Note: Near-field imaging of thermal radiation at low temperatures by passive millimeter-wave microscopy

Abstract

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.

Observation of Protein Thermodynamics in Ice by Passive Millimeter-Wave Microscopy

“…Our experiments show thermodynamics in the transition of structural states in protein as the transition in emissivity. Advantages of our thermometric method are that the measurement is noninvasive, information from ice is free, and the system can provide two-dimensional thermal images [16]. Our system is expected to overcome common problems in protein calorimetry.…”

“…where γ , T r , and T am are the reflectance of the probe, the physical temperature of the receiver, and the ambient temperature that expresses the influence of ambient thermal radiation, respectively [14,16]. The first term in the right hand side of Eq.…”

“…Our passive millimeter-wave microscope performed thermal imaging in the temperature range from room temperature down to 160 K [16]. Because water ice emits negligible thermal radiation in the millimeter-wave region [17], when our microscope system scans a sample in ice, the thermal radiation from samples other than solid water should be detected.…”

“…Thermography using millimeter-wave radiation is insufficient in its wavelength because the spatial resolution achievable with radiometric imaging system using conventional optics is limited to the wavelength of the operating frequency because of the diffraction effect [14], causing necessity for preparing a large amount of sample. We have overcome this problem by developing a tapered near-field slit probe that allows for passive thermal imaging in the millimeter-wave region with subwavelength spatial resolution below the diffraction limit [15,16].…”