We propose and demonstrate theoretically a method of achieving single lateral mode field patterns in CW broad area laser diodes at high power. The method relies on laterally profiling the thermal conductivity of the bonding solder layer and thereby thermally controlling the lateral index. Various configurations are simulated using a split step FFT beam propagation method including thermal, carrier (alpha-parameter) and optical (Kerr effect) index variations. Thermal effects are described using a 2-D finite element electro-thermal model including current spreading..