Direct imaging of a laser mode via midinfrared near-field microscopy

Fabry-Pérot standing waves inside a midinfrared quantum cascade laser have been imaged using an apertureless scanning near-field optical microscope. The devices emit at λ≈7.7 μm and they feature air-confinement waveguides, with the optical mode guided at the semiconductor-air interface. A consistent portion of the mode leaks evanescently from the device top surface and can be detected in the near field of the device. Imaging of the evanescent wave across a plane parallel to the device surface allows one to directly assess the effective light wavelength inside the laser material, yielding the effective index of refraction. Imaging across a plane perpendicular to the device surface allows one to directly measure the electric field decay length, which is found in excellent agreement with the numerical simulations.