Optical properties of erbium-doped organic polydentate cage complexes
L. H. Slooffa) and A. Polman
FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands

M. P. Oude Wolbers, F. C. J. M. van Veggel, and D. N. Reinhoudt
Supramolecular Chemistry and Technology, University of Twente, P.O. Box 217,
7500 AE Enschede, The Netherlands

J. W. Hofstraat
Akzo Nobel Central Research, Department RGL, P.O. Box 9300, 6800 SB Arnhem, The Netherlands

͑Received 7 July 1997; accepted for publication 16 September 1997͒
The optical properties of different erbium ͑Er͒-doped polydentate hemispherand organic cage complexes are studied, for use in polymer-based planar optical amplifiers. Room temperature photoluminescence at 1.54 ␮m is observed, due to an intra-4 f transition in Er3ϩ. The Er is directly excited into one of the 4 f manifolds ͑at 488 nm͒, or indirectly ͑at 287 nm͒ via the aromatic part of the cage. The luminescence spectrum is 70 nm wide ͑full width at half maximum͒, the highest known for any Er-doped material, enabling high gain bandwidth for optical amplification. The absorption cross section at 1.54 ␮m is 1.1ϫ10Ϫ20 cm2, higher than in most other Er-doped materials, which allows the attainment of high gain. Measurements were performed on complexes in KBr tablets, in which the complex is present in the form of small crystallites, or dissolved in the organic solvents dimethylformamide and butanol-OD. In KBr the luminescence lifetime at 1.54 ␮m is Ͻ0.5 ␮ s, possibly due to concentration quenching effects. In butanol-OD solution, the lifetime is
0.8 ␮s, still well below the radiative lifetime of 4 ms estimated from the measured absorption cross sections. Experiments on the selective deuteration of the near-neighbor C–H bonds around the
Er3ϩ-ion indicate that these are not the major quenching sites of the Er3ϩ luminescence.
Temperature dependent luminescence measurements indicate that temperature quenching is very small. It is therefore concluded that an alternative luminescence quenching mechanism takes place, presumably due to the presence of O–H groups on the Er-doped complex ͑originating either from the synthesis or from the solution͒. Finally a calculation is made of the gain performance of a planar polymer waveguide amplifier based on these Er complexes, resulting in a threshold pump power of
1.4 mW and a typical gain of 1.7 dB /cm. © 1998 American Institute of Physics.
͓S0021-8979͑97͒06124-0͔

I. INTRODUCTION

Erbium ͑Er͒ doped materials have attracted a lot of attention, because of their potential applications in optoelectronics.1–3 In its trivalent state the Er3ϩ ion shows an intra 4 f shell transition from its first excited state ( 4I13/2) to the ground state ( 4I15/2), which occurs at a wavelength of
1.54 ␮m. In the free Er3ϩ-ion optical intra 4 f shell transitions are parity forbidden. Incorporated in a solid host however, the crystal field of the host induces mixing of states, which makes some of the transitions allowed. These optical transitions are rather sharp, due to the fact that the partially filled 4 f shell is shielded by filled 5s and 5p shells. The luminescence lifetime of the first excited state can be as long as several milliseconds.4–6 These features make Er-doped materials very attractive for lasers and optical amplifiers operating at 1.54 ␮m, one of the standard telecommunication wavelengths. Er-doped planar optical amplifiers have been demonstrated using silica,7–12 Al2O3, 13 and LiNbO314,15 hosts.
However, as polymer waveguides are becoming more important, both as fibers and in thin film configurations, it is interesting to study the doping of planar polymer waveguides a͒ Electronic mail: Slooff@amolf.nl; http://www.amolf.nl

J. Appl. Phys. 83 (1), 1 January 1998

with Er, and to investigate if optical amplification can be achieved. Such polymer amplifiers could then be integrated in existing optical polymer devices such as splitters, switches, and multiplexers16