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Light propagation in Photonic integrated circuits (PICs), which can nowadays involve complex systems of light-guiding structures, is measured with different approaches(Nuzhdin et al., 2020, Lončar et al., 2002, Hopman et al., 2007, Morichetti et al., 2014, Sapienza et al., 2012, Vesseur et al., 2007). An interferometric polarization-sensitive measurement of the evanescent fields of these structures provides insight in the performance of the circuit detects possible malfunction with sub-wavelength precision(Engelen et al., 2007, Gersen et al., 2005, Barwick et al., 2009). We demonstrate a Near-field scanning optical microscopy (NSOM) measurement on coupled ring resonators that shows how the guided modes evolve as they propagate across the optical chip. Analysis of the measurements provides information about intensity distribution of the two polarization components (TE and TM) and their spatial confinement. The data validates our methodology and opens new possibilities for analysis of signal propagation in prototyping and optimization of integrated optical systems. Direct observation of polarization state of the guided modes allows for clear understanding of mode conversion in coupling systems.

Plasmonic nanoantennas are pushing the limits of optical imaging resolution capabilities in near-field scanning optical microscopy (NSOM). Accordingly, these techniques are driving the basic understanding of photonic and optoelectronic nanoscale devices with applications in sensing, energy conversion, solid-state lighting, and information technology. Imaging the localized surface plasmon resonance (LSPR) at the nanoscale is a key to understanding the optical responses of a given tip geometry in order to engineer better plasmonic nanoantennas for near-field experiments. In recent years the advancement of focused ion beam technology provides the ability to directly modify plasmonic structures with nanometer resolution. Also, scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) is an established technique allowing imaging of LSPR. Specifically, the combination of these two techniques provides spectrally sensitive two-dimensional (2D) image information to better visualize and understand LSPR on the nanometer scale. This can be combined with electron tomography to provide the three-dimensional LSPR distribution. Here we demonstrate the fabrication of Au nanopyramids using helium ion microscopy, and analyze the LSPR in 3D reconstructions produced by total variation (TV)-norm minimization of a set of 2D STEM-EELS maps. Additionally, a boundary element simulation method was used to verify the experimentally observed nanopyramid LSPR modes. Finally, we show that the point-spread-functions (PSF) of LSPR mode hot spots in nanopyramids differ to local electric-field enhancement under optical excitation making direct comparison to NSOM experimental resolution difficult. However, the STEM-EELS results show how LSPR modes are influenced by the tip characteristics, which can inform the development of new nanoantenna designs.

Highly evanescent waveguides with a subwavelength core thickness present a promising lab-on-chip solution for generating nanovolume trapping sites using overlapping evanescent fields. In this work, we experimentally studied Si3N4 waveguides whose sub-wavelength cross-sections and high aspect ratios support fundamental and higher order modes at a single excitation wavelength. Due to differing modal effective indices, these co-propagating modes interfere and generate beating patterns with significant evanescent field intensity. Using near-field scanning optical microscopy (NSOM), we map the structure of these beating modes in three dimensions. Our results demonstrate the potential of NSOM to optimize waveguide design for complex field trapping devices. By reducing the in-plane width, the population of competing modes decreases, resulting in a simplified spectrum of beating modes, such that waveguides with a width of 650 nm support three modes with two observed beats. Our results demonstrate the potential of NSOM to optimize waveguide design for complex field trapping devices.

This article provides a careful comparison between the electric and mechanical excitation of a tuning fork for shear force feedback in scanning probe microscopy, an analysis not found in present literature. A setup is designed and demonstrated for robust signal and noise measurements at comparable levels of physical movement of the probe. Two different signal amplification methods, combined with two excitation ways provide three possible configurations. For each method a quantitative analysis, supported by analytical elaboration and numerical simulations, is provided. Finally, it is shown that in practical circumstances electric excitation followed by detection with a transimpedance amplifier provides the best result. • Shear force feedback provides height feedback in scanning probe microscopy. • Quartz tuning forks outfitted with sharp probe suit purpose of shear force detection. • Sub-micrometer movement of a tuning fork induces interaction between tip and sample. • Possible tuning fork excitation and signal detection methods impact measurement. [ABSTRACT FROM AUTHOR]

The size of the V-defects in the GaN/InGaN-based quantum wells blue light-emitting diode (LED) was intentionally modified from 50 nm to 300 nm. High resolution photoluminescence and electroluminescence of a single large V-defect were investigated by near-field scanning optical microscopy. The current distribution along the {10-11} facets of the large defect was measured by conductive atomic force microscopy. Nearly 20 times the current injection and dominant emission from bottom quantum wells were found in the V-defect compared to its vicinity. Such enhanced current injection into the bottom part of quantum wells through V-defect results in higher light output power. Reduced external quantum efficiency droops were achieved due to more uniform carrier distribution. The un-encapsulated fabricated chip shows light output power of 172.5 mW and 201.7 mW at 400 mA, and external quantum efficiency drop of 22.3% and 15.4% for the sample without and with large V-defects, respectively. Modified V-defects provide a simple and effective approach to suppress the efficiency droop problem that occurs at high current injection, while improving overall quantum efficiency.

Subwavelength confined waveguiding is experimentally demonstrated with high refractive index dielectric nanoparticles with photon energy propagation at distances beyond 500 μm. These particles have naturally occurring electric and magnetic dipole resonances. When they are placed in a 1D chain, the magnetic resonances of adjacent elements couple to each other, providing a means to transport energy at visible or NIR wavelengths in a confined mode. Chains of nanoparticles made of silicon were fabricated and guided waves were measured with near-field scanning optical microscopy. Propagation loss is quantified at 34 dB/mm for 720 nm and 5.5 dB/mm for 960 nm wavelengths with 150 and 220 nm diameter particles, respectively. Simulations confirm the unique properties of this waveguiding in comparison with photonic crystals. The resonant nature of the waveguide lays a foundation for integrated photonics beyond nanowire waveguides of silicon and silicon nitride. This technology is promising for more compact and deeper photonic integration such as right angle bends, more compact modulators, slow light and interfacing with single photon emitters for photonic integrated circuits, quantum communications, and biosensing. [ABSTRACT FROM AUTHOR]

Long regarded as a model system for studying insulator-to-metal phase transitions, the correlated electron material vanadium dioxide (VO2) is now finding novel uses in device applications. Two of its most appealing aspects are its accessible transition temperature (~341 K) and its rich phase diagram. Strain can be used to selectively stabilize different VO2 insulating phases by tuning the competition between electron and lattice degrees of freedom. It can even break the mesoscopic spatial symmetry of the transition, leading to a quasiperiodic ordering of insulating and metallic nanodomains. Nanostructuring of strained VO2 could potentially yield unique components for future devices. However, the most spectacular property of VO2--its ultrafast transition--has not yet been studied on the length scale of its phase heterogeneity. Here, we use ultrafast near-field microscopy in the mid-infrared to study individual, strained VO2 nanobeams on the 10 nm scale. We reveal a previously unseen correlation between the local steady-state switching susceptibility and the local ultrafast response to below-threshold photoexcitation. These results suggest that it may be possible to tailor the local photoresponse of VO2 using strain and thereby realize new types of ultrafast nano-optical devices. [ABSTRACT FROM AUTHOR]

3D surface topography is simulated using Boundary Element Method (BEM) by computing far field scattered by a metallic or dielectric nanosphere which scans in the near field region of an arbitrary rough surface. Nanosphere is considered as NSOM probe's tip and two scanning modes, Constant Height Mode (CHM) and Constant Distance Mode (CDM) are simulated. Our results show that resolution and image quality are enhanced by decreasing scanning sphere's height or diameter in CHM. Also, image quality in Transverse Magnetic (TM) polarization is higher compared with Transverse Electric (TE) polarization. In TM polarization for both scanning mode CHM and CDM, Aluminum scanner makes a better image quality in comparison to dielectric sphere. However in TE polarization, electric field intensity scattered by a dielectric sphere is higher compared with Aluminum sphere. In spite of higher image quality in CHM, image quality is not satisfactory in CDM and image does not show a perfect match with the surface topography. High precision, speed, and low amount of data during the calculations are advantages of BEM in comparison to other numerical methods. [ABSTRACT FROM AUTHOR]

The migration of mesenchymal stem cells (MSCs) plays a key role in tumor-targeted delivery vehicles and tumor-related stroma formation. However, there so far has been no report on the distribution of cell surface molecules during the VEGF-induced migration of MSCs. Here, we have utilized near-field scanning optical microscopy (NSOM) combined with fluorescent quantum dot (QD)-based nano-technology to capture the functional relationship between CD44 and CD29 adhesion molecules on MSCs and the effect of their spatial rearrangements. Before VEGF-induced migration of MSCs, both CD44 and CD29 formed 200–220 nm nano-domains respectively, with little co-localization between the two types of domains. Surprisingly, the size of the CD44 nano-domain rapidly increased in size to 295 nm and apparently larger aggregates were formed following MSC treatment with VEGF for 10 min, while the area of co-localization increased to 0.327 μm 2 . Compared with CD44, CD29 was activated obviously later, for the fact that CD29 aggregation didn't appear until 30 min after VEGF treatment. Consistently, its co-localization area increased to 0.917 μm 2 . The CD44 and CD29 nano-domains further aggregated into larger nano-domains or even formed micro-domains on the membrane of activated MSCs. The aggregation and co-localization of these molecules promoted FAK formation and cytoskeleton rearrangement. All of the above changes induced by VEGF contributed to MSC migration. Taken together, our data of NSOM-based dual color fluorescent imaging demonstrated for the first time that CD44, together with CD29, involved in VEGF-induced migration of MSCs through the interaction between CD44 and its co-receptor of VEGFR-2. [ABSTRACT FROM AUTHOR]

Surface arrays of photoactive materials with subwavelength dimensions are important for energy harvesting applications due to their capacity for broadband and omnidirectional absorption of the solar radiation. It was recently proposed and demonstrated that the absorption performance of such arrays can be further amplified with the introduction of additional nano-features. In the current work we consider the incorporation of nano-features into nanopillar (NP) arrays. The considered nano-features are: quasi-nanolenses, sidewall nano-decorations, and decreasing the NP bottom diameter to transform it into a light funnel (LF). The underlying various light trapping mechanisms are examined using near-field microscopy. Near-field microscopy is used to obtained both high-resolution far-field imaging, as well as mapping of the near-field photon distributions of the arrays. Firstly, it is shown that the main contribution to far-field reflected photons is from the ambient surrounding the silicon structures. Secondly, the incorporation of the various nano-features induces a decrease in the reflected far-field photons which originate from this ambient region. Also, it is shown how the incorporation of quasi-nanolenses concludes a decrease in far-field photons reflected in the oblique directions with high polar angles, whereas the incorporation of sidewall nano-decorations leads to a decrease in far-field photons reflected in directions of small polar angles. Also, the incorporation of both qNL and sidewall nano-decorations is examined, and light trapping with the LF geometry is discussed and demonstrated. Finally, understanding the underlying light trapping mechanisms will support the deterministic design of subwavelength arrays incorporated with nano-features for efficient harvesting of the solar radiation. [Display omitted] • Nano-features for broadband absorption of the solar radiation. • High-resolution far-field imaging of subwavelength arrays incorporated with nano-features. • Near-field photon imaging for the determination of solar absorption. • Determination of light trapping mechanisms with near-field optical microscopy. [ABSTRACT FROM AUTHOR]

Recent advances in Surface Plasmon Resonance (SPR) technologies have shown the possibility of transmission enhancement of localized modes propagating through sub-diffraction wide slits and apertures, resulting in the strong near-field focusing of metallic planar nanostructures. This work presents a new approach to the fabrication of high-resolution near-field optical probes using 3D lithography in combination with numerical finite difference time domain (FDTD) simulations. A narrow 500 nm depth of field focus area was observed both by numerical analysis and near field scanning optical microscopy (NSOM) measurements. Further research and optimization are planned in order to achieve subwavelength focal regions and increased signal intensities. [ABSTRACT FROM AUTHOR]

Near-field scanning optical microscopy (NSOM) is a promising technique to perform nanoscale fluorescence measurements with a nano-sized near-field-light-source. However, much like a conventional confocal measurement, it requires illuminating the sample with the focus spot of the propagating light even when the fluorescence signal of interest is generated only from a nano-sized area of the sample, causing a serious photodegradation of the sample in a large area. Here, we propose to combine NSOM with plasmon nanofocusing to eliminate the direct illumination of the sample by a propagating light and demonstrate that the photodegradation could be significantly suppressed. This approach allows one to perform nanoscale fluorescence imaging with a negligible effect of photodegradation. • Plasmon-nanofocusing-based NSOM. • Fluorescence nanoimaging. • Suppression of photodegradation. [ABSTRACT FROM AUTHOR]

Cesium-halide perovskite quantum dots (QDs) have gained tremendous interest as quantum emitters in quantum information processing applications due to their optical and photophysical properties. However, engineering excitonic states in quantum dots requires a deep knowledge of the coherent dynamics of their excitons at a single-particle level. Here, we use femtosecond time-resolved two-photon near-field scanning optical microscopy (NSOM) to reveal coherences involving a single cesium lead bromide perovskite QD (CsPbBr 3 ) at room temperature. We show that, compared to other nonperovskite nanoparticles, the electronic coherence on a single perovskite QD has a relatively long lifetime of ca. 150 fs, whereas CdSe QDs have exciton coherence times shorter than 75 fs at room temperature. One possible explanation for the longer coherence time observed for the CsPbBr 3 perovskite system is related to the exciton fine structure of these perovskite QDs compared to other nanoparticles. These perovskite QDs exhibit interesting optical properties that differ from those of the traditional QDs including bright triplet exciton states. In fact, due to the small amplitude of the energy gap fluctuations of dipole-allowed triplet states in perovskite QDs, the coherent superposition could be preserved for longer times. Furthermore, single-particle excitation approach implemented in this work allows us to remove effects of heterogeneity that are usually present in ensemble averaging experiments at room temperature. The realization of quantum-mechanical phase-coherence of a charge carrier that can operate at room temperature is an issue of great importance for the potential application of coherent electronic phenomena in electronic and optoelectronic devices. These interesting findings provide further evidence of the great potential of these perovskite QDs as candidates for quantum computing and information processing applications.

Subsurface mapping is crucial to understanding many biological systems as well as structured thin films for (opto)electronic or photonic applications. A non‐invasive method is presented to map subsurface nanostructures from scanning near‐field optical microscopy images. The Bethe–Bouwkamp model is used to simulate imaging of buried nano‐objects or subsurface slanted planar interfaces, and it is shown how to determine their depth and size, or the interface inclination, from just one image. It is shown that the steep optical field gradient makes near‐field microscopy a particularly sensitive depth probe for thin films. [ABSTRACT FROM AUTHOR]

The size of the V-defects in the GaN/InGaN-based quantum wells blue light-emitting diode (LED) was intentionally modified from 50 nm to 300 nm. High resolution photoluminescence and electroluminescence of a single large V-defect were investigated by near-field scanning optical microscopy. The current distribution along the {10-11} facets of the large defect was measured by conductive atomic force microscopy. Nearly 20 times the current injection and dominant emission from bottom quantum wells were found in the V-defect compared to its vicinity. Such enhanced current injection into the bottom part of quantum wells through V-defect results in higher light output power. Reduced external quantum efficiency droops were achieved due to more uniform carrier distribution. The un-encapsulated fabricated chip shows light output power of 172.5 mW and 201.7 mW at 400 mA, and external quantum efficiency drop of 22.3% and 15.4% for the sample without and with large V-defects, respectively. Modified V-defects provide a simple and effective approach to suppress the efficiency droop problem that occurs at high current injection, while improving overall quantum efficiency. [ABSTRACT FROM AUTHOR]