Scanning Nanowire Probe Interferometer for Scalable Humidity Mapping

Abstract

Quantifying humidity has long been an unavoidable task in science, industry, and society. Recent developments of nanoscience and technology that deal with ultrasmall droplets have aroused interest in microscopic moisture. Utilization of nanomaterials has been emerging as a promising strategy to miniaturize hygrometers for high‐sensitive, ultrasmall‐area sensing. However, a lack of high‐precision, on‐demand position control of sensing nanomaterials makes it difficult to explore spatial distribution of humidity at the micro‐ and nanoscale. Here, a scanning probe hygrometry (SPH) is developed that enables not only micro/nanoresolution but also scalable spatial mapping of humidity distribution. The SPH is realized with an unprecedented scanning nanowire probe interferometer (NPI) that is produced by direct 3D nanoprinting of a moisture‐sensitive polymer on a tapered optical fiber. Notably, the interferometric response of the NPI probe in ultrasmall areas quantitatively depends on humidity, arising from its refractive index change and volumetric swelling. By scanning the NPI probe and reading out the interferometric signals, multiscale spatial mapping of humidity distribution with versatile scanning steps from ≈102 nm to a few mm is demonstrated. The NPI is expected to provide a new nanoscale metrology that can answer fundamental questions about evaporation‐related science and engineering.

Vapor Mapping in a Microscopic Space with a Scanning Nanoprobe Interferometer

“…34 In fact, vapor mapping in a microscopic space can be measured from the humidity values, which are then attained from the interference patterns measured with the nanoprobe interferometer. 30 Accurate measurement of the interference patterns is thus critical for enhancing humidity sensitivity. Here a central strategy for improving the measurement accuracy is to collect the interference signals with low optical loss, which can be realized by a flat tip surface of the nanowire so as to greatly enhance the reflectance of the incident beam at the tip.…”

“…Figure a illustrates the schematic of our nanoprobe, which basically consists of a humidity-responsive polymer nanowire directly grown on the tip of a tapered optical fiber by the confined growth 3D nanoprinting. − The working principle of the nanoprobe is based on the optical interference between two reflected beams from the tip of the nanowire ( I 1 ) and from the fiber tip ( I 2 ) for an incident beam. The phase difference in the interference δ ∼ 4π nL /λ (λ: wavelength) is modulated by the humidity that changes the refractive index ( n ) and the length ( L ) of the nanowire.…”

“…In fact, vapor mapping in a microscopic space can be measured from the humidity values, which are then attained from the interference patterns measured with the nanoprobe interferometer . Accurate measurement of the interference patterns is thus critical for enhancing humidity sensitivity.…”

“…The nanoprobe and microcapillaries were individually controlled by a three-axis ( x–y–z ) motorized stage with a 250 nm step resolution (KOHZU precision). The scanning process for humidity and thus vapor mapping was monitored in real time by using an optical imaging setup including a CMOS camera (Lumenera, Infinity1-2), a 10× objective lens (Mitutoyo), and a green LED (Mightex) illuminator . The spatial and temporal resolutions in vapor mapping are a few micrometers and 0.2 s, respectively.…”

Recent advances in meniscus-on-demand three-dimensional micro- and nano-printing for electronics and photonics

“…Because a meniscus can be as small as a few micrometers or even a few hundred nanometers, printed stimulus-responsive polymers can serve as micro/nanoscale sensors or actuators. Kim et al developed a highresolution (250 nm) on-demand moisture sensor by exploiting the interferometric response of a 3D-printed PS nanoprobe (figure 5(a)) [70,71]. Won et al used the meniscus-guided 3D printing method to form a stretchable poly(methyl methacrylate)/PPy gas sensor for NH 3 (figure 5(b)) [72].…”