Experimental principles of infrared passive near-field microscopy (SNoiM) and its applications
Near-field radiation at the surface of an object is difficult to detect due to its swift-wave nature (i.e., the intensity decreases sharply as it moves away from the surface of the object). In SNoiM, this problem is effectively solved using the scanning probe technique. As shown in Fig. 1(b), when the nanoprobe is not introduced (or the probe is far away from the object surface), the near-field snappy waves near the surface of the object cannot be detected, and the microscope operates in the conventional infrared thermography mode, which obtains only the far-field radiated signals.The key of the SNoiM technique is to bring the probe close to the near-surface of the sample (e.g., within 10 nm) so that the near-field snappy waves can be effectively scattered by the tip of the probe. In this detection mode, both near-field and far-field components are present in the sample signal acquired by the probe. Therefore, by controlling the probe-to-surface spacing h, a mixed near-field and far-field signal (h < 100 nm, called near-field mode) or a single far-field signal (h >> 100 nm or withdrawal of the probe, called far-field mode) can be obtained. Ultimately, the near-field information of the object can be extracted from the far-field background using the probe height modulation and demodulation techniques.
The near-field signals scattered by the probe are first collected by a high numerical aperture infrared objective lens. However, the far-field radiated signals from the environment, the DUT and the instrument itself cannot be cancelled in this process, and they are collected with the near-field signals by the infrared objective lens, resulting in the weak near-field signals of the DUT being annihilated by the large far-field background radiation. In order to minimise the far-field background signals, the researchers designed a confocal aperture with a very small aperture (~100 μm) above the infrared objective lens, which reduces the collection spot and effectively suppresses the background radiation signals. However, even with this, it is difficult to determine whether there is a sensitive enough infrared detector that can detect the weak near-field signals scattered by the nanoprobes. To this end, our team has developed an ultra-high sensitivity infrared detector to overcome this technical barrier.
Among them, the golden cylindrical cavity is a cryogenic Dewar, which carries the self-developed ultra-high sensitivity infrared detector (CSIP) and some low-temperature optical components; the white box shows the tuning fork-based atomic force microscope (AFM), the infrared collection objective and the sample stage area assembled in the lab. The spatial resolution of the IR near-field image is no longer limited by the probe wavelength, but determined by the probe tip size. By electrochemical etching method, metal (tungsten) nanoprobes with excellent morphology can be prepared, in which the tip diameter can be as small as 100 nm or less.
