THz differential near-field scanning optical microscopy for biological applications

Differential Near-Field Scanning Optical Microscopy (DNSOM) [1] represents a valuable tool for imaging with sub-wavelength resolution. In standard Near-Field Scanning Optical Microscopy (NSOM) the light is diffracted by a sub-wavelength aperture, which drastically limits the signal to noise ratio achievable. In our approach the acquisition is carried out using squared apertures with dimensions comparable with the wavelength. The aperture, mounted on a xy translational stage driven with piezo-actuators, is scanned in the near field of the object of interest, illuminated by a laser source. The total transmitted power is then recorded in the far-field for each scanning position by a suitable detector. Differently from SNOM, recover of the sample image is accomplished by performing a numerical second order derivative along the x and y directions of the image acquired by the detector. The final result consists of four independent replicas of the original object, two positive and two negative, originating from the diffraction of the aperture's corners. The precision of these measurements in principle depends only on the sharpness of the corners, on the minimum step size, and on the relative distance Δz between object and aperture. Particularly, in the Mid-Infrared and THz spectral region, DSNOM represents a non-destructive method and a valuable tool for imaging biological samples or alternatively for retrieving informations about the dielectric properties of the material, with micrometer precision. In our measurements we have employed a THz Quantum Cascade Laser (QCL) laser emitting at 2.9 THz. We fabricated, with standard photolithography, square apertures, with different sizes varying from 70×70 µm
²
to 150×150 µm
²
, in a Cr/Au metallization, with typical thickness of 10/400 nm. These apertures are obtained by metal evaporation on a GaAs substrate, or alternatively on SiN membranes of 100 nm thickness. The objects, in this case, correspond to triangular or squared metallization on GaAs, with typical lateral size of ∼ 50 µm. Fig. 1a presents an image acquired with a CCD camera in the visible spectrum of a square metallization (50 µm side) placed in the nearfield (Δz estimated ≤ 3 µm) of the square aperture (120 µm side). The total THz signal, collected by parabolic mirrors and focused on a Pyro-detector 1000 from Infrared System, is recorded as a function of the aperture's position. After performing the numerical second-order derivative of the signal, we obtained four replicas of the original object, two positive and two negative.