Abstract
We report near field imaging of the transverse lasing modes of quantum cascade lasers. A mid-infrared apertureless near-field scanning optical microscope was used to characterize the modes on the laser facet. A very stable mode pattern corresponding to a TM(00) mode was observed as function of increasing driving current for a narrow active region quantum cascade laser. Higher order modes were observed for devices with a larger active region width-to-wavelength ratio operated in pulsed mode close to threshold. A theoretical model is proposed to explain why specific transverse modes are preferred close to threshold. The model is in good agreement with the experimental results.
Highlights
Understanding how the transverse mode structure and more generally the beam quality [1] of a semiconductor laser varies as a function of pumping current or ridge width is crucial for applications that require coupling of the laser output into another optical element
We report near field imaging of the transverse lasing modes of quantum cascade lasers
A mid-infrared apertureless near-field scanning optical microscope was used to characterize the modes on the laser facet
Summary
Understanding how the transverse mode structure and more generally the beam quality (divergence, spatial coherence, etc.) [1] of a semiconductor laser varies as a function of pumping current or ridge width is crucial for applications that require coupling of the laser output into another optical element. The interesting mode features in the near field have a lateral dimension roughly equal to or smaller than the free space wavelength divided by the effective refractive index of the mode Because of their low spatial resolution due to the diffraction limit, traditional optical methods cannot be used for near-field measurements. The Near-field Scanning Optical Microscope (NSOM) was introduced to circumvent this problem [2] In this technique, a tapered fiber with a subwavelength aperture at its end is scanned in the near field of the sample. The intensity of the light collected by the fiber is directly related to the sample’s near field This method has been used to image the transverse mode of λ=1.55 μm telecommunication lasers [3,4]. The near-field images acquired show clearly that for very narrow devices, only the TM00 mode is present and that for devices with broader ridge, high order modes are preferred at a driving current close to the laser threshold
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