Scanning microscope sees quantum cascade laser modes

13 Jun 2007

A scanning microscope is allowing researchers to image the modes inside a quantum cascade laser with nanometer resolution.

A near-field scanning optical microscope (NSOM) modified to operate in the mid-infrared is helping researchers to image the modes within a quantum cascade laser (QCL). The results could be used to optimize more sophisticated structures, such as photonic crystal QCLs. (Applied Physics Letters 90 201114).

The work was carried out by research groups led by Yannick De Wilde (at ESPCI, France) and Raffaele Colombelli (at IEF, France), in collaboration with Sheffield University, UK.

"We have shown how to map the evanescent field produced at the surface of a QCL that is operating," Colombelli told optics.org. "We are now able to directly access the spatial distribution of the modes inside the cavity."

Optical setup

Standard infrared microscopy cannot be used to image the evanescent field produced at the surface of an air-guided mid-IR QCL. In comparison, IR-NSOM reveals the evanescent field by using the tip of an atomic force microscope (AFM).

"The decay length of the electric field is of the order of 500 nm," explained De Wilde and Colombelli. "It is this field that we have imaged using the IR-NSOM. In a conventional IR microscope, diffraction limits the resolution to about 10 microns. IR-NSOM achieves a resolution of the order of 50 nm."

The AFM tip in the IR-NSOM acts as a local scatterer. The evanescent field at the tip is converted into a propagating wave that is collected and focused onto a detector using standard optical components. The resulting signal at the detector is proportional to the intensity of the evanescent field at the tip location on the surface.

"The resolution of the image depends on the size of the tip - which is roughly 50 nm," said De Wilde. "In our case, a 20x20 micron image takes 15 minutes to record."

The team is now planning to use QCLs to electrically generate long-wavelength surface plasmons. "We will nano- or micro-structure the surface of the QCL in order to transfer surface plasmons from the semiconductor-metal interface to the top metal-air interface," concluded Colombelli. "These measurements will allow us to validate the principle of surface-plasmon generation with QCLs."