9:30am - 9:45am ThA4

Polymer-Conductivity Matching For Efficient Poling Towards High Second Harmonic Generation Efficiency

At 1.55 pm *Vincent Ricci, Michael Canva, and George I Stegeman,

CREOL - University of Central Florida. Tel.: 407-823 6994. Email: vincent@mail.creol.ucf.edu

There is a crucial demand for optical devices such as ultra-fast frequency shifters and network routers in the growing market of optical networks. Polymers can have very large nonlinearities and good thermal stability. Most of all, the technology associated with polymer processing and multi-layer integration is simple and inexpensive. Cascading of second-order nonlinear effects in these materials can be used for all-optical switching, spatial solitary waves, etc. However, demonstrating highly efficient second harmonic generation (SHG) is a key milestone towards practical cascading applications.

Once the design of a guiding structure meets the classical phase-matching condition, the transfer of the large microscopic nonlinearity of a polymer material into an efficient SHG device requires the optimization of three major device characteristics. They span both material properties and technology development. First, the active chromophore orientation must be optimized during poling. Second, the overall material dependent propagation losses should be decreased. Finally, the waveguide geometry should be optimized for the best mode overlap and the optimum mode confinement. In this communication, we would like to report on our successful efforts to improve the poling efficiency, by a factor of 3, through the use of conductivity matching between the layers in the guiding structure.

Clearly, the sequential spin-coating technology is well adapted to the making of multi-layer guiding structures. However, this process randomizes the orientation of the nonlinear active chromophores inside the layer. It is well known that a macroscopic second-order nonlinearity will only exist if the centro-symmetry of the nonlinear active region is broken. When the temperature has risen sufficiently close to the glass transition temperature (Tg) of the polymer matrix, the matrix becomes soft enough such that the nonlinear active dipoles can rotate fieely. Therefore, they can orient themselves along the field lines of an externally applied voltage. Once thermal equilibrium is achieved, decreasing the temperature of the waveguide down to room temperature while maintaining the external poling field freezes the angular distribution of the dipole moments, thereby creating a nonzero second order polarization. The magnitude of the nonlinearity increases with a larger internal electric field.

To the best of the authors' knowledge, the largest SHG efficiencies in polymer waveguides at 1.55 pm to date were demonstrated in DANS with a parallel plate poling geometry with a SHG figure of merit (FOM) of q=14 % / W /cm2 [l] (cf. [2] for FOM definitions and discussions). The experimental nonlinearity d,E o f 5 pmN (for a poling field of 180 V/pm) was considerably lower than the extrapolated value of d = 36 pmN, for an in-plane poling geometry where d = 60 pmN (nominal poling field of 300 V/pm) [3]. In the in-plane poling geometry, the field is directly applied to the active region. Clearly, one of the key issues in parallel plate poling is to make sure that most of the externally applied poling voltage falls across the nonlinear active layer as shown Figure 1 [4]. Moreover, the electrical properties of each polymer can vary by several orders of magnitude as the poling temperature approaches its T,. In reference [l], the transparent portion of the guiding layer was made of a polyetherimide (PEID), which has a T, = 190 "C, much higher than for the DANS nonlinear layer where T, = 142 "C. As a result, the poling field across the nonlinear DANS layer was not optimized. Matching the layer conductivities for optimum poling requires the proper choice of the glass transition temperatures for different material layers.

0-7803-4947-4/98/$10.00 01998 IEEE 9

In this work, a transparent material with a lower Tg than PEID was used to improve the poling efficiency. We are grateful to Dr. K. P. Chan who engineered a derivative of PEID, namely PEIHM, with a decreased Tg of 125 “C, lower than DANS. The index dispersion of PEIHM was measured to follow just below PEID with a 0.03 index difference in the wavelength range of our experiment. Also, because of solvent compatibility issues, a thin protective layer of cross-linkable polycarbonate PC (1 50 nm-thick) was needed between the DANS layer and the PEIHM layer as shown Figure 1 . The overall price was a decrease in the mode confinement. A poling voltage of only 400 V was applied across the 8-pm multi- layer structure with a poling temperature maintained near the DANS Tg of 142 “C. We report a measured FOM of q = 14 % / W / cm2 for this geometry. We show the corresponding SHG tuning curve Figure 2. The linear losses at the fundamental wavelength were measured to be comparable to previous studies: 5 dB / cm [l]. In summary, the same effective nonlinearity demonstrated in reference 111 was achieved but with an external poling voltage 3 times smaller: i.e. 400 V over 7 pm instead of 1260 V over 7 pm.

In conclusion, we have demonstrated the enhancement of the parallel plate poling efficiency by a factor of 3 . We matched the best SHG FOM in polymer waveguides with a poling voltage three times smaller. Considering that we are still in a linear regime for the growth of the effective nonlinearity as a function of poling field, we can expect to gain at least an order of magnitude in the SHG efficiency by applying a larger voltage.

Acknowledgements: We would like to thank M Flipse and M. Diemeer from Akzo Nobel for the DANS and PC polymer and K. P. Chan from Molecular Electronic Corporation (MOEC) for the PEIHM polymer. This research is supported by AFOSR.

I FOM=14%/W/cm2

ExternaVApplied Poling Voltage

A1 electrode

Useful Internal Poling Voltage

IT0 electrode

L a 2 a (3 I 0)

1.0

0.8

06

0 4

0.2

00 1560 1580 1600 1620 1640

Fundamental Wavelength (nm)

Figure 1: Parallel plate poling of multi-layer guiding structure.

Figure 2: SHG tuning curve of lmm-long 4-pm wide channel waveguide.

References:

[l] M. Jlger, G. Stegeman, M. Flipse, M. Diemeer, G. Mohlmann., “Modal dispersion phase matching over 7 mm length in overdampedpolymeric channel waveguides”, Appl. Phys. Lett., Vol. 69, pp. 4139-4141, (1996).

[2] M. Jager, V. Ricci, W. Cho, M. Canva, G. Stegeman., “Advantages of modal dispersion phase-matching and material requirements for polymeric devices using efJicient second harmonic generation at telecommunication wavelength ”, MRS Symposium Proceeding Vol. 488, pp. 179- 191, (Dec. 1997).

131 A. Otomo, M. Jager, G. Stegeman, M. Flipse, M. Diemeer, “Key trade-offs for second harmonic generation in poledpolymers”, Appl. Phys. Lett., Vol. 69, pp. 1991-1993, (1996).

[4] G. Rikken C. Seppen, E. Staring, A. Venhuizen, “EfJicient modal-dispersion phase-matched frequency doubling inpoledpolymer waveguides”, Appl. Phys. Lett., Vol. 62, pp. 2483-2585, (1993).

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