Imaging of thermal radiation with a spatial resolution below the diffraction limit is demonstrated with a passive millimeter-wave microscope. This technique utilizes a sensitive radiometric receiver in combination with a scanning near-field microscope. Experiments were performed at 50 GHz (λ = 6 mm) with sample temperatures ranging from room temperature down to 160 K, and the performance was shown to be superior to that achieved with passive imaging systems in the infrared region. The images are affected by non-uniformities in the transmission of thermal radiation from the sample to the receiver via the near-field probe and the reflection of thermal radiation back to the receiver from the probe. The effects of these non-uniformities were successfully removed using a sample image acquired by active measurements using a vector network analyzer.

We describe an analytical method for investigating the signal contrast obtained in near-field scanning microscopy using a metal slit probe. The probe has a slit-like aperture at the open end of a rectangular or a parallel plate waveguide. In our method, the electromagnetic field around the metal slit aperture at the probe tip is calculated from Maxwell's equations in the Fourier domain in order to derive the electrical admittance of a sample system consisting of layered dielectrics as seen from the probe tip. A simple two-port electrical circuit terminated by this admittance is then established to calculate the complex reflection coefficient of the probe as a signal. The validity of the method is verified at millimeter wavelengths by a full-wave high frequency 3-D finite element modeler and also by experiment. The signal contrast when varying the short dimension of the slit aperture, the separation between the probe tip and the sample, and the sample thickness are successfully explained in terms of the variation in the product of the admittance and the characteristic impedance of the waveguide at the probe tip. In particular, the cause of the local minimum in the signal intensity when varying the separation is clarified.

We report on the use of a knife blade as a scanning probe for apertureless near-field microscopy at millimeter wavelengths. Since the knife blade probe is a wider version of the metal tip probe commonly used in this technique, and therefore the interaction area between the probe tip and the sample is larger, an improvement in the intensity of the measured near-field signal is expected. The knife blade probe can also work as a part of a resonator in the illumination optics used in this microscopy format to enhance the strength of the near field that interacts with the sample, resulting in a further improvement in the signal intensity. A scanning method and an image reconstruction algorithm based on computerized tomography are adopted to obtain 2-D near-field images. Experiments performed at 60 GHz using a knife blade with a tip radius of 6 lm ($k=1000) show that the signal intensity is enhanced by $20 dB compared with an equivalent metal tip probe, and that an image resolution approaching the tip radius of the knife blade is achieved. V C 2012 American Institute of Physics. [http://dx.

The study of protein functions attributed to the conformation and fluctuation that are ruled by both the amino acid sequence and thermodynamics, requires thermodynamic quantities given by calorimetry using thermometric techniques. The increased need for protein function in different applications requires improvements of measurement systems assessing protein thermodynamics to handle many kinds of samples quickly. We have developed a passive millimeter-wave microscope that allows near-field imaging of thermal radiation, even in the low temperature range under room temperature where passive infrared imaging systems are ineffective. This advantage of our microscope system in combination with low thermal emission property of water ice in the millimeter-wave region enables the characterization of the thermal radiation from the proteins themselves in aqueous solution at a temperature range low enough to freeze water and to trap conformation intermediates in the proteins. Experiments performed at a millimeter-wave frequency of 50 GHz in a temperature range from 130 K to 270 K for a 20 % bovine serum albumin (BSA) aqueous solution showed a displacement between two conformational states of BSA at a temperature of approximately 190 K as a boundary. Our microscope system using this freeze-trapping method is expected to provide noninvasive thermal images to enable novel high-throughput calorimetry useful for the analysis of protein functions.3

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