Aperture Scanning of a Line Grating
In 1972, E. A. Ash and G. Nicholls, from the University College in London, demonstrated the near-field resolution of a subwavelength aperture scanning microscope operating in the microwave region of the electromagnetic spectrum. Utilizing microwaves, with a wavelength of 3 centimeters, passing through a probe-forming aperture of 1.5 millimeters, the probe was scanned over a metal grating having periodic line features. Both the 0.5-millimeter lines and 0.5-millimeter gaps in the grating were easily resolvable, demonstrating sub-wavelength resolution having approximately one-sixtieth (0.017) the period of the imaging wavelength. This interactive tutorial explores the Ash and Nicholls experiment.
The tutorial illustrates a near-field scanning experiment utilizing a microwave resonator source, and initializes in the Auto Scan mode, with a metal-on-glass specimen being scanned beneath an illuminating aperture in an opaque metal screen. Both the Reflected Signal (detector located above the metal screen) and the Transmitted Signal (detector located below the specimen) are displayed on their respective monitors. In this example, the displayed signals are simply the inverse of one another, but in an actual experiment the signals could be vastly different from one another depending upon the material properties of the specimen (in effect, the reflectivity coefficient of the metal and transmissivity of the glass at the specific radiation wavelength).
Selecting the Manual Scan radio button will enable the Specimen Movement slider. The mouse cursor can then be used to position the specimen manually in the illuminating beam. Note that the signals displayed on the upper and lower monitors will correspond to the beam's interaction with the specimen. The metal grid bars reflect the microwave radiation and produce a maximum signal on the Reflected Signal monitor. Conversely, glass areas between the metal bars transmit the illuminating beam, resulting in the maximum Transmitted Signal. In practice, the optimal position of the detector could be above the specimen, below it, or at an oblique angle, depending upon the specific specimen and the instrument configuration. The detector that can collect the greatest signal from the specimen will have the largest signal-to-noise ratio, and thus will generate the best image with the highest contrast.
In the most fundamental form of near-field scanning imaging, radiation illuminating a specimen is confined by the dimensions of a subwavelength-diameter aperture. By scanning the aperture over the specimen at a distance less than the aperture diameter, point-by-point illumination can be achieved without the limiting effects of diffraction. Ash and Nicholl's experiment that verified the feasibility of near-field imaging utilized microwave radiation for illumination, and the specimen was moved underneath a fixed screen containing the aperture. An antenna located inside the open resonator collected the reflected signal. In order to differentiate between the microwaves that were reflected by the metal grating on the glass (specimen signal) and waves reflected by the opaque metal screen, the glass and metal specimen was oscillated at a specific frequency and only the signal that exhibited this oscillation frequency was passed by the detector to form the image.