Anisotropy of optical absorption and fluorescence in Al[sub 2]O[sub 3]:C,Mg crystals

Anisotropy of optical absorption and fluorescence in Al2O3:C,Mg crystalsSubrata Sanyala� and Mark S. AkselrodLandauer Inc., Stillwater Crystal Growth Division, 723 1

2 Eastgate Street, Stillwater, Oklahoma 74074

�Received 10 November 2004; accepted 17 June 2005; published online 8 August 2005�

Spectroscopic properties of recently discovered fluorescent sapphire single crystals �Al2O3:C,Mg�,developed for volumetric optical data storage, are investigated. Polarized optical absorption,excitation-emission spectra, and quantum yield of fluorescence of the crystal in two differentphotochromic states are studied. The spatial distribution of intensity and polarization ratio of thefluorescence associated with the 435/520- and 620/750-nm excitation-emission bands,characteristic of the two photochromic states of the crystal, display strong anisotropy. The intensitydistribution, and hence the spatially averaged quantum yield of fluorescence for the two states,measured relative to standard laser dyes are similar for different crystallographic orientations. Theaverage quantum yield is found to be close to unity for both the states. The anisotropic propertiesof optical absorption and fluorescence suggest a model of double oxygen vacancy. © 2005American Institute of Physics. �DOI: 10.1063/1.1999032�

I. INTRODUCTION

Spectroscopic properties of sapphire ��-Al2O3� havebeen the subject of considerable interest because of the po-tential use of sapphire as a host material in diverse techno-logical applications, e.g., in solid-state lasers,1 radiationdetectors,2,3 dielectric substrate for microelectronics,4

ceramics,5 fusion reactors,6 fiber optics,7 etc. Recently, a ma-terial — �-Al2O3 doped with carbon and magnesium andhaving a high concentration of aggregate oxygen vacancies— Al2O3:C,Mg crystal was recognized as an efficient fluo-rescent medium suitable for optical data storage and imagingapplications.8 The past few decades have seen much progressin characterizing the structure and properties of point defects�color centers� responsible for the optical-absorption and lu-minescence properties of sapphire crystals, in both pure formand doped with impurities. The basic electronic point defectsin �-Al2O3 are single oxygen vacancy F and F+ centers �trap-ping two or one electrons� and aggregate oxygen vacancies,in the form of dimer F2 , F2

+, and F22+ centers �two nearest-

or next-nearest-neighbor oxygen vacancies trapping two tofour electrons; the superscript denotes the effective charge ofthe defects with respect to the ideal lattice�.

All previous spectroscopic studies on sapphire were per-formed either on crystals exposed to high-fluence energeticparticles or ionizing radiation or on Mg-doped sapphire withlow concentrations of oxygen vacancies. The purpose of thispaper is to study the optical-absorption, fluorescence, and theassociated polarization properties in Al2O3:C,Mg crystalsand to understand the electronic and structural properties ofthe color centers involved. In particular, the determination ofrelative quantum yield and anisotropy of the optical absorp-tion and fluorescence associated with the color centerspresent in this material is important for technological appli-cations as well as for understanding the physics of pointdefects in noncubic oxide systems.

II. SAMPLES AND EXPERIMENTAL PROCEDURES

Single crystals of sapphire or �-Al2O3 crystals, dopedwith carbon and magnesium impurities, i.e., Al2O3:C,Mgcrystals, were grown using the Czochralski method,9

whereby crystals are grown by simultaneous rotation andpulling of a single-crystalline seed, oriented along its opticalc axis �that is, the body diagonal of the rhombohedral unitcell�, from a pool of melt. A highly reducing atmospherewith a low partial pressure of oxygen is used to help theformation of the required oxygen vacancies in the �-Al2O3

crystal structure.Samples of suitable sizes, shapes, and crystallographic

orientations were cut and polished using diamond and silicasuspensions. Two parallelepipeds of dimension 8.50�8.50�13.25 mm3, a cube with a side of 4.13 mm, and a few thinrectangular plates obtained from the same crystal samplewere used. The thickness of the plates ranged from 300 �mto 1.65 mm, and the length of the rectangular sides rangedbetween 8.0 and 13.5 mm. The “annealed” samples wereheated in an oven to 680 °C and allowed to cool down toroom temperature prior to the experiments. This procedurereturns the crystal to its original “as-grown” condition.Photochromic transformation �“bleaching”� in the crystalsthrough the process of sequential two-photon absorption/ionization was performed using 435-nm laser pulses from aContinuum Panther optical parametric oscillator �OPO�pumped with the 355-nm third harmonic of Nd: yttrium alu-minum garnet �YAG� laser. The full width at half maximum�FWHM� of the pulses was 4.5 ns, and a peak power of up to300 MW/cm2 was achieved.

Optical-absorption measurements were carried out usinga UV-2401 PC spectrophotometer �Shimadzu, Inc.�, whichhas a scanning range of 1100–190 nm. Polacoat 105 dye-typeUV polarizers �API Optics, Inc.� were used to obtain appro-priate polarization of light in the spectrophotometer. Absorp-tion coefficients were calculated after subtraction of the baseline. For measurements of polarization anisotropy of differ-ent absorption bands, instead of rotating the polarization of

a�Present address: 706 Knob Hill Drive, Blacksburg, VA 24060; electronicmail: [email protected]

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http://dx.doi.org/10.1063/1.1999032

the excitation light, the crystal samples were rotated through90° under vertically polarized excitation in order to eliminatethe polarization anisotropy of the spectrophotometer’s pho-tocell sensitivity.

The fluorescence quantum yield Q and the fluorescencelifetime � are important characteristics that determine theefficiency of the fluorescence process governing the depopu-lation of the excited state to the ground state. They are re-lated to the radiative �kr� and nonradiative �knr� rate constantsvia Q=kr / �kr+knr�, and �=1/ �kr+knr�, where for conve-nience all possible nonradiative decay processes are groupedwith a single rate constant knr. Measuring absolute quantumyield is quite involved and requires special equipment.10

Generally, the relative quantum yield is satisfactory, which isdetermined by comparison of the wavelength-integrated in-tensity of the unknown sample x to that of the standardsample s,

Qx = Qs�Ks/Kx��s/�x��ax/as��nx/ns�2, �1�

where K is the absorbance at the excitation wavelength � , ais the area under the corrected emission curve �or fluores-cence intensity�, and n is the refractive index. The relativequantum yield of fluorescence associated with the color cen-ter having an absorption band at 435 nm and emission bandat 520 nm was measured against Coumarin-6 �LambdaPhysik, Inc.� in absolute ethanol, and Fluorescein �Acros Or-ganics, Inc.� in 0.1N aqueous NaOH, with the publishedquantum yield values of 0.780 �Ref. 11� and 0.925,12 respec-tively. Similarly, LDS-821 �Exciton, Inc.� in spectrophoto-metric grade methanol and diethyloxatricarbocyanine iodide,i.e., DOTC-I �Exciton, Inc.� in spectrophotometric gradedimethylsulfoxide �DMSO�, with the respective quantumyields of 0.200 �Ref. 13� and 0.630,14 were used for measur-ing the relative quantum yield of the 620-nm excitation–750-nm emission band of the crystal. The primary criteria forthe choice of these standard organic laser dyes were to havetheir absorption spectra as well as emission spectra widelyoverlap with those of the crystal. A stock solution for eachdye was prepared and then step diluted with the correspond-ing solvent to arrive at a final concentration of the dye whereits measured absorption coefficient was similar to the corre-sponding crystal sample at the excitation wavelength to beused. Sealed polymethylmethacrylate cuvettes �Fisher Scien-tific� with four clear sides, 1-cm path length, and a low cutoffwavelength of 285 nm were used as sample cells for thedyes. Thin microchambers for the dyes were prepared bysandwiching the liquid dye between two microscope coverglasses using several layers of adhesive Secure-Seal imagingspacers �9 mm diameter, 0.12 mm thick; Sigma-Aldrich�.

Excitation-emission spectra of the Al2O3:C,Mg crystalwere measured using a fluorimeter setup, constructed usingtwo scanning monochromators �Acton Research Corp.�, oneof which was equipped with a thermoelectrically cooledcharge-coupled device �CCD� detector. Deuterium �30 W;Oriel Instruments�, halogen �150 W; Titan Tool Supply�, andshort-arc xenon flash �20 W; EG&G� lamps were used asexcitation sources. This setup was also used to measure thefluorescence quantum yield of the laser dyes relative to eachother and to verify correspondence with published values.

Parallelepiped crystal samples and laser dyes in square cu-vettes were used in a 90° geometry, whereby the fluores-cence light was collected at 90° relative to the excitationbeam. Scattering of the excitation light by the sample isminimum with this geometry. In order to decrease the “innerfiltering effect” �i.e., the hindrance in penetration of the ex-citing or emitting light by strongly absorbing samples�present in optically dense Al2O3:C,Mg and dye samples,off-center illumination was used to reduce the optical pathlength to about 2 mm. UV fused silica lenses were used tofocus the excitation beam on the sample and to image thefluorescence on the slit of the emission monochromator. Be-cause Al2O3:C,Mg strongly absorbs UV and undergoesphotochromic transformations, an early UV exposure couldchange the nature of its excitation-emission spectra. This wasavoided by scanning the excitation monochromator fromhigh to low wavelengths. Suitable cutoff filters �Edmund In-dustrial Optics, OptoSigma Corp.� were used in the excita-tion beam to prohibit the second-order diffraction from theexcitation monochromator reaching the sample. Three sets ofexperiments were performed for each photochromic state ofthe crystal �annealed and bleached�: Halogen excitation withwavelengths from 700 down to 450 nm through a GG-385long-pass filter; deuterium excitation from 450 to 300 nmthrough a WG-295 long-pass filter; and deuterium excitationfrom 300 to 200 nm without any color filter. All emissionmeasurements were corrected for the base line, appropriateirradiances of the light sources, wavelength dependences ofthe excitation- and emission-monochromator’s grating effi-ciencies, the color filters’ transmission efficiencies, and theCCD quantum efficiency.

FIG. 1. Schematic diagram of the fluorescence measurement setup using alaser. S: shutter; W: half-wave plate; B: beam expander; M: power meter; H:sample on a sample holder for central illumination of 2-mm path length; L:lens; P: polarizer; I: iris; F: color filters; and D: Si photodetector.

033518-2 S. Sanyal and M. S. Akselrod J. Appl. Phys. 98, 033518 �2005�


Figure 1 shows the schematic diagram of the opticalsetup that was primarily used to measure the three-dimensional �3D� distribution of fluorescence intensity andemission polarization ratio �defined in Sec. III D� from a4.13-mm cube of Al2O3:C,Mg crystal. It has the provisionto simultaneously measure fluorescence in the forward �6°�and backward �186°� directions and at 90° with respect to theexcitation beam, all in the same optical plane. Identical setsof optical assemblies were used in all three directions toimage the center of the cube sample on the correspondingdetector iris. Each optical assembly contains an imaginglens, a polarizer, an iris, a suitable choice of color filters, anda silicon photodetector �DET-110; Thorlabs, Inc.�. The detec-tors were intercalibrated, and the fluorescence intensity mea-sured by a detector was directly read from an attached digitalmultimeter �Agilent Inc.� not shown in the figure. For mea-surements with annealed crystals, the vertically polarizedbeam from a 70 mW, 442 nm, He–Cd laser �Melles Griot,Inc.� was expanded to uniformly illuminate the entire cubesample. A combination of a long-wave-pass filter LWP-450,a short-wave-pass filter SWP-700, and a color glass cutofffilter GG-475, in that order, was used in front of the detectorsto effectively produce a bandpass between 475 and 700 nm.The use of LWP-450 drastically reduced the intensity of thestray laser light, particularly in the forward direction thatotherwise caused the GG-475 filter to fluoresce. For mea-surements with bleached crystals, a 24 mW, 638 nm, solid-state laser �Micro Laser Systems� was used in conjunctionwith a Schott glass low-pass RG-665 filter in front of eachdetector. For both the cases, the excitation intensity wasmonitored with the use of a power meter, as shown in Fig. 1.Polarization insensitivity of the detectors and the optics ineach of these three directions was checked with the help ofrandomly polarized emission from liquid dyes. A half-waveplate �Melles Griot, Inc.�, with its fast axis oriented at 45° tothe vertical, was used when the polarization state of the ex-citation beam had to be changed from vertical to horizontal.

Both the fluorimeter setup and the 90° geometry of thelaser setup in Fig. 1 were used to measure and compare therelative fluorescence yields for the laser dyes and a parallel-epiped sample of Al2O3:C,Mg crystal. In these experiments,the imaging optics for the 90° geometry in the laser setupwere rearranged to be identical to those used in the fluorim-eter. In the experiments that incorporated measurementsfrom Coumarin-6 dye a long-pass OG-455 glass filter, in-stead of the combination of color filters mentioned earlier,was used to reduce the lower cutoff wavelength for measuredemission.

III. EXPERIMENTAL RESULTS

A. Optical-absorption spectra

Optical-absorption spectra for two different photo-chromic states of the Al2O3:C,Mg crystals, annealed andbleached, are compared in Fig. 2. Spectra for polarized lightare presented for three mutually perpendicular orientations ofthe optical c axis; namely, �i� k�c �E, �ii� k�c�E, and �iii�k �c�E, where k and E denote the propagation vector andthe electric-field vector of the light illuminating the sample.

For the rest of the paper, abbreviations c �E , c�E, and c �kwill be used to denote these three crystal orientations andK� , K�, and Kc, respectively, to specify the base-line-subtracted values of the corresponding absorption coeffi-cients. Optical-absorption bands could be assigned to the dif-ferent oxygen vacancy centers present in the crystal. Theabsorption bands near 205 and 255 nm are assigned to F andF+ centers, respectively.15–19 The 230-nm absorption band,also associated with F+ centers, can be seen only at a certainorientation of the crystal when E�c, true for both cases �ii�and �iii� above, because of its strong anisotropy.18,20,21 Thepresence of the absorption band peaking near at 358 nm dueto F2

+ center15,20–22 is more apparent in Fig. 2 for the an-nealed crystal with c �E orientation.

In comparison with the undoped or carbon-doped sap-phire crystals,2,19 an absorption band at 435 nm appears inthe Mg-doped crystals, as can be seen in Fig. 2. This broadblue absorption band, responsible for the characteristicgreenish-yellow color of the as-grown and annealedAl2O3:C,Mg crystals, is assigned to the F2

2+�2Mg� colorcenters.8 Figure 2 also depicts the strong polarization depen-dence of this band, to be discussed below. Photochromictransformation of these color centers using sequential two-photon absorption can be performed using high peak powerpulsed 435-nm laser light �the bleaching procedure�,whereby the coloration and the 435-nm absorption band dis-appear, and bands near 335, 525, and 620 nm show up �Fig.2�. We note that the cw illumination with 442-nm He–Cdlaser light does not perform photochromic transformation ofthe color centers responsible for the 435-nm absorption banddue to insufficient power density.

B. Excitation-emission spectra

Unlike optical-absorption spectra, excitation-emissionspectra provide two-dimensional information and allow oneto resolve the emission bands for color centers with overlap-

FIG. 2. Optical-absorption spectra of Al2O3:C,Mg crystal measured withpolarized light for three mutually perpendicular crystal orientations, show-ing the effect of photochromic transformation �bleaching�.



ping optical-absorption bands. The data shown in Fig. 3 areobtained from three sets of experiments each using the 90°geometry in the fluorimeter for two photochromic states ofAl2O3:C,Mg parallelepiped samples: �a� annealed and �b�bleached. The bleaching was performed with the intense435-nm pulsed laser light from the OPO. The 90° geometrywas used to minimize the scattering of the excitation beamfrom the sample, but it could not be avoided altogether; itshows up as a continuous band running diagonally across inFig. 3, and provides a means to check for the intercalibrationof the excitation and emission monochromators.

It was noted in Sec. III A that several optical-absorptionbands, previously seen in sapphire crystals and assigned todifferent oxygen vacancy centers, are also present in the Mg-doped crystal. The corresponding emission bands can beidentified from the excitation-emission spectra of Fig. 3. Inmost of the previous studies these bands were induced in�-Al2O3 crystals by neutron, proton, or electron irradiationand ion-implantation methods.15,17,20,22–24 Single oxygen va-cancy centers such as F and F+ centers were studied

extensively15,18,19,25 and have well-identified excitation-emission bands. F centers show strong UV absorption at 205nm and emit light at 415 nm. F+ centers exhibit characteristicabsorption-excitation bands at 230 and 255 nm with emis-sion at 330 nm.16–18 The charge-neutral, double oxygen va-cancy F2 centers reported in the literature21,26 with anabsorption-excitation band at 302 nm and an emission bandat 516 nm were also found in a bleached Al2O3:C,Mg crys-tal, but their intensity was small. Presence of another knowncolor center, F2

+ center, showing absorption at 358 nm andemission at 380 nm �Refs. �15,19–22�� is also apparent inFig. 3.

The absorption-excitation band at 435 nm with the cor-responding emission band at 520 nm is characteristic to theas-grown and annealed Al2O3:C,Mg crystals. Comparisonof Figs. 3�a� and 3�b� clearly shows that after high intensity435-nm pulsed laser illumination, the 435-nm band disap-pears, and two other absorption-excitation bands at 335 and620 nm with a corresponding emission band at 750 nm showup as a result of a photochromic transformation of the crys-tal. Excitation-absorption bands centered near 258 and 525nm with the associated emission bands near 750 and 767 nm,respectively, also undergo some changes during the anneal-ing and bleaching procedure of the crystal, but they are noteasy to explain. The excitation-emission bands near 255/648and 358/780 nm have the same FWHM as 255/330- and358/380-nm bands, respectively, and hence seem to be theresult of a second-order diffraction of those bands. The ob-served shifts from their expected peak positions of 255/660and 358/760 nm are probably related to uncertainties in cali-bration.

Figure 4 shows a strong correlation between the 435-nmabsorption band and the excitation band for the 520-nm fluo-rescence. The emission spectrum under 435-nm excitation isalso shown in the same figure. The weak 520-nm peak in theexcitation spectrum and the 435-nm peak in the emissionspectrum are due to the excitation light scattered by thesample. Similarly, Fig. 5 depicts the correlation between theabsorption band of the bleached crystal and the excitationspectrum responsible for the 750-nm emission. This emission

FIG. 3. Excitation-emission spectra for Al2O3:C,Mg crystals in two photo-chromic states: �a� annealed and �b� bleached with 435-nm pulsed laserlight.

FIG. 4. Comparison of the 435-nm absorption and excitation bands respon-sible for 520-nm emission from annealed Al2O3:C,Mg crystals.



is much more intense when the bleached crystal is excitedwith 335-nm light compared with the one excited with620-nm light.

C. Quantum yield of fluorescence relative to standardlaser dyes

With vertically polarized excitation, the polarized ab-sorption and the unpolarized �i.e., no polarizer used� emis-sion from the 435/520-nm excitation-emission band of theannealed Al2O3:C,Mg parallelepiped crystal were measuredat the three perpendicular orientations, noted previously,c �E , c�E, and c �k, and compared with those from twostandard laser dyes, Fluorescein in 0.1N NaOH andCoumarin-6 in ethanol. The concentrations of the laser dyeswere chosen to achieve their absorption coefficients at theHe–Cd wavelength of 442 nm within the range of the valuesobtained for Al2O3:C,Mg crystals in different orientations,in order to keep the inner filter effects in the dyes and thecrystals similar. Figure 6 shows the emission spectra ob-tained using the fluorimeter setup, along with the corre-sponding absorption spectra. A similar experiment was per-formed using the 90° geometry of the laser setup having theoptical arrangement the same as that in the fluorimeter. Thearea under the emission curves in Fig. 6 and the fluorescenceintensities measured with the laser setup were used alongwith the absorption coefficients �Table I� in Eq. �1� to calcu-late the fluorescence yield at 90° for each of these samplesrelative to the two laser dyes. The quantum yield values ofthe dyes are taken from the literature; 0.925 for Fluoresceinin 0.1N aqueous NaOH solution,11 and 0.780 for Coumarin-6in polar solvent ethanol.12 The absorption and emission spec-tra for the dyes quite well reproduce their literature data aswell as the quantum yield with respect to each other. Table Ishows that Al2O3:C,Mg crystals have a high relative fluo-rescence yield at 90°, and the yield is strongly dependent onthe crystal orientation. The similarity of the measured valuesobtained from the two different experimental setups is quitesatisfactory. The highest yield of fluorescence in the 90° ge-ometry was obtained when k-vector is parallel to the opticalc axis of the crystal.

Similar measurements of fluorescence yield associatedwith the 620/750-nm excitation-emission band of thebleached crystal are prone to a few possible sources of error.Comparison of the absorption spectra of different bleachedcrystals shown in Figs. 2 and 5 clearly indicates that contri-bution from the broad 525-nm absorption peak �and maybesome additional as yet unidentified absorption peaks in thatregion of wavelength� and the bleaching history of the crys-tal introduce ambiguity in the measurement of absorptioncoefficient for the 620-nm absorption band. A “cleanup” pro-cedure for the 620-nm absorption band in the bleached crys-tal was developed. At first, freshly annealed crystals werebleached with 435-nm pulsed laser light to the optimum laserenergy at which the absorption coefficients for 620- and335-nm bands are close to their maximum values, while thatfor 435 nm is close to zero. Then the crystals were annealedto 500 °C to remove the additional absorption bands in therange between 500 and 600 nm that are produced duringbleaching and obscure the 620-nm absorption band associ-ated with 750-nm fluorescence. It was found that both the

FIG. 5. Comparison of the absorption and excitation bands responsible for750-nm emission from bleached Al2O3:C,Mg crystals.

FIG. 6. The absorption and emission spectra of annealed Al2O3:C,Mg crys-tal and two standard laser dyes: Fluorescein and Coumarin-6. These data areused for the relative fluorescence yield calculations of Table I.



620-nm absorption band and 750-nm fluorescence are ther-mally stable up to 600 °C. Details of this study will be pub-lished elsewhere.

Figure 7 compares the absorption and emission spectraof a bleached Al2O3:C,Mg crystal with the laser dyes, LDS-821 in methanol and DOTC-I in DMSO, used for the relativequantum yield measurements of the 620/750-nm band. As

before, the area below the fluorimeter emission curves andthe absorption data in Fig. 7, the 90° fluorescence intensitymeasured using the 638-nm solid-state laser setup arrangedsimilar to the fluorimeter, and the earlier publishedvalues13,14 of the quantum yield for the two laser dyes wereused in conjunction with Eq. �1� to evaluate the 620/750-nm fluorescence yield at 90° for each of these samples rela-tive to the dyes. Table II accumulates the results for threeorientations of the bleached crystal and the dyes. Again,measurements from the two different experimental setupscompare well, and the 620/750-nm fluorescence from thebleached crystal is brightest for c �k orientation.

The inner filter effect in the thick parallelepiped crystalstrongly affects its fluorescence yield. Measurements arevery sensitive to sample positioning and sample thickness.Figure 8 shows the dependence of fluorescence intensity onsample thickness for crystal samples cut along and across thec planes and for a corresponding laser dye, using the forward�6°� and backward �186°� directions of the laser setup in Fig.1. A 442-nm He–Cd laser was used for the Fluorescein dyeand the annealed samples �Fig. 8�a��, while a 638-nm laserdiode was used for the DOTC-I dye and the bleached crystals�Fig. 8�b��. The results from the forward and backward di-rections are identical. For all samples studied, the fluores-cence intensity normalized by absorption decreases with in-creasing sample thickness. The trend of these dependencesfor the crystals and the corresponding dyes is shown in Fig.8 as solid lines obtained using nonlinear least-square fittingmethods. The extrapolated fluorescence intensities to zerosample thickness from these figures are used in Tables IIIand IV to calculate the fluorescence yield of the annealed andbleached crystals, respectively, relative to the correspondinglaser dye used in forward or backward direction for the threecrystal orientations. Both the absorption coefficient at thelaser wavelength and the emission polarization ratio AI �de-fined in Sec. III D of the crystals and the dye samples foreach case are almost independent of sample thickness.

D. Anisotropy of fluorescence and emissionpolarization ratio

Figure 9 shows the 3D distribution of fluorescence inten-sity and emission polarization ratio for the 520-nm emissionband of the annealed Al2O3:C,Mg crystal that was studiedusing the He–Cd laser setup. The results were derived as

TABLE I. Yield of 520-nm fluorescence from Al2O3:C,Mg crystals measured at 90° to the excitation directionand normalized to the quantum yield of standard laser dyes: Fluorescein �Ref. �11�� and Coumarin-6 �Ref. �12��.

SampleK�cm−1�

at 442 nm

Fluorescence yield at 90°�vs Fluorescein in NaOH�

Fluorescence yield at 90°�vs Coumarin-6 in ethanol�

He–Cd laser Fluorimeter He–Cd laser Fluorimeter

Fluorescein in NaOH 1.258 0.925a 0.925a 0.930 0.905Coumarin-6 in ethanol 1.691 0.776 0.797 0.780b 0.780b

Al2O3:C,Mg �c �E� 2.884 1.028 1.057 1.033 1.034Al2O3:C,Mg �c�E� 0.967 0.416 0.453 0.418 0.443Al2O3:C,Mg �c �k� 0.902 1.351 1.369 1.358 1.340

aSee Ref. �11�.bSee Ref. �12�.

FIG. 7. The absorption and emission spectra of bleached Al2O3:C,Mg crys-tal and two standard laser dyes: LDS-821 and DOTC-I. These data are usedfor relative fluorescence yield calculations of Table II.



follows. It is evident from the schematic of the laser setup inFig. 1 that the measurements of fluorescence in three direc-tions �6°, 90°, and 186°� were always in the same horizontalplane �x -y plane in Fig. 9� with the excitation. The off-plane�vertical� fluorescence data presented in Fig. 9 �z direction�are obtained by appropriately rotating the crystal. With thevertically polarized beam illuminating one of the six faces ofthe cube, fluorescence from the cube was measured in all

three directions for three states of polarization in each direc-tion; namely, unpolarized, vertically polarized, and horizon-tally polarized. The polarization state of the laser beam ex-citation was then changed to horizontal using a half-waveplate and similar measurements were carried out. This entireset of experiments was repeated for each of the six sides ofthe cube facing the laser beam in turn. In the same session ofexperiments, the absorption coefficient for the crystal at 442nm was measured for different combinations of crystal ori-entation under vertically polarized light. The fluorescenceintensities were then normalized by the corresponding exci-tation intensities and absorption coefficients, and reduced tothree nonidentical geometries shown in Fig. 9, namely, �i�c �E, �ii� c�E, and �iii� c �k. For easier comparison, theabsorption-normalized fluorescence intensity is furtherscaled in Fig. 9 so that the value in the forward �6°� orbackward �186°� direction for each crystal orientation repre-sents the fluorescence yield in that direction relative to Fluo-rescein laser dye obtained in Table III. Thus, Fig. 9 displaysthe 3D anisotropic distribution of the fluorescence yield for520-nm unpolarized fluorescence from the crystal at threeperpendicular orientations.

As expected, illumination of opposing faces of the cuberesulted in identical anisotropic distribution of fluorescenceintensity. Additionally, the fluorescence intensity from op-posing faces was identical. For every crystal orientation, theemission from the c planes of the crystal has the smallestyield. The largest fluorescence yield from the crystal isnearly a constant ��1.352±0.022�, irrespective of the crystalorientation and/or polarization direction of the excitation. It

TABLE II. Yield of 750-nm fluorescence from Al2O3:C,Mg crystals measured at 90° to the excitation directionand normalized to the quantum yield of standard laser dyes: LDS-821 �Ref. �13�� and DOTC-I �Ref. �14��.

SampleK�cm−1�

at 638 nm

Fluorescence yield at 90°�vs LDS-821 in methanol�

Fluorescence yield at 90°�vs DOTC-I in DMSO�

Solid-statelaser Fluorimeter

Solid-statelaser Fluorimeter

LDS-821 in methanol 0.058 0.200a 0.200a 0.201 0.201DOTC-I in DMSO 0.100 0.628 0.628 0.630b 0.630b

Al2O3:C,Mg �c �E� 0.143 1.061 1.103 1.064 1.107Al2O3:C,Mg �c�E� 0.044 0.401 0.396 0.403 0.397Al2O3:C,Mg �c �k� 0.034 1.281 1.262 1.285 1.266

aSee Ref. �13�.bSee Ref. �14�.

FIG. 8. Optical path-length dependence of the forward �6°� or backward�186°� fluorescence intensity from �a� annealed and �b� bleachedAl2O3:C,Mg crystals for three crystallographic orientations and that fromthe laser dyes. The fluorescence intensity values are extrapolated to zerosample thickness for relative quantum yield calculations in Tables III andIV.

TABLE III. Quantum yield data for three orientations of annealedAl2O3:C,Mg crystals obtained from the extrapolated fluorescence intensi-ties of Fig. 8. The average quantum yield is obtained from the data presentedin Fig. 9.

Al2O3:C,Mgcrystal’s

orientation

Extrapolatednormalized

fluorescence �a.u.�under He–Cd laser

excitation

Yield relative to Fluorescein in NaOH

Forward or backwardfluorescence yield

Average quantumyield

c �E 16.780 1.373 0.839c�E 16.720 1.368 0.844c �k 7.653 0.626 0.987



is interesting to note from the cases �i� and �ii� in Fig. 9 thatdespite the fact that the crystal is about three times moreabsorbing when its c axis is oriented parallel to the electric-field vector E �Table I�, compared to that when it is orientedsuch that c�E, anisotropic distribution of fluorescence yieldfrom the crystal in both cases appears to be almost identicalto each other within the limits of experimental uncertainty. Inother words, when light propagates perpendicular to its opti-cal c axis �c�k�, the spatial distribution of the unpolarized520-nm emission from the crystal is independent of the stateof polarization of the excitation beam. The distribution offluorescence yield in space is assumed to be smooth and hasthe shape of an ellipsoid, with the relative fluorescenceyields given in Fig. 9 in x , y, and z directions being half ofthe dimension of the ellipsoid in that direction. By equatingthe volume of this fluorescence yield ellipsoid to that of asphere, the average quantum yield Q of the crystal is calcu-lated in Table III for each crystal orientation as the radius ofthat equivalent sphere. Thus, Qc�k�0.842±0.003, when thecrystal is oriented with c�k, irrespective of the polarizationstate of the excitation. When the crystal is photoexcitedalong the c axis, i.e., when c �k, the unpolarized fluorescenceellipsoid has a different shape �Fig. 9�iii��, and on an averagethe crystal fluoresces more efficiently �Qc�k�0.987�.

As in the case of absorption spectra, the emission polar-ization ratio could be defined as Al= I� / I�, where I� and I�

denote the intensities of the emitted light polarized paralleland perpendicular to the optical c axis of the crystal. Refer-ring to Fig. 9, we note that this definition does not work forthe light emitted from the c planes of the crystal. For thiscase, with the constraint of in-plane measurements, we de-fine the anisotropy ratio as �AI�c= �I��c / �I��c, with �I��c and�I��c denoting the intensities of the emitted light polarizedparallel and perpendicular to the electric-field vector E or theE plane �defined by E and the propagation vector k of theexcitation light�. As depicted in Fig. 9, the 520-nm emissionis strongly polarized in all directions �except for in the zdirection when c �E� and the distribution of the emission po-larization ratio depends on the crystal orientation. Interest-ingly enough, the anisotropic distribution of this ratio isnearly identical between two cases of crystal orientationsshown in Figs. 9�ii� and 9�iii�, for both of which the electric-field vector E is perpendicular to the c axis of the crystal, i.e.,E�c. In other words, the spatial distribution of the emissionpolarization ratio depends on the state of polarization of the

excitation beam, but is independent of its direction of propa-gation, with respect to the c axis of the crystal.

The emission polarization ratio is relatively smaller forthe fluorescence from the c planes of the crystal compared tothat from the other planes, i.e., �AI�c�AI, for all the threecases of crystal orientations shown in Fig. 9. When the ex-citation light is polarized along the c axis of the crystal�c �E�, the emission from the c planes is isotropic in polar-ization, and emission from the other two perpendicularplanes has the same anisotropic emission polarization ratio.

TABLE IV. Quantum yield data for three orientations of bleachedAl2O3:C,Mg crystals obtained from the extrapolated fluorescence intensi-ties of Fig. 8. The average quantum yield is obtained from the data presentedin Fig. 10.

Al2O3:C,Mgcrystal’s

orientation

Extrapolatednormalized

fluorescence �a.u.�under solid-statelaser excitation

Yield relative to DOTC-I in DMSO

Forward or backwardfluorescence yield

Average quantumyield

c �E 147.205 1.392 0.829c�E 146.444 1.385 0.813c �k 68.000 0.643 0.982

FIG. 9. Three-dimensional anisotropic distribution of relative fluorescenceyield and emission polarization ratio �in parentheses� for 520-nm fluores-cence from annealed Al2O3:C,Mg crystals.



Identical experimental procedures were followed to mea-sure the 3D distribution of fluorescence intensity and emis-sion polarization ratio for the 620/750-nm excitation-emission band of the bleached Al2O3:C,Mg crystal. Asmentioned earlier, the annealed cube sample of the abovestudy was bleached using the optimum energy of a 435-nmlaser beam and then annealed to 500 °C prior to using in thisstudy where the excitation source is a 638-nm solid-statelaser. The measured anisotropic distributions in Fig. 10 showmany similarities with that in Fig. 9. In particular, for eachcrystal orientation the fluorescence yield distributions are

similar in the two figures, with the smallest yield being thecharacteristic of the emission from the c planes of the crystal,and the largest yield being a constant ��1.339±0.053 in Fig.10� independent of the crystal orientation and the polariza-tion of the excitation beam. The fluorescence yield distribu-tions in Figs. 10�i� and 10�ii� are similar to each other, givingrise to their similar average quantum yield value, Qc�k

�0.821±0.008, that is smaller than the result obtained in thecase of Fig. 10�iii�, which is Qc�k�0.982 �Table IV�.

The anisotropic distribution shown in Fig. 10 of theemission polarization ratio for the 620/750-nm band is simi-lar for two crystal orientations, c�E and c �k. For each ori-entation, the emission from the c planes of the crystal exhib-its the smallest polarization ratio. These results are againvery similar to those obtained in Fig. 9 for the fluorescencefrom 435/520-nm band, although the magnitude of polariza-tion sensitivity is quite different in each direction, except forthe fact that emission from both the bands is isotropic in thez direction for the c �E crystal orientation.

IV. DISCUSSION

The structure of sapphire or �-Al2O3 crystals is a rigid,slightly distorted, hexagonal close packing of O2− sublatticewith Al3+ ions occupying two out of every three octahedralinterstices. Each O2− ion is surrounded by four tetrahedralAl3+ ions, two each as nearest- and next-nearest neighbors.Substitution of Al3+ ions with Mg2+ ions in the Al2O3 latticevia magnesium doping and crystal growth in a highly reduc-ing atmosphere introduces oxygen nonstoichiometry andstimulates production of single oxygen vacancies in the formof F and F+ color centers. The optical absorption andexcitation-emission bands characteristic to these electronicpoint defect centers are well studied in the literature andwere clearly identified in the Al2O3:C,Mg crystals.

There have been several previous experimentalstudies3,27,28 on Mg-doped sapphire, all of which were de-voted to the single oxygen vacancy F-type color centers. Theexperiments presented in this paper measure the anisotropyof absorption and luminescence from the aggregate defect, inthe form of impurity-associated dimer �F2-type� color centersin Al2O3:C,Mg crystals. To the best of our knowledge, thework of Welch et al.22 measured the polarization propertiesof absorption and luminescence from sapphire. In particular,they have studied the 358-nm absorption band and its asso-ciated 379-nm luminescence band in fast neutron- andproton-irradiated �-Al2O3 crystals. The luminescence wascollected in the so-called 90° geometry and for the propaga-tion vector perpendicular to the optical c axis of the crystal�c�k�. The eight different polarization configurations stud-ied are a subset of the 24 necessary to be measured for the3D distribution of polarization anisotropy shown in Figs. 9and 10. A simple-minded F2

+-aggregate defect center modelwas adopted in their study to successfully compare the pre-dictions with the measured absorption and luminescencedata. There are few theoretical studies29 dealing with themodeling of the aggregate color centers in Mg-doped sap-phire, primarily because of the lack of experimental resultsfor comparison. The present work is an effort to provide such

FIG. 10. Three-dimensional anisotropic distribution of relative fluorescenceyield and emission polarization ratio �in parentheses� corresponding to the620/750-nm excitation-emission band of bleached Al2O3:C,Mg crystals.



an experimental database and a call for further theoreticalresearch work in order to improve upon the understanding ofthe physics of aggregate defects in crystals.

F+ centers appear in Al2O3:C,Mg crystals for the chargecompensation of Mg2+ ions, which become negativelycharged with respect to the lattice when substituted for Al3+

ions. The aggregation of these impurity-induced vacancypairs creates color centers which were recently discovered.8

The characteristic greenish-yellow color of the as-grownAl2O3:C,Mg crystals arises from its typical broad, symmet-ric, blue absorption band centered near 435 nm. This bandwith the corresponding emission near 520 nm and a lifetimeof �=9±3 ns �Ref. �8�� is attributed to the F2

2+�2Mg� colorcenter, assumed to be a cluster of oxygen vacancy defectsformed by two F+ centers and charge compensated by twoMg-impurity atoms, as shown in the schematic model of Fig.11. The greenish-yellow color of the crystal is thermally un-stable above 700 °C,8 as opposed to the thermal stability ofthe absorption bands associated with F and F+ centers evenup to 1200 °C. This indicates that unlike F and F+ colorcenters, which are simple point defects, F2

2+�2Mg� centers arecomplex “aggregate” defects that possibly dissociate due tohigh-temperature annealing, giving rise to irreversible discol-oration of the crystal. Both absorption and emission bands ofF2

2+�2Mg� centers appear at higher energies compared to the459/563-nm excitation-emission band associated with thesimilar double vacancy F2

2+ centers created by neutron irra-diance of undoped sapphire.19

Photopumping with pulsed laser light into the 435-nmabsorption band bleaches this band and the associated colorof the crystal, along with the appearance and concomitantincrease of optical absorption bands near 335, 525, and 620nm �Fig. 2�. The 335- and 620-nm absorption bands coincidewith the excitation bands producing 750-nm emission havinga lifetime of �=80±5 ns.8 These two absorption-excitationbands have been assigned to a different charge state F2

+�2Mg�of the same aggregate center F2

2+�2Mg�. The F2+�2Mg� centers

are similar in nature to the three-electron F2+ centers observed

in undoped sapphire exposed to fast particles or ionizingradiation. The 356/380-nm excitation-emission band associ-ated with F2

+ centers was observed in Al2O3:C,Mg crystalsas well. Partial retrieval of the 435-nm absorption band andthe color of the crystal have been observed by photoexciting

the bleached crystal with pulsed 335- or 620-nm laser light.The origin of this phenomenon could be speculated as thereversible photochromic transformation of the same struc-tural defect center from one charge state to another, i.e., pho-toconversion between F2

2+�2Mg� and F2+�2Mg� color centers

as follows: in the sequential two-photon absorption of435-nm pulsed laser light, the first photon transfers one ofthe two electrons localized on the F2

2+�2Mg� center to itsexcited state which is then transferred to the conductionband, thereby photoionizing the center.

F22+�2Mg� + h�1 → F2

+�2Mg�*.

�2�F2

2+�2Mg�* + h�1 → F23+�2Mg� + e−.

The other nearby F22+�2Mg� center can capture the released

electron and is converted into a F2+�2Mg� center.

F22+�2Mg� + e− → F2

+�2Mg� . �3�

Similarly, the reverse phototransformation of the crystal us-ing a pulsed 335- or 620-nm laser light takes place accordingto the following reaction:

F2+�2Mg� + h�2 → F2

2+�2Mg� + e−. �4�

The electrons created in either of the reactions in Eq. �2� or�4� can be captured by deep traps present in the crystal or byF+ centers thus creating charge-neutral F centers.

It is expected that the absorption bands associated withdifferent color centers in sapphire structure should show dif-ferent amounts of polarization anisotropy. Because of this,the absorption spectra of the crystal and the associated po-larization anisotropy, in general, provide further insight tothe structure of the defect centers. In the classical approxi-mation, absorption and the corresponding emission of lightfrom defect centers can be viewed as a set of absorbing andemitting dipoles of definite orientation, or in quantum theory,as dipole transitions between energy states of given orienta-tional symmetry associated with that center. According toMitchell et al.,20 in the dipolar approximation the polariza-tion anisotropy ratio A for the uniaxial crystals such as sap-phire can be represented as

A = 2 cot2 � , �5�

where the transition dipole moment is inclined at an angle �from the c axis of the crystal. Equation �5� represents thepolarization anisotropy ratio A for both absorption �A�AK�and emission �A�Al� of light from defect centers. For theabsorption bands associated with the centers involving onetype of dipoles, Eq. �5� simplifies to

A � AK = 2f �/f�, �6�

where f � and f� are the oscillator strengths of the corre-sponding dipole transitions for light polarized parallel andperpendicular to the c axis, respectively.27

Figure 2 clearly shows the polarization anisotropy of dif-ferent bands in the two photochromic states of Al2O3:C,Mgcrystals. The polarization anisotropy of the F+ absorptionband at 230 nm, observed in the inset of Fig. 2, is wellknown in the literature.18,20,21 The absorption bands near 435and 335 nm show no dependence of peak position and

FIG. 11. Aggregate defect F22+ �2Mg� in Al2O3:C,Mg crystal structure,

consisting of two oxygen vacancies charge compensated by two Mg2+ ions.



FWHM on crystal orientation relative to vector E of the ex-citing light, but their peak heights, i.e., the measured absorp-tion coefficients �K� , K�, and Kc�, show rather strong aniso-tropy �Fig. 2�. When plane-polarized light propagatesperpendicular to the optical c axis �c�k�, rotation of thecrystal around the vector k direction causes strong changes inthe coefficient of absorption for the crystal.8 For the 435-nmband in the annealed crystal, it varies from K� =3.00 cm−1

when the c axis is parallel to the electric-field vector down toK�=1.01 cm−1 when the c axis is perpendicular to the Evector. Thus, the observed optical anisotropy of the 435-nmband is AK=K� /K�=2.97. When plane-polarized light propa-gates parallel to the c axis of the crystal, rotation of thesample does not change the absorption coefficient apprecia-bly, with its value Kc=0.94 cm−1 being slightly lower thanK�.

As mentioned earlier, in order to remove any ambiguityinvolved in the measurement of absorption and emissionfrom the 620/750-nm excitation-emission band, the bleachedcrystals were obtained from the freshly annealed samplesusing optimum energy bleaching followed by 500 °C anneal-ing treatment. The absorption coefficients at different orien-tations associated with the 620-nm band for such a crystalshow strong anisotropic behavior, which is very similar tothat seen for the 435-nm absorption band. The measured co-efficients for the 620-nm band are K� =0.1701 cm−1 ,K�

=0.0524 cm−1, and Kc=0.0405 cm−1, so the anisotropy isAK=3.25.

According to Eq. �5�, the observed anisotropy AK=2.97�or 3.25� for the 435-nm �or 620-nm� absorption band speci-fies that the line joining the vacancy pair, i.e., the transitiondipole moment in F2

2+�2Mg� �or F2+�2Mg�� centers, is aligned

at �=39.4° �or 38.1°� to the c axis of the crystal, which isclose to the inclination of the line connecting two oxygenatoms in an ideal lattice of Al2O3. It has been shownpreviously21,22 that an anisotropy ratio near three is charac-teristic of absorption bands associated with oxygen vacancypairs, i.e., F2-type centers in sapphire. Thus the measuredanisotropy data support the assignment of the same F2-typeaggregate defect structure to both 435- and 620-nm absorp-tion bands. The ratios of the corresponding oscillatorstrengths f � / f�=1.5 and 1.6, respectively. Figures 9 and 10show that for both of these bands, AI�AK, indicating that inboth cases the absorbing and emitting dipoles have differentorientations, i.e., the excited state looses the orientationalsymmetry during the relaxation process.

It is clear from Fig. 3 that emission centered near 750nm can also be excited with 258-nm light. The apparent dif-ference in the FWHM of this emission from the ones that canbe excited with a 335- or 620-nm light, and the overlap of its258-nm excitation band with that associated with F2 band�not shown� having a 258-nm excitation–322-nm emissionband, calls for further research to identify the origin of the258/750-nm band. Similarly the origin of the 525-nmabsorption-excitation band and its corresponding 767-nmemission band is unclear at present.

The fluorescence intensity and emission polarization ra-tio, associated with both F2

2+�2Mg� and F2+�2Mg� centers,

show anisotropic 3D distribution in space. These distribu-

tions depend on the direction of propagation �given by thepropagation vector k� and polarization state of the excitationbeam with respect to the crystallographic orientation. Thusthe yield of the corresponding unpolarized fluorescence,measured relative to standard organic laser dyes, also showsthe same anisotropic dependence on the direction of mea-surement. Results in Figs. 9 and 10 indicate that the spatialdistribution of the fluorescence yield from the crystal de-pends on the direction of propagation of the excitation beamwith respect to the c axis of the crystal, for both annealed andbleached states. For a given direction of propagation, thisdistribution is independent of the polarization state of theexcitation light. On the contrary, the spatial distribution ofthe emission polarization ratio depends on the excitation po-larization state, and for a given polarization it is independentof the direction of propagation of the excitation beam withrespect to the c axis of the crystal. It is worth noting that themeasured 3D fluorescence yield distributions are very similarfor both 435/520- and 620/750-nm excitation-emissionbands of the crystal, indicating that the same structural defectcenter at two different charge states is responsible for theirorigin. This confirms the inference drawn from the absorp-tion anisotropy data as well, thereby supporting the sug-gested model of double oxygen vacancy aggregate defectstructure.

The average quantum yield Q of the strongly anisotropicfluorescence from Al2O3:C,Mg crystal was calculated as theradius of an equivalent sphere equal in volume to that of anellipsoid with the fluorescence yield numbers obtained forthree different directions as half-axis in that direction. Forthe 520-nm fluorescence it is close to unity and depends onthe orientation of the propagation vector k of the excitationbeam relative to the optical c axis of the crystal: Qc�k

�0.842±0.003 and Qc�k�0.987. A very similar fluorescenceintensity distribution and the average quantum yield wereobtained for the 750-nm fluorescence: Qc�k�0.821±0.008and Qc�k�0.982. The physics behind these orientation de-pendences remains to be understood. The high quantum yieldof fluorescence from Al2O3:C,Mg crystals will have a directimpact, for example, on its suggested applications in volu-metric optical data storage,8 which utilizes a technique ofsequential two-photon absorption for bitwise recording andfluorescent confocal readout.

V. CONCLUSIONS

Optical-absorption and luminescence properties of theAl2O3:C,Mg single crystals were investigated. A model forthe Mg-impurity-associated aggregate oxygen vacancy de-fects was suggested. Anisotropic behavior in optical absorp-tion and fluorescence associated with the 435/520- and620/750-nm excitation-emission bands, characteristic of twodifferent photochromic states of the crystal, supports the as-signment of the same F2-type aggregate defect in two differ-ent charge states to these bands. The measured 3D spatialdistributions of fluorescence intensity and the emission po-larization ratio of these color centers show strong depen-dences on the direction of the propagation and the polariza-tion state of the excitation beam relative to the



crystallographic orientation. Quantum yield of fluorescencewas determined in comparison with standard laser dyes byaveraging over the anisotropic fluorescence intensity distri-bution in space for both photochromic states of the crystaland was found to be close to unity. The high quantum yieldof fluorescence is advantageous for the potential use of thisrecently discovered material for various luminescence appli-cations, for example, in volumetric optical data storage andradiation field imaging.

ACKNOWLEDGMENTS

The authors gratefully acknowledge Dr. Anna Akselrodand Dr. Sergei S. Orlov for stimulating discussions and Pro-fessor Douglas Magde for providing us with the Fluoresceindye sample. Thanks are also due to Thomas H. Underwoodfor the growth and preparation of the crystalline samplesused in this study.

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Anisotropy of optical absorption and fluorescence in Al[sub 2]O[sub 3]:C,Mg crystals