Colored LiF: an optical material for all seasons

Journal of Luminescence 100 (2002) 333–343

Colored LiF: an optical material for all seasons

Giuseppe Baldacchini*

Department of Advanced Technologies, Frascati Research Center, ENEA, Via E. Fermi 45, P.O. Box 65, Frascati, Rome 00044, Italy

Abstract

Colored LiF salt has been always considered a singular optical material among alkali halides and other dielectric

crystals for its peculiar characteristics, which in due time have been applied with success in thermoluminescence and

laser technology. Lately, while the two previous topics have been revived, new relevant results have been obtained in the

optoelectronic field by using both bulk crystals and newly characterized thin films. In practice, miniaturized

photoluminescent patterns can be produced rather easily by using low-energy electron beams and soft X-rays. So, LiF

salt is becoming a new interesting photonic material with promising developments in basic reasearch and applications as

well.

r 2002 Elsevier Science B.V. All rights reserved.

PACS: 78.55.Fv; 61.70.Dx; 42.55.Rz; 78.60.Kn; 78.65

Keywords: Lithium floride; Color centers; Lasers; Energy transfer; Thermoluminescence; Films; X-ray detectors

1. Introduction

Point defects in insulating crystals represent awell-known class of optical materials which havebeen and are being used in basic studies ofelementary light processes and application inoptoelectronics. Among them, color centers (CC)in alkali halides occupy a special place becausethey were the first ones to be studied carefully andthey were, and still are, considered a model casefor more complex point defects [1]. Fig. 1 showsthe schematic models of a few CCs which derivefrom anion vacancies. There are also CCs whichare based on cation vacancies, but they are not of

direct interest in this work, and will not bedescribed and mentioned further. The simplestCC is formed by an electron trapped in an anionvacancy, and it is called F center. The F center canbe aggregated to another F center and vacancy, toone cation impurity, and to one anion impurity,forming F2, F3

�, FA, and FH centers, respectively.The F�

3 center, which is an ionized F3 center, laysoutside a single lattice plane. If a third electron isadded to the F2 center, two electrons to the F+

3

center, and one electron is taken away from the F2

center, one gets the F2�, F3

�, and F2+ centers,

respectively, which are not shown in Fig. 1 toavoid further congestion in a rather limited area.

The previous CCs give rise to absorption bandsextending from the UV through the visible up tothe near-IR region of the electromagnetic spec-trum which, by the way, are at the origin of thevarious crystal colorations. Many of them, if

*Corresponding author. Tel.: +39-69400-5365; fax: +39-

69400-5337.

E-mail address: [email protected]

(G. Baldacchini).

0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 4 6 0 - X

properly excited with electromagnetic radiation,produce emission bands characterized by well-defined Stokes shifts according to the differentinteractions between the CCs and the latticevibrations. In a few notable cases, the quantumefficiency of these emissions is so high, next to 1.They have been successfully utilized for tunablelaser emission from 0.4 to 5 mm [2,3].

However, not all CCs in the various alkali halidecrystals behave in the same way and, notably, LiFamong them is a well-known exception. Indeed, itssingular properties placed it aside since long timeago. In a famed review article [4], Frederick Seitzsaid ‘‘crystals of this material (LiF) exhibit some ofthe properties of the other akali halides; however,in many ways they appear to demonstrate char-acteristics which are uniquely their own. It isdifficult to say whether these differences arecharacteristics of the pure salt, or whether theyare associated with possible contamination of thecommercially available material’’. Now, afteralmost 50 years of R&D on the matter, it is wellknown that, while impurities can play some role inthe final properties of LiF and often the crystals

are doped purposely, the unique characteristics ofLiF are of intrinsic nature. As a matter of fact, thesalt is almost insoluble in water, the cation–aniondistance is the shortest, and the Li+ and F� ionspossess the smallest radius among the alkali andhalide ions, respectively. Moreover, the index ofrefraction, n ¼ 1:3912; is one of the lowest and theband gap, B14 eV, the largest with respect to allthe other alkali halides and dielectric materials ingeneral.

LiF can be colored only by resorting toirradiation with ionizing radiation, like X-rays,g-rays, elementary particles, and ions. However,with these methods it is not possible, in general, toobtain a sample containing only one type of CC,as it is usually the case with the other alkali halidesadditively colored. A typical absorption spectrumof an LiF crystal irradiated by an electron beam isshown in Fig. 2. The first intense band centered atB250 nm is commonly attributed to F centers,while the M band is actually composed of theF2 and F3

+ bands. Although luminescence fromF centers was searched with experimental effortssince long time [4,1], and a weak emission isexpected theoretically around 900 nm, no emissionhas been detected up to now which can beunambiguously attributed to F centers [5]. Any-way, the thermal activation energy of its relaxedexcited state (the energy level from which light isemitted, if any) has been measured, and it has beenfound in disagreement with the known values ofthe other alkali halides [1]. As for the F centers,

Fig. 1. Structural models of the F center and of simple

aggregate centers. In order to show the F3+ center, three rows

of ions (thicker lines) of a second lattice plane have been added

on the right side. See text for details.

Fig. 2. Absorption spectrum at RT of an LiF crystal 0.5mm

thick colored at RT with 3MeV electrons. The band M is

actually composed by the sum of the F3+ and F2 bands.

G. Baldacchini / Journal of Luminescence 100 (2002) 333–343334

also F3 and F4 centers have not shown anyluminescence while, just to compensate this short-coming with respect to the other salts, all the CCsreported in Fig. 2 display efficient luminescences.

It is very interesting to note that CCs in LiFspan the whole range of couplings with the lattice,from very strong, F and most likely F3 and F4

centers, to very weak, F3+ and F3

� centers. Theextremely weak coupling regime of many CCs inLiF has allowed to study in great detail the zero-phonon lines associated with the absorption andemission bands at low temperature [1], and one ofthem associated with the F4 centers has been usedrecently to clarify a doubtful interpretation [6].

In conclusion, it is evident from the previousconsiderations that LiF salt sits itself aside withrespect to the other alkali halides, both as a crystalthan as a host for point defects because of itsextreme characteristics, which have been carefullystudied but not yet completely clarified. Moreover,its chemical and physical properties have been putto work in many relevant applications andrecently, new results have been obtained whichhave opened new perspectives for future develop-ments.

2. Continuing past developments

Optical properties in colored LiF salts havealways been at the very center of the scientificattention because of their unique characteristics.Indeed, it has been already mentioned that manyaggregate centers exhibit efficient emissions uponappropriate optical excitation. Fig. 3 shows theabsorption and emission bands of the CCs in LiFwhich are known to possess emissions and where,for the lack of it, the F, F3, and F4 bands havebeen omitted. Contrary to the other alkali halides,the reported emission bands posses high quantumefficiencies also at RT, and for this reason theyhave been used in tunable lasers operating at RT inpulsed regime from 0.5 to 1.3 mm [7]. EspeciallyF2�, F2

+, and to a less extent F3� centers have

produced high-energy outputs, with the recordpower of 1GW at 1.14 mm for the F2

� centers,while F2 and F3

+ centers have not been used inpractice up to now because they are prone to fade

away and are unstable under high pumpingintensities. However, the high-gain coefficients ofthe F2 and F3

+ centers and the almost totaloverlapping of their absorption bands have alwaysprompted a continuing and rich investigation onthe subject. Indeed, while their laser emission wasdemonstrated long ago for F2 [8,9] and F3

+ centers[10,11], their instability was studied only later on[12], together with the basic properties of thecomplex optical cycle of the F3

+ centers [13]. In thelatter work the role of a triplet state wasdefinitively established, and the active centersparticipating in the laser emission was shown todecrease linearly as a function of pumping power,and to increase by decreasing the temperature.These results have indicated that an F3

+ centerlaser can operate only in pumping conditions oflow-energy density. These experimental conditionshave been realized recently by resorting to pump atlow repetition rate a crystal containing F+

3 centersin a optically transverse mode, which allowsdecreasing of the pumping energy density ofseveral orders of magnitude with respect to themore common longitudinally pumped cavity [14].

Apart laser applications and other ones innonlinear optical behaviors [15], the radioactiveproperties of CCs in LiF have suggested alsoanother interesting theme of investigation regard-ing the energy transfer of the optical excitation

Fig. 3. Schematic absorption and emission bands, in a colored

LiF crystal, belonging to CCs which are known to posses also

photoluminescence.

G. Baldacchini / Journal of Luminescence 100 (2002) 333–343 335

among the various CCs. Indeed, by lookingcarefully at the various bands in Fig. 3, it isrealized at once that by moving from left to right,i.e., from high to low energy, the emission of apreceding CC mostly coincides with the absorptionof the following one, and so on. This is equivalentto say that light emitted by one center can beabsorbed and emitted again by other centers in acontinuous energy-transfer cascade process.

In general, this phenomenon is rather relevant invarious fields like spectroscopy, laser technology,phosphors, artificial solar energy conversion, andphotobiology, and in colored LiF it can be studiedin a relatively clean and easy way. Indeed, not onlythe spectral region of the energy transfer is ratherwide, and the absorption and emission bands arenarrow compared to it, but also their intensitiescan be varied up to a certain degree by increasingor decreasing the amount of CCs involved. Any-way, recently it has been possible to study in detailsome energy processes which play an importantrole in optoelectronics.

In particular, F3+ and F2 centers have been

investigated under extreme physical conditions inorder to test their usefulness for photonic applica-tions [16,17]. Presently, the emission efficiency Z ofF2 centers has been measured as a function of theirconcentration up to 2� 1018 centers/cm3 [6,17].Fig. 4 shows the relative value of Z; constant up to1017 centers/cm3 decreasing after that by morethan 50%. This decreasing could have beenattributed to interactions with F3

+ centers whichshare with F2 centers an ample portion of theabsorption band. However, experiments per-formed with different amounts of F+

3 centers seemto exclud any such interaction, while they haveuncovered an energy-transfer mechanism with F2

+

centers. Indeed, the emission of F2 centers can beabsorbed by F2

+ centers, see Fig. 3, and it has beenobserved that the lifetime, tB17 ns, of the F2

�

center decay process decreases appreciably whilethe amount of F2

+ centers increases [18], abehavior well known to happen in radiationless,self-absorption, and energy-transfer processes [19].More tangled situations are observed in Fig. 3, bymoving toward the infrared region of the electro-magnetic spectrum, a region which is beingconsidered by present optoelectronics technology

in connection with optical telecommunications.Having in mind such purposes and in order toclarify better the band attributions and theirspectral-range influence, a heavily colored crystalhas been pumped at 672 nm, and the absorptionand emission spectra measured with care at thepumping spot and elsewhere in the crystal [20].Fig. 5 shows one such result referring to a volumeof the crystal 12mm away from the front surfacewhere the crystal is excited by a diod laser. Firstly,in order to describe consistently the experimentalresults, a new absorption band at B700 nm hasbeen added besides the F4-like band at B640 nm,the F3

� band at B800 nm, and the F2� band at

B960 nm. The new band has been identified as dueto the second absorption band of the F3

� centers,although it lays at slightly shorter wavelengthsthan previously reported. Anyway, the pumpingenergy is completely outside of the absorption ofboth F3

� and F2� centers, whose emissions are

instead well observed in Fig. 5b. In order toexplain such surprising results, it has beennecessary to resort to an energy-transfer processwhere each aggregate center is excited in cascadeby the previous one, a process repeated thrice atthe end. Moreover, not only the excitation istransferred from one center to the next one,energetically speaking, but also the intensities of

Fig. 4. Quantum efficiency Z of F2 CCs emission at RT versus

their own concentration. The various crystals B1mm thick

have been colored at RT with a 5MeV electron beam at

different radiation doses. The values of Z have been arbitrarily

normalized to unity.


the three emissions display different slopes withdistance from the pumping spot, a behaviorhinting at a spatial energy diffusion which addsto the complexity of the previous system. Thesecrystals are still under investigation in order toboth clarify their spectral properties and study inmore detail the special spatial and temporalevolution of the energy-transfer processes.

Similar processes, and others connected withactivation transitions, are at the origin of thermo-luminescence (TL), i.e., light emitted duringheating a crystalline sample which has beenpreviously exposed to ionizing radiation. Ingeneral, this thermal emission of light is due toelectrons and/or holes trapped in impurities anddefect centers. For each type of trap there is acharacteristic activation energy for releasing theelectron and/or hole. So when the crystal is heated,each trap releases at a certain temperature one

electron and/or one hole which migrate throughthe crystal until they combine each other orbecome trapped again. Either of these processesmay lead to light emission which corresponds to aso-called glow peak. Usually, the whole TL band iscomposed of several glow peaks which are due tothe material itself, impurity content, ionizingradiation, type, dose and time of irradiation,defect center generated, and heating rates. Thewhole situation is so complex that up to now it hasnot been possible to associate with certainty thevarious glow peaks to well-defined impurities andlattice defects.

This situation is particularly true for LiF, whichis a well-known TL material, being almost tissueequivalent [21]. Although detailed works on ithave been performed since long time [22], they arestill continuing nowadays in order to unravel itsmany tangled facets, often with conflicting results[23,24]. Moreover, at the moment there is nogeneral agreement about the model of TL in LiF,which is believed to be different with respect to theother TL materials (more than an exception, thisnew ‘‘difference’’ seems to confirm the rule thatLiF salt stands really aside when compared withthe other alkali halides and also with the dielectriccrystals in general), also because of its large bandgap [23,25]. Anyway, Fig. 6 shows typical TLcurves of several commonly used phosphors,among them an LiF crystal containing Mg ionswas added to increase its efficiency as a dosimetermaterial [21]. The TL curve is composed appar-ently of three major and at least two minor glowpeaks, but these behaviors may change underdifferent experimental conditions and the spec-trum may extend up to 5001C [23]. For thesereasons, lately it has been decided to study TL inLiF samples pure and colored in a controlled wayin order to be able to associate the glow peaks onlyto the defect centers without the interference of theimpurities.

Fortunately for this purpose, recently basicstudies have been performed on g-rays coloredLiF crystals containing much less than 1 ppm ofimpurities with the purpose to determine exactlythe spectroscopic parameters of the absorptionand emission bands of F2 and F3

+ centers [26].Because of the almost total overlapping of the two

Fig. 5. Measured absorption spectrum: (a) at RT above 600 nm

of an LiF crystal B1 cm thick colored with a high-energy

electron beam (full line), and photoluminescence (b) excited by

a diode laser at 672 nm (arrow). An attempt of fitting (dashed

lines) the experimental data has been made with several

Gaussian bands (dotted lines).


absorption bands, it was necessary to preparevarious samples containing different amounts ofF2 and F3

+ centers in order to distinguish at bestthe two bands mixed in an unique absorption bell-shaped feature. Fig. 7 shows the final results ofseveral experiments and fitting procedures, and atthe end only four samples have been sorted outhaving significantly differently spectroscopic prop-erties. All samples are 1.15mm thick. Samples Aand B have been irradiated at RT with g-rays, andsuccessively sample B has been annealed at 2001Cfor 22min. Samples C and D have been irradiatedat �601C with g-rays, and successively sample Dhas been illuminated at RT with UV light. Apartthe evident different amounts of F2 and F+

3

centers, it is also easily observed that samples Aand B possess much more F3 and F4 centers, at

B3.3 eV (B380 nm) and B2.3 eV (B540 nm),respectively, than samples C and D.

Anyway, these four samples have been mea-sured in the range 30–5001C as far as TL isconcerned, and at least eight glow peaks have beenobserved. The four TL curves have differentshapes although the various glow peaks have moreor less the same temperatures. The analysis ofthe experimental results is just going on, and atthe moment it is not yet possible to associateall the glow peaks to the various CCs existing inthe crystals. However, it has been found that theportion of the TL curve below 2001C, composed oftwo glow peaks at least, is related to the F+

3

centers, because it is very strong in sample D andcompletely absent in sample B, as also partiallyexpected following its previous treatment [27]. Bycontinuing the analysis of the previous data,it is reasonably expected a complete attributionof the other glow peaks to the CCs existing inthe samples. Moreover, in the present case theimpurities at levels much less than 1 ppm shouldnot give any worry for the final attribution glow-peaks2color-centers, because they correspond tomuch less than 6� 1016 ions/cm3 against 8� 1016

F2 centers/cm3 and 5� 1016 F3+ centers/cm3, at

least.

3. New results

As it has been already told, colored LiF crystalshave been studied since long time for their opticalproperties but films of the same material wereinvestigated only later on. Indeed, the first knowresearch on layers of alkali halide salts colored bycathode-ray beams was stimulated by wartimeefforts for the realization of luminous displays forradar screens, finally fabricated with other materi-al phosphors having more appropriate character-istics for the job in question [28]. Later on, asystematic study of alkali halide films grown onsubstrates at RT was performed to determine theirphysical properties [29] and, more recently, poly-crystalline-doped tick LiF films were colored byX-rays to perform hole-burning spectroscopy inzero-phonon lines, having in mind applicationsin frequency domain optical storage [30]. But only

Fig. 6. Thermoluminescence curves of typical phosphors irra-

diated by *a rays from a 60Co source, among them LiF is doped

with Mg ions [21].


in the late eighties a systematic research on coloredfilms of alkali halides, and in particular in LiF,started for good.1 The reasoning behind such anenterprise was that the new photonic science atthat time was asking for miniaturization of theoptical devices in order to be easily integrated inoptoelectronic circuits, like the analogous all-electronic chips. As a result, the technology toproduce films of the desired characteristics and tocolor them effectively was developed together witha parallel effort to study the optical properties ofcolored bulk crystals, mimicking as much aspossible the extreme physical conditions existing

and/or required in micrometer and sub-micro-meter films, as high concentration of CCs, stabilityof CCs under storage and intense optical pumping,interactions among CCs, energy-transfer pro-cesses, etc. These efforts were paid off very soonwith excellent results regarding the production andthe characterization of films [31] and the opticalproperties of the same films colored with electronbeams [32].

After these very first results, research moved inso many different directions that it would beimpossible to report here the whole story of it, forwhich it is referred to elsewhere [6,33,34]. How-ever, a few results which have been and will be ofextreme importance for basic and applicationpurposes will be commented here. Fig. 8 displaysthe value of the refractive index n of a 100 nm thicklayer of LiF crystal colored with a 3 keV electronbeam, compared with the well-known bulk valuen0, as a function of the wavelength in the visibleand near-IR spectral region [35]. Apart around theF2 and F+

3 absorption bands, the value of n is

Fig. 7. Absorption spectra (full times) at RT of g-colored LiF crystals in different experimental conditions, see text for details.

Gaussian bands (dashed lines) have been used to fit (dotted lines) the measured curves.

1Research in LiF films in Frascati began in the spring of 1988

after long discussions on the subject of new optical materials

and their transformation processes under the suggestion of

Prof. Luiz Carlos Scavarda do Carmo of the Pontificia

Universidade Catolica do Rio de Janeiro, who was a guest of

the ENEA Laboratories in Frascati during his sabbatical year.

Besides the author, G. Baldacchini, and Prof. Scavarda, also

Dr. R.M. Montereali and technician A. Pace joined the new

enterprise at the beginning, and other people followed later on.


always bigger than those of n0, a result whoseconsequences will be discussed in a while.

At the same time, also the active role of the CCsin layers of LiF were probed to extend their laserproperties already well known in bulk crystals.Fig. 9 shows the emission spectrum and the gaincoefficient of a 1.8 mm thick film of LiF coloredwith a 12 keV electron beam, and in the shape of a25 mm wide strip and several mm long [36]. Besidesthe measured and corrected shape of the lumines-cence excited by the 458 nm line of an Ar laser andcorresponding to the F+

3 and F2 emissions, thegain coefficient, determined by measuring theamplified spontaneous emission along the coloredstripe as a function of its pumping length, is of theorder of 7 cm�1 from 500 to 700 nm, more or lessthe same value measured in bulk crystals [37,34].This last result added to the previous one on theincreasing of the refractive index upon coloration,had the practical consequences that light canpropagate in predefined colored waveguides inLiF, and can also be amplified along them, and inwhatsoever colored film geometry. Indeed, for thefirst time guided light propagation of two modeshas been observed with propagation losses from6 to 30 dB/cm, which are still high but fullyacceptable for the present purposes, and impro-vable in perspective [38,39]. Moreover, amplifica-tion of light in waveguiding structures has beenrecently obtained [40], and the optically active

qualities of films of colored LiF have been appliedfor the realization of optical microcavities, at themoment emitting around 670 nm [41], but able tofunction in the whole range reported in Fig. 9.

In all previous examples of this section, thecoloration of crystals or films of LiF has beenperformed by utilizing mostly low-energy electronbeams, also because of their versatility regardingbeam shape and directionality which allow toirradiate well-defined volumes and patterns of thecrystals or films. On the other side, because of theintrinsic low intensities of the electron beams forlocal and precise coloration, the irradiation timesmay become too long for practical purposes. So, itis appropriate to remind at this point that from thevery beginning of CC research in the twenties,X-rays irradiation was the most common methodutilized for the coloration of dielectric crystals, andonly later on X-rays, electron beams, chemicaladditive and electrolytic methods, and elementaryparticle beams were added and utilized accordingto the emerging technologies and required needs.Anyway, besides a few exceptions like the well-known color center School of Neuchatel [42],X-ray coloration has not been used much in thelast three decades. But, there have been recentlya few experiments that may be able to change thistrend for good.

Indeed, on one side lithographic technology ispushing its spatial resolution to less than 100 nm,mostly to increase still more the density of the

Fig. 8. Measured refractive index (dashed line) of a thin layer

of LiF crystal colored with a 3 keV electron beam. The solid line

represents the bulk value of an uncolored crystal. See text for

details.

Fig. 9. Optical gain coefficients (squares) of a colored LiF film

1.8 mm thick, 25mm wide and several mm long. Also, the

measured luminescence spectrum (solid line) is reported for a

direct comparison.


electronic circuitry in the chips, and this continu-ing miniaturization requires extreme ultraviolet(EUV), 60–6 nm (20–200 eV), and in perspectivesoft X-rays, 6–0.15 nm (200–8000 eV), radiation.On the other side, there has been great progresslately in generating intense point sources of EUVradiation and soft X-rays, by means of plasmaplumes obtained by focusing a powerful pulsedlaser beam on a solid target [43].

Given the previous ingredients and the longexpertise in CC physics, all of them alreadyexisting at the ENEA Laboratories of Frascati, itwas natural to put all together in an attempt toirradiate a layer of LiF salt with the electromag-netic radiation generated by a laser-plasma source[44]. The basic idea was to verify the possibility toproduce predefined colored patterns with highspatial resolution on relatively large surfaces ofLiF crystals. For this purpose, a copper mesh of60 mm period and wire 10 mm diameter was placedin contact with an LiF crystal in front and at adistance of about 15 cm from the laser-plasmasource, and irradiated with 1000 shots of roughly0.5mJ/cm2/shot of EUV and soft X-ray radiation.The effects of the irradiation on the crystals havebeen observed with an optical microscope bymeans of the luminescence emitted by the coloredzones of the LiF crystal illuminated by the 458 nmline of an Ar laser. Fig. 10 shows the image in theLiF crystal of the copper mesh as observed in themicroscope. The luminous zones correspond tothose colored by the ionizing radiation, and theyellowish light emitted is essentially coming fromF2 and F3

+ centers. The most important result,however, resides in the spatial resolution of theluminescent patterns which at a first analysis resultto be p1 mm, much better than shown in Fig. 10which is slightly blurred by a scarce number ofpixels.

In practice, this new method allows to producecolored patterns in LiF crystals and films withresolution smaller than 1 mm on surfaces as big as4� 4 cm2 and exposure times less than 1 s,contrary to what is possible to obtain today withother systems like electron beams, which require alonger operative time in order to obtain acomparable resolution. So, soft X-rays and LiFsalts have been mixed together with outstanding

results, opening so far new avenues in thelithographic technology, and it should not escapeto the experts and alike this potentiality for newdevelopments which can be envisaged for thenanotechnology field in general.

4. Conclusions

In the previous section, a good part of thehistorical developments of colored LiF salt hasbeen presented with the intent of demonstratingthe singular characteristics of this precious opticalmaterial. For this purpose, apart from a generaldescription of the salt itself and the CCs generatedby exposure to ionizing radiation, the big amountof information available has been divided arbi-trarily in two parts by taking as a basic criterion ofchoice the temporal scale of development. How-ever, the previous division has not proved tight-proof because, for instance, laser applicationsstarted for sure in 1960 well after film technologywas developed for the first time in 1940, but thelatter one has known such a revival later on that itcan be considered like a new starting. The sameholds for the X-ray coloration, which beganaround 1920, but again the perspectives of newuses and the addition with other technologiesrepresent much more than a simple continuation.

Anyway, although it has been always dangeroustrying to describe future developments, in thefollowing a general prevision will be attemptedfollowing more or less the layout of the paper.

Fig. 10. Luminescent image of an LiF crystal illuminated by

blue laser light under an optical microscope. The crystal has

been previously irradiated by soft X-rays through a copper

mesh clearly visible as a dark feature. See text for details.


First of all, it is clear by now that basic research oncolored LiF is still needed to clarify a fewimportant questions. The formation and transfor-mation of point defects under irradiation, andsubsequent physical and chemical treatments needto be further studied also for their consequencesfor TL and laser technology. The lacking of F-center luminescence remains a subject of utmostimportance, not only to find out the whereaboutsof the radiation and radiationless processes afterthe excitation, but also to clarify the basicmechanisms of the energy-transfer processes.Indeed, the latter ones are important also for TLand optoelectronic developments.

Different experimental techniques like time-resolved spectroscopy, short-pulse excitation,two-photon and coherent spectroscopy, nonlinearoptical spectroscopy, optical near-field spectro-scopy, and other modern technologies should beused to solve the previous topics. However, basicresearch does not get enough attention nowadaysuntil some applied development is in clear view, orbetter in progress. Fortunately, this is exactly thecase of LiF at this very moment when, by ignoringhere relatively old topics like TL and lasertechnology which are mature fields, but still thesubjects of living investigations, there are coloredthin films and image detectors for soft X-rays stillin their infancy which look very promising forvarious applications.

The feasibility of thin film devices has beendemonstrated viable at the level of laboratoryprototypes, and they have still to be integrated intoexisting products with operative lifetimes satisfy-ing the market request. However, colored LiF saltpossesses all the physical and chemical attributesto be considered a prime choice for next genera-tion of photonic materials. Indeed, the mere factthat it can be manipulated easily, colored in thedesired patterns, and high concentration of CCscan be obtained without detracting from theiroptical properties, gives to it important advantageswith respect to other solid-state and semiconduc-tor materials. In particular, they seem verypromising as waveguide amplifiers where therare-earth ion-doped glasses or crystals haveshown severe limitations as far as the optical gainis concerned [45]. Films of colored LiF have shown

optical gains an order of magnitude higher withrespect to the best-know glass amplifier [45,46].

The use of LiF as a detector for soft X-rays hasproved so successful that it is very difficult not tobe optimistic at the moment for future develop-ments. Indeed, micro-lithography technologyshould gain enormously by having a photonicmaterial that can be manipulated with good spatialresolution and high efficiency, for the realizationof miniaturized optical devices. Moreover, LiF saltproved to be a very good detector for soft X-rays,in several ways much more efficient and sharp withrespect to the detectors used at the moment for thesame purposes, as photographic plates, photore-sists, and charge coupled devices. And so also thisside aspect will produce some novelties.

In conclusion, although in this paper there wasnot enough room to describe in detail the old andnew story of LiF salt, colored and doped, it ishoped to have shown that interesting research isstill going on, and that new results have beenobtained which are very promising in the photonicscience both for basic purposes and applications.

Acknowledgements

I am very indebted to Dr. F. Bonfigli andR.M. Montereali for providing me with someof the material used in the work and forvaluable discussions. Many thanks are due toM. Cimino and L. Crescentini for their skillfulassistance in the preparation of the illustrationsand manuscript.

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Colored LiF: an optical material for all seasons