Semiconductor device and method of manufacture

Passively modelocked semiconductor laser device 19 comprises active region 4 and cladding layer 5, 7. A saturable absorbing layer 6 within the cladding layer, for example a thick quantum well layer (TQW), has at least a portion 11a which absorbs light emitted by the active region, and at least a portion 11b which is made non-absorbing by increasing the band gap in that region by introduction of vacancies through plasma irradiation, causing quantum well intermixing. Portions 11a and 11b may be arranged alternately in stripes, across the direction of lasing; the non-absorbing portions acting as a heat sink for heat produced in the absorbing regions. The fabrication method enables non-absorbing portions 11b to be produced after the device has been fabricated, so that the degree of overlap between the non-absorbing portions and the optical mode of the laser can be altered after the device has been grown.