Modification of semiconductor laser diodes by focused ion beam milling

Focused ion beam (FIB) milling has been applied to improve and tailor the emission characteristics of Fabry-Perot type semiconductor lasers. Multi-mode commercial lasers operating at 780 nm were modified by sputtering 1 μm × 1 μm depressions into the top of the ridge waveguide with a 30 keV Ga ⁺ FIB. As the hole depth approaches the active layer, three effects on device operation were observed. First, a modest increase in laser threshold (≤ 20%) occurred without any accompanying change in the diode's current versus voltage characteristic. Second, overall emission was blue-shifted by up to 20 nm. Third, the perturbation introduced in the waveguide altered the mode structure of the emission spectrum. Single-mode operation with diminished mode-hopping could be promoted by the selective placement of the depressions along the length of the waveguide, which act as localized absorbers producing periodic modulations in the laser's spectral gain curve. Feedback regarding the progression of FIB structuring was achieved via in situ monitoring of the laser threshold and spectrum. The effect of the ion beam on light generation and propagation was characterized by sub-threshold electroluminescence measurements, which indicated a relative increase in absorption at longer wavelength after FIB processing.