IN SITU PROBING OF ELECTRONIC SURFACE STRUCTURE IN IONIC CRYSTALS BY RESONANT SECOND HARMONIC GENERATION

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IN SITU PROBING OF ELECTRONIC SURFACE STRUCTURE IN IONIC CRYSTALS BY RESONANT

SECOND HARMONIC GENERATION

J. Reif, P. Tepper, O. Semmler, E. Matthias

To cite this version:

J. Reif, P. Tepper, O. Semmler, E. Matthias. IN SITU PROBING OF ELECTRONIC SURFACE STRUCTURE IN IONIC CRYSTALS BY RESONANT SECOND HARMONIC GENERATION.

Journal de Physique Colloques, 1987, 48 (C7), pp.C7-733-C7-735. �10.1051/jphyscol:19877180�. �jpa-

00227005�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n012, Tome 48, decembre 1987

IN SITU PROBING OF ELECTRONIC SURFACE STRUCTURE I N IONIC CRYSTALS BY RESONANT SECOND HARMONIC GENERATION

J. REIF, P. TEPPER, 0. SEMMLER and E. MATTHIAS

Fachbereich Physik, Freie Universitdt Berlin, Arnimallee 14, 0-1000 Berlin 33, F.R.G.

Abstract

Resonant second harmonic generation at the surface of transparent ionic crystals is used to investigate the electronic properties of these surfaces. We report on in situ measurements on barium fluoride, sodiumchloride, and potassiumchloride in air.

From the divalent crystal, the second harmonic signal should be mainly due to surface dipole interaction, since it shows spectral resonances and the dependence on azi- muthal crystal orientation reflects the surface symmetry. From the monovalent crys- tals, the purely surface signal does practically not depend on the azimuthal crystal orientation, and should h e m e be due mainly to long range interaction. If one compo- nent of the light polarization is perpendicular to the surface, a surprising asymme- try is found in the signal from the alkali salts.

Second harmonic generation (SHG) in reflection from the surface of centrosymmet- ric media is, in principle, a promising tool for investigating electronic surface structure /I/. Since it represents a purely optical method, it is not restricted to a special environment such as ultra-high vacuum. Hence, it provides a way of in situ, non destructive, and remote measurement of surface properties. It can be shown, fol- lowing the argument of Bloembergen et al.

that any higher multipolar contribu- tions to the signal from the bulk should vanish, if both fundamental and second har- monic are polarized perpendicular to the plane of incidence

.e. the respective electrical field vectors are parallel to the surface and to each other)

So, under this condition, the process is highly surface sensitive. Guyot-Sionnest and Shen /1/ have demonstrated that the signal should be due to long-range in surface electric quadrupole interaction and to local electric dipole terms. The latter are attributed to local electronic excitations in the interface 1 ayer. From our cluster calculations /3/ we expect such states at the BaF, (111) surface. Indeed, we observe a spectral dependence of the second harmonic signal, as is shown in fig. 1.

Fiqure 1 Reflected second harmonic intensity from a polished BaF, (111) surface as a func- tion of dye laser wavelength (coumarine 152).

Both fundamental and second harmonic wave are polarized perpendicular to the plane of inci - dence (s-polarization). The angle of incidence is

The azimuthal orientation of the crys- tal was not measured.

This means that at least part of the electronic surface structure is preserved de- sp.ite the fact that the surface is obviously covered with adsorbates from the sur- rounding air. Apparently, to a great extent the signal can be associated with the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877180

C7-734 JOURNAL DE PHYSIQUE

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Azimuthal dependence of the second harmonic signal from BaF, (111) (radius) projected onto the surface structure (fundamental at 532 nm), when the crys- tal is rotated about its normal. a) s-polarization, b) p-polarization. Angle of inci- dence: 4 5 ' .

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Fiqure 3. SHG from BaF, (1ll)as a Fiqure 4. SHG from NaCl (Ill), function of laser wavelength for at 532 nm. Azimuthal dependence for 45'-pol arization.

Azimuth for a) s-polarization, b) 45'-polariza- maximum signal. b) azimuth rotated tion.

by 30' relative to a). c) azimuth

rotated by

relative to a).

d i p o l e i n t e r a c t i o n . Having an i o n i c c r y s t a l , we can i d e n t i f y macroscopic dip01 a r azimuths i n t h e s u r f a c e plane. These are along t h e b a r i u m - f l u o r i n e d i r e c t i o n s . Due t o t h e C,, symmetry o f t h e (111) plane and t h e f a c t t h a t a l l p o l a r i z a t i o n s are p a r a l l e l t o t h e surface, t h i s leads t o a s i x f o l d symmetry o f t h e second harmonic gain, i f t h e c r y s t a l i s t u r n e d about t h e surface normal. The p r o j e c t i o n o f t h e observed symmetry onto t h e s t r u c t u r e o f t h e (111) s u r f a c e i n F i g . 2a confirms t h i s argument. I f , how- ever, we change t h e l i g h t p o l a r i z a t i o n s such t h a t t h e r e i s a component p o i n t i n g nor- mal t o t h e surface, t h e azimuthal dependence o f Fig. 2b i s obtained. T h i s d i s t r i b u - t i o n may be described by adding an i s o t r o p i c term t o t h e second harmonic f i e l d ampli- tude /4/, i . e .

w i t h 0 b e i n g t h e azimuthal angle. The s p e c t r a l dependence o f SHG w i t h a p o l a r i z a t i o n component p e r p e n d i c u l a r t o t h e surface i s shown i n Fig. 3 f o r b o t h E-vectors being a t 45O t o t h e p l a n e o f incidence. T h i s i s t h e s i t u a t i o n where t h e t o t a l s i g n a l i s strongest. The p r i n c i p a l f e a t u r e o f t h e spectrum, i . e . two peaks around 510 nm and 535 nm, i s s i m i l a r t o s - p o l a r i z a t i o n , however, t h e r e l a t i v e i n t e n s i t i e s are d i f f e r - e n t . T h i s might come from a d i f f e r e n t azimuthal o r i e n t a t i o n i n b o t h cases, as can be seen from t h e d i f f e r e n c e between Fig. 3a and b. Work i s p r e s e n t l y going on i n our l a b o r a t o r y , t o i n v e s t i g t e t h i s question i n more d e t a i l .

We a l s o performed f i r s t experiments on monovalent i o n i c c r y s t a l s (NaC1 and KC1).

Here, we do n o t expgct t o f i n d t h e pronounced e l e c t r o n i c s t r u c t u r e i n t h e bandgap a t t h e surface. Consequently, surface SHG should be dominated by t h e long-range e l e c t r i c quadrupole term more l i k e l y than by d i p o l e i n t e r a c t i o n . T h i s means, t h a t no p r o - nounced i n f l u e n c e o f t h e microscopic o r d e r i n g should be present. Indeed, f o r s - p o l a r - i z a t i o n we observe n e a r l y azimuthal i s o t r o p y f o r b o t h c r y s t a l s . T h i s i s shown i n Fig.

4 f o r NaC1. For 45°-polarization, however, we observe a tremendous one f o l d aniso- t r o p y i n b o t h cases. Presently, we do n o t y e t understand t h i s r e s u l t , b u t o u r f e e l i n g i s t h a t i t m i g h t .be associated t o some e f f e c t o f adsorbed water. T h i s i s s u b j e c t t o f u r t h e r i n v e s t i g a t i o n s .

Acknowledaements

We thank H. Schmelz f o r p a r t i c i p a t i o n i n p a r t o f t h e NaCl and KC1 measurements.

T h i s work was supported by t h e Deutsche Forschungsgemeinschaft, Sfb 6, and K a l i und Sal z AG (NaC1 and KC1 )

.

References

/1/ P. Guyot-Sionnest and Y.R. Shen, Phys. Rev. 835, 4420 (1987) and references t h e r e i n

/2/ N. Bloembergen, R.K. Chang, S.S. Jha, and C.H. Lee, Phys. Rev.

174,

Rosen, Resonant M u l t i p h o t o n Processes i n Laser-Surface I n t e r a c t i o n , t h i s volume, p 443

/4/ H.W.K. Tom and G.D. A u m i l l e r , Phys.Rev. 833, 8818 (1986)