Fabrication of Poled Polymer Dans Waveguides and First Observation of Shg by Counter-Propagating Guided Waves
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are written as E'0"
2L[E (x)e
1
\J~tFlcz + C.C.
where _+(x) describes the transverse guided wave field distributions. A second harmonic polarization of the form P 2 o) =lEOd22
+(x)&.(x)ei2ax + E+(x)E*(x)e-i 2 fl z + c.c.]
PO
(2)
is created where is a propagation constant of the fundamental guided waves and the ± signs denote the propagation directions along the +z and -z directions respectively. Unlike the usual copropagating SHG, the induced second harmonic polarization does not have a propagation term along the z-axis. Fields generated by the polarization can propagate only along the ± x-axis for CW wave mixing in slab waveguides. This surface emitting configuration has the advantage that + = 0, is always the wavevector matching condition parallel to the surface, Af3z = satisfied under any circumstances. The other unusual property is the spatial shape of the SHG pulses. Let us assume that the fundamental waves have spatial pulse shapes along the propagation direction, U(z) and U'(z), i.e. essentially the temporal pulse shapes frozen in time. The resulting SHG pulse shape in the z-direction is the convolution of the fundamental pulse shapes. Hence we can determine the convolution pulse shape by measuring the SHG pulse energy distribution along the z-direction with a CCD array. Several applications of these properties have previously been 13 12 demonstrated, including picosecond signal processing, picosecond optical transient digitizers. Furthermore this device can be used for real time spectrometers 14 because the sum frequency
convolution in space
convolution
in time
[-u~z- +o~e( 2ct-kx)
J
(z- a)U'(z +a)e'
da
Fig. 1. Schematic diagram of surface emitting SHG by counter propagating waves.
632
fields generated by different input frequency components propagate with unique and different angles from the normal to the film surface. The device potential of this phenomenon for ultrafast 15 signal processing, wavelength division demultiplexing, etc. has attracted considerable attention. 15 20 In the last few years, several versions in GaAs based waveguides were reported, - and the conversion efficiency was improved dramatically by using a form of quasi-phase-matching in multilayered structures. On the other hand, poled polymers have great potential for this device application because they have large non-resonant nonlinearities and ease of fabrication. Furthermore this device allows us to work at SHG wavelengths on resonance (0)0 = 20w) since the emitted SHG wave travels less than a jim through absorbing material. However, for efficient surface emission, the film is required to have large in-plane d 2 2 coefficients which are obtained by our plane-parallel poling geometry. DEVICE FABRICATION DANS side-chain polymers1 are one of the most highly developed poled polymers for applications to electro-optical switching devices. Their relatively high glass transition temperature provides long term stability of the poled state at room temperature and large poling field strengths provide nonlinear coefficients d
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