Spectrally resolved near field investigation of small aperture vertical cavity surface emitting lasers.

摘要

Spectrally resolved near field scanning optical microscopy (NSOM) has been used to study emission mode characteristics of small aperture vertical cavity surface emitting lasers (VCSELs). Two kinds of VCSELs were studied, proton implanted VCSELs and selectively oxidized VCSELs.;Overviews of optical mode analysis of VCSELs are given, and NSOM theory and the experimental configuration for spectroscopic NSOM are reviewed. In order to achieve high spatial resolution, better than the diffraction limit, subwavelength aperture NSOM tips were kept in the near field of laser aperture using shear force detection, and used to collect the local emission from the laser aperture. The local emission collected from the laser aperture was guided into a triple spectrometer and a full spectrum was obtained at each pixel position. Spectrally and spatially resolved modal structures of the VCSELs were obtained at various injection currents.;Implanted VCSELs showed a non-linear I-L curve indicating unstable mode formation at low injection currents just above threshold, consistent with the observed spatial shifts of optical modes with varying injection currents. A reduction of spatial mode width was observed suggesting the existence of induced index guiding. Using a spectrally resolved NSOM, the modal structures of individual transverse modes were identified showing a LP 01 mode with a circular Gaussian shape located on the aperture center and LP01 modes with a double lobe shape with null in the center. A resonance wavelength dependence on position was observed for all the lasing modes showing longest wavelengths on the aperture center and growing shorter with the distance from the aperture center. The observed total shift between the aperture center and the edge grew larger with larger injection currents. The resonance wavelength dependence was attributed to an induced index variation across the laser active area. Since resonance wavelength is proportional to the optical cavity length, the spatial variation of the lateral index for two samples were calculated at various injection currents. The increase in spectral mode spacing with the increase injection current was observed consistent with the observed induced index variation. The amount of the increase in mode spacing was consistent with the obtained lateral index variation profile.;The superior optical confinement of oxide VCSELs was suggested from the fact that a linear I-L curve was observed despite the multimode operation of the laser. Images of subthreshold emission and lasing emission were obtained in the near field and various heights from the laser aperture. The near field images showed detailed mode structures which are not shown in the far field, confirming the high spatial resolution of NSOM. We found that local regions with slightly high emissions at subthreshold currents served as lasing centers once above threshold, providing information on the lateral distribution of injected currents.