and electric gipole transitions between 9F and D are in%ced th8Saugh admixture of 4f 54 states. Crystals of B ~ ~ I F with dominal Sm conzentrations ranging from 0.01% to 0.5% were used. They were immersed in buperfiuid helium and irradiated with light from a cw dye laser of width 2MHz (the frequency selective light) and by an ~r+-laser (the gating light)
.
Holeburyng wgs observed in a fluorescence excitation spectrummonitoring specific DJ + emission7 liges. Fig. 2 summarizes the
holeburning behavior ObservedF?' For the F + D absorption in the absence of gat@ light 2% deep holes were burned aftep 2008 stcs of irqdiation at 6879A.
(2W/cm ). In the presence of gating light at 5145 8 (20W/c$1 ) 20% deep holes were burned in 3 secs. This gives a gating ration of 10 in this instance. Using higher gating intgnsities measureable holes could be burned in 3 msec giving a gating ratio of 10
.
For shallow holes, the hole depth was linear in the gating light intensity. Multiple holes could be burned with very little refilling of "earlier" holes by "later" holes (Fig. 2e). The holewidt$+shown in Fig. 2 is 25 MHz$p-
samples with a nominal concentration of 0.01% Sm.
(In samples with this increases to-
IGHZ.) Gated holes can also be burned in the line at 6297x, the holewidth being similar to that ingiven sample. The gating efficiency clearly shows a threshoqd behavior similar to that usually observed in photoconductivity measurements, with a &hreshold near 2.5 eV, putting the edge of the conduction band 2.5 eV above the Do state.
;5' 0
/ \ Fig.2: Gated holeburninq in a type
,
/
I1 crvstal of BaCl F containinq-pomi-
nally" 0.07% Sm". a) The
inhomogeneous line, of FWHM 13 GHz. b) A 500MHz section of the inhomo- geneous line before holeburninq. c) After burninq at "0" and "-220 MHz" with 2W/cm2 for 2000secs. d ) A single hole burned at "0" in 3secs by ths addition of "20W/ctnQf
5145 A qati q light showing a gating ratio of40': e) Multiple holes burned 1 1 OFrlHz apart.
L I I I
-0.2 0 0.2 0.4
Laser Frequency Offset IGHz)
Essentially two kinds of hole burn3g and erasing behavior were observed in different samples, one pointing to Sm as the dominant trap (type I), the other one to traps different from samarium (type 11). The most characteristic feature of type I is the action spectrum for erasing the holes. The time T required to half refill a hole of some standard depth (15%) was measured as a f&ction+of the wavelength of the erasing ligh5 proviged by the different lines of the Ar -laser at an intensity of about 10 W/cm. Fig 3 shows the "erasing efficiency"
C7-546 JOURNAL DE PHYSIQUE
1/T a s a f u n c t i o n of wavelength. A very pronounced s t r u c t u r e i s o b s y ~ e d which matthes p r e c i s e l y t h e shape of t h e lowest 4f5d a b s o r p t i o n band of Sm a s seen i n f i g . 3. T h e r e f o r ? + t h e ? e l e c t r o n s r e q u i r e d t o e r a s e h o l e s a r e provided by p h o t o e x c i t a t i o n of Sm a t m e a s d e s c r i b e d by
where A l a b e l s t h e s i t e corresponding t o t h e holeg and B t h e traps. In t h i s p r o c e s s t h e e l e c t r e n s a r e pumped t o t h e lowest 4f 5d l e v e l of Sm , t h e n drop t o t h e metasable DJ s t a t e s , frorg, where t h e y a r e e x c i t e d t o t h e conduction band. They t h e n a r e t r a p p e d by Sm i o n s , which, due t o t h e i r e x c e s s p o s i t i v e c h a r g e , a c t a s e f f i c i e n t e l e c t r o n t r a p s . Equations ( 1 ) and ( 2 ) imply t h a t , i n t y p e
I samples, holeburning and t h e revf+rse proc&ss of r e f i l l i n g c o n s i s t s of pumping e l e c t r o n s back and f o r t h between Sm and Sm
.
An e s t i m a t e of t h e e f f i c i e n c y of p h o t o i o n i z a t i o n from t h e e x c i t e d s t a t e , a , can be made based on o u r o b s e r v a t i o n t h a t a 10% deep h o l e could b e burned i n 8 msec under c o n d i t i o n s where 3 W of 5145 g a t i n g l i g h t and 20 mW of r e s o n a n t l i g h t were focussed on t h e sample with a 15 cm l e n s .
where f i s t h e holeburning r a t e (dimension f r a c t i o n p e r t i m e ) , W i s t h e pumping
r a t e o u t of t h e e x c i t e d s t a t e by t h e g a t i n g l i g h t and E i s t h e e x c i t a t i o n
p r o b a b i l i t y t o t h e conduction band p e r time. Under o u r c o n d i t i o n s we e s t i m a t e f = 1
13s- I. E = 0.1. NOW W = $0 where $ i s t h e g a t i n g f p x ( i 1 0 photons cm-2 ~ ~
sec- ) and a t h e a b s o r p t i o n c r o s s s e c t i o n from t h e D bound s t a t e t o t h e
conduction band. Although we have no information on t h e laQtter we assume i t i s
c o m ~ g r a b k to t h a t i n t h e r e g i o n o f t h e L band a b s o r p t i o n o f F c e n t e r s where p h o t o i o n i z a t i o n a l s o o c c u r s , i . e . 3 X 10-19 cm2. This g i v e s W = 3
.
104s-1 and ,,=0.4y, This a p p e a r s t o be a r a t h e r high e f f i c i e n c y and c l e a r l y a more d e f i n i t i v e measurement is d e s i r a b l e .A most s t r i k i n g and novel f e a t u r e of t y p e I samples i s t h e f a c t t h a t t h e h o l e s a r e s t a b l e upon thermal c y c l i n g t o room t e m p e r a t u r e ( f i g . 4 ) . The t y p e I c r y s t a l s a v a i l a b l e f o r s t u d y had h o l e w i d t h s of 100-250 MHz. Samples of t h i s kind w i t h h o l e s burned a t 2K could be r a i s e d t o room t e m p e r a t u r e f o r s e v e r a l days and when recooled t o 2K, t h e h o l e s had broadened by l e s s t h a n a f a c t o r of 2 with no d i s c e r n a l b e change i n a r e a . To o u r knowledge t h i s i s t h e f i r s t m a t e r i a l i n which s t a b i l i t y of " s p e c t r a l h o l e s under thermal c y c l i n g t o room t e m p e r a t u r e has been observed.
I
t
Fig. 4: Temperature c y c l i n g of h o l e s i n t h e 687.9nm a b s o r p t i o n l i n e . a ) h o l e 0.-
-
burned a t 2K, h e l d a t room t e m p e r a t u r e'Z f o r 24 hours and subsequently probed
8 a a t 2K. b ) New h o l e burned a t 2K. a (b) 8 , I
-
-2 -1 0 1 2Type I1 samples differ from type I samples in the following ways: 1. In these samples a fairly large fraction of the absorption line ( ~ 5 0 % as compared to 15%
in the first group) can
9
burned away. 2. The holy+cannot be erased by light corresponding to the 4f 5d-absorption band of Sm.
3. The holes are not table u5on cycling to room temperature. 4. By excitation of the transitions to 'D or D l , i.e. by red light only, transient holes with lifetimes shorter thin 1 ms are produced almost 100% deep. However, gated holeburning produces persistent holes. A11 these observfJions point to the fact that in these samples electronic traps different from Sm play a dominant role.
In conclusion we note that photoionization holeburning is an intrinsic property of the system inasmuch as the level scheme of the ion, the position of the levels relative to the conduction band etc. are involved, it is an extrinsic property inasmuch as electron traps determine important features of the hole burning and erasing process.
We thank H.A. Weakliem for kindly providing the samples of B~cIF:s~'+.
REFERENCES
/l/ A. Szabo, U.S. Patent No. 3, 896, 420 (1975); G. Castro, D. Haarer, R.M. Macfarlane and H.P. Trommsdorff, U.S. Patent No. 4, 101,976 (1978).
/2/ A.A. Gorokhovskii, R.K. Kaarli and L.A. Rebane, JETP Lett. 20, 216 (1974); S. Volker and R.M. Macfarlane, IBM J. Res. Dev. - 23, 547 (!gm), and references therein.
/3/ L.A. Rebane, A.A. Gorokhovskii and J.V. Kikas, Appl. Phys. 829, 235 (1982). /4/ H. de Vries and D.A. Wiersma, Phys. Rev. Lett. 36, 91 ( 1 9 7 6 r
/5/ J. Friedrich and D. Haarer, Angew. Chem. Int. Ed. Engl. 23, 113 (1984). /6/ R.M. Macfarlane, R.T. Harley and R.M. Shelby, Rad. Eff. 72, 1 (1983). /7/ R.M. Macfarlane and R.M. Shelby, Opt. Lett. 9 , 533 (1984x R.M. Macfarlane
and R.S. Meltzer, Opt. Commun. 52, 320 (19857.
/8/ G.J. Small, in "Modern ProblemsTn Solid State Physics", Molecular Spectroscopy, V.M. Agranovich and R.M. Hochstrasser, eds. (North-Holland, Amsterdam, 1983).
/9/ A. Winnacker, R.M. Shelby and R.M. Macfarlane, Opt. Lett. 10, 350 (1985). /10/ H.W.H. Lee, M. Gehrtz, E. Marinero and W.E. Moerner, subm.70 Chem. Phys.
Lett.
/II/ J.C. Gacon, G. Grenet, J.C. Souillat and M. Kibler, J. Chem. Phys. 69, 868
