Comparison of active and passive methods for the infrared scanning near-field microscopy

Abstract

We systematically compare the active and the passive methods for infrared scattering-type scanning near-field optical microscopy (s-SNOM). The active SNOM makes use of IR lasers or incoherent thermal emitters to illuminate a sample, whereas the passive method directly measures extremely weak fluctuating electromagnetic evanescent fields spontaneously generated at the sample surface without any external illumination. For this reason, our specific version of the passive SNOM is called a scanning noise microscope (SNoiM). In thermal equilibrium, the two methods are shown to be similar, both mapping the nanoscale variation of the complex dielectric constant of the sample. We demonstrate that a significant difference between the two methods emerges when the sample is driven out of thermal equilibrium, viz., the active SNOM is insensitive whereas the SNoiM is extremely sensitive to the electron temperature in locally heated nanoregions.

Anisotropic Hot-Electron Kinetics Revealed by Terahertz Fluctuation

“…Briefly, SNoiM probes the local energy density, thermally or externally excited, in nanoscale regions of a target material. This feature of SNoiM is distinguished from the well-known active s-SNOM, which probes complex dielectric constants …”

“…As a result, hot electrons are easily disturbed and cannot be probed by any contact-type nanothermometry techniques − as the contact-type nanothermometry, e.g., the so-called scanning thermal microscopy (SThM), relies on the direct thermal contact between the sample (lattice) and the probe tip. On the other hand, most of the noncontact-type nano-optical techniques − require intensive optical excitation which often perturbs the hot electron subsystem and are therefore invasive to hot electrons. For instance, Raman scattering spectroscopy − is sensitive to the lattice (phonon modes) but insensitive to conduction electrons.…”

“…For instance, Raman scattering spectroscopy − is sensitive to the lattice (phonon modes) but insensitive to conduction electrons. As a consequence, despite recent extensive interests in nanothermometry studies, experimental evidence for clarifying nanoscopic hot-electron effects − , remains to be mostly indirect so far. In fact, only lattice temperature ( T L ), rather than the effective electron temperature ( T e ), is probed in most of the previous works, making the thermal properties and microscopic kinetic behaviors of hot electrons largely unexplored up to now.…”

Recent progress and challenges on two-dimensional material photodetectors from the perspective of advanced characterization technologies

“…Characterization of the evanescent field benefits comprehension of noise. The s-SNOM has been utilized to identify noise distribution [191]. Lin et al fabricated a graphene device with a narrow channel of 700 nm [192].…”

THz Pulsed Imaging in Biomedical Applications

“…Driven by technological advances, a plethora of new modern diagnostics systems is investigated and validated, as complementary biomedical techniques to the conventional ones, covering various scales, from macromolecules to tissues/organs. Among them, worthy of special mention are: infrared (IR) imaging [1,2], scanning near-field microscopy [3,4], photoacoustic microscopy [5], ultrasonic imaging [6], optical coherence tomography [7,8], digital holography microscopy [9][10][11], Raman scattering microscopy [12], Coherent Raman Scattering (CRS) spectroscopic imaging [13][14][15][16][17][18][19][20][21][22][23], two-photon fluorescence (TPF) [19,24] and second-harmonic generation (SHG) imaging [25][26][27], and super-resolved imaging techniques [28][29][30][31].…”