Imaging a GaAlAs laser diode in operation using apertureless scanning near-field optical microscopy

We report the local study of an operating double hetero-junction GaAs/GaAlAs laser diode by apertureless Scanning Near-field Optical Microscopy (SNOM). This study allows the characterization of the spatial properties of the light emitted by the laser source. The optical probe used is a tungsten tip vibrating (at frequency f) perpendicularly to the emitting surface. Suitable experimental parameters permitting us to accede to near-field information have been defined. This has leaded to an experimental configuration in which the probe vibration amplitude is "optimal". A far-field detection at grazing angle is used and a second harmonic (at frequency 2f) lock-in detection is performed. This configuration has permitted us to obtain SNOM images in stimulated emission as well as in spontaneous emission. Above the threshold current, information concerning the emitted laser mode are obtained, whereas in spontaneous emission the SNOM images are linked to the index contrast of the laser diode front facet.