Cooperative upconversion as the gain-limiting factor in Er doped miniature Al 2 O 3 optical waveguide amplifiers

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 9 1 MAY 2003 Cooperative upconverion a the gain-limiting factor in Er doped miniature Al 2 O 3 optical waveguide amplifier P. G. Kik a) and A. Polman F.O.M. Intitute for Atomic and Molecular Phyic, P.O. Box 41883, 1009 DB Amterdam, The Netherland Received 27 November 2002; accepted 12 February 2003 Erbium doped Al 2 O 3 waveguide amplifier were fabricated uing two different doping method, namely Er ion implantation into putter depoited Al 2 O 3, and co-puttering from an Er 2 O 3 /Al 2 O 3 target. Although the Er concentration in both material i almot identical 0.28 and 0.31 at. %, the amplifier how a completely different behavior. Upon pumping with 1.48 m, the co-puttered waveguide how a trong green luminecence from the 4 S 3/2 level, indicating efficient cooperative upconverion in thi material. Thi i confirmed by pump power dependent meaurement of the optical tranmiion at 1.53 m and the pontaneou emiion at 1.53 and 0.98 m. All meaurement can be accurately modeled uing a et of rate equation that include firt order and econd order cooperative upconverion. The firt order cooperative upconverion coefficient C 24 i found to be cm 3 1 in the co-puttered material, two order of magnitude higher than the value obtained in Er implanted Al 2 O 3 of cm 3 1. It i concluded that the co-puttering proce reult in a trongly inhomogeneou atomic cale patial ditribution of the Er ion. A a reult, the co-puttered waveguide do not how optical gain, while the implanted waveguide do American Intitute of Phyic. DOI: / I. INTRODUCTION Erbium doped waveguide amplifier can be ued in optical integrated circuit to compenate for ignal loe by aborption, cattering, plitting, coupling, etc. Such waveguide amplifier ue timulated emiion from the firt excited tate ( 4 I 13/2 ) to the ground tate ( 4 I 15/2 )ofer 3 at 1.53 m ee Fig. 1 a. The intra-4 f tranition i parity forbidden, and a a reult the emiion cro ection i quite mall ( cm 2 ). Conequently, high Er concentration at. % are required to achieve ignificant amplification in waveguide tructure on a cm length cale. At thee high Er concentration the cloely paced Er 3 ion can interact. Thi caue everal effect, uch a energy migration, 1,2 in which an excited Er ion excite a neighboring unexcited Er ion which may in turn be coupled to a nonradiative quenching ite, and cooperative upconverion, 3 8 in which an excited Er ion promote a neighboring excited Er ion into a higher lying tate. Both procee are detrimental to the optical gain, a they reduce the population in the firt excited tate. We compare the optical gain characteritic of two different optical waveguide amplifier on ilicon that have been fabricated either by Er 2 O 3 /Al 2 O 3 co-puttering or by Er ion implantation into Al 2 O 3. We will how that although the Er concentration in the two amplifier are almot identical, the gain characteritic are entirely different, which i attributed to a difference in the atomic cale patial ditribution of the Er ion. a Author to whom correpondence hould be addreed: electronic mail: kik@caltech.edu II. EXPERIMENT One waveguide tructure wa fabricated uing radiofrequency Ar puttering of a powder-preed Er 2 O 3 /Al 2 O 3 putter target onto an oxidized Si ubtrate (SiO 2 thickne 5 m. The target contained 2.8 wt. % Er 2 O 3, correponding to 0.31 at. % Er. The compoition of the depoited film wa meaured by Rutherford backcattering pectrometry RBS uing 2 MeV He ion at a cattering angle of 165 Fig. 2. The film thickne and the ample compoition were determined from a fit to the data olid line. The peak oberved at channel 170 i related to the appearance of count due to ilicon atom in the SiO 2 film beneath the Al 2 O 3. The film thickne i found to be 570 nm, auming an atomic denity of cm 3. The Er concentration i contant over the full film thickne, and amount to 0.31 at. %. The Al concentration i found to be 38 at. %, cloe to the toichiometric compoition of Al 2 O 3. The film alo contain a mall amount of Ar 0.25 at. % which i incorporated during the puttering proce. Thi material wa annealed at 900 C for 1 h in vacuum at a bae preure of mbar to optimize the Er luminecence intenity and lifetime. 9 Ridge waveguide were formed in a 520-nm-thick layer uing tandard photolithography and Ar beam etching to a depth of 260 nm. The etch mak contained traight waveguide of 1 cm length, a well a a waveguide piral geometry that confine a 3-cm-long waveguide on an area of 1mm 2. Subequently, a 1- m-thick SiO 2 top cladding wa depoited to reduce cattering loe. Figure 3 how a canning electron microcopy image of a 2- m-wide Al 2 O 3 ridge waveguide, after removal of the SiO 2 cladding layer. Before waveguide fabrication the Er photoluminecence PL lifetime at m in annealed co-puttered film wa found /2003/93(9)/5008/5/$ American Intitute of Phyic

2 J. Appl. Phy., Vol. 93, No. 9, 1 May 2003 P. G. Kik and A. Polman 5009 FIG. 1. a Schematic repreentation of the Stark-plit Er 3 energy level, howing excitation at 1.48 m and emiion at 1.53 m. The energy level are indexed uing Ruell Saunder notation, and the correponding population N i are indicated. The emiion wavelength due to radiative relaxation to the ground tate i alo included. b Cooperative upconverion from the Er 3 firt excited tate, bringing one ion in the 4 I 9/2 tate. Thi i followed by rapid relaxation to the 4 I 11/2 level, cauing luminecence at 0.98 m. c Cooperative upconverion from the Er 3 econd excited tate, bringing one ion in the 2 H 11/2 tate that luminece in the green. FIG. 2. Rutherford backcattering pectrometry meaurement at a cattering angle of 165 of a 570-nm-thick Er doped Al 2 O 3 filmonsio 2 made by Er 2 O 3 /Al 2 O 3 co-puttering. The urface channel of the variou element are indicated. The olid line repreent a fit auming a contant Er concentration of 0.31 at. % over the full film thickne. The peak around channel 170 i the reult of overlapping ignal from aluminum in the top Al 2 O 3 layer and ilicon in the lower SiO 2 layer. FIG. 3. Scanning electron microcopy image of an Er doped Al 2 O 3 ridge waveguide, fabricated by mean of Er 2 O 3 /Al 2 O 3 co-puttering. The SiO 2 cladding layer have been removed. to be 6.2 m. The waveguide fabrication reduce the lifetime to 1.7 m, poibly due to the introduction of defect or impuritie uch a OH that are known to reduce the Er lifetime. A econd type of amplifier tructure wa fabricated by putter depoition of a 600-nm-thick layer of Al 2 O 3 from a pure Al 2 O 3 putter target, followed by Er ion implantation. An almot contant Er concentration of 0.28 at. % wa achieved from 25 to 450 nm below the Al 2 O 3 urface uing multiple implant at energie in the range 100 kev 1.5 MeV. Waveguide were fabricated by etching to a depth of 300 nm. Thi material wa previouly hown to exhibit a net optical gain of 2.3 db at 1.53 m in a 4-cm-long waveguide amplifier, at a coupled pump power of only 9 mw. 10 A detailed characterization of the optical propertie of thi material can be found in Ref. 9, 11 and 12. To compare the optical gain characteritic of the coputtered and ion implanted waveguide, all meaurement on the two ample type were performed under identical condition. The waveguide were pumped uing a continuou wave fiber-coupled 1.49 m laer diode with a maximum power of 60 mw in the fiber. The pump light wa butt coupled to the waveguide uing a ingle-mode tapered fiber. Optical image of the waveguide were taken uing a charge coupled device camera in combination with an optical microcope. Photoluminecence PL meaurement were done by collecting the pontaneou emiion from the waveguide normal to the ample uing a multimode fiber placed approximately 1 mm above the ample urface. The collected luminecence wa ent to a 48 cm grating monochromator. PL meaurement in the range m were done uing a 600 g/mm grating blazed at 1200 nm and a liquid nitrogen cooled Ge detector, in combination with tandard lock-in technique. PL lifetime meaurement at viible frequencie were done uing a 1200 g/mm grating blazed at 500 nm and a TE cooled AgOC photomultiplier tube in combination with a photon counting ytem. Optical gain meaurement were performed by adding a low power m ignal from a NewFocu 6262 external cavity tunable diode laer to the input fiber, and monitoring the tranmitted ignal power a a function of applied pump power uing a lock-in technique. The ignal power in the input fiber wa kept below 0.1 mw. The tranmitted ignal wa collected by imaging the waveguide end facet onto a multimode fiber uing a 40 microcope objective, and detected a in the PL meaurement. A bandpa filter in the range m wa placed in front of the monochromator entrance lit in order to uppre any pump related background ignal. III. RESULTS AND DISCUSSION Figure 4 a how an optical image of an Er implanted Al 2 O 3 piral waveguide. The waveguide i pumped at 1.49 m at a power of 60 mw in the input fiber. At thi pump

3 5010 J. Appl. Phy., Vol. 93, No. 9, 1 May 2003 P. G. Kik and A. Polman FIG. 4. Color a Optical image of a piral waveguide amplifier doped with 0.28 at. % Er uing ion implantation into Al 2 O 3. The waveguide i pumped at 1.49 m at a pump power of 60 mw in the input fiber. The waveguide emit green light a a reult of two ubequent cooperative upconverion interaction. b Optical image of a imilar waveguide doped with 0.31 at. % Er uing Er 2 O 3 /Al 2 O 3 co-puttering. The intene upconverion luminecence i acribed to a trongly inhomogeneou atomic cale Er ditribution. power the waveguide can be een to emit a faint green luminecence. Photoluminecence pectrocopy meaurement of the viible emiion not hown indicate that the green emiion originate from the 2 H 11/2 and 4 S 3/2 level of Er 3. The luminecence een in Fig. 4 alo contain red emiion from the 4 F 9/2 level at 670 nm. Thee higher lying level are populated via a two-tep cooperative upconverion proce, a depicted in Fig. 1 b and 1 c. Firt, two Er ion in the 4 I 13/2 tate interact to yield an ion in the 4 I 9/2 tate Fig. 1 b, which i followed by a nonradiative relaxation to the 4 I 11/2 level. Second, two ion in the 4 I 11/2 level interact to yield an ion in the 2 H 11/2 level Fig. 1 c, which relaxe to the 4 S 3/2 level. Figure 4 b how an image of the Er 2 O 3 /Al 2 O 3 coputtered waveguide taken under the ame pumping condition. Thi waveguide how a very intene upconverion luminecence, even though the Er concentration i only lightly higher than in the ion implanted ample 0.31 v 0.28 at. %. The amount of optically active Er 3 in the co-puttered waveguide wa determined from tranmiion meaurement in the range m not hown. The Er related peak aborption in the co-puttered waveguide wa found to be 3.1 db/cm. Uing the calculated overlap of the optical mode with the Er depth profile ( 49%), and the Er peak aborption cro ection 12 of cm 2 taken from Ref. 11, the concentration of optically active Er in the waveguide i found to be cm 3, 17% le than the total Er concentration determined by RBS. Thi ugget that 83% of the Er i in the optically active 3 valence tate. The Er related peak aborption in the ion implanted waveguide i only 2.5 db/cm, due to the lower mode overlap with the hallower Er implantation profile. Figure 5 how the meaured ignal tranmiion change at m a a function of applied pump power at 1.49 m for a 1-cm-long traight waveguide made by ion implantation, and a 0.8-cm-long traight waveguide made by coputtering. A maximum ignal tranmiion change of 2.5 db i oberved in the Er implanted waveguide, correponding to the onet of Er related gain. Indeed, net optical gain wa oberved in thi waveguide under lightly more optimized pumping condition. 9 At the ame pump power, the ignal tranmiion in the co-puttered waveguide ha changed by only 1.2 db, and conequently no Er related gain i obtained. FIG. 5. Meaured tranmiion change at 1.53 m a a function of 1.49 m pump power in a 1-cm-long Er doped Al 2 O 3 waveguide made by ion implantation and in a 0.8-cm- long Er doped Al 2 O 3 waveguide made by Er 2 O 3 /Al 2 O 3 co-puttering. The drawn line are baed on population calculation uing the parameter in Table I. The Er 2 O 3 /Al 2 O 3 co-puttered waveguide how a lower ignal change due to trong cooperative upconverion. Note that the hape of the two curve in Fig. 5 i quite different, the ion implanted waveguide howing a tronger curvature. Thi i a conequence of the trong upconverion in the co-puttered ample, a will be hown below. To invetigate the upconverion in the waveguide quantitatively, we have meaured the pontaneou emiion at 1.53 and 0.98 m normal to the waveguide a a function of pump power. The luminecence wa collected from the middle of the ample to avoid collection of pump light cattered from the input and output facet. The luminecence intenitie found in both ample type are plotted in Fig. 6. At low pump power, the 1.53 m luminecence from the ion implanted waveguide olid quare increae linearly with pump power. At pump power above 1 mw in the fiber the intenity in both ample tart to how ignificant aturation, a wa oberved before. 12 An entirely different pump power dependence i een in the co-puttered waveguide. Within FIG. 6. Photoluminecence from Er doped Al 2 O 3 waveguide pumped at 1.49 m. Solid ymbol refer to ion implanted material, open ymbol to Er 2 O 3 /Al 2 O 3 co-puttered material. The drawn line are calculation of the fraction of excited Er 3 obtained uing the parameter lited in Table I. The right axi indicate the correponding fraction of excited Er.

4 J. Appl. Phy., Vol. 93, No. 9, 1 May 2003 P. G. Kik and A. Polman 5011 the full range of applied pump power, the 1.53 m luminecence exhibit an almot perfect quare-root dependence on pump power. Furthermore, the maximum 1.53 m luminecence intenity in the co-puttered waveguide i found to be ignificantly lower than in the ion implanted waveguide. Thi i conitent with the gain meaurement in Fig. 5, which howed that the maximum achieved ignal tranmiion change in the co-puttered waveguide i lower than in the ion implanted waveguide. Figure 6 alo how the pump power dependence of the 0.98 m luminecence from the 4 I 11/2 tate for the implanted cloed circle and co-puttered open circle waveguide. While the relative power dependence i imilar, the co-puttered waveguide how a much higher luminecence intenity at 0.98 m. Thi clearly how that the co-puttered ample exhibit tronger upconverion than the ion implanted ample. Thee finding are conitent with the intene green luminecence emitted by the coputtered waveguide Fig. 4 b, which i the reult of upconverion from Er ion that are in the 4 I 11/2 tate. In order to analyze the data in Fig. 5 and 6, we model the Er 3 a a three-level ytem. Thi generally yield good reult for the population in the lower lying level, becaue the population buildup in the level above the 4 I 11/2 level i mall a a reult of their low 1 lifetime. 13 The hort lifetime wa confirmed for the 2 H 11/2 and 4 S 3/2 green emitting tate, both of which howed a decay time of 25 limited by the time reolution of our ytem. The rate equation that determine the Er population are dn 3 3 C 37 N 3 N 3 R 24 C 24 N 2 N 2, dn 2 2 R 24 R 21 2C 24 N 2 N 2 W 3 N 3 R 12 N 1, dn 1 2 R 21 C 24 N 2 N 2 R 12 N 1 C 37 N with N i (cm 3 ) the concentration of Er in level i ee Fig. 1 a, W i 1 i ( 1 ) the meaured pontaneou decay rate from level i, R ij ( 1 ) a pump induced tranition from level i to level j, and C ij (cm 3 1 ) the cooperative upconverion coefficient from level i, producing one ion in the ground tate and one ion in level j. We have aumed that pontaneou decay from level 3 i dominated by nonradiative relaxation to level 2. The pump rate R ij are related to the photon flux cm 2 1 in the waveguide and the cro ection p ij (cm 2 ) for a pump induced tranition from i j according to R ij p ij. Note that timulated emiion and excited tate aborption by the pump are included. The mean photon flux i given by P fib /(E phot A) with the coupling efficiency between the fiber and the waveguide, P fib W the power in the input fiber, E phot (J) the photon energy, and A cm 2 the effective optical mode area. The um of all population i equal to N 0, the concentration of optically active Er in the waveguide. Note that an Er ion excited to the N 7 level by econd order upconverion from the N 3 level i aumed to relax back to the N 3 tate immediately via multiphonon emiion, and hence a ingle C 37 term appear in Eq. 1 compared to a 2C 24 N 2 term in Eq. 2. The olution of Eq. 1 3 can be ued to calculate the Er related ignal change G at m according to G db/cm log e 21 N 2 12 N 1 24 N 2 f Er3, with ij the cro ection for a ignal induced tranition i j, f Er3 the fraction of Er in the optically active 3 tate, and the overlap of the optical mode with the Er doped core of the waveguide. The rate Eq. 1 3 were firt olved analytically for teady tate uing the parameter a determined for ion implanted Al 2 O 3 from Ref. 11 and 12. Upconverion from the econd excited tate coefficient C 37 ) i aumed to be negligible in thi cae, ince the green upconverion luminecence i very faint. The meaured 1.53 and 0.98 m PL intenitie for the ion implanted waveguide were then multiplied by a caling factor to match the fitted population N 2 and N 3 ; while the coupling efficiency wa ued a a fitting parameter for the power cale. Optimum agreement between the calculation and the meaurement for the implanted ample i obtained for Uing thee parameter, the 1.53 m luminecence i decribed very well. The 0.98 m luminecence at high power, however, i overetimated, uggeting that the ued cro ection for excited tate aborption taken from Ref. 11 i too high for thi ample. A good fit i obtained uing p cm 2. Uing thee parameter the gain meaurement for the ion implanted ample can be fitted auming an optically active Er fraction of 83%. All parameter ued are ummarized in Table I. The calculation for the implanted ample are included in Fig. 5 and 6 drawn line. The three experimental data et in Fig. 5 and 6 are reproduced for all applied pump power. A imilar analyi wa done for the co-puttered waveguide. The luminecence intenity curve in Fig. 6 were calculated uing the cro ection found for the ion implanted ample and the luminecence lifetime 2 and 3 a meaured in the co-puttered waveguide ee Table I. The upconverion coefficient C 24 and C 37, and the coupling efficiency were ued a fit parameter. Uing the value lited in Table I, the experimental data are reproduced quite well. In order to achieve the quare root-like behavior over the full range of pump power, a high upconverion coefficient C 24 of cm 3 / i required. The quare root-like behavior can be undertood a follow: in cae of trong upconverion, the population in the firt excited tate will remain low at mot applied pump power, and the Er decay rate will be dominated by the term 2C 24 N 2 in Eq. 2. In the low population limit, we have N 2 R 12 2 R 12 /(2C 24 N 2 ). Thi give N 2 (R 12 ) 1/2, which indeed produce the oberved quare root-like dependence of N 2 on pump power. The upconverion coefficient C 24 found for the coputtered ample ( cm 3 1 ) i two order of magnitude higher than the value meaured in ion implanted waveguide ( cm 3 1 ). Thi trong Er Er interaction at low Er concentration ugget that the Er in the co-puttered material i not ditributed homogeneouly. Re- 4

5 5012 J. Appl. Phy., Vol. 93, No. 9, 1 May 2003 P. G. Kik and A. Polman TABLE I. Parameter value ued in the gain calculation in Fig. 5 and the calculation of the pump power dependent photoluminecence intenitie in Fig. 6. Value marked by * were obtained uing the fitting procedure decribed in thi article. The remaining value relating to the ion implanted ample were taken from Ref. 10. Parameter Ion implanted Co-puttered C Er cm f Er3 % 83 * 83 * 2 m C 24 cm * C 37 cm * 12 cm id. 21 cm id. 24 cm * id. 12 cm id. 21 cm id. 24 cm * id. L cm % 3 * 7 * % 36 * 49 * gion with a high local Er concentration could form in different way. The ue of an Er 2 O 3 /Al 2 O 3 powder-mixed putter target could, for example, reult in the depoition of Er 2 O 3 molecule, which yield a mall Er Er eparation at low average Er concentration. Alternatively, during putter depoition the Er ion may migrate over the ample urface, and form region of high Er concentration. Such mechanim would be favored by the large difference in ionic radiu of Al 3 and Er and 1.03 Å, repectively, making incorporation of an Er ion onto an Al lattice ite unlikely. To ditinguih between the preence of molecular aggregate or more extended region with high Er concentration we performed ion beam mixing experiment, in which a co-puttered film wa irradiated with 2 MeV Xe ion to a fluence of cm 2. Under thee condition each atom in the film will be diplaced 5 time, a wa calculated uing the Monte Carlo imulation program TRIM. Ifthe high upconverion coefficient i caued by the preence of eparate Er 2 O 3 molecule, the ion beam mixing hould ubtantially reduce C 24. However, no influence of ion beam mixing on the gain characteritic or the pump power dependent luminecence wa oberved. It i therefore more likely that the co-puttered film contain more extenive region of relatively high Er concentration, poibly reulting from diffuion during the puttering proce. We have performed high-reolution tranmiion electron microcopy and x-ray diffraction meaurement to earch for region of high Er content but found no evidence for thee within the reolution of thee technique. Technique more enitive to the local atomic environment of the Er ion, uch a extended x-ray aborption fine tructure pectrocopy or high-reolution electron microcopy in combination with energy lo pectrocopy may reolve thi iue. The ucce of ion implantation in thi comparion lie in the nonequilibrium nature of the implantation proce, allowing for a relatively homogeneou Er ditribution. For commercial application of miniature optical amplifier, different growth method and different material compoition hould be further invetigated. A low upconverion coefficient may, for example, be obtained by increaing the Er olubility by varying the waveguide compoition. Alternatively, depoiting at low temperature or at a high depoition rate will minimize effect uch a diffuion, and may alo reduce the effect of upconverion. IV. CONCLUSIONS We have fabricated two type of miniature optical waveguide amplifier, by Er ion implantation into Al 2 O 3, and by Er 2 O 3 /Al 2 O 3 co-puttering. It i found that the cooperative upconverion coefficient C 24 in the co-puttered material i two order of magnitude higher than in the ion implanted material. A a reult the co-puttered material how no optical gain, while the implanted material doe. The high upconverion coefficient i attributed to a trongly nonhomogeneou Er ditribution reulting from the co-puttering proce. ACKNOWLEDGMENTS Koo van Uffelen Technical Univerity Delft i gratefully acknowledged for variou tep in the ample preparation. Thi work i part of the reearch program of the Foundation for Fundamental Reearch on Matter FOM and wa made poible by financial upport from the Dutch Organization for Scientific Reearch NWO, the Dutch Technology Foundation STW, and the ESPRIT program SCOOP of the European Union. 1 F. Auzel, Radiationle Procee, edited by B. DiBartolo Plenum, New York, E. Snoek, P. G. Kik, and A. Polman, Opt. Mater. 5, W. J. Minicalco, J. Lightwave Technol. 9, P. Blixt, J. Nilon, T. Carlnä, and B. Jakorzynka, IEEE Photonic Technol. Lett. 3, F. Hort, T. H. Hoektra, P. V. Lambeck, and T. J. A. Popma, Opt. Quantum Electron. 26, S F. Dipaquale and M. Federighi, J. Lightwave Technol. 13, T. Ohtuki, S. Honkanen, S. I. Najafi, and N. Peyghambarian, J. Opt. Soc. Am. B 14, B. C. Hwang, S. B. Jiang, T. Luo, J. Waton, G. Sorbello, and N. Peyghambarian, J. Opt. Soc. Am. B 17, G. N. van den Hoven, E. Snoek, A. Polman, J. W. M. van Uffelen, Y. S. Oei, and M. K. Smit, Appl. Phy. Lett. 62, G. N. van den Hoven, R. J. I. M. Koper, A. Polman, C. van Dam, J. W. M. van Uffelen, and M. K. Smit, Appl. Phy. Lett. 68, G. N. van den Hoven, J. A. van der Elken, A. Polman, C. van Dam, J. W. M. van Uffelen, and M. K. Smit, Appl. Opt. 36, G. N. van den Hoven, E. Snoek, A. Polman, C. van Dam, J. W. M. van Uffelen, and M. K. Smit, J. Appl. Phy. 79, See, e.g., E. Deurvire, Erbium-Doped Fiber Amplifier Wiley, New York, 1994.