Spectroscopy and near-infrared to visible upconversion of er3+ ions in aluminosilicate glasses manufactured with controlled optical transmission

applied sciences A icle Spec oscopy and Nea -In a ed o Visible Upcon e sion o E 3+ Ions in Aluminosilica e Glasses Manu ac u ed wi h Con olled Op ical T ansmission Daniel Sola 1,2,* , Ad ián Miguel 3, Edua do A ias-Egido 2and Jose I. Peña 2   Ci a ion: Sola, D.; Miguel, A.; A ias-Egido, E.; Peña, J.I. Spec oscopy and Nea -In a ed o Visible Upcon e sion o E 3+ Ions in Aluminosilica e Glasses Manu ac u ed wi h Con olled Op ical T ansmission. Appl. Sci. 2021, 11, 1137. h ps://doi.o g/10.3390/ app11031137 Recei ed: 23 Decembe 2020 Accep ed: 22 Janua y 2021 Published: 26 Janua y 2021 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2021 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). 1Labo a o io de Óp ica, Cen o de In es igación en Óp ica y Nano ísica, Campus Espina do, Uni e sidad de Mu cia, 30100 Mu cia, Spain 2Ins i u o de Nanociencia y Ma e iales de A agón, Uni e sidad de Za agoza-CSIC, 50018 Za agoza, Spain; ea ias@uniza .es (E.A.-E.); jipena@uniza .es (J.I.P.) 3 R&D Depa men , Ice Rail (Kno -B emse G oup), 31013 Pamplona, Spain; ad ian.miguel@kno -b emse.com *Co espondence: [email p o ec ed] Abs ac : In his wo k we epo on he spec oscopic p ope ies and he nea -in a ed o isible upcon e sion o E 3+ ions in aluminosilica e glasses manu ac u ed by di ec ionally solidi ica ion wi h he lase loa ing zone echnique. Glasses we e manu ac u ed in a con olled oxidizing a mosphe e o p o ide hem wi h high op ical ansmission in he isible spec al ange. Abso p ion and emission spec a, and li e imes we e assessed in bo h he isible and he nea in a ed spec al ange. G een upcon e sion emissions o he 2 H 11/2→4 I 15/2 and 4 S 3/2→4 I 15/2 ansi ions a 525 nm and 550 nm a ibu ed o a wo-pho on p ocess we e obse ed unde exci a ion a 800 nm. Mechanisms esponsible o he upcon e sion luminescence we e discussed in e ms o exci ed s a e abso p ion and ene gy ans e upcon e sion p ocesses. Exci a ion spec a o he upcon e ed emission sugges ha ene gy ans e upcon e sion p ocesses a e esponsible o he g een upcon e sion luminescence. Keywo ds: op ical p ope ies; e bium; upcon e sion; aluminosilica e glasses; lase loa ing zone 1. In oduc ion In ecen yea s a e-ea h-doped glasses ha e been subjec o in ense esea ch as hos ma e ials because o hei signi ican op ical p ope ies, which make hem adequa e as in a ed and upcon e sion lase s, op ical ampli ie s, and ac i e pho onic de ices [ 1 – 9 ]. In pa icula , silica e and aluminosilica e glasses p esen excellen he mal and mechanical p ope ies, and co osion esis ance o be used in p ac ical applica ions [ 10 – 17 ]. In addi ion, hei maximum phonon ene gy (~1050 cm −1 ) is much lowe han ha o phospha e and bo a e glasses, ~1300 cm −1 and ~1350 cm −1 , espec i ely, so ha quan um e iciency is less in luenced by mul iphonon elaxa ion p ocesses [16–19]. E 3+ ions a e among he mos in e es ing ac i e cen e s o be s udied because o i s po en ial applica ions in he ield o in a ed op ical ampli ica ion ela ed o he adia i e e iciency o he 4 I 13/2→4 I 15/2 emission a a ound 1.55 µ m [ 1 , 2 , 20 ]. Fu he mo e, he ich ene gy le el s uc u e o his a e-ea h allows he exci a ion o he 2 H 11/2→4 I 15/2 , 4 S 3/2→4 I 15/2 , and 4 F 9/2→4 I 15/2 upcon e sion emission bands cen e ed a a ound 530, 550, and 665 nm using wa eleng hs in he nea in a ed spec al egions [ 1 , 2 , 21 , 22 ]. NIR- o- isible ene gy con e sion mechanism in ol es he con e sion o low-exci a ion-ene gy pho ons in o high-ene gy emi ed ligh in he isible ange h ough non-linea an i-S okes p ocesses. In addi ion, hese ions can also be used as local o de ing p obe because o he close ela ion be ween hei spec oscopic p ope ies and he local s uc u e and bonding a he ion si e [23–27]. In a p e ious wo k, we epo ed on how o con ol he op ical ansmission o alumi- nosilica e glasses manu ac u ed depa ing om comme cial glass-ce amics by means o he Lase Floa ing Zone (LFZ) echnique [ 28 ]. This echnique u ilizes an in a ed lase sou ce Appl. Sci. 2021,11, 1137. h ps://doi.o g/10.3390/app11031137 h ps://www.mdpi.com/jou nal/applsci Appl. Sci. 2021,11, 1137 2 o 10 o c ea e a mol en zone in he ma e ial om which, by con olling he solidi ica ion a es, a new ma e ial wi h con olled mic os uc u e is p oduced. Su ounding medium du ing he ab ica ion p ocess, in e ms o oxidizing o educing a mosphe e, can also be con olled. We epo ed ha when he ab ica ion o hese aluminosilica e glasses ook place in an oxidizing a mosphe e, Ti 3+ cen e s con ained in he glass u ned in o Ti 4+ ions, gi ing ise o colo less glasses, he ansmi ance o which anged 80% in he isible spec al ange. The e o e, i was possible o ailo he esul ing op ical ansmission. In his wo k we ha e aken ad an age o his ea u e o ab ica e e bium-doped glasses in oxidizing a mosphe e o gi e ise o op ical ac i e glasses o high op ical ansmission. Spec oscopic and NIR- o- isible upcon e sion p ope ies ha e been s udied and he possible exci a ion mechanisms esponsible o his upcon e sion luminescence we e discussed. 2. Ma e ials and Me hods 2.1. Sample Fab ica ion Glass-ce amic powde was ob ained om a comme cial glass-ce amic, Ce an Sup ema ® , manu ac u ed by Scho . Nex , 1 w % and 4 w % o E 2 O 3 , ul a-pu e 99.99% (Sigma- Ald ich, S . Louis, MO, USA) we e mixed and isos a ically p essed a 200 MPa o 3 min and sin e ed a 1200 ◦ C o 12 h o ob ain he p ecu so ods. Glass samples we e ob ained depa ing om hese e bium-doped glass-ce amic p ecu so s by means o he lase loa ing zone (LFZ) echnique. This manu ac u ing echnique has been desc ibed elsewhe e [ 29 – 31 ]. Glass samples we e ob ained a a g ow h a e o 300 mm/h, which p o ided bo h high axial and adial cooling g adien s o manu ac u e glass samples. In addi ion, he manu- ac u ing p ocess was ca ied ou in a semi-sealed chambe , which allowed wo king in di e en a mosphe es such as oxygen, ni ogen, and ai . Speci ically, E -doped glasses we e ab ica ed in oxygen a mosphe e o ob ain samples wi h high op ical ansmission. Taking in o accoun he heo e ical weigh pe cen age he samples we e doped wi h, om now on hey will be named as E 1 and E 4. 2.2. Cha ac e iza ion Techniques Composi ion o glasses was de e mined by means o ield emission scanning elec on mic oscopy mic oscope (FESEM) wi h ene gy dispe si e X- ay de ec o (EDX) (Ca l Zeiss, Jena, Ge many). EDX echnique allows he de e mina ion o mos elemen s p esen in concen a ion abo e 0.1% wi h an es ima ed accu acy o ±5%. Abso p ion spec a we e eco ded wi h a Ca y 5 spec opho ome e . S eady-s a e emission we e ob ained by exci ing he sample wi h an a gon lase and a Ti-sapphi e ing lase (0.4 cm −1 linewid h) in he 770–920 nm spec al ange. The luo escence was analyzed wi h a 0.25 m Jobin-Ybon monoch oma o (Ho iba, Kyo o, Japan), and he signal was de ec ed by a Hamama su R928 pho omul iplie and inally ampli ied by a s anda d lock-in echnique. In a ed emission a 1.5 µ m was de ec ed wi h an ex ended IR Hamama su R5509-72 pho omul iplie (Hamama su, Hamama su-ci y, Japan). Li e ime measu emen s we e pe o med by exci ing he samples wi h a dye lase pumped by a pulsed ni ogen lase and a Ti-sapphi e lase , pumped by a pulsed equency doubled Nd:YAG lase (9 ns pulsewid h) (Cohe en , San a Cla a, USA), and de ec ing he emission wi h Hamama su R928 and R5509-72 pho omul iplie s (Hamama su, Hamama su- ci y, Japan). Da a we e p ocessed by a Tek onix MDO3104 oscilloscope (Tek onix-Inc, Bea e on, OR, USA). 3. Resul s 3.1. Composi ional Cha ac e iza ion The composi ion o he E -doped glass samples a e he ab ica ion p ocess in an oxygen a mosphe e was ca ied ou by EDX mic oanalysis. Table 1shows he composi ion o bo h he samples. I can be obse ed ha SiO 2 and Al 2 O 3 we e he majo i y componen s o he samples, which also included low pe cen ages o NaO, MgO, TiO 2 , Z O 2 , and E 2 O 3 . The con en o E 3+ ions in bo h glasses was calcula ed accoun ing he measu ed con en Appl. Sci. 2021,11, 1137 3 o 10 o E 2 O 3 and he densi y o bo h glasses, 2.35 g/cm 3 and 2.36 g/cm 3 o E 1 and E 4, espec i ely, esul ing in 7.92 ×1019 a /cm3 o E 1 and 2.71 ×1020 a /cm3 o E 4. Table 1. Composi ional analysis in a % o he E -doped samples manu ac u ed in an oxygen a mosphe e. Na Mg Al Si Ti Z E E 1 0.70 1.81 28.11 65.99 2.00 0.98 0.41 E 4 0.86 1.75 28.12 65.11 1.84 0.89 1.43 3.2. Abso p ion and Emission P ope ies The oom empe a u e abso p ion spec a we e ob ained o bo h samples in he 300–1700 nm ange. As an example, Figu e 1shows he abso p ion spec a as a unc ion o he wa eleng h o he sample doped wi h a 4 w % o E 2 O 3 . The spec um consis s o 10 abso p ion bands co esponding o he ansi ion om he 4 I 15/2 g ound s a e o he 4 G 11/2 , 2 H 9/2 , 4 F 3/2,5/2 , 4 F 7/2 , 2 H 11/2 , 4 S 3/2 , 4 F 9/2 , 4 I 9/2 , 4 I 11/2 , and 4 I 13/2 o E 3+ exci ed s a es ions [1]. Figu e 1. Room empe a u e abso p ion spec um o E 3+ in he aluminosilica e glass doped wi h 4 w % E 2O3. Visible emission spec a we e ob ained a oom empe a u e unde exci a ion o he 4 F 7/2 le el a 488 nm. Mul iphonon elaxa ion p ocesses popula ed he lowe le els esul - ing in he emission bands obse ed a a ound 530, 548, and 660 nm which co esponded o ansi ions om he 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 le els o he g ound s a e. Figu e 2shows he emission spec a o bo h glasses. The main emission co esponded o he ( 2 H 11/2 , 4 S 3/2 ) →4 I 15/2 ansi ion. A weak ed emission was also obse ed om he 4 F 9/2 le el. This le el was popula ed h ough mul iphonon elaxa ion p ocesses om he 4S3/2 le el. The expe imen al decays o he luminescence om 4 S 3/2 and 4 F 9/2 le els we e ob- ained a oom empe a u e o bo h glasses unde exci a ion a 488 nm. Table 2shows he alues ob ained by a i o a single exponen ial unc ion. I can be obse ed ha li e imes o hese le els we e ound o be simila o bo h glasses, wi h alues sligh ly highe o he glass doped wi h a 1 w % o E 2 O 3 . As an example, Figu e 3shows he expe imen al decays om he 4 S 3/2 and 4 F 9/2 le els unde exci a ion a 488 nm o he sample doped wi h 1 w %. Li e ime was also measu ed a oom empe a u e o he 4 I 13/2 le el unde exci a ion a 800 nm co esponding o he le el 4 I 9/2 . I was also ound ha li e ime was sligh ly highe o he glass doped wi h a 1 w % o E 2O3. The decay om he 4I13/2 le el was ound o beha e like a pe ec single exponen ial, whe eas decays om 4 S 3/2 and 4 F 9/2 exci ed le els sligh ly de ia ed om a pe ec exponen ial beha io . The obse ed li e imes we e in he same o de o magni ude han hose epo ed o o he silica e and aluminosilica e glasses [10,11,32]. Appl. Sci. 2021,11, 1137 4 o 10 Figu e 2. Room empe a u e emission spec a o he E -doped aluminosilica e glasses wi h 1 w % and 4 w % unde exci a ion a 488 nm. Table 2. Li e imes a oom empe a u e o he 4 S 3/2 and 4 F 9/2 le els ob ained unde exci a ion a 488 nm and 4I13/2 le els ob ained unde exci a ion a 800 nm. 548 nm (4S3/2) (λexc = 488 nm) 660 nm (4F9/2) (λexc = 488 nm) 1528 nm (4I13/2) (λexc = 800 nm) E 1 3.60 ±0.11 µs 3.06 ±0.09 µs 4.91 ±0.12 ms E 4 3.20 ±0.07 µs 3.17 ±0.03 µs 3.01 ±0.06 ms Figu e 3. Expe imen al decays o he 4 S 3/2 and 4 F 9/2 le els o he E -doped aluminosilica e glasses wi h 1 w % unde exci a ion a 488 nm. The luo escence spec a a oom empe a u e co esponding o he 4 I 13/2→4 I 15/2 ansi ion we e measu ed by exci ing he samples a 802 nm. As shown in Figu e 4bo h samples p esen ed a maximum a a ound 1528 nm. E ec i e bandwid h ( ∆λe ) was measu ed acco ding o ∆λe =ZI(λ)dλ Imax , (1) whe e I( λ )is he in ensi y o he emission spec um as a unc ion o he wa eleng h and I max is he peak in ensi y. I was ound ha he e ec i e bandwid h inc eased om 62.5 nm o he sample doped wi h 1 w % o 66.3 nm o he sample doped wi h 4 w % o E 2 O 3 . These alues a e la ge han hose ob ained o o he silica e and phospha e glasses, he bandwid h o which anges om 30–40 nm o silica e and 46 nm o phospha e glasses, espec i ely [ 33 ]. This ea u e is highly signi ican since b oadband ampli ie s and unable lase s equi e la ge bandwid h. Appl. Sci. 2021,11, 1137 5 o 10 Figu e 4. Emission spec a o he 4 I 13/2→4 I 15/2 ansi ion o E -doped aluminosilica e glasses wi h 1 w % and 4 w % unde exci a ion a 802 nm. In addi ion o he bandwid h, he s imula ed emission c oss-sec ion, σem , is ano he impo an pa ame e ha p o ides in o ma ion abou he op ical ampli ica ion. This pa- ame e was es ima ed om he abso p ion spec a by using he McCumbe app oach [ 34 ], in which he abso p ion and emission c oss-sec ion a e ela ed acco ding o σem(υ)=σabs(υ)expε−hυ KT , (2) whe e σem and σabs a e he s imula ed emission and abso p ion c oss-sec ion, espec i ely, υ is he pho on equency, his he Planck cons an , Kis he Bol zmann cons an , and ε is he ne ee ene gy equi ed o exci e one E 3+ ion om s a es 4 I 15/2 o 4 I 13/2 a empe a u e T. The abso p ion c oss-sec ion was expe imen ally ob ained and ε was de e mined by using he simpli ied p ocedu e p o ided by Miniscalco [ 35 ]. The maximum emission c oss-sec ions we e ound o be 3.51 × 10 −21 cm 2 and 4.19 × 10 −21 cm 2 a 1530 nm o he samples doped wi h 1 w % and 4 w % o E 2 O 3 , espec i ely. These alues we e simila o hose ound in o he aluminosilica e glasses [ 32 ]. As an example, Figu e 5shows he abso p ion and emission c oss-sec ion o he sample doped wi h 4 w % o E 2O3. Figu e 5. Abso p ion and emission c oss-sec ion o he aluminosilica e glass doped wi h 4 w % o E 2O3. Appl. Sci. 2021,11, 1137 6 o 10 3.3. In a ed o Visible Upcon e sion Visible upcon e sion a oom empe a u e was obse ed in bo h samples unde con inuous lase exci a ion in esonance wi h he 4 I 9/2 le el, as shown in Figu e 6. The obse ed g een emissions co espond o 2 H 11/2→4 I 15/2 and 4 S 3/2→4 I 15/2 ansi ions o E 3+ ions, which we e loca ed app oxima ely a 525 nm and 550 nm, espec i ely. Ne e heless, ed emission co esponding o he 4 F 9/2→4 I 15/2 ansi ion was no obse ed in hese glasses. This is due o he ac ha he ene gy gap be ween le els 4 F 9/2 and 4 I 9/2 is 2264 cm −1 and he maximum phonon ene gy is 1000 cm−1app oxima ely [32]. Figu e 6. Upcon e sion emissions a oom empe a u e o E 3+ ions in aluminosilica e glasses unde exci a ion a 800 nm. Exci ed s a e abso p ion (ESA) and ene gy ans e upcon e sion (ETU) a e he main p ocesses associa ed o he upcon e sion emission o a e-ea h ions [ 36 ]. Exci a ion mech- anisms o popula ing he 2 H 11/2 and 4 S 3/2 le els unde NIR exci a ion we e in es iga ed by s udying he upcon e sion emission in ensi y I up as a unc ion o he in a ed exci a ion powe I IR . I is well-known ha upcon e sion emission in ensi y inc eases p opo ionally o he n h powe o he in a ed exci a ion powe acco ding o he ela ion I up k ∝ (I IR ) n , whe e nis he numbe o pho ons in ol ed in he pumping mechanism. Figu e 7shows he loga i hmic plo o he g een upcon e sion in ensi y I up compa ed o he exci a ion powe I IR unde exci a ion a 800 nm o bo h glass samples. Linea i allowed de e mining he n- alues, esul ing in 1.47 and 1.52 o E 1 and E 4, espec i ely. Slope alues below wo indica es a sa u a ion o he in e media e le els [ 37 ]. These esul s con i m ha a wo-pho on s ep was in ol ed in he upcon e sion p ocess o popula e he emi ing le els. Figu e 7. Dependence o g een upcon e sion emission in ensi y on exci a ion powe o E 3+ ions unde exci a ion a 800 nm. Appl. Sci. 2021,11, 1137 7 o 10 Possible mechanisms accoun ing o g een upcon e sion emission unde 800 nm exci a ion is p esen ed in Figu e 8. G een upcon e ed emission equi es he popula ion o le els wi h a leas he ene gy o he 2 H 11/2 le el o highe . The 4 I 9/2 le el is esonan ly exci ed by he pumping wa eleng h a 800 nm. Nex , he he malized le els 2 H 11/2 and 4 S 3/2 can be popula ed by means o wo ESA p ocesses o wo ETU. On he i s ESA, labelled as ESA1, a non- adia i e elaxa ion om he 4 I 9/2 le el o he lowe 4 I 11/2 le el is p oduced, om which he abso p ion o one pho on popula es he 4 F 3/2,5/2 le el, ollowed by a non- adia i e de-exci a ion o he 2 H 11/2 and 4 S 3/2 le els. On he second ESA, labelled as ESA2, an addi ional non- adia i e elaxa ion om he 4 I 11/2 le el o he 4 I 13/2 le el is p oduced. Then, he abso p ion o one pho on p omo es he E 3+ ions o he 2 H 11/2 and 4 S 3/2 le els. These wo p ocesses in ol e only one E 3+ ion. Ne e heless, wo upcon e sion mechanisms in ol ing he in e ac ion o wo nea by E 3+ ions in he 4 I 11/2 le el and en ailing an ene gy ans e a e possible. In he i s ETU, deno ed as I, he mechanism can be desc ibed as E 3+ ( 4 I 11/2 ) + E 3+ ( 4 I 11/2 ) → E 3+ ( 4 I 15/2 ) + E 3+ ( 4 F 7/2 ), ollowed by a non- adia i e de- exci a ion o he 2 H 11/2 and 4 S 3/2 le els. The second ETU, deno ed as II, can be desc ibed as E 3+(4I11/2) + E 3+(4I13/2)→E 3+(4I15/2) + E 3+(2H11/2,4S3/2). Figu e 8. Ene gy le el o E 3+ ions in aluminosilica e glass and possible upcon e sion mechanisms unde exci a ion a 800 nm. A me hod o dis inguish be ween ESA and ETU mechanisms is p o ided by he exci a ion spec a o he upcon e ed luminescence [ 38 ]. In ESA p ocesses upcon e sion exci a ion spec a a e he esul o exci ed s a e abso p ion and he one pho on abso p ion. In ETU, exci a ion spec a a e p opo ional o he squa e o he g ound s a e abso p ion. Hence, exci a ion spec a o he upcon e sion g een emission we e pe o med in he 4 S 3/2→4 I 15/2 ansi ion a 550 nm in bo h E -doped aluminosilica e glasses. Figu e 9shows hese spec a o he sample doped wi h a 4 w %. I can be obse ed ha bo h exci a ion and abso p ion spec a a e simila . This beha io was also obse ed o he sample doped wi h a 1 w %. Consequen ly, upcon e sion mechanisms we e due o ene gy ans e upcon e sion p ocesses. Appl. Sci. 2021,11, 1137 8 o 10 Figu e 9. Exci a ion spec a o upcon e ed g een emission om he 4 S 3/2 le el and squa e o he one pho on abso p ion spec a o he aluminosilica e glass doped wi h 4 w % E 2O3. 4. Conclusions Di ec ionally solidi ied E -doped aluminosilica e glasses we e manu ac u ed by he lase loa ing zone in a con olled oxidizing a mosphe e o p o ide hem wi h a high op ical ansmission in he isible spec al ange. The in a ed and isible emissions we e assessed a oom empe a u e. In a ed emission co esponding o he 4 I 13/2→4 I 15/2 ansi ion a 1528 nm p esen ed an e ec i e bandwid h nea ly 30 nm b oade han o he silica e glasses, which makes hem sui able o b oadband ampli ie s. The isible emission was domina ed by he g een emission co esponding o he 2 H 11/2 and 4 S 3/2 le els. Li e imes om le els 4 S 3/2 and 4 F 9/2 a oom empe a u e unde exci a ion a 488 nm we e ound o be simila o bo h glasses, whe eas li e ime om he 4 I 13/2 le el was ound o be sho e o he glass wi h he highe con en o E 3+ ions. NIR- o- isible upcon e sion o E 3+ ions in hese glasses unde exci a ion a 800 nm p esen ed in ense g een emissions co esponding o 2 H 11/2→4 I 15/2 and 4 S 3/2→4 I 15/2 an- si ions placed a 525 nm and 550 nm which we e a ibu ed o a wo-pho on p ocess. Ne e heless, ed upcon e sion emission o he 4 F 9/2→4 I 15/2 ansi ion was no obse ed due o he high maximum phonon ene gy. Exci a ion spec a o he upcon e ed lumines- cence om he 4 S 3/2 le el sugges s he ene gy ans e upcon e sion as he mechanism esponsible o he upcon e sion p ocess in hese glasses. Au ho Con ibu ions: D.S. concei ed and planned he expe imen s. D.S., A.M., and E.A.-E. ca ied ou he expe imen s. D.S., A.M. and E.A.-E., J.I.P. con ibu ed o he in e p e a ion o he esul s and p o ided c i ical eedback. D.S. w o e he pape wi h inpu om all au ho s. All au ho s ha e ead and ag eed o he published e sion o he manusc ip . Funding: This esea ch was unded by he PIT2 p og am o he Uni e si y o Mu cia’s own esea ch plan. Fundación Séneca g an No 20647/JLI/18 and Eu opean Union’s Ho izon 2020 esea ch and inno a ion p og amme unde he Ma ie Skłodowska-Cu ie IF No 795630 a e also acknowledged. Ins i u ional Re iew Boa d S a emen : No applicable. In o med Consen S a emen : No applicable. Appl. 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