Two-photon spectroscopy of point defects

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Two-photon spectroscopy of point defects

U. Grassano

To cite this version:

U. Grassano. Two-photon spectroscopy of point defects. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-359-C6-362. �10.1051/jphyscol:1980692�. �jpa-00220130�

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JOURNAL DE PHYSIQUE Colloque C6, supplgment au no 7, Tome 41, Juillet 1980, page C6-359

Two-photon spectroscopy of point defects

U . M. Grassano

Istituto ^dlFisica, UniversitA di Roma, Rome, Italy ^andGruppo Nazionale di Struttura della Materia del Consiglio Nazio-

nale delle Ricerche, Rome, Italy

Resume. - Les informations experimentales de la spectroscopie ri deux photons dans les impuretes et les defauts dans les soiides sont ici r6sumCs. Les resultats fes plus importants sont la mesure de la probabilite d'absorption et I'attribution de la symetrie des transitions. Les effets non lineaires sont importants aussi dans d'autres cas cornme la creation des centres colores dans les cristaux d'halogenures alcalins.

Abstract. - The experimental results on two-photon spectroscopy in impurities and defects in solids are reviewed.

The main results are the measurement of absorption cross sections and in some cases the assignement of the symmetry of the transitions. Nonlinear effects can be of importance also in the process of colour centre formation in alkali halides crystals.

I. Introduction. ^-Two-photon spectroscopy has recently become a powerful tool for the investiga- tion o f the optical properties of solids. Two-photon effects, predicted theoretically by Maria Goeppcrt- Mayer in 1931 [I], have been experimentally verified by Kaiser and Garret in 1961 [2], soon after the dis- covery of the laser.

It was immediately realized that, due t o the different selection rules, the information obtained with the two-photon spectroscopy is complementary to that obtained from the conventional one-photon spectroscopy [3, 4, 51. However the intrinsic experimental difficulties, due to the small probability of this second order effect 161, have prevented u p t o now a widespread application of the two-photon techniqu s. These limitations are particularly severe in the study o f the electronic properties of localized defects in solids and most of the work on two- photon spectroscopy was devoted to intrinsic absorption in insulators and semiconductors (for a comprehensive review see [7]). Indeed in the study o f point defects o n e deals with a number of absor- bing centres of the order 10'' ~ m a n d therefore - ~ the two-photon absorption coefficients a r e at least four order of magnitude smaller than those measured in band-to-band absorption in solids with 102' electrons cm ^-

'.

Because o f these difficulties two- photon absorption in localized defects has been mostly studied monitoring the absorption through its subsequent luminescence. The main advantage of the detection of the two-photon absorption by luminescence is the much greater sensitivity, limited only by the noise of the signal. In this way however one actually measures a two-photon excitation spectrum and the interpretation of the results of this indirect method is less straigthforward because of the possi-

bility of non-radiative decays o r photoinduced reac- tions among the defects.

Another limitation of most of the experiments performed u p t o now in localized defects has been the use of only one o r two laser lines a t fixed frequency as sources of both photons. In this way two-photon effects rather than two-photon spectra have been observed. It is evident that only a spectrum over a sizable energy range can yield complete information on the positions of the excited states o f the defects.

The use of tunable lasers as excitation sources pro- mises to be most valuable in this respect.

2. Two-photon effects of impurities in solids. -

The first two-photon cxcitation was detected via luminescence by Kaiser and Garret in CaF2 doped with Eu [2]. T w o photons from a ruby laser (1.78 eV) simultaneously absorbed produced a blue fluorescence ^{( -}2.9 eV) whose intensity increased with the square of the laser power (Fig. 1). This quadratic behaviour is the most evident signature that a two-photon process takes place. Two-photon absorption coefficient o f impurities in alkali halides was calculated for NaCI : Ag+ by Gold and Her- nandez [8] and measured in a similar system (KC1 : A g + ) by the pioneering work o f Frijlich and coworkers [9]. The A-band of KC1 : A g + is d u e t o the transition of the Ag+ ion 4d10 -, 4d9 5s.

This transition is parity allowed for the absorption o f two photons. In this experiment the fundamental (1.78 eV) a n d the second harmonic frequency (3.56 eV) of a ruby laser provided the required energy. The excitation frequency is fixed ( A w l

+

fiw, = 5.34 eV) but the relative polarization of the two beams can be varied. T h e angular dependence of the luminescence intensity (see Fig. 2)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980692

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big. 1 . - Quadratic dependence of the blue fluorescence of CaF, : Eu2+ upon the intensity of the exciting laser beam (see ref. 121).

big. 2. - Luminescence intensity of KC1 : A g + , excited in the A-band, a s a function of the angle between the polarization of the two lasers beams (see ref. 191).

allows the determination of the symmetry (I'3

+ r5)

of the cxcited states involved in the A-band transition.

Using two photons of the same laser as excitation sourcc onc can gain in intensity, but one cannot change independently the polarization of the two beams. However by choosing different directions of incidence and by polarizing the laser beam linear- ly and circularly, one can perform a complete analysis of the symmetry of the levels involved in the transition. An experiment of this kind has been done in CaF2 : Eu2+ at five different wavelengths [lo]. The absorption of the Eu2+ ion (around 3.5 eV) is attributed to the transition 4f' (8S7/2)

-

4t6('F)5d where thc final level of the d electron is splittcd by the cubic crystalline field. The transition probability for two identical photons can be decomposed into contributions of

r,,

^l?, and

r5

symmetry. The experimental results rule out the

rS

contribution and help to study the symmetry of the levels originating within the f-d multiplet.

3. Two-photon effects in colour centres. - Two- photon effects in colour centres are still more diffi- cult to observe than those in impurities because of the intrinsic instability of these defects that during the experiment can be ionized, can aggregate or annihilate, also at low temperature. Probably because of this instability some results are not very reproducible and some are not yet clearly under- stood. In y-irradiated Nab' we have measured the two-photon absorption at a particular wavelength (1.06 fim) [I I]. Two luminescence bands, identified by their excitation spectra as due to F; and 1'j(NI) aggregate centres (see Fig. 3), increase with the square of the laser power. Assuming the same quantum yield of the emissions excited either by one or by two photons, we have evaluated the absorption cross section at 1.06 pm. We found

@ = 6.5 x cm4 s phot-' centre-' and

ON, = 5.5 x cm4 s phot-' centre-' respectively.

These values are somewhat uncertain because we could not check the absolute values against some known nonlinear cross section (e.g. Raman cross section 112, 13)) but we believe they are correct within a factor of two.

Two other bands are observed in figure 3 around 510 nm and 775 nm. No precise assignement for the origin of these emissions could be made. The green emission (at energy higher than twice the laser energy) decays with time of excitation at a rate that increases with the laser intensity. It can be probably a t t r i b u t e d t o a radiative recombination o f interstitial-vacancy pairs in the irradiated crystals.

Indeed this luminescence is absent in additively coloured NaF. Thc green band decreases also on increasing the temperature. A nonradiative process, with an activation energy of

-

35 meV, becomes more probable than the emission.

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TWO-PHOTON SPECTROSCOPY O F POINT DEFECTS C6-361

Fig. 3 . - Two-photon stimulated emission of NaF crystal containing F; and F,(N,) centres (see ref. [ I I]).

Two-photon excitation has been also observed in additively coloured KC1 containing only F centres.

The emission contains two peaks at 700 nm and 900 nm ; the latter is superimposed on a pro- nounced emission tail continuing up to the laser wavelength (Fig. 4). We have interpreted the broad emission tail as hot fluorescence 1141. The 900 nm peak is somewhat reminescent of the two photon excited emission observed by De Martini et al. [15]. Same differences may be due to the different laser pulse duration. Our spectra change markedly i f we record the emission at increasingly longer delay times after

I ,

Î Î Î

m ml sm m nn

w t h

Fig. 4. - Emission of KC1 crystal, containing only F centres, excited by two I.R. photons (1.06 pm).

the laser pulse. In the same system Vassiliev et a/.

[I61 reported on two-photon absorption of F centres followed by ionization. The effect was monitored through the F' centres formation.

4. Nonlinear effects in localized defects. - The results reported in the previous paragraphs, although too incomplete to yield two-photon spectra, are nevertheless aimed at the knowledge of the spectroscopic properties of defects such as the symmetry and the energy of states with the same parity of the ground state.

Nonlinear effect have also been used for produc- tion and control of point defects in alkali halides.

The weak absorption coefficient in the two photon process assures colouration or photocromic proces- ses much more uniform than those obtainable with one photon. With a N2 laser (3.68 eV) one can colour alkali halides with gap smaller than 7.36 eV such as KI, KBr, NaBr [17] or doped KC1 : I , KC1 : Br [18] but not wider gap materials such as pure KC1 or NaCI. The induced absorption spectrum, as shown in figure 5, is practically the same as that produced in pure X-rayed samples and the refore must be an intrinsic process. The same con- clusions were reported by Bradford et al. [19]

who coloured KC1 by two photons of 4.68 eV (fourth harmonic of a mode-locked YAIG : Nd

laser).

Two laser photons were used by Mollenauer el al.

[20] to produce uniform F centre colouration or to colour only selected regions of the crystals exploi- ting the controlled optics of the laser beam. KC1 crystals containing substitutional hydrogen ion H - (U centres) are transparent to the fourth harmonic of the YAIG : Nd laser. Absorption of two such a photons however, generates electron-hole pairs that induce the U

-

F conversion.

03 --

PHOTON ENERGY ( U )

Fig. 5. - Absorption spectrum obtained on a KC1 : I crystal irradiated with a N2 laser. The absorption spectrum of undoped KC1 produced by X-rays is shown by the dotted curve (see ref.

1181).

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C6-362 U. M. GRASSANO

Fig. 6. - Second harmonic intensity and absorption coefficient of the X and F band (right hand scale) of addit~vely coloured NaCl as a function of the annealing temperature. The duplicating centres arc formed only in a particular range of temperatures.

Another nonlinear effect we have recently found is the second harmonic generation in a cubic crystal (NaCI) containing anysotropic defects lacking inver- sion symmetry [21]. These defects are produced in crystals coloured at high pressure of Na vapor, and annealed at 480 "C. At this temperature the quasi- colloidal X band is destroyed and the isolated vacancies begin to form. Figure 6 shows the behaviour of the second harmonic intensity and of the X and E' absorption bands as a function of the annealing temperature. Note that the annealing at 550 "C produces a large F band but destroys the duplicating centres. This fact, beside symmetry considera- tions, rules out the F band as direct responsible for the effect.

5. Conclusion. - It is clear from the scattered results presented above that a lot more remains to be done t o perform a proper two-photon spectroscopy of localized centres. The experimental impro- vements brought recently forward by Frolich ef a/.

are really astonishing [22] (see Fig. 7) and it is quite

big. 7. - Experimental set-up for two-photon spectroscopy using fixed and tunable lasers (see ref. [22]).

likely the in the next few years two-photon absorption spectroscopy could yield a wealth of new experimental results in the study of point defects in solids.

Acknowledgements. - I would like t o acknowledge the stimulating discussions with Professor G. Chia- rotti and the essential contributions of Drs.

M . Casalboni, F. Crisanti, R. Francini and A. Tanga to the research described above.

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